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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)182+ and M2Au25(SC2Ph)182+. 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. [ABSTRACT FROM AUTHOR]

Monolayers of FePt nanoparticles (NPs) were coated with 10 nm-thick layers of Al2O3 by pulsed laser deposition (PLD). The Al2O3 coating prevents diffusion and stabilizes the NP monolayer against sintering. Annealing converts the FePt NPs from an alloy phase (A1) into intermetallic phases. Atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) shows that Pt-rich A1 Fe29Pt71 NPs convert into the FePt3 L12 intermetallic phase upon annealing and develop faceted morphologies. HAADF-STEM also reveals the presence of structural defects and Pt-rich shells. These results, in conjunction with prior studies of Al2O3 deposition by atomic layer deposition (ALD), demonstrate that coating arrays of FePt NPs with thin Al2O3 films by PLD or ALD imparts thermal stability and provides comparable results. [ABSTRACT FROM AUTHOR]

The article describes the use of polyethylene glycol ligands for high resolution electrospray ionization mass spectrometry measurements. The ligands were incorporated by ligand exchange into the monolayers of benzeneethanethiolate-protected Au nanoparticles. The authors present the high-resolution analysis of three main peaks in spectra acquired with different [Na+].

Intact molecular ions of the organothiolate-protected nanoparticle Au25(SCH2CH2Ph)18, including their isotopic resolution, can be observed at 7391 Da as 1- and 1+ ions in negative and positive mode, respectively, by MALDI-TOF mass spectrometry when using a tactic of threshold laser pulse intensities and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as matrix. Previous MALDI-TOF studies of Au nanoparticles using other matrices have encountered extensive fragmentation of nanoparticle as well as thiolate ligands. Absence of fragmentation enables precise determination of the distribution of mixed monolayer compositions on nanoparticles prepared by ligand exchange reactions and by synthesis using thiol mixtures. Reaction conditions producing mixed monolayers containing only one or a small number of usefully functional ligands can be readily identified. At increased laser pulse intensity, the first fragmentation step(s) for the Au25(SCH2CH2Ph)18 nanoparticle results in losses of AuL units and, in particular, loss of Au4(SCH2CH2Ph)4. [ABSTRACT FROM AUTHOR]