Intracellular temperature mapping with a fluorescent polymeric thermometer and fluorescence lifetime imaging microscopy. Nat Commun. 2012;3:705. [PMID: 22426226 PMCID: PMC3293419 DOI: 10.1038/ncomms1714]

Diamond nitrogen-vacancy color-centered thermometer for integrated circuit application

With the advancement of the chip industry, accurate temperature measurement and thermal management have become crucial. Traditional infrared temperature imaging has limitations in terms of resolution and accuracy. ln recent years, quantum diamond nitrogen-vacancy centers have emerged as a promising option for temperature sensing, but separating temperature from magnetic field effects remains a challenge. This paper presents a numerical approach to decouple temperature and magnetic fields using an ensemble Hamiltonian in high-current density Integrated Circuit (IC) applications. The proposed method demonstrates a temperature sensitivity of 22.9 mK/Hz1/2 and the ability to perform scanning temperature imaging with a spatial resolution of 20 µm on a typical IC.

Particle-based phasor-FLIM-FRET resolves protein-protein interactions inside single viral particles

Fluorescence lifetime imaging microscopy (FLIM) is a popular modality to create additional contrast in fluorescence images. By carefully analyzing pixel-based nanosecond lifetime patterns, FLIM allows studying complex molecular populations. At the single-molecule or single-particle level, however, image series often suffer from low signal intensities per pixel, rendering it difficult to quantitatively disentangle different lifetime species, such as during Förster resonance energy transfer (FRET) analysis in the presence of a significant donor-only fraction. In this article we investigate whether an object localization strategy and the phasor approach to FLIM have beneficial effects when carrying out FRET analyses of single particles. Using simulations, we first showed that an average of ∼300 photons, spread over the different pixels encompassing single fluorescing particles and without background, is enough to determine a correct phasor signature (SD < 5% for a 4-ns lifetime). For immobilized single- or double-labeled dsDNA molecules, we next validated that particle-based phasor-FLIM-FRET readily allows estimating fluorescence lifetimes and FRET from single molecules. Thirdly, we applied particle-based phasor-FLIM-FRET to investigate protein-protein interactions in subdiffraction HIV-1 viral particles. To do this, we first quantitatively compared the fluorescence brightness, lifetime, and photostability of different popular fluorescent protein-based FRET probes when genetically fused to the HIV-1 integrase enzyme in viral particles, and conclude that eGFP, mTurquoise2, and mScarlet perform best. Finally, for viral particles coexpressing FRET-donor/acceptor-labeled IN, we determined the absolute FRET efficiency of IN oligomers. Available in a convenient open-source graphical user interface, we believe that particle-based phasor-FLIM-FRET is a promising tool to provide detailed insights in samples suffering from low overall signal intensities.

Hydrogel Micropillar Array for Temperature Sensing in Fluid

Localized temperature sensing and control on a micron-scale have diverse applications in biological systems. We present a micron-sized hydrogel pillar array as potential temperature probes and actuators by exploiting sensitive temperature dependence of their volume change. Soft lithography-based molding processes were presented to fabricate poly N-isopropyl acrylamide (p-NIPAAm)-based hydrogel pillar array on a glass substrate. Au nanorods as light-induced heating elements were embedded within the hydrogel pillars, and near-infrared (NIR) light was used to modulate temperature in a local area. First, static responses of the micro-pillar array were characterized as a function of its temperature. It was shown that the hydrogel had a sensitive volume transition near its low critical solution temperature (LCST). Furthermore, we showed that LCST could be readily adjusted by utilizing copolymerizing with acrylamide (AAM). To demonstrate the feasibility of spatiotemporal temperature mapping and modulation using the presented pillar array, pulsed NIR light was illuminated on a local area of the hydrogel pillar array, and its responses were recorded. Dynamic temperature change in water was mapped based on the abrupt volume change characteristics of the hydrogel pillar, and its potential actuation using NIR light was successfully demonstrated. Considering that the structure can be arrayed in a two-dimensional pixel format with high spatial resolution and high sensitive temperature characteristics, the presented method and the device structure can have diverse applications to change and sense local temperatures in liquid. This is particularly useful in biological systems, where their physiological temperature can be modulated and mapped with high spatial resolution.

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.

Photothermia at the nanoscale induces ferroptosis via nanoparticle degradation

The Fe(II)-induced ferroptotic cell death pathway is an asset in cancer therapy, yet it calls into question the biocompatibility of magnetic nanoparticles. In the latter, Fe(II) is sequestered within the crystal structure and is released only upon nanoparticle degradation, a transition that is not well understood. Here, we dissect the chemical environment necessary for nanoparticle degradation and subsequent Fe(II) release. Importantly, temperature acts as an accelerator of the process and can be triggered remotely by laser-mediated photothermal conversion, as evidenced by the loss of the nanoparticles' magnetic fingerprint. Remarkably, the local hot-spot temperature generated at the nanoscale can be measured in operando, in the vicinity of each nanoparticle, by comparing the photothermal-induced nanoparticle degradation patterns with those of global heating. Further, remote photothermal irradiation accelerates degradation inside cancer cells in a tumor spheroid model, with efficiency correlating with the endocytosis progression state of the nanoparticles. High-throughput imaging quantification of Fe2+ release, ROS generation, lipid peroxidation and cell death at the spheroid level confirm the synergistic thermo-ferroptotic therapy due to the photothermal degradation at the nanoparticle level.

Ratiometric Nanothermometer Based on a Radical Excimer for In Vivo Sensing

Ratiometric fluorescent nanothermometers with near-infrared emission play an important role in in vivo sensing since they can be used as intracellular thermal sensing probes with high spatial resolution and high sensitivity, to investigate cellular functions of interest in diagnosis and therapy, where current approaches are not effective. Herein, the temperature-dependent fluorescence of organic nanoparticles is designed, synthesized, and studied based on the dual emission, generated by monomer and excimer species, of the tris(2,4,6-trichlorophenyl)methyl radical (TTM) doping organic nanoparticles (TTMd-ONPs), made of optically neutral tris(2,4,6-trichlorophenyl)methane (TTM-αH), acting as a matrix. The excimer emission intensity of TTMd-ONPs decreases with increasing temperatures whereas the monomer emission is almost independent and can be used as an internal reference. TTMd-ONPs show a great temperature sensitivity (3.4% K-1 at 328 K) and a wide temperature response at ambient conditions with excellent reversibility and high colloidal stability. In addition, TTMd-ONPs are not cytotoxic and their ratiometric outputs are unaffected by changes in the environment. Individual TTMd-ONPs are able to sense temperature changes at the nano-microscale. In vivo thermometry experiments in Caenorhabditis elegans (C. elegans) worms show that TTMd-ONPs can locally monitor internal body temperature changes with spatio-temporal resolution and high sensitivity, offering multiple applications in the biological nanothermometry field.

