Synthesis of monodisperse [Oct4N(+)][Au25(SR)18(-)] nanoparticles, with some mechanistic observations

A single phase (THF) synthesis of monodisperse [Oct(4)N(+)][Au(25)(SR)(18)(-)] nanoparticles is described that yields insights into pathways by which it is formed from initially produced larger nanoparticles. Including the Oct(4)N(+)Br(-) salt in a reported single phase synthetic procedure enables production of reduced nanoparticles having a fully occupied HOMO molecular energy level (Au(25)(SR)(18)(-), as opposed to a partially oxidized state, Au(25)(SR)(18)(0)). The revised synthesis accommodates several (but not all) different thiolate ligands. The importance of acidity, bromide, and dioxygen on Au(25) formation was also assessed. The presence of excess acid in the reaction mixture steers the reaction toward making Au(25)(SR)(18); while bromide does not seem to affect Au(25) formation, but it may play a role in maintaining the -1 oxidation state. Conducting the nanoparticle synthesis and "aging" period in the absence of dioxygen (under Ar) does not produce small nanoparticles, providing insights into the pathway of reaction product "aging" in the synthesis solvent, THF. The "aging" process favors the Au(25)(-) moiety as an end point and possibly involves degradation of larger nanoparticles by hydroperoxides formed from THF and oxygen.