Correlations between the fundamentals and applications of ultrasmall metal nanoclusters: Recent advances in catalysis and biomedical applications

Ultrasmall metal nanoparticles (NPs) with a well-defined composition and structure, the so-called metal nanoclusters (NCs, usually smaller than 3 nm in size), represent the forefront subclass of functional NPs. In the past two decades, great progress has been made in precision synthesis, cluster structure resolving, and the discovery of the intriguing physical and chemical properties of metal NCs. Metal NCs with unique electronic structure and atomic precision have broad prospects in various applications. However, there is still a lack of systematic and in-depth discussion on the design rules of metal NCs for various applications and effective methods for engineering metal NCs to directly meet application requirements. In this review, we focus our discussions on the correlation between the applications and structures (and the physical and chemical properties) of metal NCs, highlighting the basic principles of structure- and property-oriented applications of metal NCs. This correlation will provide guidance for the design of metal NCs with the desirable properties to meet application needs. First, we introduce the physical and chemical properties of metal NCs, including discrete electronic structure, precise composition, and chirality, which are distinctly different from larger metal NPs (typically > 3 nm). Then, we discuss the efforts on controlled synthesis and modulation of metal NCs from the atomic to molecular level, followed by highlighting structural- and property-oriented applications. Finally, we put forward some ideas for future work of metal NCs for catalysis and biomedical applications.