At the micro-and nanoscale, thermometry has been regarded as fundamental to develop underlying laws of heat transfer, [8][9][10][11] to design and improve new circuits, [12,13] to comprehend quantitatively heat exchange within cells and tissues, [14][15][16] and to harmlessly apply hyperthermia therapy. [17][18][19] In this context, ratiometric approaches for molecular (or nano) thermometry are widely employed because of their precision, easiness, and high relative thermal sensitivity (S r ). [3,[20][21][22][23] Usually, a ratio of emission (integrated) intensities is used as the thermometric parameter (Δ) due to the high sensitivity of emission-based techniques allowing, for instance, singleparticle spectroscopy. [24] Indeed, photoluminescence of lanthanide trivalent ions (Ln(III)) is especially suitable for these measurements [3,25,26] due to their unique features such as high emission intensities, narrow excitation, and emission bands, long emission lifetimes (µs to ms), and spectral ranges from UV-vis to nearinfrared (NIR). [27,28] In addition, the energies of the intra-4f transitions are nearly independent of their surroundings, e.g., ligands, host crystal, solvent, etc. [29] This allows for prompt assignments of excitation and emission spectra and predictable energy differences. [27][28][29] These features have contributed to the accelerated development of Ln(III)-based thermometers in recent years, [3,25] including
Remote sensing through ratiometric luminescence thermometry based on trivalent lanthanide ions (Ln(III)) has lately become a promising technique due to its numerous applications. Most available Ln(III)-based luminescentthermometers require a calibration process with a reference thermal probe (secondary thermometers) and recurrent calibrations are mandatory, particularly when the thermometers are used in different media. This is sometimes impractical and a medium-independent calibration relation is postulated, which is potentially inaccurate. Thus, the determination of the temperature based on well-grounded physical principles by primary thermometers is the only way to overcome these challenges. Despite being considered one of the most important developments in luminescence thermometry, primary luminescent thermometers are scarce. Primary thermometers requiring calibration are proposed, implemented, and validated at one known temperature (primary-T), which are also self-referencing, employing ratiometric data from the excitation spectrum of Ln(III). By combining with the emission spectrum, thermometers not requiring calibration (primary-S) are devised. An Eu(III)-β-diketonate complex is used as a proof-of-concept, but the approach is universal and other Ln(III)-based materials can be explored. Because many thermometric parameters are employed for temperature prediction an unprecedented very high accuracy of 0.2% in the physiological range is obtained.
