New Horizons for Temperature Sensing and Bioimaging Using Er³⁺‐Doped Core@Shell Nanoparticles and Effects of H₂O and D₂O Solvents on Their Spectroscopic Properties

Abstract Lanthanide‐doped nanoparticles (NPs) exhibit temperature‐dependent luminescence, enabling the design of luminescent nanothermometers for industrial and medical applications. This research demonstrates the temperature‐sensing properties of NaYF₄:7.5%Er³⁺@NaYF₄ and NaErF₄@NaYF₄ NPs, which have a hexagonal shape and average size of 17 nm. Their core@shell structure is confirmed using high‐resolution transmission electron microscopy, and they exhibit intense upconversion (UC) emission under 1532 nm excitation in H₂O and D₂O colloids. The recorded spectra show Er³⁺ emission bands with varying intensity ratios depending on the Er³⁺ concentration, chosen solvent, and temperature. The spectroscopic properties of the studied NPs allow for their excitation and observation of emission within biological windows, which makes them useful for bio‐related applications. The emission of prepared NPs is analyzed as a function of temperature from 298 up to 358/363 K in H₂O and D₂O. The ratios for thermally‐coupled levels and non‐TCLs and their relative sensitivities are studied. For the high dopant concentration sample in water, the O─H vibrations and blue shift in the absorption spectrum lead to a record relative sensitivity of 2.50% K⁻¹ (at 363 K) for the ²H_11/2/⁴I_11/2 ratio. The use of synthesized NPs for bioimaging under 1550 nm excitation is also demonstrated to observe their accumulation in the guts of Daphnia magna.