**3. Results and discussions**

length) convergent lens on the copper plate (thickness: 0.6 mm, size: 18 × 30 mm2

**(Phen) complex**

**2.3. Preparation of polymer film with Tb3+ complex and Bi nanoparticles**

was obtained by dissolving Tb<sup>4</sup>

**Figure 4.** Experimental setup of laser ablation for preparation of Bi nanoparticles.

hydrochloric acid.

132 Acrylic Polymers in Healthcare

0.010 mmol Tb<sup>4</sup>

prepared Tb(Sal)<sup>3</sup>

**Figure 5**.

**2.2. Preparation of Tb(Sal)3**

O7

ously for an hour to achieve Tb(Sal)<sup>3</sup>

beaker with water 2.5 mm deep from the top surface. The liquid was continuously stirred using a magnetic stirrer. Laser ablation was continued for 45 min. The Bi plate was taken out of the liquid, and the sample was used for optical measurements and preparation of thin film samples. The same procedure was repeated for water + sodium hydroxide (HN), and water +

O7

tion. 0.015 mmol of salicylic acid and 0.005 mmol 1,10-phenanthroline were independently dissolved in 2.0 ml ethanol to get its ethanolic solution. Both were added and stirred rigor-

Polyvinyl alcohol was dissolved in double distilled water to attain its 0.011 mmol transparent homogeneous solution. From the already-prepared laser ablated Bi nanoparticles in water, 5 ml of colloidal NPs were mixed separately with ethanolic solution of the as-

rer for 2 hours at room temperature and dispensed in aqueous solution of PVA. The resulting mixture was stirred thoroughly for 4–5 h and later poured in the polypropylene Petri dish and allowed to dry at its own without any heating agent to obtain the thin films. Flowchart depicting the steps for the preparation of polymer samples is shown in

in hydrochloric acid to obtain TbCl<sup>3</sup>

(Phen) complex as explained by Kaur et al. [21].

(Phen) complex. The mixture was homogenized using a magnetic stir-

) kept in a

solu-

#### **3.1. Structural analysis using transmission electron microscopy (TEM)**

TEM micrograph of Bi NPs in water, i.e., H (pH = 7) is shown in **Figure 6(a)**. The particles with average particle size of 14 nm and spherical in nature are observed. **Figure 6(b)** shows the size distribution of the particles as a histogram. The nanoparticles are poly-crystalline as seen from their selected area electron diffraction (SAED) patterns as shown in **Figure 6(c)**. Later, we supplemented it with significant amounts of acid and base to water separately to examine the consequence of pH on the size/dimension, contour/shape, and arrangement of NPs. At pH 9.7, namely, the medium is basic now, one monitors agglomerated core shell type NPs that are depicted in **Figure 6(d)**. An enlarged vision of core shell NPs is shown in its inset, verifying core diameter ~8 nm and shell thickness ~6 nm for the core shell NPs. **Figure 6(e)** demonstrates the histogram for its size distribution and the average size of NPs was found to be 18 nm. The SAED pattern for the same is given in **Figure 6(e)**. Here, the agglomeration of nanoparticles exists that leads to formation of bigger nanoparticles. This is due to the occurrence of the opposite polarity (Na+ and OH− ) on the different NPs.

Spherical nanoparticles are formed when the pH of the sample is 7 (i.e., NPs prepared in pure water). Interestingly, when we add HCl in water and make the pH of the solution to 2.3,

**Figure 6.** (a–c) TEM images of Bi NPs, particle size distribution, and SAED pattern in aqueous solution of H. (d–f) TEM images of Bi NPs, particle size distribution and SAED pattern in aqueous solution of HN. Inset of figure in (d) shows formation of core shell NPs. (g–i) TEM images of Bi NPs aqueous solution in water + HC with enlarged view of NPs with core size in the inset, particle size distribution and its SAED pattern (reproduced from Kumar et al. [48]).

the prepared NPs appear as hollow core shell NPs as shown in the TEM micrograph in **Figure 6(f)**. The average size of the NPs ranges between 15 and 45 nm (see **Figure 6(g)**). A few particles are bigger as they have swollen up. The diameter of the hollow core and thickness of shell is measured to be 20 and 10 nm, respectively, as shown in the inset of **Figure 6(g)**. **Figure 6(h)** shows the size distribution of the particles with average size of 21 nm. The SAED pattern presenting polycrystalline character is revealed in **Figure 6(i)**. All annotations provide evident confirmation that the shape of NPs depends on the pH of the used medium. The character of NPs transforms to hollow from core shell if we alter the pH and change it to acidic from the basic one.

