Impact of Noise and Background on Measurement Uncertainties in Luminescence Thermometry

Operando Luminescence Thermometry for Hydrocarbon Conversion Catalysis: Dealing with Dynamic Changes in Catalyst Optical Properties

Luminescence thermometry offers an attractive strategy for temperature sensing of a catalytic process, whereby the temperature-dependent luminescence of a phosphor material embedded in the reactor is excited and recorded remotely. However, changes in the optical properties of the catalyst materials can distort the luminescence recording, resulting in inaccurate temperature sensing. This is particularly problematic in hydrocarbon conversion catalysis due to the buildup of carbon deposits, which color the material and cause fluorescence upon excitation. In this work, we developed operando reflectance-corrected time-gated luminescence thermometry, using a highly dynamic and industrially relevant Pt-Sn-based propane dehydrogenation (PDH) catalyst and an Eu3+-based phosphor as a showcase. PDH catalyst materials deactivate via the deposition of carbon, which is subsequently removed by an oxidative regeneration treatment. During these alternating reaction-regeneration processes, the background fluorescence and the color of the catalyst material change continuously. This skews the Eu3+ luminescence, leading to temperature-readout artifacts. We solved these problems by rejecting background fluorescence using time-gated detection following pulsed excitation and by correcting for the wavelength-dependent absorption changes of the PDH catalyst. This method offers accurate temperature sensing of the PDH catalyst material and is a step forward in the development of luminescence thermometry for samples with highly dynamic optical properties.

What makes β-NaYF4:Er3+,Yb3+ such a successful luminescent thermometer?

Luminescence thermometry has emerged as a promising approach for remote, non-invasive temperature sensing at the nanoscale. One of the simplest approaches in that regard is single-ion luminescence Boltzmann thermometry that exploits thermal coupling between two radiatively emitting levels. The working horse example for this type of luminescence thermometry is undoubtedly the green-emitting upconversion phosphor β-NaYF4:Er3+,Yb3+ exploiting the thermal coupling between the two excited 2H11/2 and 4S3/2 levels of Er3+ for this purpose. Within this tutorial article, I would like to give a theoretically motivated account on the underlying reasons for the experimentally recorded success of this material for Boltzmann thermometry referring to time-resolved data on both the bulk and nanocrystalline material. Guidelines are established and both advantages and potential pitfalls in β-NaYF4:Er3+,Yb3+ for luminescence thermometry are given.

Excitation-Power Dependence of Lanthanide-Based Ratiometric Luminescent Nanothermometry

Ratiometric luminescent nanothermometry has emerged as a promising tool for remote thermal mapping at the nanoscale, yet its dependence on excitation power has been largely overlooked. Herein, we investigate the excitation power dependence of lanthanide-based ratiometric luminescent nanothermometers by examining two nonlinear pumping processes of Tm3+, where the differing slope factors of two emissions introduce significant intrinsic deviations in the luminescence intensity ratio (LIR) under varying excitation power densities. The robustness of the observed exponential relationship between excitation power density and LIR across different temperatures enables the derivation of a new calibration curve, applicable to any excitation power density. Additionally, analyzing the effect of excitation power on thermometric performance reveals that the absolute thermal sensitivity will change with the excitation power density, while the relative thermal sensitivity remains constant. This study provides valuable insights for optimizing ratiometric luminescent nanothermometry, offering a pathway to more accurate and reliable temperature measurements.

Multimodal Temperature Readout Boosts the Performance of CuInS2/ZnS Quantum Dot Nanothermometers

Fluorescent nanothermometers are positioned to revolutionize research into cell functions and provide strategies for early diagnostics. Fluorescent nanostructures hold particular promise to fulfill this potential if nontoxic, stable varieties allowing for precise temperature measurement with high thermal sensitivities can be fabricated. In this work, we investigate the performance of micelle-encapsulated CuInS2/ZnS core/shell colloidal quantum dots (QDs) as fluorescent nanothermometers. We demonstrate four temperature readout modes, which are based on variations in the photoluminescence intensity, energy, and lifetime and on a specific ratio of excitation efficiencies. We further leverage this multimodal readout to construct a fifth, multiparametric thermometer calibration based on the multiple linear regression (MLR) model. We show that the MLR approach boosts the thermometer sensitivity by up to 7-fold while reducing the readout error by about a factor of 3. As a result, our QDs offer the highest sensitivities among semiconducting QDs emitting in the first biological window. The obtained results indicate that CuInS2/ZnS QDs are excellent candidates for intracellular in vivo thermometry and provide guidelines for further optimization of their performance.

