Carbon nanotubes regulated by oxidizing functional groups as peroxidase mimics for total antioxidant capacity determination

With the rapid development of nanozymes research, many carbon nanotubes (CNTs) and their derivatives have been reported as mimics of peroxidases. Despite this, their low peroxidase-like activity severely limited their ability to be applied. In this study, multi-walled CNTs were mixed with oxidizing agents to produce oxygenated carbon nanotubes (O-CNTs) that are rich in oxidizing functional groups. After the simple and green oxidation treatment, the CNTs' peroxidase-like activity was significantly improved. Intriguingly, the degree of oxidation affected the content of oxygen-containing functional groups, which gave O-CNTs different peroxidase-like functions. Since O-CNTs-7 (0.5 g CNTs treated with 3.5 g NaClO3) exhibited the best peroxidase-like activity, its steady-state dynamics and catalytic mechanism were analyzed. It is found that that hydroxyl radical may be the key oxygen active species in the reaction process. Furthermore, on the basis that ascorbic acid can inhibit the oxidation reaction of TMB, O-CNTs-7 was successfully applied to assay the total antioxidant capacity of fresh juice, beverages, and vitamin C tablets. It not only demonstrated how simple oxidation treatments can enhance the peroxidase-like activity of carbon-based nanozymes, but also demonstrated how these nanozymes can be applied to the design of biosensors with colorimetric analysis.