In Situ Local Temperature Mapping in Microscopy Nano‐Reactors with Luminescence Thermometry

High-sensitivity luminescent temperature sensors: MFX:1%Sm2+ (M = Sr, Ba, X = Cl, Br)

The use of lanthanide luminescence has advanced the field of remote temperature sensing. Luminescence intensity ratio methods relying on emission from two thermally coupled energy levels are popular but suffer from a limited temperature range. Here, we present a versatile luminescent thermometer: Ba(Sr)FBr(Cl):Sm2+. The Sm2+ ion benefits from multiple thermally coupled excited states to extend the temperature range and has strong parity-allowed 4f6→4f55d1 absorption to increase brightness. We conduct a comparative analysis of the temperature sensing performance of Sm2+ in BaFBr, BaFCl, SrFBr, and SrFCl and address the role of concentration, host, and Boltzmann equilibration. Different thermal coupling schemes, 5D1-5D0 and 4f55d1-5D0, and temperature-dependent lifetimes enable accurate sensing between 350 and 800 kelvin. Differences in 4f55d1-5D0 energy gap allows optimization for a temperature range of interest. This type of Sm2+-based thermometer holds great potential for temperature monitoring in the wide and relevant range up to 500°C.

A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds

Trivalent europium (Eu3+) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu3+-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu3+:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu3+ ion, can directly affect the optical properties of Eu3+. We review the luminescence properties of Eu3+ ions in mixed-anion compounds of the oxynitride YSiO2N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the 5D0 → 7F0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu3+ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu3+-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu3+-doped phosphors are presented.

Luminescence Thermometry Beyond the Biological Realm

As the field of luminescence thermometry has matured, practical applications of luminescence thermometry techniques have grown in both frequency and scope. Due to the biocompatibility of most luminescent thermometers, many of these applications fall within the realm of biology. However, luminescence thermometry is increasingly employed beyond the biological realm, with expanding applications in areas such as thermal characterization of microelectronics, catalysis, and plasmonics. Here, we review the motivations, methodologies, and advances linked to nonbiological applications of luminescence thermometry. We begin with a brief overview of luminescence thermometry probes and techniques, focusing on those most commonly used for nonbiological applications. We then address measurement capabilities that are particularly relevant for these applications and provide a detailed survey of results across various application categories. Throughout the review, we highlight measurement challenges and requirements that are distinct from those of biological applications. Finally, we discuss emerging areas and future directions that present opportunities for continued research.

Operando Electron Microscopy of Catalysts: The Missing Cornerstone in Heterogeneous Catalysis Research?

Heterogeneous catalysis in thermal gas-phase and electrochemical liquid-phase chemical conversion plays an important role in our modern energy landscape. However, many of the structural features that drive efficient chemical energy conversion are still unknown. These features are, in general, highly distinct on the local scale and lack translational symmetry, and thus, they are difficult to capture without the required spatial and temporal resolution. Correlating these structures to their function will, conversely, allow us to disentangle irrelevant and relevant features, explore the entanglement of different local structures, and provide us with the necessary understanding to tailor novel catalyst systems with improved productivity. This critical review provides a summary of the still immature field of operando electron microscopy for thermal gas-phase and electrochemical liquid-phase reactions. It focuses on the complexity of investigating catalytic reactions and catalysts, progress in the field, and analysis. The forthcoming advances are discussed in view of correlative techniques, artificial intelligence in analysis, and novel reactor designs.

