**1. Introduction**

### **1.1 Research background**

As we know that the heavy metals in solution in the form of heavy metal ions (HMIs) contaminating water in a little quantity not only be risky to public health, but also may disturb aquatic life, i.e. the lowest level of heavy metal ions (HMIs) and its compounds are very toxic, dangerous for liver, brain, heart muscle, kidneys, human nervous system, blood circulation system and can damage skeletal system resulting in skeletal disease. In response to the needs of modern society and rapid industrial development, it is necessary to design high-efficiency, environmentally friendly, and low-cost electrochemical sensor. Nevertheless, major issues are associated with these sources, such as rapidly increasing prices, environmental consequences, and global climate change. These serious problems have necessitated the development of alternative energy sources.

### *Heavy Metals – Recent Advances*

In the past several decades, heavy metal ions (HMIs) have been of great attention, as they are enormously injurious in the biosphere and even their minute aggregate possesses an unfavorable threat to human health [1–3].

### **1.2 Methods used for the detection of heavy metal ions**

The sensing and quantification of heavy metal ions (HMIs) are important in many applications, including waste management, environmental monitoring, developmental biology, and clinical toxicology. Several techniques/methods have been incorporated over the years for heavy metal ion (HMIs) such as atomic absorption spectroscopy (AAS), [4] inductively coupled plasma-mass spectrometry (ICP-MS), [5] inductively coupled plasma atomic emission spectrometry (ICP-AES) [6] X-ray fluorescence (XRF) spectrometry, [7] and so on. As these spectroscopic practices are time-consuming, their instruments are expensive and complicated in operation. Furthermore, the individual as well as simultaneous sensing/detection of heavy metal ions (HMIs) of great sensitivity and selectivity is the need of today. In this favor, electrochemical techniques/methods especially anodic stripping voltammetric (ASV) has been reflected to be a powerful, most sensitive, extremely rapid, and cost-effective method [8–12].

### **1.3 Electrochemical methods in sensing/detection of heavy metal ions**

So many electrochemical techniques have been practiced for sensing chemical biomolecules and contaminants [13–15]. Normally, voltammetric methods for instance cyclic voltammetry (CV), linear sweep voltammetry (LSV), square wave voltammetry (SWV), differential pulse voltammetry (DPV) etc., potentiometric techniques and electrochemical impedance (EIS) techniques are employed in the sensing/detecting of analytes. In this regard, electrochemical behavior for the detecting of analytes are extremely applicable in micro fluidics and wider field of separation/partition science for the purposes of detection, valving and pumping. Few of the microscopic techniques are joined with electrochemical principles, such as scanning electrochemical microscopy (SECM) and chemically selective scanning tunneling microscopy (CSSTM), which are actually spatially resolved electrochemical sensors, even though they are classified as microscopic techniques [16]. An electrochemical sensor comprises of two constituents: (i) a recognition/perception element also called target receptor may be chemical or biological; and (ii) a material transducer/sensor usually a modified electrode that convert the sensing signals to an electronic signal. Collaborations between the detecting/sensing element of the substrate (HMIs) and the analytes are calculated through the quick reply, sensitivity, discrimination, and flexibility of the modified sensors (electrode) [17]. Durable contacts/interactions are usually linked with greater sensitivity and selectivity; however exemplary adjustability needs weak interactions. The serious factors like sensitivity, selectivity, response time, detection limit, signal-to-noise ratio, linearity, and stability are responsible for the performance of electrochemical sensors [18].

Among these methods square wave anodic stripping voltammetry method and differential normal pulse voltammetry method have been tested and are recommended for the individual analysis as well as simultaneous analysis of heavy metal ions (HMIs) by various researchers. We also endorse the said methods as the best choice for sensing of heavy metal ions however the material phase (plane) is also of specific importance in this regard.

