Balancing the Rate of Cluster Growth and Etching for Gram- Scale Synthesis of Thiolate- Protected Au25 Nanoclusters with Atomic Precision

We report a NaOH-mediated NaBH4 reduction method for the synthesis of mono-, bi-, and tri-thiolate-protected Au-25 nanoclusters (NCs) with precise control of both the Au core and thiolate ligand surface. The key strategy is to use NaOH to tune the formation kinetics of Au NCs, i.e., reduce the reduction ability of NaBH4 and accelerate the etching ability of free thiolate ligands, leading to a well-balanced reversible reaction for rapid formation of thermodynamically favorable Au-25 NCs. This protocol is facile, rapid (3h), versatile (applicable for various thiolate ligands), and highly scalable (>1g Au NCs). In addition, bi- and tri-thiolate-protected Au-25 NCs with adjustable ratios of hetero-thiolate ligands were easily obtained. Such ligand precision in molecular ratios, spatial distribution and uniformity resulted in richly diverse surface landscapes on the Au NCs consisting of multiple functional groups such as carboxyl, amine, and hydroxy. Analysis based on NMR spectroscopy revealed that the hetero-ligands on the NCs are well distributed with no ligand segregation. The unprecedented synthesis of multi-thiolate-protected Au-25 NCs may further promote the practical applications of functional metal NCs.