Dual-Mode Optical Thermometry Based on the Fluorescence Intensity Ratio Excited by a 915 nm Wavelength in LuVO₄:Yb³⁺/Er³⁺@SiO₂ Nanoparticles

The optical thermometry properties of LuVO₄:Yb³⁺/Er³⁺@SiO₂ nanoparticles (NPs) are studied in detail. In order to avoid the overheating effect for biological tissue caused by 980 nm radiation, 915 nm is employed as the excitation wavelength to investigate the upconversion (UC) and optical thermometry properties of the as-prepared NPs. In the visible region, the fluorescence intensity ratio (FIR) of the ²H_11/2 and ⁴S_3/2 levels of Er³⁺ is utilized to measure the temperature. The relative sensitivity S_R in this case can be written as 1077/ T², which is higher than that of β-NaYF₄:Yb³⁺/Er³⁺ NPs, β-NaLuF₄:Yb³⁺/Er³⁺ NPs, YVO₄:Yb³⁺/Er³⁺ NPs, etc. In the near-infrared (NIR) region, an anomalous enhancement of the ⁴I_13/2 → ⁴I_15/2 transition with increasing temperature is observed. What is more, the FIR of peak 2 (located at 1496 nm) to peak 1 (located at 1527 nm) is changed regularly with increasing temperature, which can also be used to measure the temperature. The combination of the visible and NIR regions for optical thermometry can provide a self-referenced temperature determination to make measurement of the temperature more precise. In addition, the UC mechanism is also investigated, especially the population route of the ⁴F_9/2 level of Er³⁺. Through analysis of the decay curves, we propose that the dominant way for populating the Er^{3+ 4}F_9/2 level is energy transfer from the Yb^{3+ 2}F_5/2 level to the Er^{3+ 4}I_13/2 level. All of the results reveal the potential application of LuVO₄:Yb³⁺/Er³⁺@SiO₂ NPs for dual-mode optical thermometry.