Colorimetric sensor arrays for antioxidant recognition based on Co₃O₄ dual-enzyme activities

Antioxidants are considered as essential compounds for monitoring human health. In this work, a colorimetric sensor array was developed using oxidase-like (OXD) and peroxidase-like (POD) activities of Co₃O₄ nanoflowers as sensing elements, together with a substrate, 3,3',5,5'-tetramethylbenzidine dihydrochloride (TMB), as a signal reader to effectively identify different antioxidants. In the presence of Co₃O₄, colorless TMB can be oxidized into blue oxTMB to different degrees in the presence and absence of H₂O₂. Interestingly, after the addition of antioxidants, the sensor array showed cross-reactions, and different changes in color and absorbance were observed, as TMB and antioxidants competed for binding. Different colorimetric responses were obtained on the sensor array and identified by linear discriminant analysis (LDA). The LDA result indicated that the sensor array can be used to distinguish 4 antioxidants, namely, dopamine (DA), glutathione (GSH), ascorbic acid (AA), and cysteine (Cys) at seven different concentrations, namely, 10, 20, 30, 50, 100, 200, and 250 nM. Different concentrations of antioxidants and proportions of mixed antioxidants were determined. This demonstrates the potential application of sensor arrays in diagnosis and food monitoring.