Upconversion and downconversion nanoparticles for biophotonics and nanomedicine

•UCNPs and DCNPs have become important with an exponential increase in published works.•Advantages in optical properties of these NPs suit the biomedical applications.•Imaging and biosensing are enhanced by the high resolution and low noise of these NPs.•Combining therapeutic elements and these NPs facilitate in vivo theranostics.•Study on the long-term toxicity of these NPs on human is needed before clinical use. In the last decade, upconversion and downconversion nanoparticles (UCNPs and DCNPs) have exhibited attractive unique optical properties and are used in various fields, including photonics, energy and chemical engineering. They recently showed potential in biophotonics and nanomedicine research, such as ultrasensitive biosensing and light-controlled cancer therapy. This review first summarizes the synthesis, chemical and optical properties and the functionalization of different UCNPs and DCNPs, and then focuses on recent advances in employing functional UCNPs and DCNPs for bio-imaging and -sensing in disease diagnostics. It also highlights the in-depth research of UCNPs and DCNPs for nanomedicine applications, especially both in vitro and in vivo cancer therapy, such as light-controlled drug and gene delivery. Finally, it provides concluding perspectives, challenges and future directions on the use of UCNPs and DCNPs in biomedical and clinical research. The objective of this review is to introduce UCNPs and DCNPs available in biomedical research to the general readers to facilitate further research and development of both fundamental sciences and applications.