variable cost includes cost of water use, electricity (parasitic energy), labor, fertilizers (N & P) and waste water. V: culture volume.

**Table 1.** A comparative study [21] on algal (*Chlorella vulgaris*) cultivation technologies which include open pond and closed photobioreactors (PBRs: Tubular and flat panels) and economics of algal biomass production. The high productivities of flat panels compared to the other systems are reflected in the lower cost price.

and the cyanobacterium/green alga spirulina and *B. braunii,* respectively. The loading densities in the tests were 0.0178 (*R. sphaeroides*), 0.0792 (spirulina), and 0.050 (*B.braunii*) nmol/cm2 .

#### **4.2. Experimental test systems**

The wavelength dependence of photosynthesis by purple bacteria and microalgae has been known since the early twentieth century and confirmed many times in different species. As shown in **Figure 4**, green algae/cyanobacteria/higher plants show the greatest activity with red light, whereas purple bacteria are most active under near-infrared (NIR) [38–42]. The effect is so powerful that these organisms have developed an apparent "phototaxis" response, accumulating in the optically optimal part of a natural water column [38]. This ability is very important because red light is absorbed strongly by water, and hence, the availability of useful light drops markedly with depth. Blue light has a far greater penetration (see later).

The action spectra of photosynthetic microorganisms have been extensively surveyed. Green microorganisms (and plant chloroplasts) conform to a generic action spectrum, while purple bacteria conform to a distinctly different generic action spectrum (see [35] and **Figure 4**)

One method of "upgrading" "waste" light of a particular wavelength is to use the light-emitting properties of quantum dots (QDs). QDs are single crystals of uniform size and shape of ~2–10 nm diameter and usually comprising pairs of semiconductors (e.g., CdSe, PbSe). QDs are replacing fluor dyes in cell biology due to their high brightness and photostability [43]. The properties and potential applications of QDs are described elsewhere in this volume, and indeed, QDs are commercially available in appropriate delivery systems for boosting horticulture and small-scale crop production [44] but have yet to find application in large-scale photobioreactor systems. However, for bioenergy applications and bulk-scale animal feed production, large scale constructions would be required (e.g., see **Figure 3b** and **Table 1**). Hence, a feasibility study was undertaken using the three microorganisms described above to indicate whether photosynthetic boosting via QDs is feasible for algal and bacterial growth systems. The use of LEDs to supply additional lighting at the optimal wavelengths is wellestablished technology [44, 45], and it is assumed to be intrinsically scalable, although a full

The concept of photonic enhancement is to increase the proportion of the solar spectrum that corresponds to the major peak(s) of the organismal action spectrum (**Figure 4**), at the expense of other irradiance at less active wavelengths. The part of the spectrum to be intensified is referred to as the target band. **Figure 4** (top panel) shows the boundaries of the target band corresponding to the half maximum of the major peak in the organismal action spectrum. Using generic action spectra derived previously [43], the target bands of 640–690 nm and 790–940 nm were determined for algae/cyanobacteria and purple bacteria, respectively. These

The study used test quantum dots purchased from Invitrogen: Qdot'792 (ITK carboxyl, no Q21371, lot 834,674; quantum yield (QY) 72%; full width height maximum (FWHM): 82 nm) and QD'652 (ITK carboxyl, no. Q21321MP, lot 891,174; QY 78%; FWHM 26 nm) for cultures of *R. sphaeroides*

attributable to the different chlorophylls evolved in the taxanomic groups.

156 Nonmagnetic and Magnetic Quantum Dots

**4. Quantum dots as a potential means of "upgrading" light**

cost-benefit analysis is required for applications in biofuels production.

**4.1. Boosting of three photosystems using quantum dots**

bands account for 25 and 67% of the total action, respectively.

#### *4.2.1. Rhodobacter sphaeroides for biohydrogen production*

*R. sphaeroides* was used in a test system of mounted vials as shown in **Figure 5**, using simulated sunlight.

#### *4.2.2. Spirulina for biomass production*

*Arthrospira platensis* (spirulina) was used in a test system (**Figure 6**) using a close-match solar simulator (supplementary material in Redwood et al. [35]).

#### *4.2.3. B. braunii: A single-celled alga for bio-oil production*

*B. braunii* was cultured routinely in shake flask cultures. Q dot'652 was added directly into small 25 ml cultures to 10 nm. Cultures were shaken in a temperature-controlled greenhouse (average solar photon flux was 11 μmol/m2 /s). Photosynthetic action was inferred from growth at 21 days as estimated by OD600.

#### **4.3. Photosynthetic enhancement using commercial quantum dots**

The first test, using *R. sphaeroides* to produce H2 , showed a photonic enhancement of ~10% (**Table 2**; **Figure 8c**). This was a close fit to the increase predicted by the known QD quantum efficiency and the QD loading/cm2 . Photonic enhancement of the growth of *A. platensis* (spirulina) doubled the biomass yield, which was ~25% higher than a predicted stimulation on the basis of quantum yield and QD loading density (**Figure 8d**). In this example, the QDs were held separate from the culture (**Figure 6**), which rules out stimulation via components leaching from the QD preparation. Finally, using *B. braunii* with QDs added directly into the culture and incubated in sunlight, the biomass yield was increased by 2.4-fold (**Figure 8b**). This photonic enhancement, 2.4-fold with respect to optical density, was >50% higher than that predicted on the basis of quantum yield and loading. A growth stimulatory effect of contaminants was largely ruled out on the basis of the test using spirulina, which was held separate from the cells (above). However, as *B. braunii* becomes heavily loaded with oil globules during growth (**Figure 7**), their contribution to increasing the size (and hence OD600) of the cells cannot be precluded. The effect of photosynthetic enhancement on oil production was not examined in this study. It was concluded that the use of QDs as photonic enhancers has potential, but the light emission from the commercial QDs was not at the ideal wavelength (**Figure 4**), while the high cost of commercial QDs would currently be prohibitive in large scale systems, although the potential cost reduction at bulk scale is not known.


Data are means ± SEM for the number of experiments shown in parentheses. Photonic enhancements are modest due to the low dose of QDs used but were in accordance with theoretical predictions. The maximum enhancement was not tested. Enhancements are statistically significant at P = 0.95.

\* The criterion for the algae was biomass content/ml (OD600) that for *R. sphaeroides* was production of hydrogen. # Data from shake flask tests.

**Table 2.** Photosynthetic boosting using quantum dots.

**5. Factors affecting development of quantum dots for enhancement** 

This feasibility study (carried out in 2012) was limited by the suitable but nonideal properties of commercially available QDs. Small shifts in the emission peaks could improve the photonic

**Figure 8.** Enhancement of photosynthetic activity using QDs in three test systems. (a) Spectrum showing absorbance of chlorophylls a and b, and QD emission. Red emission was used (circled) for the algae and near infra red emission for *R. sphaeroides* (not shown). (b), (d) Photoproductivity of *B. braunii* (b) and *A. platensis* (d) with QDs (c) Hydrogen production

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For cyanobacteria and algae, the available Qdot'652 was suitable as the major peak in the action spectra of cyanobacteria and algae occurs inside 640–690 nm (**Figure 4**), while the main part of the emission of Qdot'652 was within 639–665 nm (boundaries placed at half-maximum). Further development would aim to adjust the emission peak to ~655 nm while maintaining high quantum yield (QY) and full width height maximum (FWHM) ≤35 nm. For purple bacteria, the major peak in the action spectrum occurs inside the 790–940 nm region (**Figure 4**), while the main part of the emission of Qdot'792 was within 751–833 nm (boundaries placed at

**of photosynthetic biomass and biofuel processes**

enhancement, particularly in purple bacteria.

by *R. sphaeroides* with QDs. N is the number of tests in each case.

**Figure 7.** *Botryococcus braunii*. (A) Twenty-day-old *Botryococcus braunii* culture in uplift photobioreactor. (B) Bright field image of *Botryococcus braunii*, Race B. Pyriform *B. braunii* cells held together by a hydrocarbon-polysaccharide matrix. Oil containing vesicles are clearly visible inside the cells, which contain a single chloroplast. Images were acquired using an Olympus BX51 System Microscope with an attached DP71 digital CCD camera. Image processing and analysis software used was Cell F version 2.8 from Olympus).The scalebar represents 5 μm.

and the QD loading/cm2

158 Nonmagnetic and Magnetic Quantum Dots

. Photonic enhancement of the growth of *A. platensis* (spirulina) doubled

the biomass yield, which was ~25% higher than a predicted stimulation on the basis of quantum yield and QD loading density (**Figure 8d**). In this example, the QDs were held separate from the culture (**Figure 6**), which rules out stimulation via components leaching from the QD preparation. Finally, using *B. braunii* with QDs added directly into the culture and incubated in sunlight, the biomass yield was increased by 2.4-fold (**Figure 8b**). This photonic enhancement, 2.4-fold with respect to optical density, was >50% higher than that predicted on the basis of quantum yield and loading. A growth stimulatory effect of contaminants was largely ruled out on the basis of the test using spirulina, which was held separate from the cells (above). However, as *B. braunii* becomes heavily loaded with oil globules during growth (**Figure 7**), their contribution to increasing the size (and hence OD600) of the cells cannot be precluded. The effect of photosynthetic enhancement on oil production was not examined in this study. It was concluded that the use of QDs as photonic enhancers has potential, but the light emission from the commercial QDs was not at the ideal wavelength (**Figure 4**), while the high cost of commercial QDs would currently be prohibitive in large scale systems, although the potential cost reduction at bulk scale is not known.

**Microbial group Organism QD-free controls Experiment with QDs\* Photonic enhancement\***

Data are means ± SEM for the number of experiments shown in parentheses. Photonic enhancements are modest due to the low dose of QDs used but were in accordance with theoretical predictions. The maximum enhancement was not

**Figure 7.** *Botryococcus braunii*. (A) Twenty-day-old *Botryococcus braunii* culture in uplift photobioreactor. (B) Bright field image of *Botryococcus braunii*, Race B. Pyriform *B. braunii* cells held together by a hydrocarbon-polysaccharide matrix. Oil containing vesicles are clearly visible inside the cells, which contain a single chloroplast. Images were acquired using an Olympus BX51 System Microscope with an attached DP71 digital CCD camera. Image processing and analysis software

The criterion for the algae was biomass content/ml (OD600) that for *R. sphaeroides* was production of hydrogen.

(13)

(4)

(3)

1.1-fold

2.1-fold

2.4-fold

Purple nonsulfur bacteria *Rhodobacter sphaeroides* 15.54 ± 0.31 (13) 17.00 ± 0.16

Cyanobacteria *A. platensis* 0.025 ± 0.002 (6) 0.052 ± 0.011

True algae# *B. braunii* 0.106 ± 0.032 (3) 0.251 ± 0.011

tested. Enhancements are statistically significant at P = 0.95.

**Table 2.** Photosynthetic boosting using quantum dots.

used was Cell F version 2.8 from Olympus).The scalebar represents 5 μm.

\*

#

Data from shake flask tests.

**Figure 8.** Enhancement of photosynthetic activity using QDs in three test systems. (a) Spectrum showing absorbance of chlorophylls a and b, and QD emission. Red emission was used (circled) for the algae and near infra red emission for *R. sphaeroides* (not shown). (b), (d) Photoproductivity of *B. braunii* (b) and *A. platensis* (d) with QDs (c) Hydrogen production by *R. sphaeroides* with QDs. N is the number of tests in each case.

## **5. Factors affecting development of quantum dots for enhancement of photosynthetic biomass and biofuel processes**

This feasibility study (carried out in 2012) was limited by the suitable but nonideal properties of commercially available QDs. Small shifts in the emission peaks could improve the photonic enhancement, particularly in purple bacteria.

For cyanobacteria and algae, the available Qdot'652 was suitable as the major peak in the action spectra of cyanobacteria and algae occurs inside 640–690 nm (**Figure 4**), while the main part of the emission of Qdot'652 was within 639–665 nm (boundaries placed at half-maximum). Further development would aim to adjust the emission peak to ~655 nm while maintaining high quantum yield (QY) and full width height maximum (FWHM) ≤35 nm. For purple bacteria, the major peak in the action spectrum occurs inside the 790–940 nm region (**Figure 4**), while the main part of the emission of Qdot'792 was within 751–833 nm (boundaries placed at half-maximum). Therefore, a significant fraction of the emission fell outside the target band. Further development using this method would aim to adjust the emission peak to ~855 nm while maintaining high QY and FWHM ≤83 nm, thereby placing almost all QD emissions within the target band. For cyanobacteria and algae, this would require a FWHM for the QD of ≤35 nm, which matches the manufacturer specifications for Qdot'655. However, for purple bacteria, this could be more challenging as the longer wavelength emitting QDs typically produce broader emission peaks. However, the major action peak for purple bacteria is also broad (770–940 nm; half-maximum; **Figure 4**), suggesting an ideal emission peak of ~855 nm with FWHM ≤83 nm, which is similar to published specifications.

However, given poor penetration of the red component of sunlight in water (**Table 3**), it is apparent that a deep QD-reactor system with irradiation from above would be unsuitable for purple bacteria as they use red-infrared light. An algal system is less sensitive to culture depth, as it can utilize blue light; the loss of light at 655 nm was calculated to be ~30%, which would still be a factor to consider in photobioreactor design. However in the blue region, corresponding to an absorption maximum of chlorophyll b (575 nm: **Figure 4**), very little light is lost, while at 430 nm (optimum for chlorophyll a), the available light intensity is still acceptable with depth, meaning that "point" sources of QD light could be used (insets or roof panels).

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**6. Potential alternative strategy for economic production of quantum** 

leaching and recovery as metal sulfides), which produces excess waste H2

form ZnS quantum dots by combination with Zn2+ ions. This strategy was tested in principle. Zinc sulfide has a bandgap varying from, in bulk material, 3.7 eV to, in nanoparticles, 4.2 eV [52, 53]. It has large exciton energy (~ 40 meV) and has been used in light-emitting diodes and, for example, flat panel displays [54]. The nanoparticles have to be stabilized during synthesis in order to minimize extensive agglomeration. This is important because the quantum yield

Methods of QD nanoparticle synthesis commonly use organic solvent [55], capping agent, and/or surfactant in order to control agglomeration. These methods may introduce problems of reproducibility as well as complexity and cost, as well as leaving residual chemicals and hence being nonsustainable (see [56] for overview). Looking toward large scale manufacturing, various "traditional" methods could reduce the high cost of ZnS NP-synthesis. Khani et al.

have also been used [58]. Here, refluxing with tetrapropyl ammonium hydroxide resulted in QDs of nanoparticle size 4.5 nm, and the respective absorption and emission peaks were 315

sulfate-reducing bacteria. These convert sulfate (dilute H2

[57] incorporated 2-mercaptoethanol as a capping agent; Na2

is lower in larger particles [54].

For incorporating quantum dots into photosynthesizing cultures, some forms of QD encapsulation or barrier method are likely to be required (see above), while the use of toxic materials *per se* is unattractive for manufacturing, even assuming that the QDs are held separate from the cells, are easily recovered and are re-usable. Given the high cost of commercial quantum dots, the possibility to use more traditional metallic-based semiconductors was revisited, since these can be made economically at scale, but the use of highly toxic metals such as Cd should still be avoided. The waste hydrogen sulfide off-gas from an (unrelated) bioremediation process was considered for use to promote the formation of zinc sulfide nanoparticles which are well-known QDs. Using a waste from a remediation process (which is, in itself, used to recover Zn and Cu from acidic mine wastes [50, 51]) is a paradigm example toward realizing a circular economy. The liquid minewater wastes are obtained via the activity of microorganisms that leach the metals out of ore residues and closed mines. They also lower the pH (by formation of sulfuric acid), and hence, they are acid-loving bacteria (acidophiles). The acidophilic bacteria are fed by using additional nutrients derived from an algal source, *Coccomyxa onubensis,* and hence, development of a method for enhancing growth of this alga via a QD-enhancement approach would impact positively on the economics of the primary metal recovery process (combined metal bio-

SO4

S from the activity of

) to sulfide, which is available to

S and mercaptopropionic acid

**dots at scale**

The discussion does not consider other potential impacts of QDs on the photobiological apparatus. The absorbance of less useful solar wavelengths by QDs could protect against damage from heat and UV irradiation, a benefit that would not be apparent from the experiments described here, as the temperature was actively controlled and much of the UV element of sunlight was absorbed by several layers of glass before reaching the QDs or the culture.

In this feasibility study, a single type of QD was selected to align as closely as possible with the major action peak of the organism. Further development could combine different QDs to further enrich the solar spectrum, according to the minor action peaks (**Figure 4**). There is also further potential in using combinations to further enrich the spectrum at ~680 or ~850 nm above the model presented here. Nature has evolved complex but optimal systems, for example, purple bacteria have ancillary pigments which absorb light in the visible region (e.g., 400–500 nm) and transfer energy very rapidly onto the bacteriochlorophylls [46, 47]. A biomimicry approach could use alternative QDs to construct a spectrum that precisely mirrors the action spectrum.

One important technical factor affecting practical photonic enhancement would be the stability of QDs. QDs can be affected by photobleaching [48], and the leaching of QD degradation products could have a potential negative environmental impact. Therefore, further investigations should focus on QD immobilization methods, aiming to make QDs a permanently encapsulated part of a photobiological installation and to enable low-risk handling in large quantities and recovery for multiple uses.

Finally, since the photobiotechnologies envisaged here would necessarily be at large scale to supply energy carriers for replacement fuels (as compared to the relatively small PBRs used for high-value products), maximum light transfer from sunlight to the QDs and light upgrading to the cells is essential, while maintaining a minimum QD loading for economy.


**Table 3.** Light attenuation in water.

However, given poor penetration of the red component of sunlight in water (**Table 3**), it is apparent that a deep QD-reactor system with irradiation from above would be unsuitable for purple bacteria as they use red-infrared light. An algal system is less sensitive to culture depth, as it can utilize blue light; the loss of light at 655 nm was calculated to be ~30%, which would still be a factor to consider in photobioreactor design. However in the blue region, corresponding to an absorption maximum of chlorophyll b (575 nm: **Figure 4**), very little light is lost, while at 430 nm (optimum for chlorophyll a), the available light intensity is still acceptable with depth, meaning that "point" sources of QD light could be used (insets or roof panels).

half-maximum). Therefore, a significant fraction of the emission fell outside the target band. Further development using this method would aim to adjust the emission peak to ~855 nm while maintaining high QY and FWHM ≤83 nm, thereby placing almost all QD emissions within the target band. For cyanobacteria and algae, this would require a FWHM for the QD of ≤35 nm, which matches the manufacturer specifications for Qdot'655. However, for purple bacteria, this could be more challenging as the longer wavelength emitting QDs typically produce broader emission peaks. However, the major action peak for purple bacteria is also broad (770–940 nm; half-maximum; **Figure 4**), suggesting an ideal emission peak of ~855 nm

The discussion does not consider other potential impacts of QDs on the photobiological apparatus. The absorbance of less useful solar wavelengths by QDs could protect against damage from heat and UV irradiation, a benefit that would not be apparent from the experiments described here, as the temperature was actively controlled and much of the UV element of sunlight was absorbed by several layers of glass before reaching the QDs or the culture.

In this feasibility study, a single type of QD was selected to align as closely as possible with the major action peak of the organism. Further development could combine different QDs to further enrich the solar spectrum, according to the minor action peaks (**Figure 4**). There is also further potential in using combinations to further enrich the spectrum at ~680 or ~850 nm above the model presented here. Nature has evolved complex but optimal systems, for example, purple bacteria have ancillary pigments which absorb light in the visible region (e.g., 400–500 nm) and transfer energy very rapidly onto the bacteriochlorophylls [46, 47]. A biomimicry approach could use alternative QDs to construct a spectrum that precisely mirrors the action spectrum. One important technical factor affecting practical photonic enhancement would be the stability of QDs. QDs can be affected by photobleaching [48], and the leaching of QD degradation products could have a potential negative environmental impact. Therefore, further investigations should focus on QD immobilization methods, aiming to make QDs a permanently encapsulated part of a photobiological installation and to enable low-risk handling in large

Finally, since the photobiotechnologies envisaged here would necessarily be at large scale to supply energy carriers for replacement fuels (as compared to the relatively small PBRs used for high-value products), maximum light transfer from sunlight to the QDs and light upgrad-

> 4.2 1.8 4.0 8.7 16.7 71.0 82.0

ing to the cells is essential, while maintaining a minimum QD loading for economy.

**Color Wavelength, nm Percentage absorbed in 1 m of water (%)**

with FWHM ≤83 nm, which is similar to published specifications.

160 Nonmagnetic and Magnetic Quantum Dots

quantities and recovery for multiple uses.

**Violet Blue Green Yellow Orange Red Infrared**

Adapted from [49]

**Table 3.** Light attenuation in water.

## **6. Potential alternative strategy for economic production of quantum dots at scale**

For incorporating quantum dots into photosynthesizing cultures, some forms of QD encapsulation or barrier method are likely to be required (see above), while the use of toxic materials *per se* is unattractive for manufacturing, even assuming that the QDs are held separate from the cells, are easily recovered and are re-usable. Given the high cost of commercial quantum dots, the possibility to use more traditional metallic-based semiconductors was revisited, since these can be made economically at scale, but the use of highly toxic metals such as Cd should still be avoided. The waste hydrogen sulfide off-gas from an (unrelated) bioremediation process was considered for use to promote the formation of zinc sulfide nanoparticles which are well-known QDs. Using a waste from a remediation process (which is, in itself, used to recover Zn and Cu from acidic mine wastes [50, 51]) is a paradigm example toward realizing a circular economy. The liquid minewater wastes are obtained via the activity of microorganisms that leach the metals out of ore residues and closed mines. They also lower the pH (by formation of sulfuric acid), and hence, they are acid-loving bacteria (acidophiles). The acidophilic bacteria are fed by using additional nutrients derived from an algal source, *Coccomyxa onubensis,* and hence, development of a method for enhancing growth of this alga via a QD-enhancement approach would impact positively on the economics of the primary metal recovery process (combined metal bioleaching and recovery as metal sulfides), which produces excess waste H2 S from the activity of sulfate-reducing bacteria. These convert sulfate (dilute H2 SO4 ) to sulfide, which is available to form ZnS quantum dots by combination with Zn2+ ions. This strategy was tested in principle.

Zinc sulfide has a bandgap varying from, in bulk material, 3.7 eV to, in nanoparticles, 4.2 eV [52, 53]. It has large exciton energy (~ 40 meV) and has been used in light-emitting diodes and, for example, flat panel displays [54]. The nanoparticles have to be stabilized during synthesis in order to minimize extensive agglomeration. This is important because the quantum yield is lower in larger particles [54].

Methods of QD nanoparticle synthesis commonly use organic solvent [55], capping agent, and/or surfactant in order to control agglomeration. These methods may introduce problems of reproducibility as well as complexity and cost, as well as leaving residual chemicals and hence being nonsustainable (see [56] for overview). Looking toward large scale manufacturing, various "traditional" methods could reduce the high cost of ZnS NP-synthesis. Khani et al. [57] incorporated 2-mercaptoethanol as a capping agent; Na2 S and mercaptopropionic acid have also been used [58]. Here, refluxing with tetrapropyl ammonium hydroxide resulted in QDs of nanoparticle size 4.5 nm, and the respective absorption and emission peaks were 315 and ~415 nm [58]. Other work reported QDs with absorbance and emission peaks at 279 and 435 nm, respectively; this method utilizes thiolactic acid with Zn2+ solution and Na2 S [59]. Being very close to the absorption peak of chlorophyll a at 430 nm (**Figure 4**), this raises the possibility to use ZnS NPs as a quantum dot ancillary to Qdot'655 (above) or, indeed as a substitute for the latter, using emitted blue light via the other absorbance region for chlorophyll a (see above).

via irradiation of chlorophyll a in the blue region, as an alternative to visible-red wavelengths, also noting the preferred use of blue light for deep culture (above). The emission of biogenic ZnS QDs prepared in 50 mM citrate buffer, pH 6, was reported at 410 nm [60], whereas the optimal absorbance wavelength of chlorophyll a is ~430 nm (**Figure 4**); respective molar extinction coefficients at 410 and 431.66 nm were calculated as 70,733 and 110,789 cm−1/M, respectively [66, 67]. At ~425 nm, this was given as 93,099 and 98,874 cm−1/M at 424.8 and 426.15 nm, respectively [66]. This illustrates the need to redshift the QD emission of the bio-

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The early study [60] used 50 mM citrate buffer (pH 6) to prevent uncontrolled precipitation of ZnS by chelating the Zn2+ in solution and acting as a passivant for the ZnS nanoparticles.

**Figure 9.** Excitation (a) and Emission (b) Spectra of ZnS Quantum dots. QDs were synthesized using excess waste

rate 132 ml/min). Example scans are shown. ZnS QDs were made in the presence of 10 mM, 25 mM, 50 mM and 100 mM citrate buffer (pH 6) as shown. Note excitation redshift to ~ 310 nm and emission from ~ 410 to ~425 nm (dotted lines) with decreased citrate concentration from 50 to 10 mM. Buffering at pH 6 without citrate (MES-NaOH buffer) gave

S from a metal bioremediation process [60]. Samples were sparged with the culture off-gas for 30 min (flow

biogenic H2

material with negligible emission.

genic ZnS QDs by up to 15–20 nm to realize the full potential.

**7. Toward realizing useful quantum dots from biogenic ZnS**

A first report [60] showed that the characteristics and the light emitting properties of ZnS quantum dots made by use of bacterially made waste H2 S left over from the metal bioremediation process [50, 51] were comparable to those made by "classical" methods, which required more complex procedures. As a potential synthesis method at scale, this shows potential for commercial QD production and introduces the possibility to use these biogenic ZnS QDs to promote algal growth for the applications described above and also to provide algal feedstock as a nutrient source for other processes (e.g. high-value chemicals); algae as biomass feedstock *per se* for pyrolysis oil production has also been reported (e.g., [5]).

The price of commercial QDs discourages development above small-scale and a full-scale energy plant is probably currently unfeasible. However, QDs are rapidly developing from niche markets into consumer electronics [61], which is expected to bring substantial increases in production scale, and hence, reduction in cost may be expected in the future. From the tests and data shown above, the QD cost would have to be reduced by up to 100-fold in order for photonic photobioreactors to achieve parity with standard PBRs in terms of capital cost (M.D. Redwood, unpublished). This estimation was based on early published values for capital costs of different photobiological systems [62–65] and a survey (in 2012) of market prices for commercial QDs. Because open systems or raceways present much lower capital costs (per unit area) than enclosed PBRs, the estimated minimum QED cost reduction would be ~10 fold more attractive for raceways, suggesting that such enhancement would be first tested in PBRs then developed in raceways at scale as costs fall.

However, these estimations do not consider the reduced land requirement and reduced running costs of photonically enhanced photobioreactors, which may lessen the cost impact. On the other hand, end-of-life decommissioning may be more costly if potentially toxic metals have been used. QD retention via immobilization/encapsulation and re-use would be a key strategy. The extent to which biofouling of transparent surfaces in contact with the culture may impact adversely on QD-enhanced PBR useful life has not been taken into account (nor tested). Common methods to remove biofouling deposits (e.g., scraping) may damage surfaces that have been precision-machined or polished for optical transmission. Hence, an air gap between the QD enclosure and the culture liquid may prove beneficial. In practice, as long as there is sufficient stirring, the shear force is sufficient to prevent fouling problems. This means that sufficient shear force being produced by sparging of the PBRs can prevent the algae being able to settle on the (e.g., perspex) surface. However, if the perspex is scratched, then algae will adhere more readily. In some cases, fouling can be a major problem; some algal species are more adherent than others, but if the circulation in the PBR is sufficiently high, the algae will not adhere. Conversely, if the shear forces are too high, this may damage the algae. Most of the species that are grown commercially are fairly robust, but some species are shear sensitive; hence, this would need to be tested on a case by case basis (D. McKenzie, Xanthella Ltd., personal communication).

Based on this discussion, and the ease and potential scalability of bimanufacture of ZnS quantum dots, these were considered as a possible alternative to boost photosynthetic output via irradiation of chlorophyll a in the blue region, as an alternative to visible-red wavelengths, also noting the preferred use of blue light for deep culture (above). The emission of biogenic ZnS QDs prepared in 50 mM citrate buffer, pH 6, was reported at 410 nm [60], whereas the optimal absorbance wavelength of chlorophyll a is ~430 nm (**Figure 4**); respective molar extinction coefficients at 410 and 431.66 nm were calculated as 70,733 and 110,789 cm−1/M, respectively [66, 67]. At ~425 nm, this was given as 93,099 and 98,874 cm−1/M at 424.8 and 426.15 nm, respectively [66]. This illustrates the need to redshift the QD emission of the biogenic ZnS QDs by up to 15–20 nm to realize the full potential.

### **7. Toward realizing useful quantum dots from biogenic ZnS**

and ~415 nm [58]. Other work reported QDs with absorbance and emission peaks at 279 and

very close to the absorption peak of chlorophyll a at 430 nm (**Figure 4**), this raises the possibility to use ZnS NPs as a quantum dot ancillary to Qdot'655 (above) or, indeed as a substitute for the latter, using emitted blue light via the other absorbance region for chlorophyll a (see above). A first report [60] showed that the characteristics and the light emitting properties of ZnS

ation process [50, 51] were comparable to those made by "classical" methods, which required more complex procedures. As a potential synthesis method at scale, this shows potential for commercial QD production and introduces the possibility to use these biogenic ZnS QDs to promote algal growth for the applications described above and also to provide algal feedstock as a nutrient source for other processes (e.g. high-value chemicals); algae as biomass feedstock

The price of commercial QDs discourages development above small-scale and a full-scale energy plant is probably currently unfeasible. However, QDs are rapidly developing from niche markets into consumer electronics [61], which is expected to bring substantial increases in production scale, and hence, reduction in cost may be expected in the future. From the tests and data shown above, the QD cost would have to be reduced by up to 100-fold in order for photonic photobioreactors to achieve parity with standard PBRs in terms of capital cost (M.D. Redwood, unpublished). This estimation was based on early published values for capital costs of different photobiological systems [62–65] and a survey (in 2012) of market prices for commercial QDs. Because open systems or raceways present much lower capital costs (per unit area) than enclosed PBRs, the estimated minimum QED cost reduction would be ~10 fold more attractive for raceways, suggesting that such enhancement would be first

However, these estimations do not consider the reduced land requirement and reduced running costs of photonically enhanced photobioreactors, which may lessen the cost impact. On the other hand, end-of-life decommissioning may be more costly if potentially toxic metals have been used. QD retention via immobilization/encapsulation and re-use would be a key strategy. The extent to which biofouling of transparent surfaces in contact with the culture may impact adversely on QD-enhanced PBR useful life has not been taken into account (nor tested). Common methods to remove biofouling deposits (e.g., scraping) may damage surfaces that have been precision-machined or polished for optical transmission. Hence, an air gap between the QD enclosure and the culture liquid may prove beneficial. In practice, as long as there is sufficient stirring, the shear force is sufficient to prevent fouling problems. This means that sufficient shear force being produced by sparging of the PBRs can prevent the algae being able to settle on the (e.g., perspex) surface. However, if the perspex is scratched, then algae will adhere more readily. In some cases, fouling can be a major problem; some algal species are more adherent than others, but if the circulation in the PBR is sufficiently high, the algae will not adhere. Conversely, if the shear forces are too high, this may damage the algae. Most of the species that are grown commercially are fairly robust, but some species are shear sensitive; hence, this would need to be tested on a case by case basis (D. McKenzie, Xanthella Ltd., personal communication).