Spotlight on Luminescence Thermometry: Basics, Challenges, and Cutting-Edge Applications

Luminescence (nano)thermometry is a remote sensing technique that relies on the temperature dependency of the luminescence features (e.g., bandshape, peak energy or intensity, and excited state lifetimes and risetimes) of a phosphor to measure temperature. This technique provides precise thermal readouts with superior spatial resolution in short acquisition times. Although luminescence thermometry is just starting to become a more mature subject, it exhibits enormous potential in several areas, e.g., optoelectronics, photonics, micro- and nanofluidics, and nanomedicine. This work reviews the latest trends in the field, including the establishment of a comprehensive theoretical background and standardized practices. The reliability, repeatability, and reproducibility of the technique are also discussed, along with the use of multiparametric analysis and artificial-intelligence algorithms to enhance thermal readouts. In addition, examples are provided to underscore the challenges that luminescence thermometry faces, alongside the need for a continuous search and design of new materials, experimental techniques, and analysis procedures to improve the competitiveness, accessibility, and popularity of the technology.

Bifunctional Europium for Operando Catalyst Thermometry in an Exothermic Chemical Reaction

Often the reactor or the reaction medium temperature is reported in the field of heterogeneous catalysis, even though it could vary significantly from the reactive catalyst temperature. The influence of the catalyst temperature on the catalytic performance and vice versa is therefore not always accurately known. We here apply EuOCl as both solid catalyst and thermometer, allowing for operando temperature determination. The interplay between reaction conditions and the catalyst temperature dynamics is studied. A maximum temperature difference between the catalyst and oven of +16 °C was observed due to the exothermicity of the methane oxychlorination reaction. Heat dissipation by radiation appears dominating compared to convection in this set-up, explaining the observed uniform catalyst bed temperature. Application of operando catalyst thermometry could provide a deeper mechanistic understanding of catalyst performances and allow for safer process operation in chemical industries.

Demonstration of Neural Networks to Reconstruct Temperatures from Simulated Fluorescent Data Toward Use in Bio-microfluidics

Biological systems often have a narrow temperature range of operation, which require highly accurate spatially resolved temperature measurements, often near ±0.1 K. However, many temperature sensors cannot meet both accuracy and spatial distribution requirements, often because their accuracy is limited by data fitting and temperature reconstruction models. Machine learning algorithms have the potential to meet this need, but their usage in generating spatial distributions of temperature is severely lacking in the literature. This work presents the first instance of using neural networks to process fluorescent images to map the spatial distribution of temperature. Three standard network architectures were investigated using non-spatially resolved fluorescent thermometry (simply-connected feed-forward network) or during image or pixel identification (U-net and convolutional neural network, CNN). Simulated fluorescent images based on experimental data were generated based on known temperature distributions where Gaussian white noise with a standard deviation of ±0.1 K was added. The poor results from these standard networks motivated the creation of what is termed a moving CNN, with an RMSE error of ±0.23 K, where the elements of the matrix represent the neighboring pixels. Finally, the performance of this MCNN is investigated when trained and applied to three distinctive temperature distributions characteristic within microfluidic devices, where the fluorescent image is simulated at either three or five different wavelengths. The results demonstrate that having a minimum of 10 ^3.5 data points per temperature and the broadest range of temperatures during training provides temperature predictions nearest to the true temperatures of the images, with a minimum RMSE of ±0.15 K. When compared to traditional curve fitting techniques, this work demonstrates that greater accuracy when spatially mapping temperature from fluorescent images can be achieved when using convolutional neural networks.

A cell design for correlative hard X-ray nanoprobe and electron microscopy studies of catalysts under in situ conditions

To improve the understanding of catalysts, and ultimately the ability to design better materials, it is crucial to study them during their catalytic active states. Using in situ or operando conditions allows insights into structure-property relationships, which might not be observable by ex situ characterization. Spatially resolved X-ray fluorescence, X-ray diffraction and X-ray absorption near-edge spectroscopy are powerful tools to determine structural and electronic properties, and the spatial resolutions now achievable at hard X-ray nanoprobe beamlines make them an ideal complement to high-resolution transmission electron microscopy studies in a multi-length-scale analysis approach. The development of a system to enable the use of a commercially available gas-cell chip assembly within an X-ray nanoprobe beamline is reported here. The novel in situ capability is demonstrated by an investigation of the redox behaviour of supported Pt nanoparticles on ceria under typical lean and rich diesel-exhaust conditions; however, the system has broader application to a wide range of solid-gas reactions. In addition the setup allows complimentary in situ transmission electron microscopy and X-ray nanoprobe studies under identical conditions, with the major advantage compared with other systems that the exact same cell can be used and easily transferred between instruments. This offers the exciting possibility of studying the same particles under identical conditions (gas flow, pressure, temperature) using multiple techniques.

