Enzymes participate in all biological processes in nature. For example, many proteases use non-metallic or metallic objects as catalytic activity centers (cofactors), contributing to the efficiency of biochemical interactions and sustaining regular movements in biological systems on earth [1]. Recently, nanocatalysts based on enzymatic reactions have been widely applied in the biomedical field [2], [3], [4], [5], [6]. Numerous applications of nanotechnology and enzymatic catalysis have been extensively used for disease diagnosis [7], cancer therapy [8], [9], [10], and wound healing [11], [12], [13], [14]. For instance, nanozymes with peroxidase (POD)-like activity can cause irreversible damage to tumor cells or bacteria by specifically catalyzing the overexpression of hydrogen peroxide (H2O2) to produce free radicals in cells [15], [16], [17], [18].

Compared to natural enzymes, nanozymes display considerable catalytic stability, but the majority of them are derived from metallic oxides (e.g., Fe3O4, Mn3O4, CeO2, and CuO) [19], [20], [21], [22] or noble metals (e.g., Au, Pt, and Ag) [23], [24], [25]. In a vast proportion of catalytic processes, metallic or metallic oxide nanoparticles have shown the same significant promise as classical heterogeneous catalysts. Nevertheless, the active centers of metallic or metallic oxide nanoparticles usually lie at the corners, edges, and facets of the crystals, resulting in the reduction of the normal catalytic efficiency. Moreover, the catalytic performance of nanozymes can be affected by several factors, including the size, morphology, and surface properties of nanoparticles. Optimal utilization of metallic nanoparticles and normalization of catalytic performance can be achieved by reducing the size of nanoparticles to increase catalytic activity and selectivity. Since most biochemical processes occur on the surface, it is also essential to regulate the size and surface area of nanozymes. The low-ligand central atoms, as catalytic sites, intensify sharply as the nanoparticle size decreases. In this way, increased catalytic efficiency can be achieved because most catalytic reactions normally occur on the nanocatalyst surface. For example, when the size of a nanoparticle reduced from 30 to 3 nm, the number of exposed atoms on the surface increases tenfold [26]. Moreover, size variation tends to alter the electronic structure, atomic conformation, and surface defects of the catalysts.

When the size of nanoparticles covering the surface of the substrate is reduced to a single-atomic scale, they are referred to as single-atomic nanozymes (SANZs) [27]. In recent years, SANZs with atomic-level metallic sites have been regarded as an influential research discipline in the field of catalysis. Unlike other nanoscale catalysts, SANZs offer great catalytic activity due to their atomic-level metallic catalytic sites and have emerged as one of the most promising biomedical catalysts. SANZs, a new generation of synthetic enzymes, possess the advantages of both natural enzymes and nanomaterials, including superior enzymatic catalytic properties, low production costs, and good stability [7,26]. For instance, Cheng et al. reported the use of Cu single-atom sites/N-doped porous carbon (Cu SASs/NPC) for photothermal-catalytic antibacterial treatment [28]. In the presence of H2O2, Cu SASs/NPC can effectively induce a Fenton-like reaction, contributing to a large number of hydroxyl radicals (•OH) and having a certain bacteria-killing effect that makes bacteria more susceptible to temperature. Liu et al. constructed Mn-based SANZs for cancer therapy by inducing •OH generation and glutathione oxidation in the tumor microenvironment [29]. Our group also designed single-atom Fe-dispersed N-doped mesoporous carbon nanospheres (SAFe-NMCNs) nanozymes with high H2O2 affinity for use in photothermal-reinforced nanocatalytic therapy [30]. In these reports, all the metal atoms of these bio-SANZs have four nitrogen coordination numbers, and the coordination environment of the single-atom sites determines the POD-like activity of SANZs. An increase in the POD-like activity of these bio-SANZs can be achieved by the photothermal effect. However, high-quality multifunctional artificial nanomaterials with photothermal and catalytic therapy properties are still in the laboratory stage. In addition, photothermal-enhanced catalytic therapy is restricted by the depth of the NIR laser [31].

In this study, we developed a variety of single-atomic Co nanozymes (SACNZs-Nx-C) with different Co-Nx coordination numbers (x = 2, 3, 4). To understand the reactive sites and reactant species, we investigated the Fenton-like catalytic performance of a range of cobalt nanozymes at the atomic level with different N coordination numbers. After pyrolysis at different temperatures, the central single-atom sites of the prepared SACNZs-Nx-C possessed different surrounding N numbers (Scheme 1a). The reduction in the coordinating N number results in more unoccupied 3d orbitals for the Co atoms, which enhances the adsorption of *H2O2 and improves the Fenton reaction efficiency. Therefore, catalysts with two N coordination numbers possess higher activity and selectivity than those with four N coordination numbers. SACNZs-Nx-C were modified with polyvinylpyrrolidone (PVP) to improve biocompatibility. The PVP-modified SACNZs-Nx-C (PSACNZs-Nx-C) can initiate a heterogeneous Fenton reaction for efficient nanocatalytic antibacterial therapy and wound healing (Scheme 1b). Our research highlights the relevance of coordination trimming of reactive sites in single-atomic catalytic therapy and expands its application further in biomedicine.