A Critical Evaluation of the Role of the Precursor Complex and Counterions in Synthesis of Gold Nanoparticles in Micellar Media

We demonstrate here that the gold nanoparticles (AuNPs) obtained in micellar solutions are susceptible to the precursor complex nature and specific counterion effects controlling the reaction kinetics, particles size and shape. The tested approach consisted in reducing the HAuCl4 complex with ascorbic acid or citric acid in dodecyltrimethylammonium (DTA) solutions, at 318 K, in the presence of bromide (Br), methanesulfonate (Ms), trifluoroacetate (TFA) or trifluoromethanesulfonate (Tf) ions. Bromide ions replaced the chloride ions in the precursor gold complex, and no nanoparticle formation could be observed in DTA-Br media, in the absence of seeds. In contrast, scanning electron microscopy images and atomic force microscopy measurements showed that quasi-spherical AuNPs were formed in DTA-Ms and DTA-TFA solutions, while flat disk-like or triangular AuNPs were obtained in DTA-Tf solutions, with no need of adding seeds in all such cases. The results indicated that the reduction of the precursor gold complex takes place at the micellar interface and that the specific counterion effects should be taken into account for controlling nanoparticles formation in micellar media.