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The increasing popularity of luminescent nanothermometry in recent years can be attributed to its application potential in biomedicine. In response to this need, we describe a biocompatible bimodal luminescent thermometer that operates in ratiometric and luminescence lifetime modes based on particles of MgTiO3:Cr3+,Nd3+@SiO2. The introduction of Cr3+ and Nd3+ dopants enabled the luminescence of Ti3+ ions to be observed, and the difference in the thermal quenching rates of Cr3+ (4T2→4A2), Ti3+ (2T2→2E) and Nd3+ (4F3/2 → 4I11/2) ions enabled the ratiometric thermometers. The highest sensitivity reaching SR = 1.00%K−1 was obtained for MgTiO3:0.1 % Cr3+, 0.1 % Nd3+ at 203 K. The shortening of the lifetime of the 4T2 level of Cr3+ ions associated with its thermal depopulation allows to develop a lifetime-based thermometer with a relative sensitivity reaching 0.85–1.18%K−1 in the physiological temperature range. The deposition of a SiO2 shell on a MgTiO3:Cr3+,Nd3+ did not introduce significant changes in the shape of the emission spectrum and slightly elongates the lifetime by reducing the probability of surface-related nonradiative processes. More importantly, the thermometric performance of this luminescence thermometer was preserved. The low cytotoxicity of the obtained materials underlines their potential in bioapplications of the described luminescent thermometers.

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.

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)

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.

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

Despite the numerous advantages of lanthanide based luminescent thermometers, a major limitation is the limited brightness associated with the low absorption cross section of the 4f–4f transitions. In this paper, co-doping with Cr3+/4+ is proposed as a strategy to enhance the near-infrared luminescence emission of Nd3+/Er3+-based luminescent thermometers, as well as to modulate the temperature sensitivity of the ratio of Nd3+ to Er3+ in YAG microcrystals. It was found that an increase in the Cr3+ concentration leads to the enhancement of the YAG:Nd3+,Er3+ emission intensity via the Cr3+ → Ln3+ sensitization process. The quantitative analysis of the absolute brightness indicates the 30-fold enhancement of their near infrared emission upon broad white light excitation for 20% Cr3+ with respect to the non co-doped with the Cr3+ counterpart. In addition, the phonon-assisted nature of the simultaneously occurring Cr3+ → Ln3+ and Nd3+ → Cr4+ → Er3+ energy transfers leads to a 7-fold increase of the relative temperature sensitivity of the ratio of Nd3+ to Er3+ emission. In conclusion, co-doping with transition metals can enhance simultaneously the luminescence brightness and the temperature sensitivity of NIR luminescent thermometers, holding promise for sensitive and robust remote temperature sensing at physiological temperatures.

The co-doping of SrTiO3:Mn4+ luminescent nanocrystals with trivalent lanthanide ions (Ln3+ = Lu3+, Tm3+, Er3+, Ho3+, Dy3+, Eu3+, and La3+) is demonstrated as a new strategy for significant sensitivity improvement of lifetime-based luminescent thermometers. SrTiO3:Mn4+:Ln3+ nanocrystals of about 25 nm diameter were prepared by the modified Pechini method and characterized using X-ray powder diffraction and electron microscopy techniques. The temperature dependence of Mn4+ emission in SrTiO3 was considerably altered by the co-doping of Ln3+ due to the cooperating effects of Mn4+ → Ti3+ and Mn4+ → Ln3+ energy transfers. A substantial enhancement of the relative sensitivity of lifetime-based thermometry as high as 145% with respect to the unco-doped nanocrystals reveals the success of the approach. The obtained values of maximal relative sensitivity for different Ln3+ co-dopants are 5.10% K−1 at 290 K for Er3+, 5.00% K−1 at 301 K for Eu3+, 4.84% K−1 at 303 K for Dy3+, 4.71% K−1 at 289 K for Ho3+, and 3.87% K−1 at 290.7 K for Lu3+.

This paper presents four new temperature readout approaches to luminescence nanothermometry in spectral regions of biological transparency demonstrated on Yb3+/Er3+-doped yttrium aluminum garnet nanoparticles. Under the 10 638 cm-1 excitation, down-shifting near infrared emissions (>10 000 cm-1) are identified as those originating from Yb3+ ions' 2F5/2 → 2F7/2 (∼9709 cm-1) and Er3+ ions' 4I13/2 → 4I15/2 (∼6494 cm-1) electronic transitions and used for 4 conceptually different luminescence thermometry approaches. Observed variations in luminescence parameters with temperature offered an exceptional base for studying multiparametric temperature readouts. These include the temperature-dependence of: (i) intensity ratio between emissions from Stark components of Er3+ 4I13/2 level; (ii) intensity ratio between emissions of Yb3+ (2F5/2 → 2F7/2 transition) and Er3+ (4I13/2 → 4I15/2 transition); (iii) band shift and bandwidth and (iv) lifetime of the Yb3+ emission (2F5/2 → 2F7/2 transition) with maximal sensitivities of 1% K-1, 0.8% K-1, 0.09 cm-1 K-1, 0.46% K-1 and 0.86% K-1, respectively. The multimodal temperature readout provided by this material enables its application in different luminescence thermometry setups as well as improved the reliability of the temperature sensing by the cross-validation between measurements. © 2021 The Royal Society of Chemistry.

