Atomically Precise Water Soluble Gold Nanoclusters Straightforward Synthesis and Tuning of Physicochemical Properties

Abstract

Metal clusters with ultra-small size offer many novel properties such as molecule-like optical transitions, photoluminescence, quantized charging, chirality, and so on. Ligand-protected metal nanoclusters with precise compositions appear to be promising building units of various functional materials. Novel properties and promising applications make metal clusters a prolific target of fundamental and applied research. The major challenges of the nanocluster research are developing synthesis protocols for precise control of cluster size, achieving tunability of nanocluster properties, and imparting stability and desired functionality to the clusters particularly in aqueous and biological media. Cluster synthesis often involves multi-step, two-phase methods and results in mixed-sized products that require time-consuming post-synthesis size-separation and surface-modification steps. Therefore, we need to develop straightforward synthesis protocols that can directly produce stable, atomically precise metal nanoclusters in high yields. Further, we need to develop guiding principles for the rational design of desired nanoclusters and tailoring the properties of the nanoclusters. The thesis work attempts to address some of these challenges by taking the cases of two representative nanoclusters, Au18(SCH2CH2COOH)14 and Au25(SCH2CH2COOH)18 as examples. newlineWe have developed straightforward direct synthesis methods capable of producing atomically precise gold nanoclusters in high yield in an aqueous medium. We have synthesized two representative nanoclusters, Au18(SCH2CH2COOH)14 and Au25(SCH2CH2COOH)18. Among various gold nanoclusters, Au25L18 (where L= ligand) is a ubiquitous nanocluster because of its extraordinarily high stability, unlike Au18L14. Our synthesis of atomically-precise water-soluble gold nanoclusters involved a common reducing agent, NaBH4, and a short carbon chain bifunctional ligand, HSCH2CH2COOH (3-mercaptopropionic acid, MPA) under ambient conditions. We have explored the essential factors that influence th