Strongly emissive individual DNA-encapsulated Ag nanoclusters as single-molecule fluorophores

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Abstract
The water-soluble, near-IR-emitting DNA-encapsulated silver nanocluster presented herein exhibits extremely bright and photostable emission on the single-molecule and bulk levels. The photophysics have been elucidated by intensity-dependent correlation analysis and suggest a heavy atom effect of silver that rapidly depopulates an excited dark level before quenching by oxygen, thereby conferring great photostability, very high single-molecule emission rates, and essentially no blinking on experimentally relevant time scales (0.1 to >1,000 ms). Strong antibunching is observed from these biocompatible species, which emit >10 9 photons before photobleaching. The significant dark-state quantum yield even enables bunching from the emissive state to be observed as a dip in the autocorrelation curve with only a single detector as the dark state precludes emission from the emissive level. These species represent significant improvements over existing dyes, and the nonpower law blinking kinetics suggest that these very small species may be alternatives to much larger and strongly intermittent semiconductor quantum dots.