Copper-doped zinc oxide nanoparticle-modified electrode for sensitive electrochemical detection of ascorbic acid in food samples

Ascorbic acid (AA) is an important antioxidant widely used in food and pharmaceutical products, necessitating sensitive, simple, and cost-effective methods for its determination. Although numerous electrochemical sensors have been developed for AA detection, limitations related to electrode fabrication, material cost, sensitivity, and reliable performance in complex matrices remain. In this study, a cost-effective and highly sensitive carbon paste electrode (CPE) modified with copper-doped zinc oxide nanoparticles (Cu-ZnO NPs) was developed for the electrochemical determination of AA in food samples. The Cu-ZnO NPs were synthesised via a co-precipitation method and characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) demonstrated enhanced AA oxidation currents and a slight negative shift in oxidation potential at the modified electrode. The electrochemically active surface areas of the bare and modified electrodes were 0.047 and 0.065 cm 2 , respectively. A 0.1 M phosphate buffer at pH 5.5 provided optimum conditions for AA oxidation. Scan-rate studies indicated predominantly diffusion-controlled oxidation. Under optimised DPV conditions, the sensor showed a linear response from 20 nM to 100 μM (R 2 = 0.998), with a limit of detection (LOD) and a limit of quantitation (LOQ) of 6 and 18 nM, respectively. Good selectivity, repeatability (RSD = 2.9%), and reproducibility (RSD = 2.0%) were obtained. Fruit juice and vitamin C tablet analyses yielded recoveries of 92.8–110.6%. These results demonstrate the potential of the Cu–ZnO/CPE sensor as a simple, rapid, cost-effective, and reliable platform for AA determination in food and pharmaceutical products.