Rise and Decay of Photoluminescence in Upconverting Lanthanide-Doped Nanocrystals

Nanocrystals (NCs) doped with lanthanides are capable of efficient photon upconversion, i.e., absorbing long-wavelength light and emitting shorter-wavelength light. The internal processes that enable upconversion are a complex network of electronic transitions within and energy transfer between dopant centers. In this work, we study the rise and decay dynamics of upconversion emission from β-NaYF4 NCs codoped with Er3+ and Yb3+. The rise dynamics of the red and green upconverted emissions are nonlinear, reflecting the nonlinear nature of upconversion and revealing the mechanisms that populate the emitting states. The excited-state decay dynamics are nonexponential. We unravel the underlying decay pathways using photonic experiments. These reveal the contributions of different upconversion pathways visually, as each pathway exhibits a distinct response to systematic variation of the local density of optical states. Moreover, the effect of the local density of optical states on core-only NCs is qualitatively different from core-shell NCs. This is due to the different balance between feeding and decay of the electronic levels that produce upconverted emission. The understanding of the upconversion dynamics provided here could lead to better imaging and sensing methods relying on upconversion lifetimes or guide the rational optimization of the dopant concentrations for brighter upconversion.

Toward Accurate Photoluminescence Nanothermometry Using Rare-Earth Doped Nanoparticles for Biomedical Applications

Photoluminescence nanothermometry can detect the local temperature at the submicrometer scale with minimal contact with the object under investigation. Owing to its high spatial resolution, this technique shows great potential in biomedicine in both fundamental studies as well as preclinical research. Photoluminescence nanothermometry exploits the temperature-dependent optical properties of various nanoscale optical probes including organic fluorophores, quantum dots, and carbon nanostructures. At the vanguard of these diverse optical probes, rare-earth doped nanoparticles (RENPs) have demonstrated remarkable capabilities in photoluminescence nanothermometry. They distinguish themselves from other luminescent nanoprobes owning to their unparalleled and versatile optical properties that include narrow emission bandwidths, high photostability, tunable lifetimes from microseconds to milliseconds, multicolor emissions spanning the ultraviolet, visible, and near-infrared (NIR) regions, and the ability to undergo upconversion, all with excitation of a single, biologically friendly NIR wavelength. Recent advancements in the design of novel RENPs have led to new fundamental breakthroughs in photoluminescence nanothermometry. Moreover, driven by their excellent biocompatibility, both in vitro and in vivo, their implementation in biomedical applications has also gained significant traction. However, these nanoprobes face limitations caused by the complex biological environments, including absorption and scattering of various biomolecules as well as interference from different tissues, which limit the spatial resolution and detection sensitivity in RENP temperature sensing. Among existing approaches in RENP photoluminescence nanothermometry, the most prevalent implemented mechanisms either leverage the changes in the relative intensity ratio of two emission bands or exploit the lifetimes of various excited states. Photoluminescence intensity ratio (PLIR) nanothermometry has been the mainstream method owing to the readily available spectrometers for photoluminescence acquisition. Despite offering high temperature sensitivity and spatial resolution, this technique is restricted by tedious calibration and undesirable fluctuation in photoluminescence intensity ascribed to factors such as probe concentration, excitation power density, and biochemical surroundings. Lifetime-based nanothermometry uses the lifetime of a specific transition as the contrast mechanism to infer the temperature. This modality is less susceptible to various experimental factors and is compatible with a broader range of photoluminescence nanoprobes. However, due to relatively expensive and complex instrumentation, long data acquisition, and sophisticated data analysis, lifetime-based nanothermometry is still breaking ground with recently emerging techniques lightening its path. In this Account, we provide an overview of RENP nanothermometry and their applications in biomedicine. The architectures and luminescence mechanisms of RENPs are examined, followed by the principles of PLIR and lifetime-based nanothermometry. The in-depth description of each approach starts with its basic principle of accurate temperature sensing, followed by a critical discussion of the representative techniques, applications as well as their strengths and limitations. Special emphasis is given to the emerging modality of lifetime-based nanothermometry in light of the important new developments in the field. Finally, a summary and an outlook are provided to conclude this Account.

