MnFe2O4 Nanoparticles Synthesized via Citrus paradisi Extract: A Novel Green Platform for High-Sensitivity Electrochemical Sulfite Detection

Developing sustainable paradigms for nanomaterial synthesis is essential to reduce the environmental impact of conventional chemical routes. Herein, we report a green, phytochemically mediated synthesis of MnFe2O4NPs using aqueous grapefruit (Citrus paradisi) peel extracts. The bioflavonoid and polyphenolic constituents functioned as dual-functional biogenic reducing and stabilizing agents. FTIR analysis verified spinel lattice metal–oxygen bonds and surface-bound biogenic groups, while SEM imaging revealed the surface morphology and structural aggregation of the nanoparticles. Once integrated into an electroanalytical platform (MnFe2O4NPs/GCE), the modified interface exhibited robust electrocatalytic activity toward sulfite oxidation in an acetate buffer (pH = 4.0). Mixed-valence dynamics (Mn2+/Mn3+ and Fe2+/Fe3+) significantly amplified anodic currents and lowered the kinetic overpotential. Under optimized chronoamperometric conditions, the sensor demonstrated a wide linear range (1–100 μM, r = 0.999), a low detection limit (LOD = 0.045 μM), a limit of quantification (LOQ = 0.136 μM) and excellent selectivity against co-existing ionic and organic interferents. This study provides a simple, eco-friendly, and technically robust strategy for fabricating high-performance electrochemical platforms for food and environmental monitoring.