Deep Tissue Imaging with Highly Fluorescent Near-Infrared Nanocrystals after Systematic Host Screening

Photoluminescent inorganic nanoparticles are attractive as bioimaging contrast agents because they do not degrade by photobleaching and do not suffer from concentration quenching as clinically applied organic dyes. Here, for the first time, a large variety of oxide, phosphate, and vanadate nanocrystals doped with Nd 3+ are systematically examined and compared as down-converting photoluminescent contrast agents to understand underlying physical properties and to identify the brightest composition. These inorganic crystals are particularly attractive for bioimaging in the near-infrared (NIR) window, where absorption and scattering by human tissue are reduced drastically. Through close control of their crystal size, the resulting fluorescence properties are quantitatively compared under NIR excitation. Most interestingly, BiVO 4 doped with Nd 3+ is shown to be the most efficient composition. Its application as a photoluminescent NIR imaging contrast agent is demonstrated ex vivo with chicken skeletal muscle and bovine liver tissues. Under a harmless laser power density (0.2 W/cm 2 ), fluorescent BiVO 4 particles could be clearly detected at an injection depth of 20 mm by a simple commercial camera.