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.

Improper Background Treatment Underestimates Thermometric Performance of Rare Earth Vanadate and Phosphovanadate Nanocrystals

Luminescence thermometry is the state-of-the-art technique for remote nanoscale temperature sensing, offering numerous promising cutting-edge applications. Advancing nanothermometry further requires rational design of phosphors and well-defined, comprehensive mathematical treatment of spectral information. However, important questions regarding improper signal processing in ratiometric luminescence thermometry are continuously overlooked in the literature. Here, we demonstrate that systematic errors arising from background/signal superposition impact the calculated thermometric quality parameters of ratiometric thermometers. We designed ultraviolet-excitable (Y,Eu)VO4 and (Y,Eu)(P,V)O4 nanocrystals showing overlapped VO4 3- and Eu3+ emissions to discuss systematically how uncorrected background emissions cause magnified (∼10×) temperature uncertainties and undervalued (∼60%) relative thermal sensitivities. Adequate separation of spectral contributions from the VO4 3- background and the Eu3+ signals via baseline correction is necessary to prevent underestimation of the thermometric performances. The described approach can be potentially extended to other luminescent thermometers to account for signal superposition, thus enabling to circumvent computation of apparent, miscalculated thermometric parameters.

Lanthanide-based nanomaterials for temperature sensing in the near-infrared spectral region: illuminating progress and challenges

Being first proposed as a method to overcome limitations associated with conventional contact thermometers, luminescence thermometry has been extensively studied over the past two decades as a sensitive and fast approach to remote and minimally invasive thermal sensing. Herein, lanthanide (Ln)-doped nanoparticles (Ln-NPs) have been identified as particularly promising candidates, given their outstanding optical properties. Known primarily for their upconversion emission, Ln-NPs have also been recognized for their ability to be excited with and emit in the near-infrared (NIR) regions matching the NIR transparency windows. This sparked the emergence of the development of NIR-NIR Ln-NPs for a wide range of temperature-sensing applications. The shift to longer excitation and emission wavelengths resulted in increased efforts being put into developing nanothermometers for biomedical applications, however most research is still preclinical. This mini-review outlines and addresses the challenges that limit the reliability and implementation of luminescent nanothermometers to real-life applications. Through a critical look into the recent developments from the past 4 years, we highlight attempts to overcome some of the limitations associated with excitation wavelength, thermal sensitivity, calibration, as well as light-matter interactions. Strategies range from use of longer excitation wavelengths, brighter emitters through strategic core/multi-shell architectures, exploitation of host phonons, and a shift from double- to single-band ratiometric as well as lifetime-based approaches to innovative methods based on computation and machine learning. To conclude, we offer a perspective on remaining gaps and where efforts should be focused towards more robust nanothermometers allowing a shift to real-life, e.g., in vivo, applications.

