Ligand-regulated catalytic activity in fluorescent gold nanoclusters

Ligand engineering is crucial for tailoring the catalytic and fluorescence properties of gold nanoclusters (AuNCs). However, the mechanisms through which ligands synergistically affect these properties remain largely unexplored. Here we employed single-molecule fluorescence microscopy and ensemble fluorescence spectroscopy to investigate how water-soluble thiol ligands modulate the fluorescence and catalytic activities of AuNCs. Our findings reveal that small ligands, such as 3-mercaptopropionic acid (MPA), provide high catalytic activity, despite lacking fluorescence. Medium-sized ligands, like glutathione (GSH) and the lysine-cysteine-lysine (KCK) peptide, enhance fluorescence and maintain catalytic activity through their stable ligand structures and non-competitive mechanisms. Large protein ligands, such as bovine serum albumin (BSA), significantly increase fluorescence and catalytic activity by controlling their conformation to enhance substrate adsorption but experience reduced catalytic rates due to their competitive binding at high substrate concentrations. Additionally, pH levels influence these properties, with fluorescence intensity increasing under alkaline conditions and catalytic activity peaking in acidic environments for small and medium-sized ligand-AuNCs. For BSA-AuNCs, optimal activity is observed at neutral pH due to favorable protein conformation, where enhanced substrate adsorption has a more significant impact on the catalytic rate than merely exposing gold active sites. These insights into the ligand size and structure-dependent modulation of the catalytic and fluorescence properties of AuNCs lay a foundation for designing efficient fluorescent catalysts. The catalytic activity of fluorescent gold nanoclusters was investigated using single-molecule fluorescence microscopy, revealing the synergistic regulatory effects of ligand size and structure on their fluorescence and catalytic activities.