Thermal transport across membranes and the Kapitza length from photothermal microscopy

An analytical model is presented for light scattering associated with heat transport near a cell membrane that divides a complex system into two topologically distinct half-spaces. Our analysis is motivated by experiments on vibrational photothermal microscopy which have not only demonstrated remarkably high contrast and resolution, but also are capable of providing label-free local information of heat transport in complex morphologies. In the first Born approximation, the derived Green's function leads to the reconstruction of a full 3D image with photothermal contrast obtained using both amplitude and phase detection of periodic excitations. We show that important fundamental parameters including the Kapitza length and Kapitza resistance can be derived from experiments. Our goal is to spur additional experimental studies with high-frequency modulation and heterodyne detection in order to make contact with recent theoretical molecular dynamics calculations of thermal transport properties in membrane systems.

Nanoparticles for Interrogation of Cell Signaling

Cell functions rely on signal transduction-the cascades of molecular interactions and biochemical reactions that relay extracellular signals to the cell interior. Dissecting principles governing the signal transduction process is critical for the fundamental understanding of cell physiology and the development of biomedical interventions. The complexity of cell signaling is, however, beyond what is accessible by conventional biochemistry assays. Thanks to their unique physical and chemical properties, nanoparticles (NPs) have been increasingly used for the quantitative measurement and manipulation of cell signaling. Even though research in this area is still in its infancy, it has the potential to yield new, paradigm-shifting knowledge of cell biology and lead to biomedical innovations. To highlight this importance, we summarize in this review studies that pioneered the development and application of NPs for cell signaling, from quantitative measurements of signaling molecules to spatiotemporal manipulation of cell signal transduction.

Photothermal Effects of High-Energy Photobiomodulation Therapies: An In Vitro Investigation

The purpose of this study was to investigate photothermal aspects of photobiomodulation therapies (PBMT) in vitro to assist in the development of safe clinical parameters with respect to higher-power devices with large surface applicators. Laser wavelengths in the range of 650 nm-1064 nm were investigated using a thermal camera. Thermographic measures of surface and sub-surface temperature variations of similar lean porcine muscle tissue samples were recorded for a series of calibrated experiments. A thermal comparison was then made between Flat-top and Gaussian beam spatial distribution devices. Outcome data were subjected to statistical analysis using an ANOVA model. Results acquired at similar parameters of irradiance indicated that the application of the 980 nm wavelength was associated with the highest rise in temperature, which decreased with other wavelengths in the order 980 > 1064 ≈ 650 >>> 810 nm (p < 5 × 10^-20). All wavelengths assessed were associated with a significant temperature increase, and with the exception of 810 nm, all exceeded the threshold of a 6 °C rise within the prescribed parameter limits. Optical scanning by movement of the applied source over a relevant area was found to offer effective mitigation of these temperature increases. An extended discussion is presented, analysing the clinical significance of the study outcomes. Recommendations are made within the limits of this in vitro study in order to assist future clinical investigations.

Measurement of cellular thermal properties and their temperature dependence based on frequency spectra via an on-chip-integrated microthermistor

To understand the mechanism of intracellular thermal transport, thermal properties must be elucidated, particularly thermal conductivity and specific heat capacity. However, these properties have not been extensively studied. In this study, we developed a cellular temperature measurement device with a high temperature resolution of 1.17 m °C under wet conditions and with the ability to introduce intracellular local heating using a focused infrared laser to cultured cells on the device surface. Using this device, we evaluated the thermal properties of single cells based on their temperature signals and responses. Measurements were taken using on-chip-integrated microthermistors with high temperature resolution at varying surrounding temperatures and frequencies of local infrared irradiation on cells prepared on the sensors. Frequency spectra were used to determine the intensities of the temperature signals with respect to heating times. Signal intensities at 37 °C and a frequency lower than 2 Hz were larger than those at 25 °C, which were similar to those of water. The apparent thermal conductivity and specific heat capacity, which were determined at different surrounding temperatures and local heating frequencies, were lower than and similar to those of water at 37 °C and 25 °C, respectively. Our results indicate that the thermal properties of cells depend on both temperatures and physiological activities in addition to local heating frequencies.

Accurate Visualization of Metabolic Aberrations in Cancer Cells by Temperature Mapping with Quantum Coherence Modulation Microscopy

Specific metabolic aberrations of cancer cells rapidly generate energy with a minuscule but detectable temperature variation, which is a typical characteristic providing insight into cancer pathogenesis. However, to date, intracellular temperature mapping of cancer cell metabolism with high temporal and spatial resolution has not been realized. In this study, we mapped and monitored in real-time the intracellular temperature variations of mitochondria and cytoplasm at a subcellular scale via a single-molecule coherent modulation microscopy coupling targeted molecule labeling technique. According to the variation of the decoherence processes of targeted molecules as a function of intracellular temperature, we achieved a high temperature resolution (<0.1 K) and proved that this technique could eliminate interference from fluorescence intensity disturbance and external pH change. Furthermore, we showed a positive correlation between the determined temperature and the adenosine triphosphate production rate of mitochondrial metabolism in combination with a cell energy metabolic analyzer. This technology enables accurate real-time temporal and spatial visualization of cancer metabolism and establishes diagnoses and therapies for cancer.

Quinolinium-Based Fluorescent Probes for Dynamic pH Monitoring in Aqueous Media at High pH Using Fluorescence Lifetime Imaging

Spatiotemporal pH imaging using fluorescence lifetime imaging microscopy (FLIM) is an excellent technique for investigating dynamic (electro)chemical processes. However, probes that are responsive at high pH values are not available. Here, we describe the development and application of dedicated pH probes based on the 1-methyl-7-amino-quinolinium fluorophore. The high fluorescence lifetime and quantum yield, the high (photo)stability, and the inherent water solubility make the quinolinium fluorophore well suited for the development of FLIM probes. Due to the flexible fluorophore-spacer-receptor architecture, probe lifetimes are tunable in the pH range between 5.5 and 11. An additional fluorescence lifetime response, at tunable pH values between 11 and 13, is achieved by deprotonation of the aromatic amine at the quinolinium core. Probe lifetimes are hardly affected by temperature and the presence of most inorganic ions, thus making FLIM imaging highly reliable and convenient. At 0.1 mM probe concentrations, imaging at rates of 3 images per second, at a resolution of 4 μm, while measuring pH values up to 12 is achieved. This enables the pH imaging of dynamic electrochemical processes involving chemical reactions and mass transport.

