Upconversion Thermometry Using Yb³⁺/Er³⁺ Co-Doped KY₃F₁₀ Nanoparticles

We report high-sensitivity luminescence thermometry using Yb3+/Er3+ co-doped KY3F10 core and core–shell upconverting nanoparticles. The highest thermal sensitivities of 1.510 ± 0.015%·K–1 (300 K) and 1.245 ± 0.013%·K–1 (316 K) and temperature uncertainty of 0.11 K were achieved by tuning the excitation wavelength to 975 nm to be in-resonant with optical transitions of the Yb3+ ion. The high-resolution Yb3+ excitation spectra were recorded by observing the Er3+, 4S3/2, 4F9/2 → 4I15/2 upconversion fluorescence, with the highest fluorescence yield obtained at 10 254 cm–1 (975 nm) for both nanoparticles. This coincides with the ground-state absorption maximum at 297 K. However, at 10 K, the Yb3+ upconversion excitation spectra also exhibit excited-state absorption peaks. The Yb3+ absorption spectra for the core and core–shell nanoparticles are dominated by single-ion centers (of C4v symmetry). Resonant excitation of the core and core–shell nanoparticles is observed to yield a 5-fold enhancement in Er3+ upconversion fluorescence intensity in comparison to excitation at 980 nm, increasing the fluorescence intensity ratio and the thermal sensitivity of a KY3F10 nanothermometer operating at the physiological temperatures.