*Electrochemical Techniques for the Detection of Heavy Metals DOI: http://dx.doi.org/10.5772/intechopen.110411*

The facet-dependent electrochemical behavior of Co3O4 nanoplates and nanocubes based on their adsorption/sensing behaviors toward heavy metal ions (HMIs) has been practiced. The Co3O4 nanoplates with plane (111) were better in electrochemical sensing than Co3O4 nanocubes with plane (001). Both adsorption quantities and density-functional theory (DFT) calculations were in accordance with the concept that the variance in electrochemical properties was due to the sensing of heavy metal ions (HMIs) [19]. It is prominent from the study that sensing interface modified electrodes play a key role in the sensing/detection of heavy metal ions (HMIs).

The graphene-analogue carbon nitride (GA-C3N4) has been recommended as suitable sensor for Cu2+ purpose. So far graphene base nanocomposites are considered promising candidate for heavy metal ions (HMIs) determination in water environment [20].

The electrochemical system for the sensing/detection of heavy metals in soil has also been reported. The electrochemical sensor with screen-sprinted electrode (SPE) adapted by ionic liquid (IL) n-octylpyridinum hexafluorophosphate (OPFP) and graphene (GR) was tested for sensing/detection of Cd (II) in soil [21]. The system was further tested for sensitive detection of trace cadmium ions by square wave anodic stripping voltammetry (SWASV).

We are trying to describe the electrochemical stand by combing the reduced graphene oxides/metallic oxides (rGO/MOx) nanocomposites for the analysis of heavy metal ions (HMIs) in solution by electrochemical methods. The detection simultaneous limit (3σ method) used for HMIs of the rGO/MOx nanocomposite modified electrode can be calculated for electrochemical methods on individual analysis as well as simultaneous analysis.

For electrochemical sensing Electrochemical Workstation (Potentiostat/ Galvanostat) with three electrode system or multiple channel system may be used as shown in **Figure 1(a)**. The pH of the solution may be maintained from (5–12) for different metal ions. The electrode composition also plays important role in the sensing/detection of HMIs as illustrated in **Figure 1(b)** larger surface area greater will be adsorption etc. **Figure 1(c, d)** describe the active planes playing important roles in sensing/detection. The Co3O4 nanoplates with plane (111) were better in electrochemical sensing than Co3O4 nanocubes with plane (001) as described in introduction. So far we experienced the reduced graphene oxide/Metallic oxides (rGO/MOx) nanocomposites are the best choice for the sensing/detection of HMIs of boilers (high temperature). Where reduced graphene oxide (rGO) act as a base and prevent metallic oxides (MOx) from aggregation. The reduced Graphene oxide/ Metallic oxides (rGO/MOx) nanocomposites facets (planes) also have influence on the sensing mechanism **Figure 1(d)**. **Figure 1(e, f )** illustrate the sensing/detection through square wave anodic stripping voltammetry (SWV) method and differential normal pulse voltammetry (DNPV) voltammetric peaks of reduced graphene oxides/ metallic oxides (rGO/MOx) nanocomposites modified electrode for the analysis of heavy metal ions (HMIs) or if someone may practice will get similar peaks at different potential specified for each ion. On the other hand, reduced graphene oxide/Metallic oxides with conducting polymers may be practiced with cold water solution system as conducting polymers are not stable at high temperature. Simply in **Figure 1(a–f )** we are try to explain that the sensing/detection of HMIs by nanocomposites are influenced by composition, active planes of the nanocomposites, the Co3O4 nanoribbons keep chemical sensitivity than Co3O4 nanoparticles with active (110), (220) planes and play key role to adsorb heavy metal ions than the latter [22].

### **Figure 1.**

*Electrochemical workstation, (a) nanocomposite modified electrode, (b) reduced graphene oxide/metallic oxide phase (plane), (c, d) sensing mechanism (e) Voltammetric signals of square wave anodic stripping voltammetry (SWASV) and differential normal pulse voltammetry response of rGO/MOx modified electrode for the simultaneous analysis of heavy metal ions (HMIs) over a specified concentration range (1-10 μM) in acetate buffer (pH 5–12) (f).*
