Conventional fluorescent labels are of limited in vivo utility because short-wavelength radiation penetrates tissues poorly and excites strong background autofluorescence. Upconversion nanoparticles (UCN) convert low-energy exciting photons to visible emissions, resulting in deep tissue penetration and high detection sensitivity in vivo. Here, we report the synthesis and characterization of fluorescent UCN and demonstrate their use in molecular imaging in vitro and in vivo in small animals.
