Noble Metal Clusters: Applications in Energy, Environment, and Biology

Sub‐nanometer‐sized metal clusters, having dimensions between metal atoms and nanoparticles, have attracted tremendous attention in the recent past due to their unique physical and chemical properties. As properties of such materials depend strongly on size, development of synthetic routes that allows precise tuning of the cluster cores with high monodispersity and purity is an area of intense research. Such materials are also interesting owing to their wide variety of applications. Novel sensing strategies based on these materials are emerging. Owing to their extremely small size, low toxicity, and biocompatibility, they are widely studied for biomedical applications. Primary focus of this review is to provide an account of the recent advances in their applications in areas such as environment, energy, and biology. With further experimental and theoretical advances aimed at understanding their novel properties and solving challenges in their synthesis, an almost unlimited field of applications can be foreseen. Unique applications of sub‐nanometer clusters of noble metals protected with ligands are reviewed in the context of their chemistry. Size‐dependent luminescence in the visible region is their single most important property exploited in such applications. A concomitant discrete electronic structure leads to unusual catalytic and energy harvesting properties. Chemistry of the ligands in conjunction with the electronic structure of the core allows applications in biology.