Stepwise Preparation of Spherical Gold Nanoparticles Passivated with Cationic Amphiphiles.

Spherical gold nanoparticles with cationic surfaces were prepared in hexadecyltrimethylammonium (CTA(+)) chloride (CTAC) and CTA(+) bromide (CTAB) solutions. In a CTAC solution, ascorbic acid reduction of gold ions (AuCl4(-)) induced spontaneous nucleation of gold clusters, which resulted in the formation of small gold nanoparticles (<5 nm). In a CTAB solution, the combination of ascorbic acid and AuBr4(-) induced low spontaneous nucleation, and therefore controllable crystal growth of seed particles was possible. To obtain uniform gold nanoparticles, seed particles (<5 nm) were first prepared in a CTAC solution using NaBH4 as a reducing agent. Subsequent growth reactions of the seeds in CTAB solutions were controllable to obtain gold nanoparticles with diameters ranging from 7 to 60 nm.