 Lanthanidoped upconversion nanoparticles, UCNPs, are a promising alternative to traditional fluorescent probes due to their unique properties such as non-autofluorescence, high chemical stability, deep penetration into tissues, long lifetimes, and reduced damage to samples. Since 2014, significant advances have been made in using these particles for bio-detection, including the detection of inorganic ions, gases, reactive oxygen species, and thiols and hydrogen sulfide. These developments have enabled the use of UCNPs as powerful tools for detecting biomarkers in various applications, such as medical diagnostics, environmental monitoring, food safety, and water quality control. This article was authored by Bingyu and Chichu and Zhang.