Local Temperature Increments and Induced Cell Death in Intracellular Magnetic Hyperthermia

The generation of temperature gradients on nanoparticles heated externally by a magnetic field is crucially important in magnetic hyperthermia therapy. But the intrinsic low heating power of magnetic nanoparticles, at the conditions allowed for human use, is a limitation that restricts the general implementation of the technique. A promising alternative is local intracellular hyperthermia, whereby cell death (by apoptosis, necroptosis, or other mechanisms) is attained by small amounts of heat generated at thermosensitive intracellular sites. However, the few experiments conducted on the temperature determination of magnetic nanoparticles have found temperature increments that are much higher than the theoretical predictions, thus supporting the local hyperthermia hypothesis. Reliable intracellular temperature measurements are needed to get an accurate picture and resolve the discrepancy. In this paper, we report the real-time variation of the local temperature on γ-Fe₂O₃ magnetic nanoheaters using a Sm³⁺/Eu³⁺ ratiometric luminescent thermometer located on its surface during exposure to an external alternating magnetic field. We measure maximum temperature increments of 8 °C on the surface of the nanoheaters without any appreciable temperature increase on the cell membrane. Even with magnetic fields whose frequency and intensity are still well within health safety limits, these local temperature increments are sufficient to produce a small but noticeable cell death, which is enhanced considerably as the magnetic field intensity is increased to the maximum level tolerated for human use, consequently demonstrating the feasibility of local hyperthermia.

Water-Soluble Cationic Eu3+-Metallopolymer with High Quantum Yield and Sensitivity for Intracellular Temperature Sensing

Lanthanide-based (Ln³⁺) luminescent materials are ideal candidates for use in fluorescence intracellular temperature sensing. However, it remains a great challenge to obtain a Ln³⁺-ratiometric fluorescence thermometer with high sensitivity and quantum yield in an aqueous environment. Herein, a cationic Eu³⁺-metallopolymer was synthesized via the coordination of Eu(TTA)₃·2H₂O with an AIE active amphipathic polymer backbone that contains APTMA ((3-acrylamidopropyl) trimethylammonium) and NIPAM (N-isopropylacrylamide) units, which can self-assemble into nanoparticles in water solution with APTMA and NIPAM as the hydrophilic shell. This polymer exhibited highly efficient dual-emissive white-light emission (Φ = 34.3%). Particularly, when the temperature rises, the NIPAM units will transform from hydrophilic to hydrophobic in the spherical core of the nanoparticle, while the VTPE units are moved from inside the nanoparticle to the shell, activating its nonradiative transition channel and thereby decreasing its energy transfer to Eu³⁺ centers, endowing the Eu³⁺-metallopolymer with an extremely high temperature sensing sensitivity within the physiological temperature range. Finally, the real-time monitoring of the intracellular temperature variation is further conducted.

Thermal conductivity and conductance of protein in aqueous solution: Effects of geometrical shape

Considering the importance of elucidating the heat transfer in living cells, we evaluated the thermal conductivity κ and conductance G of hydrated protein through all-atom non-equilibrium molecular dynamics simulation. Extending the computational scheme developed in earlier studies for spherical protein to cylindrical one under the periodic boundary condition, we enabled the theoretical analysis of anisotropic thermal conduction and also discussed the effects of protein size correction on the calculated results. While the present results for myoglobin and green fluorescent protein (GFP) by the spherical model were in fair agreement with previous computational and experimental results, we found that the evaluations for κ and G by the cylindrical model, in particular, those for the longitudinal direction of GFP, were enhanced substantially, but still keeping a consistency with experimental data. We also studied the influence by salt addition of physiological concentration, finding insignificant alteration of thermal conduction of protein in the present case.

Thermoresponse of Odd-Even Effect in n-Alkanethiolate Self-Assembled Monolayers on Gold Substrates

This study examines thermoresponse of odd-even effect in self-assembled monolayers (SAMs) of n-alkanethiolates (SC_n , n=3-18) formed on template-stripped gold (Au^TS ) using macro- and microscopic analytical techniques, contact angle goniometry (CAG) and vibrational sum frequency generation (VSFG) spectroscopy, respectively. Both CAG and VSFG analyses showed that the odd-even effect in liquid-like SAMs (n=3-9) disappeared upon heating at 50-70 °C, indicating that the heating led to increased structural disorder regardless of odd and even carbon numbers. In contrast, the opposite thermoresponse was observed for odd and even SC_n molecules in wax- and solid-like SAMs (n=10-18). Namely, temperature-dependent orientational change of terminal CH₃ relative to the surface normal was opposite for the odd and even molecules, thereby leading to mitigated odd-even effect. Our work offers important insights into thermoresponse of supramolecular structure in condensed organic matter.

Vast heterogeneity in cytoplasmic diffusion rates revealed by nanorheology and Doppelgänger simulations

The cytoplasm is a complex, crowded, actively driven environment whose biophysical characteristics modulate critical cellular processes such as cytoskeletal dynamics, phase separation, and stem cell fate. Little is known about the variance in these cytoplasmic properties. Here, we employed particle-tracking nanorheology on genetically encoded multimeric 40 nm nanoparticles (GEMs) to measure diffusion within the cytoplasm of individual fission yeast (Schizosaccharomyces pombe) cellscells. We found that the apparent diffusion coefficients of individual GEM particles varied over a 400-fold range, while the differences in average particle diffusivity among individual cells spanned a 10-fold range. To determine the origin of this heterogeneity, we developed a Doppelgänger simulation approach that uses stochastic simulations of GEM diffusion that replicate the experimental statistics on a particle-by-particle basis, such that each experimental track and cell had a one-to-one correspondence with their simulated counterpart. These simulations showed that the large intra- and inter-cellular variations in diffusivity could not be explained by experimental variability but could only be reproduced with stochastic models that assume a wide intra- and inter-cellular variation in cytoplasmic viscosity. The simulation combining intra- and inter-cellular variation in viscosity also predicted weak nonergodicity in GEM diffusion, consistent with the experimental data. To probe the origin of this variation, we found that the variance in GEM diffusivity was largely independent of factors such as temperature, the actin and microtubule cytoskeletons, cell-cyle stage, and spatial locations, but was magnified by hyperosmotic shocks. Taken together, our results provide a striking demonstration that the cytoplasm is not "well-mixed" but represents a highly heterogeneous environment in which subcellular components at the 40 nm size scale experience dramatically different effective viscosities within an individual cell, as well as in different cells in a genetically identical population. These findings carry significant implications for the origins and regulation of biological noise at cellular and subcellular levels.

