*2.2.2 Electrochemical adsorption*

As the HMs exist as ions in the solution same nature can be exploited to achieve the electrochemical adsorption. It works on the electrostatic force of attraction principle. Basically, the working electrode will be modified with a material which has got sufficient opposite charges w.r.t. HMIs or its compounds. In addition to this material which also got greater surface area, hence, more charges on it will be preferred. It is obvious that nanomaterials are the competent candidates for this purpose. First comes the carbon nano substrates such as graphene, CNT, graphite flakes etc. Reason is twofold; Inertness of the carbon substrate hence it can just act as a platform for the HMIs adsorption and for the same reason it can be used for the construction of electrode as well. Ease with which the charge bearing functional groups can be covalently bonded on to it. There exist well-established procedures to introduce various functional groups such as carboxylic acid, amine etc. onto the carbon substrates. Now the materials, functionalized graphene or CNT, having greater surface area and also charge on it are suitable for the electrodeposition of the HMIs. Apart from these qualities, defects introduced during the functionalization process and their inherent good electrical conductivity have the added advantage for this application [25–27]. Likewise, nanomaterials other than carbon substrate can also be used. Positive or negative potential is applied to the electrode to enhance the rate of adsorption.

#### **2.3 Different materials based electrochemical sensors**

### *2.3.1 Carbon substrates*

Graphitic carbon, glassy carbon, can be considered as the bulk form, itself has got good conductivity, stability have been used as an electrode material for a long time. Nanomaterials (NMs) derived out of graphite's single or few layers resulted in graphene, CNT, fullerene, carbon nanoflakes, etc. possessed extraordinary conductivity, excellent electrocatalytic property. Each carbon NM has been extensively studied for the electrochemical sensing of HMIs either in their pristine or modified form or as composite. Considering few examples out of large number of articles each carbon NM is discussed below.

### *2.3.1.1 Graphene*

Two-dimensional single atomic thick monolayer is metallic in nature which inherited large surface area and very high conductivity [28]. In addition, hydroxyl and carboxylic groups originally present to a more or less extent depending on how the material is synthesized. If not present, those functional groups can be introduced on its surface with ease by following well established methods. Mentioned functional groups facilitate the interaction between the graphene and HMIs through either coordination or electrostatic or both. On the other hand, these inherent functional groups are exploited to introduce organic molecules, NMs etc. to enhance the sensitivity and selectivity. Li et al. reviewed the synthesis protocols and analytical applications of the graphene with exhaustive literature [29]. GCE modified with fluorinated graphene oxide was used to detect four HMIs such as Cu2+, Pb2+, Cd2+, and Hg2+simultaneously [30]. Nafion is used in many of the reported works to assemble the graphene onto the electrode i.e. it acts as a binder. At the same time,

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

it also plays a role of ion exchange membrane hence, interference can be considerably overcome. Combination of the duo lead to many methods for the simultaneous determination of HMIs [31–33]. Simultaneous analysis of Zn2+, Cd2+, Pb2+, and Cu2+ was exhibited by a composite nafion, graphene and in situ prepared mercury film. Distinct SWASV peaks were observed for the mentioned HMIs [34]. It is worth mentioning that despite the mentioned synergetic advantages of the nafion & graphene there is problem of restacking of the graphene layers due to van der Waals force of attraction and also irreversible adsorption of HMIs onto the film was observed [35]. Gong et al. developed a strategy to overcome the restacking a graphene layers by introducing Au nanoparticles in between them. In addition, Au nanoparticles improved the analytical figures of merit for the sensing of Hg2+ions [36]. In a similar approach SnO2 and reduced graphene oxide composite lead to simultaneous determination of Cd2+, Pb2+, Cu2+, and Hg2+ as shown in **Figure 2** [37]. Composite of reduced graphene oxide with cysteic acid lead to highly sensitive DPASV method for Ag<sup>+</sup> ion with detection limit of 1 nM [38]. Jingbo Chang et al. exclusively reviewed the sensing strategies of HMIs using graphene and its composites their more examples on this topic can be found [35].

#### *2.3.1.2 CNTs*

Properties, modification strategies discussed w.r.t. graphene holds good for CNTs. Since CNTs can be considered as rolled up structure of graphene and carbon atom is sp3 hybridized in both the cases. CNTs are supposed to be equally competent for the analysis of HMIs compared to graphene but there are minor differences [39]. But, hybrid of graphene and CNTs yielded much better results compared to CNTs alone. Three-dimensional structure of graphene and CNT was able to simultaneously analyze pb2+ and Cd2+ions [40]. CNTs are used in combination with bismuth film for the detection of HMIs [41]. CNTs and Pt nanoparticle together resulted in a highly sensitive electrochemical method for the ppb to ppt level As3+ determination [42].