One ion to catch them all: Targeted high-precision Boltzmann thermometry over a wide temperature range with Gd3

Ratiometric luminescence thermometry with trivalent lanthanide ions and their 4fn energy levels is an emerging technique for non-invasive remote temperature sensing with high spatial and temporal resolution. Conventional ratiometric luminescence thermometry often relies on thermal coupling between two closely lying energy levels governed by Boltzmann's law. Despite its simplicity, Boltzmann thermometry with two excited levels allows precise temperature sensing, but only within a limited temperature range. While low temperatures slow down the nonradiative transitions required to generate a measurable population in the higher excitation level, temperatures that are too high favour equalized populations of the two excited levels, at the expense of low relative thermal sensitivity. In this work, we extend the concept of Boltzmann thermometry to more than two excited levels and provide quantitative guidelines that link the choice of energy gaps between multiple excited states to the performance in different temperature windows. By this approach, it is possible to retain the high relative sensitivity and precision of the temperature measurement over a wide temperature range within the same system. We demonstrate this concept using YAl3(BO3)4 (YAB):Pr3+, Gd3+ with an excited 6PJ crystal field and spin-orbit split levels of Gd3+ in the UV range to avoid a thermal black body background even at the highest temperatures. This phosphor is easily excitable with inexpensive and powerful blue LEDs at 450 nm. Zero-background luminescence thermometry is realized by using blue-to-UV energy transfer upconversion with the Pr3+-Gd3+ couple upon excitation in the visible range. This method allows us to cover a temperature window between 30 and 800 K.

Upconverting Er3+-Yb3+ Inorganic/Covalent Organic Framework Core-Shell Nanoplatforms for Simultaneous Catalysis and Nanothermometry

Lanthanide-based luminescent nanoparticles that are thermally responsive can be used to probe temperature changes at a nanoscale regime. However, materials that can work as both a nanothermometer and a catalyst are limited. Herein, we show that covalent organic frameworks (COFs), which is an emerging class of porous crystalline materials, can be grown around lanthanide nanoparticles to create unique core-shell nanostructures. In this way, the COF (shell) supports copper metal ions as catalytic sites and simultaneously lanthanide nanoparticles (β-NaLuF₄:Gd,Er,Yb-core) locally measure the temperature during the catalytic reaction. Moreover, β-NaLuF₄:Gd,Er,Yb nanoparticles are upconverting materials and hence can be excited at longer wavelengths (975 nm), which do not affect the catalysis substrates or the COF. As a proof-of-principle, a three-component addition reaction of benzaldehyde, indole, and malononitrile was studied. The local temperature was probed using luminescence nanothermometry during the catalytic reaction.

Improving performance of luminescent nanothermometers based on non-thermally and thermally coupled levels of lanthanides by modulating laser power

This work sheds light on the pump power impact on the performance of luminescent thermometers, which is often underestimated by researchers. An up-converting, inorganic nanoluminophore, YVO₄:Yb³⁺,Er³⁺ (nanothermometer) was synthesized using the hydrothermal method and a subsequent calcination. This nanomaterial appears as a white powder composed of small nanoparticles (≈20 nm), exhibiting a very intense, green upconverted luminescence (λ_ex = 975 nm), visible to the naked eye. Its emission spectrum consists of four Er³⁺ bands (500-850 nm) and one Yb³⁺ band (>900 nm). The obtained compound exhibits temperature-dependent luminescence properties, hence it is used as an optical nanosensor of temperature. The determined band intensity ratios of the non-thermally coupled levels (non-TCLs) of Yb³⁺/Er³⁺ and thermally coupled levels (TCLs) of Er³⁺ are correlated with temperature, and they are used for ratiometric sensing of temperature. The effects of the pump (NIR laser) power on the luminescence properties of the material, including band intensity ratios, absolute and relative sensitivities and temperature resolution are analysed. It was pointed out that the applied laser power has a huge impact on the values of the aforementioned thermometric parameters, and manipulating the laser power can significantly improve the performance of optical nanothermometers.

