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.

By adjusting the coordination environment of single-atom catalysts, the enzyme-like activity can be finely tuned for highly sensitive biosensing. Herein, we demonstrated that coordinatively unsaturated cobalt-nitrogen sites doped within porous carbon (SA-CoN₃) could serve as highly efficient oxidase mimic. Compared with the typical planar four-coordination structure (SA-CoN₄), the as-obtained single-atom Co nanozymes anchored by three nitrogen atoms are found to display much higher oxidase-like catalytic efficiency. Combined theoretical and experimental analysis revealed that the coordinatively unsaturated Co sites could facilitate adsorption and activation of O₂ molecule and thus improve their oxidase-like activity. Based on the enhanced oxidase-like activity of SA-CoN₃, a paper/smartphone sensor for organophosphorus pesticides (OPs) was successfully constructed and used to quantify glyphosate in environmental and food samples with a low detection limit of 0.66 μM. This work not only highlights the important role of coordination unsaturation of SA nanozymes for promoting oxidase-like activity, but also provides an easy and cost-effective way to conduct effective quantification of OPs in the field.

Pd-based nanomaterials have shown great promise as potential mimic enzymes, but conventional catalysts use only a small fraction of the Pd content that located on the catalyst’s surface. Herein, we demonstrated that maximum atom utilization could be achieved by using single-atom Pd catalysts as oxidase mimic. The single-atom Pd nanozymes exhibit significantly enhanced catalytic efficiency, with a catalytic rate constant (K_cat) and the catalytic efficiency (K_cat/K_m) values more than 625 and 4,837 times higher than those of horseradish peroxidase, respectively. A combined experimental and theoretical calculation reveals reactive oxygen species involved catalytic mechanism which endows single-atom Pd catalysts with excellent colorimetric analysis performance. Benefiting from the maximum atom utilization efficiency and well-defined structural features, the single-atom Pd nanozymes could be successfully applied for the total antioxidant capacity of fruit, determining the serum acid phosphatase activity as well as constructing NAND logic gate. This finding not only provides an effective strategy to maximize the noble-metal atom utilization efficiency as enzyme mimics, but also provides a new idea for extending their possible applications.

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.