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

Abstract

We report reactivity of the gold nanoparticle [TOA+] [Au25(SC2Ph)18]1− (TOA+ = tetraoctylammonium; SC2Ph = phenylethanethiolate = L; [Au25(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 [Au25(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 [Au25(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.