Smartphone-based colorimetric assay of antioxidants in red wine using oxidase-mimic MnO₂ nanosheets

In this work, we report a fast and colorimetric method for the detection of antioxidants based on the oxidase-like activity of two-dimensional MnO₂ nanosheets. Specifically, MnO₂ nanosheets can transform the colourless substrate, 3,3',5,5'-tetramethylbenzidine (TMB), into a deep blue product (oxTMB) via catalytic oxidation, with a specific absorption peak arising at 652 nm. In the presence of antioxidants, both the dramatic colour fading and the decrease in absorbance at 652 nm are observed owing to the inhibition of catalytic oxidation of TMB, where MnO₂ nanosheets can be transformed into Mn²⁺ with the aid of antioxidants. Based on the linear relationship between the absorbance at 652 nm and the concentration of the antioxidants, quantitative determination of antioxidants in the range of 0.1-12 μM has been achieved. Moreover, combined with smartphone-based technology, a portable colorimetric analytical platform for assay of antioxidants is further developed. This method shows advantages including easy operation, low cost, rapid detection and high sensitivity and has been successfully applied in the determination of total antioxidants in red wine.