On the Chemical\nStability of DNA-Stabilized Silver\nNanoclusters

DNA-stabilized silver\nnanoclusters (DNA-AgNCs) are atomically precise\nemitters, whose chemical stability is commonly defined by their stable\noptical response over time. Here, two isotopically pure versions (<sup>107</sup>Ag and <sup>109</sup>Ag) of DNA<sub>2</sub>-[Ag<sub>16</sub>Cl<sub>2</sub>]<sup>8+</sup> were mixed, and we demonstrate for the\nfirst time for DNA-AgNCs that silver atoms are continuously exchanged\nbetween individual clusters. This atom exchange was monitored by time-resolved\nmass spectrometry. Depending on the temperature, the exchange happens\non a time scale of minutes to hours, while DNA<sub>2</sub>-[Ag<sub>16</sub>Cl<sub>2</sub>]<sup>8+</sup> retains its ensemble spectroscopic\nfeatures. Based on the time constants and the available crystal structure,\nwe hypothesized the following exchange mechanism: formation of transient\ndimers followed by atom exchange between the two entities. A further\nand slower exchange of the swapped Ag atoms within the rest of the\nclusters occurs at a much slower time scale. Our findings shed new\nlight on the meaning of chemical stability for this class of fluorophores\nand show that these systems can be structurally dynamic at the molecular\nlevel, while maintaining stable ensemble spectroscopic properties.\nOur results also explain why DNA-AgNCs are good sensors, as the dynamic\nnature of DNA-AgNCs allows for competing interactions, altering their\noptical response.