Fluorescence lifetime: Beating the IRF and interpulse window

Fluorescence lifetime imaging captures the spatial distribution of chemical species across cellular environments employing pulsed illumination confocal setups. However, quantitative interpretation of lifetime data continues to face critical challenges. For instance, fluorescent species with known in vitro excited-state lifetimes may split into multiple species with unique lifetimes when introduced into complex living environments. What is more, mixtures of species, which may be both endogenous and introduced into the sample, may exhibit 1) very similar lifetimes as well as 2) wide ranges of lifetimes including lifetimes shorter than the instrumental response function or whose duration may be long enough to be comparable to the interpulse window. By contrast, existing methods of analysis are optimized for well-separated and intermediate lifetimes. Here, we broaden the applicability of fluorescence lifetime analysis by simultaneously treating unknown mixtures of arbitrary lifetimes-outside the intermediate, Goldilocks, zone-for data drawn from a single confocal spot leveraging the tools of Bayesian nonparametrics (BNP). We benchmark our algorithm, termed BNP lifetime analysis, using a range of synthetic and experimental data. Moreover, we show that the BNP lifetime analysis method can distinguish and deduce lifetimes using photon counts as small as 500.

A Guide to Plant Intracellular Temperature Imaging using Fluorescent Thermometers

All aspects of plant physiology are influenced by temperature. Changes in environmental temperature alter the temperatures of plant tissues and cells, which then affect various cellular activities, such as gene expression, protein stability and enzyme activities. In turn, changes in cellular activities, which are associated with either exothermic or endothermic reactions, can change the local temperature in cells and tissues. In the past 10 years, a number of fluorescent probes that detect temperature and enable intracellular temperature imaging have been reported. Intracellular temperature imaging has revealed that there is a temperature difference >1°C inside cells and that the treatment of cells with mitochondrial uncoupler or ionomycin can cause more than a 1°C intracellular temperature increase in mammalian cultured cells. Thermogenesis mechanisms in brown adipocytes have been revealed with the aid of intracellular temperature imaging. While there have been no reports on plant intracellular temperature imaging thus far, intracellular temperature imaging is expected to provide a new way to analyze the mechanisms underlying the various activities of plant cells. In this review, I will first summarize the recent progress in the development of fluorescent thermometers and their biological applications. I will then discuss the selection of fluorescent thermometers and experimental setup for the adaptation of intracellular temperature imaging to plant cells. Finally, possible applications of intracellular temperature imaging to investigate plant cell functions will be discussed.

Total Luminescence Spectroscopy for Quantification of Temperature Effects on Photophysical Properties of Photoluminescent Materials

Quantification of the temperature effects on the optical properties of photoluminescent (PL) materials is important for a fundamental understanding of both materials optical processes and rational PL materials design and applications. However, existing techniques for studying the temperature effects are limited in their information content. Reported herein is a temperature-dependent total photoluminescence (TPL) spectroscopy technique for probing the temperature dependence of materials optical properties. When used in combination with UV-vis measurements, this TPL method enables experimental quantification of temperature effects on fluorophore fluorescence intensity and quantum yield at any combination of excitation and detection wavelengths, including the fluorophore Stokes-shifted and anti-Stokes-shifted fluorescence. All model polyaromatic hydrocarbon (PAH) and xanthene fluorophores exhibited a strong excitation- and emission-wavelength dependence in their temperature effects. However, the heavy-atom effects used for explaining the strong temperature dependence of brominated anthracenes are not operative with xanthene fluorophores that have heavy atom substitutions. The insights from TPL measurements are important not only for enhancing the fundamental understandings of the materials photophysical properties but also for rational measurement design for applications where the temperature sensitivity of the fluorophore fluorescence is critical. An example application is demonstrated for developing a sensitive and robust ratiometric fluorescence thermometric method for in situ real-time monitoring of sample temperatures inside a fluorescence cuvette placed in a temperature-controlled sample holder.

Visible light responsive photoacids for subcellular pH and temperature correlated fluorescence sensing

Liao's photoacids (PAs) are a well-known type of visible light-responsive photoswitches. Here, taking advantage of the temperature-dependent thermal relaxation from the ring-closed to the ring-opened forms, PAs are proposed for the first time as a fluorescent temperature sensor in cells. The logarithmic lifetime (ln τ) of the ring-closed spiro-form exhibited an excellent linear response to the reciprocal of the temperature. In addition, the fluorescent ring-opened PAs were able to highlight lysosomes and responded to lysosomal pH changes. These properties made the PAs promising fluorescent probes in the sensing of subcellular pH and temperature.

ZnTe Crystal Multimode Cryogenic Thermometry Using Raman and Luminescence Spectroscopy

In this study, ZnTe crystal was applied to provide precise thermal sensing for cryogenic temperatures. Multiple techniques, namely Raman and photoluminescence spectroscopies, were used to broaden the operating temperature range and improve the reliability of the proposed thermometers. Raman-based temperature sensing could be applied in the range of 20-100 K, while luminescence-based thermometry could be utilized in a narrower range of 20-70 K. However, the latter strategy provides better relative thermal sensitivity and temperature resolution. The best thermal performances based on a single temperature-dependent parameter attain S_r = 3.82% K^-1 and ΔT = 0.12 K at T = 50 K. The synergy between multiple linear regression and multiparametric thermal sensing demonstrated for Raman-based thermometry results in a ten-fold improvement of _Sr and a two-fold enhancement of ΔT. All studies performed testify that the ZnTe crystal is a promising multimode contactless optical sensor for cryogenic thermometry.

Single-cell thermometry with a nanothermocouple probe

Herein, a nanopipette-based thermocouple probe that possesses high temperature resolution, rapid response, good reversibility and stability was constructed and successfully applied for single-cell temperature sensing. Different intracellular temperatures were observed in diverse types of cells, which reveals differences in their metabolism levels. Temperature responses of cancer and normal cells against various exogenous drugs were also demonstrated. The spatially resolved temperature sensing of three-dimensional cell culture models unveils the existence of their inner temperature gradients. This work would facilitate drug screening and disease diagnosis.

