A Dual–Mode Nanozyme Assay Based on Photoactivated Mn–Doped Carbon Nanosheets for Rapid Evaluation of Antioxidant Responses in Tea Beverages and Dairy Tea Matrices

Rapid and matrix-aware evaluation of antioxidant responses in dairy-containing tea beverages remains challenging because milk components may interfere with the detectable activity of tea polyphenols. In this study, a photoactivated Mn-doped carbon nanosheet (Mn-CNS)-based dual-mode nanozyme assay was developed for the rapid evaluation of antioxidant responses in tea beverages and dairy tea matrices. The Mn-CNSs exhibited light-triggered oxidase-like activity toward 3,3',5,5'-tetramethylbenzidine, enabling synchronized colorimetric and fluorescence readouts within 1 min. Using epigallocatechin gallate as a model antioxidant, the assay showed limits of detection of 0.22 and 0.29 μg/mL in the colorimetric and fluorescence modes, respectively, together with good selectivity and applicability to tea infusions. When applied to milk tea systems, the measurable antioxidant response decreased to 56.9-72.2% of that in corresponding tea infusions, indicating a pronounced matrix-dependent attenuation. Matrix-matched calibration and dual-indicator recovery analysis further distinguished tea infusion recovery from antioxidant response recovery, while molecular docking suggested that β-lactoglobulin-EGCG interactions contributed to the reduced detectable response. These results demonstrate that the proposed dual-mode assay is a rapid tool for antioxidant-response evaluation and highlight the importance of matrix-aware interpretation in complex dairy tea beverages.