Size- and shape-controlled synthesis of colloidal gold through autoreduction of the auric cation by poly(ethylene oxide)-poly(propylene oxide) block copolymers in aqueous solutions at ambient conditions

Gold nanoparticles with an average diameter in the range 5-20 nm have been synthesized from hydrogen tetrachloroaureate (III) hydrate (HAuCl ₄·3H₂O) in air-saturated aqueous PEO-PPO-PEO block copolymer solutions at ambient temperature in the absence of any other reducing agent (PEO: poly(ethylene oxide), PPO: poly(propylene oxide)). The particle size was controlled by the block copolymer concentration and PEO and PPO block lengths. Our findings indicate that longer PEO blocks lead to an increase in particle size because of an increase in reaction activity. Adsorption of PO segments on gold nanoparticles seems to prevent particle growth from aggregation, and results in small particle size and high colloidal stability. An increase of the HAuCl₄ concentration causes a change in the particle shape from spherical to triangular or hexagonal nanoplates.