Beam and sample movement compensation for robust spectro-microscopy measurements on a hard X-ray nanoprobe

Static and in situ nanoscale spectro-microscopy is now routinely performed on the Hard X-ray Nanoprobe beamline at Diamond and the solutions implemented to provide robust energy scanning and experimental operation are described. A software-based scheme for active feedback stabilization of X-ray beam position and monochromatic beam flux across the operating energy range of the beamline is reported, consisting of two linked feedback loops using extremum seeking and position control. Multimodal registration methods have been implemented for active compensation of drift during an experiment to compensate for sample movement during in situ experiments or from beam-induced effects.

Mapping Elevated Temperatures with a Micrometer Resolution Using the Luminescence of Chemically Stable Upconversion Nanoparticles

The temperature-sensitive luminescence of nanoparticles enables their application as remote thermometers. The size of these nanothermometers makes them ideal to map temperatures with a high spatial resolution. However, high spatial resolution mapping of temperatures >373 K has remained challenging. Here, we realize nanothermometry with high spatial resolutions at elevated temperatures using chemically stable upconversion nanoparticles and confocal microscopy. We test this method on a microelectromechanical heater and study the temperature homogeneity. Our experiments reveal distortions in the luminescence spectra that are intrinsic to high-resolution measurements of samples with nanoscale photonic inhomogeneities. In particular, the spectra are affected by the high-power excitation as well as by scattering and reflection of the emitted light. The latter effect has an increasing impact at elevated temperatures. We present a procedure to correct these distortions. As a result, we extend the range of high-resolution nanothermometry beyond 500 K with a precision of 1-4 K. This work will improve the accuracy of nanothermometry not only in micro- and nanoelectronics but also in other fields with photonically inhomogeneous substrates.

Lanthanide-Based Nanosensors: Refining Nanoparticle Responsiveness for Single Particle Imaging of Stimuli

Lanthanide nanoparticles (LNPs) are promising sensors of chemical, mechanical, and temperature changes; they combine the narrow-spectral emission and long-lived excited states of individual lanthanide ions with the high spatial resolution and controlled energy transfer of nanocrystalline architectures. Despite considerable progress in optimizing LNP brightness and responsiveness for dynamic sensing, detection of stimuli with a spatial resolution approaching that of individual nanoparticles remains an outstanding challenge. Here, we highlight the existing capabilities and outstanding challenges of LNP sensors, en-route to nanometer-scale, single particle sensor resolution. First, we summarize LNP sensor read-outs, including changes in emission wavelength, lifetime, intensity, and spectral ratiometric values that arise from modified energy transfer networks within nanoparticles. Then, we describe the origins of LNP sensor imprecision, including sensitivity to competing conditions, interparticle heterogeneities, such as the concentration and distribution of dopant ions, and measurement noise. Motivated by these sources of signal variance, we describe synthesis characterization feedback loops to inform and improve sensor precision, and introduce noise-equivalent sensitivity as a figure of merit of LNP sensors. Finally, we project the magnitudes of chemical and pressure stimulus resolution achievable with single LNPs at nanoscale resolution. Our perspective provides a roadmap for translating ensemble LNP sensing capabilities to the single particle level, enabling nanometer-scale sensing in biology, medicine, and sustainability.

