The challenge of intracellular temperature

Optical super-resolution nanothermometry via stimulated emission depletion imaging of upconverting nanoparticles

From engineering improved device performance to unraveling the breakdown of classical heat transfer laws, far-field optical temperature mapping with nanoscale spatial resolution would benefit diverse areas. However, these attributes are traditionally in opposition because conventional far-field optical temperature mapping techniques are inherently diffraction limited. Optical super-resolution imaging techniques revolutionized biological imaging, but such approaches have yet to be applied to thermometry. Here, we demonstrate a super-resolution nanothermometry technique based on highly doped upconverting nanoparticles (UCNPs) that enable stimulated emission depletion (STED) super-resolution imaging. We identify a ratiometric thermometry signal and maintain imaging resolution better than ~120 nm for the relevant spectral bands. We also form self-assembled UCNP monolayers and multilayers and implement a detection scheme with scan times >0.25 μm2/min. We further show that STED nanothermometry reveals a temperature gradient across a joule-heated microstructure that is undetectable with diffraction limited thermometry, indicating the potential of this technique to uncover local temperature variation in wide-ranging practical applications.

Thermally Prolonged NIR-II Luminescence Lifetimes by Cross-Relaxation

Temperature regulates nonradiative processes in luminescent materials, fundamental to luminescence nanothermometry. However, elevated temperatures often suppress the radiative process, limiting the sensitivity of thermometers. Here, we introduce an approach to populating the excited state of lanthanides at elevated temperatures, resulting in a sizable lifetime lengthening and intensity increase of the near-infrared (NIR)-II emission. The key is to create a five-energy-level system and use a pair of lanthanides to leverage the cross-relaxation process. We observed the lifetime of NIR-II emission of Er3+ has been remarkably increased from 3.85 to 7.54 ms by codoping only 0.5 mol % Ce3+ at 20 °C and further increased to 7.80 ms when increasing the temperature to 40 °C. Moreover, this concept is universal across four ion pairs and remains stable within aqueous nanoparticles. Our findings emphasize the need to design energy transfer systems that overcome the constraint of thermal quenching, enabling efficient imaging and sensing.

Limitations of Bulk Diamond Sensors for Single-Cell Thermometry

The present paper reports on a Finite Element Method (FEM) analysis of the experimental situation corresponding to the measurement of the temperature variation in a single cell plated on bulk diamond by means of optical techniques. Starting from previous experimental results, we have determined-in a uniform power density approximation and under steady-state conditions-the total heat power that has to be dissipated by a single cell plated on a glassy substrate in order to induce the typical maximum temperature increase ΔTglass=1 K. While keeping all of the other parameters constant, the glassy substrate has been replaced by a diamond plate. The FEM analysis shows that, in this case, the maximum temperature increase is expected at the diamond/cell interface and is as small as ΔTdiam=4.6×10-4 K. We have also calculated the typical decay time in the transient scenario, which resulted in τ≈ 250 μs. By comparing these results with the state-of-the-art sensitivity values, we prove that the potential advantages of a longer coherence time, better spectral properties, and the use of special field alignments do not justify the use of diamond substrates in their bulk form.

Phasor identifier: A cloud-based analysis of phasor-FLIM data on Python notebooks

This paper introduces an innovative approach utilizing Google Colaboratory for the versatile analysis of phasor fluorescence lifetime imaging microscopy (FLIM) data collected from various samples (e.g., cuvette, cells, tissues) and in various input file formats. In fact, phasor-FLIM widespread adoption has been hampered by complex instrumentation and data analysis requirements. We mean to make advanced FLIM analysis more accessible to researchers through a cloud-based solution that 1) harnesses robust computational resources, 2) eliminates hardware limitations, and 3) supports both CPU and GPU processing. We envision a paradigm shift in FLIM data accessibility and potential, aligning with the evolving field of artificial intelligence-driven FLIM analysis. This approach simplifies FLIM data handling and opens doors for diverse applications, from studying cellular metabolism to investigating drug encapsulation, benefiting researchers across multiple domains. The comparative analysis of freely distributed FLIM tools highlights the unique advantages of this approach in terms of adaptability, scalability, and open-source nature.

