Single-atom catalysts (SACs) featuring the complete atomic utilization of metal, high-efficient catalytic activity, superior selectivity, and excellent stability have been emerged as a frontier in the catalytic field. Recently, increasing interests have been drawn to apply SACs in biomedical fields for enzyme-mimic catalysis and disease therapy. To fulfill the demand of precision and personalized medicine, precisely engineering the structure and active site toward atomic levels is a trend for nanomedicines, promoting the evolution of metal-based biomedical nanomaterials, particularly biocatalytic nanomaterials, from nanoparticles to clusters and now to SACs. This review outlines the syntheses, characterizations, and catalytic mechanisms of metal clusters and SACs, with a focus on their biomedical applications including biosensing, antibacterial therapy, and cancer therapy, as well as an emphasis on their in vivo biological safeties. Challenges and future perspectives are ultimately prospected for SACs in diverse biomedical applications.

Developing robust and highly active bifunctional electrocatalysts for overall water splitting is critical for efficient sustainable energy conversion. Herein, heteroatom‐doped amorphous/crystalline ruthenium oxide‐based hollow nanocages (M‐ZnRuOx (MCo, Ni, Fe)) through delicate control of composition and structure is reported. Among as‐synthesized M‐ZnRuOx nanocages, Co‐ZnRuOx nanocages deliver an ultralow overpotential of 17 mV at 10 mA cm−2 and a small Tafel slope of 21.61 mV dec−1 for hydrogen evolution reaction (HER), surpassing the commercial Pt/C catalyst, which benefits from the synergistic coupling effect between electron regulation induced by Co doping and amorphous/crystalline heterophase structure. Moreover, the incorporation of Co prevents Ru from over‐oxidation under oxygen evolution reaction (OER) operation, realizing the leap from a monofunctional to multifunctional electrocatalyst and then Co‐ZnRuOx nanocages exhibit remarkable OER catalytic activity as well as overall water splitting performance. Combining theory calculations with spectroscopy analysis reveal that Co is not only the optimal active site, increasing the number of exposed active sites while also boosting the long‐term durability of catalyst by modulating the electronic structure of Ru atoms. This work opens a considerable avenue to design highly active and durable Ru‐based electrocatalysts.

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Gadolinium chelates for tumor magnetic resonance imaging (MRI) face challenges such as inadequate sensitivity, lack of selectivity, and risk of Gd leakage. This study presents a single-atom Gd nano-contrast agent (Gd-SA) that enhances tumor MRI. Isolated Gd atoms coordinated by six N atoms and two O atoms are atomically dispersed on a hollow carbon nanosphere, allowing the maximum utilization of Gd atoms with reduced risk of toxic Gd ion leakage. Owning to the large surface area and fast exchange of relaxed water molecules, Gd-SA shows excellent T 1 -weighted magnetic resonance enhancement with a r 1 value of 11.05 mM −1 s −1 at 7 T, which is 3.6 times that of the commercial gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA). In vivo MRI results show that the Gd-SA has a higher spatial resolution and a wider imaging time window for tumors than Gd-DTPA, with low hematological, hepatic, and nephric toxicities. These advantages demonstrate the great potential of single-atom Gd-based nanomaterials as safe, efficient, and long-term MRI contrast agents for cancer diagnosis.