Reaction controlled kinetic assembly of small gold nanoclusters with high NIR extinction

Nanoclusters with sizes of ~50nm with high NIR extinction at wavelengths beyond 800 nm are of interest in various fields including biomedical optical imaging, microelectronics, plasmonic sensors, and catalysis. Herein we report gold nanoclusters with hydrodynamic diameters of ~50 nm composed of ~10 nm primary particles. The kinetically controlled assembly of clusters occurs simultaneously with the reaction to synthesize the primary particles. The clustering is induced by attractive van der Waals forces that dominate over the steric and electrostatic repulsive forces present. Stability is provided using a single, biocompatible polysaccharide in either carboxymethyl dextran or dextran. High NIR shifts of the surface Plasmon resonance are achieved through close interparticle spacings of primary particles, deviations in morphology from that of a sphere of primaries, and the surface roughness that results from the clustering process. The cluster size is mediated by controlling the relative nucleation and growth rates of primary particles using a moderate reducing agent in NH2OH and glucose at pH 8.7. It will be shown that cluster size is also dependent on Au concentrations in solution. Maintaining low Au concentrations will allow for smaller clusters. In particular, the small size and high NIR extinction at longer wavelengths (800-1100 nm) makes these particles of interest for optical imaging applications in biology, as particles with a hydrodynamic diameter of ~50 nm have long blood lifetimes.