(439c) Sub 50 Nm Au Nanoclusters With Absorbance Up to 1200 Nm Made By Favoring Kinetically Versus Equilibrium Favored Morphologies

Plasmonic nanoparticles with high NIR absorbance are of great interest in fields including biomedical optical imaging, microelectronics, plasmonic sensors and catalysis. Herein we synthesize 35 to 50 nm Au nanoclusters coated with carboxymethyldextran (CMD) with high NIR absorbance at wavelengths ranging from 700 nm to 1200 nm. Nanoclusters, composed of Au primary particles with diameters of ~ 10 nm, were assembled by finely tuning the rate of reduction of Au with pH in the presence of a polymeric stabilizer. To limit extemely fast nucleation and growth, we utilize a low Au precursor concentration together with a mild reducing agent. Strong effects of pH on NIR absorbance are observed and can be understood in terms of growth processes (kinetic vs. thermodynamic) and the resulting particle morphologies. As pH is decreased,  the growth becomes more kinetically controlled leading to “finger-like” protrusions that increse the NIR extinction markedly. At higher pH with slower growth, a more thermodynamically controlled parthway is dominant with much smaller protrusions and thus much weaker NIR extinction. In the case of two iterations of growth, the additional protrusions and asymmetry from the sperhical boundary condition lead to even greater NIR absorbace from multi-polar interactions.   The particles also have a high negative zeta potential, providing good colloidal stability.