Thermodynamic definition of mean temperature

The notion of mean temperature is crucial for a number of fields, including climate science, fluid dynamics, and biophysics. However, so far its correct thermodynamic foundation is lacking or even believed to be impossible. A physically correct definition should not be based on mathematical notions of the means (e.g., the mean geometric or mean arithmetic), because they are not unique, and they ignore the fact that temperature is an ordinal level variable. We offer a thermodynamic definition of the mean temperature that is based upon the following two assumptions. First, the correct definition should necessarily involve equilibration processes in the initially nonequilibrium system. Among such processes, reversible equilibration and fully irreversible equilibration are the two extreme cases. Second, within the thermodynamic approach we assume that the mean temperature is determined mostly by energy and entropy. Together with the dimensional analysis, the two assumptions lead to a definition of the mean temperature that is determined up to a weight factor that can be fixed to 1/2 due to the maximum ignorance principle. The mean temperature for ideal and (van der Waals) nonideal gases with temperature-independent heat capacity is given by a general and compact formula that (besides the initial temperatures) only depends on the heat capacities and concentration of gases. Our method works for any nonequilibrium initial state, not only two-temperature states.

Luminescent Single-Molecule Magnets as Dual Magneto-Optical Molecular Thermometers

Luminescent thermometry allows the remote detection of the temperature and holds great potential in future technological applications in which conventional systems could not operate. Complementary approaches to measuring the temperature aiming to enhance the thermal sensitivity would however represent a decisive step forward. For the first time, we demonstrate the proof-of-concept that luminescence thermometry could be associated with a complementary temperature readout related to a different property. Namely, we propose to take advantage of the temperature dependence of both magnetic (canonical susceptibility and relaxation time) and luminescence features (emission intensity) found in Single-Molecule Magnets (SMM) to develop original dual magneto-optical molecular thermometers to conciliate high-performance SMM and Boltzmann-type luminescence thermometry. We highlight this integrative approach to concurrent luminescent and magnetic thermometry using an air-stable benchmark SMM [Dy(bbpen)Cl] (H2 bbpen=N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-methylpyridyl)ethyl-enediamine)) exhibiting Dy3+ luminescence. The synergy between multiparametric magneto-optical readouts and multiple linear regression makes possible a 10-fold improvement in the relative thermal sensitivity of the thermometer over the whole temperature range, compared with the values obtained with the single optical or magnetic devices.

Trans-scale thermal signaling in biological systems

Biochemical reactions in cells serve as the endogenous source of heat, maintaining a constant body temperature. This process requires proper control; otherwise, serious consequences can arise due to the unwanted but unavoidable responses of biological systems to heat. This review aims to present a range of responses to heat in biological systems across various spatial scales. We begin by examining the impaired thermogenesis of malignant hyperthermia in model mice and skeletal muscle cells, demonstrating that the progression of this disease is caused by a positive feedback loop between thermally driven Ca2+ signaling and thermogenesis at the subcellular scale. After we explore thermally driven force generation in both muscle and non-muscle cells, we illustrate how in vitro assays using purified proteins can reveal the heat-responsive properties of proteins and protein assemblies. Building on these experimental findings, we propose the concept of 'trans-scale thermal signaling'.

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.

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.

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.

Nanothermometry with Enhanced Sensitivity and Enlarged Working Range Using Diamond Sensors