Effect of annealing conditions on the luminescence properties and thermometric performance of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphors

We report on the synthesis, photoluminescence optimization and thermometric properties of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphor powders. The photoluminescence of Sr2.9Al2O5Cl2:0.1Eu2+ phosphors exhibits a blue-shift with an increasing annealing temperature owing to a decrease in the crystal field strength of the host caused by evaporation of Cl from the material. The quenching of the blue band in favour of the red band observed in the luminescence spectra of Sr2.9Al2O5Cl2:0.1Eu2+ with an increased annealing temperature was explained using the mechanism of the Landau-Zener transitions. The quantum yield and the lifetime of the phosphors depend on the annealing temperature. Phosphor samples annealed at 850 °C, 1000 °C, 1200 °C and 1500 °C were found to be potential luminescence thermometers using the luminescence spectral method. For Sr3Al2O5Cl2:Eu2+ annealed at 1000 °C, the temperature-dependent dual-band intensity ratio demonstrated a high-temperature sensitivity of ∼1.47%/°C in the temperature range of 23 °C to 40 °C which is superior to other reported phosphors with a microsecond decay time, suggesting that the material has potential for sensitive thermometry applications at ambient temperatures.

Short-wave Infrared Photoluminescence Lifetime Mapping of Rare-Earth Doped Nanoparticles Using All-Optical Streak Imaging

The short-wave infrared (SWIR) photoluminescence lifetimes of rare-earth doped nanoparticles (RENPs) have found diverse applications in fundamental and applied research. Despite dazzling progress in the novel design and synthesis of RENPs with attractive optical properties, existing optical systems for SWIR photoluminescence lifetime imaging are still considerably restricted by inefficient photon detection, limited imaging speed, and low sensitivity. To overcome these challenges, SWIR photoluminescence lifetime imaging microscopy using an all-optical streak camera (PLIMASC) is developed. Synergizing scanning optics and a high-sensitivity InGaAs CMOS camera, SWIR-PLIMASC has a 1D imaging speed of up to 138.9 kHz in the spectral range of 900-1700 nm, which quantifies the photoluminescence lifetime of RENPs in a single shot. A 2D photoluminescence lifetime map can be acquired by 1D scanning of the sample. To showcase the power of SWIR-PLIMASC, a series of core-shell RENPs with distinct SWIR photoluminescence lifetimes is synthesized. In particular, using Er3+ -doped RENPs, SWIR-PLIMASC enables multiplexed anti-counterfeiting. Leveraging Ho3+ -doped RENPs as temperature indicators, this system is applied to SWIR photoluminescence lifetime-based thermometry. Opening up a new avenue for efficient SWIR photoluminescence lifetime mapping, this work is envisaged to contribute to advanced materials characterization, information science, and biomedicine.

Ultrasensitive NIR-II Ratiometric Nanothermometers for 3D In Vivo Thermal Imaging

Luminescent nanothermometry, particularly the one based on ratiometric, has sparked intense research for non-invasive in vivo or intracellular temperature mapping, empowering their uses as diagnosis tools in biomedicine. However, ratiometric detection still suffers from biased sensing induced by wavelength-dependent tissue absorption and scattering, low thermal sensitivity (Sr ), and lack of imaging depth information. Herein, this work constructs an ultrasensitive NIR-II ratiometric nanothermometer with self-calibrating ability for 3D in vivo thermographic imaging, in which temperature-insensitive lanthanide nanocrystals and strongly temperature-quenched Ag2 S quantum dots are co-assembled to form a hybrid nanocomposite material. Precise control over the amount ratio between two sub-materials enables the manipulation of heat-activated back energy transfer from Ag2 S to Yb3+ in lanthanide nanoparticles, thereby rendering Sr up to 7.8% °C-1 at 43.5 °C, and higher than 6.5% °C-1 over the entire physiological temperature range. Moreover, the luminescence intensity ratio between two separated spectral regions within the narrow Yb3+ emission peak is used to determine the depth information of nanothermometers in living mice and correct the effect of tissue depth on 2D thermographic imaging, and therefore allows a proof-of-concept demonstration of accurate 3D in vivo thermographic imaging, constituting a solid step toward the development of advanced ratiometric nanothermometry for biological applications.

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.

