Development of a robust method for Cd(II) ions analysis using CeO₂- and CeO₂-Cu-BTC-based electrochemical sensors

Simple and sensitive electrochemical sensors were fabricated from cerium oxide (CeO₂) and copper-benzene tricarboxylic acid-modified cerium oxide (CeO₂-Cu-BTC) materials for differential pulse voltammetric analysis of toxic cadmium (Cd) ions in aqueous solutions. The materials were prepared by hydrothermal method and structurally characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with energy-dispersive X-ray (SEM-EDX), thermogravimetric analysis (TGA), and X-ray diffraction analysis (XRD). The CeO₂-modified carbon paste electrode (CeCPE) and the CeO₂-Cu-BTC-modified carbon paste electrode (CeBCPE) were electrochemically characterized by their cyclic voltammetry and electrochemical impedance study in standard K₃[Fe(CN)₆] single-electron redox process. Their electrochemical surface areas, electrode surface coverages, and charge transfer resistances were calculated to be 1.46 cm², 2.338 × 10^-5 mol∙cm^-2, and 2790 Ω and 5.48 cm², 2.476 × 10^-5 mol∙cm^-2, and 1254.65 Ω for CeCPE and CeBCPE, respectively. These fabricated electrodes were used as electrochemical sensors for cadmium ion estimation by optimizing the experimental parameters through differential pulse voltammetry. The optimized conditions included 10% modifier for CeCPE and 5% modifier for CeBCPE in 0.12 M HCl solution of pH 5 as supporting electrolyte at - 1.2 V deposition for 30 s in 0.01 to 10 mg L^-1 linear cadmium solution range. Under these conditions, the limit of quantification (LOQ) of 0.368 mg L^-1 and 0.005 mg L^-1 was calculated for CeCPE and CeBCPE electrodes, respectively. The limit of detection (LOD) was calculated to be 0.121 mg L^-1 and 0.002 mg L^-1 for CeCPE and CeBCPE, respectively. All the experimental results indicated that electrodes fabricated from CeO₂-Cu-BTC show better performance as compared to CeO₂-based electrodes. Both these types of electrochemical sensors presented good repeatability and performance in the presence of interfering ions as well. From these findings, it can also be inferred that these electrochemical sensors can provide a simple and very sensitive method for approximation of toxic cadmium ions in aqueous solutions.