Facile biosynthesis of spinel NiCo ₂ O ₄ nanoparticles for electrochemical sensor development: highly sensitive and selective detection of metol in environmental samples

The development of eco-friendly electrochemical sensors for monitoring toxic phenolic contaminants like metol (4-methylaminophenol sulfate) is critical for environmental and industrial safety. Here, we report a novel biosynthesized spinel NiCo₂O₄ nanoparticle-modified carbon paste electrode (NiCo₂O₄ NPs/CPE) for the ultrasensitive, selective, and low-cost detection of metol in complex real-world matrices. The NiCo₂O₄ NPs were synthesized <i>via</i> a facile plant-mediated green approach using mint (<i>Mentha piperita</i>) leaf extract, yielding well-dispersed, crystalline spinel structures (confirmed by FTIR, XRD, and SEM techniques). The modified electrode exhibited enhanced electrocatalytic activity toward metol oxidation, achieving a record-low detection limit (2.22 nM) and wide linear range (0.008-11.0 μM). The sensor demonstrated exceptional anti-interference capability against common contaminants (<i>e.g.</i>, hydroquinone, catechol, metal ions) and stability (>93% signal retention after 30 days). Crucially, the NiCo₂O₄ NPs/CPE was validated in real water samples (Nile River, tap water), showing excellent recovery rates (95.5-100.25%) and reproducibility (RSD < 2%). This work not only advances the design of sustainable nanozymes for electroanalysis but also provides a field-deployable, cost-effective solution for metol monitoring in environmental and consumer safety applications.