Based on this discussion, and the ease and potential scalability of bimanufacture of ZnS quantum dots, these were considered as a possible alternative to boost photosynthetic output

S [59]. Being

S left over from the metal bioremedi-

435 nm, respectively; this method utilizes thiolactic acid with Zn2+ solution and Na2

quantum dots made by use of bacterially made waste H2

162 Nonmagnetic and Magnetic Quantum Dots

*per se* for pyrolysis oil production has also been reported (e.g., [5]).

tested in PBRs then developed in raceways at scale as costs fall.

The early study [60] used 50 mM citrate buffer (pH 6) to prevent uncontrolled precipitation of ZnS by chelating the Zn2+ in solution and acting as a passivant for the ZnS nanoparticles.

**Figure 9.** Excitation (a) and Emission (b) Spectra of ZnS Quantum dots. QDs were synthesized using excess waste biogenic H2 S from a metal bioremediation process [60]. Samples were sparged with the culture off-gas for 30 min (flow rate 132 ml/min). Example scans are shown. ZnS QDs were made in the presence of 10 mM, 25 mM, 50 mM and 100 mM citrate buffer (pH 6) as shown. Note excitation redshift to ~ 310 nm and emission from ~ 410 to ~425 nm (dotted lines) with decreased citrate concentration from 50 to 10 mM. Buffering at pH 6 without citrate (MES-NaOH buffer) gave material with negligible emission.

However, the use of citrate should be minimized for process economy. Omission of citrate or its substitution by 50 mM MES-NaOH buffer gave a ZnS nanomaterial with poor light emission at 410 nm. By using lower concentrations of citrate in preparation (10 and 25 mM), the light emission was observed to increase by up to 5-fold, together with a redshift from 410 to

425 nm, i.e., into the absorption peak for chlorophyll a (**Figure 9**). Increasing the concentration of citrate (to 100 mM) during QD synthesis gave a similar effect; the reason for this was not investigated but future development would require the minimum amount of citrate. Further tests showed that further reduction of the citrate concentration to 7 mM retained the emission

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It is well known that increasing the size of quantum dots produces a redshift in the emission spectrum [68]. Hence, the ZnS QD material produced from the biogas from Zn2+ solution using high (50 mM) and low (7 mM) concentrations of citrate was examined using two methods: high resolution transmission electron microscopy (HRTEM) and differential centrifuga-

**Figure 11.** High resolution TEM study and population size analysis of ZnS QDs made under two citrate concentrations.

respective nanoparticle sizes are 3–4 and ~ 10 nm as shown. The population from 7 mM citrate solution had additional small nanoparticles of size ~5 nm (a, inset). Bars are 2 and 5 nm as shown. b,c: Estimation of nanoparticle sizes using an analytical disc centrifuge as described in [60] using light scattering. The nanoparticle size in 50 mM citrate buffer was calculated (from log plots) as ~4–5 nm and in 7 mM citrate buffer was ~ 8–10 nm with a smaller population of size <5 nm. The small nanoparticles are too close to the lower size cutoff of the instrument [60] to be an exact measurement, but the

two independent methods report the same result. **Figure 11a** was in collaboration with J. Gomez-Bolivar.

S was used to make ZnS in 50 mM citrate buffer (a, main image) and 7 mM citrate buffer (a, inset). Approximate

tion analysis for determining the size distribution of native nanoparticles.

peak at 425 nm.

Biogenic H2

**Figure 10.** Transmission electron microscopy of ZnS Quantum Dots. QDs were synthesized in 50 mM citrate buffer, pH6 using biogenic H2 S [60]. Accelerating voltage was 80 kV which is optimal for contrast and shows (a) agglomerations of ~60 nm containing discrete small nanoparticles. (b) High-resolution TEM study (300 kV) of a single area of an agglomerations how nina.Lattice details are visible showing facets of ZnS:(111) and (220) corresponding to interplanar spacings of 0.320 nm and 0.196 nm. a (inset): Selected area diffraction of a single nanoparticle at accelerating voltage of 300 kV. The diffraction rings correspond to the (111), (220), and (311) facets of ZnS by reference to the JCPDS database. Calculations from Image J software, in collaboration with J. Gomez-Bolivar.

425 nm, i.e., into the absorption peak for chlorophyll a (**Figure 9**). Increasing the concentration of citrate (to 100 mM) during QD synthesis gave a similar effect; the reason for this was not investigated but future development would require the minimum amount of citrate. Further tests showed that further reduction of the citrate concentration to 7 mM retained the emission peak at 425 nm.

However, the use of citrate should be minimized for process economy. Omission of citrate or its substitution by 50 mM MES-NaOH buffer gave a ZnS nanomaterial with poor light emission at 410 nm. By using lower concentrations of citrate in preparation (10 and 25 mM), the light emission was observed to increase by up to 5-fold, together with a redshift from 410 to

**Figure 10.** Transmission electron microscopy of ZnS Quantum Dots. QDs were synthesized in 50 mM citrate buffer, pH6

~60 nm containing discrete small nanoparticles. (b) High-resolution TEM study (300 kV) of a single area of an agglomerations how nina.Lattice details are visible showing facets of ZnS:(111) and (220) corresponding to interplanar spacings of 0.320 nm and 0.196 nm. a (inset): Selected area diffraction of a single nanoparticle at accelerating voltage of 300 kV. The diffraction rings correspond to the (111), (220), and (311) facets of ZnS by reference to the JCPDS database. Calculations from Image J

S [60]. Accelerating voltage was 80 kV which is optimal for contrast and shows (a) agglomerations of

using biogenic H2

software, in collaboration with J. Gomez-Bolivar.

164 Nonmagnetic and Magnetic Quantum Dots

It is well known that increasing the size of quantum dots produces a redshift in the emission spectrum [68]. Hence, the ZnS QD material produced from the biogas from Zn2+ solution using high (50 mM) and low (7 mM) concentrations of citrate was examined using two methods: high resolution transmission electron microscopy (HRTEM) and differential centrifugation analysis for determining the size distribution of native nanoparticles.

**Figure 11.** High resolution TEM study and population size analysis of ZnS QDs made under two citrate concentrations. Biogenic H2 S was used to make ZnS in 50 mM citrate buffer (a, main image) and 7 mM citrate buffer (a, inset). Approximate respective nanoparticle sizes are 3–4 and ~ 10 nm as shown. The population from 7 mM citrate solution had additional small nanoparticles of size ~5 nm (a, inset). Bars are 2 and 5 nm as shown. b,c: Estimation of nanoparticle sizes using an analytical disc centrifuge as described in [60] using light scattering. The nanoparticle size in 50 mM citrate buffer was calculated (from log plots) as ~4–5 nm and in 7 mM citrate buffer was ~ 8–10 nm with a smaller population of size <5 nm. The small nanoparticles are too close to the lower size cutoff of the instrument [60] to be an exact measurement, but the two independent methods report the same result. **Figure 11a** was in collaboration with J. Gomez-Bolivar.

Examination of the ZnS material (from 50 mM citrate) revealed agglomerations, within which small NPs were visible [60] (**Figure 10a**). Examination of these confirmed their identity as ZnS (**Figure 10a**, **b**). Use of 7 mM citrate produced larger nanoparticles (**Figure 11a**) consistent with the redshift observation in **Figure 9** (see above). Electron microscopy can produce artifacts due to drying [60]. Hence, independent confirmation was provided via analytical centrifugation of the liquid suspension in conjunction with light scattering (**Figure 11b**, **c**).

of available sunlight at the earth's surface is ~ half of that obtained at the maximum transmittance of light of ~625 nm [72]. Inspection of **Figure 9** shows that the larger QDs made in 7–10 mM citrate (**Figure 11**) absorb at the interface between UVA and UVB, and further work using lower concentrations of citrate may repay study to redshift both the absorbance and emission peaks by, ideally, a further 5–10 nm. The "preferred" UV light for irradiation also

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From **Table 4**, it is clear that several types of bulk materials would have potential application in large scale photobioreactor technology. LLDPE materials, transparent to both UVA and UVB, are used extensively in bottles and liquid sachets, and a simple approach might involve flotation or suspension of suitable sachet bags into various types of culture as shown in **Figure 2**. However, LLDPE polymers degrade in UV light with a useful life of only about 3 years [73] and would not provide a durable solution, although they would provide a route to easy separation of QDs for re-use. It is also routine to use Perspex™ for photobioreactor materials, e.g., for enclosed inserts. Indeed, Perspex™ is routinely used in numerous applications such as glazing, and its properties are well described, including hardness and scratch resistance; indeed, it is recommended for use as a flooring material [74]. Should scratches occur they can be easily polished out using a proprietary polishing material [74], although the degree of polishing required to achieve a near-perfect optical transmission for optimal use of quantum dots would need to

**Material Type Examples Transparency UVB Transparency UVA** Building window materials Glasses Clear glass Opaque Transparent Reflective glass Tinted/wire Tinted glasses Nonwindow materials Quartz glass Transparent Transparent

Furniture glass<sup>1</sup>

Polyethylene terephthalate Plastic bottle "Standard water" Opaque Transparent

"BelAquah" "Coca Cola"

"Voltic", "Ice Pak"

Storage bags "Acqua fil" "Cool Pak" (Sachet bags) "Ahenpon", "Kenro"

Containers for liquid storage

**Table 4.** UV transparency of some common materials used in bulk applications.

Perspex Perspex is the best. Perspex is the best.

Liquid Blue crystal Transparent Transparent

drives the choice of reactor materials (**Table 4**).

be determined experimentally.

Transparent linear low density polyethylene

(LLDPE)

Examination of nanoparticle sizes by the two methods gave similar results (**Figure 11**). The nanoparticles made with 7-mM citrate were of size in the region of ~10-12 nm with a subpopulation of small NPs of 5 nm or less. In contrast, when made in the presence of 50 mM citrate, the population comprised small NPs of size 3–5 nm. Accurate sizing of the latter was precluded by the limitations of the analytical centrifugation method (see [60] for discussion), but it is clear that this simple method gives the potential to "steer" the ZnS QDs for size optimization.

## **8. Considerations for large scale process using ZnS quantum dots**

A comparison was made of the cost/benefit analysis of electricity production based on the microalga *C. vulgaris* (**Table 1**). Open ponds are feasible and are used routinely, with process intensification achieved using raceways (see earlier), but the best photobioreactor format was concluded to be a flat panel arrangement which, although of volume of 20% of that of an open pond, gave >3-fold more biomass production for ~ twice the cost. (**Table 1**), also outperforming a tubular reactor arrangement. Consideration of algal growth for biofuels production has been reviewed elsewhere [69]. Calculations were made here based on using LEDs to boost light delivery, projecting a system of daytime solar irradiation via solar panels (parallel to the PBR) along with modern battery technology (energy storage) to permit LED-illumination (for culture 'tickover') at night (**Table 1**). A full cost-benefit and life cycle analysis is in progress, but here, it should be noted that ZnS quantum dots have also found application in solar cells [70, 71], giving further scope for cost reduction of solar panels which is not factored into **Table 1**. Similarly, **Table 1** does not take into account any increase in photoproductivity via use of QDs, and the values shown (made on the basis of published values) may reveal further benefits (e.g., 2-fold) via addition of the QD technology described here. Hence, the calculations shown in **Table 1** are taken to form a conservative "baseline."

Although immobilized QDs in suspension in a circulating reactor (raceways) might be appropriate for large scale, well-mixed growth, this may give challenges with respect to stability of the encapsulation material under shear, recovery of the QDs, and importantly, ensuring transparency to UV irradiation in the region of interest (absorbance maximum was ~ 310–315 nm with a peak emission at 425 nm; **Figure 4**). This redshift with respect to early work [60] is very relevant: UV light is divisible into UVA (315–400 nm), UVB (280–315 nm), and UVC (100–289 nm). The latter (the most damaging to living cells due to absorbance by DNA and proteins) is almost all absorbed by the atmosphere. Of the total UV radiance reaching the earth's surface, 85% comprises UVA, while only 5% comprises UVB; At 310 nm, the portion of available sunlight at the earth's surface is ~ half of that obtained at the maximum transmittance of light of ~625 nm [72]. Inspection of **Figure 9** shows that the larger QDs made in 7–10 mM citrate (**Figure 11**) absorb at the interface between UVA and UVB, and further work using lower concentrations of citrate may repay study to redshift both the absorbance and emission peaks by, ideally, a further 5–10 nm. The "preferred" UV light for irradiation also drives the choice of reactor materials (**Table 4**).

Examination of the ZnS material (from 50 mM citrate) revealed agglomerations, within which small NPs were visible [60] (**Figure 10a**). Examination of these confirmed their identity as ZnS (**Figure 10a**, **b**). Use of 7 mM citrate produced larger nanoparticles (**Figure 11a**) consistent with the redshift observation in **Figure 9** (see above). Electron microscopy can produce artifacts due to drying [60]. Hence, independent confirmation was provided via analytical centrifugation of the liquid suspension in conjunction with light scattering (**Figure 11b**, **c**). Examination of nanoparticle sizes by the two methods gave similar results (**Figure 11**). The nanoparticles made with 7-mM citrate were of size in the region of ~10-12 nm with a subpopulation of small NPs of 5 nm or less. In contrast, when made in the presence of 50 mM citrate, the population comprised small NPs of size 3–5 nm. Accurate sizing of the latter was precluded by the limitations of the analytical centrifugation method (see [60] for discussion), but it is clear that this simple method gives the potential to "steer" the ZnS QDs for size

**8. Considerations for large scale process using ZnS quantum dots**

shown in **Table 1** are taken to form a conservative "baseline."

A comparison was made of the cost/benefit analysis of electricity production based on the microalga *C. vulgaris* (**Table 1**). Open ponds are feasible and are used routinely, with process intensification achieved using raceways (see earlier), but the best photobioreactor format was concluded to be a flat panel arrangement which, although of volume of 20% of that of an open pond, gave >3-fold more biomass production for ~ twice the cost. (**Table 1**), also outperforming a tubular reactor arrangement. Consideration of algal growth for biofuels production has been reviewed elsewhere [69]. Calculations were made here based on using LEDs to boost light delivery, projecting a system of daytime solar irradiation via solar panels (parallel to the PBR) along with modern battery technology (energy storage) to permit LED-illumination (for culture 'tickover') at night (**Table 1**). A full cost-benefit and life cycle analysis is in progress, but here, it should be noted that ZnS quantum dots have also found application in solar cells [70, 71], giving further scope for cost reduction of solar panels which is not factored into **Table 1**. Similarly, **Table 1** does not take into account any increase in photoproductivity via use of QDs, and the values shown (made on the basis of published values) may reveal further benefits (e.g., 2-fold) via addition of the QD technology described here. Hence, the calculations

Although immobilized QDs in suspension in a circulating reactor (raceways) might be appropriate for large scale, well-mixed growth, this may give challenges with respect to stability of the encapsulation material under shear, recovery of the QDs, and importantly, ensuring transparency to UV irradiation in the region of interest (absorbance maximum was ~ 310–315 nm with a peak emission at 425 nm; **Figure 4**). This redshift with respect to early work [60] is very relevant: UV light is divisible into UVA (315–400 nm), UVB (280–315 nm), and UVC (100–289 nm). The latter (the most damaging to living cells due to absorbance by DNA and proteins) is almost all absorbed by the atmosphere. Of the total UV radiance reaching the earth's surface, 85% comprises UVA, while only 5% comprises UVB; At 310 nm, the portion

optimization.

166 Nonmagnetic and Magnetic Quantum Dots

From **Table 4**, it is clear that several types of bulk materials would have potential application in large scale photobioreactor technology. LLDPE materials, transparent to both UVA and UVB, are used extensively in bottles and liquid sachets, and a simple approach might involve flotation or suspension of suitable sachet bags into various types of culture as shown in **Figure 2**. However, LLDPE polymers degrade in UV light with a useful life of only about 3 years [73] and would not provide a durable solution, although they would provide a route to easy separation of QDs for re-use. It is also routine to use Perspex™ for photobioreactor materials, e.g., for enclosed inserts. Indeed, Perspex™ is routinely used in numerous applications such as glazing, and its properties are well described, including hardness and scratch resistance; indeed, it is recommended for use as a flooring material [74]. Should scratches occur they can be easily polished out using a proprietary polishing material [74], although the degree of polishing required to achieve a near-perfect optical transmission for optimal use of quantum dots would need to be determined experimentally.


**Table 4.** UV transparency of some common materials used in bulk applications.

## **9. Conclusions**

Photobiotechnologies are maturing rapidly from small-scale high-value applications to large scale operations for biofuels. The major challenge remains optimal use of solar light, since photosynthesis is intrinsically inefficient and effective solar-biotechnologies are currently limited geographically to areas of high and constant solar irradiance. LEDs are already used to supply light into photobioreactors, but their use at large scale requires a careful cost-benefit analysis, especially with regard to the overall energy balance and especially if the biomass is used to make biofuels. Quantum dot technologies, until now used at small scale for niche applications such as imaging, are entering the global commodity market, but traditional QDs are costly. We have shown that commercial QDs can be used to double the photoproductivity, and we also show an economic route to QD manufacture via harnessing a waste from another biotechnology process into the QD manufacturing without compromising quality or performance.

[2] Alfstad T. World Biofuels Study Scenario Analysis of Global Biofuel Markets. BNL-80238-2000. New York NY, USA: Brookhaven National Laboratory; 2008, 2008

Enhancement of Photosynthetic Productivity by Quantum Dots Application

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169

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## **Acknowledgements**

We acknowledge the support of NERC (Grant No NE/L014076/1) in the research presented here (AJM and RLO) to develop ZnS-based quantum dots technology via resource recovery from waste. The underpinning evaluation of commercial quantum dots in the three test photobiological systems was supported by the Discipline Hopping Award scheme co-funded by EPSRC, BBSRC, and MRC. The support of BBSRC is acknowledged for MRes studentships (AG and FW). We also thank Drs D.J. Binks and M. Dickinson of the Photonics Institute, University of Manchester, for collaborations, photonics expertise and hosting the secondment of Dr. M.D. Redwood. We acknowledge with thanks the discussions and collaboration with Mr. J. Gomez-Bolivar and Dr. M. Merroun of University of Granada, Spain.

## **Author details**

Angela Janet Murray1 , John Love2 , Mark D. Redwood1 , Rafael L. Orozco1 , Richard K. Tennant2 , Frankie Woodhall<sup>1</sup> , Alex Goodridge1 and Lynne Elaine Macaskie1 \*


2 Biocatalysis Centre, University of Exeter, Exeter, UK

#### **References**

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[2] Alfstad T. World Biofuels Study Scenario Analysis of Global Biofuel Markets. BNL-80238-2000. New York NY, USA: Brookhaven National Laboratory; 2008, 2008

**9. Conclusions**

168 Nonmagnetic and Magnetic Quantum Dots

performance.

**Acknowledgements**

**Author details**

Angela Janet Murray1

Richard K. Tennant2

**References**

Photobiotechnologies are maturing rapidly from small-scale high-value applications to large scale operations for biofuels. The major challenge remains optimal use of solar light, since photosynthesis is intrinsically inefficient and effective solar-biotechnologies are currently limited geographically to areas of high and constant solar irradiance. LEDs are already used to supply light into photobioreactors, but their use at large scale requires a careful cost-benefit analysis, especially with regard to the overall energy balance and especially if the biomass is used to make biofuels. Quantum dot technologies, until now used at small scale for niche applications such as imaging, are entering the global commodity market, but traditional QDs are costly. We have shown that commercial QDs can be used to double the photoproductivity, and we also show an economic route to QD manufacture via harnessing a waste from another biotechnology process into the QD manufacturing without compromising quality or

We acknowledge the support of NERC (Grant No NE/L014076/1) in the research presented here (AJM and RLO) to develop ZnS-based quantum dots technology via resource recovery from waste. The underpinning evaluation of commercial quantum dots in the three test photobiological systems was supported by the Discipline Hopping Award scheme co-funded by EPSRC, BBSRC, and MRC. The support of BBSRC is acknowledged for MRes studentships (AG and FW). We also thank Drs D.J. Binks and M. Dickinson of the Photonics Institute, University of Manchester, for collaborations, photonics expertise and hosting the secondment of Dr. M.D. Redwood. We acknowledge with thanks the discussions and collaboration with

Mr. J. Gomez-Bolivar and Dr. M. Merroun of University of Granada, Spain.

, Mark D. Redwood1

1 School of Biosciences, University of Birmingham, Edgbaston, Birmingham, UK

, Alex Goodridge1

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\*

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\*Address all correspondence to: l.e.macaskie@bham.ac.uk

2 Biocatalysis Centre, University of Exeter, Exeter, UK


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**Section 2**

**Magnetic Quantum Dots**


**Magnetic Quantum Dots**

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174 Nonmagnetic and Magnetic Quantum Dots

Perspex\_Glazing\_236.pdf

**Chapter 10**

**Provisional chapter**

**Magnetization Dynamics in Arrays of Quantum Dots**

The possibility of preparing materials based on quantum dots with fine-tuned magnetic properties has opened up the door for designing new and more efficient devices where the interplay of different microscopic phenomena balances out in useful ways. Nevertheless, our knowledge of the precise interaction of complex objects built from a great number of such nanometric magnetic components is still limited. The investigation of the spin or magnetization dynamics in such materials represents an important opportunity to better comprehend and predict some missing pieces for the advancement of a great deal of

**Keywords:** magnetism, quantum dots, magnetization dynamics, arrays, macrospin

Since quantum dots were first discovered [1] and later fabricated, they have attracted a great deal of attention given how, just as single atoms or simple molecules, they depict quantum behavior at the level of their electronic and optical properties, but at the same time allow their tuning as a function of their shape, size, and composition––reason that has led some to refer

Depending on whether quantum dots are made of a semiconductor, a metal, or another material, different intrinsic properties of a system based on them can be tailored to exhibit specific values or signatures. By themselves, single quantum dots are of prominent relevance and have found applications in diverse scientific and technological fields [3]. Nonetheless, it is really the properties of arrays of these particles which outline a unique landscape in the design space of new materials. Arrays of regularly ordered *magnetic quantum dots* (MQDs) provide an opportunity to develop materials with characteristics different from those exhibited by traditional solid-state systems.

**Magnetization Dynamics in Arrays of Quantum Dots**

DOI: 10.5772/intechopen.73008

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Pablo F. Zubieta Rico, Daniel Olguín and

Daniel Olguín and Yuri V. Vorobiev

http://dx.doi.org/10.5772/intechopen.73008

promising technologies.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Yuri V. Vorobiev

**Abstract**

**1. Introduction**

to them as *artificial atoms* [2].

Pablo F. Zubieta Rico,

**Provisional chapter**

## **Magnetization Dynamics in Arrays of Quantum Dots Magnetization Dynamics in Arrays of Quantum Dots**

DOI: 10.5772/intechopen.73008

Pablo F. Zubieta Rico, Daniel Olguín and Yuri V. Vorobiev Daniel Olguín and Yuri V. Vorobiev

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73008

**Abstract**

Pablo F. Zubieta Rico,

The possibility of preparing materials based on quantum dots with fine-tuned magnetic properties has opened up the door for designing new and more efficient devices where the interplay of different microscopic phenomena balances out in useful ways. Nevertheless, our knowledge of the precise interaction of complex objects built from a great number of such nanometric magnetic components is still limited. The investigation of the spin or magnetization dynamics in such materials represents an important opportunity to better comprehend and predict some missing pieces for the advancement of a great deal of promising technologies.

**Keywords:** magnetism, quantum dots, magnetization dynamics, arrays, macrospin

## **1. Introduction**

Since quantum dots were first discovered [1] and later fabricated, they have attracted a great deal of attention given how, just as single atoms or simple molecules, they depict quantum behavior at the level of their electronic and optical properties, but at the same time allow their tuning as a function of their shape, size, and composition––reason that has led some to refer to them as *artificial atoms* [2].

Depending on whether quantum dots are made of a semiconductor, a metal, or another material, different intrinsic properties of a system based on them can be tailored to exhibit specific values or signatures. By themselves, single quantum dots are of prominent relevance and have found applications in diverse scientific and technological fields [3]. Nonetheless, it is really the properties of arrays of these particles which outline a unique landscape in the design space of new materials. Arrays of regularly ordered *magnetic quantum dots* (MQDs) provide an opportunity to develop materials with characteristics different from those exhibited by traditional solid-state systems.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Arrays––and more generally assemblies––of MQDs comprise all of those systems in which the magnetic nanoparticles are embedded in, dispersed into, or arranged over a different nonmagnetic medium––either a liquid or a solid, e.g., some polymeric material. More concisely, all of them could be simply classified as magnetic nanocomposite or hybrid materials, which depending on the spatial relative placement of the MQDs can form one-, two-, or threedimensional architectures [4, 5].

where γ is the gyromagnetic ratio, *MS*

<sup>→</sup> (r <sup>→</sup>) \_\_\_\_\_

interactions, and external field terms.

*<sup>i</sup>*), where *<sup>m</sup>* → *i*

> → \_\_\_\_*<sup>i</sup>*

*dt* <sup>=</sup> <sup>−</sup> *<sup>γ</sup>* \_\_\_\_

<sup>1</sup> <sup>+</sup> *<sup>α</sup>*<sup>2</sup>{*<sup>m</sup>* → *<sup>i</sup>* × *H* → *eff*(*r* → *<sup>i</sup>*) <sup>+</sup> \_\_\_*<sup>α</sup> MS* [(*m* → *<sup>i</sup>* ⋅ *H* → *eff*(*r* → *<sup>i</sup>*)) *m* → *<sup>i</sup>* − *H* → *eff*(*r* →

*dt* <sup>=</sup> <sup>−</sup> *<sup>γ</sup>* \_\_\_\_

→ *eff* (*r*

<sup>1</sup> <sup>+</sup> *<sup>α</sup>*<sup>2</sup>{<sup>m</sup> <sup>→</sup> (r <sup>→</sup>) × *H* → *eff*(r <sup>→</sup>) + \_\_\_*<sup>α</sup> MS* [(m <sup>→</sup> (r <sup>→</sup>) ⋅ *H* → *eff*(*r* →) )*m* <sup>→</sup> (*r* <sup>→</sup>) − *H* → *eff*(*r* →)

*<sup>d</sup>*<sup>m</sup>

*m* <sup>→</sup> (*r* <sup>→</sup>) <sup>=</sup> <sup>∑</sup>*<sup>i</sup> m* → *i δ*(*r* <sup>→</sup> − *r* →

and *δ*(*<sup>r</sup>* <sup>→</sup> − *r* →

*<sup>d</sup> <sup>m</sup>*

The total effective field *H*

the viscous damping coefficient [25]. Here, the magnetization *m*

Equation (1) can be solved analytically in very few cases and needs, in general, to be inte-

the systems, it includes magnetic anisotropy contributions, interparticle or substrate-particle

Depending on the nature, shape, and size of the MQDs, there are two general approaches to solve Eq. (2). The first one, *micromagnetics*, [24] describes the spatial magnetization distribution in the regime where the MQDs are large enough for domain walls and vertices to appear. In this scenario, the interactions between pairs of intra-dot domains render quite involved and time-consuming the computational task of solving the LLG equation, requiring highly technical numerical expertise or specialized software to handle the computational complexity. The second approach, called *macrospin model* [24, 26], is appropriate to describe the magnetization time evolution of assemblies of small nanoparticles, such that each dot magnetization can be described by a single magnetic moment within the QD volume. What we mean by small is unfortunately system dependent, but some known data for spherical MQDs can be found in **Figure 1** as reference. In the macrospin limit, the magnetization distribution of an array of MQDs becomes

grated numerically, which is usually done by recasting it into the following form:

also referred to as the Landau-Lifshitz equation or explicit form of LLG equation.

*<sup>i</sup>*) is the Dirac delta function. Plugging the above expression for *<sup>m</sup>*

**Figure 1.** Threshold diameters for maximum monodomain size for spherical nanoparticles. Adapted from Majetich et al. [27].

is the saturation magnetization of the material, and α is

<sup>→</sup>) is normalized over *MS*

Magnetization Dynamics in Arrays of Quantum Dots http://dx.doi.org/10.5772/intechopen.73008

.

179

]}, (2)

<sup>→</sup> (*r*

<sup>→</sup>) should be specified for each system of interest, but for most of

denotes the monodomain magnetization of the *i*th particle in the array

<sup>→</sup> (*r*

<sup>→</sup>) into Eq. (2), we obtain

*<sup>i</sup>*)]}, (3)

Assemblies of MQDs are attractive for their rich spectra of potential technological applications that range from biomedical uses such as magnetic resolution imaging, magnetic hyperthermia, and drug delivery [3–7] to magnetic refrigeration and energy-harvesting devices [8]. Notwithstanding, the major driving forces behind the study of arrays of MQDs are related to the area of information storage technologies [9, 10]. In particular, there is a great hope of achieving through these magnetic storage media of ultrahigh densities [11–13], nonvolatile magnetic random-access memories (MRAM) [14, 15], and logic devices [16–18]. Moreover, the importance of these systems does not stop here; arrays of MQDs are also of special interest as model systems for better understanding interactions and transport processes of magnetic materials in general [19–22].

In this chapter, we present the results from several––mainly theoretical and numerical––works focused on the analysis of magnetization dynamics and interactions of a representative variety of two-dimensional systems of MQDs arrays. Throughout each of the sections, we will review some of the physical models that are used to study, predict, and understand the dynamics and how interparticle interactions affect the overall behavior of these systems. We will also take the opportunity for introducing some of the limitations that have hindered progress on the realization of some of the applications mentioned above and will discuss based on the aforementioned works what possible strategies could be followed in order to steer away the status quo in the field.