The formation of hollow NPs can be understood on the basis of the Kirkendall effect [25]. The creation of bismuth nano particles initiate as soon as the laser beam is incident on the metallic plate of bismuth and focused carefully. Now the pH of the medium plays a crucial role. Spherical Bi NPs are formed in neutral pH. But the case differs for the acidic medium, as a few of the formed bismuth NPs react with the medium to produce bismuth oxychloride (BiOCl) that gets coated on the bismuth core NPs with the passage of time of ablation. It is noteworthy that the diffusion of bismuth ions is exceedingly rapid when compared to that of chloride ions. The difference in the diffusion rate generates vacancies in the core of bismuth NPs, and when these vacancies are formed in surplus, these join together to form voids. Because of these analogous reasons and facts, core-shell configuration is commonly observed to form voids in larger NPs. Even the created voids are not proportional in every case. This is probably owing to formation of partial hollow nanospheres as observed in CdS/Cd hollow NPs by researchers [5].

One does not observe the formation of hollow NPs when pH of media changes to basic. The reason accounted for this is possibly that in this case, bismuth hydroxide forms the shell and gets deposited on the bismuth core. Here in this case, the diffusivity of hydroxide ion is greatly better than that of bismuth ion. Consequently, there is a superior possibility of formation of vacancies in the core to acquire a core-shell type structure. This, moreover, augments the density of particles in the core and diminishes their amount in the shell.

## **3.2. Structural study using scanning electron microscopy (SEM)**

Bi NPs in aqueous solution of H, HN, and HC, respectively, were added to the polymer films and their SEM images are shown in **Figure 7**(**a**–**c**). The nanoparticles appear embedded in the case of water and basic medium, but in acidic media, some of the bismuth NPs reacts with acid to form flower-like clusters of BiOCl that are quite obvious in **Figure 7(c)**.

#### **3.3. Raman measurement**

the prepared NPs appear as hollow core shell NPs as shown in the TEM micrograph in **Figure 6(f)**. The average size of the NPs ranges between 15 and 45 nm (see **Figure 6(g)**). A few particles are bigger as they have swollen up. The diameter of the hollow core and thickness of shell is measured to be 20 and 10 nm, respectively, as shown in the inset of **Figure 6(g)**. **Figure 6(h)** shows the size distribution of the particles with average size of 21 nm. The SAED pattern presenting polycrystalline character is revealed in **Figure 6(i)**. All annotations provide evident confirmation that the shape of NPs depends on the pH of the used medium. The character of NPs transforms to hollow from core shell if we alter the pH and change it to acidic

core size in the inset, particle size distribution and its SAED pattern (reproduced from Kumar et al. [48]).

**Figure 6.** (a–c) TEM images of Bi NPs, particle size distribution, and SAED pattern in aqueous solution of H. (d–f) TEM images of Bi NPs, particle size distribution and SAED pattern in aqueous solution of HN. Inset of figure in (d) shows formation of core shell NPs. (g–i) TEM images of Bi NPs aqueous solution in water + HC with enlarged view of NPs with

The formation of hollow NPs can be understood on the basis of the Kirkendall effect [25]. The creation of bismuth nano particles initiate as soon as the laser beam is incident on the metallic plate of bismuth and focused carefully. Now the pH of the medium plays a crucial role. Spherical Bi NPs are formed in neutral pH. But the case differs for the acidic medium,

from the basic one.

134 Acrylic Polymers in Healthcare

To understand the formation of Bi NPs and their compounds attributed to different environments (H, HN, and HC), the Raman spectra of colloidal Bi NPs were measured. Raman spectrum of Bi NPs in aqueous solution of water (H), water + NaOH (HN), and water + HCl (HC),

**Figure 7.** SEM images of Bi NPs in polymer films with H (a), HN (b) and HC (c) respectively (reproduced from Kumar et al. [48]).

respectively, in the range 50–400 cm−1 are depicted in **Figure 8(a)**–**(c)**. The spectra demonstrate numerous active vibrational modes in this frequency range. These bands are customized by the environment and bestow lucid information about molecules formed in different media.

**Figure 8(a)** shows the Raman vibrational peaks owing to Bi NPs in water. Two vibrational bands at 66.48 and 92.51 cm−1 are observed that can be attributed to pure Bi NPs in the host [11, 18, 30, 31]. Both these peaks depend upon the particle size. The former lower frequency band is assigned to the Eg mode and the later corresponds to A1g mode. Generally, the vibrational frequencies of A1g mode of Bi NPs lie between 85 and 95 cm−1, and the Raman frequencies of the lower Eg mode lie between 59 and 75 cm−1, which get somewhat modified with the size of the nanoparticles and transfer toward lower frequencies for smaller nanoparticles.

