Advances in Synthesis Strategies for Lanthanide-Based NaYF4 Upconversion Nanocrystals and Their Applications in Ratiometric Thermometry

The optical characteristics and varied applications of lanthanide-doped NaYF4 upconversion nanocrystals have received considerable interest in recent years, such as in ratiometric thermometry. This review thoroughly examines the various synthesis processes utilized in producing these nanocrystals and their application in temperature sensing. Synthesis of NaYF4 upconversion nanocrystals is a complex procedure that requires careful management of dopant concentrations, crystal phase, size, and shape. The distinctive luminescent characteristics of lanthanide ions, which facilitate the transformation of photons with low energy into emissions with higher energy, render NaYF4 nanocrystals suitable for ratiometric thermometry applications. We explore the fundamental concepts underlying upconversion luminescence in developing ratiometric temperature sensors. In this discourse, we examine the selection of lanthanide dopants, the mechanics underlying their energy transmission, and the development of customized sensor architectures. This review covers the recent progress and utilization of NaYF4 upconversion nanocrystals in ratiometric thermometry, including diverse areas such as biological temperature detection, environmental surveillance, and materials research. We evaluate the obstacles and potential advancements in this domain, specifically emphasizing approaches to improving temperature sensors' precision, responsiveness, and applicability based on upconversion.