While it is not the goal of the current exposition to dive into the experimental aspects that entail the fabrication of arrays of MQDs, it is worth mentioning that numerous efforts have been devoted to the development of different synthetic pathways for MQD arrays, as it can represent a highly challenging task to prepare some of these systems in such a way that the targeted properties of the system are obtained within a desired precision in a predictable fashion. Indeed, at the characteristic length scale of MQDs, i.e., down to less than ten to a few hundreds of nanometers, variations on the shape, size, and distribution of MQDs can significantly impact the magnetic behavior of the whole array.

## **2. Theoretical modeling of the magnetization dynamics**

In general, the starting point for the description of the magnetization dynamics of arrays of MQDs is to consider the relevant fields that modify the magnetization of each individual QD. If we denote the total effective field "felt" at every point in space as *H* → *eff* (*r* →), then the equation of motion of the magnetization *m* <sup>→</sup> (*r* <sup>→</sup>) is the Landau-Lifshitz-Gilbert (LLG) equation [23, 24]:

$$\frac{d\vec{m}(\vec{r})}{dt} = -\gamma \vec{m}(\vec{r}) \times \vec{H}\_{eq}(\vec{r}) + \frac{\alpha}{M\_s} \left( \vec{m}(\vec{r}) \times \frac{d\vec{m}(\vec{r})}{dt} \right),\tag{1}$$

where γ is the gyromagnetic ratio, *MS* is the saturation magnetization of the material, and α is the viscous damping coefficient [25]. Here, the magnetization *m* <sup>→</sup> (*r* <sup>→</sup>) is normalized over *MS* .

Equation (1) can be solved analytically in very few cases and needs, in general, to be integrated numerically, which is usually done by recasting it into the following form:

$$\frac{d\vec{\text{m}}(\vec{\text{r}})}{dt} = -\frac{\mathcal{V}}{1+a^2} \left\{ \vec{\text{m}}(\vec{\text{r}}) \times \vec{H}\_{\text{eff}}(\vec{\text{r}}) + \frac{a}{M\_s} \left[ \left( \vec{\text{m}}(\vec{\text{r}}) \cdot \vec{H}\_{\text{eff}}(\vec{\text{r}}) \right) \vec{m}(\vec{\text{r}}) - \vec{H}\_{\text{eff}}(\vec{\text{r}}) \right] \right\},\tag{2}$$

also referred to as the Landau-Lifshitz equation or explicit form of LLG equation.

Arrays––and more generally assemblies––of MQDs comprise all of those systems in which the magnetic nanoparticles are embedded in, dispersed into, or arranged over a different nonmagnetic medium––either a liquid or a solid, e.g., some polymeric material. More concisely, all of them could be simply classified as magnetic nanocomposite or hybrid materials, which depending on the spatial relative placement of the MQDs can form one-, two-, or three-

Assemblies of MQDs are attractive for their rich spectra of potential technological applications that range from biomedical uses such as magnetic resolution imaging, magnetic hyperthermia, and drug delivery [3–7] to magnetic refrigeration and energy-harvesting devices [8]. Notwithstanding, the major driving forces behind the study of arrays of MQDs are related to the area of information storage technologies [9, 10]. In particular, there is a great hope of achieving through these magnetic storage media of ultrahigh densities [11–13], nonvolatile magnetic random-access memories (MRAM) [14, 15], and logic devices [16–18]. Moreover, the importance of these systems does not stop here; arrays of MQDs are also of special interest as model systems for better understanding interactions and transport processes of magnetic

In this chapter, we present the results from several––mainly theoretical and numerical––works focused on the analysis of magnetization dynamics and interactions of a representative variety of two-dimensional systems of MQDs arrays. Throughout each of the sections, we will review some of the physical models that are used to study, predict, and understand the dynamics and how interparticle interactions affect the overall behavior of these systems. We will also take the opportunity for introducing some of the limitations that have hindered progress on the realization of some of the applications mentioned above and will discuss based on the aforementioned works what possible strategies could be followed in order to steer away the status quo in the field.

While it is not the goal of the current exposition to dive into the experimental aspects that entail the fabrication of arrays of MQDs, it is worth mentioning that numerous efforts have been devoted to the development of different synthetic pathways for MQD arrays, as it can represent a highly challenging task to prepare some of these systems in such a way that the targeted properties of the system are obtained within a desired precision in a predictable fashion. Indeed, at the characteristic length scale of MQDs, i.e., down to less than ten to a few hundreds of nanometers, variations on the shape, size, and distribution of MQDs can signifi-

In general, the starting point for the description of the magnetization dynamics of arrays of MQDs is to consider the relevant fields that modify the magnetization of each individual

> → *eff* (*r*

*dt* ), (1)

<sup>→</sup>) is the Landau-Lifshitz-Gilbert (LLG) equation [23, 24]:

→), then the equa-

cantly impact the magnetic behavior of the whole array.

<sup>→</sup> (*r* <sup>→</sup>) \_\_\_\_\_

*dt* <sup>=</sup> <sup>−</sup>*γm*

tion of motion of the magnetization *m*

*dm*

**2. Theoretical modeling of the magnetization dynamics**

QD. If we denote the total effective field "felt" at every point in space as *H*

<sup>→</sup> (*r*

<sup>→</sup> (*r* <sup>→</sup>) × *H* → *eff*(*r* <sup>→</sup>) + \_\_\_*<sup>α</sup> MS* (*m* <sup>→</sup> (*r* <sup>→</sup>) <sup>×</sup> *dm* <sup>→</sup> (*r* <sup>→</sup>) \_\_\_\_\_

dimensional architectures [4, 5].

178 Nonmagnetic and Magnetic Quantum Dots

materials in general [19–22].

The total effective field *H* → *eff* (*r* <sup>→</sup>) should be specified for each system of interest, but for most of the systems, it includes magnetic anisotropy contributions, interparticle or substrate-particle interactions, and external field terms.

Depending on the nature, shape, and size of the MQDs, there are two general approaches to solve Eq. (2). The first one, *micromagnetics*, [24] describes the spatial magnetization distribution in the regime where the MQDs are large enough for domain walls and vertices to appear. In this scenario, the interactions between pairs of intra-dot domains render quite involved and time-consuming the computational task of solving the LLG equation, requiring highly technical numerical expertise or specialized software to handle the computational complexity. The second approach, called *macrospin model* [24, 26], is appropriate to describe the magnetization time evolution of assemblies of small nanoparticles, such that each dot magnetization can be described by a single magnetic moment within the QD volume. What we mean by small is unfortunately system dependent, but some known data for spherical MQDs can be found in **Figure 1** as reference.

In the macrospin limit, the magnetization distribution of an array of MQDs becomes *m* <sup>→</sup> (*r* <sup>→</sup>) <sup>=</sup> <sup>∑</sup>*<sup>i</sup> m* → *i δ*(*r* <sup>→</sup> − *r* → *<sup>i</sup>*), where *<sup>m</sup>* → *i* denotes the monodomain magnetization of the *i*th particle in the array and *δ*(*<sup>r</sup>* <sup>→</sup> − *r* → *<sup>i</sup>*) is the Dirac delta function. Plugging the above expression for *<sup>m</sup>* <sup>→</sup> (*r* <sup>→</sup>) into Eq. (2), we obtain

$$\frac{d\vec{m}\_i}{dt} = -\frac{\mathcal{V}}{1 + a^2} \left\{ \vec{m}\_i \times \vec{H}\_{el}(\vec{r}\_i) + \frac{a}{M\_s} \left[ \left( \vec{m}\_i \cdot \vec{H}\_{el}(\vec{r}\_i) \right) \vec{m}\_i - \vec{H}\_{el}(\vec{r}\_i) \right] \right\},\tag{3}$$

**Figure 1.** Threshold diameters for maximum monodomain size for spherical nanoparticles. Adapted from Majetich et al. [27].

which is an i-dimensional nonlinear system of ODEs whose solutions can be approximated by traditional numerical integration schemes, e.g., Heun's or Runge-Kutta methods.

Within the same limit, the total effective field *H* → *eff* can be usually written as

$$
\vec{H}\_{\text{eff}} = \vec{H}\_{\text{aul}} + \vec{H}\_{\text{dev}} + \vec{H}\_{\text{int}} + \vec{H}\_{\text{ext}} \tag{4}
$$

of an external field while achieving some stability for the magnetic moment at the same time has motivated the study of the influence of the geometrical parameters––such as the inter-dot separation and crystal structure of arrays of MQDs––on the magnetic collective behavior of

Magnetization Dynamics in Arrays of Quantum Dots http://dx.doi.org/10.5772/intechopen.73008 181

In 2014, Meza et al. [28] reported an analysis of the coercivity fields for two-dimensional clusters of ellipsoidal cobalt nanoparticles in two different crystalline configurations (square and hexagonal) as a function of the inter-dot spacing and the frequency of a switching continuous external applied field. For this work oblate nanoparticles of semiaxis lengths of 3 nm × 3 nm × 1.5 nm were chosen. The easy axis of the particles, oriented in plane, was chosen parallel to the boundary of the cluster, and the external field was applied along the same direction. Given that these MQDs lie in the monodomain regime for cobalt, it is safe to assume that they can be properly described by the macrospin

By simulating hysteresis cycles for small clusters of cobalt nanoparticles (3 × 3, 5 × 5, and 10 × 10) at different frequencies for the applied oscillating external field (**Figure 2**), it was found that the hexagonal configuration stabilizes the magnetization reversal and narrows the coercivity with respect to the square crystal structure; this is clearly a consequence of the

For both crystal systems, it was also observed that as the cluster size gets bigger the coercivity narrows at all frequencies and it is much lower than the coercivity for a single MQD. This is

**Figure 2.** Coercivity histograms for macrospins arranged in a square (left) and hexagonal (right) lattice with different numbers of particles: (a) 3 × 3, (b) 5 × 5, and (c) 10 × 10. The distance between the particles is marked as *d* = 2 *a* 0, 4 *a* 0,

interaction strength promoted by the greater number of the nearest neighbors.

such systems.

model.

and 8 *a* 0 (see [28]).

that is, the total field is the sum of the contributions from the magnetocrystalline anisotropy *H* → *ani*, the demagnetizing field *<sup>H</sup>* → *dem*, the particle interaction term *<sup>H</sup>* → *int*, and the applied external magnetic field *H* → *ext*.

Depending on the crystal structure of a ferromagnetic material, one or more privileged axes––known as *easy axes*––that energetically favor the alignment of the magnetization along their direction may exist. This energy contribution results from spin-orbit interactions and is referred to as magnetocrystalline anisotropy energy.

The demagnetization energy is the energy of the magnetization in the magnetic field created by the magnetization itself. This means that this energy contribution accounts for the dipoledipole interaction of the elementary magnets.

For MQDs modeled as macrospins and in the absence of other interactions, the field *H* → *int* is simply the dipolar field induced by all of the particles, that is

$$\vec{H}\_{int}(\vec{r}\_i) = -\frac{VM\_s}{4\pi} \sum\_{\neq \neq} \left( \frac{\vec{m}\_j}{r\_{ij}^3} - 3 \frac{\left( \vec{m}\_j \cdot \vec{r}\_{\neq} \right) \vec{r}\_{\neq}}{r\_{\neq}^5} \right) . \tag{5}$$

For the micromagnetic description of a system, Eq. (4) should also include the quantummechanical exchange interaction.

## **3. The effect of the geometry of MQD arrays**

At this point, it should be clear that when analyzing the magnetic behavior of MQD arrays, one must take into account a lot of different factors. Just to mention a few, let us consider both the chemical composition and the crystalline structure of the nanoparticles themselves, as these factors determine––among other things––the magnetic anisotropy of each MQD; the shape of a single QD gives rise to a demagnetizing field; the spatial distribution among particles in an array establishes how particles will interact via pairs of dipole-dipole potentials; and, equally, any external factor that modifies the array, e.g., some applied external field, will impact the time response of the system.

Given the wide variety of contributing factors to the global magnetic properties of an array of MQDs, let's restrict our attention first to just one of them: the geometric distribution of particles in a two-dimensional assembly.

#### **3.1. Single-domain limit**

The possibility to synchronously manipulate the motion of all the magnetic moments in a cluster of magnetic nanoparticles in order to attain the fastest dynamic response in the presence of an external field while achieving some stability for the magnetic moment at the same time has motivated the study of the influence of the geometrical parameters––such as the inter-dot separation and crystal structure of arrays of MQDs––on the magnetic collective behavior of such systems.

which is an i-dimensional nonlinear system of ODEs whose solutions can be approximated by

→

that is, the total field is the sum of the contributions from the magnetocrystalline anisotropy

Depending on the crystal structure of a ferromagnetic material, one or more privileged axes––known as *easy axes*––that energetically favor the alignment of the magnetization along their direction may exist. This energy contribution results from spin-orbit interactions and is

The demagnetization energy is the energy of the magnetization in the magnetic field created by the magnetization itself. This means that this energy contribution accounts for the dipole-

For MQDs modeled as macrospins and in the absence of other interactions, the field *H*

For the micromagnetic description of a system, Eq. (4) should also include the quantum-

At this point, it should be clear that when analyzing the magnetic behavior of MQD arrays, one must take into account a lot of different factors. Just to mention a few, let us consider both the chemical composition and the crystalline structure of the nanoparticles themselves, as these factors determine––among other things––the magnetic anisotropy of each MQD; the shape of a single QD gives rise to a demagnetizing field; the spatial distribution among particles in an array establishes how particles will interact via pairs of dipole-dipole potentials; and, equally, any external factor that modifies the array, e.g., some applied external field, will

Given the wide variety of contributing factors to the global magnetic properties of an array of MQDs, let's restrict our attention first to just one of them: the geometric distribution of

The possibility to synchronously manipulate the motion of all the magnetic moments in a cluster of magnetic nanoparticles in order to attain the fastest dynamic response in the presence

*<sup>i</sup>*) <sup>=</sup> <sup>−</sup>\_\_\_\_ *V MS* <sup>4</sup>*<sup>π</sup>* ∑*<sup>j</sup>*≠*<sup>i</sup>* ( *m* → *<sup>j</sup>* \_\_\_ *rij* <sup>3</sup> <sup>−</sup> <sup>3</sup> (*<sup>m</sup>* → *<sup>j</sup>* ⋅ *r* → *ji*) *r* → *ji* \_\_\_\_\_\_\_\_\_ *rji*

*dem*, the particle interaction term *<sup>H</sup>*

*eff* can be usually written as

→

*ext*, (4)

<sup>5</sup> ). (5)

*int*, and the applied external

→ *int* is

traditional numerical integration schemes, e.g., Heun's or Runge-Kutta methods.

→ *eff* = *H* → *ani* + *H* → *dem* + *H* → *int* + *H* →

→

simply the dipolar field induced by all of the particles, that is

→ *int*(*r* →

**3. The effect of the geometry of MQD arrays**

referred to as magnetocrystalline anisotropy energy.

dipole interaction of the elementary magnets.

Within the same limit, the total effective field *H*

*H*

180 Nonmagnetic and Magnetic Quantum Dots

*ani*, the demagnetizing field *<sup>H</sup>*

→ *ext*.

*H*

mechanical exchange interaction.

impact the time response of the system.

particles in a two-dimensional assembly.

**3.1. Single-domain limit**

*H* →

magnetic field *H*

In 2014, Meza et al. [28] reported an analysis of the coercivity fields for two-dimensional clusters of ellipsoidal cobalt nanoparticles in two different crystalline configurations (square and hexagonal) as a function of the inter-dot spacing and the frequency of a switching continuous external applied field. For this work oblate nanoparticles of semiaxis lengths of 3 nm × 3 nm × 1.5 nm were chosen. The easy axis of the particles, oriented in plane, was chosen parallel to the boundary of the cluster, and the external field was applied along the same direction. Given that these MQDs lie in the monodomain regime for cobalt, it is safe to assume that they can be properly described by the macrospin model.

By simulating hysteresis cycles for small clusters of cobalt nanoparticles (3 × 3, 5 × 5, and 10 × 10) at different frequencies for the applied oscillating external field (**Figure 2**), it was found that the hexagonal configuration stabilizes the magnetization reversal and narrows the coercivity with respect to the square crystal structure; this is clearly a consequence of the interaction strength promoted by the greater number of the nearest neighbors.

For both crystal systems, it was also observed that as the cluster size gets bigger the coercivity narrows at all frequencies and it is much lower than the coercivity for a single MQD. This is

**Figure 2.** Coercivity histograms for macrospins arranged in a square (left) and hexagonal (right) lattice with different numbers of particles: (a) 3 × 3, (b) 5 × 5, and (c) 10 × 10. The distance between the particles is marked as *d* = 2 *a* 0, 4 *a* 0, and 8 *a* 0 (see [28]).

an interesting finding that could be exploited by trying to switch the magnetization only for of small subgroups of MQDs from a bigger array without affecting the rest of the magnetic system.

In all cases, by increasing the proximity of the nanoparticles, i.e., with a tighter packing of the assembly, and the intensity of the dipole interactions becomes more relevant, this in turn stabilizes the reversal at higher frequencies; so, in a macrospin system, the minimum of the switching time for a given frequency of an oscillating external field is limited by the packing of the nanoparticles.

As an additional note, it is noteworthy that experimentally, it is certainly possible to synthesize magnetic nanoparticles of these sizes by chemical methods. Nevertheless, the fabrication of colloidal nanoparticles and its self-assembly via solvent evaporation cannot yet attain the level of control that would be required to fabricate small uniform clusters of the sizes studied. On the other hand, lithographic techniques that allow a more refined control over the position, sizes, and number of particles in an array are still far from reaching the length scales of the system proposed.

#### **3.2. Micromagnetics**

To contrast the time evolution of a single-domain small-sized array, let us now consider the work done by Semenova et al. in 2013. In [29], the quasistatic hysteresis of close-packed arrays of NiFe nanodisks is studied. The motivation of this work is quite different from the one presented above and relates to the desire of understanding the phenomena in the field of *magnonics*, where there is a great interest in better grasping the propagation and confinement of spin waves in (among other materials) ordered assemblies of MQDs, which would have potential applications in the fabrication of reprogrammable crystals, magnetoelectronic devices, and metamaterials to name a few.

In the assemblies described above, the dipolar interactions play a less important role than the internal micromagnetics of each particle near almost null applied fields, and to some extent, the inter-dot dipolar interaction can be thought in an effective way contributing only to the

**Figure 3.** Simulated and measured hysteresis loops for the (a) the tightly packed array and (b) the almost closed-packed

Magnetization Dynamics in Arrays of Quantum Dots http://dx.doi.org/10.5772/intechopen.73008 183

The lesson learned so far is that the net effect of the geometry of arrays of MQDs is of greater importance for monodomain systems where one of the dominant interactions is the dipolar one––for sufficiently packed systems––and that it enters in a much less important fashion for large systems where intra-dot micromagnetics dominate the magnetization reversal properties.

To conclude the section, let us look one final assembly of MQD where the particle's size is of the order of other multi-domain nanostructures but for which the array geometry and the

In 2010, Redondo et al. [32] reported experimental observations of the magnetization reversal mechanism for arrays of equally irregularly shaped MQDs aligned and distributed over an ordered square grid. The research team found that for different directions of an applied external field, the magnetization reversal would behave in some cases as if the switching mechanisms were dominated by nucleation and displacement of vortices, but in other directions, for

For such systems one of the demagnetizing fields or the exchange interactions will benefit

displacement of the vertices out of the disks.

macrospin description might play an important part.

the same system, the behavior would be that of a macrospin array.

from the presence of the external field to balance out the contention.

**3.3. In between domains**

array from [29].

**Conflict of interest**

The authors declare no conflicts of interest.

Although part of the work is focused on obtaining the spin-wave excitation spectra on the arrays of MQDs, we will only discuss the results on the role of the array geometry and packing over the hysteresis loops measured.

The specific set of MQD assemblies studied consisted in two two-dimensional hexagonal arrays of Ni80Fe20 nanodisks prepared by etched nanosphere lithography. The disks were of near 350 nm in diameter with an edge-to-edge separation of 65 and 12 nm in average for the two arrays, respectively. Accordingly, there is one tightly packed array, and another one is almost closed-packed array.

The system analyzed requires resolving over much larger distances than it is customary, making the problem of numerically predicting the magnetization dynamics a challenging exercise––mainly because of the complexity required to adequately consider periodic boundary conditions for the long-range interactions of the QDs.

The experimental results and magnetic simulations in [29] indicated that the overall hysteretic behavior in the system is dictated by the nucleation and escape of vortices within each nanodisk. Indeed, the observed sudden drop (see **Figure 3**) in the magnetization near an applied null external field is attributed to the formation of vortices near the center of the QDs; eventually, as the applied field pushes the system toward saturation, it manages to move around the center of every vertex to the edge of its disk. Similar phenomenology has been observed in square arrays of MQDs with some degree of anisotropy on the vortex nucleation [30, 31].

**Figure 3.** Simulated and measured hysteresis loops for the (a) the tightly packed array and (b) the almost closed-packed array from [29].

In the assemblies described above, the dipolar interactions play a less important role than the internal micromagnetics of each particle near almost null applied fields, and to some extent, the inter-dot dipolar interaction can be thought in an effective way contributing only to the displacement of the vertices out of the disks.

The lesson learned so far is that the net effect of the geometry of arrays of MQDs is of greater importance for monodomain systems where one of the dominant interactions is the dipolar one––for sufficiently packed systems––and that it enters in a much less important fashion for large systems where intra-dot micromagnetics dominate the magnetization reversal properties.

#### **3.3. In between domains**

an interesting finding that could be exploited by trying to switch the magnetization only for of small subgroups of MQDs from a bigger array without affecting the rest of the magnetic system. In all cases, by increasing the proximity of the nanoparticles, i.e., with a tighter packing of the assembly, and the intensity of the dipole interactions becomes more relevant, this in turn stabilizes the reversal at higher frequencies; so, in a macrospin system, the minimum of the switching time for a given frequency of an oscillating external field is limited by the packing of the nanoparticles. As an additional note, it is noteworthy that experimentally, it is certainly possible to synthesize magnetic nanoparticles of these sizes by chemical methods. Nevertheless, the fabrication of colloidal nanoparticles and its self-assembly via solvent evaporation cannot yet attain the level of control that would be required to fabricate small uniform clusters of the sizes studied. On the other hand, lithographic techniques that allow a more refined control over the position, sizes, and number of particles in an array are still far from reaching the length scales of the system proposed.

To contrast the time evolution of a single-domain small-sized array, let us now consider the work done by Semenova et al. in 2013. In [29], the quasistatic hysteresis of close-packed arrays of NiFe nanodisks is studied. The motivation of this work is quite different from the one presented above and relates to the desire of understanding the phenomena in the field of *magnonics*, where there is a great interest in better grasping the propagation and confinement of spin waves in (among other materials) ordered assemblies of MQDs, which would have potential applications in the fabrication of reprogrammable crystals, magnetoelectronic devices, and

Although part of the work is focused on obtaining the spin-wave excitation spectra on the arrays of MQDs, we will only discuss the results on the role of the array geometry and pack-

The specific set of MQD assemblies studied consisted in two two-dimensional hexagonal arrays of Ni80Fe20 nanodisks prepared by etched nanosphere lithography. The disks were of near 350 nm in diameter with an edge-to-edge separation of 65 and 12 nm in average for the two arrays, respectively. Accordingly, there is one tightly packed array, and another one is

The system analyzed requires resolving over much larger distances than it is customary, making the problem of numerically predicting the magnetization dynamics a challenging exercise––mainly because of the complexity required to adequately consider periodic boundary

The experimental results and magnetic simulations in [29] indicated that the overall hysteretic behavior in the system is dictated by the nucleation and escape of vortices within each nanodisk. Indeed, the observed sudden drop (see **Figure 3**) in the magnetization near an applied null external field is attributed to the formation of vortices near the center of the QDs; eventually, as the applied field pushes the system toward saturation, it manages to move around the center of every vertex to the edge of its disk. Similar phenomenology has been observed in square arrays

of MQDs with some degree of anisotropy on the vortex nucleation [30, 31].

**3.2. Micromagnetics**

182 Nonmagnetic and Magnetic Quantum Dots

metamaterials to name a few.

almost closed-packed array.

ing over the hysteresis loops measured.

conditions for the long-range interactions of the QDs.

To conclude the section, let us look one final assembly of MQD where the particle's size is of the order of other multi-domain nanostructures but for which the array geometry and the macrospin description might play an important part.

In 2010, Redondo et al. [32] reported experimental observations of the magnetization reversal mechanism for arrays of equally irregularly shaped MQDs aligned and distributed over an ordered square grid. The research team found that for different directions of an applied external field, the magnetization reversal would behave in some cases as if the switching mechanisms were dominated by nucleation and displacement of vortices, but in other directions, for the same system, the behavior would be that of a macrospin array.

For such systems one of the demagnetizing fields or the exchange interactions will benefit from the presence of the external field to balance out the contention.

## **Conflict of interest**

The authors declare no conflicts of interest.

## **Author details**

Pablo F. Zubieta Rico<sup>1</sup> , Daniel Olguín<sup>2</sup> and Yuri V. Vorobiev<sup>1</sup> \*

\*Address all correspondence to: vorobiev@cinvestav.mx

1 Materials Department, Center for Research and Advanced Studies of the National Polytechnic Institute, Querétaro, Mexico

[12] Ross CA. Patterned magnetic recording media. Annual Review of Materials Research.

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2 Physics Department, Center for Research and Advanced Studies of the National Polytechnic Institute, Mexico City, Mexico

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, Daniel Olguín<sup>2</sup>

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2 Physics Department, Center for Research and Advanced Studies of the National

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**Chapter 11**

**Provisional chapter**

**Dilute Magnetic Semiconducting Quantum Dots: Smart**

The present day world involved in the fabrication of miniaturized smart devices is in continuous quest of materials with better optoelectronic and magneto-electronic efficiency. Effective incorporation of dopants into semiconductor lattices have been accepted as a primary means of controlling electrical, optical, magnetic and other physico-chemical properties of semiconductors. Manipulations in magnetic spin within a semiconducting material have lead to an effective research for potential ferromagnets with semiconducting properties, leading to an important field, dilute magnetic semiconductors (DMS). On the other hand, quantum dots (QDs) have been registered to be quantum confined nanocrystals with unique optoelectronic properties, having a wide range of potential applications. QDs experienced rapid development leading to the concept of dilute magnetic semiconducting quantum dots (DMSQDs), where transition metals with a few to several atomic percentages, having unpaired d-electrons, are doped in order to manipulate their opto-magnetic properties. These materials are fabricated by alloying transition metals with Group II-VI, III-V and IV-IV elements resulting in multi-component systems. They have tremendous applications in the spintronics industry, where electronic properties are controlled by spin degree of freedom. The present report reveals the significance, electronic origination of the fact, synthesis and their applications toward the fabrication of spintronics devices.

**Dilute Magnetic Semiconducting Quantum Dots: Smart** 

DOI: 10.5772/intechopen.73286

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

B or A(M)B or AB(M), where A is often a

and reproduction in any medium, provided the original work is properly cited.

The dilute magnetic semiconductor quantum dots (DMSQDs) are basically the combination of semiconducting quantum dots, where transition metals are introduced as impurity or dop-

**Keywords:** dilute magnetic semiconductors, quantum dots, dilute magnetic semiconductors quantum dots, spintronics, opto-magnetic properties

**Materials for Spintronics**

**Materials for Spintronics**

http://dx.doi.org/10.5772/intechopen.73286

**Abstract**

**1. Introduction**

ants. They are symbolically represented by A1−xM<sup>x</sup>

Jejiron Maheswari Baruah and Jyoti Narayan

Additional information is available at the end of the chapter

Jejiron Maheswari Baruah and Jyoti Narayan

Additional information is available at the end of the chapter


**Provisional chapter**

## **Dilute Magnetic Semiconducting Quantum Dots: Smart Materials for Spintronics Materials for Spintronics**

**Dilute Magnetic Semiconducting Quantum Dots: Smart** 

DOI: 10.5772/intechopen.73286

Jejiron Maheswari Baruah and Jyoti Narayan Additional information is available at the end of the chapter

Jejiron Maheswari Baruah and Jyoti Narayan

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73286

#### **Abstract**

[27] Majetich SA, Wen T, Mefford OT. Magnetic nanoparticles. MRS Bulletin. 2013 Nov;**38**(11):

[28] Meza MM, Zubieta Rico PF, Horley PP, Sukhov A, Vieira VR. Multi-parameter optimization of a nanomagnetic system for spintronic applications. Physica B: Condensed Matter.

[29] Semenova EK, Montoncello F, Tacchi S, Dürr G, Sirotkin E, Ahmad E, Madami M, Gubbiotti G, Neusser S, Grundler D, Ogrin FY. Magnetodynamical response of largearea close-packed arrays of circular dots fabricated by nanosphere lithography. Physical

[30] Zhu X, Grütter P, Metlushko V, Ilic B. Magnetization reversal and configurational anisotropy of dense permalloy dot arrays. Applied physics letters. Jun 24, 2002;**80**(25):4789-4791

[31] Natali M, Lebib A, Chen Y, Prejbeanu IL, Ounadjela K.Configurational anisotropy in square lattices of interacting cobalt dots. Journal of Applied Physics. May 15, 2002;**91**(10):7041-7043

[32] Redondo C, Sierra B, Moralejo S, Castano F. Magnetization reversal induced by irregular shape nanodots in square arrays. Journal of Magnetism and Magnetic Materials. Jul 31,

899-903

Nov 15, 2014;**453**:136-139

186 Nonmagnetic and Magnetic Quantum Dots

2010;**322**(14):1969-1972

Review B. May 28, 2013;**87**(17):174432

The present day world involved in the fabrication of miniaturized smart devices is in continuous quest of materials with better optoelectronic and magneto-electronic efficiency. Effective incorporation of dopants into semiconductor lattices have been accepted as a primary means of controlling electrical, optical, magnetic and other physico-chemical properties of semiconductors. Manipulations in magnetic spin within a semiconducting material have lead to an effective research for potential ferromagnets with semiconducting properties, leading to an important field, dilute magnetic semiconductors (DMS). On the other hand, quantum dots (QDs) have been registered to be quantum confined nanocrystals with unique optoelectronic properties, having a wide range of potential applications. QDs experienced rapid development leading to the concept of dilute magnetic semiconducting quantum dots (DMSQDs), where transition metals with a few to several atomic percentages, having unpaired d-electrons, are doped in order to manipulate their opto-magnetic properties. These materials are fabricated by alloying transition metals with Group II-VI, III-V and IV-IV elements resulting in multi-component systems. They have tremendous applications in the spintronics industry, where electronic properties are controlled by spin degree of freedom. The present report reveals the significance, electronic origination of the fact, synthesis and their applications toward the fabrication of spintronics devices.