Nanothermometry is increasingly demanded in frontier research in physics, chemistry, materials science and engineering, and biomedicine. An ideal thermometer should have features of reliable temperature interpretation, high sensitivity, fast response, minimum disturbance of the target's temperature, applicability in a variety of environments, and a large working temperature range. For applications in nanosystems, high spatial resolution is also desirable. Such requirements impose great challenges in nanothermometry since the shrinking of the sensor volume usually leads to a reduction in sensitivity.Diamond with nitrogen-vacancy (NV) centers provides opportunities for nanothermometry. NV center spins have sharp resonances due to their superb coherence. NV centers are multimodal sensors. They can directly sense magnetic fields, electric fields, temperature, pressure, and nuclear spins and, through proper transduction, measure other quantities such as the pH and deformation. In particular, their spin resonance frequencies vary with temperature, making them a promising thermometer. The high thermal conductivity, high hardness, chemical stability, and biocompatibility of diamond enable reliable and fast temperature sensing in complex environments ranging from erosive liquids to live systems. Chemical processing of diamond surfaces allows various functionalities such as targeting. The small size and the targeting capability of nanodiamonds then enable site-specific temperature sensing with nanoscale spatial resolution. However, the sensitivity of NV-based nanothermometry is yet to meet the requirement of practical systems with a large gap of a few orders of magnitude. On the other hand, although NV-based quantum sensing works well from 0.3 to 600 K, extending the sensing scheme to high temperature remains challenging due to uncertainty in identifying the exact physical limits and possible solution at elevated temperatures.This Account focuses on our efforts to enhance the temperature sensitivity and widen the working temperature range of diamond-based nanothermometry. We start with explaining the working principle and features of NV-based thermometry with examples of applications. Then a transducer-based concept is introduced with practical schemes to improve the sensitivity of the nanodiamond thermometer. Specifically, we show that the temperature signal can be transduced and amplified by adopting hybrid structures of nanodiamond and magnetic nanoparticles, which results in a record temperature sensitivity of 76 μK/√Hz. We also demonstrate quantum sensing with NV at high temperatures of up to 1000 K by adopting a pulsed heating-cooling scheme to carry out the spin polarization and readout at room temperature and the spin manipulation (sensing) at high temperatures. Finally, unsolved problems and future endeavors of diamond nanothermometry are discussed.

Nanodiamond-Quantum Sensors Reveal Temperature Variation Associated to Hippocampal Neurons Firing

Temperature is one of the most relevant parameters for the regulation of intracellular processes. Measuring localized subcellular temperature gradients is fundamental for a deeper understanding of cell function, such as the genesis of action potentials, and cell metabolism. Notwithstanding several proposed techniques, at the moment detection of temperature fluctuations at the subcellular level still represents an ongoing challenge. Here, for the first time, temperature variations (1 °C) associated with potentiation and inhibition of neuronal firing is detected, by exploiting a nanoscale thermometer based on optically detected magnetic resonance in nanodiamonds. The results demonstrate that nitrogen-vacancy centers in nanodiamonds provide a tool for assessing various levels of neuronal spiking activity, since they are suitable for monitoring different temperature variations, respectively, associated with the spontaneous firing of hippocampal neurons, the disinhibition of GABAergic transmission and the silencing of the network. Conjugated with the high sensitivity of this technique (in perspective sensitive to < 0.1 °C variations), nanodiamonds pave the way to a systematic study of the generation of localized temperature gradients under physiological and pathological conditions. Furthermore, they prompt further studies explaining in detail the physiological mechanism originating this effect.

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.

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.

Raman Thermometry Nanopipettes in Cancer Photothermal Therapy

Raman thermometry based on surface-enhanced Raman scattering has been developed using nanopipettes in cancer cell photothermal therapy (PTT). Gold nanorods (AuNRs) are robustly epoxied on glass pipettes with a high surface coverage of ∼95% and less than 10 nm-wide nanogaps for intracellular thermometry and photothermal cancer therapy. The temperature changes could be estimated from the N≡C band shifts of 4-fluorophenyl isocyanide (FPNC)-adsorbed AuNRs on the Raman thermometry nanopipette (RTN) surfaces. An intracellular temperature change of ∼2.7 °C produced by altering the [Ca²⁺] in A431 cells was detected using the RTN in vitro, as checked from fura-2 acetoxymethyl ester (fura-2 AM) fluorescence images. For in vivo experiments, local temperature rises of ∼19.2 °C were observed in the mouse skin, whereas infrared camera images could not tract due to spatial resolution. In addition, a tumor growth suppression was observed in the PTT processes after an administration of the three AuNR-coated nanopipettes combined with a 671 nm laser irradiation for 5 min in 30 days. These results demonstrate not only the localized temperature sensing ability of FPNC-tagged AuNR nanopipettes in cell biology but also anti-cancer effects in photothermal cancer therapy.

