Ultrasmall Ag⁺-rich nanoclusters as highly efficient nanoreservoirs for bacterial killing

Metallic silver (Ag) and its ability to combat infection have been known since ancient history. In the wake of nanotechnology advancement, silver's efficacy to fight broad spectrum bacterial infections is further improved in the form of Ag nanoparticles (NPs). Recent studies have ascribed the broad spectrum antimicrobial properties of Ag NPs to dissociation of Ag⁺ ions from the NPs, which may not be entirely applicable when the size of Ag NPs decreases to the sub-2 nm range [denoted Ag nanoclusters (NCs)]. In this paper we report that ultrasmall glutathione (GSH)-protected Ag⁺-rich NCs (Ag⁺-R NCs for short, with a predominance of Ag⁺ species in the NCs) have much higher antimicrobial activities towards both gram-negative and gram-positive bacteria than the reference NC, GSH–Ag⁰-R NCs. They have the same size and surface ligand, but with different oxidation states of the core silver. This interesting finding suggests that the undissociated Ag⁺-R NCs armed with abundant Ag⁺ ions on the surface are highly active in bacterial killing, which was not observed in the system of their larger counterpart, Ag NPs.