**Keywords:** dilute magnetic semiconductors, quantum dots, dilute magnetic semiconductors quantum dots, spintronics, opto-magnetic properties

## **1. Introduction**

The dilute magnetic semiconductor quantum dots (DMSQDs) are basically the combination of semiconducting quantum dots, where transition metals are introduced as impurity or dopants. They are symbolically represented by A1−xM<sup>x</sup> B or A(M)B or AB(M), where A is often a

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

non-magnetic cation, A and B can be from Group II-VI, Group III-V and Group IV-IV elements. These nanoscale materials play an important role in microelectronics and magnetic storage devices [1–3]. Additionally, these materials have the quality to exist in both Curie temperatures (T<sup>c</sup> ) as well as in room temperature (RT) with high saturation of magnetization (M<sup>s</sup> ) [4–6]. These quantum confined materials has unique magneto-optical and optically controlled magnetism properties, which make them essentially important in today's research on materials for spintronics (spin-based electronics) [7, 8] device making. The device making includes miniaturization of electronic devices, magnetic fluids and high density data storage systems [9–11]. The semiconducting quantum dots which are being in research are from the Group II-VI and the dopants are transition metals. The individual potentiality of these materials, generated by coupling of diluted magnetic semiconductor (DMS) and quantum dots (QDs), is expected to be a path breaking one in the future field of optoelectronics and magneto-optoelectronic devices. To understand better, we will first look into the concept of DMS and QDs separately.

structure as per their compositional range [17]. It is noticed that the change in Manganese (Mn) content can lead the crystal structure to cubic or hexagonal orientation. It has been observed that lower the input of Mn element in the host composition, the resultant structure tends to acquire cubic crystallinity, whereas, with higher the amount of Mn doping wurtzite crystallinity is observed [18]. Crystal structure of the various compositional ranges of the material suggests that, although their symmetry is different, both achieve tetrahedral geometry (*s-p3* bonding) with the involvement of 2*s* valence electrons from Group II element and 6*p* valence

Dilute Magnetic Semiconducting Quantum Dots: Smart Materials for Spintronics

http://dx.doi.org/10.5772/intechopen.73286

bonding arrangement, although Mn differs from Group II material with an exactly half-filled d-orbital. The Hund's rule suggests that introduction of an unpaired electron of opposite spin will require a lot of input of energy (~ 6–7 eV) and hence Mn is acting as complete *3d* shell material [19]. This ability makes Mn eligible for the replacement of Group II elements in the tetrahedral structure. There is another crucial reason behind the establishment of Mn as a replacement of other Group II materials, which is the exact half-filled *3d* orbital configuration of Mn. The fact is as important as, there is a possibility of forming stable phase by other elements too, although, dimer formation is very much a possibility for other than Mn of the Group II element [18].

electrons to the *s-p3*

189

) system is

revealed at RT [23, 24].

films (0 ≤ x ≤ 0.08), on LaAlO<sup>3</sup>

O2

can be accommodated

(0 ≤ x ≤ 0.05) onto α-Al<sup>2</sup>

and

O3

electrons from Group VI element. Mn acts as a contributor of its valence *4s2*

Among the oxide base DMS materials, Cobalt (Co) intruded Titanium dioxide (TiO<sup>2</sup>

exhibiting the expected necessary property as Co-doped TiO<sup>2</sup>

system was reported at Anatase phase Ti1−xCo<sup>x</sup>

the thin films of rutile phase TiO<sup>2</sup>

suggest two-dimensional smooth surfaces [32].

bohedral orientations [28]. Therefore the thin films of Co-doped TiO<sup>2</sup>

one of the most consistently researched *n-type* semiconductor, to achieve ferromagnetism far above the RT (T<sup>c</sup> > 650 K) [20]. The importance of temperature in this kind of system is because the system of magnetic semiconductor is hard to achieve at RT [21, 22]. The reason behind such concept is the faced difficulty in the introduction of both electronic and magnetic dopants in the system and functionalization of the designed material as a good balanced material between dopant spins and free carriers of electrons. Hence, the synthesized material achieves the coupling as a thermally strong dopant spin-carrier coupling [22]. The most recent research suggest that the ternary cited materials of Mn-doped Group II-VI semiconductors are also capable of

 is a wide direct forbidden band gap (3.03 eV) material, used for optoelectronic devices and solar cell applications [25–27]. Its crystal symmetry is found to be in tetragonal and rhom-

 substrates, using laser molecular beam epitaxy, at substrate growth temperatures between 680 and 720°C [29]. Same research group also found the satisfactory results with

with a composition of Ti1−xCo<sup>x</sup>

substrates, using the same deposition technique [30]. After this achievement, a good number work on this composition was done with various thin film deposition techniques, viz., pulsed laser deposition (PLD) [31–36], laser molecular beam epitaxy (LMBE) [37–39], metal–organic chemical vapor deposition (MOCVD) [40], reactive co-sputtering, oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) [41] and sol–gel [42] method. Researchers observed that the pressure of Oxygen applied during the thin film deposition is also a very important factor and suggested that at PO2 ≥ 1.3 × 10−5 mbar, we can have clear streaky RHEED patterns, which

O2

in the applicative DMS devices. The first observation of RT ferromagnetism in the Co-doped

**3.2. TiO2**

TiO<sup>2</sup>

TiO<sup>2</sup>

SrTiO<sup>3</sup>

 **base DMS**

## **2. Dilute magnetic semiconductor (DMS)**

Magnetic semiconductors are the semiconducting materials, which can exhibit ferromagnetism. Doping of the transition metals in these materials are said to be dilute magnetic semiconductors (DMS). The DMS are therefore semi-magnetic due to the introduction of magnetic elements in their lattices. Basically, the spintronics property of these materials has attracted the present day research for possible technological applications. By definition, spintronics is a combination of electrons' spin and their associated electronic charge and magnetic moment.

The first generation spintronics devices are derived from passive magnetoresistive sensors [12], but the second generations devices are expectedly achieved with the active spin-based devices, which are manipulated in the host semiconductor with spin-polarized electrons [13, 14]. The thought behind a spintronics device is the presence of spin-polarized electrons which travels through the host. Although the introduction of the ferromagnetic material in the semiconductor material through doping is extensively studied, yet the electronic spin is difficult to preserve throughout the material interface due to the difference in electrical conductivity in both the doped as well as the host material [15]. Hence, to present these materials as expected material, better than both ferromagnetic and semiconductor individually, research is very much crucial and warranted in spin electronic carrier device industries. DMS is next concept to meet the vital applicative operation to establish the spintronics carrier devices, an efficient one. In DMS, the host is non-magnetic semiconductor, whereas, the magnetic material is from transition metal series. These are powerful integrated devices having highly spin-polarized capacity.

## **3. DMS materials and their applications**

#### **3.1. AII 1−xMn<sup>x</sup> BVI ternary materials**

One of the most extensively studied DMS is alloys of AII 1−xMn<sup>x</sup> BVI (*A- Group II element & B-Group VI element*) [16]. This ternary material exhibits both wurtzite and zinc blende crystalline structure as per their compositional range [17]. It is noticed that the change in Manganese (Mn) content can lead the crystal structure to cubic or hexagonal orientation. It has been observed that lower the input of Mn element in the host composition, the resultant structure tends to acquire cubic crystallinity, whereas, with higher the amount of Mn doping wurtzite crystallinity is observed [18]. Crystal structure of the various compositional ranges of the material suggests that, although their symmetry is different, both achieve tetrahedral geometry (*s-p3* bonding) with the involvement of 2*s* valence electrons from Group II element and 6*p* valence electrons from Group VI element. Mn acts as a contributor of its valence *4s2* electrons to the *s-p3* bonding arrangement, although Mn differs from Group II material with an exactly half-filled d-orbital. The Hund's rule suggests that introduction of an unpaired electron of opposite spin will require a lot of input of energy (~ 6–7 eV) and hence Mn is acting as complete *3d* shell material [19]. This ability makes Mn eligible for the replacement of Group II elements in the tetrahedral structure. There is another crucial reason behind the establishment of Mn as a replacement of other Group II materials, which is the exact half-filled *3d* orbital configuration of Mn. The fact is as important as, there is a possibility of forming stable phase by other elements too, although, dimer formation is very much a possibility for other than Mn of the Group II element [18].

#### **3.2. TiO2 base DMS**

non-magnetic cation, A and B can be from Group II-VI, Group III-V and Group IV-IV elements. These nanoscale materials play an important role in microelectronics and magnetic storage devices [1–3]. Additionally, these materials have the quality to exist in both Curie tempera-

) as well as in room temperature (RT) with high saturation of magnetization (M<sup>s</sup>

These quantum confined materials has unique magneto-optical and optically controlled magnetism properties, which make them essentially important in today's research on materials for spintronics (spin-based electronics) [7, 8] device making. The device making includes miniaturization of electronic devices, magnetic fluids and high density data storage systems [9–11]. The semiconducting quantum dots which are being in research are from the Group II-VI and the dopants are transition metals. The individual potentiality of these materials, generated by coupling of diluted magnetic semiconductor (DMS) and quantum dots (QDs), is expected to be a path breaking one in the future field of optoelectronics and magneto-optoelectronic devices.

To understand better, we will first look into the concept of DMS and QDs separately.

Magnetic semiconductors are the semiconducting materials, which can exhibit ferromagnetism. Doping of the transition metals in these materials are said to be dilute magnetic semiconductors (DMS). The DMS are therefore semi-magnetic due to the introduction of magnetic elements in their lattices. Basically, the spintronics property of these materials has attracted the present day research for possible technological applications. By definition, spintronics is a combination of electrons' spin and their associated electronic charge and magnetic moment. The first generation spintronics devices are derived from passive magnetoresistive sensors [12], but the second generations devices are expectedly achieved with the active spin-based devices, which are manipulated in the host semiconductor with spin-polarized electrons [13, 14]. The thought behind a spintronics device is the presence of spin-polarized electrons which travels through the host. Although the introduction of the ferromagnetic material in the semiconductor material through doping is extensively studied, yet the electronic spin is difficult to preserve throughout the material interface due to the difference in electrical conductivity in both the doped as well as the host material [15]. Hence, to present these materials as expected material, better than both ferromagnetic and semiconductor individually, research is very much crucial and warranted in spin electronic carrier device industries. DMS is next concept to meet the vital applicative operation to establish the spintronics carrier devices, an efficient one. In DMS, the host is non-magnetic semiconductor, whereas, the magnetic material is from transition metal series. These are powerful integrated devices having highly spin-polarized capacity.

1−xMn<sup>x</sup>

*Group VI element*) [16]. This ternary material exhibits both wurtzite and zinc blende crystalline

BVI (*A- Group II element & B-*

**2. Dilute magnetic semiconductor (DMS)**

**3. DMS materials and their applications**

One of the most extensively studied DMS is alloys of AII

**BVI ternary materials**

**3.1. AII**

**1−xMn<sup>x</sup>**

) [4–6].

tures (T<sup>c</sup>

188 Nonmagnetic and Magnetic Quantum Dots

Among the oxide base DMS materials, Cobalt (Co) intruded Titanium dioxide (TiO<sup>2</sup> ) system is one of the most consistently researched *n-type* semiconductor, to achieve ferromagnetism far above the RT (T<sup>c</sup> > 650 K) [20]. The importance of temperature in this kind of system is because the system of magnetic semiconductor is hard to achieve at RT [21, 22]. The reason behind such concept is the faced difficulty in the introduction of both electronic and magnetic dopants in the system and functionalization of the designed material as a good balanced material between dopant spins and free carriers of electrons. Hence, the synthesized material achieves the coupling as a thermally strong dopant spin-carrier coupling [22]. The most recent research suggest that the ternary cited materials of Mn-doped Group II-VI semiconductors are also capable of exhibiting the expected necessary property as Co-doped TiO<sup>2</sup> revealed at RT [23, 24].

TiO<sup>2</sup> is a wide direct forbidden band gap (3.03 eV) material, used for optoelectronic devices and solar cell applications [25–27]. Its crystal symmetry is found to be in tetragonal and rhombohedral orientations [28]. Therefore the thin films of Co-doped TiO<sup>2</sup> can be accommodated in the applicative DMS devices. The first observation of RT ferromagnetism in the Co-doped TiO<sup>2</sup> system was reported at Anatase phase Ti1−xCo<sup>x</sup> O2 films (0 ≤ x ≤ 0.08), on LaAlO<sup>3</sup> and SrTiO<sup>3</sup> substrates, using laser molecular beam epitaxy, at substrate growth temperatures between 680 and 720°C [29]. Same research group also found the satisfactory results with the thin films of rutile phase TiO<sup>2</sup> with a composition of Ti1−xCo<sup>x</sup> O2 (0 ≤ x ≤ 0.05) onto α-Al<sup>2</sup> O3 substrates, using the same deposition technique [30]. After this achievement, a good number work on this composition was done with various thin film deposition techniques, viz., pulsed laser deposition (PLD) [31–36], laser molecular beam epitaxy (LMBE) [37–39], metal–organic chemical vapor deposition (MOCVD) [40], reactive co-sputtering, oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) [41] and sol–gel [42] method. Researchers observed that the pressure of Oxygen applied during the thin film deposition is also a very important factor and suggested that at PO2 ≥ 1.3 × 10−5 mbar, we can have clear streaky RHEED patterns, which suggest two-dimensional smooth surfaces [32].

The ferromagnetism in Co-doped TiO<sup>2</sup> is a topic of interest for the research accompanying spintronics devices. The oxide base DMS materials have extrinsic or intrinsic effect, which is the root of their device driven capability, is still a matter of discussion. The extrinsic effect may be attributed to the interaction of local magnetic moments with magnetic impurities. The intrinsic magnetism may be due to the exchange coupling between the spin of carriers and local magnetic moments. Since, spintronics takes place only in polarized charge carriers, which is possible only when the ferromagnetism is intrinsic. The issue is of great concern because the experimental evidence is not yet available behind the actual reason of magnetism of DMS in TiO<sup>2</sup> . Anomalous Hall effect (AHE) and electric field induced modulation by magnetization suggests, for rutile phased Co-doped TiO<sup>2</sup> system, the carrier-mediated ferromagnetism with a value of 13.5% [43, 44].

due to the fact of their atom like structures. These particles in this confinement have 10–1000 numbers of atoms within one particle. Therefore, the energy levels of each particle have the merging levels of only some of the atoms in comparison to their bulk entity, where millions of atoms coincide. Because of this fact, very less energy levels can merge with each other in a QD and hence the band gap energy increases drastically. The QDs have another specific property of showing blunt and broad absorption peak. The primary cause behind the phenomenon resides in their size effect. At the atomic level, the slight change in the size of a particle (viz., 0.5 nm) can change their HOMO-LUMO gap drastically. Therefore, whenever there is a solution of QDs, the particle size is never homogeneously uniform in the solution. Hence, for every particle the band gap energy will be different and therefore the absorption maximum shifts accordingly. As a consequence of the presence of differently sized particles in the solution, togetherness of these absorption maxima can be observed and hence a broad peak. Therefore, by tuning the size, we can meet the desired application with these particles. Apart from the size factor, shape phenomenon also plays a significant role in deciding the characteristic features in the field of quantum dot physics. The electrons, which are the driving force behind every electronic transition in a physical matter, have received different orientations in terms of surface of the particle. In the quantum range of physics, the QDs are forced to adapt the required application by modifying their surface. The reason behind such observation is the attachment of the surface electrons for differently shaped particles is different, which is again as an outcome of releasing surface energy of the particle to make it stable. The introduction of capping agents (the ligands) is also having a capability of taming the particle according to their preferred shape. This phenomenon is addressed as surface functionalization. The surface modification can lead us to the

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191

fabrication of the particles with better efficacy in different applicative devices.

The property of showing high luminescence by these QD metamaterials is one of the most aspired properties. The generation of double excitons leads the materials toward more promising luminescent material. This extraordinary property blesses these materials to show higher emission range than the traditional dyes and hence they become more appropriate with the fact of getting more emission with the excitation of only one electron. The size and, of course the shape, both have an important role in making them suitable for these applications. Most of the time the tunable size property of these quantum dots is mentioned, due to which one can access the whole light spectrum. The devices such as LEDs and solar cell require these nano dots in such a manner that they have the ability to absorb the whole visible and UV region and emit the same in higher wavelength. Therefore, the luminescence property of these fluorescent dots has to have the tenability to perform in the whole region. Fortunately, researchers found that for different semiconducting quantum dots, we can achieve the luminescence as per our requirement. Another interesting concept of wastage of the solar energy as thermal energy during the absorption of sun light by a photovoltaic cell comes into play, in the present day photo voltaic research. It is observed that a photovoltaic material, such as, QDs (although being the most promising one), cannot absorb the whole sun light as conversion efficiency of the cell becomes less. The reason behind this is the material, that we are using, can absorb the light in the desired range but cannot emit the same in the desired wavelength. To tackle this difficulty, the concept of large stock shift quantum dots has come up. This large stock shift materials can absorb the sun light in short wavelength and emit the same in the long wavelength, which make these functionalized quantum dots more efficient toward these kind of applications [55].

Recent theoretical studies propose the creation and distribution of oxygen vacancies in Co-doped TiO<sup>2</sup> is responsible for the ferromagnetism in these systems. The ferromagnetism is suppressed when the oxygen content is increased in the unit cell [45]. In a nutshell, for the TiO<sup>2</sup> crystal, in the event of an oxygen vacancy, Ti atoms will give away their electrons to oxygen and hence they will be in the scarcity of electrons to get bind with the oxygen vacancy sites by their own atoms and therefore a situation of hydrogen-like orbital occurs, hence constitutes a Polaron. This phenomenon is supported by a percolation model named bond magnetic polaron (BMP), which was used to study the magnetically doped oxides [46].

In the interaction of the magnetic cations with the hydrogenic electrons in the impurity band, the donors tend to form BMPs, coupling the 3d moments of the ions within their orbits. Depending on whether the cation 3d orbital is less than half filled, or half filled or more, the coupling between the cation and the donor electron is ferromagnetic or anti-ferromagnetic, respectively. Either way, the coupling between two similar impurities within the same donor orbital is ferromagnetic. The polaron radius is a function of the host material's dielectric constant and electron effective mass. If the polaron concentration in the material is large enough to achieve percolation, an entire network of polarons and magnetic cations become interconnected and we observe macroscopic ferromagnetic behavior [47].

Thus, the incorporation of impurities/dopants in the semiconducting lattices have been realized as an important primary means of controlling the magnetic and electrical conductivities, besides having an immense effect on magnetic, magneto-optical and other physical properties of semiconductors.

## **4. Quantum dots**

Quantum dots (QDs) of semiconducting materials have attracted the research community due to their potential application in various fields of humanity, viz., optoelectronics, solar cell, bioimaging and biosensors, cosmetics, space science, photocatalytic activity, etc. [48–54]. The QDs can be defined with respect to their size, which is supposed to be less than excitons Bohr radius. The material specific Bohr radius also leads to the property of that material. The size factor is supported by differently shaped particles. The size of the QDs leads to the significant change in band gap of the semiconductors than the bulk. The enhanced band energy of the particles is due to the fact of their atom like structures. These particles in this confinement have 10–1000 numbers of atoms within one particle. Therefore, the energy levels of each particle have the merging levels of only some of the atoms in comparison to their bulk entity, where millions of atoms coincide. Because of this fact, very less energy levels can merge with each other in a QD and hence the band gap energy increases drastically. The QDs have another specific property of showing blunt and broad absorption peak. The primary cause behind the phenomenon resides in their size effect. At the atomic level, the slight change in the size of a particle (viz., 0.5 nm) can change their HOMO-LUMO gap drastically. Therefore, whenever there is a solution of QDs, the particle size is never homogeneously uniform in the solution. Hence, for every particle the band gap energy will be different and therefore the absorption maximum shifts accordingly. As a consequence of the presence of differently sized particles in the solution, togetherness of these absorption maxima can be observed and hence a broad peak. Therefore, by tuning the size, we can meet the desired application with these particles. Apart from the size factor, shape phenomenon also plays a significant role in deciding the characteristic features in the field of quantum dot physics. The electrons, which are the driving force behind every electronic transition in a physical matter, have received different orientations in terms of surface of the particle. In the quantum range of physics, the QDs are forced to adapt the required application by modifying their surface. The reason behind such observation is the attachment of the surface electrons for differently shaped particles is different, which is again as an outcome of releasing surface energy of the particle to make it stable. The introduction of capping agents (the ligands) is also having a capability of taming the particle according to their preferred shape. This phenomenon is addressed as surface functionalization. The surface modification can lead us to the fabrication of the particles with better efficacy in different applicative devices.

The ferromagnetism in Co-doped TiO<sup>2</sup>

190 Nonmagnetic and Magnetic Quantum Dots

netism with a value of 13.5% [43, 44].

netization suggests, for rutile phased Co-doped TiO<sup>2</sup>

of DMS in TiO<sup>2</sup>

Co-doped TiO<sup>2</sup>

of semiconductors.

**4. Quantum dots**

TiO<sup>2</sup>

is a topic of interest for the research accompanying

system, the carrier-mediated ferromag-

spintronics devices. The oxide base DMS materials have extrinsic or intrinsic effect, which is the root of their device driven capability, is still a matter of discussion. The extrinsic effect may be attributed to the interaction of local magnetic moments with magnetic impurities. The intrinsic magnetism may be due to the exchange coupling between the spin of carriers and local magnetic moments. Since, spintronics takes place only in polarized charge carriers, which is possible only when the ferromagnetism is intrinsic. The issue is of great concern because the experimental evidence is not yet available behind the actual reason of magnetism

Recent theoretical studies propose the creation and distribution of oxygen vacancies in

is suppressed when the oxygen content is increased in the unit cell [45]. In a nutshell, for the

In the interaction of the magnetic cations with the hydrogenic electrons in the impurity band, the donors tend to form BMPs, coupling the 3d moments of the ions within their orbits. Depending on whether the cation 3d orbital is less than half filled, or half filled or more, the coupling between the cation and the donor electron is ferromagnetic or anti-ferromagnetic, respectively. Either way, the coupling between two similar impurities within the same donor orbital is ferromagnetic. The polaron radius is a function of the host material's dielectric constant and electron effective mass. If the polaron concentration in the material is large enough to achieve percolation, an entire network of polarons and magnetic cations become intercon-

Thus, the incorporation of impurities/dopants in the semiconducting lattices have been realized as an important primary means of controlling the magnetic and electrical conductivities, besides having an immense effect on magnetic, magneto-optical and other physical properties

Quantum dots (QDs) of semiconducting materials have attracted the research community due to their potential application in various fields of humanity, viz., optoelectronics, solar cell, bioimaging and biosensors, cosmetics, space science, photocatalytic activity, etc. [48–54]. The QDs can be defined with respect to their size, which is supposed to be less than excitons Bohr radius. The material specific Bohr radius also leads to the property of that material. The size factor is supported by differently shaped particles. The size of the QDs leads to the significant change in band gap of the semiconductors than the bulk. The enhanced band energy of the particles is

netic polaron (BMP), which was used to study the magnetically doped oxides [46].

nected and we observe macroscopic ferromagnetic behavior [47].

 crystal, in the event of an oxygen vacancy, Ti atoms will give away their electrons to oxygen and hence they will be in the scarcity of electrons to get bind with the oxygen vacancy sites by their own atoms and therefore a situation of hydrogen-like orbital occurs, hence constitutes a Polaron. This phenomenon is supported by a percolation model named bond mag-

. Anomalous Hall effect (AHE) and electric field induced modulation by mag-

is responsible for the ferromagnetism in these systems. The ferromagnetism

The property of showing high luminescence by these QD metamaterials is one of the most aspired properties. The generation of double excitons leads the materials toward more promising luminescent material. This extraordinary property blesses these materials to show higher emission range than the traditional dyes and hence they become more appropriate with the fact of getting more emission with the excitation of only one electron. The size and, of course the shape, both have an important role in making them suitable for these applications. Most of the time the tunable size property of these quantum dots is mentioned, due to which one can access the whole light spectrum. The devices such as LEDs and solar cell require these nano dots in such a manner that they have the ability to absorb the whole visible and UV region and emit the same in higher wavelength. Therefore, the luminescence property of these fluorescent dots has to have the tenability to perform in the whole region. Fortunately, researchers found that for different semiconducting quantum dots, we can achieve the luminescence as per our requirement. Another interesting concept of wastage of the solar energy as thermal energy during the absorption of sun light by a photovoltaic cell comes into play, in the present day photo voltaic research. It is observed that a photovoltaic material, such as, QDs (although being the most promising one), cannot absorb the whole sun light as conversion efficiency of the cell becomes less. The reason behind this is the material, that we are using, can absorb the light in the desired range but cannot emit the same in the desired wavelength. To tackle this difficulty, the concept of large stock shift quantum dots has come up. This large stock shift materials can absorb the sun light in short wavelength and emit the same in the long wavelength, which make these functionalized quantum dots more efficient toward these kind of applications [55].

## **5. Dilute magnetic semiconductor quantum dots (DMSQDs)**

Discussions on DMS and QDs have made it easier to understand the concept of DMSQDs. They are quantum dots of semiconducting materials doped with transition metals having magnetic behavior. Due to specific significance of QDs, researchers are tremendously focusing on the ferromagnetic material doped QDs. Since, semiconductors do not possess high magnetism in any level of their atomic growth, it becomes essential to incorporate the magnetic nature of DMS in nanoscale so as to improve its efficiency in the various fields of spintronics applications. It has been observed that the effectiveness of interaction of *sp-d* for the exchange of carrier and magnetic ions in terms of hole energy depends on the high and low magnetic field induces from outside. Hence, it is expected that due to the small size of a quantum dot, the exchange and interaction of *d* electrons with *sp* shelled electrons will be extensive in DMSQDs [56]. Therefore, the spintronics devices developed from DMSQDs are expected to be efficient as well as miniaturized one, probably due to the quantum confinement effect of DMS and therefore better than the single DMS materials.

of *sp-d* between the dopants magnetic material and the host semiconductor, although proper mechanism of origin of the effect and the governance of ferromagnetism are not yet confirmed. Secondly, it has been observed that due to the presence of quantitatively unknown weights of ligands within the synthesized material makes it difficult to calculate the conversion of magnetic moments from magnetic ions, [60] however there have been improvement toward the production of DMSQDs from time to time. Early reports on quantification showed the presence of a few magnetic moments in emu/gram (memu/g) [61] due to the doping of magnetic ions, instead of much more as expected. The effect of unknown amount of magnetic moment hinders the knowledge of comparison between the absolute magnetism of bulk and nano materials. The plausible reason to this effect may be the clustering of magnetic dopants

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One of the most advantageous finding on DMSQDs shows an exceptionally different nature of magnetism. It is the co-doping of ZnO with Cu and Fe [62]. Interestingly, ZnO individually doped with Fe or Cu showed an anti-ferromagnetic behavior without a trace of ferromagnetism. Whereas, the co-doping of both the transition metals in ZnO showed high quality ferromagnetism with magnetic moment as high as 600 memu/g. This work has proved the anti-ferromagnetism of Cu doped ZnO with the help of *M* versus *H* (Magnetization versus Magnetic field intensity) plot and anti-ferromagnetism of Fe doped ZnO with the inverse susceptibility plot as function of temperature, showing a negative intercept. But, in co-doped ZnO with Cu and Fe, X-ray absorption spectroscopy (XAS) clearly showed the presence of both Fe+2 and Fe+3 and its relative percentage is dependent on the presence of Cu as a dopant. Another research revealed that the size of Fe doped CdS QD was responsible for the magnetic moment [57]. They have achieved a magnetic moment of 80 memu/g at RT for doped CdS and undoped CdS showed negligible amount of magnetic moment with the same scale reaction. It has been observed that in the presence of an external magnetic field, a non-magnetic substance showed a small internal magnetic field due to Zeeman splitting (having an order of 2), whereas, materials like DMSQDs, the intensity of internal field is very high in the presence of external magnetic field [56]. It is also observed that along with the large internal field, a small external magnetic field also gets generated during this process. This happens due to the presence of the magnetic ions inside the material and the tendency to align themselves in the direction of the applied magnetic field. Theoretical modeling of magneto-optical and electronic property of core-shell nanoparticles of CdS-ZnS, doped with magnetic impurities of Mn showed that, these nanocrystals can give an attuned value of *g* over a wide range and make them suitable for spintronics devices, if the position of the magnetic impurities can be controlled [63]. Spectral fingerprints of the spin–spin interactions between the host excitons and the dopant is also revealed by single particle spectroscopy with discrete projections of individual Mn+2 ions observed from emission peaks. These QDs showed enhancement in exchange splitting at elevated temperatures by an order of magnitude compared to their epitaxial counterparts, which is useful for solotronics applications. The circularly polarized photoluminescence in the presence of magnetic field (MCPL) for bulk DMS is very much different than the QDs. In case of DMS material, the emission band edge of the host material showed a polarization due to the splitting of the band, but doped material (Mn+2) do not show any band polarization due to spin and orbital forbidden emission [64]. But, in DMSQDs, along with the host, the dopant also showed polarized emission band edge in the presence of magnetic field. This surprised effect was although not yet properly understood, but expected to be due to quantum confinement,

or/and inherent *sp-d* exchange interactions.

#### **5.1. Synthesis of DMSQDs**

The synthesis procedures are very much similar to those for the synthesis of QDs. The only exception is to incorporate the metallic materials as impurity during the reaction process. Among a vast number of procedures, the chemical route to synthesis DMSQDs is the most commonly use deficient method. The size and shape of such QDs can be easily tailored through this method. Unwanted oxidation can also be prevented during the adopted during the synthesis process. Fe, Co, Ni and Mn are the main doping elements used for the preparation of DMSQDs of semiconductors of Group II-VI [56].

Clustering and surface doping are two main issues that are faced during the synthetic process for obtaining uniformed DMSQDs. To eliminate these key issues, one has to overcome selfpurification [57] of the materials and understand the reactivity between the host-guest materials [58, 59]. The self-purification is a process where host molecule expels the guest molecule from the surface to attain a thermodynamically stable state by reducing its defect energy. Selfpurification can be resolved by making the magnetic core at first, followed by coating with the semiconducting material and then annealing at higher temperature for a longer time to diffuse the dopant inside the host properly before it get expelled by the host. The reactivity issue can be sorted out by two ways: a) Nucleation doping and b) Growth doping. Successive ionic layer adsorption reaction (SILAR) method was used to dope Fe in CdS in one of the methods of its preparation. This was attained at high temperature. This method showed excellent result with the homogeneous diffusion of Fe in CdS shell. It was also reported that the oxidation state of Fe was reduced to 2 from 2.44 due to the presence of reducing reagent and replaced the Cd site with substitutional doping.

