A ratiometric nanozyme sensor with smartphone-assisted visual detection for rapid nitrite detection in meat products...

Rational design of high-performance nanozymes to replace natural enzymes remains a significant challenge in food safety monitoring. Herein, hierarchical Mn₂O₃/MnNiO₃ microspheres were developed via a facile solvothermal strategy for the sensitive detection of nitrite. The obtained dual-phase Mn₂O₃/MnNiO₃ composite exhibited favorable oxidase-like activity toward TMB oxidation. Structural characterization indicates that the optimized composite possesses a specific surface area of 16.29 m²/g and a mesoporous architecture. Mechanistic investigations using Electron Paramagnetic Resonance (EPR) and radical scavenging assays reveal that the introduced lattice defects promote the generation of superoxide anion radicals (·O₂^-), which serve as the primary active species. The Mn₂O₃/MnNiO₃ microspheres showed oxidase-like activity, enabling rapid chromogenic signal generation for the subsequent assay. Leveraging this activity, a ratiometric colorimetric sensor was constructed by coupling the enzymatic oxidation of TMB with a nitrite-specific diazotization reaction. The assay demonstrates a linear range of 0-300 µM with an ultra-low detection limit (LOD) of 0.17 µM. Furthermore, a smartphone-assisted visual detection platform was developed, achieving an LOD of 2.5 µM based on the single B-channel response.