Fragmentation patterns of DNA-stabilized silver nanoclusters under mass spectrometry

DNA-stabilized silver nanoclusters (Ag_N-DNAs) are emitters with tuneable structures and photophysical properties. While understanding of the sequence-structure-property relationships of Ag_N-DNAs has advanced significantly, their chemical transformations and degradation pathways are far less understood. To advance understanding of these pathways, we analysed the fragmentation products of 21 different red and NIR Ag_N-DNAs using negative ion mode electrospray ionization mass spectrometry (ESI-MS). Ag_N-DNAs were found to lose Ag⁺ under ESI-MS conditions, and sufficient loss of silver atoms can lead to a transition to a lesser number of effective valence electrons, N₀. Of more than 400 mass spectral peaks analysed, only even values of N₀ were identified, suggesting that solution-phase Ag_N-DNAs with odd values of N₀ are unlikely to be stable. Ag_N-DNAs stabilized by three DNA strands were found to fragment significantly more than Ag_N-DNAs stabilized by two DNA strands. Moreover, the fragmentation behaviour depends strongly on the DNA template sequence, with diverse fragmentation patterns even for Ag_N-DNAs with similar molecular formulae. Molecular dynamics simulations, with forces calculated from density functional theory, of the fragmentation of (DNA)₂(Ag₁₆Cl₂)⁸⁺ with a known crystal structure show that the 6-electron Ag₁₆Cl₂ core fragments into a 4-electron Ag₁₀ and a 2-electron Ag₆, preserving electron-pairing rules even at early stages of the fragmentation process, in agreement with experimental observation. These findings provide new insights into the mechanisms by which Ag_N-DNAs degrade and transform, with relevance for their applications in sensing and biomedical applications.