Surface Properties of Water-Soluble Glycine-Cysteamine-Protected Gold Clusters

We report the synthesis of water-soluble, nearly monodisperse glycine-cysteamine (Gly-CSA) gold monolayer protected clusters (MPCs) via base deprotection of Fmoc-Gly-CSA MPCs. The resulting Gly-CSA MPCs, which have terminal primary amine groups, are fully characterized by (1)H and (13)C NMR, UV-vis spectroscopy, and TEM, and their surface properties were probed by dynamic light scattering and acid-base titrations. The characterization methods indicate that the as-synthesized particles are nearly monodisperse with an average particle size of 1.8 +/- 0.3 nm, but are only stable to aggregation in water at pHs of 4 and below. Acid-base titrations of the Gly-CSA MPCs show that the primary ammonium groups have a pK(a) of approximately 5.5, which is several orders of magnitude lower than the pK(a2) for the ammonium group of glycine (9.6). Thus, the particles are only partially protonated at intermediate pH's, which then drives the aggregation of the nanoparticles via hydrogen-bond formation. Dynamic light scattering results confirm the pH-driven aggregation of the nanoparticles, and studies with ninhydrin confirm that the primary amine groups are reactive and have potential for further functionalization. These results show that amine-terminated MPCs can be synthesized; however, their aggregation at intermediate pH's can limit their utility as building blocks for multifunctional nanoparticle syntheses.