Identifying the Oxidase-like Behavior of N-Doped Carbon Nanozymes via Postsynthetic Modification for Colorimetric Sensing.

N-doped carbon nanomaterials (NCMs) have attracted significant interest as metal-free nanozymes for sensing due to their exceptional stability and biocompatibility. However, the controversial active sites and catalytic pathways severely hinder the application of NCM-based nanozymes. Here, postsynthetic modification methods have been developed to study the catalytic mechanism, including selective deactivation, chemical grafting, and surface doping. Model NCMs are synthesized through the direct pyrolysis of conjugated porous polymers, which exhibit remarkable oxidase-like activity. It has been demonstrated that the -C atom of pyridinic-N serves as the active site for the O adsorption, while a moderate increase in graphitic-N can promote the interfacial electron transfer to enhance the oxidase-like activity. In contrast, the two N-doping types contribute to the peroxidase properties. Furthermore, the metal-free characteristic and enzyme-like activity of PTPAV-C render it an appropriate choice for the assay of total antioxidant capacity. The colorimetric sensor demonstrates excellent selectivity, as only reductive analytes induce significant color changes. These findings provide comprehensive insights into the catalytic mechanisms of NCMs, thereby enabling the rational design of enzyme mimics.