Recent advances in luminescence properties of lanthanide-doped up-conversion nanocrystals and applications for bio-imaging, drug delivery, and optosensing

The lanthanide-doped up-conversion (UC) nanocrystals exhibit unique luminescence properties, such as the conversion of near-infrared long wavelength excitation into short visible wavelength emission. These UC nanoparticles symbolize a new class of phosphor materials. Recently, lanthanide-doped UC nanocrystals have been developed as a biological class for various applications including bio-imaging, drug delivery, and sensing which require low auto-fluorescence background, large anti-Stokes shifts, sharp emission bandwidths, high resistance to photo-bleaching, high penetration depth tissue, and high temporal resolution. Furthermore, these UC nanocrystals show promising potential for improving the selectivity and sensitivity compared with those in common traditional methods. This review focuses on the fundamental mechanism of UC optical processes, synthetic methods with focus on the different techniques used, energy transfer mechanisms of lanthanide-doped UC nanocrystals, and in particular recent developments in biological applications, such as bio-imaging, drug delivery, fluorescence resonance energy transfer sensing, and heavy metal ion detections.