Excited State Absorption for Ratiometric Thermal Imaging

Luminescence thermometry, an alternative to thermal imaging using the thermovision technique, requires the development of new approaches and a thorough understanding of the physical phenomena involved, in order to improve the temperature readout parameters. A phenomenon that has recently been shown to cause an extremely strong increase in the emission intensity for the temperature elevation is the thermally induced excited state absorption. This work demonstrates that taking advantage of the strong thermal dependence of the thermally induced excited state absorption process, the limitation associated with the two thermally coupled excited levels usually involved in the ratiometric temperature readout can be overcome, improving the thermometric properties of the luminescent thermometer. The same excitation wavelength was used to induce the emission resulting from the thermally induced excited state absorption of the Tb³⁺ ions and ground-state absorption of the other type of co-dopant ions causing the opposite nature of the thermal dependence of their emission intensities. Moreover, thanks to the strong color changes exhibited by the phosphors, it was possible to demonstrate the applicability of the proposed approach for through-object 2D thermal imaging of a microelectronic printed circuit board covered with a glass plate using an ordinary commercial digital camera, where the thermovision camera fails.

Luminescent Ratiometric Thermometers Based on a 4f-3d Grafted Covalent Organic Framework to Locally Measure Temperature Gradients During Catalytic Reactions

Covalent Organic Frameworks (COFs), an emerging class of crystalline porous materials, are a new type of support for grafting lanthanide ions (Ln³⁺ ), which can be employed as ratiometric luminescent thermometers. In this work we have shown that COFs co-grafted with lanthanide ions (Eu³⁺ , Tb³⁺ ) and Cu²⁺ (or potentially other d-metals) can synchronously be employed both as a nanothermometer and catalyst during a chemical reaction. The performance of the thermometer could be tuned by changing the grafted d-metal and solvent environment. As a proof of principle, the Glaser coupling reaction was investigated. We show that temperature can be precisely measured during the course of the catalytic reaction using luminescence thermometry. This concept could be potentially easily extended to other catalytic reactions by grafting other d-metal ions on the Ln@COF platform.

Luminescent Nanothermometer Operating at Very High Temperature-Sensing up to 1000 K with Upconverting Nanoparticles (Yb3+/Tm3+)

Lanthanide-based luminescent nanothermometers play a crucial role in optical temperature determination. However, because of the strong thermal quenching of the luminescence, as well as the deterioration of their sensitivity and resolution with temperature elevation, they can operate in a relatively low-temperature range, usually from cryogenic to ≈800 K. In this work, we show how to overcome these limitations and monitor very high-temperature values, with high sensitivity (≈2.1% K-1) and good thermal resolution (≈1.4 K) at around 1000 K. As an optical probe of temperature, we chose upconverting Yb3+-Tm3+ codoped YVO4 nanoparticles. For ratiometric sensing in the low-temperature range, we used the relative intensities of the Tm3+ emissions associated with the 3F2,3 and 3H4 thermally coupled levels, that is, 3F2,3 → 3H6/3H4 → 3H6 (700/800 nm) band intensity ratio. In order to improve sensitivity and resolution in the high-temperature range, we used the 940/800 nm band intensity ratio of the nonthermally coupled levels of Yb3+ (2F5/2 → 2F7/2) and Tm3+ (3H4 → 3H6). These NIR bands are very intense, even at extreme temperature values, and their intensity ratio changes significantly, allowing accurate temperature sensing with high thermal and spatial resolutions. The results presented in this work may be particularly important for industrial applications, such as metallurgy, catalysis, high-temperature synthesis, materials processing and engineering, and so forth, which require rapid, contactless temperature monitoring at extreme conditions.

