**3. Optical sensing of heavy metal ions**

#### **3.1 Spectrophotometric/ Fluorometric methods**

Wide range of materials including organic molecules, nanomaterials, hybrid of earlier two, and quantum dots are used to sense HMIs by generating either color or fluorescence signal. Simultaneous signal generation for multiple HMIs is more often observed in electrochemical sensing. Whereas in case of optical methods, generally, signal will be generated exclusively due to target HMI. Hence, below are the sections categorized w.r.t. type of the material used for sensing. Lead i.e. Pb is considered as an example to explain the commonly followed strategies for the optical sensing of HMIs.

#### *3.1.1 Cyclodextrins*

In the year 1996 Czarnik et al. proposed a bench mark work for the fluorescence sensing of Pb2+ ions using a small molecule, heteroatom containing ligands. On complexation with Pb2+ ions proposed ligand exhibited enhanced fluorescence signal for about 15-fold [63]. Cyclodextrin molecule in binary solvent system exhibited 20-fold enhancement in the fluorescence signal after the addition of Pb2+ ions [64]. Similar cyclodextrin molecules are proposed by Chen et al. and Hayashita et al. for fluorometric sensing of the HMI as shown in **Figure 3** [65, 66].

#### *3.1.2 Organic molecules*

Color resulting dibromo-p-methyl-methylsulfonazo molecule was explored by Li et al. for spectrophotometric detection of the Pb2+ions [67]. Inspired by this strategy Meng et al. worked out similar organic molecule for the colorimetric sensing of HMI [68].

#### **Figure 3.**

*(A) Representation of the structure of the ligand. (B) Image of the fluorescence response of the proposed ligand. (C) Binding mechanism of the Pb2+ ions with the proposed ligand (reprinted with permission from [65] copyright 2002 American Chemical Society).*

#### *3.1.3 Rhodamines*

Rhodamine family molecules and its derivatives have been utilized extensively for the optical sensing of not only Pb2+ ions but most of the HMIs. Majority of such works result the optical signal through spirolactum ring opening or forming mechanism after interaction between the rhodamine and HMI. Yoon et al. reported rhodamine B derivative for both fluorometric and colorimetric sensing of Pb2+ ion in methyl cyanide medium [69].

#### *3.1.4 Calixarenes*

Calixarene family molecules are another category contributed significantly for the HMIs sensing. Calixarene structures generally from a dative bond with HMI through the functional groups to result an optical signal. Calix [4]arene derivatives are reported for the Pb2+ ion sensing can be observed as a regular method [70]. Switchable i.e. on–off–on fluorescent sensor observed due to the binding capacity of the calixarene molecule with two different cations i.e. Pb2+ and K+ as shown in **Figure 4**. Observed phenomenon is due to the interaction of the K<sup>+</sup> ions with the molecular orbitals whereas that is absent in the Pb2+ ions case [71].

Apart from the above-mentioned categories there exist another variety of organic and bioorganic molecule extensively studied for the HMI. To name a few polymers based, peptide based, DNAzyme based sensors.

#### *3.1.5 Carbon quantum dots*

Carbon quantum dots (CQDs): Again, inherent functional groups on its surface and their deceptive role in anchoring desired molecules, nanoparticles etc.

*Electrochemical and Optical Methods for the Quantification of Lead and Other Heavy Metal… DOI: http://dx.doi.org/10.5772/intechopen.95085*

#### **Figure 4.**

*Representation of the switchable fluorescence of the calix [4] arene derivative and its binding mechanism with Pb2+ and K+ (reprinted with permission from [71] copyright 2004 American Chemical Society).*

made CQDs as emerging optosensors for HMIs. It is prerequisite to understand the mechanism of sensing of HMIs using CQDs before considering the individual articles on the same. Interaction between the light (electromagnetic radiation) of suitable wavelength and CQDs leads to the generation of charge carriers i.e. due to the excitation of electron from valence band to the conduction band. Generated charge carriers are utilized in the sensing of HMIs depending on the interaction of HMI with the ligand on the CQDs surface. Photo induced charge transfer, fluorescence enhancing/quenching, inner filter effects, phosphorescence etc. [72]. Sodium citrate and polyacrylamide sodium citrate resulted blue fluorescent CQDs which was quenched upon the addition of Pb2+ ion. By adding pyrophosphoric acid to the quenched solution, fluorescence was regenerated. This fluorescence off–on method was able to detect the Pb2+ of the order of 4.6 nM [73]. CQDs synthesized from the green approach using *Lantana camara* berries were exhibited sensitive and wide linear range up to 200 nM Pb2+ ion concentration [74]. CQDs synthesized from chocolate source also exhibited nanomolar HMI detection [75]. Household sugar was used to prepare the CQDs and found to be sensitive and selective for the naked eye sensing of Pb2+ ions in water [76].

#### *3.1.6 Nanoparticles*

Nanoparticles forms an interesting domain for the HMIs sensing. Though there exist a wide range of metal nanoparticles majority of the work has been done on Ag and noble metals Pt, Au nanoparticles in spite of their costly affair. Surface plasmon resonance is the corner stone of the optical sensing of HMIs using Ag, Au, and Pt. Hupp et al. used 11-mercaptoundecanoic acid capped Au nanoparticles as color generating agent for the detection of Pb2+, Cd2+, and Hg2+ ions [77]. Thomas et al. proposed quite a different approach wherein they started with Au3+/Ag+ ions and gallic acid solution. After the addition of Pb2+ Au/Ag nanoparticles are formed to result pink or red color respectively [78]. Non-aggregation-based sensor is developed by Huang et al. Originally Gold nanoparticles surrounded by thiosulphate and

2-mercaptoethanol in a solution. After the addition of Pb2+ due to the formation of Pb-Au alloy gold nanoparticles dissolved into solution and hence Surface plasmon resonance is decreased [79]. Opposite to that, i.e. aggregation based, sensing method was developed by Su et al. using glutathione modified Au nanoparticles. Upon the addition of HMI Au nanoparticles aggregate to result a color change from red to blue [80]. Theme of this work is summarized in the **Figure 5**.

Above discussed category of organic molecules, bioorganic molecules, nanoparticles have also been extensively used for the sensing of remaining HMIs. Functional moieties in the sensing molecules are different for different HMIs. But, conceptual wise it will be a repetition if the discussion is extended for the other HMIs. Ha Na Kim et al. has reviewed all the above discussed type of materials except CQDs for HMIs with an exhaustive literature survey and detailed discussion [81]. One can get more details and literature on CQDs for HMIs sensing in the review article presented by Pooja Devi et al. [72].

**Figure 5.**

*(A) Scheme represents the preparation of glutathione capped Au nanoparticles. (B) Uv–Vis spectra, inset shows the images, (C) TEM images of Au nanoparticles in the absence (a) and presence of Pb2+ ions (b). (D) Images of the Au nanoparticle in the presence of various metal cations and Pb2+ ions (reprinted with permission from [80] copyright 2010 American Chemical Society).*