This paper presents four new temperature readout approaches to luminescence nanothermometry in spectral regions of biological transparency demonstrated on Yb3+/Er3+-doped yttrium aluminum garnet nanoparticles. Under the 10 638 cm-1 excitation, down-shifting near infrared emissions (>10 000 cm-1) are identified as those originating from Yb3+ ions' 2F5/2 → 2F7/2 (∼9709 cm-1) and Er3+ ions' 4I13/2 → 4I15/2 (∼6494 cm-1) electronic transitions and used for 4 conceptually different luminescence thermometry approaches. Observed variations in luminescence parameters with temperature offered an exceptional base for studying multiparametric temperature readouts. These include the temperature-dependence of: (i) intensity ratio between emissions from Stark components of Er3+ 4I13/2 level; (ii) intensity ratio between emissions of Yb3+ (2F5/2 → 2F7/2 transition) and Er3+ (4I13/2 → 4I15/2 transition); (iii) band shift and bandwidth and (iv) lifetime of the Yb3+ emission (2F5/2 → 2F7/2 transition) with maximal sensitivities of 1% K-1, 0.8% K-1, 0.09 cm-1 K-1, 0.46% K-1 and 0.86% K-1, respectively. The multimodal temperature readout provided by this material enables its application in different luminescence thermometry setups as well as improved the reliability of the temperature sensing by the cross-validation between measurements. © 2021 The Royal Society of Chemistry.

The co-doping of SrTiO3:Mn4+ luminescent nanocrystals with trivalent lanthanide ions (Ln3+ = Lu3+, Tm3+, Er3+, Ho3+, Dy3+, Eu3+, and La3+) is demonstrated as a new strategy for significant sensitivity improvement of lifetime-based luminescent thermometers. SrTiO3:Mn4+:Ln3+ nanocrystals of about 25 nm diameter were prepared by the modified Pechini method and characterized using X-ray powder diffraction and electron microscopy techniques. The temperature dependence of Mn4+ emission in SrTiO3 was considerably altered by the co-doping of Ln3+ due to the cooperating effects of Mn4+ → Ti3+ and Mn4+ → Ln3+ energy transfers. A substantial enhancement of the relative sensitivity of lifetime-based thermometry as high as 145% with respect to the unco-doped nanocrystals reveals the success of the approach. The obtained values of maximal relative sensitivity for different Ln3+ co-dopants are 5.10% K−1 at 290 K for Er3+, 5.00% K−1 at 301 K for Eu3+, 4.84% K−1 at 303 K for Dy3+, 4.71% K−1 at 289 K for Ho3+, and 3.87% K−1 at 290.7 K for Lu3+.

Despite the numerous advantages of lanthanide based luminescent thermometers, a major limitation is the limited brightness associated with the low absorption cross section of the 4f–4f transitions. In this paper, co-doping with Cr3+/4+ is proposed as a strategy to enhance the near-infrared luminescence emission of Nd3+/Er3+-based luminescent thermometers, as well as to modulate the temperature sensitivity of the ratio of Nd3+ to Er3+ in YAG microcrystals. It was found that an increase in the Cr3+ concentration leads to the enhancement of the YAG:Nd3+,Er3+ emission intensity via the Cr3+ → Ln3+ sensitization process. The quantitative analysis of the absolute brightness indicates the 30-fold enhancement of their near infrared emission upon broad white light excitation for 20% Cr3+ with respect to the non co-doped with the Cr3+ counterpart. In addition, the phonon-assisted nature of the simultaneously occurring Cr3+ → Ln3+ and Nd3+ → Cr4+ → Er3+ energy transfers leads to a 7-fold increase of the relative temperature sensitivity of the ratio of Nd3+ to Er3+ emission. In conclusion, co-doping with transition metals can enhance simultaneously the luminescence brightness and the temperature sensitivity of NIR luminescent thermometers, holding promise for sensitive and robust remote temperature sensing at physiological temperatures.