Neural Networks Push the Limits of Luminescence Lifetime Nanosensing

Luminescence lifetime-based sensing is ideally suited to monitor biological systems due to its minimal invasiveness and remote working principle. Yet, its applicability is limited in conditions of low signal-to-noise ratio (SNR) induced by, e.g., short exposure times and presence of opaque tissues. Herein this limitation is overcome by applying a U-shaped convolutional neural network (U-NET) to improve luminescence lifetime estimation under conditions of extremely low SNR. Specifically, the prowess of the U-NET is showcased in the context of luminescence lifetime thermometry, achieving more precise thermal readouts using Ag2 S nanothermometers. Compared to traditional analysis methods of decay curve fitting and integration, the U-NET can extract average lifetimes more precisely and consistently regardless of the SNR value. The improvement achieved in the sensing performance using the U-NET is demonstrated with two experiments characterized by extreme measurement conditions: thermal monitoring of free-falling droplets, and monitoring of thermal transients in suspended droplets through an opaque medium. These results broaden the applicability of luminescence lifetime-based sensing in fields including in vivo experimentation and microfluidics, while, hopefully, spurring further research on the implementation of machine learning (ML) in luminescence sensing.

Operando time-gated Raman spectroscopy of solid catalysts

Operando Raman spectroscopy is a powerful analytical tool to provide new insights in the working and deactivation principles of solid catalysts. Intense fluorescence can obscure Raman spectra to the extent that they become uninterpretable. Time-gated Raman spectroscopy, based on pulsed excitation and time-gated detection, suppresses background fluorescence based on its slower time dynamics compared to Raman scattering. In this work, we demonstrate and quantify the benefit of time gating for operando Raman spectroscopy, using the propane dehydrogenation reaction over Pt-Sn-based catalyst materials as a case study. Experimental time-gated Raman spectroscopy data are fitted to a time-trace model that is used to optimize time gating for the maximum signal-to-background-noise ratio. Time-gated Raman spectra of a spent propane dehydrogenation catalyst material show lower background fluorescence compared to the time-integrated Raman spectra counterparts. Simultaneous operando time-gated and time-integrated Raman spectroscopy experiments demonstrate the benefit of time gating to obtain more distinct Raman features, especially in the early coking stages where spectra are dominated by background fluorescence.

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.

Multiple site occupancy induced yellow-orange emission in an Eu2+-doped KSr6Sc(SiO4)4 phosphor towards optical temperature sensors

Phosphors have attracted significant interest as potential optical temperature sensors in recent years. In our work, a new blue-light stimulated KSr₆Sc(SiO₄)₄:Eu²⁺ phosphor with decorative kröhnkite-like octahedral tetrahedral chains was successfully synthesized. Multiple site occupancy occurred in KSr₆Sc(SiO₄)₄:Eu²⁺ and induced a yellow-orange emission band with a peak at 571 nm and an FWHM of 91 nm. Gaussian fitting and time-resolved photoluminescence mapping were combined to analyze the occupation of Eu²⁺ in five Sr²⁺ sites. In the meantime, the site occupation preference, energy transfer process, and thermal quenching mechanism of Eu²⁺ emission centers have been comprehensively examined. Under 450 nm excitation, the optimal sample possesses an acceptable quantum efficiency (EQE = 17.3%) and a high sensitivity between luminescence properties and temperature variation ranging from 200 to 475 K. The optimal sample's relative sensor sensitivity achieves a maximum value of 3.53% K^-1 at 475 K. The phosphor KSr₆Sc(SiO₄)₄:0.07Eu²⁺ presents the potentiality as an optical thermometer.

Comparison of Performance between Single- and Multiparameter Luminescence Thermometry Methods Based on the Mn5+ Near-Infrared Emission

