Ion mobility–tandem mass spectrometry of bulky tert-butyl thiol ligated gold nanoparticles

Gold nanoparticles (AuNPs) synthesized in the 1–3 nm range have a specific number of gold core atoms and outer protecting ligands. They have become one of the “hot topics” in recent decades because of their interesting physical and chemical properties. The characterization of their structures is usually achieved by crystal X-ray diffraction although the structures of some AuNPs remain unknown because they have not been successfully crystallized. An alternative method for studying the structure of AuNPs is electrospray ionization–ion mobility–tandem mass spectrometry (ESI-IM-MSMS). This research evaluated how effectively ESI-IM-MSMS using the commercially available Waters Synapt XS instrument yielded useful structural information from two AuNPs; Au₂₃(S-tBu)₁₆ and Au₃₀(S-tBu)₁₈. The study used the maximum range of available collision energies along with ion mobility separation to measure the energy-dependence of the product ions and their drift times which is a measure of their spatial size. For Au₂₃(S-tBu)₁₆, the dissociation gave the masses of the outer protecting monomeric [RS–Au–SR] and trimeric [SR–Au–SR–Au–SR–Au–SR] staples where R = tBu, and complete dissociation of the outer layer Au and tBu groups to reveal the Au₁₅S₈ core. For Au₃₀(S-tBu)₁₈, the dissociation products was primarily through the loss of the partial ligands S-tBu and tBu from the outer protecting layer and the loss of single Au₄(S-tBu)₄ unit. These results showed the that ESI-IM-MSMS analysis of the smaller Au₂₃(S-tBu)₁₆ gave information on all it major structural components whereas for Au₃₀(S-tBu)₁₈, the overall structural information was limited to the ligands of the outer layer.