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 singlemolecule 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.correlation ͉ photophysics ͉ silver nanoclusters ͉ single-molecule spectroscopy ͉ fluorescence intermittency W hile myriad dyes exist with varying photophysical properties (1, 2), organic dye-based single-molecule and even bulk in vivo imaging dynamics studies suffer from low probe brightness, poor photostability (3), and oxygen sensitivity (4). Advances in nanotechnology such as the use of quantum dots (5, 6) have ameliorated some of these issues but at the cost of toxicity (7), broad excitation (8, 9), power-law blinking (10-12), and large probe size (13,14). While quantum dots are readily excited with low-intensity sources, their fluorescence exhibits intermittency on all time scales (10-12), thereby causing problems when used for tracking or imaging studies. Arising from Auger processes (15), these photophysical dynamics are apparent at all excitation intensities and appear without characteristic times. While functionalization, large size (Ϸ10-20 nm in diameter), and cellular uptake are potential problems, the strong nonmolecular power-law fluorescence intermittency is a major drawback of these materials as single-molecule reporters (10-12). Recently, Ϸ35-nm-sized fluorescent nanodiamonds have also been reported as single-molecule emitters, but these also raise concerns about label size (16). Consequently, for both in vitro and in vivo single-molecule studies, fluorophores with high emission rates and excellent photostability must be identified that are completely devoid of blinking on all relevant time scales, while maintaining small overall sizes.By combining the virtues of chemistry and nanotechnology, we have developed few-atom, molecular-scale noble metal nanoclusters as a class of emitters that simultaneously exhibit bright, highly polarizable discrete transitions, good photostability, and small size, all within biocompatible scaffolds (17-20). Recent observations that DNA encapsulates Ag nanoclust...

With discrete states, several-atom Ag(n) nanoclusters exhibit molecule-like behavior with strong visible fluorescence and robust optical properties. This new class of single-molecule fluorophores has been created and electrically contacted in thin films to produce the first electroluminescent single molecules. A direct reporter of nanoscale charge injection and transport through discrete energy levels, bright Ag(n) electroluminescence has been harnessed to create single-molecule light-emitting diodes (LEDs) and optoelectronic logic gates and even to demonstrate full addition operations. These experiments utilizing the small size and quantum behavior of individual Ag nanoclusters usher in the new field of single-molecule optoelectronics.

Individual, strongly electroluminescent Agn molecules (n ‫؍‬ 2ϳ8 atoms) have been electrically written within otherwise nonemissive silver oxide films. Exhibiting characteristic single-molecule behavior, these individual room-temperature molecules exhibit extreme electroluminescence enhancements (>10 4 vs. bulk and dc excitation on a per molecule basis) when excited with specific ac frequencies. Occurring through field extraction of electrons with subsequent reinjection and radiative recombination, single-molecule electroluminescence is enhanced by a general mechanism that avoids slow bulk material response. Thus, while we detail strong electroluminescence from single, highly fluorescent Agn molecules, this mechanism also yields strong ac-excited electroluminescence from similarly prepared, but otherwise nonemissive, individual Cu nanoclusters.

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