Single-atom cobalt catalysts have been recognized as promising alternatives to natural enzymes. However, their relatively low catalytic activity greatly limits their further application. Herein, we demonstrate that single cobalt sites immobilized on  defective carbon nanosheets (2D Co-CN(H)) can act as efficient oxidase mimics with high atom utilization efficiency. In particular, the 2D Co-CN(H) catalysts are found to be twice as effective as defect-free Co-CN catalysts. Combined experimental and theoretical analyses reveal that the defects around atomic cobalt sites can rationally regulate the electronic distribution, significantly promoting the cleavage of O-O bonds and thus improving their oxidase-like performance. The oxidase-like activity of 2D Co-CN(H) catalysts can effectively catalyze the oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) into oxidized TMB (oxTMB) with sensitive colorimetric readout. And the oxTMB generated can also serve as a photothermal agent to convert the colorimetric readout into heat under near-infrared (NIR) irradiation. Taking advantage of the excellent oxidase-like activity of 2D Co-CN(H) catalysts and the good photothermal properties of oxTMB, an innovative dual-mode colorimetric-photothermal sensing platform toward effective discrimination and detection of dihydroxybenzene isomers has been successfully constructed. This study not only highlights the important role of defects on the oxidase-like activity of single-atom nanozymes, but also broadens their potential applications in environmental conservation.

Electrochemical synthesis of hydrogen peroxide (H₂O₂) through two-electron oxygen reduction represents an attractive alternative for on-site H₂O₂ generation. Here, we develop a facile thermally activated-persulfate approach to obtain oxidized carbon nanotubes (O-CNTs-x, x represents oxidation time) with enhanced H₂O₂ electrosynthesis performance. Electrochemical studies have demonstrated that the optimized O-CNTs-6 (i.e., oxidation time is 6 h) could deliver a sustained high selectivity of around 92% for H₂O₂ over a wide voltage window in 0.1 mol/L KOH and a high H₂O₂ production rate of 296.84 mmol/L g^-1 _cat h^-1. Compared with pristine CNTs, the enhanced catalytic activity primarily stems from the newly-generated oxygen-containing functional groups and some defects created on the surface of O-CNTs-x. Importantly, the proposed oxidation process is proved to be valid for promoting H₂O₂ electrosynthesis performance of the Ketjen black. This study provides an universal oxidation method to obtain highly active carbon-based catalysts and initiates new opportunities for the exploration of high-performance electrosynthesis H₂O₂ catalysts.