Fluorescence temperature sensing of NaYF 4 :Yb 3+ /Tm 3+ @NaGdF 4 :Nd 3+ /Yb 3+  nanoparticles at low and high temperatures.

NaYF 4 :Yb 3+ /Tm 3+ @NaGdF 4 :Nd 3+ /Yb 3+ upconversion nanoparticles were prepared using a solvothermal method, and the effects of key factors such as the content of sensitiser Nd 3+ and Yb 3+ on their luminescence properties were investigated. The nanoparticles are homogeneous in size and well dispersed. Under 808 nm excitation, it can produce strong upconversion fluorescence. At the same time, the nanoparticles have good temperature sensing properties at the thermally coupled energy levels of 700 and 646 nm for Tm 3+ . Using its fluorescence intensity ratio, accurate temperature measurements can be performed, and it has been found that it exhibits different temperature sensing properties in low and high-temperature regions. The maximum relative sensitivity was found to be 0.88% K -1 and 1.89% K -1 for the low-temperature region of 285-345 K and the high-temperature region of 345-495 K. The nanoparticles were applied to the internal temperature measurement of lithium batteries and the actual high-temperature environment, respectively, and were found to have good temperature measurement performance.