Fe 3+ -codoped ultra-small NaGdF 4 :Nd 3+ nanophosphors: enhanced near-infrared luminescence, reduced particle size and bioimaging applications.

Small-sized lanthanide-doped nanoparticles (NPs) exhibiting superior near-infrared (NIR) luminescence properties are highly desired for bioimaging applications. Herein, Fe 3+ ions are codoped in NaGdF 4 :Nd 3+ nanocrystals via a simple coprecipitation method, which can simultaneously reduce the particle size and enhance the downconverting NIR luminescence of the NPs. The NIR luminescence intensity reaches the maximum for the obtained sub-5 nm NPs when the doping concentration of Fe 3+ is tuned to 20 mol%, which is ∼1.7 times higher than that of the pristine 8.7 nm NPs without Fe 3+ doping. After being modified with targeting molecules, the ultra-small NaGdF 4 :Nd 3+ ,Fe 3+ NPs were successfully applied as luminescent probes for targeted NIR imaging of tumors in biological tissues. Moreover, they also show great potential as a high contrast agent for T2-weighted MRI imaging.