Novel and High‐Sensitive Primary and Self‐Referencing Thermometers Based on the Excitation Spectra of Lanthanide Ions

Abstract

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.

Energy Gap Linear Superposition of Thermally Coupled Levels toward Enhanced Relative Sensitivity of Ratiometric Thermometry

“…An accurate temperature measurement is of great significance and triggers a great demand for versatile thermometers. − Thereinto, ratiometric luminescence thermometry (RLT) has attracted considerable attention by virtues of non-invasive, fast response, high precision, and substantial electromagnetic interference resistance. , Generally, conventional RLT is performed by decoding the functional relationship between the temperature and fluorescence intensity ratio (FIR) of two emission bands detonated by thermally coupled levels (TCLs) of the luminescence centers . To evaluate the performance of optical thermometry, the relative sensitivity ( S R ) is proposed and expressed as a percentage change per temperature, reflecting the thermally induced relative change of FIR .…”

Lanthanide Luminescence Thermometry and Slow Magnetic Relaxation in 3-D Polycyanidometallate-Based Materials

“…The compound can be used as a luminescence thermometer based on the temperature dependence of their excitation spectra (as the emission ones show a very weak temperature dependence). Examples of ratiometric luminescent thermometers based on the excitation spectra of Ln III ions are scarce − as these thermometers require more elaborated equipment and longer acquisition times, being this a disadvantage relatively to those based on the emission spectra . We consider the ratio between the 240–450 nm ( I A ) and 450–480 nm ( I B ) integrated intensity areas of the excitation spectra (Figure S9B).…”

“…Examples of ratiometric luminescent thermometers based on the excitation spectra of Ln III ions are scarce 68−71 as these thermometers require more elaborated equipment and longer acquisition times, being this a disadvantage relatively to those based on the emission spectra. 72 We consider the ratio between the 240−450 nm (I A ) and 450−480 nm (I B ) integrated intensity areas of the excitation spectra (Figure S9B). The defined thermometric parameter is well described by a polynomial function, eq S5, allowing compound 2 to operate as a thermometer in the 25− 325 K range with a temperature uncertainty (δT) 73 lower than 1 K in the 75−325 K range and S r = 0.6%•K −1 at around 325 K (Table S8 and Figures S10 and S11).…”

Single-center LnIII ratiometric luminescent temperature probes based on singlet → singlet and singlet → triplet sensitizations

“…From a practical perspective, the two-wavelength excitation method is more promising since it displays superior thermal sensitivity and it requires only two excitation sources emitting two different wavelengths (such as a laser or a LED), an emission monochromator, and the detector, which is the typical setup of luminescence thermometry measurements while the excitation band intensity ratio approach requires a broadband emitter lighting source, excitation and emission monochromators, as well as the detector, although it has also been investigated. 39…”