Triethylamine Solution for the Intractability of Aqueous Gold-Thiolate Cluster Anions: How Ion Pairing Enhances ESI-MS and HPLC of aq-Aun(pMBA)p

Herein we disclose methods :that greatly improve the solution- and gas-phase handling properties of larger aqueous-phase gold thiolate clusters, which previously presented extreme technical obstacles to molecular analysis and size control, even as they have enjoyed ever-wider applications in materials science and biomedicine. The methods are based upon an analogy between the polyacidic surface structure of the pMBA-protected clusters (pMBA = p-mercaptobenzoic acid) and that of oligonucleotides. A volatile ion-pairing reagent, TEA = triethylamine, greatly improves solution-phase stability near neutral pH and thus facilitates both electrospray generation of the gas-phase ions and the in-line reversed phase ion-pairing HPLC-ESI-MS approach to analyzing complex mixtures of Au-pMBA oligomers and clusters. Previously anticipated but never established compounds,; including Au-36(pMBA)(24), are thereby demonstrated: These results are in accord with recent theoretical simulations Of ion pairing of model. Au-144(pMBA)(60) clusters in aqueous solutions. This advance complements our recent work On their nonaqueous (hydrophobic) counterparts, in which redox electrochemistry is sufficient to support the efficient LC-ESI processes,,enabling various precise measurements on the intact molecular ions. Here, we report (i) novel conditions for enhanced ESI generation of polyanions of the aqueous clusters and by extension (ii) a notably improved method by which mixtures of these clusters may be successfully separated and detected by ion-pair reversed-phase HPLC-MS.