The past four years have witnessed booming progress in single-atom nanozymes (SANs), one of the newest generations of nanozymes with atomically dispersed metal sites for catalytic biomedical uses. They show distinct advantages over their nanoparticle-based counterparts, such as well-defined electronic/geometric structures and complete atomic utilization efficiency, thus offering opportunities to develop advanced nanozymes for practical uses. The atomically dispersed active centers in SANs could also facilitate the precise regulation of catalytic performance, while probing structure–activity relationship for in-depth understanding of mechanism. In this review, we first introduce the synthetic approaches, surface engineering, and characterization techniques of SANs. Subsequently, we discuss the enzyme-like properties of SANs, including some strategies for boosting their catalytic activities. Furthermore, we present their biomedical applications, ranging from biosensors, antibacterial uses, antioxidants, to therapeutics. Finally, the challenges and opportunities of SANs are prospected.

When the dimensionality of layered compounds decreases to the physical limit, ultimate two-dimensional (2D) anisotropy and/or quantum confinement effects may lead to extraordinary physicochemical attributes. Here, we report single-layer Rh nanosheets (NSs) exhibiting ultrahigh peroxidase-like activity, far exceeding that of horseradish peroxidase (HRP) and of most known layered nanomaterial-based peroxidase mimics. Considering per NS as an active subunit, the Rh NSs displayed a catalytic rate constant (K_cat) as high as 4.45 × 10⁵ s^–1 to H₂O₂, two orders of magnitude higher than those of HRP and Rh nanoparticles. The high atom efficiency of the Rh NSs can be attributed to the full exposure of surface-active Rh atoms, which greatly facilitates electron transfer and formation of superoxide anions, representing reactive oxygen species in the catalytic process. As a proof-of-concept application, the Rh NSs were successfully used as peroxidase mimics for the colorimetric detection of H₂O₂ and xanthine, with high sensitivity and selectivity. Moreover, a simple, rapid, and sensitive Rh-based paper sensor for ascorbic acid was also developed. In summary, this work provides a novel example of single-layer metallic NSs for biosensing.

We report a facile protocol for the one-pot preparation of monodisperse Pd nanoparticles (NPs) supported on ultrathin NiCl₂ nanosheets (NSs). The effective protocol can be described as in situ reduction–oxidation–assembly to create Pd/NiCl₂ nanocomposites and is applicable for the development of stable yet highly active Pd-based heterogeneous catalysts for organic transformations. The Pd/NiCl₂ composite displayed synergistically enhanced catalytic activity, high stability, and good recyclability for the tested model oxidation reaction. The in situ nucleation and growth of NiCl₂ NS around Pd NPs guaranteed a clean metal–support interface and greatly facilitated the catalytic reaction. This work provides a novel synthesis method for supported Pd nanocomposites suitable for many important applications.

Enhancing the activity of Pt-based nanocatalysts is of great significance yet a challenge for the oxygen reduction reaction (ORR). In this work, a series of porous Pt/Ag nanoparticles (NPs) were fabricated from regular Pt_xAg_100–x (x = 25, 50, 75) octahedra by a facile and economical dealloying process. Remarkable enhancement in multiple enzyme-mimic activities related to ORR was observed for the dealloyed Pt₅₀Ag₅₀ (D-Pt₅₀Ag₅₀) NPs. This effect can be attributed to the resulting Pt-rich surface structure, increased surface area, and a synergistic effect of Pt and Ag atoms in the D-Pt₅₀Ag₅₀ NPs. Furthermore, the D-Pt₅₀Ag₅₀ NPs exerted excellent antibacterial effects on two model bacteria (gram-negative Escherichia coli and gram-positive Staphylococcus aureus). The present work represents a significant advance in the exploration of the relation between controllable synthesis of high-quality nanoalloys and their novel catalytic properties for various promising applications, including catalysts, biosensors, and biomedicine.