Nanomaterials with enzyme‐like activities, coined nanozymes, have been researched widely as they offer unparalleled advantages in terms of low cost, superior activity, and high stability. The complex structure and composition of nanozymes has led to extensive investigation of their catalytic sites at an atomic scale, and to an in‐depth understanding of the biocatalysis occurring. Single‐atom catalysts (SACs), characterized by atomically dispersed active sites, have provided opportunities for mimicking metalloprotease and for bridging the gap between natural enzymes and nanozymes. In this Minireview, we illustrate the unique properties of nanozymes and we discuss recent advances in the synthesis, characterization, and applications of SACs. Subsequently, we outline the impressive progress made in single‐atom nanozymes and we discuss their applications in sensing, degradation of organic pollutants, and in therapeutic roles. Finally, we present the major challenges and opportunities remaining for a successful marriage of nanozymes and SACs.

The traditional luminol–H2O2 electrochemiluminescence (ECL) sensing platform suffers from self‐decomposition of H2O2 at room temperature, hampering its application for quantitative analysis. In this work, for the first time we employ iron single‐atom catalysts (Fe‐N‐C SACs) as an advanced co‐reactant accelerator to directly reduce the dissolved oxygen (O2) to reactive oxygen species (ROS). Owing to the unique electronic structure and catalytic activity of Fe‐N‐C SACs, large amounts of ROS are efficiently produced, which then react with the luminol anion radical and significantly amplify the luminol ECL emission. Under the optimum conditions, a Fe‐N‐C SACs–luminol ECL sensor for antioxidant capacity measurement was developed with a good linear range from 0.8 μm to 1.0 mm of Trolox.

Recently, in situ detection of hydrogen peroxide (H 2 O 2 ) generated from live cells have caused tremendous attention, because it is of great significance in the control of multiple biological processes. Herein, Fe−N−C single-atom nanozymes (Fe−N−C SAzymes) with intrinsic peroxidase-like activity were successfully prepared via high-temperature calcination using FeCl 2 , glucose, and dicyandiamide as precursors. The Fe−N−C SAzymes with FeN x as active sites were similar to natural metalloproteases, which can specifically enhance the peroxidase-like activity rather than oxidase-like activity. Accordingly, owing to the excellent catalytic efficiency of the Fe−N−C SAzymes, colorimetric biosensing of H 2 O 2 in vitro was performed via a typical 3,3′,5,5′-tetramethylbenzidine induced an allochroic reaction, demonstrating the satisfactory specificity and sensitivity. With regard to the practical application, in situ detection of H 2 O 2 generated from the Hela cells by the Fe−N−C SAzymes was also performed, which can expand the applications of the newborn SAzymes.

Despite the breakthroughs of transition-metal catalysts in enzyme mimicking, fundamental investigation on the design of efficient nanozymes at the atomic scale is still required for boosting their intrinsic activities to fill in gaps from enzymes to nanozymes. Herein, we developed a universal salt-template strategy for the fabrication of atomically dispersed Fe atoms on ultrathin nitrogen-doped carbon nanosheets characterized by a dramatically high concentration of 13.5 wt %. The proposed Fe-N-C nanozymes with densely isolated FeN 4 sites show high peroxidase-like activities and exhibit a specific activity of 25.33 U/mg, superior to Zn(Co)-N-C nanozymes. Both experiments and theoretical analysis revealed that FeN 4 sites not only lead to the strong adsorption of H 2 O 2 molecules but also weaken the bonding interaction between single Fe atom and two absorbed hydroxyl groups, lowering the energy barrier of the formation of hydroxyl radicals and therefore boosting their peroxidase-like activities. As expected, utilizing the peroxidase-like activity of Fe-N-C nanozymes, good sensitivity and selectivity for the intracellular H 2 O 2 monitoring were realized. It offers a versatile approach for the construction of densely isolated M-N-C single-atom catalysts and achieves better understanding of single sites for the peroxidase-like catalytic mechanisms.

Single‐atom catalysts (SACs) have attracted extensive attention in the catalysis field because of their remarkable catalytic activity, gratifying stability, excellent selectivity, and 100% atom utilization. With atomically dispersed metal active sites, Fe‐N‐C SACs can mimic oxidase by activating O2 into reactive oxygen species, O2−• radicals. Taking advantages of this property, single‐atom nanozymes (SAzymes) can become a great impetus to develop novel biosensors. Herein, the performance of Fe‐N‐C SACs as oxidase‐like nanozymes is explored. Besides, the Fe‐N‐C SAzymes are applied in biosensor areas to evaluate the activity of acetylcholinesterase based on the inhibition toward nanozyme activity by thiols. Moreover, this SAzymes‐based biosensor is further used for monitoring the amounts of organophosphorus compounds.