On the Chemical Stability of DNA-Stabilized Silver Nanoclusters

DNA-stabilized silver nanoclusters (DNA-AgNCs) are atomically precise emitters, whose chemical stability is commonly defined by their stable optical response over time. Here, two isotopically pure versions (107Ag and 109Ag) of DNA2-[Ag16Cl2]8+ were mixed, and we demonstrate for the first time for DNA-AgNCs that silver atoms are continuously exchanged between individual clusters. This atom exchange was monitored by time-resolved mass spectrometry. Depending on the temperature, the exchange happens on a time scale of minutes to hours, while DNA2-[Ag16Cl2]8+ retains its ensemble spectroscopic features. Based on the time constants and the available crystal structure, we hypothesized the following exchange mechanism: formation of transient dimers followed by atom exchange between the two entities. A further and slower exchange of the swapped Ag atoms within the rest of the clusters occurs at a much slower time scale. Our findings shed new light on the meaning of chemical stability for this class of fluorophores and show that these systems can be structurally dynamic at the molecular level, while maintaining stable ensemble spectroscopic properties. Our results also explain why DNA-AgNCs are good sensors, as the dynamic nature of DNA-AgNCs allows for competing interactions, altering their optical response.