Gas-phase fragmentation in mass spectrometry informs the shape and topology of thiolated metal nanoclusters.

Abstract

Thiolated metal nanoclusters (NCs), comprising a metal core protected by outer-layer staple motifs, have attracted widespread interest due to their promising applications in catalysis, biosensing, and biomedicine. While mass spectrometry (MS) is widely used to determine the chemical composition of metal NCs via intact mass measurement, its potential for in-depth structural analysis via gas-phase fragmentation in tandem MS (MS/MS) has been relatively underexplored and mostly focused on the collision-induced dissociation (CID) technique involving gas collisions. Herein, we systematically compared the fragmentation pathways between CID and an alternative method, surface-induced dissociation (SID), using model Au NCs featuring distinct symmetries and structural motifs. We show that gas-phase fragments in CID and SID overall shared the common fragmentation behavior of breaking the outer staples of Au NCs, but with different preferences in the location of bond cleavages. In addition, SID showed characteristic Au losses in ellipsoidal Au NCs with trimeric staples; and fragmentation efficiency in CID and SID varied differentially according to the shape of the NCs. Building on these findings, we applied the method to study non-crystallizable Au-Ag NCs with heterogeneous stoichiometry. The SID fragmentation data helped locate the Au and Ag atoms, providing valuable insights into the synthetic pathway of the NCs, which was otherwise inaccessible experimentally. Together, these results support SID as a complementary MS/MS tool for metal NCs, with particular promise for heterogeneous systems like the mixed NCs that cannot be easily crystallized.

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