Carbon-based single-atom catalysts (SACs) have been widely studied in the field of biomedicine due to their excellent catalytic performance. However, carbon-based SACs usually aggregate during pyrolysis, which leads to the reduction of catalytic activity. Here, we describe a method to improve the monodispersion of SACs using silicon dioxide as a protective layer. The decoration of silicon dioxide serves as a buffer layer for individual nanoparticles, which is not destroyed during the pyrolysis process, ensuring the single-particle dispersion of the nanoparticles after etching. This approach increased the hydroxyl groups on the surface of Fe-SAC (Fe-SAC-SE) and improved its water solubility, resulting in a four times enhancement of the peroxidase (POD)-like activity of Fe-SAC-SE (58.4 U/mg) than that of non-protected SACs (13.9 U/mg). The SiO₂-protection approach could also improve the catalytic activities of SACs with other metals such as Mn, Co, Ni, and Cu, indicating its generality for SACs preparation. Taking advantage of the high POD-like activity, photothermal properties, and large specific surface area of Fe-SAC-SE, we constructed a synergistic therapeutic system (Fe-SAC-SE@DOX@PEG) for combining the photothermal therapy, catalytic therapy, and chemotherapy. It was verified that the photothermal properties of Fe-SAC-SE@DOX@PEG could effectively improve its POD-like activity, exhibiting excellent tumor-killing performance at the cellular level. This work may provide a general approach to improve the performances of SACs for disease therapy and diagnosis.

The local coordination environment of catalysts has been investigated for an extended period to obtain enhanced catalytic performance. Especially with the advancement of single-atom catalysts (SACs), research on the coordination environment has been advanced to the atomic level. The surrounding coordination atoms of central metal atoms play important roles in their catalytic activity, selectivity and stability. In recent years, remarkable improvements of the catalytic performance of SACs have been achieved by the tailoring of coordination atoms, coordination numbers and second- or higher-coordination shells, which provided new opportunities for the further development of SACs. In this review, the characterization of coordination environment, tailoring of the local coordination environment, and their related adjustable catalytic performance will be discussed. We hope this review will provide new insights on further research of SACs.

Control of surface structure at the atomic level can effectively tune catalytic properties of nanomaterials. Tuning surface strain is an effective strategy for enhancing catalytic activity; however, the correlation studies between the surface strain with catalytic performance are scant because such mechanistic studies require the precise control of surface strain on catalysts. In this work, a simple strategy of precisely tuning compressive surface strain of atomic-layer Cu₂O on Cu@Ag (AL-Cu₂O/Cu@Ag) nanoparticles (NPs) is demonstrated. The AL-Cu₂O is synthesized by structure evolution of Cu@Ag core-shell nanoparticles, and the precise thickness-control of AL-Cu₂O is achieved by tuning the molar ratio of Cu/Ag of the starting material. Aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) and EELS elemental mapping characterization showed that the compressive surface strain of AL-Cu₂O along the [111] and [200] directions can be precisely tuned from 6.5% to 1.6% and 6.6% to 4.7%, respectively, by changing the number of AL-Cu₂O layer from 3 to 6. The as-prepared AL-Cu₂O/Cu@Ag NPs exhibited excellent catalytic property in the synthesis of azobenzene from aniline, in which the strained 4-layers Cu₂O (4.5% along the [111] direction, 6.1% along the [200] direction) exhibits the best catalytic performance. This work may be beneficial for the design and surface engineering of catalysts toward specific applications.