Synthesis and self-organization of Au nanoparticles

Monodisperse (diameter 10 nm) Au nanoparticles have been synthesized by the reduction of AuCl3 at high temperatures, in commercial oleyl amine, in the presence of tri-octyl phosphine oxide (TOPO). The oleyl amine molecule has multiple roles, acting as high-boiling-point solvent, capping agent and reducing agent. The Au nanocrystalline particles have been studied by powder x-ray diffraction (XRD) and transmission electron microscopy (TEM). The nanoparticles can disperse well in non-polar solvents such as hexane, toluene and chloroform. The as-received Au nanoparticles, capped with the alkylamine-TOPO molecules, form two-dimensional superlattices via solvent evaporation. Moreover, they can be converted into a water-soluble derivative via a simple procedure based on the formation of a physisorbed layer of an ionic (cationic or anionic) surfactant around the capped surface. The charged particles can assemble in aqueous solution into three-dimensional structures via electrostatic interactions.