Although there are many types of fluorophores available for biological studies, single-molecule studies and in vivo imaging still can be challenging with today’s options. Fluorophores based on organic molecules are not always bright enough, they exhibit photosensitivity, and they may be sensitive to environmental oxygen levels. Quantum dots offer another option, but their blinking fluorescence makes tracking studies difficult, and they are also large (10 to 20 nm).In the May 22 online issue of PNAS, researchers at Georgia Institute of Technology in Atlanta report on water-soluble, DNA-encapsulated silver nanoclusters that overcome some of the problems with organic fluorophores and quantum dots. The nanoclusters consist of silver bound to single-stranded DNA (see figure) and have an excitation maximum of ~650 nm and an emission maximum of ~700 nm. In addition, the biocompatible emitters are the size of only a few atoms.The researchers, led by Robert M. Dickson, tested aspects of the nanoclusters and found that they had bright and photostable emission when used in single-molecule and bulk experiments. They emitted more than 109 photons before photobleaching, and experiments showed them to be at least twice as bright as Cy5.29. The nanoclusters exhibited almost no blinking from 0.1 to >1000 ms — timescales relevant for experiments. The researchers believe that these nanoclusters could allow in vitro, and possibly in vivo, experiments at faster as well as longer timescales.