Non-contact optical temperature sensors are highly sought after by researchers due to their satisfactory temperature resolution (δ(T) < 0.1 °C), high relative thermal sensitivity (S_r > 1% °C^-1), fast temporal response (t < 0.1 s), and long-term optical stability. In this study, NaYF₄:Yb³⁺/Ho³⁺/Tm³⁺ upconversion nanoparticles were prepared by a solvothermal method, and their crystal structure, microscopic morphology, and luminescence mechanism, together with the temperature sensing properties of the specimens, were investigated. Under 980 nm laser excitation, the specimens exhibited strong upconversion luminescence, and the emission peaks corresponded to the characteristic energy level jumps of Ho³⁺ and Tm³⁺, respectively. The temperature-dependent luminescence spectra of the samples were investigated based on the fluorescence intensity ratio (FIR) technique over a temperature gradient of 295-495 K. The samples are based on thermally coupled energy levels (TCLs: ¹G_4(1,2) → ³H₆(Tm³⁺)) and non-thermally coupled energy levels (NTCLs: ³F₃ → ³H₆(Tm³⁺) and ⁵F₃ → ⁵I₈(Ho³⁺), ³F₃ → ³H₆(Tm³⁺) and ¹G₄ → ³H₆(Tm³⁺), ³F₃ → ³H₆(Tm³⁺) and ⁵F₅ → ⁵I₈(Ho³⁺), ³F₃ → ³H₆(Tm³⁺) and ⁵F₄ → ⁵I₈(Ho³⁺)) for temperature sensing performance. The maximum absolute sensitivity (S_a), relative sensitivity (S_r), and minimum temperature resolution δ(T) were found to be 0.0126 K^-1 (495 K), 1.7966% K^-1 (345 K), and 0.0167 K, respectively, which are better than those of most sensing materials, and the simultaneous action of multiple coupling energy levels can further improve the temperature precision. This study indicates that the sample has a good value for optical temperature measurement and also provides new ideas for the exploration of other high-quality optical temperature sensing materials.

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