Perspective: a stirring role for metabolism in cells

Based on recent findings indicating that metabolism might be governed by a limit on the rate at which cells can dissipate Gibbs energy, in this Perspective, we propose a new mechanism of how metabolic activity could globally regulate biomolecular processes in a cell. Specifically, we postulate that Gibbs energy released in metabolic reactions is used to perform work, allowing enzymes to self‐propel or to break free from supramolecular structures. This catalysis‐induced enzyme movement will result in increased intracellular motion, which in turn can compromise biomolecular functions. Once the increased intracellular motion has a detrimental effect on regulatory mechanisms, this will establish a feedback mechanism on metabolic activity, and result in the observed thermodynamic limit. While this proposed explanation for the identified upper rate limit on cellular Gibbs energy dissipation rate awaits experimental validation, it offers an intriguing perspective of how metabolic activity can globally affect biomolecular functions and will hopefully spark new research.

Pushing the limits of luminescence thermometry: probing the temperature of proteins in cells

Proteins are involved in numerous cellular activities such as transport and catalysis. Misfolding during biosynthesis and malfunctioning as a molecular machine may lead to physiological disorders and metabolic problems. Protein folding and mechanical work may be viewed as thermodynamic energetically favorable processes in which stochastic nonequilibrium intermediate states may be present with conditions such as thermal fluctuations. In my opinion, measuring those thermal fluctuations may be a way to access the energy exchange between the protein and the physiological environment and to better understand how those nonequilibrium states may influence the misfolding/folding process and the efficiency of the molecular engine cycle. Here, I discuss luminescence thermometry as a possible way to measure those temperature fluctuations from a single-molecule experimental perspective with its current technical limitations and challenges.

Accurate and Real-Time Detection Method for the Exothermic Behavior of Enzymatic Nano-Microregions Using Temperature-Sensitive Amino-AgInS2 Quantum Dots

The thermal behavior of enzymes in nanoscale is of great significance to life phenomena. This nonequilibrium state real-time thermal behavior of enzymes at nanoscale cannot be accurately detected by existing methods. Herein, a novel method is developed for the detection of this thermal behavior. The enzyme-quantum dot (QD) conjugates can be obtained by chemically grafting temperature-sensitive amino-AgInS₂ QDs to the enzyme, where the QDs act as nanothermometers with a sensitivity of -2.82% °C^-1 . Detecting the photoluminescence intensity changes of the enzyme-QD conjugates, the real-time thermal behavior of enzymes can be obtained. The enzyme-QD conjugates show a temperature difference as high as 6 °C above ambient temperature in nano-microregions with good reproducibility (maximum error of 4%) during catalysis, while solution temperature hardly changed. This method has a temperature resolution of ≈0.5 °C with a detection limit of 0.02 mg mL^-1 of enzyme, and spatially ensured that the amino-AgInS₂ QDs are quantitatively bound to the enzyme; thus, it can accurately detect the exothermic behavior of the enzyme and can be extended to other organisms' detection. This method has high sensitivity, good stability, and reliability, indicating its great potential application in investigating the thermal behavior of organisms in nanoscale and related life phenomena.

Intracellular thermometry uncovers spontaneous thermogenesis and associated thermal signaling

Conventional thermal biology has elucidated the physiological function of temperature homeostasis through spontaneous thermogenesis and responses to variations in environmental temperature in organisms. In addition to research on individual physiological phenomena, the molecular mechanisms of fever and physiological events such as temperature-dependent sex determination have been intensively addressed. Thermosensitive biomacromolecules such as heat shock proteins (HSPs) and transient receptor potential (TRP) channels were systematically identified, and their sophisticated functions were clarified. Complementarily, recent progress in intracellular thermometry has opened new research fields in thermal biology. High-resolution intracellular temperature mapping has uncovered thermogenic organelles, and the thermogenic functions of brown adipocytes were ascertained by the combination of intracellular thermometry and classic molecular biology. In addition, intracellular thermometry has introduced a new concept, "thermal signaling", in which temperature variation within biological cells acts as a signal in a cascade of intriguing biological events.

