 Lanthanide-doped upconversion nanoparticles, UCNPs, are a promising class of materials for biotetection due to their unique properties such as non-autofluorescence, high chemical stability, deep penetration into tissues, long lifetimes, and reduced sample damage. Recent advances in this field include the development of UCNP-based probes for detecting inorganic ions, gases, reactive oxygen species, and thiols and hydrogen sulfide. These probes can be used for applications such as medical imaging, environmental monitoring, food safety testing, and biological sensors. This article was authored by Bingu and Qiqiu and Zhang.