Mn defect enhanced oxidase-mimicking activity of MnO₂ for colorimetric detection of total antioxidant capacity

Total antioxidant capacity (TAC) is a critical biomarker for evaluating the nutritional quality of food and efficacy of pharmaceutical formulations. Colorimetric detection offers operational simplicity and visual readout, yet its sensitivity depends on efficient catalyst development. Nanozymes have emerged as stable, cost-effective alternatives to natural enzymes. Manganese dioxide (MnO₂) exhibits intrinsic oxidase-like activity but suffers from poor conductivity and sluggish electron transfer kinetics. Engineering efficient MnO₂-based oxidase mimics without hydrogen peroxide dependence remains a significant challenge for practical TAC sensing applications.

This study developed a manganese vacancy-engineered MnO₂ (MnO₂-V_Mn) with significantly enhanced oxidase-like activity for H₂O₂-free colorimetric TAC detection. Comprehensive spectroscopic analyses (EPR, XPS, XAS) and DFT calculations confirmed successful Mn vacancy introduction, which reduced the work function from 5.11 eV to 4.30 eV and induced half-metallic behavior, thereby accelerating electron transfer to adsorbed O₂ (0.42 e^- vs. 0.32 e^- for pristine Mn O₂). The MnO₂-V_Mn exhibited a 5-fold enhancement in substrate affinity (K_m = 0.20 mM) compared to pristine MnO₂ (K_m = 1.00 mM). Using ascorbic acid and gallic acid as models, a dual-mode colorimetric platform integrating UV-Vis spectroscopy and smartphone-based RGB analysis was established, achieving detection limits of 0.13 μM and 3.24 μM, respectively. The sensor demonstrated excellent reproducibility (RSD = 1.46%), stability (16 days), and successful TAC quantification in vitamin C tablets, beverages, and fruit juices, correlating strongly with standard CUPRAC assays.

This work presents the systematic demonstration of Mn vacancy engineering to enhance the oxidase-like activity of MnO₂. We conclude that cationic vacancies modulate electronic structure, reducing work function and promoting interfacial electron transfer for efficient O₂ activation. This significant finding provides a new design strategy for high-performance nanozymes and establishes a reliable, field-deployable platform for antioxidant quality assessment in food and pharmaceutical products.