Ultrasensitive\nand\nAdjustable Nanothermometers Based\non Er<sup>3+</sup>-Sensitized Core@Shell Nanoparticles for Use in\nthe First Biological Window

In recent years,\nintensive research has focused on lanthanide-doped\nnanoparticles (NPs) used as noncontact temperature sensors, particularly\nin nanomedicine. These NPs must be capable of excitation and emission\nwithin biological windows, where biological materials usually show\nbetter transparency for radiation. In this article, we propose that\nNPs sensitized with Er<sup>3+</sup> ions can be applied as temperature\nsensors in biological materials. We synthesized the NPs through a\nreaction in high-boiling solvents and confirmed their crystal structure\nand the formation of core@shell NPs by using X-ray diffraction, high-resolution\ntransmission electron microscopy, and element distribution mapping\nwithin the NPs. NaErF<sub>4</sub>@NaYF<sub>4</sub>, NaYF<sub>4</sub>:12.5% Er<sup>3+</sup>, 2.5% Tm<sup>3+</sup>@NaYF<sub>4</sub>, NaYF<sub>4</sub>:7.5% Er<sup>3+</sup>@NaYF<sub>4</sub>, and NaYF<sub>4</sub>:12.5% Er<sup>3+</sup>, 2.5% Ho<sup>3+</sup>@NaYF<sub>4</sub> exhibited\nintense upconversion (UC) emission under 1532 nm laser excitation\ndetectable also in the whole human blood. We propose that this UC\nresults from energy transfer between Er<sup>3+</sup> ions and from\nEr<sup>3+</sup> to Tm<sup>3+</sup> or Ho<sup>3+</sup> codopants. To\ndetermine the mechanism of UC, we measured the dependence of the emission\nband intensities on the laser power densities. Importantly, we also\nanalyzed the temperature-dependent emission of the NPs within the\n295–360 K range. Based on the collected emission spectra, we\ncalculated the luminescence intensity ratios (LIRs) of the emission\nbands to assess their potential for optical temperature sensing. The\ntemperature-sensing properties varied with the concentration of Er<sup>3+</sup> ions and the presence of additional Tm<sup>3+</sup> or Ho<sup>3+</sup> codopants. Depending on the NP composition and the emission\nbands used for luminescence ratio calculations, the maximum relative\ntemperature sensitivity ranged from 4.55%·K<sup>–1</sup> to 1.12%·K<sup>–1</sup>, with temperature resolution\nbetween 0.05 and 2.53 K at room temperature. Finally, as proof of\nusing NPs as temperature sensors in biomedicine, we successfully measured\nthe temperature-dependent emission of NaYF<sub>4</sub>:7.5% Er<sup>3+</sup>@NaYF<sub>4</sub> NPs dispersed in whole blood under 1532\nnm excitation. We demonstrated that the ratio of Er<sup>3+</sup> ion\nemission bands changes with temperature, indicating that these NPs\nhave potential applications in temperature sensing within biological\nenvironments. We also confirmed the properties of NPs as temperature\nsensors by measuring the temperature reading uncertainty and the repeatability\nof the LIR readings during heating–cooling cycles, thereby\nconfirming the excellent properties of the studied systems as temperature\nsensors.