Controlling reduction degree of graphene oxide-based electrode for improving the sensing performance toward heavy metal ions

The influences of electrochemical reduction degrees of reduced graphene oxide (rGO)-based electrode on its electrochemical sensing performance toward heavy metal ions (HMIs) were investigated. Initially, graphene oxide was synthesized by the modified Hummers' method. Then, rGO films with different degrees of electrochemical reduction were prepared by using the cyclic voltammetry (CV) technique in different numbers of voltammetric cycles x (where x = 3, 6, 9, and 12). The structural and morphological characterizations of different degrees of reduction for rGOx were carried out by using ultraviolet–visible spectroscopy, attenuated total reflection-infrared, X-ray diffraction, atomic force microscopy, field emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The charge transfer rate of rGOx modified glassy carbon electrodes (rGOx/GCEs) was investigated by CV and electrical impedance spectroscopy measurements using a standard ferri/ferrocyanide system. These modified electrodes were further investigated to achieve the best electrochemical performance toward HMIs detection. The modified electrode with six voltammetric cycles (rGO6/GCE) exhibited considerable improvements related to the stability, sensitivity, and well-oxidation potential definition for cadmium ion (Cd${}^{2+}$) and lead ion (Pb${}^{2+}$). The deposition potential, pH value, and accumulation time were optimized. The simultaneous electrochemical detection of Cd${}^{2+}$ and Pb${}^{2+}$ was performed using differential anodic stripping voltammetry technique under optimized conditions in the presence of bismuth ion (Bi${}^{3+}$). The Bi/rGO6/GCE was selected as the desired electrode and employed to detect the Cd${}^{2+}$ and Pb${}^{2+}$ at different concentrations within a linear range between 10 and 50 μgL${}^{-1}$. The detection limits for Cd${}^{2+}$ and Pb${}^{2+}$ were 1.2 and 0.2 μgL${}^{-1}$, respectively; with a signal to noise ratio (S/N = 3). Finally, the repeatability and reproducibility were investigated which exhibited excellent stability with relative standard deviations equal to 2.9 and 0.7% for Cd${}^{2+}$ and Pb${}^{2+}$ respectively, and similar linear range with detection limits.