Reactivity of [Au25(SCH2CH2Ph)18]1- Nanoparticles with Metal Ions

We report reactivity of the gold nanoparticle [TOA(+)] [Au-25(SC2Ph)(18)](1-) (TOA(+) = tetraoctylammonium; SC2Ph = phenylethanethiolate = L; [Au-25(SC2Ph)(18)](1-) = Au25L181-) with Ag+, Cu2+, and Pb2+ ions. Titration of solutions of Au25L181- in CH2Cl2 with one and two equivalents of Ag+ produces changes in absorbance spectra with isosbestic points, and a titration curve break at 1:1 mol ratio, indicating a stoichiometric interaction. Similar effects are seen with Cu2+ and Pb2+ additions, but the break occurs at 0.5:1 mol ratio metal/nanoparticle. Changes in Au25L181- absorbance and fluorescence spectra are qualitatively similar to those accompanying oxidation of the Au25L181- nanoparticle anion, but the spectra of the stoichiometric products differ slightly according to the metal ion. Addition of higher excess of Ag+ or Cu2+ causes loss of characteristic [Au-25(SC2Ph)(18)](1-) UV-vis spectral fine structure and apparent irreversible refining into larger nanoparticles. Voltammetric currents for nanoparticle 0/-1 and +1/0 redox waves are depressed by Ag+ addition. Electrospray ionization mass spectra of products of addition of up to two equivalents Ag+ show prominent peaks for [Au-25(SC2Ph)(18)](1-) but also peaks corresponding to bimetal nanoparticles [Au24Ag(SC2Ph)(18)](2+), [Au23Ag2(SC2Ph)(18)](2+), and [Au22Ag3(SC2Ph)(18)](2+). We propose a redox model of reaction of the Au25L181- nanoparticle with metal ions, in which the Au25L181- nanoparticle acts as a reductant toward the metal ion, forming Au25M(SC2Ph)(18) adducts that become oxidatively dissociated in the mass spectral cationization environment to yield the bimetals observed.