Single-atom catalysts (SACs) with atomically dispersedactive sitesand maximum atomic utilization efficiency provide new opportunitiesto develop high-performance nanozymes. However, the preparation ofSACs with a high metal content still faces serious challenges. Herein,we report a pyrolysis method for achieving a Cu single-atom catalyst(Cu-SAC) on nitrogen-doped carbon with a Cu loading of 7.98 wt %.Isolated Cu atoms with Cu-N-3-C coordinationwere confirmed by spherical aberration-corrected transmission electronmicroscopy and X-ray absorption spectroscopy. The synthesized Cu-SACdisplayed outstanding peroxidase-like activity by effectively catalyzingthe reaction between 3,3 ',5,5 '-tetramethylbenzidine(TMB) and H2O2 to show a blue color. Based onthe inhibition effect on the oxidation of TMB by ascorbic acid (AA),a colorimetric assay for AA was established with a limit of detection(LOD) of 0.63 mu M. Furthermore, the total antioxidant capacities(TACs) of practical samples including vitamin C tablets, commercialbeverages, and fresh fruit were successfully detected by the proposedcolorimetric assay. This study provided a facile method for the preparationof high-metal-loading SACs, which exhibited high enzymatic propertieswith promising applications in food and drug safety.

Single-atom catalysts (SACs) have shown broad application prospects in different area due to their maximum atomic utilization and excellent catalytic properties. However, how to realize large-scale synthesis of high loading SACs is still a problem for various application requirements. Herein, we report a novel strategy to synthesize Co single-atom catalyst (Co-SAC) by controllable releasing of Co atoms at high temperature, which can be effective to avoid the formation of Co nanoparticles. Particularly, the synthesized Co-SAC with Co content of 7.04 wt% contained exclusive single Co atoms coordinated Co-N4 active sites, demonstrated by inductively coupled plasma optical emission spectroscopy (ICP-OES) and X-ray absorption spectroscopy (XAS). The obtained Co-SAC exhibited excellent intrinsic oxidase-like activity by catalyzing the dissolved oxygen to superoxide radicals. Based on the antioxidant of ascorbic acid (AA) and the enzymatic property of acid phosphatase (ACP), a colorimetric assay for AA and ACP was established by utilizing the oxidase-like activity of Co-SAC. The results indicated that the limit of detection (LOD) of AA and ACP was 1.31 μM and 0.08 U/L, respectively. Considering the large-scale synthetic method and its excellent catalytic property, this kind of Co-SAC would have a good prospect in biosensor fields.