**2. Electrochemical sensing of heavy metal ions**

### **2.1 Different electrochemical techniques**

### *2.1.1 Voltammetry*

Generally, in this case resultant current due to faradaic reaction [s] of the analyte is recorded by sweeping the potential between the two chosen potentials. As a result, the graph of current vs. voltage will be obtained that is referred to as voltammogram. Below are the various Voltammetric techniques commonly followed to measure the heavy metal ion[s] concentration.

### *2.1.1.1 Cyclic voltammetry [CV]*

Current is recorded in the forward and backward directions by sweeping the potential in the fixed potential window. Faradaic reaction of the metal ion will result the oxidation and [or] reduction peaks at a particular potential where it underwent redox reaction. By referring to cyclic voltammogram


In this three-electrode system platinum wire is used as counter electrode, calomel or Ag/AgCl electrode is used as reference electrode and glassy carbon electrode (GCE)/gold or platinum electrode/carbon paste electrode etc. is used as working electrode. Devi et al. used gold nanoparticles modified GCE as the working electrode for the quantification of Hg2+ ions using CV [8]. Authors exploited the well-known interaction of thiol and gold for the functionalization of gold on the GCE. Micro molar concentration of Hg2+ can be quantified using this CV method.

#### *2.1.1.2 Pulse voltammetry*

In this technique series of super imposing pulse of voltage are generated to result the potential sweep. Because of the applied voltage, HMI will undergo redox reaction to result the faradaic current and that is measured. Differential pulse voltammetry (DPV) is more opted out of various pulse voltammetric techniques such as normal pulse voltammetry and reverse pulse voltammetry. Xia et al. proposed DPV method for the simultaneous determination of Pb2+, Cd2+ and Cu2+. Wherein they used carbon paste electrode modified with hexagonal mesoporous silica and quercetin [9].

#### *2.1.1.3 Square wave voltammetry [SWV]*

SWV is another voltammetric technique which is also very often used to quantify the HMIs with some advantages like fast scan rate, less adsorption on the working electrode and reduced measurement time over DPV.

In the case of CV, DPV and SWV more than one HMI can be detected simultaneously.

Generally, HMIs quantification by voltammetric techniques is performed in conjunction with electrochemical deposition followed by stripping. Hence, few examples for the above mentioned voltammetric techniques are discussed under stripping voltammetry section.

#### *2.1.1.4 Stripping voltammetry*

It is a two-step procedure. First step involves the electrodeposition of the HMI[s] from the electrolyte solution onto the working electrode surface. Second, by applying any of the voltammetric techniques [discussed above], HMI[S] on the electrode will be stripped off into electrolyte solution. Based on the applied scan i.e. anodic or cathodic or during which stripping take place technique is named as anodic stripping or cathodic stripping voltammetry [10]. Following cases can be considered as an example for how stripping step is combined with various voltammetric techniques. Yao et al. proposed square wave anodic stripping voltammetry [SWASV] for the quantification of Zn2+, Cd2+, Pb2+, Cu2+and Hg2+. Fe3O4 nanocrystals of two different shapes are used as a modifier to obtain a sensitive and selective signal for the HMI [11]. **Figure 1** depicts the nature of SWASV. Serious interference of Cu2+ ions in the electrochemical detection of Cd2+ ions was effectively overcome by introducing Bi film on the GCE. As a result, stripping peaks were found to be intact even in the presence of Cu2+ ions for SWASV [12]. Raghu et al. developed DPASV method to achieve the quantification of Hg2+ ions down to picomolar concentration. Multiwalled carbon nanotubes [MWCNT] were functionalized with Fast Violet B salt through diazotization. Functionalized MWCNT then drop casted onto GCE to sense Hg2+ ions by DPASV in drinking water and industrial effluents [13]. Pandurangappa Malingappa and coworkers have published few exemplary works in which stripping voltammetric analysis has been systematically utilized for the analysis of HMIs from the various samples [14–16].

#### *2.1.2 Amperometry*

It is a potentiostatic technique. Electrochemical measurements are carried out at a fixed potential to measure the resultant current due to the redox reaction at the

#### **Figure 1.**

*SWASV recorded for the (A) octahedral and (B) cubic Fe3O4 modified electrodes in the presence of varied concentration of Pb2+ion (reprinted with permission from [11] copyright 2014 American Chemical Society).*

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

electrode electrolyte interface. Based on the analyte, here a particular metal ion, to be detected value of the potential needs to be applied will be decided. Hence, the measured current will be exclusively due to faradaic reaction of that particular analyte. To quote the recent example, Sannegowda and his coworkers developed iminephthalocyanine based amperometric sensor for the quantification of Pb2+ ions. That exhibited the linear range and detection limit in the nanomolar Pb2+ ion concentration [17]. Amperometric biosensor based on the urease was developed for the detection of Pb2+ and Hg2+ ions in river water samples also exhibited the analytical figures of merits closer to nanomolar levels [18]. Simultaneous quantification of more than one HMI is not possible i.e. the drawback of this method.

Another similar work can be quoted here. This case acridono-crown ether played a role of ionophore and. Poly(vinyl chloride) membrane again acted as a host. The potentiometric sensor works in a range of pH 4-7 but suffered a much-required sensitivity [19].

#### *2.1.3 Potentiometry*

Developed potential or electromotive force (EMF) is measured without applying external current. Experimental setup required for the potentiometric measurements is inexpensive. But, sensitivity of this technique is not appreciable when the routine electrodes are used. Efforts are in progress to improve the sensitivity by making use of electrodes constructed out of advanced materials such as graphene, CNT, and nanomaterials or reducing the size of the electrode itself i.e. nanoelectrodes [20, 21]. Ionophore is a corner stone of the potentiometric experimental setup that decides the selectivity and sensitivity of the procedure. Xin-Gui Li et al. developed a ionophore based on conducting copolymer microparticles. Poly[vinyl chloride] membrane acted as a platform to host the ionophore. Presence of functional molecules such as –NH–, –N, –NH2, and –SO3H in the microparticles resulted high selectivity towards Pb2+ ions. It's worth mentioning that potentiometric sensor exhibited sub micromolar detection limit towards Pb2+ ions [22]. Another similar work can be quoted here. This case acridono-crown ether played a role of ionophore and poly[vinyl chloride] membrane again acted as a host. The potentiometric sensor works in a range of pH 4-7 but suffered a much-required sensitivity [23].

#### **2.2 Electrochemical preconcentration**

Generally, concentration of HMIs is very low in drinking water, food and biological samples. In addition to that, sample matrices will be complex in nature and many other ions and molecules will be present. In this regard it is very important to separate the HMI[S] from the sample matrix by enriching the same on to the working electrode surface. There by the interference from various electrochemically active species can be overcome and sensitivity can be significantly enhanced. Below two important electrochemical preconcentration methods are discussed in brief.

#### *2.2.1 Electrochemical deposition*

Electrochemical deposition is done by taking sample solution containing the HMI[s]in a three-electrode electrochemical cell. Then, suitable potential is applied to working electrode (most of the times modified working electrode) w.r.t. reference electrode. As a result of the applied potential metal ion will get reduced to metal atom and simultaneously deposited onto the working electrode. Prior to electrochemical deposition cyclic voltammetric experimental data will be helpful in deciding the deposition potential. Suitable buffer solution and pH are necessary to fine tune the selective deposition of the particular HMI(S) [24].