Herein, we investigate the performance of single- and multiparametric luminescence thermometry founded on the temperature-dependent spectral features of Ca₆BaP₄O₁₇:Mn⁵⁺ near-infrared emission. The material was prepared by a conventional steady-state synthesis, and its photoluminescence emission was measured from 7500 to 10,000 cm^-1 over the 293-373 K temperature range in 5 K increments. The spectra are composed of the emissions from ¹E → ³A₂ and ³T₂ → ³A₂ electronic transitions and Stokes and anti-Stokes vibronic sidebands at 320 cm^-1 and 800 cm^-1 from the maximum of ¹E → ³A₂ emission. Upon temperature increase, the ³T₂ and Stokes bands gained in intensity while the maximum of ¹E emission band is redshifted. We introduced the procedure for the linearization and feature scaling of input variables for linear multiparametric regression. Then, we experimentally determined accuracies and precisions of the luminescence thermometry based on luminescence intensity ratios between emissions from the ¹E and ³T₂ states, between Stokes and anti-Stokes emission sidebands, and at the ¹E energy maximum. The multiparametric luminescence thermometry involving the same spectral features showed similar performance, comparable to the best single-parameter thermometry.

Extending the dynamic temperature range of Boltzmann thermometers

Lanthanide-doped (nano)crystals are an important class of materials in luminescence thermometry. The working mechanism of these thermometers is diverse but most often relies on variation of the ratio of emission intensities from two thermally coupled excited states with temperature. At low temperatures, nonradiative coupling between the states can be slow compared to radiative decay, but, at higher temperatures, the two states reach thermal equilibrium due to faster nonradiative coupling. In thermal equilibrium, the intensity ratio follows Boltzmann statistics, which gives a convenient model to calibrate the thermometer. Here, we investigate multiple strategies to shift the onset of thermal equilibrium to lower temperatures, which enables Boltzmann thermometry in a wider dynamic range. We use Eu³⁺-doped microcrystals as a model system and find that the nonradiative coupling rates increase for host lattices with higher vibrational energies and shorter lanthanide-ligand distances, which reduces the onset temperature of thermal equilibrium by more than 400 K. We additionally reveal that thermometers with excited states coupled by electric-dipole transitions have lower onset temperatures than those with magnetic-dipole-coupled states due to selection rules. These insights provide essential guidelines for the optimization of Boltzmann thermometers to operate in an extended temperature range.

2D Thermal Maps Using Hyperspectral Scanning of Single Upconverting Microcrystals: Experimental Artifacts and Image Processing

Whereas lanthanide-based upconverting particles are promising candidates for several micro- and nanothermometry applications, understanding spatially varying effects related to their internal dynamics and interactions with the environment near the surface remains challenging. To separate the bulk from the surface response, this work proposes and performs hyperspectral sample-scanning experiments to obtain spatially resolved thermometric measurements on single microparticles of NaYF₄: Yb³⁺,Er³⁺. Our results showed that the particle's thermometric response depends on the excitation laser incidence position, which may directly affect the temperature readout. Furthermore, it was noticed that even minor temperature changes (<1 K) caused by room temperature variations at the spectrometer CCD sensor used to record the luminescence signal may significantly modify the measurements. This work also provides some suggestions for building 2D thermal maps that shall be helpful for understanding surface-related effects in micro- and nanothermometers using hyperspectral techniques. Therefore, the results presented herein may impact applications of lanthanide-based nanothermometers, as in the understanding of energy-transfer processes inside systems such as nanoelectronic devices or living cells.

Less is more: dimensionality reduction as a general strategy for more precise luminescence thermometry

Thermal resolution (also referred to as temperature uncertainty) establishes the minimum discernible temperature change sensed by luminescent thermometers and is a key figure of merit to rank them. Much has been done to minimize its value via probe optimization and correction of readout artifacts, but little effort was put into a better exploitation of calibration datasets. In this context, this work aims at providing a new perspective on the definition of luminescence-based thermometric parameters using dimensionality reduction techniques that emerged in the last years. The application of linear (Principal Component Analysis) and non-linear (t-distributed Stochastic Neighbor Embedding) transformations to the calibration datasets obtained from rare-earth nanoparticles and semiconductor nanocrystals resulted in an improvement in thermal resolution compared to the more classical intensity-based and ratiometric approaches. This, in turn, enabled precise monitoring of temperature changes smaller than 0.1 °C. The methods here presented allow choosing superior thermometric parameters compared to the more classical ones, pushing the performance of luminescent thermometers close to the experimentally achievable limits.