Tailored lanthanide-doped upconversion nanoparticles and their promising bioapplication prospects

Lanthanide-doped upconversion nanoparticles (UCNPs) which could show unique upconversion photoluminescence (UCPL) have attracted considerable attention due to their excellent chemical and optical characteristics. Compared with down-shifting luminescence, UCPL shows its bright future in the bioapplication where sequential absorption of multiple low-energy photons (such as near-infrared (NIR) excitation) by the ladder-like energy levels of the lanthanide ions (Ln3+) leads to the production of higher energy photons (such as ultraviolet, visible light). UCPL shows a variety of advantages such as large anti-Stokes shifts, sharp emissions, long luminescence lifetimes, and high resistance to photobleaching. Furthermore, NIR irradiation has lower photo damage effect, a large penetration depth in tissues, and at the same time, it can avoid the auto-fluorescence interference of biological sample and light scattering phenomenon, which can reduce the background light and improve the signal-to-noise ratio. Thus, UCNPs have emerged as more appropriate nanomaterials for bioapplications. To date, many scientists have focused on the research of the bioapplications of UCNPs, including bioimaging, drug release, and therapies after the special surface modification. In this critical review, recent advances regarding the mechanism, synthesis, modification, and promising bioapplications of UCNPs are reviewed. In particular, the studies related to sensing and bioimaging (UCPL, MR, CT, PET and SPECT), drug release, and therapies (photothermal therapy, photodynamic therapy, and radiotherapy) are presented in detail. For specific bioapplication, tailored UCNPs can be designed and synthesized according to the different synthesis and modification methods as summarized in this review. Finally, we discuss the prospects and challenges of UCNPs in the biomedical and biotechnological fields.