Etching colloidal gold nanocrystals with hyperbranched and multivalent polymers: A new route to fluorescent and water-soluble atomic clusters

We report a novel ligand-induced etching process in which hyperbranched and multivalent coordinating polymers such as polyethylenimine (PEI) react with preformed gold nanocrystals to form atomic gold clusters that are soluble in water and are highly fluorescent upon UV light excitation. These as-prepared fluorescent clusters appear to be in an oxidized electronic state, with their excitation and emission maxima located at 421 and 505 nm, respectively. Upon treatment by strong reducing agents such as NaBH4, the cluster's excitation and emission peaks are shifted to 353 and 445 nm. Electrospray ionization (ESI) mass spectrometry data indicate that the light-emitting species are small atomic nanoclusters consisting of only 8 gold atoms (Au8). There is also evidence to suggest that a ligand-to-metal electronic transition in the ultraviolet is superimposed on the cluster absorption spectra, but it is not directly related to the intrinsic fluorescent properties of these atomic clusters. These surprising results suggest that ligand-induced etching could be used to prepare new types of fluorescent metal nanoclusters for applications in catalysis and biological labeling.