Measurement of Intracellular Temperature in Brown Adipocytes Using a Cationic Fluorescent Polymeric Thermometer

Brown adipose tissue specializes in expending energy through non-shivering thermogenesis, and many studies have associated its activity with protection and treatment of obesity and metabolic diseases. To reveal the mechanisms involved in heat production, primary cultured brown adipose cells (BACs) have been used because of their ease of genetic engineering and similarity to living tissue. However, thermogenic activity has often been evaluated as an indirect method, such as the measurement of oxygen consumption. Recently, fluorescent nanothermometers for the direct measurement of intracellular temperature have been developed and applied to elucidate the mechanisms of heat production in BACs. In this chapter, we introduce a protocol that uses a cationic fluorescent polymeric thermometer to directly measure the temperature within primary cultured BACs. We anticipate that this protocol will be beneficial in elucidating the mechanism of thermogenesis in BACs.

Time-resolved spectroscopic mapping of vibrational energy flow in proteins: Understanding thermal diffusion at the nanoscale

Vibrational energy exchange between various degrees of freedom is critical to barrier-crossing processes in proteins. Hemeproteins are well suited for studying vibrational energy exchange in proteins because the heme group is an efficient photothermal converter. The released energy by heme following photoexcitation shows migration in a protein moiety on a picosecond timescale, which is observed using time-resolved ultraviolet resonance Raman spectroscopy. The anti-Stokes ultraviolet resonance Raman intensity of a tryptophan residue is an excellent probe for the vibrational energy in proteins, allowing the mapping of energy flow with the spatial resolution of a single amino acid residue. This Perspective provides an overview of studies on vibrational energy flow in proteins, including future perspectives for both methodologies and applications.

Heat Release by Isolated Mouse Brain Mitochondria Detected with Diamond Thermometer

The production of heat by mitochondria is critical for maintaining body temperature, regulating metabolic rate, and preventing oxidative damage to mitochondria and cells. Until the present, mitochondrial heat production has been characterized only by methods based on fluorescent probes, which are sensitive to environmental variations (viscosity, pH, ionic strength, quenching, etc.). Here, for the first time, the heat release of isolated mitochondria was unambiguously measured by a diamond thermometer (DT), which is absolutely indifferent to external non-thermal parameters. We show that during total uncoupling of transmembrane potential by CCCP application, the temperature near the mitochondria rises by 4-22 °C above the ambient temperature with an absolute maximum of 45 °C. Such a broad variation in the temperature response is associated with the heterogeneity of the mitochondria themselves as well as their aggregations in the isolated suspension. Spontaneous temperature bursts with comparable amplitude were also detected prior to CCCP application, which may reflect involvement of some mitochondria to ATP synthesis or membrane potential leaking to avoid hyperproduction of reactive oxygen species. The results obtained with the diamond temperature sensor shed light on the "hot mitochondria" paradox.

Full-Color-Tunable Nanohydrogels as High-Stability Intracellular Nanothermometers

Full-color-tunable hydrogels with ultrahigh stability can be used in various fields, including intracellular temperature sensing. However, constructing full-color-tunable organic nanohydrogels with excellent biocompatibility and stability for intracellular temperature sensing remains a great challenge. Here, we report a full-color-tunable nanohydrogel with ultrahigh stability as an intracellular nanothermometer. Three types of temperature-sensitive polymers with red, green, and blue fluorescence were synthesized. Through easy mixing of these three polymers with regulation of the mass ratio, these polymers can be encoded to full-color-tunable fluorescent nanohydrogels, including nanohydrogels with white-light emission (NWLEs), with sizes of about 200 nm in aqueous media. Further study suggested that the as-obtained NWLEs exhibited good performance in intracellular temperature sensing because of their ultrahigh stability on their fluorescence properties and morphologies.

Heat-Induced Conformational Transition Mechanism of Heat Shock Factor 1 Investigated by Tryptophan Probe

A transcriptional regulatory system called heat shock response (HSR) has been developed in eukaryotic cells to maintain proteome homeostasis under various stresses. Heat shock factor-1 (Hsf1) plays a central role in HSR, mainly by upregulating molecular chaperones as a transcription factor. Hsf1 forms a complex with chaperones and exists as a monomer in the resting state under normal conditions. However, upon heat shock, Hsf1 is activated by oligomerization. Thus, oligomerization of Hsf1 is considered an important step in HSR. However, the lack of information about Hsf1 monomer structure in the resting state, as well as the structural change via oligomerization at heat response, impeded the understanding of the thermosensing mechanism through oligomerization. In this study, we applied solution biophysical methods, including fluorescence spectroscopy, nuclear magnetic resonance, and circular dichroism spectroscopy, to investigate the heat-induced conformational transition mechanism of Hsf1 leading to oligomerization. Our study showed that Hsf1 forms an inactive closed conformation mediated by intramolecular contact between leucine zippers (LZs), in which the intermolecular contact between the LZs for oligomerization is prevented. As the temperature increases, Hsf1 changes to an open conformation, where the intramolecular LZ interaction is dissolved so that the LZs can form intermolecular contacts to form oligomers in the active form. Furthermore, since the interaction sites with molecular chaperones and nuclear transporters are also expected to be exposed in the open conformation, the conformational change to the open state can lead to understanding the regulation of Hsf1-mediated stress response through interaction with multiple cellular components.

Comprehensive Investigation of Parameters Influencing Fluorescence Lifetime Imaging Microscopy in Frequency- and Time-Domain Illustrated by Phasor Plot Analysis

Having access to fluorescence lifetime, researchers can reveal in-depth details about the microenvironment as well as the physico-chemical state of the molecule under investigation. However, the high number of influencing factors might be an explanation for the strongly deviating values of fluorescent lifetimes for the same fluorophore reported in the literature. This could be the reason for the impression that inconsistent results are obtained depending on which detection and excitation scheme is used. To clarify this controversy, the two most common techniques for measuring fluorescence lifetimes in the time-domain and in the frequency-domain were implemented in one single microscopy setup and applied to a variety of fluorophores under different environmental conditions such as pH-value, temperature, solvent polarity, etc., along with distinct state forms that depend, for example, on the concentration. From a vast amount of measurement results, both setup- and sample-dependent parameters were extracted and represented using a single display form, the phasor-plot. The measurements showed consistent results between the two techniques and revealed which of the tested parameters has the strongest influence on the fluorescence lifetime. In addition, quantitative guidance as to which technique is most suitable for which research task and how to perform the experiment properly to obtain consistent fluorescence lifetimes is discussed.

