Green synthesis of zinc oxide nanoparticles for copper-ion detection and energy storage in aqueous media: an electrochemical and DFT study

Abstract

This study compares the detection of copper ions using two carbon paste electrodes (CPEs) modified with ZnO nanoparticles: one synthesized by conventional chemical methods and the other via a green biosynthesis approach using Calamintha nepeta leaf extract. Electrochemical techniques including cyclic voltammetry (CV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS) were employed to evaluate sensor performance. The bio-synthesized ZnO nanoparticle-modified electrode showed superior sensitivity, with a linear detection range from 3 to 9 μM (R² = 0.99707) and a low detection limit of 0.41 μM. Recovery tests yielded rates between 96% and 111%, confirming the sensor's reliability for Cu²⁺ monitoring in drinking water. DFT calculations show that ZnO-functionalized graphite has better electronic properties and a stronger ability to adsorb Cu²⁺ at −0.5 eV, making it a promising material for electrochemical copper detection. Moreover, charge–discharge (CD) experiments demonstrated the electrochemical stability and performance of the bio-derived electrode with a capacity retention of 99.2%, highlighting its potential for application in aqueous energy storage systems. This multifunctional ZnO@graphite nanocomposite thus offers a versatile platform for both sensitive environmental sensing and sustainable energy storage.