#### **5.2. Properties and applications of DMSQDs**

DMSQDs possess unique properties which make them suitable for wide range of applications. Their properties are primarily divided into magnetic and magneto-optical as well as magneto-electrical properties. These properties are attributed to the exchanged interaction of *sp-d* between the dopants magnetic material and the host semiconductor, although proper mechanism of origin of the effect and the governance of ferromagnetism are not yet confirmed. Secondly, it has been observed that due to the presence of quantitatively unknown weights of ligands within the synthesized material makes it difficult to calculate the conversion of magnetic moments from magnetic ions, [60] however there have been improvement toward the production of DMSQDs from time to time. Early reports on quantification showed the presence of a few magnetic moments in emu/gram (memu/g) [61] due to the doping of magnetic ions, instead of much more as expected. The effect of unknown amount of magnetic moment hinders the knowledge of comparison between the absolute magnetism of bulk and nano materials. The plausible reason to this effect may be the clustering of magnetic dopants or/and inherent *sp-d* exchange interactions.

**5. Dilute magnetic semiconductor quantum dots (DMSQDs)**

DMS and therefore better than the single DMS materials.

tion of DMSQDs of semiconductors of Group II-VI [56].

**5.1. Synthesis of DMSQDs**

192 Nonmagnetic and Magnetic Quantum Dots

with substitutional doping.

**5.2. Properties and applications of DMSQDs**

Discussions on DMS and QDs have made it easier to understand the concept of DMSQDs. They are quantum dots of semiconducting materials doped with transition metals having magnetic behavior. Due to specific significance of QDs, researchers are tremendously focusing on the ferromagnetic material doped QDs. Since, semiconductors do not possess high magnetism in any level of their atomic growth, it becomes essential to incorporate the magnetic nature of DMS in nanoscale so as to improve its efficiency in the various fields of spintronics applications. It has been observed that the effectiveness of interaction of *sp-d* for the exchange of carrier and magnetic ions in terms of hole energy depends on the high and low magnetic field induces from outside. Hence, it is expected that due to the small size of a quantum dot, the exchange and interaction of *d* electrons with *sp* shelled electrons will be extensive in DMSQDs [56]. Therefore, the spintronics devices developed from DMSQDs are expected to be efficient as well as miniaturized one, probably due to the quantum confinement effect of

The synthesis procedures are very much similar to those for the synthesis of QDs. The only exception is to incorporate the metallic materials as impurity during the reaction process. Among a vast number of procedures, the chemical route to synthesis DMSQDs is the most commonly use deficient method. The size and shape of such QDs can be easily tailored through this method. Unwanted oxidation can also be prevented during the adopted during the synthesis process. Fe, Co, Ni and Mn are the main doping elements used for the prepara-

Clustering and surface doping are two main issues that are faced during the synthetic process for obtaining uniformed DMSQDs. To eliminate these key issues, one has to overcome selfpurification [57] of the materials and understand the reactivity between the host-guest materials [58, 59]. The self-purification is a process where host molecule expels the guest molecule from the surface to attain a thermodynamically stable state by reducing its defect energy. Selfpurification can be resolved by making the magnetic core at first, followed by coating with the semiconducting material and then annealing at higher temperature for a longer time to diffuse the dopant inside the host properly before it get expelled by the host. The reactivity issue can be sorted out by two ways: a) Nucleation doping and b) Growth doping. Successive ionic layer adsorption reaction (SILAR) method was used to dope Fe in CdS in one of the methods of its preparation. This was attained at high temperature. This method showed excellent result with the homogeneous diffusion of Fe in CdS shell. It was also reported that the oxidation state of Fe was reduced to 2 from 2.44 due to the presence of reducing reagent and replaced the Cd site

DMSQDs possess unique properties which make them suitable for wide range of applications. Their properties are primarily divided into magnetic and magneto-optical as well as magneto-electrical properties. These properties are attributed to the exchanged interaction One of the most advantageous finding on DMSQDs shows an exceptionally different nature of magnetism. It is the co-doping of ZnO with Cu and Fe [62]. Interestingly, ZnO individually doped with Fe or Cu showed an anti-ferromagnetic behavior without a trace of ferromagnetism. Whereas, the co-doping of both the transition metals in ZnO showed high quality ferromagnetism with magnetic moment as high as 600 memu/g. This work has proved the anti-ferromagnetism of Cu doped ZnO with the help of *M* versus *H* (Magnetization versus Magnetic field intensity) plot and anti-ferromagnetism of Fe doped ZnO with the inverse susceptibility plot as function of temperature, showing a negative intercept. But, in co-doped ZnO with Cu and Fe, X-ray absorption spectroscopy (XAS) clearly showed the presence of both Fe+2 and Fe+3 and its relative percentage is dependent on the presence of Cu as a dopant. Another research revealed that the size of Fe doped CdS QD was responsible for the magnetic moment [57]. They have achieved a magnetic moment of 80 memu/g at RT for doped CdS and undoped CdS showed negligible amount of magnetic moment with the same scale reaction. It has been observed that in the presence of an external magnetic field, a non-magnetic substance showed a small internal magnetic field due to Zeeman splitting (having an order of 2), whereas, materials like DMSQDs, the intensity of internal field is very high in the presence of external magnetic field [56]. It is also observed that along with the large internal field, a small external magnetic field also gets generated during this process. This happens due to the presence of the magnetic ions inside the material and the tendency to align themselves in the direction of the applied magnetic field. Theoretical modeling of magneto-optical and electronic property of core-shell nanoparticles of CdS-ZnS, doped with magnetic impurities of Mn showed that, these nanocrystals can give an attuned value of *g* over a wide range and make them suitable for spintronics devices, if the position of the magnetic impurities can be controlled [63]. Spectral fingerprints of the spin–spin interactions between the host excitons and the dopant is also revealed by single particle spectroscopy with discrete projections of individual Mn+2 ions observed from emission peaks. These QDs showed enhancement in exchange splitting at elevated temperatures by an order of magnitude compared to their epitaxial counterparts, which is useful for solotronics applications. The circularly polarized photoluminescence in the presence of magnetic field (MCPL) for bulk DMS is very much different than the QDs. In case of DMS material, the emission band edge of the host material showed a polarization due to the splitting of the band, but doped material (Mn+2) do not show any band polarization due to spin and orbital forbidden emission [64]. But, in DMSQDs, along with the host, the dopant also showed polarized emission band edge in the presence of magnetic field. This surprised effect was although not yet properly understood, but expected to be due to quantum confinement, where wave functions are overlapped extensively [65]. Magneto-optical response in Cu doped chalcogenide QDs is also a tremendous effect observed in DMSQDs. This photo-excitation phenomenon in these DMSQDs has come as a result of strong spin-exchanged interaction between the valence band-conduction band (VB-CB) of the host and the paramagnetic Cu dopant. The magnetic circular dichroism (MCD) studies revealed the enhancement of paramagnetism up to 100% in these Cu doped ZnSe/CdSe QDs under the UV light excitation. Again, in dark, these materials retained a photo-magnetization memory for timescales of hours [66]. Another application is reported for Mn-doped CdSe QDs as light-induced spontaneous magnetization, where spin effect is controlled to generate, manipulate and read out spins [67]. In this case, no external magnetic field was applied but still showed large Zeeman splitting as a result of photo-excitation. The reason behind these giant splitting is the generation of large dopant-carriers exchange fields. These materials are having potential applications in the field of magneto-optical storage and optically controlled magnetism. DMSQDs are also known to respond to charged carriers. The carrier-mediated ferromagnetic interaction in Mn-doped CdSe QDs are also reported, which arise due to photo-excited carriers from surface defect states of smaller QDs (~3 nm) [68]. Mn-doped ZnO QDs also exhibited ferromagnetic exchange interaction due to photo-excitation in the absence of oxygen [69] and as is reported in air-stable Fe–Sn co-doped In<sup>2</sup> O3 [70]and Mn–Sn co-doped In<sup>2</sup> O3 [71]. Research has proved the conduction band electron–dopant ferromagnetic exchange interaction, offers magneto-electric and magneto-plasmonic properties which helps in wide scale spintronics applications.

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## **Author details**

Jejiron Maheswari Baruah and Jyoti Narayan\*

\*Address all correspondence to: jnarayan.nehu@gmail.com

Department of Basic Sciences & Social Sciences (Chemistry division), Synthetic Nanochemistry Laboratory, School of Technology, North Eastern Hill University, Shillong, India

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where wave functions are overlapped extensively [65]. Magneto-optical response in Cu doped chalcogenide QDs is also a tremendous effect observed in DMSQDs. This photo-excitation phenomenon in these DMSQDs has come as a result of strong spin-exchanged interaction between the valence band-conduction band (VB-CB) of the host and the paramagnetic Cu dopant. The magnetic circular dichroism (MCD) studies revealed the enhancement of paramagnetism up to 100% in these Cu doped ZnSe/CdSe QDs under the UV light excitation. Again, in dark, these materials retained a photo-magnetization memory for timescales of hours [66]. Another application is reported for Mn-doped CdSe QDs as light-induced spontaneous magnetization, where spin effect is controlled to generate, manipulate and read out spins [67]. In this case, no external magnetic field was applied but still showed large Zeeman splitting as a result of photo-excitation. The reason behind these giant splitting is the generation of large dopant-carriers exchange fields. These materials are having potential applications in the field of magneto-optical storage and optically controlled magnetism. DMSQDs are also known to respond to charged carriers. The carrier-mediated ferromagnetic interaction in Mn-doped CdSe QDs are also reported, which arise due to photo-excited carriers from surface defect states of smaller QDs (~3 nm) [68]. Mn-doped ZnO QDs also exhibited ferromagnetic exchange interaction due to photo-excitation in the absence of oxygen [69] and as is reported in air-stable

Fe–Sn co-doped In<sup>2</sup>

194 Nonmagnetic and Magnetic Quantum Dots

**Author details**

India

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Jejiron Maheswari Baruah and Jyoti Narayan\*

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\*Address all correspondence to: jnarayan.nehu@gmail.com

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Chemical Society. 2005;**127**:17586-17587

[71] Tandon B, Yadav A, Nag A. Delocalized electrons mediated magnetic coupling in Mn– Sncodoped In2O3 nanocrystals: Plasmonics shows the way. Chemistry of Materials. 2016;**28**:3620-3624

**Chapter 12**

**Provisional chapter**

**Mn-Doped ZnSe Quantum Dots as Fluorimetric**

DOI: 10.5772/intechopen.70669

**Mn-Doped ZnSe Quantum Dots as Fluorimetric Mercury** 

Quantum dots (QDs), because of their exciting optical properties, have been explored as alternative fluorescent sensors to conventional organic fluorophores which are routinely employed for the detection of various analytes via fluorometry. QD probes can detect toxic metal ions, anions, organic molecules with good selectivity and sensitivity. This chapter investigates the synthesis of Mn-doped ZnSe QDs using nucleation-doping strategy. The as-synthesized QDs were characterized by various analytical tools such as ultravioletvisible (UV-vis) absorption, photoluminescence (PL) spectroscopy, X-ray diffractometry (XRD) and transmission electron microscopy (TEM). It was found that Mn doping of QDs significantly increases the PL intensity. The PL of the resulting QDs was examined in the presence of different metal ions to check its selective response. Among the various metal ions, Hg2+ exhibits a drastic quenching of the QD's emission intensity. A Stern-Volmer plot of [Hg2+] sensing using the as-synthesized QDs showed linearity in the range of 0–30 × 10−6

10−7 ML−1. Thus, the present Mn-doped ZnSe QDs represent a simple, non-toxic fluorescent probe for the qualitative and quantitative detection of mercury ions in aqueous samples.

**Keywords:** quantum dots, ZnSe, fluorimetry, doping, heavy metal detection

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

= 0.99. The detection limit was found to be 6.63 ×

and reproduction in any medium, provided the original work is properly cited.

Continuous and high-concentration exposure of heavy metals can cause various physiological and biochemical problems to the environment and human being. Thus, the detection of these harmful metal ions has become very important from industrial, environmental and biological point of view. This is a challenging subject for analytical chemists due to the

**Mercury Sensor**

**Sensor**

Oluwatobi S. Oluwafemi

**Abstract**

**1. Introduction**

and Oluwatobi S. Oluwafemi

Sundararajan Parani, Ncediwe Tsolekile,

Sundararajan Parani, Ncediwe Tsolekile, Bambesiwe M.M. May, Kannaiyan Pandian

Bambesiwe M.M. May, Kannaiyan Pandian and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70669

ML−1 with the regression coefficient R<sup>2</sup>

**Provisional chapter**

## **Mn-Doped ZnSe Quantum Dots as Fluorimetric Mercury Sensor Sensor**

DOI: 10.5772/intechopen.70669

**Mn-Doped ZnSe Quantum Dots as Fluorimetric Mercury** 

Sundararajan Parani, Ncediwe Tsolekile, Bambesiwe M.M. May, Kannaiyan Pandian and Oluwatobi S. Oluwafemi Sundararajan Parani, Ncediwe Tsolekile, Bambesiwe M.M. May, Kannaiyan Pandian and Oluwatobi S. Oluwafemi

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70669

#### **Abstract**

Quantum dots (QDs), because of their exciting optical properties, have been explored as alternative fluorescent sensors to conventional organic fluorophores which are routinely employed for the detection of various analytes via fluorometry. QD probes can detect toxic metal ions, anions, organic molecules with good selectivity and sensitivity. This chapter investigates the synthesis of Mn-doped ZnSe QDs using nucleation-doping strategy. The as-synthesized QDs were characterized by various analytical tools such as ultravioletvisible (UV-vis) absorption, photoluminescence (PL) spectroscopy, X-ray diffractometry (XRD) and transmission electron microscopy (TEM). It was found that Mn doping of QDs significantly increases the PL intensity. The PL of the resulting QDs was examined in the presence of different metal ions to check its selective response. Among the various metal ions, Hg2+ exhibits a drastic quenching of the QD's emission intensity. A Stern-Volmer plot of [Hg2+] sensing using the as-synthesized QDs showed linearity in the range of 0–30 × 10−6 ML−1 with the regression coefficient R<sup>2</sup> = 0.99. The detection limit was found to be 6.63 × 10−7 ML−1. Thus, the present Mn-doped ZnSe QDs represent a simple, non-toxic fluorescent probe for the qualitative and quantitative detection of mercury ions in aqueous samples.

**Keywords:** quantum dots, ZnSe, fluorimetry, doping, heavy metal detection

## **1. Introduction**

Continuous and high-concentration exposure of heavy metals can cause various physiological and biochemical problems to the environment and human being. Thus, the detection of these harmful metal ions has become very important from industrial, environmental and biological point of view. This is a challenging subject for analytical chemists due to the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

sensitivity, detection limits and acceptable toxicity levels set by global regulation bodies [1, 2]. In addition, similar chemistry of heavy metals is fastidious with respect to selectivity of the determination method. A variety of standardized analytical methods for the detection of metal ions are available. However, only some of them have found application in routine analysis. Recommended procedures for the detection of heavy metals in water samples include photometric methods, flame or graphite furnace atomic absorption spectroscopy (AAS), inductively coupled plasma emission or mass spectrometry (ICP-ES, ICP-MS), total reflection X-ray fluorimetry (TXRF) and anodic-stripping voltammetry (ASV) [3–5]. These methods offer good limits of detection and wide linear ranges, but they are time consuming, require high cost analytical instruments trained operating personals and high maintenance which is a financial burden to analytical laboratories. Furthermore, the required sample pretreatment and preparation time introduce systematic errors in the analysis. However, the development of fast, economical and portable devices for metal ion detection has grown tremendously over the past 10 years. Particularly, chemo-sensors, which offer the ability for both on-line and field monitoring, has attracted many industries in the detection of metal ions in water [6]. This has allowed for quick and continuous supervision monitoring of drinking or ground water and lentic or lotic watercourses. An ideal sensor should provide good sensitivity, high selectivity towards the target analyte, mathematical relationship of signal output to the amount of analyte, fast response time, good signal-to-noise ratio and longterm stability [7]. A variety of sensors have been developed, and these include DNAzymes sensors, optical sensors, electrochemical, colorimetric and fluorescent sensors [1–10] just to mention a few. This chapter aims to introduce the reader to the use of quantum dots (QDs) as metal ion sensors.

single-use test strips for various ions, including heavy metals, are commercially available [19], which have their limitations in accuracy and reversibility. In recent years, activities applying optical sensors for the determination of heavy metals increased [20]. The most significant

Mn-Doped ZnSe Quantum Dots as Fluorimetric Mercury Sensor

http://dx.doi.org/10.5772/intechopen.70669

203

Semiconductor quantum dots (QD) are nanocrystalline materials that confines the motion of the charge carriers in three spatial directions. These crystals are intermediate state of matter that display properties present in neither bulk nor molecular systems. The physical and electronic properties of QDs are strongly dependent on size (number of atoms). Their importance was recognized by the 2000 Nobel Prize in Physics awarded to Alferov and Kroemerin for their basic work on information and communication technology using the semiconductor heterostructures. QDs are generally made as binary semiconductor nanocrystals from groups II–VI (e.g. CdSe, ZnS, etc.), groups IV–VI (e.g. PbTe, PbS) or groups III–V (e.g. InAs, InP) in the periodic table [21]. Moreover, QDs of Si, Ge, Ag, also of carbon and graphene [22–24] and ternary QDs (from I–VI) have been reported [25, 26]. QDs have received much attraction because of their stable, tunable, bright and narrow photoemission, high chemical and photo bleaching stability, processability and surface functionality and they offer many advantages in comparison with conventional fluorophore. Thus, a new generation of QDs based sensor containing the unique optical proper-

methods are the application of quenchable fluorophores or indicator dyes.

ties of QDs has been constructed for sensing molecules and ions in ultratrace level.

by colloidal synthesis, which are difficult to obtain by the other methods.

Colloidal synthesis of QDs widely employs a 'bottom-up' approach where the crystals are nucleated and grown from the precursor materials dissolved in a suitable solvent in the presence of a stabilizing agent. This approach was pioneered by L. Brus, an American scientist when he was at Bell laboratories, New Jersey, in the late 1970s and carried over by some of his post docs notably Moungi Bawendi and Paul Alivisatos. Because of its mild preparative conditions, this method is also called as a wet chemical route and has become popular among the scientists and industrialists for their usefulness in the biomedical and analytical fields. Different types of QDs such as alloyed QDs, core/shell QDs, impurity-doped QDs, polymer-QD composites with desired size and desired functional group on the surface can be prepared

High-quality colloidal quantum dot crystals can be prepared in organic medium. Organic QDs obtained by this method have good degree of monodispersity and high photoluminescence quantum efficiency. This method became familiar after the synthesis of CdSe QD by Murray et al. in 1993 [27, 28]. The precursors for CdSe QDs chosen by Murray were dimethylcadmium

Cd) and TOPSe (Se dissolved in trioctylphosphine (TOP). The rapid injection of both the precursors together into the hot solution of trioctylphosphine oxide (TOPO) at ~300°C produced yellow/orange CdSe nanocrystallites. However, the use of expensive and/or hazardous organic reagents, harsh reaction conditions, and hydrophobicity of the as-prepared QDs are some of the shortcomings of the organic synthetic routes. To make the QDs water soluble, hydrophobic nature of the QDs surface should be converted into hydrophilic nature by surface encapsulation or ligand exchange. Surface modification processes are tedious, involving

**1.2. Quantum dots**

(Me<sup>2</sup>

#### **1.1. Optical sensors**

A chemical sensor can be defined as 'a portable miniaturized analytical device, which can deliver real-time and on-line information in the presence of specific compounds or ions in complex samples' [11]. Chemical sensors can be categorized into electrochemical, optical, mass-sensitive and heat-sensitive, according to the types of transducer. Of these classifications, optical sensors have been the most widely used as contact-less detectors, counting or positioning of parts. An optical sensor device consists of the following components: (a) the recognition element, where specific interaction and identification of the analyte takes place; (b) the transducer element that converts the recognition process into a measurable optical signal; (c) an optical device (process unit) which consists of at least a light source and finally (d) a detector which detects and converts the change of optical properties and amplifies the signal into a unit readout. The optical properties measured can be absorbance, reflectance, luminescence, light polarization, Raman and others. Optical sensors have found many applications in various fields, including biomedical, clinical, environmental monitoring and process controlling [12–18]. They are an attractive analytical tool, whenever continuous monitoring and real-time information is desired. They can track sources of contamination in an industrial process, follow the formation and movement of environmental pollutants and can raise the alarm when a toxic species exceeds an expected level of exposure. For environmental analysis, single-use test strips for various ions, including heavy metals, are commercially available [19], which have their limitations in accuracy and reversibility. In recent years, activities applying optical sensors for the determination of heavy metals increased [20]. The most significant methods are the application of quenchable fluorophores or indicator dyes.

#### **1.2. Quantum dots**

sensitivity, detection limits and acceptable toxicity levels set by global regulation bodies [1, 2]. In addition, similar chemistry of heavy metals is fastidious with respect to selectivity of the determination method. A variety of standardized analytical methods for the detection of metal ions are available. However, only some of them have found application in routine analysis. Recommended procedures for the detection of heavy metals in water samples include photometric methods, flame or graphite furnace atomic absorption spectroscopy (AAS), inductively coupled plasma emission or mass spectrometry (ICP-ES, ICP-MS), total reflection X-ray fluorimetry (TXRF) and anodic-stripping voltammetry (ASV) [3–5]. These methods offer good limits of detection and wide linear ranges, but they are time consuming, require high cost analytical instruments trained operating personals and high maintenance which is a financial burden to analytical laboratories. Furthermore, the required sample pretreatment and preparation time introduce systematic errors in the analysis. However, the development of fast, economical and portable devices for metal ion detection has grown tremendously over the past 10 years. Particularly, chemo-sensors, which offer the ability for both on-line and field monitoring, has attracted many industries in the detection of metal ions in water [6]. This has allowed for quick and continuous supervision monitoring of drinking or ground water and lentic or lotic watercourses. An ideal sensor should provide good sensitivity, high selectivity towards the target analyte, mathematical relationship of signal output to the amount of analyte, fast response time, good signal-to-noise ratio and longterm stability [7]. A variety of sensors have been developed, and these include DNAzymes sensors, optical sensors, electrochemical, colorimetric and fluorescent sensors [1–10] just to mention a few. This chapter aims to introduce the reader to the use of quantum dots (QDs)

A chemical sensor can be defined as 'a portable miniaturized analytical device, which can deliver real-time and on-line information in the presence of specific compounds or ions in complex samples' [11]. Chemical sensors can be categorized into electrochemical, optical, mass-sensitive and heat-sensitive, according to the types of transducer. Of these classifications, optical sensors have been the most widely used as contact-less detectors, counting or positioning of parts. An optical sensor device consists of the following components: (a) the recognition element, where specific interaction and identification of the analyte takes place; (b) the transducer element that converts the recognition process into a measurable optical signal; (c) an optical device (process unit) which consists of at least a light source and finally (d) a detector which detects and converts the change of optical properties and amplifies the signal into a unit readout. The optical properties measured can be absorbance, reflectance, luminescence, light polarization, Raman and others. Optical sensors have found many applications in various fields, including biomedical, clinical, environmental monitoring and process controlling [12–18]. They are an attractive analytical tool, whenever continuous monitoring and real-time information is desired. They can track sources of contamination in an industrial process, follow the formation and movement of environmental pollutants and can raise the alarm when a toxic species exceeds an expected level of exposure. For environmental analysis,

as metal ion sensors.

202 Nonmagnetic and Magnetic Quantum Dots

**1.1. Optical sensors**

Semiconductor quantum dots (QD) are nanocrystalline materials that confines the motion of the charge carriers in three spatial directions. These crystals are intermediate state of matter that display properties present in neither bulk nor molecular systems. The physical and electronic properties of QDs are strongly dependent on size (number of atoms). Their importance was recognized by the 2000 Nobel Prize in Physics awarded to Alferov and Kroemerin for their basic work on information and communication technology using the semiconductor heterostructures. QDs are generally made as binary semiconductor nanocrystals from groups II–VI (e.g. CdSe, ZnS, etc.), groups IV–VI (e.g. PbTe, PbS) or groups III–V (e.g. InAs, InP) in the periodic table [21]. Moreover, QDs of Si, Ge, Ag, also of carbon and graphene [22–24] and ternary QDs (from I–VI) have been reported [25, 26]. QDs have received much attraction because of their stable, tunable, bright and narrow photoemission, high chemical and photo bleaching stability, processability and surface functionality and they offer many advantages in comparison with conventional fluorophore. Thus, a new generation of QDs based sensor containing the unique optical properties of QDs has been constructed for sensing molecules and ions in ultratrace level.

Colloidal synthesis of QDs widely employs a 'bottom-up' approach where the crystals are nucleated and grown from the precursor materials dissolved in a suitable solvent in the presence of a stabilizing agent. This approach was pioneered by L. Brus, an American scientist when he was at Bell laboratories, New Jersey, in the late 1970s and carried over by some of his post docs notably Moungi Bawendi and Paul Alivisatos. Because of its mild preparative conditions, this method is also called as a wet chemical route and has become popular among the scientists and industrialists for their usefulness in the biomedical and analytical fields. Different types of QDs such as alloyed QDs, core/shell QDs, impurity-doped QDs, polymer-QD composites with desired size and desired functional group on the surface can be prepared by colloidal synthesis, which are difficult to obtain by the other methods.

High-quality colloidal quantum dot crystals can be prepared in organic medium. Organic QDs obtained by this method have good degree of monodispersity and high photoluminescence quantum efficiency. This method became familiar after the synthesis of CdSe QD by Murray et al. in 1993 [27, 28]. The precursors for CdSe QDs chosen by Murray were dimethylcadmium (Me<sup>2</sup> Cd) and TOPSe (Se dissolved in trioctylphosphine (TOP). The rapid injection of both the precursors together into the hot solution of trioctylphosphine oxide (TOPO) at ~300°C produced yellow/orange CdSe nanocrystallites. However, the use of expensive and/or hazardous organic reagents, harsh reaction conditions, and hydrophobicity of the as-prepared QDs are some of the shortcomings of the organic synthetic routes. To make the QDs water soluble, hydrophobic nature of the QDs surface should be converted into hydrophilic nature by surface encapsulation or ligand exchange. Surface modification processes are tedious, involving multiple steps and usually produce materials with reduced optical properties compared to the parent organic materials [29, 30].

On the other hand, QDs can also be synthesized in aqueous medium directly. For example, in aqueous synthesis of thiol-stabilized CdTe QDs, Cd2+ dissolved in water medium would be reacted with a HTe− solution in the presence of water-soluble thiol ligands. Refluxing of the above mixture produces CdTe QDs dispersion. Rajh et al. used a thiol (3-mercapto-l,2 propane-diol) as a stabilizing agent to prepare CdTe QD with 20% photoluminescence quantum yield (PLQY) [31]. Afterwards, numerous thiols were investigated as stabilizing agents [32–34]. Compared with organic phase synthesis, aqueous synthesis involves less toxic precursors, inexpensive and produces excellent water-soluble and biocompatible products.

#### **1.3. Modifications for PL enhancement**

Most of the QDs, which are prepared in aqueous conditions, have low stability and low PLQY. As the surface of QDs is highly reactive, they have a high possibility to aggregate in the presence of heat, light, air or some ions. This may cause surface, which further reduced the PLQY. A number of techniques have been used to improve the particle stability, PL efficiency and biocompatibility of the QDs. These include photo-irradiation [35, 36], ultrasonic irradiation [37], doping with transition metals [38–40] and inorganic passivation [41]. Among them, inorganic passivation and doping techniques are the most widely investigated.

material, whereas in the growth-doping process, the host material alone is nucleated and

Mn-Doped ZnSe Quantum Dots as Fluorimetric Mercury Sensor

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205

QDs possess high surface-to-volume ratio and any change occurring at the surface can influence its surface-dependent properties; hence, luminescence of QDs is sensitive to surface states. Any species that interact directly with the QDs surface by physical or chemical means interferes with radiative recombination of the exciton leading to 'quenching' or enhancement of QDs fluorescence. Following this approach, QDs fluorescent probes can be designed by modifying their surface with suitable ligands so that they can selectively recognize the target

QD-based sensors are most frequently used to detect metal cations. Generally, metal ion quenches the QDs fluorescence via three different types of interaction: (i) by exchanging the metal cation of QDs, (ii) by displacing the capping ligand from the QDs surface and (iii) by binding with surface ligands [48]. The first two types are based on the competitive interaction between the analyte metal ion and the surface metal ion of QDs to bind with counterpart anion or surface-capping ligand, respectively, thus forming a stable lower solubility product. For instance, CuSe has a lower solubility than CdSe. As a result, surface Cd2+ ions in CdSe QDs can be easily exchanged by analyte Cu2+ ions to form CuSe particles on the surface of QDs. Similarly, in the case of glutathione (GSH)-capped CdSe QDs, Pb2+ ion binds with the thiol group of the capping GSH followed by displacement from QDs surface due to its higher binding affinity than Cd2+. However, the quenching mechanism in both cases is

grow for certain time, followed by doping and re-growth of host material again.

**Figure 1.** Schematic representations of nucleation- and growth-doping techniques.

**1.4. QDs as sensor**

analyte.

Doped semiconductor nanocrystals have been studied extensively in the past two decades since Bhargava et al. [42] reported on Mn-doped ZnS (Mn:ZnS). They stated that it could be possible to obtain efficient emission from the dopant centres even if the host nanocrystals were not of high quality. The PL of doped QDs is purely dopant-specific. Based on this, different colour-emitting (blue to red) QDs doped with metals (Al3+), transition metals (Cu+ , Mn2+) and halides (F− ) have been reported in the literature [43–45]. Doped nanocrystalline II–VI semiconductors incorporating rare earth (RE) ions such as Tb3+, Eu3+ and Er3+ have also been reported [46, 47]. However, due to the dissimilar chemical properties (e.g. ionic radius, valence state) between the RE ion and host cation (Cd2+, Zn2+), efficient doping of RE ions into II–VI semiconductor host is not favourable.