Pushing the Limit of Boltzmann Distribution in Cr3+-Doped CaHfO3 for Cryogenic Thermometry

Luminescence Boltzmann thermometry is one of the most reliable techniques used to locally probe temperature in a contactless mode. However, to date, there is no report on cryogenic thermometers based on the highly sensitive and reliable Boltzmann-based ⁴T₂ → ⁴A₂/²E → ⁴A₂ emission ratio of Cr³⁺. On the basis of structural information of the local HfO₆ octahedral site we demonstrated the potential of the CaHfO₃:Cr³⁺ system by combining deep theoretical and experimental investigation. The material exhibits simultaneous emission from both the ²E and ⁴T₂ excited states, following the Boltzmann law in a cryogenic temperature range of 40-150 K. The promising thermometric performance corroborates the potential of CaHfO₃:Cr³⁺ as a Boltzmann cryothermometer, being characterized by a high relative sensitivity (∼ 2%·K^-1 at 40 K) and exceptional thermal resolution (0.045-0.77 K in the 40-150 K range). Moreover, by exploiting the flexibility of the ⁴T₂-²E energy gap controlled by the crystal field of the local octahedral site, the design proposed herein could be expanded to develop new Cr³⁺-doped cryogenic thermometers.

Operando Nanoscale Sensors in Catalysis: All Eyes on Catalyst Particles

An era of circularity requires robust and flexible catalysts and reactors. We need profound knowledge of catalytic surface reactions on the local scale (i.e., angstrom-nanometer), whereas the reaction conditions, such as reaction temperature and pressure, are set and controlled on the macroscale (i.e., millimeter-meter). Nanosensors operating on all relevant length scales can supply this information in real time during operando working conditions. In this Perspective, we demonstrate the potential of nanoscale sensors, with special emphasis on local molecular sensing with shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and local temperature sensing with luminescence thermometry, to acquire new insights of the reaction pathways. We also argue that further developments should be focused on local pressure measurements and on expanding the applications of these local sensors in other areas, such as liquid-phase catalysis, electrocatalysis, and photocatalysis. Ideally, a combination of sensors will be applied to monitor catalyst and reactor "health" and serve as feedback to the reactor conditions.

Making Nd3+ a Sensitive Luminescent Thermometer for Physiological Temperatures-An Account of Pitfalls in Boltzmann Thermometry

Ratiometric luminescence thermometry employing luminescence within the biological transparency windows provides high potential for biothermal imaging. Nd3+ is a promising candidate for that purpose due to its intense radiative transitions within biological windows (BWs) I and II and the simultaneous efficient excitability within BW I. This makes Nd3+ almost unique among all lanthanides. Typically, emission from the two 4F3/2 crystal field levels is used for thermometry but the small ~100 cm-1 energy separation limits the sensitivity. A higher sensitivity for physiological temperatures is possible using the luminescence intensity ratio (LIR) of the emissive transitions from the 4F5/2 and 4F3/2 excited spin-orbit levels. Herein, we demonstrate and discuss various pitfalls that can occur in Boltzmann thermometry if this particular LIR is used for physiological temperature sensing. Both microcrystalline, dilute (0.1%) Nd3+-doped LaPO4 and LaPO4: x% Nd3+ (x = 2, 5, 10, 25, 100) nanocrystals serve as an illustrative example. Besides structural and optical characterization of those luminescent thermometers, the impact and consequences of the Nd3+ concentration on their luminescence and performance as Boltzmann-based thermometers are analyzed. For low Nd3+ concentrations, Boltzmann equilibrium starts just around 300 K. At higher Nd3+ concentrations, cross-relaxation processes enhance the decay rates of the 4F3/2 and 4F5/2 levels making the decay faster than the equilibration rates between the levels. It is shown that the onset of the useful temperature sensing range shifts to higher temperatures, even above ~ 450 K for Nd concentrations over 5%. A microscopic explanation for pitfalls in Boltzmann thermometry with Nd3+ is finally given and guidelines for the usability of this lanthanide ion in the field of physiological temperature sensing are elaborated. Insight in competition between thermal coupling through non-radiative transitions and population decay through cross-relaxation of the 4F5/2 and 4F3/2 spin-orbit levels of Nd3+ makes it possible to tailor the thermometric performance of Nd3+ to enable physiological temperature sensing.