Xiong, L. et al. Nat. Commun. 3, 1193 (2012).

Imaging in vivo is complicated by tissue autofluorescence and light scattering; there is much interest in near-infrared (NIR) probes to reduce these effects. Xiong et al. report polymer nanoparticles that incorporate two steps of energy transfer for in vivo imaging with a signal-to-noise ratio an order of magnitude higher than that of standard fluorescence imaging. A variant of luciferase is the bioluminescence resonance energy transfer (BRET) donor, the nanoparticle itself is the BRET acceptor and fluorescence resonance energy transfer (FRET) donor, and an NIR dye is the FRET acceptor. The resulting particles are coated with poly(ethylene glycol) for increased solubility and can be conjugated with tumor-targeting peptides. The particles self-illuminate upon application of the luciferase substrate coelenterazine, and they do not contain potentially toxic metals. Using these particles, Xiong et al. imaged the lymphatic system and tumors as small as 2–3 millimeters in the mouse.