In contrast to RE ions, the chemical properties of Mn2+ are very similar to those of Cd2+ (or Zn2+); thus, incorporating Mn2+ into II–VI semiconductor host is much easier. Mn-doped semiconductors are potential luminescent and spintronic materials. The Mn2+ ion, used in many luminescent materials, has a d5 configuration. The Mn2+ ion exhibits a broad emission peak, whose position depends strongly on the host lattice due to changes in crystal field strength with host. The emission colour can vary from green to deep red, corresponding to a <sup>4</sup> T1 –6 A1 transition. Since this transition is spin-forbidden, the typical luminescent relaxation time of this emission is of the order of milliseconds.

Doping of the impurity in the host material can be carried out via nucleation-doping or growth-doping processes [48] as shown in **Figure 1**. In a former process, both host and dopant materials are subjected to nucleation at the same time followed by growth of the host

**Figure 1.** Schematic representations of nucleation- and growth-doping techniques.

material, whereas in the growth-doping process, the host material alone is nucleated and grow for certain time, followed by doping and re-growth of host material again.

#### **1.4. QDs as sensor**

,

T1 –6 A1

multiple steps and usually produce materials with reduced optical properties compared to

On the other hand, QDs can also be synthesized in aqueous medium directly. For example, in aqueous synthesis of thiol-stabilized CdTe QDs, Cd2+ dissolved in water medium would

the above mixture produces CdTe QDs dispersion. Rajh et al. used a thiol (3-mercapto-l,2 propane-diol) as a stabilizing agent to prepare CdTe QD with 20% photoluminescence quantum yield (PLQY) [31]. Afterwards, numerous thiols were investigated as stabilizing agents [32–34]. Compared with organic phase synthesis, aqueous synthesis involves less toxic precursors, inexpensive and produces excellent water-soluble and biocompatible

Most of the QDs, which are prepared in aqueous conditions, have low stability and low PLQY. As the surface of QDs is highly reactive, they have a high possibility to aggregate in the presence of heat, light, air or some ions. This may cause surface, which further reduced the PLQY. A number of techniques have been used to improve the particle stability, PL efficiency and biocompatibility of the QDs. These include photo-irradiation [35, 36], ultrasonic irradiation [37], doping with transition metals [38–40] and inorganic passivation [41]. Among them,

Doped semiconductor nanocrystals have been studied extensively in the past two decades since Bhargava et al. [42] reported on Mn-doped ZnS (Mn:ZnS). They stated that it could be possible to obtain efficient emission from the dopant centres even if the host nanocrystals were not of high quality. The PL of doped QDs is purely dopant-specific. Based on this, different colour-emitting (blue to red) QDs doped with metals (Al3+), transition metals (Cu+

II–VI semiconductors incorporating rare earth (RE) ions such as Tb3+, Eu3+ and Er3+ have also been reported [46, 47]. However, due to the dissimilar chemical properties (e.g. ionic radius, valence state) between the RE ion and host cation (Cd2+, Zn2+), efficient doping of RE ions into

In contrast to RE ions, the chemical properties of Mn2+ are very similar to those of Cd2+ (or Zn2+); thus, incorporating Mn2+ into II–VI semiconductor host is much easier. Mn-doped semiconductors are potential luminescent and spintronic materials. The Mn2+ ion, used in many

whose position depends strongly on the host lattice due to changes in crystal field strength with host. The emission colour can vary from green to deep red, corresponding to a <sup>4</sup>

transition. Since this transition is spin-forbidden, the typical luminescent relaxation time of

Doping of the impurity in the host material can be carried out via nucleation-doping or growth-doping processes [48] as shown in **Figure 1**. In a former process, both host and dopant materials are subjected to nucleation at the same time followed by growth of the host

) have been reported in the literature [43–45]. Doped nanocrystalline

configuration. The Mn2+ ion exhibits a broad emission peak,

inorganic passivation and doping techniques are the most widely investigated.

solution in the presence of water-soluble thiol ligands. Refluxing of

the parent organic materials [29, 30].

204 Nonmagnetic and Magnetic Quantum Dots

**1.3. Modifications for PL enhancement**

II–VI semiconductor host is not favourable.

this emission is of the order of milliseconds.

luminescent materials, has a d5

be reacted with a HTe−

Mn2+) and halides (F−

products.

QDs possess high surface-to-volume ratio and any change occurring at the surface can influence its surface-dependent properties; hence, luminescence of QDs is sensitive to surface states. Any species that interact directly with the QDs surface by physical or chemical means interferes with radiative recombination of the exciton leading to 'quenching' or enhancement of QDs fluorescence. Following this approach, QDs fluorescent probes can be designed by modifying their surface with suitable ligands so that they can selectively recognize the target analyte.

QD-based sensors are most frequently used to detect metal cations. Generally, metal ion quenches the QDs fluorescence via three different types of interaction: (i) by exchanging the metal cation of QDs, (ii) by displacing the capping ligand from the QDs surface and (iii) by binding with surface ligands [48]. The first two types are based on the competitive interaction between the analyte metal ion and the surface metal ion of QDs to bind with counterpart anion or surface-capping ligand, respectively, thus forming a stable lower solubility product. For instance, CuSe has a lower solubility than CdSe. As a result, surface Cd2+ ions in CdSe QDs can be easily exchanged by analyte Cu2+ ions to form CuSe particles on the surface of QDs. Similarly, in the case of glutathione (GSH)-capped CdSe QDs, Pb2+ ion binds with the thiol group of the capping GSH followed by displacement from QDs surface due to its higher binding affinity than Cd2+. However, the quenching mechanism in both cases is different. The electron and/or hole transfer from the CdSe to the CuSe energy levels (case i) is much faster than the process of fluorescence generation in the CdSe moiety, thus leading to quenching of QD fluorescence. The CuSe particles chemisorbed on CdSe QDs alters the bandgap energy level and crystal lattice. Hence, the shift in absorption, emission and X-ray diffractometry (XRD) patterns of the QDs after the interaction with the analyte metal ions is the indicative of this metal ion exchange process. By contrast, removing the capping ligand from the QDs surface by analyte metal ion (case ii) depassivates the QDs surface and creates drastic imperfections on the QDs surface, resulting in fluorescence quenching. In the ionbinding type (case iii), divalent analyte metal ions adsorb to the QD surface by electrostatic interaction with the capping carboxylic ligands, thus coordinating several QDs together, leading to the formation of closely packed QD aggregates. This causes a decrease in the luminescence intensity due to self-quenching mechanism. The quenching by this interaction is sensitive to pH and ionic strength of the medium. Under acidic conditions, quenching by metal ions is very low due to non-availability of carboxylate for the interaction with metal ions.

(CH<sup>3</sup>

COO) <sup>2</sup>

• 4H2

O (MnAc<sup>2</sup>

**2.2. Synthesis of Mn-doped ZnSe QDs**

and 0.0015 or 0.003 mmol of MnAc<sup>2</sup>

**Figure 2.** Synthesis of Mn-doped ZnSe QDs**.**

solution and purged with N<sup>2</sup>

**2.3. Characterizations**

loaded in a round-bottomed flask containing 10 mL of N<sup>2</sup>

mixture was heated at 100°C for 15 min under N<sup>2</sup>

) were obtained from Sisco Research Laboratories (SRL) Pvt. Ltd.,

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were

207


flow until the black selenium powder disap-

were dissolved in 25 mL of DD water followed by the

for 20 min. Then, freshly prepared NaHSe solution was added

Mumbai. Selenium powder (99.99%) 3-mercaptopropionic acid and sodium borohydride (NaBH4) were the AR reagents from Sigma Aldrich, Bangalore. Phosphate buffer at a pH of 7.0 was prepared according to literature. The chloride solutions of different metal ions were

Aqueous colloidal solution of Mn:ZnSe QDs was synthesized using MPA as capping agent as depicted in **Figure 2**. Typically, 0.25 mmol of selenium powder and 0.6 mmol of NaBH<sup>4</sup>

peared completely to give a clear colourless solution. On the other hand, 0.5 mmol of ZnAc<sup>2</sup>

addition of MPA. The pH of the reaction mixture was adjusted to 10.3 by adding 1 M NaOH

followed by stirring at 50°C for 2 h. The molar ratio of Zn to Se to MPA was set at 1:0.5:2, whereas Zn to Mn was 1:0.03 or 1:0.06. The as-synthesized nanoparticles were purified by

To investigate the optical, crystal structure and morphological properties of QDs, they were characterized by various analytical techniques. ultraviolet-visible (UV-vis) absorption spectra were recorded with a Shimadzu, (Model UV-1800) UV-visible spectrophotometer, Japan. The samples were dispersed in doubly distilled water and loaded in a 4.5-mL precleaned quartz cuvette with 1-cm optical path. The entire spectrum was scanned against the background spectrum of water. PL measurements of the samples were performed in a 4.5-mL quartz cuvette at ambient conditions on a Perkin Elmer LS5B spectrofluorimeter. For a given sample, the excitation wavelength was identified from the absorption spectrum

prepared at the concentration of 1 × 10−4 M using doubly distilled (DD) water.

precipitation with ethanol, followed by centrifugation and vacuum drying.

Apart from the metal ions, molecules such as phenolic compounds, H<sup>2</sup> O2 [49], 2,4,6-trinitrotoluene (TNT) [50, 51] and glucose [52] can also be detected by fluorometric titration with QDs. The electron-accepting nature of phenolic compounds and TNT shuttled the electron from the conduction band to the valence band of the excited QDs, whereas H<sup>2</sup> O2 oxidizes the surface of QD and destroys its lattice structure resulting in the PL quenching. Glucose can be indirectly detected knowing that glucose can produce H<sup>2</sup> O2 on catalytic oxidation by glucose oxidase.

Most of the QDs that were investigated for fluorometric sensing are based on cadmium QDs; however, a major drawback for their application is the toxicity of cadmium ion. Less toxic particles like doped ZnS or ZnSe QDs may be interesting alternatives for biological imaging and other applications. Mn2+-doped ZnS quantum dots have been extensively investigated in various fields [53]. Fang et al. synthesized high-quality water-dispersible Mn2+-doped ZnSe core/ZnS shell (Mn:ZnSe/ZnS) nanocrystals directly in aqueous media with MPA as the capping ligand [54]. They observed that there was almost no dopant Mn emission in the Mn:ZnSe d-dots and bright Mn luminescence was observed after overcoating the ZnS shell around the Mn:ZnSe dots. In the present work, Mn-doped ZnSe (Mn:ZnSe) QDs have been synthesized by a wet chemical method using 3-mercaptopropionic acid (3-MPA) as capping agent and characterized by various analytical tools. The nucleation-doping method was adopted because it would form a structure similar to core-shell (MnSe/ZnSe) with a diffuse interface. The PL of the resulting QDs was examined in the presence of different metal ions to check its selective response.

## **2. Methodology**

#### **2.1. Materials**

All chemicals were of analytical grade and were used without further purification. All solutions were prepared using doubly distilled water. Zn (CH3COO) <sup>2</sup> •2H2 O (ZnAc<sup>2</sup> ) and Mn (CH<sup>3</sup> COO) <sup>2</sup> • 4H2 O (MnAc<sup>2</sup> ) were obtained from Sisco Research Laboratories (SRL) Pvt. Ltd., Mumbai. Selenium powder (99.99%) 3-mercaptopropionic acid and sodium borohydride (NaBH4) were the AR reagents from Sigma Aldrich, Bangalore. Phosphate buffer at a pH of 7.0 was prepared according to literature. The chloride solutions of different metal ions were prepared at the concentration of 1 × 10−4 M using doubly distilled (DD) water.

#### **2.2. Synthesis of Mn-doped ZnSe QDs**

Aqueous colloidal solution of Mn:ZnSe QDs was synthesized using MPA as capping agent as depicted in **Figure 2**. Typically, 0.25 mmol of selenium powder and 0.6 mmol of NaBH<sup>4</sup> were loaded in a round-bottomed flask containing 10 mL of N<sup>2</sup> -purged DD water. The reaction mixture was heated at 100°C for 15 min under N<sup>2</sup> flow until the black selenium powder disappeared completely to give a clear colourless solution. On the other hand, 0.5 mmol of ZnAc<sup>2</sup> and 0.0015 or 0.003 mmol of MnAc<sup>2</sup> were dissolved in 25 mL of DD water followed by the addition of MPA. The pH of the reaction mixture was adjusted to 10.3 by adding 1 M NaOH solution and purged with N<sup>2</sup> for 20 min. Then, freshly prepared NaHSe solution was added followed by stirring at 50°C for 2 h. The molar ratio of Zn to Se to MPA was set at 1:0.5:2, whereas Zn to Mn was 1:0.03 or 1:0.06. The as-synthesized nanoparticles were purified by precipitation with ethanol, followed by centrifugation and vacuum drying.

#### **2.3. Characterizations**

different. The electron and/or hole transfer from the CdSe to the CuSe energy levels (case i) is much faster than the process of fluorescence generation in the CdSe moiety, thus leading to quenching of QD fluorescence. The CuSe particles chemisorbed on CdSe QDs alters the bandgap energy level and crystal lattice. Hence, the shift in absorption, emission and X-ray diffractometry (XRD) patterns of the QDs after the interaction with the analyte metal ions is the indicative of this metal ion exchange process. By contrast, removing the capping ligand from the QDs surface by analyte metal ion (case ii) depassivates the QDs surface and creates drastic imperfections on the QDs surface, resulting in fluorescence quenching. In the ionbinding type (case iii), divalent analyte metal ions adsorb to the QD surface by electrostatic interaction with the capping carboxylic ligands, thus coordinating several QDs together, leading to the formation of closely packed QD aggregates. This causes a decrease in the luminescence intensity due to self-quenching mechanism. The quenching by this interaction is sensitive to pH and ionic strength of the medium. Under acidic conditions, quenching by metal ions is very low due to non-availability of carboxylate for the interaction with

uene (TNT) [50, 51] and glucose [52] can also be detected by fluorometric titration with QDs. The electron-accepting nature of phenolic compounds and TNT shuttled the electron from the

QD and destroys its lattice structure resulting in the PL quenching. Glucose can be indirectly

Most of the QDs that were investigated for fluorometric sensing are based on cadmium QDs; however, a major drawback for their application is the toxicity of cadmium ion. Less toxic particles like doped ZnS or ZnSe QDs may be interesting alternatives for biological imaging and other applications. Mn2+-doped ZnS quantum dots have been extensively investigated in various fields [53]. Fang et al. synthesized high-quality water-dispersible Mn2+-doped ZnSe core/ZnS shell (Mn:ZnSe/ZnS) nanocrystals directly in aqueous media with MPA as the capping ligand [54]. They observed that there was almost no dopant Mn emission in the Mn:ZnSe d-dots and bright Mn luminescence was observed after overcoating the ZnS shell around the Mn:ZnSe dots. In the present work, Mn-doped ZnSe (Mn:ZnSe) QDs have been synthesized by a wet chemical method using 3-mercaptopropionic acid (3-MPA) as capping agent and characterized by various analytical tools. The nucleation-doping method was adopted because it would form a structure similar to core-shell (MnSe/ZnSe) with a diffuse interface. The PL of the resulting QDs was examined in the presence of different metal ions to check its

All chemicals were of analytical grade and were used without further purification. All solu-

tions were prepared using doubly distilled water. Zn (CH3COO) <sup>2</sup>

O2

O2

O2

on catalytic oxidation by glucose oxidase.

•2H2

O (ZnAc<sup>2</sup>

) and Mn

[49], 2,4,6-trinitrotol-

oxidizes the surface of

Apart from the metal ions, molecules such as phenolic compounds, H<sup>2</sup>

conduction band to the valence band of the excited QDs, whereas H<sup>2</sup>

detected knowing that glucose can produce H<sup>2</sup>

metal ions.

206 Nonmagnetic and Magnetic Quantum Dots

selective response.

**2. Methodology**

**2.1. Materials**

To investigate the optical, crystal structure and morphological properties of QDs, they were characterized by various analytical techniques. ultraviolet-visible (UV-vis) absorption spectra were recorded with a Shimadzu, (Model UV-1800) UV-visible spectrophotometer, Japan. The samples were dispersed in doubly distilled water and loaded in a 4.5-mL precleaned quartz cuvette with 1-cm optical path. The entire spectrum was scanned against the background spectrum of water. PL measurements of the samples were performed in a 4.5-mL quartz cuvette at ambient conditions on a Perkin Elmer LS5B spectrofluorimeter. For a given sample, the excitation wavelength was identified from the absorption spectrum

**Figure 2.** Synthesis of Mn-doped ZnSe QDs**.**

and it was fixed to scan the emission wavelength. X-ray powder diffraction patterns of the samples were recorded at ambient conditions by using PANanalytical X'Pert PRO diffractometer with monochromatic Cu-Kα1 radiation (λ = 1.5418 Å), 2θ ranging from 10 to 80° in steps of 0.017°/s. The accelerating voltage was set at 40 kV and the current flux was 30 mA. Transmission electron microscopy (TEM) images of QDs were obtained from the FEI Tecnai G2 (T-30) instrument with the operating voltage of 250–300 kV. A small amount of sample for TEM analysis was ultra-sonicated in ethanol or water for a few minutes and then dropped on carbon-coated copper grids. The sample grid was then kept in vacuum desiccators prior to the analysis.

#### **2.4. Metal ion sensing**

Fluorescence sensitivity of the QDs towards different metal ions was carried out on a fluorescence microplate reader (Turners Biosystems-Modulus Microplate Multimode Reader-9300-010). Stock solutions of different metal ions (Li<sup>+</sup> , Na<sup>+</sup> , K<sup>+</sup> , Mg2+, Ca2+, Ba2+, Al3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag<sup>+</sup> , Cd2+, Hg2+ and Pb2+) were prepared by dissolving their respective nitrate or chloride salts. Aqueous solution of QDs with the OD = 0.1 was prepared by diluting the stock solution of QDs. The solution of 50 μL was dropped into each well of a 96-well plate followed by filling with different metal solutions to obtain the final volume of 200 μL. The excitation wavelength was selected according to the absorption spectrum of QDs and the relative fluorescence intensity was measured with the plate reader. The standard deviations were calculated from at least six measurements.

at 480 nm and a small shoulder at 398 nm assigned to trap-state emission and band-edge emission, respectively. These two emissions appearing together are often observed in CdSe, ZnSe nanocrystals and their sulphur analogues. As discussed by Denzler et al [56], the bulk defects such as vacancies (Schottky defects) and interstitials (Frenkel defects) are the main source of trap states in the aqueous ZnSe QDs. With cubic zinc blende structure (by XRD), ZnSe usually has Schottky defects predominant over Frenkel defects. Therefore, the observed photoluminescence of QDs could be ascribed to a recombination of electrons at the selenium vacancy energy levels because of low Se/Zn ratio synthetic conditions. There are many possible recombination paths through many trap-state emissions, each with different emission energy, causing the relatively wide emission peak. The PL spectrum of as-prepared ZnSe QDs has the full

**Figure 3.** (a) Absorption and (b) PL spectra of undoped and Mn-doped ZnSe QDs.

The PL spectrum of 3% Mn:ZnSe QDs exhibits a new band at 579 nm in addition to trap- and band-edge emissions. The appearance of this new emission in the Mn-doped QDs is attributed

crystal field effects [55]. Fang et al [54] notified that if the dopant ions are adsorbed on the surface of the host nanocrystal instead of being incorporated into its lattice, no dopant emission is observed and further the host emission is drastically quenched because a loosely adsorbed dopant ion can easily act as a surface trap that quenches the host PL [57]. However, in the present system, after doping with 3% Mn, the luminescence of host emission is enhanced by 7.7 times along with the appearance of Mn emission. This indicates the successful incorporation of Mn2+ in the host ZnSe lattice. The enhancement of host PL is mainly attributed to the reduction of non-radiative energy centres by Mn2+ doping. **Table 1** shows the PL-integrated

With further increase in the Mn concentration from 3 to 6%, the resultant doped QDs show increased Mn emission followed by subtle red-shift indicating the increase in Mn content in host. However, the host-trap emission is decreased and no band-edge emission is significantly observed. Furthermore, instead of the expected two-fold increase in the Mn emission

emission. This transition is spin-forbidden but is allowed because of

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209

width at half maximum of about 110 nm.

intensity ratios calculated from the spectra.

T1 →<sup>6</sup> A1

to Mn2+-related <sup>4</sup>

## **3. Results and discussion**

#### **3.1. Absorption spectra**

**Figure 3a** shows the absorption spectra of 3 and 6% Mn:ZnSe QDs along with undoped QDs. The Mn content is represented here as the mole % against the host metal (Zn) which is considered as 100% according to the experiment. Compared with bulk ZnSe having the bandgap of ~2.8 eV, the absorption band edge of both doped and undoped QDs is blue-shifted (400–500 meV) indicating quantum confinement of the particles. Furthermore, the bandgap of Mn:ZnSe is blue-shifted with respect to undoped ZnSe QDs under the same experimental conditions, which reveal the formation of smaller-sized particles. This is consistent with the results by Mahamuni et al [55]. The synthesis process itself is affected by Mn additive. In addition, the spectrum of 3% Mn:ZnSe QDs is slightly blue-shifted from that of 6% Mn:ZnSe QDs. Because of lower Mn/Se content, the former has relatively smaller-sized nuclei formed in the nucleation step which leads to a slight blue shift.

#### **3.2. PL spectra**

The PL spectra of the 3 and 6% Mn:ZnSe QDs along with undoped QDs (λexc = 365 nm) are shown in **Figure 3b**. The spectrum of undoped ZnSe QDs shows broad emission band centred

**Figure 3.** (a) Absorption and (b) PL spectra of undoped and Mn-doped ZnSe QDs.

and it was fixed to scan the emission wavelength. X-ray powder diffraction patterns of the samples were recorded at ambient conditions by using PANanalytical X'Pert PRO diffractometer with monochromatic Cu-Kα1 radiation (λ = 1.5418 Å), 2θ ranging from 10 to 80° in steps of 0.017°/s. The accelerating voltage was set at 40 kV and the current flux was 30 mA. Transmission electron microscopy (TEM) images of QDs were obtained from the FEI Tecnai G2 (T-30) instrument with the operating voltage of 250–300 kV. A small amount of sample for TEM analysis was ultra-sonicated in ethanol or water for a few minutes and then dropped on carbon-coated copper grids. The sample grid was then kept in vacuum desicca-

Fluorescence sensitivity of the QDs towards different metal ions was carried out on a fluorescence microplate reader (Turners Biosystems-Modulus Microplate Multimode Reader-9300-010).

ride salts. Aqueous solution of QDs with the OD = 0.1 was prepared by diluting the stock solution of QDs. The solution of 50 μL was dropped into each well of a 96-well plate followed by filling with different metal solutions to obtain the final volume of 200 μL. The excitation wavelength was selected according to the absorption spectrum of QDs and the relative fluorescence intensity was measured with the plate reader. The standard deviations were calculated from at

**Figure 3a** shows the absorption spectra of 3 and 6% Mn:ZnSe QDs along with undoped QDs. The Mn content is represented here as the mole % against the host metal (Zn) which is considered as 100% according to the experiment. Compared with bulk ZnSe having the bandgap of ~2.8 eV, the absorption band edge of both doped and undoped QDs is blue-shifted (400–500 meV) indicating quantum confinement of the particles. Furthermore, the bandgap of Mn:ZnSe is blue-shifted with respect to undoped ZnSe QDs under the same experimental conditions, which reveal the formation of smaller-sized particles. This is consistent with the results by Mahamuni et al [55]. The synthesis process itself is affected by Mn additive. In addition, the spectrum of 3% Mn:ZnSe QDs is slightly blue-shifted from that of 6% Mn:ZnSe QDs. Because of lower Mn/Se content, the former has relatively smaller-sized nuclei formed in the

The PL spectra of the 3 and 6% Mn:ZnSe QDs along with undoped QDs (λexc = 365 nm) are shown in **Figure 3b**. The spectrum of undoped ZnSe QDs shows broad emission band centred

, Cd2+, Hg2+ and Pb2+) were prepared by dissolving their respective nitrate or chlo-

, Mg2+, Ca2+, Ba2+, Al3+, Mn2+, Fe2+, Co2+, Ni2+,

, Na<sup>+</sup> , K<sup>+</sup>

tors prior to the analysis.

208 Nonmagnetic and Magnetic Quantum Dots

Stock solutions of different metal ions (Li<sup>+</sup>

**2.4. Metal ion sensing**

least six measurements.

**3.1. Absorption spectra**

**3.2. PL spectra**

**3. Results and discussion**

nucleation step which leads to a slight blue shift.

Cu2+, Zn2+, Ag<sup>+</sup>

at 480 nm and a small shoulder at 398 nm assigned to trap-state emission and band-edge emission, respectively. These two emissions appearing together are often observed in CdSe, ZnSe nanocrystals and their sulphur analogues. As discussed by Denzler et al [56], the bulk defects such as vacancies (Schottky defects) and interstitials (Frenkel defects) are the main source of trap states in the aqueous ZnSe QDs. With cubic zinc blende structure (by XRD), ZnSe usually has Schottky defects predominant over Frenkel defects. Therefore, the observed photoluminescence of QDs could be ascribed to a recombination of electrons at the selenium vacancy energy levels because of low Se/Zn ratio synthetic conditions. There are many possible recombination paths through many trap-state emissions, each with different emission energy, causing the relatively wide emission peak. The PL spectrum of as-prepared ZnSe QDs has the full width at half maximum of about 110 nm.

The PL spectrum of 3% Mn:ZnSe QDs exhibits a new band at 579 nm in addition to trap- and band-edge emissions. The appearance of this new emission in the Mn-doped QDs is attributed to Mn2+-related <sup>4</sup> T1 →<sup>6</sup> A1 emission. This transition is spin-forbidden but is allowed because of crystal field effects [55]. Fang et al [54] notified that if the dopant ions are adsorbed on the surface of the host nanocrystal instead of being incorporated into its lattice, no dopant emission is observed and further the host emission is drastically quenched because a loosely adsorbed dopant ion can easily act as a surface trap that quenches the host PL [57]. However, in the present system, after doping with 3% Mn, the luminescence of host emission is enhanced by 7.7 times along with the appearance of Mn emission. This indicates the successful incorporation of Mn2+ in the host ZnSe lattice. The enhancement of host PL is mainly attributed to the reduction of non-radiative energy centres by Mn2+ doping. **Table 1** shows the PL-integrated intensity ratios calculated from the spectra.

With further increase in the Mn concentration from 3 to 6%, the resultant doped QDs show increased Mn emission followed by subtle red-shift indicating the increase in Mn content in host. However, the host-trap emission is decreased and no band-edge emission is significantly observed. Furthermore, instead of the expected two-fold increase in the Mn emission


**Table 1.** PL properties of undoped and Mn-doped ZnSe QDs.

(from 3% doped), only 1.4-fold increase is observed. The reason might be the combination of the following three processes: (i) non-radiative energy transfer between neighbouring Mn2+-dopant ions which quenches Mn emission [58], (ii) adsorption of some Mn ions on the surface instead of incorporation into the host lattice which quenches both host and Mn emission and (iii) initially formed larger MnSe. As the environment of the doping ions in larger nuclei is not as uniform as that in smaller ones, the PL emission performance of doped QDs with larger nuclei will not be satisfactory as those with smaller nuclei [57]. Overall, the PL peak position of the host is in tune with the corresponding band-edge absorption. The 3% doped sample has the highest emission peak intensity, followed by the 6% and then the 0% sample. The above results are interesting that the ratio of the dual-colour emissions (blue and orange) of the Mn-doped ZnSe QDs could be controlled by tuning the Mn-doping levels in the QDs.

#### **3.3. Structural and morphological analysis**

X-ray diffraction pattern of the 3% Mn:ZnSe QDs is presented in **Figure 4**. Broad diffraction peaks are observed and are attributed to the nanocrystalline nature of the material. The XRD peaks are close to the characteristic peaks corresponding to the (111), (220) and (311) planes of cubic zinc blende ZnSe. Cubic structures are often obtained in the low-temperature aqueous synthesis of ZnSe QDs. This indicates that the incorporation of Mn2+ into the host ZnSe does not bring any obvious change in the crystal lattice and the structure [37, 54]. TEM image of the 3% Mn:ZnSe QDs (**Figure 5a**) shows that QDs are spherical in nature with the average diameter of ~4 nm. Some of the aggregates of the particles are also seen. The energy-dispersive X-ray spectroscopy (EDS) spectrum (**Figure 5b**) confirms the presence of Mn, Zn and Se and the purity of the sample.

almost no response to other metal ions. About 74% of the PL intensity is quenched after the

The influence of [Hg2+] ion on the QDs fluorescence was studied. **Figure 6b** shows the quenching behaviour of Hg2+ ions on the PL intensity of QDs. The PL intensity is quenched drastically and then slightly with the increase in [Hg2+] ions. The fluorescence quenching with respect to the concentration of quencher was analysed using the Stern-Volmer equation. The

/F versus [Hg2+] as shown in **Figure 6c** exhibits a good linear relationship up to 30

= 0.9918. The limit of detection (LOD) was calculated

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addition of 30 μM L−1 of Hg2+ ions.

**Figure 5.** (a) TEM image and (b) EDS of 3% Mn-doped ZnSe QDs.

**Figure 4.** XRD patterns of 3% Mn-doped ZnSe QDs.

μM L−1 with a correlation coefficient R<sup>2</sup>

according to the following equation:

plot of F0

#### **3.4. Effect of metal ions on the PL intensity**

The fluorescence titrations of the 3% Mn:ZnSe QDs (PL, λmax = 461 nm) with various metal ions are shown in **Figure 6a**. The fluorescence intensity (F) is measured with excitation at 365 nm. The PL of blank QDs is used as a control (F0 ). From the figure, it is observed that QDs show maximum response to mercury ions (Hg2+), little response to Ni2+, Cu2+ and Pb2+ ions and

**Figure 4.** XRD patterns of 3% Mn-doped ZnSe QDs.

(from 3% doped), only 1.4-fold increase is observed. The reason might be the combination of the following three processes: (i) non-radiative energy transfer between neighbouring Mn2+-dopant ions which quenches Mn emission [58], (ii) adsorption of some Mn ions on the surface instead of incorporation into the host lattice which quenches both host and Mn emission and (iii) initially formed larger MnSe. As the environment of the doping ions in larger nuclei is not as uniform as that in smaller ones, the PL emission performance of doped QDs with larger nuclei will not be satisfactory as those with smaller nuclei [57]. Overall, the PL peak position of the host is in tune with the corresponding band-edge absorption. The 3% doped sample has the highest emission peak intensity, followed by the 6% and then the 0% sample. The above results are interesting that the ratio of the dual-colour emissions (blue and orange) of the Mn-doped ZnSe QDs could be controlled by tuning the Mn-doping levels

t = trap, b = band edge. (PL peak position and the corresponding integrated intensity are obtained by Gaussian fitting.)

