Highly luminescent silver nanoclusters with tunable emissions: cyclic reduction–decomposition synthesis and antimicrobial properties

This paper reports a facile aqueous-based synthesis method for highly luminescent Ag nanoclusters (NCs) with tunable emissions. The key strategy was to use a reduction–decomposition–reduction cycle to modify the Ag NC intermediates that were further subjected to size/structure focusing. The as-modified Ag NC intermediates were more robust in water against the subsequent etching by thiol ligands, making a mild size-/structure-focusing environment possible, which produced highly luminescent Ag NCs with a well-defined size and structure. Ag NCs with intense red and green emission were successfully synthesized by this method. These Ag NCs possessed a well-defined size and structure (Ag16(SG)9 and Ag9(SG)6), and exhibited excellent stability in the aqueous phase. Our highly luminescent Ag NCs also possessed superior antimicrobial properties against the multidrug-resistant bacteria Pseudomonas aeruginosa, via generating a high concentration of intracellular reactive oxygen species. Jianping Xie, David Tai Leong and co-workers from the National University of Singapore have devised a method to prepare silver nanoclusters that exhibit interesting luminescence and antimicrobial properties. Small metallic clusters of sizes between single atoms and nanoparticles show properties different from both of these, and have been attracting increasing attention. Gold and silver clusters, for example, are investigated as optical probes in bioimaging and biosensing. The researchers used a two-step process — a reduction–decomposition–reduction cycle then an etching step — to assemble well-defined silver clusters stabilized by glutathione ligands through a thiol linkage. The clusters, which comprise Ag16 and Ag9 cores, are stable in aqueous solutions and show intense red and green luminescence, respectively. Furthermore, they efficiently inhibited the growth of multidrug resistant Pseudomonas aeruginosa in cell culture. This antimicrobial activity arises through the intracellular generation of reactive oxygen species. This paper reports a facile cyclic reduction–decomposition method for the aqueous-based synthesis of highly luminescent Ag nanoclusters (NCs) with tunable emissions. A reduction–decomposition–reduction cycle was used to modify the non-luminescent Ag NC intermediates with improved stability against subsequent etching by thiol ligands, which created a mild size-/structure-focusing (or etching) environment and produced highly luminescent Ag NCs with intense red (Ag16(SG)9) and green (Ag9(SG)6) emission. The highly luminescent Ag NCs also possessed superior antimicrobial properties against the multidrug-resistant bacteria Pseudomonas aeruginosa via generating a high concentration of intracellular reactive oxygen species.