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Gold nanoclusters (AuNCs) are of great potential in antibacterial field, but their unsatisfactory stability as a result of high surface free energy limits their practical applications. One efficient solution is loading the ultrasmall AuNCs onto a substrate or matrix. In this work, graphitic carbon nitride (CN) nanoparticles were firstly functionalized with imidazole groups via a photocatalytic reaction. Then, AuNCs were in-situ synthesized on surface of CN by using imidazole moieties as the reducing and stabilizing ligands via formation of Au-N bonds. Transmission electron microscopy (TEM) analysis of the obtained CN-AuNC nanocomposites verified the uniform distribution of AuNCs with a mean size of 1.8 nm on CN surface. Since the coordination strength of Au-N bond is weaker than that of Au-S bond, ligand exchange can occur for the imidazole-protected AuNCs due to ligand exchange in the presence of thiol compounds. Accordingly, the obtained CN-AuNC nanocomposites exhibited thiol depletion ability. The AuNCs slowly released from the CN surface can enter Escherichia coli (E. coli) due to their ultrasmall size, consume bio-thiols reducing species and increase the oxidative stress, which effectively killed the bacteria and inhibited subsequent biofilm formation. This paper provides a promising bactericidal nanocomposite with intrinsic properties for sterilization. [Display omitted] [ABSTRACT FROM AUTHOR]

N-Heterocyclic carbene (NHC) is regarded as a versatile ligand to stabilize gold nanoclusters (AuNCs) via forming strong Au-C bonds. However, reports on NHC-protected AuNCs are rare, especially in aqueous systems. Herein, we present a facile one-step approach to fabricate hydrophilic AuNCs in an aqueous medium using a NHC-based ligand as both a reducing agent and stabilizer. The coordination mechanism involves formation of covalent Au-C bond, as evidenced by nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy. Taking advantages of their ultrasmall size and positively charged surface, these AuNCs can readily penetrate the cell wall and accumulate in bacteria, leading to an obvious increase in intracellular reactive oxygen species. Accordingly, these AuNCs have wide-spectrum antibacterial activity toward Staphylococcus aureus (gram-positive bacterium), Escherichia coli (gram-negative bacterium), and Candida albicans (fungi). [Display omitted] • N-Heterocyclic carbene-protected gold nanoclusters (AuNCs) are facilely synthesized in aqueous solution. • The stability of AuNCs is enhanced by formation of covalent Au-C bond and auxiliary binding from imidazole ring. • The AuNCs can promote generation of intracellular reactive oxygen species to give wide-spectrum antibacterial activity. [ABSTRACT FROM AUTHOR]

Surface ligand plays a crucial role in regulating the physicochemical properties and functionalities of gold nanoclusters (AuNCs). Compared to the widely studied thiolate-stabilized AuNCs, the reports on imidazole-based ligands to protect AuNCs are still rare. Herein, we report the facile preparation of imidazole-stabilized AuNCs using 1-vinylimidazole as both a reducing and capping agent. The coordination pattern of imidazole ring to Au core was proved by formation of Au-N bond, which can protect the AuNCs with moderate stability against different pH and temperature. Owing to the weaker binding strength of Au-N bond than Au-S bond, the obtained AuNCs possessed thiol depletion capacity due to ligand exchange in the presence of thiol compounds. By consuming the cellular bio-thiols that act as the reducing species in redox balance, the imidazole-protected AuNCs can cause the accumulation of oxidizing species and the elevation of the oxidative stress. Accordingly, the obtained AuNCs exhibited intrinsic antibacterial activity toward Staphylococcus aureus , which could be a promising bactericidal nanomaterial for sterilization. [Display omitted] • Imidazole-stabilized AuNCs were facilely synthesized using 1-vinylimidazole as a reducing and capping agent. • Owing to formation of Au-N bond, they showed good stability and thiol depletion capacity. • They exhibited intrinsic antibacterial activity toward Staphylococcus aureus by consuming cellular bio-thiols. [ABSTRACT FROM AUTHOR]