**PL-integrated intensity ratios of Mn:ZnSe QDs**

**Trap/Band edge**

**/Ib It**

**Trap/ Mn**

**/IMn**

**Band-edge Trap Mn Cumulative**

**Peak (nm) I/I0% Peak (nm) I/I0% Peak (nm) I/I3% I/I0% It**

0% 397 1.0 480 1.0 – – 1.0 82.2 – 3% 386 6.7 461 7.7 579 1.0 10.1 95.7 3.2 6% – – 473 3.1 584 1.4 6.5 – 0.9

X-ray diffraction pattern of the 3% Mn:ZnSe QDs is presented in **Figure 4**. Broad diffraction peaks are observed and are attributed to the nanocrystalline nature of the material. The XRD peaks are close to the characteristic peaks corresponding to the (111), (220) and (311) planes of cubic zinc blende ZnSe. Cubic structures are often obtained in the low-temperature aqueous synthesis of ZnSe QDs. This indicates that the incorporation of Mn2+ into the host ZnSe does not bring any obvious change in the crystal lattice and the structure [37, 54]. TEM image of the 3% Mn:ZnSe QDs (**Figure 5a**) shows that QDs are spherical in nature with the average diameter of ~4 nm. Some of the aggregates of the particles are also seen. The energy-dispersive X-ray spectroscopy (EDS) spectrum (**Figure 5b**) confirms the presence of Mn, Zn and Se and

The fluorescence titrations of the 3% Mn:ZnSe QDs (PL, λmax = 461 nm) with various metal ions are shown in **Figure 6a**. The fluorescence intensity (F) is measured with excitation at 365 nm.

maximum response to mercury ions (Hg2+), little response to Ni2+, Cu2+ and Pb2+ ions and

). From the figure, it is observed that QDs show

in the QDs.

**Mn doping**

210 Nonmagnetic and Magnetic Quantum Dots

\*

the purity of the sample.

**3.3. Structural and morphological analysis**

**Table 1.** PL properties of undoped and Mn-doped ZnSe QDs.

**3.4. Effect of metal ions on the PL intensity**

The PL of blank QDs is used as a control (F0

**Figure 5.** (a) TEM image and (b) EDS of 3% Mn-doped ZnSe QDs.

almost no response to other metal ions. About 74% of the PL intensity is quenched after the addition of 30 μM L−1 of Hg2+ ions.

The influence of [Hg2+] ion on the QDs fluorescence was studied. **Figure 6b** shows the quenching behaviour of Hg2+ ions on the PL intensity of QDs. The PL intensity is quenched drastically and then slightly with the increase in [Hg2+] ions. The fluorescence quenching with respect to the concentration of quencher was analysed using the Stern-Volmer equation. The plot of F0 /F versus [Hg2+] as shown in **Figure 6c** exhibits a good linear relationship up to 30 μM L−1 with a correlation coefficient R<sup>2</sup> = 0.9918. The limit of detection (LOD) was calculated according to the following equation:

**Figure 6. (**a) Effect of metal ions (30 μM L−1) on the PL intensity of 3% Mn-doped ZnSe QDs at pH 10.8. (b) The plot of PL intensity of 3% Mn-doped ZnSe QDs versus [Hg2+] ions and (c) the corresponding Stern-Volmer relationship**.**

$$\text{LOD} = \frac{3\,\text{S}\_0}{\text{K}\_w} \tag{1}$$

recombination centres [59]. However, mere Ksp values are not the sole factors in fluorescence quenching. In addition, surface ligands have profound effects on the fluorescence response of QDs to metal ions [60]. They play a critical role in metallic ion selectivity [61]. Also, the quenching of luminescence of the QDs can occur partly through ion binding followed by photoinduced electron transfer process from the thiol ligand to Hg2+ ions on the surface of QDs. The schematic representation of fluorescence quenching is shown in **Figure 7**. Theoretical calculation and further study are in progress to gain an insight into the mechanism of fluo-

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**Figure 7.** Schematic representation of fluorescence quenching of QDs by metal io**ns.**

In summary, water-soluble MPA-capped Mn-doped ZnSe QDs were synthesized using nucleation-doping method. The absorption spectra of the as-synthesized QDs were blue-shifted in relation to the bulk counterparts due to quantum confinement. The QDs exhibited dual-colour emission (blue and orange). The intensity ratio of the dual-colour emission can be varied by tuning the Mn-doping percentage. It was found that 3% Mn doping in the ZnSe crystal lattice increases the fluorescence (blue) of ZnSe QDs by 10-fold

> T1 →<sup>6</sup> A1

due to the reduction of non-radiative energy centres. Furthermore, Mn2+-related <sup>4</sup>

(orange) emission characteristic of Mn doped in the ZnSe host was observed. The as-prepared QDs exhibited a cubic crystal structure according to XRD patterns. TEM images showed aggregates of tiny spherical particles with the average diameter of ~4 nm. The as-prepared Mn3%:ZnSe QDs were treated with different metal ions and were found to be highly selective to Hg2+ ions and exhibit pleasing LOD. The possible mechanism of sensing by quenching was also proposed. These studies on metal ion sensing by fluorescent QDs have demonstrated their potential as chemical sensor which can be developed for

rescence quenching.

**4. Conclusions**

industrial applications.

where 3 is the signal-to-noise ratio, S0 is the standard deviation of blank measurements (n = 6) and Ksv is the slope of calibration graph. LOD of the present probe towards [Hg2+] under the experimental conditions is found to be 6.63 × 10−7 M L−1.

#### **3.5. Mechanism of sensing by quenching**

The fluorescence quenching can be explained in terms of strong binding of quencher metal ions (Hg2+) on the surface of QDs. It is known that the solubility HgSe (Ksp = 2 × 10−53) is extremely lower than that of ZnSe (Ksp= 3.6 × 10−26). The low solubility product is always preferred in a solution and hence the quencher metal ions (Hg2+) displace the Zn on the surface of QDs and form a lower solubility product (HgSe) which deposit on the surface of the QDs

$$\mathrm{Zn}\_{\mathrm{n}}\mathrm{Se}\_{\mathrm{n}} + \mathrm{xHg^{2+}} \rightarrow \mathrm{Zn}\_{\mathrm{n}\times\mathrm{e}}\mathrm{Hg}\_{\mathrm{n}}\mathrm{Se}\_{\mathrm{n}} + \mathrm{xZn^{2+}} \tag{2}$$

The formed particles of HgSe, both isolated and aggregated, can quench the luminescence of QDs by facilitating non-radiative annihilation of charge carriers, which act as electron-hole

**Figure 7.** Schematic representation of fluorescence quenching of QDs by metal io**ns.**

recombination centres [59]. However, mere Ksp values are not the sole factors in fluorescence quenching. In addition, surface ligands have profound effects on the fluorescence response of QDs to metal ions [60]. They play a critical role in metallic ion selectivity [61]. Also, the quenching of luminescence of the QDs can occur partly through ion binding followed by photoinduced electron transfer process from the thiol ligand to Hg2+ ions on the surface of QDs. The schematic representation of fluorescence quenching is shown in **Figure 7**. Theoretical calculation and further study are in progress to gain an insight into the mechanism of fluorescence quenching.

#### **4. Conclusions**

(1)

LOD <sup>=</sup> <sup>3</sup> <sup>S</sup> \_\_\_<sup>0</sup>

experimental conditions is found to be 6.63 × 10−7 M L−1.

where 3 is the signal-to-noise ratio, S0

212 Nonmagnetic and Magnetic Quantum Dots

**3.5. Mechanism of sensing by quenching**

Ksv

and Ksv is the slope of calibration graph. LOD of the present probe towards [Hg2+] under the

**Figure 6. (**a) Effect of metal ions (30 μM L−1) on the PL intensity of 3% Mn-doped ZnSe QDs at pH 10.8. (b) The plot of PL

intensity of 3% Mn-doped ZnSe QDs versus [Hg2+] ions and (c) the corresponding Stern-Volmer relationship**.**

The fluorescence quenching can be explained in terms of strong binding of quencher metal ions (Hg2+) on the surface of QDs. It is known that the solubility HgSe (Ksp = 2 × 10−53) is extremely lower than that of ZnSe (Ksp= 3.6 × 10−26). The low solubility product is always preferred in a solution and hence the quencher metal ions (Hg2+) displace the Zn on the surface of QDs and form a lower solubility product (HgSe) which deposit on the surface of the QDs

Zn<sup>m</sup> Se<sup>n</sup> + xHg2+ → Zn<sup>m</sup>‐<sup>x</sup> Hgx Se<sup>n</sup> + xZn2+ (2)

The formed particles of HgSe, both isolated and aggregated, can quench the luminescence of QDs by facilitating non-radiative annihilation of charge carriers, which act as electron-hole

is the standard deviation of blank measurements (n = 6)

In summary, water-soluble MPA-capped Mn-doped ZnSe QDs were synthesized using nucleation-doping method. The absorption spectra of the as-synthesized QDs were blue-shifted in relation to the bulk counterparts due to quantum confinement. The QDs exhibited dual-colour emission (blue and orange). The intensity ratio of the dual-colour emission can be varied by tuning the Mn-doping percentage. It was found that 3% Mn doping in the ZnSe crystal lattice increases the fluorescence (blue) of ZnSe QDs by 10-fold due to the reduction of non-radiative energy centres. Furthermore, Mn2+-related <sup>4</sup> T1 →<sup>6</sup> A1 (orange) emission characteristic of Mn doped in the ZnSe host was observed. The as-prepared QDs exhibited a cubic crystal structure according to XRD patterns. TEM images showed aggregates of tiny spherical particles with the average diameter of ~4 nm. The as-prepared Mn3%:ZnSe QDs were treated with different metal ions and were found to be highly selective to Hg2+ ions and exhibit pleasing LOD. The possible mechanism of sensing by quenching was also proposed. These studies on metal ion sensing by fluorescent QDs have demonstrated their potential as chemical sensor which can be developed for industrial applications.

## **Author details**

Sundararajan Parani1,2, Ncediwe Tsolekile1,2,3, Bambesiwe M.M. May1,2, Kannaiyan Pandian<sup>4</sup> and Oluwatobi S. Oluwafemi1,2\*

[7] Deng G. Principles of chemical and biological sensors. Materials and Manufacturing Processes [Internet]. 1999;**14**(4):623-625 Available from: http://www.tandfonline.com/

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215

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foodchem.2014.06.045

\*Address all correspondence to: oluwafemi.oluwatobi@gmail.com

1 Department of Applied Chemistry, University of Johannesburg, Doornfontein, Johannesburg, South Africa

2 Centre for Nanomaterials Science Research, University of Johannesburg, Johannesburg, South Africa

3 Department of Chemistry, Cape Peninsula University of Technology, Cape Town, South Africa

4 Department of Inorganic Chemistry, University of Madras, Chennai, India

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**Author details**

South Africa

**References**

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and Oluwatobi S. Oluwafemi1,2\*

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2 Centre for Nanomaterials Science Research, University of Johannesburg, Johannesburg,

3 Department of Chemistry, Cape Peninsula University of Technology, Cape Town, South

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**Chapter 13**

**Provisional chapter**

**Quantum Dots and Fluorescent and Magnetic**

**Nanocomposites: Recent Investigations and** 

**Quantum Dots and Fluorescent and Magnetic** 

DOI: 10.5772/intechopen.70614

This chapter presents a comprehensive and updated review on the ongoing research area of nanostructures with a focus on quantum dots (QDs), fluorescent and magnetic nanocomposites, and their applications in biological and medical field. The study includes the essential characteristics of QDs and fluorescent and magnetic nanocomposites, their structure, properties, and methods that are utilized for their characterization. Some interesting qualities of CdSe/ZnS QDs with reference to the research of the microorganism are emphasized. The bioimaging applications of QDs and fluorescent and magnetic nanocomposites and their role as nanoprobes and as contrast enhancing agents are discussed. So, in this work, an overview is exhibited including the case of the most commonly studied QD-based hybrid NPs, which are called MQDs, such as a dual "two-in-one" fluorescent-magnetic nanocomposite materials, that blend both fluorescent and magnetic properties in a unique concept and show the feasibility for clinical diagnostics, drug delivery, and therapy.

**Keywords:** quantum dots, magnetic quantum dots, nanocomposites, fluorescence property,

Fast progresses in nanotechnology and nanoscience have offered a diversity of nanoscale materials possessing very controlled and distinctive optical, electrical, magnetic, or catalytic properties. The diversity of the composition (organic or inorganic, semiconductors or metals), form (particles, rods, wires, cubes, or triangles), and the availability for the functionalization of the surface (physical, chemical, or biological) allowed the production of the different functional nanoscale tools [1, 2]. The scientists have expanded new types of nanoscale tools

magnetic property, microorganism labeling, biological imaging, drug delivery

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Nanocomposites: Recent Investigations and**

**Applications in Biology and Medicine**

**Applications in Biology and Medicine**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70614

Anca Armăşelu

**Abstract**

**1. Introduction**

Anca Armăşelu

**Provisional chapter**

## **Quantum Dots and Fluorescent and Magnetic Nanocomposites: Recent Investigations and Applications in Biology and Medicine Nanocomposites: Recent Investigations and Applications in Biology and Medicine**

**Quantum Dots and Fluorescent and Magnetic** 

DOI: 10.5772/intechopen.70614

Anca Armăşelu Anca Armăşelu Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70614

#### **Abstract**

This chapter presents a comprehensive and updated review on the ongoing research area of nanostructures with a focus on quantum dots (QDs), fluorescent and magnetic nanocomposites, and their applications in biological and medical field. The study includes the essential characteristics of QDs and fluorescent and magnetic nanocomposites, their structure, properties, and methods that are utilized for their characterization. Some interesting qualities of CdSe/ZnS QDs with reference to the research of the microorganism are emphasized. The bioimaging applications of QDs and fluorescent and magnetic nanocomposites and their role as nanoprobes and as contrast enhancing agents are discussed. So, in this work, an overview is exhibited including the case of the most commonly studied QD-based hybrid NPs, which are called MQDs, such as a dual "two-in-one" fluorescent-magnetic nanocomposite materials, that blend both fluorescent and magnetic properties in a unique concept and show the feasibility for clinical diagnostics, drug delivery, and therapy.

**Keywords:** quantum dots, magnetic quantum dots, nanocomposites, fluorescence property, magnetic property, microorganism labeling, biological imaging, drug delivery

## **1. Introduction**

Fast progresses in nanotechnology and nanoscience have offered a diversity of nanoscale materials possessing very controlled and distinctive optical, electrical, magnetic, or catalytic properties. The diversity of the composition (organic or inorganic, semiconductors or metals), form (particles, rods, wires, cubes, or triangles), and the availability for the functionalization of the surface (physical, chemical, or biological) allowed the production of the different functional nanoscale tools [1, 2]. The scientists have expanded new types of nanoscale tools

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

that could be utilized for forensic science, biology, medicine, electronic technology, environmental science, computer fabrication, and food industries. The researchers in biological and medical fields already used these nanodevices in an assortment of uses varying from the diagnosis of disease to gene therapies. The combination of biomaterials (proteins, peptides, nucleic acids with semiconductor quantum dots (QDs), and metal nanoparticles is expected to generate important advances in molecular biology, bioengineering, medical, and therapeutic diagnostics.

and against photo- and chemical degradation [19, 20]. QDs have great potential in many applications such as solar cells, light-emitting devices, and photobio-labeling technologies. The unique optical properties of QDs make them appealing *in vivo* and *in vitro* in different biological and clinical applications, such as cell labeling [17, 20], cell tracking, *in vivo* imaging,

Quantum Dots and Fluorescent and Magnetic Nanocomposites: Recent Investigations...

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223

The combination of dual-natured parts of optical and magnetic properties on nanometer system can bring new advances in molecular imaging and medical theranosis, which are fundamental for early detection and rapid disease treatment. QDs represent an exciting and versatile category of fluorophores with a bright future, thus increasing the interest in blending the advantages of QDs with those of other materials to obtain composites with multifunc-

Nanocomposites, which comprise fluorescent and magnetic particles, represent a basis for multiplexed nanoprobe designs. The area of nanocomposite applications is still in a state of development [24]. The magnetic materials constitute one of the most frequent materials, which when amalgamated with QDs form a captivating class of new materials for bioimaging. In this sense, QD biosensing can be further improved by combination with MNPs (e.g., superparamagnetic iron oxide nanoparticles, SPIONs) or ions (e.g., gadolinium). The fluorescent property of the QDs allows visualization, while the magnetic property of the composite allows imaging, magnetic separation and can bring therapeutic advantages [23]. In this paper, actual investigations using only QDs or MNPs will be reviewed in situations where the appli-

This review examines the properties of QDs and magnetic QDs (MQDs) comprising the applications of these materials. Because the properties of these materials continue to enhance, QDs and MQDs possess the capacity to considerably determine biological imaging, diagnosis, and treatment. The application of QDs for combined targeting and delivery of diagnostic and therapeutic agents can be further developed by a combination with magnetic separation techniques via the recent evolution of MQDs. In this chapter, among many benefits of MQDsbased separation to current procedures, one should also mention the small dimension of MQDs, which are small enough to possibly interact with single-cell biomarkers/cell surface receptors which results in corresponding quantification of the results [22, 25]. These multifunctional fluorescent and magnetic nanoparticles of small dimension, which are MQDs, can target any biomolecule and can be separately controlled by engineering magnetic fields. Some authors reported very useful research papers which refer to a combined result of both types of fluorescent and magnetic properties to approach important biological issues [24, 26, 27]. This work provides a survey of the different application of biosensing technologies

In the last decades, NPs were produced as interesting materials, with outstanding results for many applications. As a type of NPs, QDs represent the excellent competitors for optical

and DNA detection [19, 21].

tional properties [22, 23].

cations can be expanded to nanocomposites.

which are based on QDs and their MQD correspondents.

**2. Properties of quantum dots**

The current progresses comprise the evolution of the functional nanoparticles (electronic, optical, and magnetic) which are conjugated to biological molecules such as peptides, proteins, and nucleic acids. Today, the magnetic nanoparticles (MNPs) are considered to be primary components in therapies and screening methods that are gradually included in many areas of medical practice. Due to the dimension-dependent properties and dimensional similitudes with biomolecules, the magnetic nanoparticles and their bioconjugates are highly appropriate for intracellular tagging [3, 4] and for image contrast-improving agents in magnetic resonance imaging (MRI) [5–8], magnetic separation [7, 9], targeted drug delivery [7, 8, 10], and for usage in hyperthermia [7, 10–12].

QDs are a unique class of fluorescent nanoparticles that are crystalline semiconductors of variable sizes (1–100 nm) and consist of only a few hundred to a few thousand atoms, in spite of the fact that QDs exhibit the same crystal structure as the bulk semiconductor material. The highest-quality QDs are typically composed of atoms from groups II and VI or groups III and V or groups IV and VI of the periodic table [1, 13, 14].

These nanoparticles, compared to their bulk, have smaller exciton Bohr radius which characterizes their definition [15]. This thing establishes what is described as quantum confinement when distinctive optical and electronic properties are created [15]. The nanometric dimension of QDs determines the quantum-confinement effect, which results in unique optical and electronic properties. Due to the effects of quantum confinement, QDs possess distinct photophysical properties that give QDs tremendous advantages over the conventional organic fluorophores [16–18]. Traditional organic dyes exhibit chemical and photophysical limitations such as pH dependence, susceptibility to photo-bleaching, narrow absorption windows of wavelengths, asymmetric emission spectra broadened by a red tail, small Stokes shifts, and short excited state fluorescent lifetimes [17–19].

The researchers in the field have found many production techniques for QDs, from photolithography to wet chemical synthesis. The most utilized QD construction consists of two materials from group II–VI materials, namely a CdSe core with a thin, protective shell of ZnS. Colloidal QDs are fabricated utilizing surfactant micelles, coprecipitation or organic solvent synthesis at high temperature. The last technique is used to manufacture the highestquality materials [1, 14].

In contrast with the organic dyes and fluorescent proteins, QDs have distinct electronic and optical properties that comprise high quantum yield, broad absorption, large effective Stokes shifts, the ability to size-tune fluorescent emission as a function of core dimension, simultaneous excitation of multiple fluorescence colors, and high resistance against photo-bleaching and against photo- and chemical degradation [19, 20]. QDs have great potential in many applications such as solar cells, light-emitting devices, and photobio-labeling technologies. The unique optical properties of QDs make them appealing *in vivo* and *in vitro* in different biological and clinical applications, such as cell labeling [17, 20], cell tracking, *in vivo* imaging, and DNA detection [19, 21].

The combination of dual-natured parts of optical and magnetic properties on nanometer system can bring new advances in molecular imaging and medical theranosis, which are fundamental for early detection and rapid disease treatment. QDs represent an exciting and versatile category of fluorophores with a bright future, thus increasing the interest in blending the advantages of QDs with those of other materials to obtain composites with multifunctional properties [22, 23].

Nanocomposites, which comprise fluorescent and magnetic particles, represent a basis for multiplexed nanoprobe designs. The area of nanocomposite applications is still in a state of development [24]. The magnetic materials constitute one of the most frequent materials, which when amalgamated with QDs form a captivating class of new materials for bioimaging. In this sense, QD biosensing can be further improved by combination with MNPs (e.g., superparamagnetic iron oxide nanoparticles, SPIONs) or ions (e.g., gadolinium). The fluorescent property of the QDs allows visualization, while the magnetic property of the composite allows imaging, magnetic separation and can bring therapeutic advantages [23]. In this paper, actual investigations using only QDs or MNPs will be reviewed in situations where the applications can be expanded to nanocomposites.

This review examines the properties of QDs and magnetic QDs (MQDs) comprising the applications of these materials. Because the properties of these materials continue to enhance, QDs and MQDs possess the capacity to considerably determine biological imaging, diagnosis, and treatment. The application of QDs for combined targeting and delivery of diagnostic and therapeutic agents can be further developed by a combination with magnetic separation techniques via the recent evolution of MQDs. In this chapter, among many benefits of MQDsbased separation to current procedures, one should also mention the small dimension of MQDs, which are small enough to possibly interact with single-cell biomarkers/cell surface receptors which results in corresponding quantification of the results [22, 25]. These multifunctional fluorescent and magnetic nanoparticles of small dimension, which are MQDs, can target any biomolecule and can be separately controlled by engineering magnetic fields. Some authors reported very useful research papers which refer to a combined result of both types of fluorescent and magnetic properties to approach important biological issues [24, 26, 27]. This work provides a survey of the different application of biosensing technologies which are based on QDs and their MQD correspondents.

## **2. Properties of quantum dots**

that could be utilized for forensic science, biology, medicine, electronic technology, environmental science, computer fabrication, and food industries. The researchers in biological and medical fields already used these nanodevices in an assortment of uses varying from the diagnosis of disease to gene therapies. The combination of biomaterials (proteins, peptides, nucleic acids with semiconductor quantum dots (QDs), and metal nanoparticles is expected to generate important advances in molecular biology, bioengineering, medical, and thera-

The current progresses comprise the evolution of the functional nanoparticles (electronic, optical, and magnetic) which are conjugated to biological molecules such as peptides, proteins, and nucleic acids. Today, the magnetic nanoparticles (MNPs) are considered to be primary components in therapies and screening methods that are gradually included in many areas of medical practice. Due to the dimension-dependent properties and dimensional similitudes with biomolecules, the magnetic nanoparticles and their bioconjugates are highly appropriate for intracellular tagging [3, 4] and for image contrast-improving agents in magnetic resonance imaging (MRI) [5–8], magnetic separation [7, 9], targeted drug delivery [7, 8, 10], and for

QDs are a unique class of fluorescent nanoparticles that are crystalline semiconductors of variable sizes (1–100 nm) and consist of only a few hundred to a few thousand atoms, in spite of the fact that QDs exhibit the same crystal structure as the bulk semiconductor material. The highest-quality QDs are typically composed of atoms from groups II and VI or groups III and

These nanoparticles, compared to their bulk, have smaller exciton Bohr radius which characterizes their definition [15]. This thing establishes what is described as quantum confinement when distinctive optical and electronic properties are created [15]. The nanometric dimension of QDs determines the quantum-confinement effect, which results in unique optical and electronic properties. Due to the effects of quantum confinement, QDs possess distinct photophysical properties that give QDs tremendous advantages over the conventional organic fluorophores [16–18]. Traditional organic dyes exhibit chemical and photophysical limitations such as pH dependence, susceptibility to photo-bleaching, narrow absorption windows of wavelengths, asymmetric emission spectra broadened by a red tail, small Stokes shifts, and

The researchers in the field have found many production techniques for QDs, from photolithography to wet chemical synthesis. The most utilized QD construction consists of two materials from group II–VI materials, namely a CdSe core with a thin, protective shell of ZnS. Colloidal QDs are fabricated utilizing surfactant micelles, coprecipitation or organic solvent synthesis at high temperature. The last technique is used to manufacture the highest-

In contrast with the organic dyes and fluorescent proteins, QDs have distinct electronic and optical properties that comprise high quantum yield, broad absorption, large effective Stokes shifts, the ability to size-tune fluorescent emission as a function of core dimension, simultaneous excitation of multiple fluorescence colors, and high resistance against photo-bleaching

peutic diagnostics.

222 Nonmagnetic and Magnetic Quantum Dots

usage in hyperthermia [7, 10–12].

V or groups IV and VI of the periodic table [1, 13, 14].

short excited state fluorescent lifetimes [17–19].

quality materials [1, 14].

In the last decades, NPs were produced as interesting materials, with outstanding results for many applications. As a type of NPs, QDs represent the excellent competitors for optical bioanalysis by virtue of their electronic and optical properties, which can be adjusted by modifying the dimension, morphology, and composition of NPs [18, 28]. A new biological labeling material, QDs are considered that they can bring several important advantages in their applications in comparison with the organic fluorophores [29]. In this paper, these tremendous advantages are considered that determine in special, the fluorescent label comportment and hence the utilization in different cases, which comprise the spectral position, the width of the excitation spectrum, the width of the excitation spectrum, the Stokes shift, the molar absorption coefficient, the fluorescence quantum yield, the photo stability, and the decay lifetime.

to the overlapping of the spectra [41]. QDs exhibit high resistance to physical and chemical degradation (suitable for long-term imaging) high quantum yields and high molar extinction coefficients which are 10–50 times greater than that of organic dyes, which make them much

Quantum Dots and Fluorescent and Magnetic Nanocomposites: Recent Investigations...

http://dx.doi.org/10.5772/intechopen.70614

QDs present photoluminescence (PL), when photons are utilized to excite QDs and another photon of lower frequency is released [42]. QDs are famous for eye-catching photos of differently dimensioned QDs under ultraviolet lighting which exhibit a shining rainbow of photoluminescence [33]. This brilliant PL is obtained like a consequence of high quantum

[32, 33, 42]. The values of the molar extinction coefficients of QDs are 10–100 times greater than those for most organic fluorophores [41]. In contrast with the organic dyes, another beneficial characteristic of QDs is represented by the very large two-photon action cross section [43].

The majority of QD applications in biology utilize this feature for cellular/molecular tracking and imaging [42]. If a photon excites a QD, but the energy is collected as electricity, QD is utilized as photovoltaic material and represents a good possible choice for the case of the current applications in solar cells [41, 42]. In addition, QD blends the conveniences of inorganic and organic materials. In many QD-based solar cells, QDs do not only help like a light-collecting material but also have numerous purposes in order to assist in load separation and transportation [44]. QDs have also been extensively utilized in solar cells for sensitization. Quantum dot-sensitized solar cells (QDSSCs) have lately captivated a lot of interest due to their benefits over the dye-sensitized solar cells (DSSCs), comprising higher molar extinction coefficient of QDs, tunable energy gaps, and multiple exciton generation [45, 46]. It is also worth mentioning the case in which the higher voltage electrical energy can also segregate electrons from holes to form excitons, and when energy is released in the form of light, QDs are utilized in a

Compared to organic fluorophores, QDs are about 10–100 times brighter and about 100–1000 times more stable against photo-bleaching [2, 47, 48]. Due to these properties, QDs are excel-

The fluorescence time is defined like the average time in which a fluorophore will stay in its excited state before it emits light to return to its ground state [43, 49]. The usual values of the fluorescence lifetimes are from 1 to 10 ns for organic dyes and 10 to 100 ns for QDs [43, 49]. An important property that is used for the diminution of the auto-fluorescence of the biological samples is the possibility to choose any wavelength shorter than the wavelength of fluorescence. This quality of QDs can be obtained by electing the most suitable excitation wavelength for which the auto-fluorescence is reduced to a minimum. Previous studies show that the pushing of the emission wavelength into the near-infrared (650–950-nm) range led to the enhancement of the tissue penetration depth and the decrease of the fluorescence at these wavelengths [14, 50].

QDs have shown other many remarkable advantages compared to traditional fluorophores, such as organic dyes, fluorescent proteins, and lanthanide chelates [15, 32]. One of the most significant properties of QD is the red shift of the emission spectra, called the QD Stokes shift. QD Stokes shift, which can be as large as 300–400 nm, depending on the wavelength of the

–107 M−1 cm−1)

225

yields (ϕ = 0.1–0.9) combined with substantial molar extinction coefficients (10<sup>5</sup>

brighter in photon-limited *in vivo* conditions [14].

new light-emitting diode variation, the QD-LEDs [30, 42].

lent for single molecule or, more accurately, particle measurement [33].

The most valuable feature influencing the optical properties is the dimension of the QDs. QDs of varying dimensions change the color emitted or absorbed by the crystal thanks to the energy levels of the crystal [30]. QDs present an electronic structure analogous to atoms, due to the tight confinement of charge carriers in them [30, 31]. So, the discrete size-dependent energy levels of QDs represent the effect of the confinement of the charge carriers (electrons, holes) in three dimensions [18, 20, 31, 32]. As a result, the energy difference between excited and ground state (the bandgap energy) of a QD is a function of the QD size and composition: the smaller the bandgap of QD, the larger the QD [18, 20, 33, 34]. This means that the fluorescence wavelength is a function of the bandgap and therefore a function of the QD size [18, 20, 31]. By modifying the dimension, coating, and composition of the QDs, the emission wavelength can be adjusted from the ultraviolet (UV) to the infrared range of the spectrum such that smaller dots emit higher-energy light that is in the blue range and the larger dots emit lower-energy light that is in the red and near-infrared (NIR) region [15]. Because the dimension of QD is inversely proportional to the bandgap energy level, the frequency light emitted changes and an effect on the color occurs [30, 35].

