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Nanozymes with intrinsic enzyme-like properties and excellent stability are promising alternatives to natural enzymes. Yet, their low density of active sites and unclear crystal structure have been the major obstacles that impede their progress. Single-atom nanozymes (SAzymes) have emerged as a unique system to mitigate these issues, due to maximal atomic utilization, well-defined electronic and geometric structures, and outstanding catalytic activity distinct from their nanosized counterparts. Furthermore, the homogeneously dispersed active sites and well-defined coordination structures provide rare pathways to shed light on the catalytic mechanisms. In this review, we summarize the latest progress in the rational design and engineering of SAzymes and their applications in biomedicine and biosensing. We then conclude the review with highlights of the remaining challenges and perspectives of this emerging technology. 

Production of clean hydrogen energy from water splitting is vital for the future fuel industry, and nanocomposites have emerged as effective catalysts for the hydrogen evolution reaction (HER). In this study, Ru-CoO@SNG nanocomposites are prepared by controlled pyrolysis where Ru-CoO heterostructured nanoparticles are supported on nitrogen and sulfur codoped graphene oxide nanosheets. With a large surface area, the obtained composites exhibit a remarkable electrocatalytic activity toward HER in 1.0 M KOH with an overpotential of only −90 mV to reach the current density of 10 mA cm−2 , in comparison to −60 mV for commercial Pt/C benchmark, along with high stability. Mechanistically, codoping of sulfur and nitrogen facilitates the dispersion of the nanoparticles, and the formation of Ru-CoO heterostructures increases the active site density, reduces the electron-transfer kinetics and boosts the catalytic performance. Results from this study highlight the unique potential of structural engineering in enhancing the electrocatalytic performance of heterostructured nanocomposites.