Regulating oxygen activation and catalytic kinetics in ZnCo2O4 nanozymes via Mn substitution: insights into the oxidase-like activity mechanism.

Mn-substituted ZnCo2O4 (Mn-ZnCo2O4) nanozymes were rationally engineered to regulate the intrinsic redox behavior and oxidase-like activity of pure ZnCo2O4. Partial substitution of Mn ions into the spinel lattice altered the local electronic structure and surface redox couples, increased the availability of mixed-valence states and enhanced surface reactivity. These changes promoted oxygen activation and accelerated interfacial electron-transfer processes during catalytic oxidation. Steady-state kinetic analysis experimentally confirmed that the optimized Mn-ZnCo2O4 nanozyme exhibited a reduced Michaelis-Menten constant (K m = 0.860 mM) and an increased maximum reaction velocity (V max = 127 × 10-8 M s-1), indicating a marked improvement in catalytic efficiency relative to unsubstituted ZnCo2O4. Density functional theory (DFT) calculations were further performed to provide mechanistic insights into the reaction mechanism of the Mn-ZnCo2O4/TMB/AA system. Building on this mechanistic understanding, a colorimetric sensing strategy for ascorbic acid (AA) was developed based on the Mn-ZnCo2O4/TMB system. The proposed method enabled AA quantification over a linear concentration range of 50-1000 µM with a detection limit of 26 µM. Its practical applicability was verified using real orange juice samples, and the obtained results agreed well with those from the classical iodine titration method. Collectively, these findings demonstrate that Mn substitution provides an effective route for tailoring the catalytic properties of ZnCo2O4 nanozymes, while also enabling their practical use in colorimetric analysis of redox-active analytes in complex matrices.