A Novel, Effective and Low-Cost Sensor Based on Recycled Eggshell Waste Supported on Chitosan for the Voltammetric Detection of the Antibiotic Metronidazole in Milk and Drug Samples

Herein, an eco-friendly, simple, and inexpensive electrochemical sensor for the determination of the antibiotic metronidazole (MET) was developed based on a carbon paste electrode (CPE) modified by a CaO-Chitosan nanocomposite (CaO-Chi-NC/CPE). Eggshell waste was utilized to prepare CaO nanoparticles, which were then combined with chitosan to obtain CaO-Chi-NC/CPE. The prepared CaO-Chi-NC/CPE was characterized using different techniques such as X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The produced sensor showed higher electrocatalytic activity toward MET in the Britton–Robinson buffer (pH 7.0) compared to the unmodified CPE. The influence of pH and scanning rate on the reduction peak of MET implies that the reduction process of MET at CaO-Chi-NC/CPE surface was a diffusion-controlled reaction with four electrons and four protons. Additionally, under ideal conditions, differential pulse voltammetry revealed that the cathodic current was directly proportional to MET concentrations within two various detection ranges of 0.003–0.35 and 0.35–3.0 μM. The limit of quantification and the limit of detection were 2.88 and 0.86 nM, respectively. Moreover, the fabricated sensor provided acceptable selectivity towards MET, good stability, and repeatable response, with recoveries ranging from 94.6 to 100.4%. This electrode was also successful in detecting MET in commercial tablets and milk samples.