Polymeric Nanoparticles with Embedded Eu(III) Complexes as Molecular Probes for Temperature Sensing

Three novel luminescent Eu(III) complexes, Eu1-Eu3, have been synthesized and characterized with CHN analysis, mass-spectrometry and ¹H NMR spectroscopy. The complexes display strong emission in dichloromethane solution upon excitation at 405 and 800 nm with a quantum yield from 18.3 to 31.6%, excited-state lifetimes in the range of 243-1016 ms at 20 °C, and lifetime temperature sensitivity of 0.9%/K (Eu1), 1.9%/K (Eu2), and 1.7%/K (Eu3). The chromophores were embedded into biocompatible latex nanoparticles (NPs_Eu1-NPs_Eu3) that prevented emission quenching and kept the photophysical characteristics of emitters unchanged with the highest temperature sensitivity of 1.3%/K (NPs_Eu2). For this probe cytotoxicity, internalization dynamics and localization in CHO-K1 cells were studied together with lifetime vs. temperature calibration in aqueous solution, phosphate buffer, and in a mixture of growth media and fetal bovine serum. The obtained data were then averaged to give the calibration curve, which was further used for temperature estimation in biological samples. The probe was stable in physiological media and displayed good reproducibility in cycling experiments between 20 and 40 °C. PLIM experiments with thermostated CHO-K1 cells incubated with NPs_Eu2 indicated that the probe could be used for temperature estimation in cells including the assessment of temperature variations upon chemical shock (sample treatment with mitochondrial uncoupling reagent).

Simulation of a Particle Domain in a Continuum, Fluctuating Hydrodynamics Reservoir

In molecular simulation and fluid mechanics, the coupling of a particle domain with a continuum representation of its embedding environment is an ongoing challenge. In this Letter, we show a novel approach where the latest version of the adaptive resolution scheme (AdResS), with noninteracting tracers as particles' reservoir, is combined with a fluctuating hydrodynamics (FHD) solver. The resulting algorithm, supported by a solid mathematical model, allows for a physically consistent exchange of matter and energy between the particle domain and its fluctuating continuum reservoir. Numerical tests are performed to show the validity of the algorithm. Differently from previous algorithms of the same kind, the current approach allows for simulations where, in addition to density fluctuations, also thermal fluctuations can be accounted for, thus large complex molecular systems, as, for example, hydrated biological membranes in a thermal field, can now be efficiently treated.

A palette of site-specific organelle fluorescent thermometers

Intracellular micro-temperature is closely related to cellular processes. Such local temperature inside cells can be measured by fluorescent thermometers, which are a series of fluorescent materials that convert the temperature information to detectable fluorescence signals. To investigate the intracellular temperature fluctuation in various organelles, it is essential to develop site-specific organelle thermometers. In this study, we develop a new series of fluorescent thermometers, Thermo Greens (TGs), to visualize the temperature change in almost all typical organelles. Through fluorescence lifetime-based cell imaging, it was proven that TGs allow the organelle-specific monitoring of temperature gradients created by external heating. The fluorescence lifetime-based thermometry shows that each organelle experiences a distinct temperature increment which depends on the distance away from the heat source. TGs are further demonstrated in the quantitative imaging of heat production at different organelles such as mitochondria and endoplasmic reticulum in brown adipocytes. To date, TGs are the first palette batch of small molecular fluorescent thermometers that can cover almost all typical organelles. These findings can inspire the development of new fluorescent thermometers and enhance the understanding of thermal biology in the future.

Micromodel of a Gas Diffusion Electrode Tracks In-Operando Pore-Scale Wetting Phenomena

Utilizing carbon dioxide (CO₂ ) as a resource for carbon monoxide (CO) production using renewable energy requires electrochemical reactors with gas diffusion electrodes that maintain a stable and highly reactive gas/liquid/solid interface. Very little is known about the reasons why gas diffusion electrodes suffer from unstable long-term operation. Often, this is associated with flooding of the gas diffusion electrode (GDE) within a few hours of operation. A better understanding of parameters influencing the phase behavior at the electrolyte/electrode/gas interface is necessary to increase the durability of GDEs. In this work, a microfluidic structure with multi-scale porosity featuring heterogeneous surface wettability to realistically represent the behavior of conventional GDEs is presented. A gas/liquid/solid phase boundary was established within a conductive, highly porous structure comprising a silver catalyst and Nafion binder. Inoperando visualization of wetting phenomena was performed using confocal laser scanning microscopy (CLSM). Non-reversible wetting, wetting of hierarchically porous structures and electrowetting were observed and analyzed. Fluorescence lifetime imaging microscopy (FLIM) enabled the observation of reactions on the model electrode surface. The presented methodology enables the systematic evaluation of spatio-temporally evolving wetting phenomena as well as species characterization for novel catalyst materials under realistic GDE configurations and process parameters.

A Lanthanide Upconversion Nanothermometer for Precise Temperature Mapping on Immune Cell Membrane

Cell temperature monitoring is of great importance to uncover temperature-dependent intracellular events and regulate cellular functions. However, it remains a great challenge to precisely probe the localized temperature status in living cells. Herein, we report a strategy for in situ temperature mapping on an immune cell membrane for the first time, which was achieved by using the lanthanide-doped upconversion nanoparticles. The nanothermometer was designed to label the cell membrane by combining metabolic labeling and click chemistry and can leverage ratiometric upconversion luminescence signals to in situ sensitively monitor temperature variation (1.4% K^-1). Moreover, a purpose-built upconversion hyperspectral microscope was utilized to synchronously map temperature changes on T cell membrane and visualize intracellular Ca²⁺ influx. This strategy was able to identify a suitable temperature status for facilitating thermally stimulated calcium influx in T cells, thus enabling high-efficiency activation of immune cells. Such findings might advance understandings on thermally dependent biological processes and their regulation methodology.

How sticky? How Tight? How Hot? Imaging probes for fluid viscosity, membrane tension and temperature measurements at the cellular level

We review the progress made in imaging probes for three important physical parameters: viscosity, membrane tension, and temperature, all of which play important roles in many cellular processes. Recent evidences showed that cell migration speed can be modulated by extracellular fluid viscosity; membrane tension contributes to the regulation of cell motility, exo-/endo-cytosis, and cell spread area; and temperature affects neural activity and adipocyte differentiation. We discuss the techniques implementing imaging-based probes to measure viscosity, membrane tension, and temperature at subcellular resolution dynamically. The merits and shortcomings of each technique are examined, and the future applications of the recently developed techniques are also explored.

