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.