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

BACKGROUND

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.

RESULTS

This study developed a manganese vacancy-engineered MnO2 (MnO2-VMn) 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-VMn exhibited a 5-fold enhancement in substrate affinity (Km = 0.20 mM) compared to pristine MnO2 (Km = 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.

SIGNIFICANCE: 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.