Tuning the d-band center of spinel Mn3O4 via tetrahedral substitution toward enhanced oxidase-like activity for colorimetric detection of total antioxidant capacity in food.

Accurate and sensitive detection of total antioxidant capacity (TAC) is of great practical importance for assessing the overall quality and health benefits. Mn3O4-based nanozymes are promising candidates for high-performance colorimetric detection with the tunable electronic structure. In this work, tetrahedral atomic substitutions (e.g., Zn2+, Co2+, Cu2+) were employed to modulate the electronic structure and d-band center of Mn3O4 to enhance its catalytic performance and thus achieving sensitive detection. A systematic comparative study between the three substitutions and the d-band center-involved mechanistic was conducted. The optimal CoMn2O4 shows a significant acceleration of oxidase-like activity with Km of 0.064 mM and Vmax of 6.29 × 10-8 M s-1. An efficient and visual colorimetric assay of ascorbic acid (AA) assay was developed with the limit of detection of 0.455 μM and the linear range of 1.25-45 μM. TAC detection in real food matrices was achieved by utilizing AA as a representative antioxidant model. Density functional theory (DFT) calculations revealed that Co incorporation at tetrahedral sites endows it with decent d-band center and moderate adsorption and desorption abilities for oxygen-related intermediates, thus enhancing the oxidase-like activity. This tetrahedral-site modulation strategy offers a feasible approach to tailor the oxidase-like activity of Mn3O4 through microscopic d-band electronic structure engineering for sensitive colorimetric sensing.