Small, water-soluble, ligand-stabilized gold nanoparticles synthesized by interfacial ligand exchange reactions

A general synthetic approach leading to well-defined, water-soluble gold nanoparticles is described that involves a simple, interfacial ligand exchange reaction between a 1.4 nm phosphine-passivated precursor and an anionic or cationic thiol-containing ligand. We demonstrate the utility of this route by synthesizing water-soluble gold nanoparticles that are stabilized by either an anionic ligand (2-mercaptoethanesulfonate), a cationic ligand (2-(dimethylamino)ethanethiol hydrochloride), or a mixture of both ionic and phosphine ligands. Although the course of the ligand exchange process depends on the nature of the incoming ligand, each of these nanoparticle products retain the small core size and narrow size distribution of the starting particle (1.4 +/- 0.4 nm). The stabilities of these nanoparticles to elevated temperature, extremes of pH, and added salt are reported and found to depend on the nature of the exposed headgroups on the ligand shell. Salt-induced aggregation is not observed in any of the cases investigated. Resistance to aggregation is attributed to the protective nature of the ligand shell.