Green Synthesis of ZnO Nanoparticles using Lemon Leaf Extract for Dual Applications: Electrochemical Glucose Sensing and Supercapacitors

This study focuses on the synthesis and characterization of zinc oxide nanoparticles (ZnO NPs) obtained through an eco-friendly method, using an aqueous lemon leaf extract as a reducing and stabilizing agent. The experimental protocol involved the preparation of the plant extract, followed by its mixing with zinc chloride and deposition onto polished copper substrates by pyrolytic spray at 350°C, then annealed at 400°C. The formation of ZnO NPs was confirmed by various analysis techniques, including X-ray diffraction (XRD), UV–Vis spectrophotometry, and scanning electron microscopy (SEM). The synthesized nanoparticles exhibited a homogeneous morphology, with an average crystalline size of 21.92 nm and a bandgap energy of 2.95 eV, indicating significant ultraviolet absorption. The electrochemical properties of the ZnO NPs/Cu were evaluated for glucose detection in an alkaline medium, showing distinct oxidation and reduction peaks. A decrease in current was observed with increasing glucose concentration, attributed to the saturation of active sites and parasitic reactions. The detection sensitivity was 947.37 µA/cm2 mM before annealing and 2703.43 µA/cm2 mM after annealing, representing a 2.85-fold improvement following thermal treatment. Moreover, their performance as supercapacitors was analyzed, revealing a specific capacitance of 175 F/g before annealing and the peak current was plotted 365 F/g after annealing, thus confirming a marked increase in energy storage capacity after annealing. These results demonstrate the potential of ZnO NPs synthesized via this hybrid green-spray pyrolysis approach for dual applications in glucose sensing and energy storage.