**2. Synthesis, modification and electrochemical use of rGO/MOx modified electrode**

The reduced graphene oxide/metallic oxide (rGO/MOx) nanocomposites may be synthesized by the hydrothermal method or by chemical vapor deposition (CVD) method.

Electrochemical experiments may be performed at Electrochemical Analyzer/ Workstation (Potentiostat/Galvanostat) with a conventional three or multi-electrode system. The working electrode may be glassy carbon (coated with reduced graphene oxide/Metallic oxides nanocomposites electrode. The silver/silver chloride (Ag/AgCl) or Calomel (Hg/Hg2Cl2) electrode and a platinum electrode may be tested as the reference and the auxiliary electrode, respectively.

### **2.1 Modified electrode preparation**

The carbon electrode (CE) as a working electrode may be practiced after sonication in alcohol and soaking in deionsed water. The suspension of rGO/MOx nanocomposites after dispersing with ultrasonic agitation into alcohol may be coated on the electrode. So the nanocomposites film modified electrodes will sense/detect heavy metal ions.

### **2.2 Heavy metal ions detection**

Among the voltammetry methods Square wave anodic stripping voltammetry (SWASV) and differential normal pulse voltammetry (DNPV) method if practiced for the individual analysis as well as simultaneous analysis of heavy metal ions will give better results. The heavy metals may be sensed at the definite potential range (V), frequency (Hz), amplitude (V), increment potential (mV) for the simultaneous and selective detection of heavy metal ions.

### **2.3 Square wave anodic stripping voltammetry (SWASV)**

Among electroanalytical techniques stripping voltammetry (SV) is considered to be the most sensitive and extensively used method for the sensing/detection of heavy metal ions (HMIs). This type of analytical investigation comprises a two-step practice: pre-concentration buildup and voltammetry sensing/detection. In the initial stage, the working electrode is plunged in metal ions solution of desired concentration. The metal ions after assortment on the surface of working electrode are reduced with voltage optimization. In the final stage, the potential is drifted to positive value to re-oxidize the metal into metal ions and voltammogram means peak current and peak potential (i-E) are recorded. Each peak of voltammogram indicate the metal and the height of peak indicate the concentration metal ion. Various factors such as synthesis method, working of electrode, practical method etc. may affect the detection limit and the sensitivity of heavy metal sensors.

Wither square wave voltammetry (SWV) or differential normal pulse voltammetry (DNPV) electrochemical analysis are practiced to detect heavy metal ions (HMIs) of various concentration (μM) solution with increased potential of millivolt (mV) for SWV and DNPV at (pH 5–12) various peaks will appear. For the stripping analysis of HMIs using both SWV and DNPV the obtained results in the form of voltammetric peaks are shown in **Figure 2** with rGO/MOx nanocomposites when experienced for the simultaneous analysis of HMIs.

The different peaks at different value (−1.0 to +0.8 V) are assigned for of potential (V) and may be attributed to HMIs on SWV and DNPV respectively. **Figure 2(a)** shows SWV with red line and DNPV with black line. Where X and Y represent the potential of various metal ions, similarly, some other peaks may arise when concentration is increased due to metallic interaction [23]. **Figure 2(b)** Linearization equations, Adj. R2 response may be developed for rGO/MOx nanocomposites and then at the last voltammetry analysis while changing concentration (A, B and C)

### **Figure 2.**

*The respective calibration curves of simultaneous detection of heavy metal ions (HMIs) with current intensity (X, Y) (a) linearization equations, Adj. R2 response of rGO/MOx nanocomposites modified glassy carbon electrode. (b) Stripping voltammetry intensity peaks of various concentration (A, B & C) (c).*

(i.e. 1-10 μM) in acetate buffer (pH 5–12) of the selected metal ions and are recorded as **Figure 2(c)**. The elongated peaks in **Figure 2(c)** show the different metal ions sensed/detected.

### **2.4 Individual as well as stripping behavior toward HMIs using SWV and DNPV**

The square wave voltammetry (SWV) or differential normal pulse voltammetry (DNPV) response of rGO/MOx modified electrode or any modified electrode for the individual voltammetry investigation or simultaneous detection of heavy metal ions (HMIs) can be measured in (μM), their linearization equations and calibration curves and the limit of detection (LOD) can be easily calculated (**Tables 1** and **2**).