Opto-thermal technologies for microscopic analysis of cellular temperature-sensing systems

Could enzymatic activities and their cooperative functions act as cellular temperature-sensing systems? This review introduces recent opto-thermal technologies for microscopic analyses of various types of cellular temperature-sensing system. Optical microheating technologies have been developed for local and rapid temperature manipulations at the cellular level. Advanced luminescent thermometers visualize the dynamics of cellular local temperature in space and time during microheating. An optical heater and thermometer can be combined into one smart nanomaterial that demonstrates hybrid function. These technologies have revealed a variety of cellular responses to spatial and temporal changes in temperature. Spatial temperature gradients cause asymmetric deformations during mitosis and neurite outgrowth. Rapid changes in temperature causes imbalance of intracellular Ca2+ homeostasis and membrane potential. Among those responses, heat-induced muscle contractions are highlighted. It is also demonstrated that the short-term heating hyperactivates molecular motors to exceed their maximal activities at optimal temperatures. We discuss future prospects for opto-thermal manipulation of cellular functions and contributions to obtain a deeper understanding of the mechanisms of cellular temperature-sensing systems.

Diamond quantum thermometry: from foundations to applications

Diamond quantum thermometry exploits the optical and electrical spin properties of colour defect centres in diamonds and, acts as a quantum sensing method exhibiting ultrahigh precision and robustness. Compared to the existing luminescent nanothermometry techniques, a diamond quantum thermometer can be operated over a wide temperature range and a sensor spatial scale ranging from nanometres to micrometres. Further, diamond quantum thermometry is employed in several applications, including electronics and biology, to explore these fields with nanoscale temperature measurements. This review covers the operational principles of diamond quantum thermometry for spin-based and all-optical methods, material development of diamonds with a focus on thermometry, and examples of applications in electrical and biological systems with demand-based technological requirements.

A highly-sensitive genetically encoded temperature indicator exploiting a temperature-responsive elastin-like polypeptide

Genetically encoded temperature indicators (GETIs) allow for real-time measurement of subcellular temperature dynamics in live cells. However, GETIs have suffered from poor temperature sensitivity, which may not be sufficient to resolve small heat production from a biological process. Here, we develop a highly-sensitive GETI, denoted as ELP-TEMP, comprised of a temperature-responsive elastin-like polypeptide (ELP) fused with a cyan fluorescent protein (FP), mTurquoise2 (mT), and a yellow FP, mVenus (mV), as the donor and acceptor, respectively, of Förster resonance energy transfer (FRET). At elevated temperatures, the ELP moiety in ELP-TEMP undergoes a phase transition leading to an increase in the FRET efficiency. In HeLa cells, ELP-TEMP responded to the temperature from 33 to 40 °C with a maximum temperature sensitivity of 45.1 ± 8.1%/°C, which was the highest ever temperature sensitivity among hitherto-developed fluorescent nanothermometers. Although ELP-TEMP showed sensitivity not only to temperature but also to macromolecular crowding and self-concentration, we were able to correct the output of ELP-TEMP to achieve accurate temperature measurements at a subcellular resolution. We successfully applied ELP-TEMP to accurately measure temperature changes in cells induced by a local heat spot, even if the temperature difference was as small as < 1 °C, and to visualize heat production from stimulated Ca²⁺ influx in live HeLa cells induced by a chemical stimulation. Furthermore, we investigated temperatures in the nucleus and cytoplasm of live HeLa cells and found that their temperatures were almost the same within the temperature resolution of our measurement. Our study would contribute to better understanding of cellular temperature dynamics, and ELP-TEMP would be a useful GETI for the investigation of cell thermobiology.

A new approach to precise mapping of local temperature fields in submicrometer aqueous volumes

Nanodiamonds hosting temperature-sensing centers constitute a closed thermodynamic system. Such a system prevents direct contact of the temperature sensors with the environment making it an ideal environmental insensitive nanosized thermometer. A new design of a nanodiamond thermometer, based on a 500-nm luminescent nanodiamond embedded into the inner channel of a glass submicron pipette is reported. All-optical detection of temperature, based on spectral changes of the emission of "silicon-vacancy" centers with temperature, is used. We demonstrate the applicability of the thermometric tool to the study of temperature distribution near a local heater, placed in an aqueous medium. The calculated and experimental values of temperatures are shown to coincide within measurement error at gradients up to 20 °C/μm. Until now, temperature measurements on the submicron scale at such high gradients have not been performed. The new thermometric tool opens up unique opportunities to answer the urgent paradigm-shifting questions of cell physiology thermodynamics.

