Electrospray Ionization Mass Spectrometry of Uniform and Mixed Monolayer Nanoparticles:  Au25[S(CH2)2Ph]18 and Au25[S(CH2)2Ph]18 - x (SR) x

• Published in: Journal of the American Chemical Society 2007-12, Vol.129 (51), p.16209-16215
• Main Authors: Tracy, Joseph B, Crowe, Matthew C, Parker, Joseph F, Hampe, Oliver, Fields-Zinna, Christina A, Dass, Amala, Murray, Royce W
• Format: Article
• Language: English
• Subjects: Nanoparticles; Spectrometry, Mass, Electrospray Ionization - methods
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja076621a

New approaches to electrospray ionization mass spectrometry (ESI-MS)with exact compositional assignmentsof small (Au25) nanoparticles with uniform and mixed protecting organothiolate monolayers are described. The results expand the scope of analysis and reveal a rich chemistry of ionization behavior. ESI-MS of solutions of phenylethanethiolate monolayer-protected gold clusters (MPCs), Au25(SC2Ph)18, containing alkali metal acetate salts (MOAc) produce spectra in which, for Na+, K+, Rb+, and Cs+ acetates, the dominant species are MAu25(SC2Ph)18 2+ and M2Au25(SC2Ph)18 2+. Li+ acetates caused ligand loss. This method was extended to the analysis of Au25 MPCs with mixed monolayers, where thiophenolate (−SPh), hexanethiolate (−SC6), or biotinylated (−S−PEG−biotin) ligands had been introduced by ligand exchange. In negative-mode ESI-MS, no added reagents were needed in order to observe Au25(SC2Ph)18 - and to analyze mixed monolayer Au25 MPCs prepared by ligand exchange with 4-mercaptobenzoic acid, HSPhCOOH, which gave spectra through deprotonation of the carboxylic acids. Adducts of tetraoctylammonium (Oct4N+) with −SPhCOO- sites were also observed. Mass spectrometry is the only method that has demonstrated capacity for measuring the exact distribution of ligand-exchange products. The possible origins of the different Au25 core charges (1−, 0, 1+, 2+) observed during electrospray ionization are discussed.