Surfactant-free Synthesis of Ultrasmall Gold Nanoclusters

The role of surfactant (most often tetraoctylammonium bromide) in the two-phase Brust gold nanocluster synthesis (Brust, W Walker, M.; Bethell, D. Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801) has been Unclear. The results of the surfactant-free synthesis of gold nanoclusters in methylene chloride employing NaBH4 a, a reducing agent with the Usual synthetic conditions such as excess thiol:gold ratio (>3) and ice-cold reaction temperature are reported. A nanocluster Mixture, similar to 5% yield with Au(I)SR by products, was obtained, The isolated Au nanoclusters were characterized by mass spectrometry, UV-visible, and nuclear magnetic resonance spectroscopy. MALDI-TOF mass spectrometry reveals the presence of mixtures of nanoclusters in the Au-16-Au-31 size range. Discrete features in the UV-visible spectrum Suggested the presence of small <2 nm particles. Relatively sharp peaks in NMR confirm the small size of the nanoclusters as suggested by MALDI-TOF MS. These results are in contrast to the commonly observed mixture of gold cores Such as 25, 38, 102, and 144. It is proposed that the surfactant plays a major role in the gold nanocluster synthesis by its influence on the solubility of Au(I) species. The observed nanoclusters were analyzed in terms of the, superatom complex model and found to contain 4 e(-), 5 e(-), 8 e(-), and 10 e(-) nanoclusters.