Hierarchical Ni-rGO nanocomposite derived from e-waste graphite for trace-level electrochemical sensing of...

Metronidazole (MTZ) accumulation in aquatic systems presents severe environmental and public health risks, requiring sensitive trace-level monitoring techniques. Here, we report the development of an electrochemical sensor based on a reduced graphene oxide (rGO) nanocomposite decorated with nickel-containing nanophases (Ni-rGO). The graphene oxide support was synthesized from graphite sustainably recovered from discarded lithium-ion batteries prior to integration via a one-pot, hydrazine-free hydrothermal method. Morphological and structural analyses (SEM, TEM, XRD, FTIR) reveal a hierarchical architecture where highly dispersed, ultra-small (1.5-3 nm) nickel-containing domains coexist with larger crystalline domains anchored to a porous rGO network. This dual-scale structure likely prevents graphene restacking, resulting in a 41.2% increase in electrochemically active surface area and a 43.35% reduction in interfacial charge-transfer resistance compared to unmodified glassy carbon electrodes. Voltammetric studies demonstrate that MTZ reduction at the Ni-rGO interface proceeds via a diffusion-dominated, proton-coupled electron transfer mechanism at physiological pH (7.4). Under optimal conditions, the sensor exhibits a wide linear range of 2-200 µM, a low detection limit of 122.6 nM, and a high sensitivity of 5111.9 µA mM^-1 cm^-2. The sensor exhibited high fabrication reproducibility (3.12% relative standard deviation across five independent electrodes), operational repeatability (95.4% signal retention over 10 continuous cycles), and storage stability (91.9% retention after 7 days). The analytical response remained unaffected by 10-fold excesses of common inorganic ions, endogenous biological species (dopamine, ascorbic acid, uric acid), and the structural analog imidazole, as well as 5-fold excesses of commonly used antibiotics, confirming the sensor's strict specificity toward the target analyte. Recovery rates of 87-108% across tap water and human urine matrices confirm the practical analytical utility of the Ni-rGO sensor for real-world MTZ monitoring in both environmental and clinical contexts.