Size Characterization of Glutathione-Protected Gold Nanoclusters in the Solid, Liquid and Gas Phases

Monolayer-protected metal nanoclusters belonging to the size regime between two and a few hundred atoms represent unique building blocks for new materials. Because of the extremely tiny size of such clusters (smaller than ∼3 nm in diameter) and the stabilizing layer which varies in size and composition, traditional techniques used for characterizing the size of large metal nanoparticles are not very reliable. In this work, we compare solid, liquid and gas phase size characterization methods for selected glutathione- (SG-) protected gold clusters (namely Au15(SG)13, Au18(SG)14, and Au25(SG)18). We also investigate the effect of nanocluster ion-pairing with bulky counterions on their size characterization. We show that X-ray powder diffraction, ion mobility mass spectrometry, and time-resolved fluorescence anisotropy can be a useful addition to tools available for ultrasmall nanoparticle characterization.