Extensive use of FRET in biological imaging.

Förster resonance energy transfer (FRET) is a phenomenon used for bioimaging ranging from single molecules to in vivo scale. A large variety of organic dyes and fluorescent proteins are available for FRET probes. In this review, we introduce the representative pairs of FRET probes developed thus far. The efficiency of FRET is depending on the spectral overlap of donor emission and acceptor absorption, the orientation of donor and acceptor and their distance. For FRET-based indicators composed of fluorescent proteins, their orientation and dimeric property of donor and acceptor largely affect the FRET efficiency, indicating the effect for the performance of indicators. In addition, three major applications of FRET, including genetically encoded indicators, single-molecule FRET, and enhancement of chemiluminescent proteins, have been introduced and their functions have also been discussed.