But the case is very different on changing the pH of the medium by adding NaOH (HN). **Figure 8(b)** depicts the Raman vibrational peaks of Bi NPs in HN. The peaks for the Eg and A1g modes in the basic medium shift to 65.25 and 92.73 cm−1 with full width at half maxima 9.86 and 10.51 cm−1, respectively. Two other peaks also emerge at 121.82 and 312.56 cm−1 in the spectrum. These Raman bands are assigned to Bi─O stretching vibrations confirming that the species formed as nanoparticles are due to α-Bi2 O3 [8, 15]. Similarly, by decreasing the pH of the medium on the addition of considerable quantity of HCl to make it acidic, namely HC, the intensity of the Raman bands gets reduced when compared with the bands of Bi NPs in H and HN (see **Figure 8(c)**). In this case, an intense peak is seen at 141.24 cm−1 along with weak

**Figure 8.** (a–c) Raman spectrum of Bi NPs in aqueous solution of water (H), water + NaOH (HN), and water + HCl (HC), respectively (reproduced from Kumar et al. [48]).

bands observed at 59.69, 73.27, 92.85, and 105.53 cm−1. The weaker Raman bands are assigned to the Eg and A1g modes of vibration for Bi in α-Bi2 O3 . The band observed at 105.53 cm−1 may be attributed to Bi for α-Bi2 O3 phase [12, 32]. The two peaks observed in HN at 121.82 and 312.56 cm−1 disappear here. This shows that the vibrational modes of Bi in α-Bi2 O3 dominate over Bi─O vibration in α-Bi2 O3 . In this case (i.e., HC), two new Raman peaks are observed at 141.24 (stronger) and 59.69 cm−1 (weaker) due to BiOCl molecule [45, 47]. The former peak is attributed to internal stretching of Bi─Cl, and the later weaker one is assigned to its external stretching [39–41]. A weak band appears at 396 cm−1, which is a consequence of the motion of the oxygen atom and designated as B1g band, but it is not optically considerable in this case. It is necessary to mention that in the laser ablation synthesis in solution, momentous chemical processes occur after the ablation of the target. Consequently, the intensity of the peaks is correlated to the concentration of a particular species developed at that instant. Thus, one may infer that the intense band at 141.24 cm−1 due to BiOCl evidently suggests its larger concentration. The positions of the Raman peaks for Bi NPs in H, HN, and HC along with their full width at half maxima (FWHM) and their respective intensity are tabulated in **Table 1**.

#### **3.4. UV-Vis absorption**

respectively, in the range 50–400 cm−1 are depicted in **Figure 8(a)**–**(c)**. The spectra demonstrate numerous active vibrational modes in this frequency range. These bands are customized by the environment and bestow lucid information about molecules formed in different media.

**Figure 8(a)** shows the Raman vibrational peaks owing to Bi NPs in water. Two vibrational bands at 66.48 and 92.51 cm−1 are observed that can be attributed to pure Bi NPs in the host [11, 18, 30, 31]. Both these peaks depend upon the particle size. The former lower frequency

tional frequencies of A1g mode of Bi NPs lie between 85 and 95 cm−1, and the Raman frequen-

But the case is very different on changing the pH of the medium by adding NaOH (HN). **Figure 8(b)** depicts the Raman vibrational peaks of Bi NPs in HN. The peaks for the Eg

A1g modes in the basic medium shift to 65.25 and 92.73 cm−1 with full width at half maxima 9.86 and 10.51 cm−1, respectively. Two other peaks also emerge at 121.82 and 312.56 cm−1 in the spectrum. These Raman bands are assigned to Bi─O stretching vibrations confirming that

of the medium on the addition of considerable quantity of HCl to make it acidic, namely HC, the intensity of the Raman bands gets reduced when compared with the bands of Bi NPs in H and HN (see **Figure 8(c)**). In this case, an intense peak is seen at 141.24 cm−1 along with weak

**Figure 8.** (a–c) Raman spectrum of Bi NPs in aqueous solution of water (H), water + NaOH (HN), and water + HCl (HC),

O3

size of the nanoparticles and transfer toward lower frequencies for smaller nanoparticles.

