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.