Redox and Fluorophore Functionalization of Water-Soluble, Tiopronin-Protected Gold Clusters

Place-exchange and amide-forming coupling reactions represent two facile and efficient routes to poly-functionalization of water-soluble nanoparticles. In this paper, place-exchange and amide-forming coupling reactions with water-soluble tiopronin-MPCs are described and their products characterized by 1 H and 31 P NMR, capillary electrophoresis, electrochemistry, and fluorescence spectroscopy. Place-exchange reactions of ligands with tiopronin-MPCs yield products with about half of the ligand exchange expected on the basis of solution stoichiometry and a nonselective exchange and were not noticeably affected by steric encumbrances. Tiopronin-MPCs to which viologens are coupled (avg 36/MPC) adsorb as monolayers on Au electrodes as shown in electrochemical quartz crystal microbalance experiments. Multilayer adsorption occurs on long experimental time scales. The strong adsorption of the viologen-functionalized clusters is ascribed to increased interaction and stability in the viologen reduction products. Capillary electrophoresis experiments with tiopronin-MPCs and viologen-functionalized tiopronin-MPCs reveal a number of separable core size/charge state combinations. Fluorescence measurements show ∼50% quenching of fluorescein upon attachment (avg 3.7/cluster) to tiopronin-MPCs, relative to the monomer under the same conditions. The results of this paper provide a pathway to explore poly-functionalized water-soluble nanoparticles in a variety of applications, including their use as biosensors.