Aggregation-resistant water-soluble gold nanoparticles.

Stable, water-soluble gold nanoparticles, Au NPs, having an average diameter of ca. 4 nm, were prepared using place exchange reactions. The nanoparticles, capped with novel zwitterionic disulfide ligands, showed remarkable stability in saline media with salt concentrations as high as 3.0 M. Similarly, the Au NPs did not precipitate out of solution when charged polyelectrolytes or biopolymers were added, indicating the absence of nonspecific interactions. The stability and degree of association of Au NPs were characterized using UV-vis absorption spectroscopy, quasi-elastic light scattering, and surface-enhanced Raman scattering.