Gold Nanocluster Prospecting via Capillary Liquid Chromatography-Mass Spectrometry: Discovery of Three Quantized Gold Clusters in a Product Mixture of "2 nm Gold Nanoparticles"

A nonaqueous reversed phase liquid chromatography-mass spectrometry (LC-MS) method has been developed for extremely hydrophobic MPCs (monolayer-protected clusters), and has been applied to the efficient separation of gold-dodecanethiolate (ddt) assemblies, leading to the identification of three dodecanethiolate-protected gold clusters, Au₁₃₀(ddt)₅₀, Au₁₃₇(ddt)₅₆, and Au₁₄₄(ddt)₆₀, as prominent components of a commercial product of nominally 2 nm (core-diameter) protected gold nanoparticles obtained from nanoComposix, Inc. Various components were separated, according to hydrophobic character, using a linear gradient of methanol-dichloromethane mobile phases, on a C₁₈ HPLC column. Varying concentrations of mobile-phase modifier (triethylammonium acetate) were compared for effect on chromatographic peak shape and cluster retention. Positive electrospray ionization (ESI) was used to ionize all components in the sample. LC separation prior to inline + ESI-MS detection facilitated sample analysis via production of simplified mass spectra for each eluting cluster species and provided insight into the relative polarity of the clusters shown here. UV-vis detection facilitated method development and allowed determination of nonionizing, and/or polydisperse components.