Background: Upconverting nanophosphors (UCNPs) are composed of certain nanosized inorganic host materials doped with specific rare earth ions. They exhibit the phenomenon of photon upconversion, whereby the sequential absorption of two or more low energy photons is followed by luminescent emission of multiple photons with higher energy. In addition, they have other attractive optical features such as NIR-absorption, sharp emission bands, exceptional photostability and high quantum yield. These properties make UCNPs lucrative probes for number of optical applications, such as imaging, sensing and theranostics. Overview: UCNPs have been used as specific and ultrasensitive probes for sensing of variety of analytes, such as gas molecules, metal ions, fine particles, pH and a number of molecules, biochemicals and macromolecules. Their absorption and emission in the NIR range facilitates the background free biosensing in situ, in vitro and in vivo. Their multicoloured emissions allow a suitable emission band to serve as a donor for energy transfer to another attached optical probe, while a separate emission band to serve as an internal reference in ratiometric sensing applications. This comprehensive review provides an in-depth exploration of the versatile applications of UCNPs in the optical sensing of various analytes based on energy transfer mechanisms, with emphasis on the recent developments. . The focus is on synthesizing UCNPs, surface functionalization, and their effective utilization especially in detecting gas molecules, pH changes, ions, free radicals, and disease biomarkers. Through a meticulous examination of the use of UCNPs, this review aims to contribute to a deeper understanding of their potential and pave the way for future advancements and challenges in optical sensing.
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