Mitochondrial signal transduction

The analogy of mitochondria as powerhouses has expired. Mitochondria are living, dynamic, maternally inherited, energy-transforming, biosynthetic, and signaling organelles that actively transduce biological information. We argue that mitochondria are the processor of the cell, and together with the nucleus and other organelles they constitute the mitochondrial information processing system (MIPS). In a three-step process, mitochondria (1) sense and respond to both endogenous and environmental inputs through morphological and functional remodeling; (2) integrate information through dynamic, network-based physical interactions and diffusion mechanisms; and (3) produce output signals that tune the functions of other organelles and systemically regulate physiology. This input-to-output transformation allows mitochondria to transduce metabolic, biochemical, neuroendocrine, and other local or systemic signals that enhance organismal adaptation. An explicit focus on mitochondrial signal transduction emphasizes the role of communication in mitochondrial biology. This framework also opens new avenues to understand how mitochondria mediate inter-organ processes underlying human health.

Optical heating and luminescence thermometry combined in a Cr3+-doped YAl3(BO3)4

The possibility of optical heating with simultaneous control of the generated light within a single phosphor is particularly attractive from the perspective of multiple applications. This motivates the search for new solutions to enable efficient optical heating. In response to these requirements, based on the high absorption cross-section of Cr³⁺ ions, the optical heater based on YAl₃(BO₃)₄:Cr³⁺ exhibiting highly efficient heating is developed. At the same time, the emission intensity ratio of ²E_(g) → ⁴A_2(g) and ⁴T_2(g) → ⁴A_2(g) of Cr³⁺ bands, thanks to the monotonic temperature dependence, enables remote temperature readout of the phosphor using luminescence thermometry technique. The combination of these two functionalities within a single phosphor makes YAl₃(BO₃)₄:Cr³⁺ a promising, self thermally controlled photothermal agent.

Fluorescent Molecular Logic Gates and Pourbaix Sensors in Polyacrylamide Hydrogels

Polyacrylamide hydrogels formed by free radical polymerisation were formed by entrapping anthracene and 4-amino-1,8-naphthalimide fluorescent logic gates based on photoinduced electron transfer (PET) and/or internal charge transfer (ICT). The non-covalent immobilisation of the molecules in the hydrogels resulted in semi-solid YES, NOT, and AND logic gates. Two molecular AND gates, examples of Pourbaix sensors, were tested in acidic aqueous methanol with ammonium persulfate, a strong oxidant, and displayed greater fluorescence quantum yields than previously reported. The logic hydrogels were exposed to aqueous solutions with chemical inputs, and the fluorescence output response was viewed under 365 nm UV light. All of the molecular logic gates diffuse out of the hydrogels to some extent when placed in solution, particularly those with secondary basic amines. The study exemplifies an effort of taking molecular logic gates from homogeneous solutions into the realm of solid-solution environments. We demonstrate the use of Pourbaix sensors as pE-pH indicators for monitoring oxidative and acidic conditions, notably for excess ammonium persulfate, a reagent used in the polymerisation of SDS-polyacrylamide gels.

Fluorescence saturation imaging microscopy: molecular fingerprinting in living cells using two-photon absorption cross section as a contrast mechanism

Imaging of molecular-specific photophysical parameters such as fluorescence intensity, emission band shape, or fluorescence decay is widely used in biophysics. Here we propose a method for quantitative mapping of another molecular-specific parameter in living cells, two-photon absorption cross section, based on the fluorescence saturation effect. Using model dye solutions and cell culture, we show that the analysis of the fluorescence signal dependencies on the intensity of two-photon excitation within the range typical for routine two-photon microscopy experiments allows one to reconstruct two-photon absorption cross section maps across the sample. We believe that the absorption cross section contrast visualized by the proposed fluorescence saturation imaging microscopy could be a new tool for studying processes in living cells and tissues.

Penetration Depth of Propylene Glycol, Sodium Fluorescein and Nile Red into the Skin Using Non-Invasive Two-Photon Excited FLIM

The stratum corneum (SC) forms a strong barrier against topical drug delivery. Therefore, understanding the penetration depth and pathways into the SC is important for the efficiency of drug delivery and cosmetic safety. In this study, TPT-FLIM (two-photon tomography combined with fluorescence lifetime imaging) was applied as a non-invasive optical method for the visualization of skin structure and components to study penetration depths of exemplary substances, like hydrophilic propylene glycol (PG), sodium fluorescein (NaFl) and lipophilic Nile red (NR) into porcine ear skin ex vivo. Non-fluorescent PG was detected indirectly based on the pH-dependent increase in the fluorescence lifetime of SC components. The pH similarity between PG and viable epidermis limited the detection of PG. NaFl reached the viable epidermis, which was also proved by laser scanning microscopy. Tape stripping and confocal Raman micro-spectroscopy were performed additionally to study NaFl, which revealed penetration depths of ≈5 and ≈8 μm, respectively. Lastly, NR did not permeate the SC. We concluded that the amplitude-weighted mean fluorescence lifetime is the most appropriate FLIM parameter to build up penetration profiles. This work is anticipated to provide a non-invasive TPT-FLIM method for studying the penetration of topically applied drugs and cosmetics into the skin.

Enhanced electrochemiluminescence at microgel-functionalized beads

Bead-based assays are successfully combined with electrochemiluminescence (ECL) technology for detection of a wide range of biomarkers. Herein, we demonstrate a novel approach to enhance the ECL signal by decorating micrometric beads with [Ru(bpy)₃]²⁺-grafted microgels (diameter ∼100 nm). Rapid and stable light emission was spatially resolved at the level of single functionalized beads. An enhancement of the ECL signal of microgel-labeled beads by 9-fold was observed in comparison to molecularly linked [Ru(bpy)₃]²⁺ beads prepared by a sandwich immunoassay or an amide bond. Imaging the ECL signal at the single bead level shows that the size of the ECL-emitting layer is extended using the microgels. The reported method offers a great promise for the optimization of bead-based ECL detection and subsequent development of ECL microscopy.