#### *2.3.2 Nanoparticles*

As is well known, large surface area, more exposed catalytic sites, enhanced conductivity, greater electron & mass transport and faster electrode kinetics attracts

#### **Figure 2.**

*SWASV peaks recorded in the presence of HMIs for SnO2-reduced graphene oxide modified electrodes (reprinted with permission from [37] copyright 2002 American Chemical Society).*

the scientists to modify the working electrode with nanomaterials (NMs) for the better electrochemical sensing of HMIs. Though wide verity of NMs have been reported in the literature but metal and metal oxide nanoparticles are considered for discussion as their contribution is major among other NMs.

### *2.3.2.1 Metal nanoparticles*

Gold nanoparticles and its derivatives have been extensively investigated as a transducer material for sensing HMIs because of its excellent conductivity, catalytic properties and inertness. There is a one more property i.e. affinity of gold towards thiol. This one property lead to plenty of works which all played around the triangular interaction between gold-thiol molecules-HMIs. As it can be observed gold component is common in below discussed cases but based on the material with which it is composed, capping agent, functionalized molecule and other experimental conditions sensitivity and selectivity towards HMIs will be totally different. Bin Zhang et al. modified carbon nanofibers with gold nanoparticles' size around 15 nm. That material was used on the GCE to simultaneously detect Cd2+, Pb2+ and Cu2+ by SWASV [43]. Whereas the same Au nanoparticles capped with tannic acid modified GCE lead to highly sensitive and selective electrochemical sensor for Hg2+down to 100 fM in the presence of Zn2+, Al3+etc. [44]. Handful number of articles can be found in which Au nanoparticles are used for the estimation of HMIs [45–47]. Inspired by the excellent electrochemical results two noble metals are combined to obtain the bimetallic composite. Same has shown promising results for the estimation of Hg2+ ions in the ppb range with the limit of detection down to ppt [48].

Earlier mercury electrodes were used to detect the HMIs because of the amalgamation reaction between the two. Thanks to the multicomponent alloy formation property of the bismuth and antimony with the HMIs. Because, highly toxic mercury electrodes were successfully replaced by the bismuth and antimony film modified electrodes. Nanoparticles of bismuth and antimony are proved to be an environmentally friendly and efficient platform for the quantification of HMIs [49–52]. Even the experiments were carried out to understand the effect of different morphology of the bismuth nanoparticles on the HMIs detection [53].

### *2.3.2.2 Metal oxide nanomaterials (MONMs)*

Come into focus as a result of finding an inexpensive alternative for the noble metal nanoparticles in spite of their excellent electrochemical results as the latter are highly expensive. Other technical reasons are being as same as that any nanomaterials such as greater surface area and hence greater adsorption of HMIs, enhanced electrocatalytic activity etc. Oxides of iron, nickel, magnesium, manganese, zirconium etc. have been extensively studied for the quantification of HMIs. Majority of the reported methods are focused on experimenting with the morphology of the MONMs. Generally wide variety of morphologies can be achieved through hydrothermal synthesis. Then the MONM powder will be drop casted onto GCE for the electrochemical sensing of HMIs. Problem with this approach is modified electrode will not be sufficiently robust. Abdul Waheed et al. discussed the same with detailed literature in their review [54]. To overcome this, Lie et al. electrochemically deposited Co3O4 nanoparticles onto the indium tin oxide electrode followed by annealing. As prepared modified electrode is further used for the electrochemical sensing of Pb2+ ions in the presence of various divalent ions [55].

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

#### *2.3.3 Mesoporous materials*

Ordered mesoporous silicas (OMSs) can be considered as a representative candidate for mesoporous materials and their application in HMIs sensing. Because OMS exhibit uniform pore size, highly ordered pore, high volume and surface area which was explored by Mobil oil corporation [56, 57]. OMS provide a better platform for the physisorption of HMIs. As the OMS can be synthesized through various approaches different functional groups are inherently present on their surface. Out of which silanol is often present and most useful because of its reactive nature. Wide variety of organic molecules can be introduced through silanol group to result hybrid OMS. Because of the newly introduced organic molecules chemisorption of HMIs is achieved in addition to physisorption onto the hybrid OMS [58]. In majority of the reported methods particular OMS is combined with graphitic powder and mineral oil to obtain the modified carbon paste electrode to sense the HMIs. Thiol self-assembled monolayers on mesoporous supports [SAMMS]are used to prepare the carbon paste electrode for simultaneous analysis of Pb2+ and Hg2+ ion in 0.2 M HNO3 [59]. Glycinylurea-SAMMS lead to SWASV method for the detection of Pb2+ with a detection limit down to 1 μg/L. It must be noted that detection can be performed over a wide range of pH 4.5 to 6.5 without using any specific buffer solution [60]. Yantasee et al. developed advanced remote accessible automated DPASV coupled with flow injection analysis of Pb2+ ions using Phosphonic-SAMMS [61]. Detailed literature on synthetic approaches and analytical applications of mesoporous materials can be found elsewhere [62].
