Near-infrared light-mediated rare-earth nanocrystals: recent advances in improving photon conversion and alleviating the thermal effect

With the rapid development of nanotechnology, the unique rare-earth lanthanide-doped upconversion nanocrystals (UCNs), which can convert tissue-penetrable near-infrared (NIR) photonic irradiation into ultraviolet, visible, and NIR emissions, have a significant potential in bioimaging, diagnosis, and therapy, as well as in photovoltaic systems and optical data storage. Despite the promising achievements made in the past decade, critical challenges associated with low upconversion efficiencies and the overheating effect induced by NIR laser-irradiation still remain in the biomedical fields. In high demand are more well-defined material design and unique structural modifications that are capable of solving these technical concerns and promoting such promising NIR light-mediated upconversion nanocrystals for their further application in the medical sciences. Recent advances in upconversion nanomaterials have witnessed a tremendous development towards enhancing their photon conversion efficiency, which provides great opportunities in expanding the potential of the UCNs in bioimaging diagnosis and anticancer therapy. Hence, this review is mainly focused on summarizing the fundamental principles and strategies that improve upconversion luminescence and the approaches to reduce the local thermal effect on the basis of a rational design of UCNs. In addition, the future perspectives in the development of UCNs for biomedical applications are also proposed.