Deep-tissue photoacoustic tomography of Förster resonance energy transfer.

Förster resonance energy transfer (FRET) is a distance-dependent process that transfers excited state energy from a donor molecule to an acceptor molecule without the emission of a photon. The FRET rate is determined by the proximity between the donor and the acceptor molecules; it becomes significant only when the proximity is within several nanometers. Therefore, FRET has been applied to visualize interactions and conformational changes of biomolecules, such as proteins, lipids, and nucleic acids that cannot be resolved by optical microscopy. Here, we report photoacoustic tomography of FRET efficiency at a 1-cm depth in chicken breast tissue, whereas conventional high-resolution fluorescence imaging is limited to <0.1  cm. Photoacoustic tomography is expected to facilitate the examination of FRET phenomena in living organisms.