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Finding thermal history phosphors with high sensitivity and a consistent readout is required for reliable thermal history determination with high temperature resolution. This work presents a new thermal history phosphor based on the luminescence of Eu3+ ions in LaVO4 to meet these requirements. As demonstrated, raising the annealing temperature causes a structural phase transition from a low-temperature tetragonal phase to a high-temperature single-stranded phase. The associated change in the local point symmetry of the crystallographic site occupied by Eu3+ ions result in a significant decrease in the emission intensity ratio of the 5D0 → 7F2 band relative to the 5D0 → 7F1 band, which enables the development of the ratiometric thermal history phosphor with the relative sensitivity of 0.38% °C−1 at 800 °C. Its applicative potential for thermal history readout was proved in the proof-of-concept experiment.

Lifetime-based luminescence thermometry has been shown to enable accurate deep-tissue monitoring of temperature changes – even at the in vivo level – in a minimally invasive way. However, major limiting factors to the performance of this approach are short lifetimes and poor brightness. These are characteristics, respectively, of semiconductor nanocrystals and lanthanide-doped nanoparticles, of which most luminescent nanothermometers are made. To address these limitations, the composition of luminescent nanothermometers co-doped with transition metal (Mn5+) and Er3+ ions are designed and optimized. The salient features of these nanothermometers are strong, near-infrared emission and long, temperature-dependent photoluminescence lifetime. The potential of these luminescent nanophosphors for thermal sensing is then showcased by monitoring a thermal gradient using a one-of-a-kind piece of equipment designed for lifetime-based luminescence thermometry measurements. The combination of the newly developed nanothermometers and the custom-made instrument allows for obtaining 2D thermal maps both in the absence and presence of tissue phantoms mimicking the optical properties of the skin. The results presented in this study thus provide credible foundations for the deployment of lifetime-based thermometry for accurate deep-tissue thermal mapping at the preclinical level.

Luminescence thermometry exploiting luminescence kinetics as a thermometric parameter is regarded as one of the most reliable temperature readout techniques. Transition metal ions are of particular interest in this application which is due to the possibility of modulating their spectroscopic properties by changing the strength of the crystal field of the matrix. In this work, we present a strategy to modulate the thermometric parameters including the relative sensitivity and useful temperature range of luminescent thermometers based on the lifetime of Ti3+ ions by introducing lanthanide (Ln3+) doping in SrTiO3 and CaTiO3. The mutual effect of the distortion of the local symmetry of the Ti3+ ions associated with the introduction of Ln3+ ions and/or the Ti3+ → Ln3+ energy transfer enabled relative sensitivities of SR = 5.87% K−1 at 140 K for SrTiO3:Dy3+ and SR = 4.51% K−1 at 251 K for CaTiO3:Gd3+. © 2022 The Author(s)

Almost all existing luminescent thermometers rely on the temperature-dependent processes such as multi-phonon relaxation and phonon-assisted energy transfers, thermal population, or coupling between energy levels of ground and excited states of luminescent species (lanthanides, transition metals, quantum dots, fluorescent molecules, etc.). Although such phenomena are in principle suitable for straightforward calibration, aiming to offer high temperature sensitivity, high temperature resolution and the widest possible temperature sensitivity range, their performance is often dependent on the excitation intensity or sample dispersive properties and often suffers from insufficient brightness, which further becomes dimmer at increased temperatures. Exploiting temperature-dependent continuous phase transitions that modify the same near-infrared (NIR) emission band under the same NIR excitation wavelength may provide an alternative reading method for temperature sensing. Here, such a new principle of luminescent nano-thermometry (LNT) using a Nd3+ doped nanocrystalline LiYO2 matrix is studied, significant sensitivities of up to 6%/K are achieved, and other issues found in conventional LNT are circumvented. Due to the hysteresis found in this class of LNT, they may find applications in studies of temperature gradients and can be integrated with modern nanophotonic devices. © 2022 Wiley-VCH GmbH

Herein, we show that the substitution of Sr2+ by trivalent lanthanide ions (Lu3+, La3+, Tb3+) in SrTiO3 nanocrystals stabilizes and enhances Ti3+ near-infrared emission (around 800 nm). This emission occurs from the 6-fold coordinated Ti3+ luminescent centers that appear in the vacancy–Ti3+–O form after lanthanide doping into the Sr2+ site. The strong dependence of the Ti3+ emission on temperature provided means for the tailored chemical engineering of luminescent nanonthermometers able to read the temperature in three ways: from the changes in Ti3+ emission intensity, excited-state lifetime, and from the ratio of Tb3+ and Ti3+ emission intensities. We demonstrated the unprecedented temperature sensitivity of the lifetime-based luminescent thermometer (8.83% K−1) with SrTiO3:Tb3+, along with exceptional repeatability in measurements.

Currently available temperature measurements or imaging at nano-micro scale are limited to fluorescent molecules and luminescent nanocrystals, whose spectral properties respond to temperature variation. The principle of operation of these conventional temperature probes is typically related to temperature induced multiphonon quenching or temperature dependent energy transfers, therefore, above 12%/K sensitivity and high thermal resolution remain a serious challenge. Here we demonstrate a novel class of highly sensitive thermographic phosphors operating in room temperature range with sub-kelvin thermal resolution, whose temperature readings are reproducible, luminescence is photostable and brightness is not compromised by thermal quenching. Corroborated with phase transition structural characterization and high spatio-temporal temperature imaging, we demonstrated that optically active europium ions are highly and smoothly susceptible to monoclinic to tetragonal phase transition in nanocrystalline (54 ± 14 nm) LiYO2 host, which is evidenced by changed number and the splitting of Stark components as well as by smooth variation of contribution between magnetic and electric dipole transitions. Further, reducing the size of phosphor from bulk to nanocrystalline matrix, shifted the phase transition temperature from 100 °C down to room temperature. These findings provide insights into the mechanism underlaying phase transition based luminescence nanothermometry and motivate future research toward new, highly sensitive, high temporal and spatial resolution nano-thermometers aiming at precise studying heat generation or diffusion in numerous biological and technology applications.