Novel determination of Cd and Zn in soil extract by sequential application of bismuth and gallium thin films at a modified screen-printed carbon electrode.

This study describes a novel electrochemical technique for the detection of bioavailable cadmium(II) (Cd) and zinc(II) (Zn), in real soil samples. This was made possible by the sequential deposition of bismuth and gallium thin films on modified screen-printed carbon electrodes (SPEs). A range of graphitic modifications were evaluated, and a reduced graphene oxide/graphitic carbon nitride (RGO/g-CN) modification proved most suitable. Electrochemical characterisation demonstrated superior stability, attributed to the functional groups of GO, and an increased electron transfer rate, attributed to the intercalated g-CN. For voltammetric analysis, cadmium was determined in acetate buffer (pH 4.6) using a bismuth thin film (BiTF). Zn was then determined in the same cell, after adjustment of the pH to 5.1, using a gallium thin film (GaTF). The rationale for two separate thin films is described. Optimisations, such as concentration of bismuth(III), gallium(III), potassium ferrocyanide, pH and deposition potentials, were conducted in the matrix of real samples. The LODs and LOQs were determined in the extracted soil matrix as 0.01 and 0.03 mg kg, respectively, for bioavailable Cd and 0.01 and 0.04 mg kg, respectively, for bioavailable Zn. Good agreement was observed for Cd and Zn levels in numerous soil samples when compared to the established technique of ICP-OES. This approach opens up the possibility for rapid on-site portable testing of Cd and Zn in real soil samples to determine the probability of Cd uptake by crops.