Many of the conventional fluorophores (organic dyes and protein- based fluorophores) exhibit narrow excitation spectra which necessitate excitation by light of a particular wavelength, which fluctuate between certain fluorophores and present broad red-tailed emission spectra which suggest that the spectra of various conventional fluorophores may overlay to a large extent [20, 32]. Three important properties of QDs are considered to be of interest to specialists in biology, namely the capability of QDs to size-tune the fluorescent emission depending on core size, the broad excitation spectra of QDs, which permit excitation of mixed QDs at a single wavelength [36] and the long luminescent life of QDs, which allows their usage for dynamic imaging of living cells [37].

The narrow emission spectra of QDs permit the multicolor excitation involving the potentiality for simultaneous usage of various functionalized QDs for a number of biological targets at the same time. This fact is suited for the usage of QDs in multiplex immunohistochemistry tests [38–40].

QDs are defined by broadband excitation wavelength, very bright fluorescence even when irradiated only with a light-emitting diode (LED) flashlight [2, 17]. These characteristics of QDs, which have been mentioned earlier, allow simultaneous imaging of many entities in a unique biological experiment. This fact represents a difficult mission with common fluorophores since their relatively narrow excitation and broad emission spectra many times lead to the overlapping of the spectra [41]. QDs exhibit high resistance to physical and chemical degradation (suitable for long-term imaging) high quantum yields and high molar extinction coefficients which are 10–50 times greater than that of organic dyes, which make them much brighter in photon-limited *in vivo* conditions [14].

bioanalysis by virtue of their electronic and optical properties, which can be adjusted by modifying the dimension, morphology, and composition of NPs [18, 28]. A new biological labeling material, QDs are considered that they can bring several important advantages in their applications in comparison with the organic fluorophores [29]. In this paper, these tremendous advantages are considered that determine in special, the fluorescent label comportment and hence the utilization in different cases, which comprise the spectral position, the width of the excitation spectrum, the width of the excitation spectrum, the Stokes shift, the molar absorption coefficient, the fluorescence quantum yield, the photo

The most valuable feature influencing the optical properties is the dimension of the QDs. QDs of varying dimensions change the color emitted or absorbed by the crystal thanks to the energy levels of the crystal [30]. QDs present an electronic structure analogous to atoms, due to the tight confinement of charge carriers in them [30, 31]. So, the discrete size-dependent energy levels of QDs represent the effect of the confinement of the charge carriers (electrons, holes) in three dimensions [18, 20, 31, 32]. As a result, the energy difference between excited and ground state (the bandgap energy) of a QD is a function of the QD size and composition: the smaller the bandgap of QD, the larger the QD [18, 20, 33, 34]. This means that the fluorescence wavelength is a function of the bandgap and therefore a function of the QD size [18, 20, 31]. By modifying the dimension, coating, and composition of the QDs, the emission wavelength can be adjusted from the ultraviolet (UV) to the infrared range of the spectrum such that smaller dots emit higher-energy light that is in the blue range and the larger dots emit lower-energy light that is in the red and near-infrared (NIR) region [15]. Because the dimension of QD is inversely proportional to the bandgap energy level, the frequency light

Many of the conventional fluorophores (organic dyes and protein- based fluorophores) exhibit narrow excitation spectra which necessitate excitation by light of a particular wavelength, which fluctuate between certain fluorophores and present broad red-tailed emission spectra which suggest that the spectra of various conventional fluorophores may overlay to a large extent [20, 32]. Three important properties of QDs are considered to be of interest to specialists in biology, namely the capability of QDs to size-tune the fluorescent emission depending on core size, the broad excitation spectra of QDs, which permit excitation of mixed QDs at a single wavelength [36] and the long luminescent life of QDs, which allows their usage for

The narrow emission spectra of QDs permit the multicolor excitation involving the potentiality for simultaneous usage of various functionalized QDs for a number of biological targets at the same time. This fact is suited for the usage of QDs in multiplex immunohistochemistry

QDs are defined by broadband excitation wavelength, very bright fluorescence even when irradiated only with a light-emitting diode (LED) flashlight [2, 17]. These characteristics of QDs, which have been mentioned earlier, allow simultaneous imaging of many entities in a unique biological experiment. This fact represents a difficult mission with common fluorophores since their relatively narrow excitation and broad emission spectra many times lead

stability, and the decay lifetime.

224 Nonmagnetic and Magnetic Quantum Dots

emitted changes and an effect on the color occurs [30, 35].

dynamic imaging of living cells [37].

tests [38–40].

QDs present photoluminescence (PL), when photons are utilized to excite QDs and another photon of lower frequency is released [42]. QDs are famous for eye-catching photos of differently dimensioned QDs under ultraviolet lighting which exhibit a shining rainbow of photoluminescence [33]. This brilliant PL is obtained like a consequence of high quantum yields (ϕ = 0.1–0.9) combined with substantial molar extinction coefficients (10<sup>5</sup> –107 M−1 cm−1) [32, 33, 42]. The values of the molar extinction coefficients of QDs are 10–100 times greater than those for most organic fluorophores [41]. In contrast with the organic dyes, another beneficial characteristic of QDs is represented by the very large two-photon action cross section [43].

The majority of QD applications in biology utilize this feature for cellular/molecular tracking and imaging [42]. If a photon excites a QD, but the energy is collected as electricity, QD is utilized as photovoltaic material and represents a good possible choice for the case of the current applications in solar cells [41, 42]. In addition, QD blends the conveniences of inorganic and organic materials. In many QD-based solar cells, QDs do not only help like a light-collecting material but also have numerous purposes in order to assist in load separation and transportation [44]. QDs have also been extensively utilized in solar cells for sensitization. Quantum dot-sensitized solar cells (QDSSCs) have lately captivated a lot of interest due to their benefits over the dye-sensitized solar cells (DSSCs), comprising higher molar extinction coefficient of QDs, tunable energy gaps, and multiple exciton generation [45, 46]. It is also worth mentioning the case in which the higher voltage electrical energy can also segregate electrons from holes to form excitons, and when energy is released in the form of light, QDs are utilized in a new light-emitting diode variation, the QD-LEDs [30, 42].

Compared to organic fluorophores, QDs are about 10–100 times brighter and about 100–1000 times more stable against photo-bleaching [2, 47, 48]. Due to these properties, QDs are excellent for single molecule or, more accurately, particle measurement [33].

The fluorescence time is defined like the average time in which a fluorophore will stay in its excited state before it emits light to return to its ground state [43, 49]. The usual values of the fluorescence lifetimes are from 1 to 10 ns for organic dyes and 10 to 100 ns for QDs [43, 49]. An important property that is used for the diminution of the auto-fluorescence of the biological samples is the possibility to choose any wavelength shorter than the wavelength of fluorescence. This quality of QDs can be obtained by electing the most suitable excitation wavelength for which the auto-fluorescence is reduced to a minimum. Previous studies show that the pushing of the emission wavelength into the near-infrared (650–950-nm) range led to the enhancement of the tissue penetration depth and the decrease of the fluorescence at these wavelengths [14, 50].

QDs have shown other many remarkable advantages compared to traditional fluorophores, such as organic dyes, fluorescent proteins, and lanthanide chelates [15, 32]. One of the most significant properties of QD is the red shift of the emission spectra, called the QD Stokes shift. QD Stokes shift, which can be as large as 300–400 nm, depending on the wavelength of the excitation light, can be utilized for *in vivo* imaging [14, 36, 50]. The high Stokes shift values diminish the phenomenon of auto-fluorescence that enlarges the sensitivity [41].

Considering these advantageous optical properties shown earlier, it is not surprising that QDs have been and are still studied as some alternative fluorophores to conventional organic dyes.

#### **2.1. Quantum dots as fluorescent probes for the bioimaging applications**

These benefits of QDs are used in an emerging area of science and technology which blends the biological chemical and engineering sciences and guarantees the achievement of nanometric scale methods in order to study the biological systems in the field of health and disease [51]. The best available QDs for biological applications comprise a semiconductor core (e.g., CdSe, CdS, and CdTe) overcoated with a shell of a semiconductor material with a wider bandgap than the material of the core (e.g., ZnS, CdS) in order to obtain a considerable enhancement in the quantum efficiency [17, 18, 33, 52]. The leader QD material, which is used in almost all biological applications, is certainly represented by CdSe/ZnS core-shell, whose celebrity is imputed to well-determined synthetic protocols, emissions that can be scattered in the visible/ NIR region and commercial accessibility [33, 53].

In [17, 20, 54], it has also been shown that CdSe/ZnS core-shell QDs represent an excellent substitute to fluorescent fluorophores utilized to label the microbial cells. In this context, the fluorescence of the CdSe/ZnS core-shell QDs, dispersed in toluene with long-chain aminecapping agents, was studied [17, 20, 54, 55]. The different semiconductor nanocrystals, which were utilized in various research works [17, 20, 54, 55], were purchased from EVIDENT Technologies. The sizes of these QDs are in the field of (3–5) nm and their emission is situated in the domain (490–600) nm. The emission properties of diverse QDs were examined and estimated utilizing Fourier transform visible spectroscopy [17, 18, 54, 55] for two excitation sources (a UV laser or a blue LED). **Figure 1** presents the case of the fluorescence of CdSe/ZnS core-shell QDs with the dimensions of 3.2, 3.8, and 5.0 nm [17].

transmission in white light or by epifluorescence, with the help of the blue and green filters [17, 54]. **Figures 2** and **3** show the easy visualization of the individual cyanobacterial cells due to the green fluorescence of QDs. These two figures present the case of the individual filamentous cells in two types of natural samples indicated in the visible microscopy (A), the case of microscopic aspect of the QD-labeled cyanobacterial cells for the blue filter (B) and the case of

Yellow type) and 0600 QDs fluid (with the crystal diameter of 5.0 nm and the color of Fort Orange type) [17].

**Figure 1.** The fluorescence of CdSe/ZnS core-shell, suspended in toluene with long-chain amine-capping agent, under ultraviolet illumination. The various colors are correlated with the various wavelengths of the fluorescence maxim according to the sizes of the QD in the suspension. From left to right, according to the specifications of the EVIDENT Technologies catalog three kinds of CdSe/ZnS core-shell QDs were used: 0490 QDs fluid (with the crystal diameter of 3.2 nm and the color of Lake Placid Blue type), 0520 QDs fluid (with the crystal diameter of 3.8 nm and the color of Hops

Quantum Dots and Fluorescent and Magnetic Nanocomposites: Recent Investigations...

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227

Another research paper examined the nonspecific labeling of cyanobacteria in natural samples and enriched cultures with CdSe/ZnS core-shell and the impact of CdSe/ZnS core-shell QDs on the global color of epifluorescence microscopy images [20]. The same paper [20] exhibited the use of the digital color analysis method for the study of the epifluorescence microscopy images, demonstrating the color transformation of the epifluorescence images of filamentous cyanobacteria and showing in this way the potential toxic effects of QDs on cyanobacteria.

**Figure 2.** Microscopic aspect of the labeling of the cyanobacterial cells with QDs in the case of the sample 1: A: Transmission in white light; B: Epifluorescence utilizing a blue filter; C: Epifluorescence utilizing a green filter [54].

the microscopic aspect of the same samples for the green filter (C) [17, 54].

In this section of the chapter, some recent important bioimaging applications of QDs, which have to do with the study of the microorganisms and toxin detection, are reviewed.

QDs have proven to be convenient in the morphological examination of microorganisms in order to show their shape, position, evolution, number, and so on [17, 20, 54, 56]. The QD-based technologies, which are used for the operations of labeling with QDs, are very relevant in biotechnology medical diagnosis and food safety [17, 20, 54, 56].

Numerous authors described successfully linked QDs to biorecognition molecules such as peptide, antibodies, nucleic acids, or small-molecule ligands for further applications as fluorescent nanoprobes [57, 58], whereas few researchers have reported results obtained in using CdSe/ZnS core-shell QDs in the field of the microbial labeling, for both pure cultures of cyanobacteria (Synechocystis PCC 6803) and mixed cultures of phototrophic and heterotrophic microorganisms [17, 20, 54, 59, 60]. In these last works, the labeling of the biological samples, comprising the cultures of the microorganisms with QDs of 0520 Evidot suspension type, which were incubated in darkness at room temperature, was described. The natural samples including filamentous cyanobacterial cells were microscopically studied (B-352 LD2) by

excitation light, can be utilized for *in vivo* imaging [14, 36, 50]. The high Stokes shift values

Considering these advantageous optical properties shown earlier, it is not surprising that QDs have been and are still studied as some alternative fluorophores to conventional organic dyes.

These benefits of QDs are used in an emerging area of science and technology which blends the biological chemical and engineering sciences and guarantees the achievement of nanometric scale methods in order to study the biological systems in the field of health and disease [51]. The best available QDs for biological applications comprise a semiconductor core (e.g., CdSe, CdS, and CdTe) overcoated with a shell of a semiconductor material with a wider bandgap than the material of the core (e.g., ZnS, CdS) in order to obtain a considerable enhancement in the quantum efficiency [17, 18, 33, 52]. The leader QD material, which is used in almost all biological applications, is certainly represented by CdSe/ZnS core-shell, whose celebrity is imputed to well-determined synthetic protocols, emissions that can be scattered in the visible/

In [17, 20, 54], it has also been shown that CdSe/ZnS core-shell QDs represent an excellent substitute to fluorescent fluorophores utilized to label the microbial cells. In this context, the fluorescence of the CdSe/ZnS core-shell QDs, dispersed in toluene with long-chain aminecapping agents, was studied [17, 20, 54, 55]. The different semiconductor nanocrystals, which were utilized in various research works [17, 20, 54, 55], were purchased from EVIDENT Technologies. The sizes of these QDs are in the field of (3–5) nm and their emission is situated in the domain (490–600) nm. The emission properties of diverse QDs were examined and estimated utilizing Fourier transform visible spectroscopy [17, 18, 54, 55] for two excitation sources (a UV laser or a blue LED). **Figure 1** presents the case of the fluorescence of CdSe/ZnS

In this section of the chapter, some recent important bioimaging applications of QDs, which

QDs have proven to be convenient in the morphological examination of microorganisms in order to show their shape, position, evolution, number, and so on [17, 20, 54, 56]. The QD-based technologies, which are used for the operations of labeling with QDs, are very rel-

Numerous authors described successfully linked QDs to biorecognition molecules such as peptide, antibodies, nucleic acids, or small-molecule ligands for further applications as fluorescent nanoprobes [57, 58], whereas few researchers have reported results obtained in using CdSe/ZnS core-shell QDs in the field of the microbial labeling, for both pure cultures of cyanobacteria (Synechocystis PCC 6803) and mixed cultures of phototrophic and heterotrophic microorganisms [17, 20, 54, 59, 60]. In these last works, the labeling of the biological samples, comprising the cultures of the microorganisms with QDs of 0520 Evidot suspension type, which were incubated in darkness at room temperature, was described. The natural samples including filamentous cyanobacterial cells were microscopically studied (B-352 LD2) by

have to do with the study of the microorganisms and toxin detection, are reviewed.

evant in biotechnology medical diagnosis and food safety [17, 20, 54, 56].

diminish the phenomenon of auto-fluorescence that enlarges the sensitivity [41].

**2.1. Quantum dots as fluorescent probes for the bioimaging applications**

NIR region and commercial accessibility [33, 53].

226 Nonmagnetic and Magnetic Quantum Dots

core-shell QDs with the dimensions of 3.2, 3.8, and 5.0 nm [17].

**Figure 1.** The fluorescence of CdSe/ZnS core-shell, suspended in toluene with long-chain amine-capping agent, under ultraviolet illumination. The various colors are correlated with the various wavelengths of the fluorescence maxim according to the sizes of the QD in the suspension. From left to right, according to the specifications of the EVIDENT Technologies catalog three kinds of CdSe/ZnS core-shell QDs were used: 0490 QDs fluid (with the crystal diameter of 3.2 nm and the color of Lake Placid Blue type), 0520 QDs fluid (with the crystal diameter of 3.8 nm and the color of Hops Yellow type) and 0600 QDs fluid (with the crystal diameter of 5.0 nm and the color of Fort Orange type) [17].

transmission in white light or by epifluorescence, with the help of the blue and green filters [17, 54]. **Figures 2** and **3** show the easy visualization of the individual cyanobacterial cells due to the green fluorescence of QDs. These two figures present the case of the individual filamentous cells in two types of natural samples indicated in the visible microscopy (A), the case of microscopic aspect of the QD-labeled cyanobacterial cells for the blue filter (B) and the case of the microscopic aspect of the same samples for the green filter (C) [17, 54].

Another research paper examined the nonspecific labeling of cyanobacteria in natural samples and enriched cultures with CdSe/ZnS core-shell and the impact of CdSe/ZnS core-shell QDs on the global color of epifluorescence microscopy images [20]. The same paper [20] exhibited the use of the digital color analysis method for the study of the epifluorescence microscopy images, demonstrating the color transformation of the epifluorescence images of filamentous cyanobacteria and showing in this way the potential toxic effects of QDs on cyanobacteria.

**Figure 2.** Microscopic aspect of the labeling of the cyanobacterial cells with QDs in the case of the sample 1: A: Transmission in white light; B: Epifluorescence utilizing a blue filter; C: Epifluorescence utilizing a green filter [54].

magnetic nanocomposites can be used as instruments in the nanomedicine realm. Thus, fluorescent magnetic nanocomposites are utilized to visualize and synchronously treat diverse

Quantum Dots and Fluorescent and Magnetic Nanocomposites: Recent Investigations...

Koole et al. described four diverse techniques of obtaining a single nanoparticle, which comprises the fluorescence and magnetic properties and represents new sensitive bimodal contrast instrument for two extremely powerful and highly complementary imaging methods:

In some research papers [21, 53], new approaches of the therapeutic procedures and imaging modalities (such as the technique of the correlation between MRI and ultrasensitive optical imaging) are exhibited in order to be developed and to become mainstream clinical methods for the visual detection of the microscopic tumors during an operation and the complete elimination of the diseased cells and tissues. The authors of these research works show that medical imaging methods can detect the illness, but do not furnish a visual template during a certain surgery. This matter can be resolved with the help of some MQD probes. MQDs are a form of magnetic contrast agents in MRI. In this regard, paramagnetic and superpara-

nanoparticle form are attached to QDs to utilize in a variety of MRI applications with the

Many researchers have investigated paramagnetic QDs (pQDs) [62–65]. So, in Refs. [62, 64], a model of multifunctional fluorescent magnetic nanocomposites, comprising silica-coated

Ahmed et al. [66] developed a novel technique for the manufacture of QDs enclosed MNPs based on layer-by-layer (LbL) self-assembly method in order to be used for cancer cells imaging. In a research study, Park's group [67] reported long-circulating, micellar hybrid nanoparticles (MHNs) which include MNs, QDs, and the anticancer drug doxorubicin (DOX) in a single poly(ethylene glycol) (PEG)-phospholipid micelle and furnish the first models of concomitant targeted drug delivery and dual-mode near-infrared fluorescence imaging and MRI

A very useful review study, which includes the mention of the specialized literature on the applications of MQDs, which represent one of the most currently explored QD-based hybrid NPs, was realized by Wegner and his colleague [68]. In this study, a principal result is indicated, which was obtained by Qiu et al. [69]. These researchers blended QDs, superparamagnetic iron oxide nanoparticles (SPIONs), and gold (Au) NPs in a single poly(lactic-co-glycolic acid) (PLGA) NP. This type of particle is significant for imaging, tracking, and manipulating

neutrophils and is used for *in vivo* applications and localized photothermal treatment.

 and TGA-capped CdTe QDs, was communicated. This type of nanocomposite has been used for the labeling and imaging of HeLa cells in a magnetic separation. In Refs. [63, 65], pQDs were improved by coating CdSe/ZnS core-shell QDs with a PEGylated phospholipid and a Gd lipid, making the particles biocompatible and MRI active. The pQDs were also conjugated by maleimide to cyclic RGD peptides for targeting angiogenic vascular endothelium as proved by *in vitro* investigations with human umbilical vein endothelial cells

O3

) in molecular form and in

http://dx.doi.org/10.5772/intechopen.70614

229

diseases [61].

Fe3 O4

(HUVECs).

fluorescence imaging and MRI [27].

magnetic agents Gd(III) and different forms of iron oxide (Fe<sup>2</sup>

scope of the improving image contrast [21, 53].

of diseased tissue *in vitro* and *in vivo* [24, 67].

**Figure 3.** Microscopic aspect of the labeling of the cyanobacterial cells with QDs in the case of the sample 2: A: Transmission in white light; B: Epifluorescence utilizing a blue filter; C: Epifluorescence utilizing a green filter [54].

## **3. Fluorescent and magnetic nanocomposites as nanoprobes for multimodal imaging applications**

QDs represent a new category of molecular imaging instruments which created a significant effect in biological and medical research, thus helping to further develop new applications. Using new layers, QD appears as a fundamental element for further manufacture of multifunctional nanostructures and nanodevices which can be manufactured by integrating QDs with NIR emission, paramagnetic or superparamagnetic nanomaterials [15]. In the next part of the paper, the latest applications of the nanocomposites are exhibited, which comprise fluorescent and magnetic particles and help for the construction of the novel multiplexed nanoprobe models.

Fluorescent-magnetic nanocomposites comprise a diversity of materials which integrate silica-based, dye-functionalized MNPs and QDs-MNPs composites. Different papers have described various types of techniques to fabricate composites of fluorescent semiconductor QDs and MNPs, such as the mixing of the two materials for the construction of a single heteromeric particle with optical and magnetic properties, the enclosing of separately synthesized fluorescent and magnetic particles in a polymer or silica matrix, the enclosing of the single particles in a polymer or silica gel, magnetically doped QDs and ionic aggregates, which are composed of a magnetic core and fluorescent ionic composites [24, 61].

These multifunctional fluorescent magnetic nanocomposites can be utilized in a range of biological and biomedical applications in nanobiotechnology, such as imaging and therapy, cell tracking and sorting, separation, and drug delivery.

Corr et al. [61] emphasized the fact that the merging of a magnetic and fluorescent entity offers novel two-in-one multifunctional nanomaterials, with a wide gamut of feasible applications, for two reasons: the first reason is that multimodal magnetic-fluorescent tests would be an advantage for *in vitro* and *in vivo* bioimaging applications (magnetic resonance imaging and fluorescence microscopy) and the second reason is that these multifunctional fluorescent magnetic nanocomposites can be used as instruments in the nanomedicine realm. Thus, fluorescent magnetic nanocomposites are utilized to visualize and synchronously treat diverse diseases [61].

Koole et al. described four diverse techniques of obtaining a single nanoparticle, which comprises the fluorescence and magnetic properties and represents new sensitive bimodal contrast instrument for two extremely powerful and highly complementary imaging methods: fluorescence imaging and MRI [27].

In some research papers [21, 53], new approaches of the therapeutic procedures and imaging modalities (such as the technique of the correlation between MRI and ultrasensitive optical imaging) are exhibited in order to be developed and to become mainstream clinical methods for the visual detection of the microscopic tumors during an operation and the complete elimination of the diseased cells and tissues. The authors of these research works show that medical imaging methods can detect the illness, but do not furnish a visual template during a certain surgery. This matter can be resolved with the help of some MQD probes. MQDs are a form of magnetic contrast agents in MRI. In this regard, paramagnetic and superparamagnetic agents Gd(III) and different forms of iron oxide (Fe<sup>2</sup> O3 ) in molecular form and in nanoparticle form are attached to QDs to utilize in a variety of MRI applications with the scope of the improving image contrast [21, 53].

**3. Fluorescent and magnetic nanocomposites as nanoprobes for** 

QDs represent a new category of molecular imaging instruments which created a significant effect in biological and medical research, thus helping to further develop new applications. Using new layers, QD appears as a fundamental element for further manufacture of multifunctional nanostructures and nanodevices which can be manufactured by integrating QDs with NIR emission, paramagnetic or superparamagnetic nanomaterials [15]. In the next part of the paper, the latest applications of the nanocomposites are exhibited, which comprise fluorescent and magnetic particles and help for the construction of the novel multiplexed

**Figure 3.** Microscopic aspect of the labeling of the cyanobacterial cells with QDs in the case of the sample 2: A: Transmission in white light; B: Epifluorescence utilizing a blue filter; C: Epifluorescence utilizing a green filter [54].

Fluorescent-magnetic nanocomposites comprise a diversity of materials which integrate silica-based, dye-functionalized MNPs and QDs-MNPs composites. Different papers have described various types of techniques to fabricate composites of fluorescent semiconductor QDs and MNPs, such as the mixing of the two materials for the construction of a single heteromeric particle with optical and magnetic properties, the enclosing of separately synthesized fluorescent and magnetic particles in a polymer or silica matrix, the enclosing of the single particles in a polymer or silica gel, magnetically doped QDs and ionic aggregates, which are

These multifunctional fluorescent magnetic nanocomposites can be utilized in a range of biological and biomedical applications in nanobiotechnology, such as imaging and therapy, cell

Corr et al. [61] emphasized the fact that the merging of a magnetic and fluorescent entity offers novel two-in-one multifunctional nanomaterials, with a wide gamut of feasible applications, for two reasons: the first reason is that multimodal magnetic-fluorescent tests would be an advantage for *in vitro* and *in vivo* bioimaging applications (magnetic resonance imaging and fluorescence microscopy) and the second reason is that these multifunctional fluorescent

composed of a magnetic core and fluorescent ionic composites [24, 61].

tracking and sorting, separation, and drug delivery.

**multimodal imaging applications**

228 Nonmagnetic and Magnetic Quantum Dots

nanoprobe models.

Many researchers have investigated paramagnetic QDs (pQDs) [62–65]. So, in Refs. [62, 64], a model of multifunctional fluorescent magnetic nanocomposites, comprising silica-coated Fe3 O4 and TGA-capped CdTe QDs, was communicated. This type of nanocomposite has been used for the labeling and imaging of HeLa cells in a magnetic separation. In Refs. [63, 65], pQDs were improved by coating CdSe/ZnS core-shell QDs with a PEGylated phospholipid and a Gd lipid, making the particles biocompatible and MRI active. The pQDs were also conjugated by maleimide to cyclic RGD peptides for targeting angiogenic vascular endothelium as proved by *in vitro* investigations with human umbilical vein endothelial cells (HUVECs).

Ahmed et al. [66] developed a novel technique for the manufacture of QDs enclosed MNPs based on layer-by-layer (LbL) self-assembly method in order to be used for cancer cells imaging. In a research study, Park's group [67] reported long-circulating, micellar hybrid nanoparticles (MHNs) which include MNs, QDs, and the anticancer drug doxorubicin (DOX) in a single poly(ethylene glycol) (PEG)-phospholipid micelle and furnish the first models of concomitant targeted drug delivery and dual-mode near-infrared fluorescence imaging and MRI of diseased tissue *in vitro* and *in vivo* [24, 67].

A very useful review study, which includes the mention of the specialized literature on the applications of MQDs, which represent one of the most currently explored QD-based hybrid NPs, was realized by Wegner and his colleague [68]. In this study, a principal result is indicated, which was obtained by Qiu et al. [69]. These researchers blended QDs, superparamagnetic iron oxide nanoparticles (SPIONs), and gold (Au) NPs in a single poly(lactic-co-glycolic acid) (PLGA) NP. This type of particle is significant for imaging, tracking, and manipulating neutrophils and is used for *in vivo* applications and localized photothermal treatment.

## **4. Conclusions**

QDs and fluorescent and magnetic nanocomposites, which have essential physicochemical characteristics, represent some excellent candidates for numerous applications in bioanalysis and bioimaging. In this review, the important properties of these nanoprobes are summarized and the recent developments of the applications of the QD-based techniques in biomedical uses (biological imaging, cell tracking, magnetic bioseparation, and bio- and chemo-sensoring) are highlighted.

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www.marin-berovic.si/znanost/files/doktorat\_peter\_dusak.pdf

2012;**100A**(3):728-737. DOI: 10.1002/jbm.a.34011

DOI: 10.2217/17435889

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Print: ISSN 1842-6573. On-line: ISSN 2065-3824

uta\_2502M\_10608.pdf?sequence=1

## **Author details**

Anca Armăşelu

Address all correspondence to: anca\_armaselu@yahoo.com

Department of Electrical Engineering and Applied Physics, Faculty of Electrical Engineering and Computer Science, Transilvania University of Brașov, Braşov, Romania

## **References**


[9] Dušak P. Magnetic Nanoparticles for Selective Magnetic Separation in Biotechnology [Thesis]. Jožef Stefan International Postgraduate School; 2015 Available from: http:// www.marin-berovic.si/znanost/files/doktorat\_peter\_dusak.pdf

**4. Conclusions**

230 Nonmagnetic and Magnetic Quantum Dots

**Author details**

Anca Armăşelu

**References**

ISBN: 978-0-12-385089-8

978-3-319-44139-9\_2

org/10.1201/b11760

DOI: 10.1016/j.addr.2009.11.002

Address all correspondence to: anca\_armaselu@yahoo.com

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QDs and fluorescent and magnetic nanocomposites, which have essential physicochemical characteristics, represent some excellent candidates for numerous applications in bioanalysis and bioimaging. In this review, the important properties of these nanoprobes are summarized and the recent developments of the applications of the QD-based techniques in biomedical uses (biological imaging, cell tracking, magnetic bioseparation, and bio- and chemo-sensoring) are highlighted.

Department of Electrical Engineering and Applied Physics, Faculty of Electrical Engineering

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## *Edited by Vasilios N. Stavrou*

The book entitled *Nonmagnetic and Magnetic Quantum Dots* is divided into two sections. In Section 1, the chapters are related to nonmagnetic quantum dots and their applications. More specifically, exact models and numerical methods have been presented to describe the analytical solution of the carrier wave functions, the quantum mechanical aspects of quantum dots, and the comparison of the latter to experimental data. Furthermore, methods to produce quantum dots, synthesis techniques of colloidal quantum dots, and applications on sensors and biology, among others, are included in this section. In Section 2, a few topics of magnetic quantum dots and their applications are presented. The section starts with a theoretical model to describe the magnetization dynamics in magnetic quantum dot array and the description of dilute magnetic semiconducting quantum dots and their applications. Additionally, a few applications of magnetic quantum dots in sensors, biology, and medicine are included in Section 2.

Photo by StationaryTraveller / iStock

Nonmagnetic and Magnetic Quantum Dots

Nonmagnetic and Magnetic

Quantum Dots

*Edited by Vasilios N. Stavrou*