Ultrasensitive and Adjustable Nanothermometers Based on Er³⁺-Sensitized Core@Shell Nanoparticles for Use in the First Biological Window

In recent years, intensive research has focused on lanthanide-doped nanoparticles (NPs) used as noncontact temperature sensors, particularly in nanomedicine. These NPs must be capable of excitation and emission within biological windows, where biological materials usually show better transparency for radiation. In this article, we propose that NPs sensitized with Er³⁺ ions can be applied as temperature sensors in biological materials. We synthesized the NPs through a reaction in high-boiling solvents and confirmed their crystal structure and the formation of core@shell NPs by using X-ray diffraction, high-resolution transmission electron microscopy, and element distribution mapping within the NPs. NaErF₄@NaYF₄, NaYF₄:12.5% Er³⁺, 2.5% Tm³⁺@NaYF₄, NaYF₄:7.5% Er³⁺@NaYF₄, and NaYF₄:12.5% Er³⁺, 2.5% Ho³⁺@NaYF₄ exhibited intense upconversion (UC) emission under 1532 nm laser excitation detectable also in the whole human blood. We propose that this UC results from energy transfer between Er³⁺ ions and from Er³⁺ to Tm³⁺ or Ho³⁺ codopants. To determine the mechanism of UC, we measured the dependence of the emission band intensities on the laser power densities. Importantly, we also analyzed the temperature-dependent emission of the NPs within the 295-360 K range. Based on the collected emission spectra, we calculated the luminescence intensity ratios (LIRs) of the emission bands to assess their potential for optical temperature sensing. The temperature-sensing properties varied with the concentration of Er³⁺ ions and the presence of additional Tm³⁺ or Ho³⁺ codopants. Depending on the NP composition and the emission bands used for luminescence ratio calculations, the maximum relative temperature sensitivity ranged from 4.55%·K^-1 to 1.12%·K^-1, with temperature resolution between 0.05 and 2.53 K at room temperature. Finally, as proof of using NPs as temperature sensors in biomedicine, we successfully measured the temperature-dependent emission of NaYF₄:7.5% Er³⁺@NaYF₄ NPs dispersed in whole blood under 1532 nm excitation. We demonstrated that the ratio of Er³⁺ ion emission bands changes with temperature, indicating that these NPs have potential applications in temperature sensing within biological environments. We also confirmed the properties of NPs as temperature sensors by measuring the temperature reading uncertainty and the repeatability of the LIR readings during heating-cooling cycles, thereby confirming the excellent properties of the studied systems as temperature sensors.