**4. Electrochemical determination of cadmium (Cd2+) using cyclic voltammetry**

Cadmium ion (Cd2+) are one of most toxic heavy metals that contaminates water and causes deadliest diseases like cancer, kidney dysfunction, cardiovascular disease, bone degeneration, lung, and liver damage to humans [32]. Cadmium ions are considered to have a greater solubility in water than any other heavy metals and offering a great threat to the biosystem. The standard level of Cd2+ in drinking water is 0.003 mg/L as per the World Health Organization (WHO). Therefore, the detection of Cd2+ in drinking water samples is of utmost important.

Attaallah and Amine reported the use of enzymic membrane as a electrode without any pre-treatment to detect cadmium ions in drinking water using cyclic voltammetric method [32]. **Figure 9** depicts the fabrication of enzymic membrane electrode to detect cadmium ions. They also modified the fabricated electrode with screen printed electrodes to further increase the selectivity, sensitivity, reproducibility. They modified electrode has showed the linear calibration range between 0.02–100 ppb (R2 = 0.990) and a detection limit of 50 ppt respectively. **Figure 10a** depicts the cyclic voltammogram of TMB/H2O2, HRP/TMB/H2O2, and HRP/Cd2+/TMB/ H2O2. [TMB = 3,3′,5,5′-Tetram ethylbenzidine, HRP = horseradish peroxidase] and **Figure 10b** represents the cyclic voltammograms of variation in the cadmium ions from 0.02 to 100 ppb, in 0.1 M acetate buffer.

*A Review on Cyclic Voltammetric Investigation of Toxic Heavy Metals DOI: http://dx.doi.org/10.5772/intechopen.108411*

#### **Figure 10.**

*(a) Cyclic voltammogram of TMB/H2O2, HRP/TMB/H2O2, and HRP/Cd2+/TMB/ H2O2, and (b) cyclic voltammograms of variation in the cadmium ions from 0.02 to 100 ppb, in 0.1 M acetate buffer [32].*

#### **Figure 11.**

*The procedure diagrams for fabrication of the electrode [33].*

Wang et al. electropolymerized ion imprinted poly (o-phenylenediamine) PoPD/ electrochemical reduced graphene (ERGO) composite on glass carbon electrode (GCE) to detect Cd2+ in water using cyclic voltammetry [33]. **Figure 11** depicts the procedure diagrams for fabrication of the electrode.

The prepared electrode exhibited a excellent selectivity toward the target Cd(II) ions in the presence multi heavy metal ions. Under optimized conditions, the electrochemical sensor showed a good linear relationship between Cd (II) concentration in the range of 1 to 50 ng/mL, with the limit of detection of 0.13 ng/mL respectively. **Figure 12** depicts the cyclic voltammetric curves of bare GCE, ERGO/GCE, Cd(II)- IIP/ERGO/GCE and IIP/ERGO/GCE.

#### **Figure 12.**

*CV curves on bare GCE (curve a), ERGO/GCE (curve b), Cd(II)-IIP/ERGO/GCE (curve c), and IIP/ERGO/ GCE (curve d) between −0.2 V and 0.6 V at a scan rate of 50 mV/s [33].*

**Figure 13.**

*Schematic representation of fabricting the electrochemical aptasensor used to determine Cd2+ [34].*

#### **Figure 14.**

*The CV of (a) bare electrode, (b) Co2Ti1 modified electrode, (c) Co2Ti1/aptamer modified electrode, and (d) Co2Ti1/aptamer/Cd2+ modified electrode respectively at 100 mV/s scan rate [34].*

#### *A Review on Cyclic Voltammetric Investigation of Toxic Heavy Metals DOI: http://dx.doi.org/10.5772/intechopen.108411*

Liu et al. prepared Ti-modified Co3O4-based electrochemical aptasensor to detect Cd (II) as shown in **Figure 13** [34]. Authors studied the effect of aptamer concentration, incubation time and pH of the solution to optimize the experimental condition to get good results. Under these conditions, peak current was proportional to the Cd (II) concentration over a wide linear range of 0.20 to 15 ng/mL, with a detection limit of 0.49 ng/mL respectively. Authors reported that, they have used cyclic voltammetry method not only to characterize each preparation and optimization step, but also to profile the bindings of aptamer to Cd2+. **Figure 14** depicts the CV of bare electrode, Co2Ti1 modified electrode, Co2Ti1/aptamer modified electrode, and Co2Ti1/aptamer/ Cd2+ modified electrode respectively.