mode and the later corresponds to A1g mode. Generally, the vibra-

and

[8, 15]. Similarly, by decreasing the pH

mode lie between 59 and 75 cm−1, which get somewhat modified with the

band is assigned to the Eg

the species formed as nanoparticles are due to α-Bi2

respectively (reproduced from Kumar et al. [48]).

cies of the lower Eg

136 Acrylic Polymers in Healthcare

**Figure 9** shows the absorption spectra of Bi NPs in H, HN, and HC solutions. The absorption peak for Bi NPs in water emerges at 265 nm. This is accredited to the plasmonic peak frequency of Bi NPs. In the case of HN, two peaks are observed. The first peak at 233 nm is attributed to the plasmon frequency of Bi2 O3 and the second peak at 274 nm is associated with that of Bi NPs. There is a shift in the Bi plasmon peak toward higher wavelength in this case as compared to the Bi plasmon peak in pure water (265 nm) that supports the agglomeration of Bi NPs. Absorption


**Table 1.** Raman peaks position, FWHM, and normalized intensity of Bi NPs in water (H), water + NaOH (HN), and water + HCL (HC).

**Figure 9.** UV-Vis-NIR absorption spectrum of Bi NPs in aqueous solution of H, HN, and HC and inset figure shows their corresponding color (a) (reproduced from Kumar et al. [48]).

spectra for Bi NPs in HC sample show a weird behavior with absorption peaks at 245, 285, and 309 nm that are attributed to Bi2 O3 , Bi NPs, and BiOCl. The main plasmon peak is shifted slightly further toward higher wavelengths. The shifting of the absorption peak to the higher wavelength region is an indication of increased agglomeration of NPs. Wang et al. [41] have reported the absorption peak at 267 nm due to semimetal bismuth NPs in PVP solution. Creighton and Desmond [6] have reported the absorption band around 270–280 nm for bismuth particles of size ranging in 10 nm. This inconsistency in intensity of the absorption peaks appears as an outcome of the concentration of the species in solution. The inset to **Figure 9** shows the color of the colloidal solution after ablation. Different colors signify formation of Bi NPs and their conversion to additional forms, namely Bi2 O3 , BiOCl, or Bi NPs and its core shell structure.

The surface Plasmon resonance is the characteristic of NPs embedded in a dielectric host and is ascribed to combined oscillations of the electrons responding to the optical excitation energy. Optical absorption spectrum of Bi NPs prepared in water by laser ablation for 20 min is shown in **Figure 10**, which depicts its characteristic surface plasmon resonance peak at 267 nm together with a diminutive band at 283 nm that may be assigned to <sup>4</sup> S3/2→<sup>2</sup> P3/2 of Bi<sup>0</sup> transitions [33]. On ablation of the bismuth target for 40 min, the peak at 267 nm is observed to diminish, and the intensity of the peak at 283 nm started increasing, which indicates that the Bi3+ ions are reduced completely to bismuth NPs [9, 17, 40, 41] Also, it is observed that the NPs get agglomerated giving bigger-sized NPs on ablation for a longer period and, hence, absorption at a longer wavelength. Gutierrez and Henglein [14] reported that nanometer-sized bismuth particles exhibited an absorption at ∼253 nm, and according to Creighton and Desmond [6], the first absorption band of 10 nm bismuth particles should appear around 270–280 nm. Polyvinylpyrroldone-stabilized bismuth NPs have been synthesized by Wang et al. [41] with an absorption peak at 281 nm. Our result fits well with the above two literature values.

The absorption spectrum of (Tb(Sal)<sup>3</sup> (Phen)) complex in PVA with and without laser-ablated Bi NPs are also shown in **Figure 10**. The (Tb(Sal)<sup>3</sup> (Phen)) complex in the PVA film shows a band centered at 315 nm that may be attributed to the S<sup>0</sup> →S1 singlet state absorption of salicylic acid. Also, the absorption band due to π→π\* transition of PVA exists in this region, so

**Figure 10.** Absorption spectra of Bi NPs in water ablated for 20 min, (Tb(Sal)<sup>3</sup> (Phen)) complex in PVA and (Tb(Sal)<sup>3</sup> (Phen)) complex with Bi NPs in PVA (reproduced from Kaur et al. [24]).

there may be an overlapping of the absorption bands of PVA and Sal. The (Tb(Sal)<sup>3</sup> (Phen)) complex with Bi NPs in PVA illustrates an absorption peak for Bi NPs along with the absorption band of Sal. It is observed that this band of Sal shows a red shift of ∼8 nm on addition of Bi NPs along with the (Tb(Sal)<sup>3</sup> (Phen)) complex in PVA film. The shift can be ascribed to aggregation of the complex through NPs.

