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− = Au 25 L 18 1− ) with Ag +, Cu 2+, and Pb 2+ ions. Titration of solutions of Au 25 L 18 1− in CH 2 Cl 2 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 Cu 2+ and Pb 2+ additions, but the break occurs at 0.5:1 mol ratio metal/nanoparticle. Changes in Au 25 L 18 1− absorbance and fluorescence spectra are qualitatively similar to those accompanying oxidation of the Au 25 L 18 1− nanoparticle anion, but the spectra of the stoichiometric products differ slightly according to the metal ion. Addition of higher excess of Ag + or Cu 2+ 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 [Au 24 Ag(SC2Ph) 18 ] 2+, [Au 23 Ag 2 (SC2Ph) 18 ] 2+, and [Au 22 Ag 3 (SC2Ph) 18 ] 2+ . We propose a redox model of reaction of the Au 25 L 18 1− nanoparticle with metal ions, in which the Au 25 L 18 1− nanoparticle acts as a reductant toward the metal ion, forming Au 25 M(SC2Ph) 18 adducts that become oxidatively dissociated in the mass spectral cationization environment to yield the bimetals observed.