Gold nanoparticles coated with stabilizing thiolate monolayers of N-(2-mercaptopropionyl)glycine (tiopronin) and N, N-trimethyl(undecylmercapto)ammonium quench the fluorescence of [Ru(bPY)(3)](2+) in aqueous solution. The tiopronin monolayer protected clusters (MPCs) exert energy-transfer quenching by both collisional quenching and by static quenching in electrostatic complexes formed between [Ru(bPY)(3)](2+) and the tiopronin carboxylate groups. The electrostatic binding is pH-dependent and can be released by addition of electrolyte (e.g., K+, Bu4N+, and Ca2+ salts, which are increasingly effective in the order given). The tiopronin MPC quenching of [Ru(bpY)(3)](2+) fluorescence follows Stern-Volmer relations for plots of F-o/F against MPC concentration and F-o/F against electrolyte concentration at constant MPC concentration. Quenching and electrolyte binding constants were derived. MPCs with average Au core diameters of 2.2 nm (average) exhibited quenching constants that are 20-fold larger than those for 1.8-nm diameter.