Stable oligomeric clusters of gold nanoparticles: preparation, size distribution, derivatization, and physical and biological properties.

Reducing dilute aqueous HAuCl4 with NaSCN under alkaline conditions produces 2–3 nm diameter yellow nanoparticles without the addition of extraneous capping agents. We here describe two very simple methods for producing highly stable oligomeric grape-like clusters (oligoclusters) of these small nanoparticles. The oligoclusters have well-controlled diameters ranging from ∼5 to ∼30 nm, depending mainly on the number of subunits in the cluster. Our first [“delay-time”] method controls the size of the oligoclusters by varying from seconds to hours the delay time between making the HAuCl4 alkaline and adding the reducing agent, NaSCN. Our second [“add-on”] method controls size by using yellow nanoparticles as seeds onto which varying amounts of gold derived from “hydroxylated gold”, Na+[Au(OH4–x)Clx]−, are added-on catalytically in the presence of NaSCN. Possible reaction mechanisms and a simple kinetic model fitting the data are discussed. The crude oligocluster preparations have narrow size distributions, an...