Luminescent PMMA Films and PMMA@SiO2 Nanoparticles with Embedded Ln3+ Complexes for Highly Sensitive Optical Thermometers in the Physiological Temperature Range**

Abstract In recent years, luminescent materials doped with Ln 3+ ions have attracted much attention for their application as optical thermometers based on both downshifting and upconversion processes. This study presents research done on the development of highly sensitive optical thermometers in the physiological temperature range based on poly(methyl methacrylate) (PMMA) films doped with two series of visible Ln 3+ complexes (Ln 3+ =Tb 3+ , Eu 3+ , and Sm 3+ ) and SiO 2 nanoparticles (NPs) coated with these PMMA films. The best performing PMMA film doped with Tb 3+ and Eu 3+ complexes was the PMMA[TbEuL 1 tppo]1 film (L 1 =4,4,4‐trifluoro‐1‐phenyl‐1,3‐butadionate; tppo=triphenylphosphine oxide), which showed good temperature sensing of S r =4.21 % K −1 at 313 K, whereas for the PMMA films doped with Tb 3+ and Sm 3+ complexes the best performing was the PMMA[TbSmL 2 tppo]3 film (L 2 =4,4,4‐trifluoro‐1‐(4‐chlorophenyl)‐1,3‐butadionate), with S r =3.64 % K −1 at 313 K. Additionally, SiO 2 NPs coated with the best performing films from each of the series of PMMA films (Tb–Eu and Tb–Sm) and their temperature‐sensing properties were studied in water, showing excellent performance in the physiological temperature range (PMMA[TbEuL 1 tppo]1@SiO 2 : S r =3.84 % °C at 20 °C; PMMA[TbSmL 2 tppo]3@SiO 2 : S r =3.27 % °C at 20 °C) and the toxicity of these nanoparticles on human cells was studied, showing that they were nontoxic.