Ligand-Regulated Catalytic Activity in Fluorescent Gold Nanoclusters

Abstract

Ligand engineering is crucial for tailoring the catalytic and fluorescent properties of gold nanoclusters (AuNCs). However, the mechanisms through which ligands synergistically affect both properties remain largely unexplored. Here, we employ single-molecule fluorescence microscopy and ensemble fluorescence spectroscopy to reveal how water-soluble thiol ligands modulate the fluorescence and catalytic activity of AuNCs. We found that small ligands, such as 3-mercaptopropionic acid (MPA), provide high catalytic activity but lack fluorescence. Medium-sized ligands, like glutathione (GSH) and lysine-cysteine-lysine (KCK) peptide, enhance fluorescence and maintain catalytic activity through 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, though they experience reduced catalytic rates due to 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 catalytic rate than merely exposing gold active sites. These insights into the ligand size- and structure-dependent modulation of AuNCs' catalytic and fluorescent properties lay a foundation for designing efficient fluorescent catalysts.