A light-controlled DNA nanothermometer for temperature sensing in the cellular membrane microenvironment

Precise sensing of cellular temperature is one significant yet challenge task for studying miscellaneous biological processes. Herein, we report a light-controlled DNA nanothermometer that allow for real-time thermal sensing in extracellular microscope with high spatiotemporal resolution. The light-controlled DNA nanothermometer three key elements: a thermal-sensitive molecular beacon (MB) labelled with fluorophore Cy5 and Cy3 at its 5' and 3' termini, an inhibitor strand containing two photocleavable linkers (pc-linker), and a biotin modified strand, which could modify this three-strand hybridization complex onto the cell surface. Upon exposing to UV light irradiation, the light-controlled DNA nanothermometer could be remotely activated and enable to perform highly sensitive and practical ratiometric temperature sensing. Meanwhile, the light-controlled DNA nanothermometer could conduct temperature sensing in the extracellular microscope and demonstrates desirable sensitivity, excellent reversibility, and quantitative ability for extracellular temperature measurement. Therefore, this light-controlled DNA can serve as a promising tool for elucidating thermal-related cell physiological and pathological processes.

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.

Intracellular Heat Transfer and Thermal Property Revealed by Kilohertz Temperature Imaging with a Genetically Encoded Nanothermometer

Despite improved sensitivity of nanothermometers, direct observation of heat transport inside single cells has remained challenging for the lack of high-speed temperature imaging techniques. Here, we identified insufficient temperature resolution under short signal integration time and slow sensor kinetics as two major bottlenecks. To overcome the limitations, we developed B-gTEMP, a nanothermometer based on the tandem fusion of mNeonGreen and tdTomato fluorescent proteins. We visualized the propagation of heat inside intracellular space by tracking the temporal variation of local temperature at a time resolution of 155 μs and a temperature resolution 0.042 °C. By comparing the fast in situ temperature dynamics with computer-simulated heat diffusion, we estimated the thermal diffusivity of live HeLa cells. The present thermal diffusivity in cells was about 1/5.3 of that of water and much smaller than the values reported for bulk tissues, which may account for observations of heterogeneous intracellular temperature distributions.

Microrobotic Swarms for Intracellular Measurement with Enhanced Signal-to-Noise Ratio

In cell biology, fluorescent dyes are routinely used for biochemical measurements. The traditional global dye treatment method suffers from low signal-to-noise ratios (SNR), especially when used for detecting a low concentration of ions, and increasing the concentration of fluorescent dyes causes more severe cytotoxicity. Here, we report a robotic technique that controls how a low amount of fluorescent-dye-coated magnetic nanoparticles accurately forms a swarm and increases the fluorescent dye concentration in a local region inside a cell for intracellular measurement. Different from existing magnetic micromanipulation systems that generate large swarms (several microns and above) or that cannot move the generated swarm to an arbitrary position, our system is capable of generating a small swarm (e.g., 1 μm) and accurately positioning the swarm inside a single cell (position control accuracy: 0.76 μm). In experiments, the generated swarm inside the cell showed an SNR 10 times higher than the traditional global dye treatment method. The high-SNR robotic swarm enabled intracellular measurements that had not been possible to achieve with traditional global dye treatment. The robotic swarm technique revealed an apparent pH gradient in a migrating cell and was used to measure the intracellular apparent pH in a single oocyte of living C. elegans. With the position control capability, the swarm was also applied to measure calcium changes at the perinuclear region of a cell before and after mechanical stimulation. The results showed a significant calcium increase after mechanical stimulation, and the calcium increase was regulated by the mechanically sensitive ion channel, PIEZO1.

Operando optical fiber monitoring of nanoscale and fast temperature changes during photo-electrocatalytic reactions

In situ and continuous monitoring of thermal effects is essential for understanding photo-induced catalytic processes at catalyst's surfaces. However, existing techniques are largely unable to capture the rapidly changing temperatures occurring in sub-μm layers at liquid-solid interfaces exposed to light. To address this, a sensing system based on a gold-coated conventional single-mode optical fiber with a tilted fiber Bragg grating inscribed in the fiber core is proposed and demonstrated. The spectral transmission from these devices is made up of a dense comb of narrowband resonances that can differentiate between localized temperatures rapid changes at the catalyst's surface and those of the environment. By using the gold coating of the fiber as an electrode in an electrochemical reactor and exposing it to light, thermal effects in photo-induced catalysis at the interface can be decoded with a temperature resolution of 0.1 °C and a temporal resolution of 0.1 sec, without perturbing the catalytic operation that is measured simultaneously. As a demonstration, stable and reproducible correlations between the light-to-heat conversion and catalytic activities over time were measured for two different catalysis processes (linear and nonlinear). These kinds of sensing applications are ideally suited to the fundamental qualities of optical fiber sensors, such as their compact size, flexible shape, and remote measurement capability, thereby opening the way for various thermal monitoring in hard-to-reach spaces and rapid catalytic reaction processes.

Luminescence Thermometry for Brain Activity Monitoring: A Perspective

Minimally invasive monitoring of brain activity is essential not only to gain understanding on the working principles of the brain, but also for the development of new diagnostic tools. In this perspective we describe how brain thermometry could be an alternative to conventional methods (e.g., magnetic resonance or nuclear medicine) for the acquisition of thermal images of the brain with enough spatial and temperature resolution to track brain activity in minimally perturbed animals. We focus on the latest advances in transcranial luminescence thermometry introducing a critical discussion on its advantages and shortcomings. We also anticipate the main challenges that the application of luminescent nanoparticles for brain thermometry will face in next years. With this work we aim to promote the development of near infrared luminescence for brain activity monitoring, which could also benefit other research areas dealing with the brain and its illnesses.

Intracellular Physical Properties with Small Organic Fluorescent Probes: Recent Advances and Future Perspectives

The intracellular physical parameters i. e., polarity, viscosity, fluidity, tension, potential, and temperature of a live cell are the hallmark of cellular health and have garnered immense interest over the past decade. In this context, small molecule organic fluorophores exhibit prominent useful properties including easy functionalizability, environmental sensitivity, biocompatibility, and fast yet efficient cellular uptakability which has made them a popular tool to understand intra-cellular micro-environmental properties. Throughout this discussion, we have outlined the basic design strategies of small molecules for specific organelle targeting and quantification of physical properties. The values of these parameters are indicative of cellular homeostasis and subtle alteration may be considered as the onset of disease. We believe this comprehensive review will facilitate the development of potential future probes for superior insight into the physical parameters that are yet to be quantified.

Thermal Probing Techniques for a Single Live Cell

Temperature is a significant factor in determining and characterizing cellular metabolism and other biochemical activities. In this study, we provide a brief overview of two important technologies used to monitor the local temperatures of individual living cells: fluorescence nano-thermometry and an array of micro-/nano-sized thin-film thermocouples. We explain some key technical issues that must be addressed and optimised for further practical applications, such as in cell biology, drug selection, and novel antitumor therapy. We also offer a method for combining them into a hybrid measuring system.