Antibacterial Mechanisms of Negatively and Positively Charged Ligands on Gold Nanoclusters

Gold nanoclusters (AuNCs) with surface ligand modifications have been developed as antibacterial agents. While understanding the mechanisms and efficacy of bacterial killing is crucial for determining the clinical applications of AuNCs, the effects of different surface charges on their antibacterial mechanisms are still not well understood. Herein, the AuNCs were synthesized with a negatively charged ligand, 6-mercaptohexanoic acid (AuNCs-MHA), and with a positively charged ligand, 4,6-diamino-2-mercaptopyrimidine (AuNCs-DAMP), via a simple one-pot method. The successful preparations of both AuNCs were confirmed by their optical and structural characterizations. Antibacterial activities of the positive and negative surface charges of AuNCs against Gram-negative bacterium Escherichia coli and Gram-positive bacterium Staphylococcus aureus were observed by analyzing bacterial growth curves and reactive oxygen species (ROS) generation. The bacterial growth curves revealed that the antibacterial activity of AuNCs increased in direct proportion to their weight concentration, and the generation of ROS confirmed this finding. In agar plate assays, the antibacterial activity of positively charged AuNCs-DAMP was more potent than that of negatively charged AuNCs-MHA. Based on the Scanning Electron Microscopy (SEM) observation, the positively charged AuNCs-DAMP had a better antibacterial effect compared to the negative surface charge of AuNCs-MHA, showing that the clusters had electrostatic interactions and van der Waals forces between the negatively charged bacterial membrane and cationic AuNCs.