## **4. Excitation spectra**

spectra for Bi NPs in HC sample show a weird behavior with absorption peaks at 245, 285, and

**Figure 9.** UV-Vis-NIR absorption spectrum of Bi NPs in aqueous solution of H, HN, and HC and inset figure shows their

further toward higher wavelengths. The shifting of the absorption peak to the higher wavelength region is an indication of increased agglomeration of NPs. Wang et al. [41] have reported the absorption peak at 267 nm due to semimetal bismuth NPs in PVP solution. Creighton and Desmond [6] have reported the absorption band around 270–280 nm for bismuth particles of size ranging in 10 nm. This inconsistency in intensity of the absorption peaks appears as an outcome of the concentration of the species in solution. The inset to **Figure 9** shows the color of the colloidal solution after ablation. Different colors signify formation of Bi NPs and their conver-

The surface Plasmon resonance is the characteristic of NPs embedded in a dielectric host and is ascribed to combined oscillations of the electrons responding to the optical excitation energy. Optical absorption spectrum of Bi NPs prepared in water by laser ablation for 20 min is shown in **Figure 10**, which depicts its characteristic surface plasmon resonance peak at 267 nm

[33]. On ablation of the bismuth target for 40 min, the peak at 267 nm is observed to diminish, and the intensity of the peak at 283 nm started increasing, which indicates that the Bi3+ ions are reduced completely to bismuth NPs [9, 17, 40, 41] Also, it is observed that the NPs get agglomerated giving bigger-sized NPs on ablation for a longer period and, hence, absorption at a longer wavelength. Gutierrez and Henglein [14] reported that nanometer-sized bismuth particles exhibited an absorption at ∼253 nm, and according to Creighton and Desmond [6], the first absorption band of 10 nm bismuth particles should appear around 270–280 nm. Polyvinylpyrroldone-stabilized bismuth NPs have been synthesized by Wang et al. [41] with an

cylic acid. Also, the absorption band due to π→π\* transition of PVA exists in this region, so

absorption peak at 281 nm. Our result fits well with the above two literature values.

, Bi NPs, and BiOCl. The main plasmon peak is shifted slightly

, BiOCl, or Bi NPs and its core shell structure.

S3/2→<sup>2</sup>

(Phen)) complex in PVA with and without laser-ablated

→S1

(Phen)) complex in the PVA film shows a

singlet state absorption of sali-

P3/2 of Bi<sup>0</sup>

transitions

O3

corresponding color (a) (reproduced from Kumar et al. [48]).

O3

together with a diminutive band at 283 nm that may be assigned to <sup>4</sup>

309 nm that are attributed to Bi2

138 Acrylic Polymers in Healthcare

sion to additional forms, namely Bi2

The absorption spectrum of (Tb(Sal)<sup>3</sup>

Bi NPs are also shown in **Figure 10**. The (Tb(Sal)<sup>3</sup>

band centered at 315 nm that may be attributed to the S<sup>0</sup>

The photoluminescence excitation spectra of Tb3+ ions in PVA, Tb3+ ions with Bi NPs in PVA and (Tb(Sal)<sup>3</sup> (Phen)) complex with Bi NPs in PVA corresponding to the 5 D4 →<sup>7</sup> F5 transition of Tb3+ ion monitored at 544 nm were recorded and are shown in **Figure 11**.

**Figure 11.** Excitation spectra of Tb3+ ions in PVA, Tb3+ ions with Bi NPs in PVA and the (Tb(Sal)<sup>3</sup> (Phen)) complex with Bi NPs in PVA corresponding to the 5 D4 →<sup>7</sup> F5 transition of Tb3+ ions monitored at 544 nm (reproduced from Kaur et al. [24]).

The excitation spectrum of Tb3+ ion in PVA sample also shows a few weak bands at 341, 352, 358, 369, 377, and 488 nm wavelengths corresponding to absorption of Tb3+ ions. When Bi NPs were added to TbCl<sup>3</sup> , the excitation intensity of the bands got improved. There also appears a weak broad plasmonic band for Bi NPs at 285 nm. The excitation spectrum (Tb(Sal)<sup>3</sup> (Phen)) complex and laser-ablated Bi NPs in PVA depicts an extensive excitation band between 275 and 375 nm that may be attributed to n→π\* transition of the salicylate ligands. There seems to be appreciable enhancement in the intensity of the bands corresponding to the Tb3+ ion emission. This vividly signifies effectual sensitization of Tb3+ ions by the ligands pointing to a competent antenna effect [20].