Toward Quantitative Bio-sensing with Nitrogen-Vacancy Center in Diamond

The long-dreamed-of capability of monitoring the molecular machinery in living systems has not been realized yet, mainly due to the technical limitations of current sensing technologies. However, recently emerging quantum sensors are showing great promise for molecular detection and imaging. One of such sensing qubits is the nitrogen-vacancy (NV) center, a photoluminescent impurity in a diamond lattice with unique room-temperature optical and spin properties. This atomic-sized quantum emitter has the ability to quantitatively measure nanoscale electromagnetic fields via optical means at ambient conditions. Moreover, the unlimited photostability of NV centers, combined with the excellent diamond biocompatibility and the possibility of diamond nanoparticles internalization into the living cells, makes NV-based sensors one of the most promising and versatile platforms for various life-science applications. In this review, we will summarize the latest developments of NV-based quantum sensing with a focus on biomedical applications, including measurements of magnetic biomaterials, intracellular temperature, localized physiological species, action potentials, and electronic and nuclear spins. We will also outline the main unresolved challenges and provide future perspectives of many promising aspects of NV-based bio-sensing.

Lanthanide doped luminescence nanothermometers in the biological windows: strategies and applications

The development of lanthanide-doped non-contact luminescent nanothermometers with accuracy, efficiency and fast diagnostic tools attributed to their versatility, stability and narrow emission band profiles has spurred the replacement of conventional contact thermal probes. The application of lanthanide-doped materials as temperature nanosensors, excited by ultraviolet, visible or near infrared light, and the generation of emissions lying in the biological window regions, I-BW (650 nm-950 nm), II-BW (1000 nm-1350 nm), III-BW (1400 nm-2000 nm) and IV-BW (centered at 2200 nm), are notably growing due to the advantages they present, including reduced phototoxicity and photobleaching, better image contrast and deeper penetration depths into biological tissues. Here, the different mechanisms used in lanthanide ion-doped nanomaterials to sense temperature in these biological windows for biomedical and other applications are summarized, focusing on factors that affect their thermal sensitivity, and consequently their temperature resolution. Comparing the thermometric performance of these nanomaterials in each biological window, we identified the strategies that allow boosting of their sensing properties.

Anti-Stokes Thermometry in Nanoplasmonics

Whereas heating nanoparticles with light is straightforward, measuring the resulting nanoscale temperature increase is intricate and still a matter of active research in plasmonics, with envisioned applications in nanochemistry, biomedicine, and solar light harvesting, among others. Interestingly, this research line mostly belongs to the optics community today because light is not only used for heating but also often for probing temperature. In this Perspective, I present and discuss recent advances in the search for efficient and reliable thermometry techniques for nanoplasmonic systems by the nano-optics community. I focus on the recently proposed approach based on the spectral measurement of anti-Stokes emission from the plasmonic nanoparticles themselves.

Cellular Thermometry Considerations for Probing Biochemical Pathways

Temperature is a fundamental thermodynamic property that can serve as a probe of biochemical reactions. Extracellular thermometry has previously been used to probe cancer metabolism and thermoregulation, with measured temperature changes of ~1-2 K in tissues, consistent with theoretical predictions. In contrast, previous intracellular thermometry studies remain disputed due to reports of >1 K intracellular temperature rises over 5 min or more that are inconsistent with theory. Thus, the origins of such anomalous temperature rises remain unclear. An improved quantitative understanding of intracellular thermometry is necessary to provide a clearer perspective for future measurements. Here, we develop a generalizable framework for modeling cellular heat diffusion over a range of subcellular-to-tissue length scales. Our model shows that local intracellular temperature changes reach measurable limits (>0.1 K) only when exogenously stimulated. On the other hand, extracellular temperatures can be measurable (>0.1 K) in tissues even from endogenous biochemical pathways. Using these insights, we provide a comprehensive approach to choosing an appropriate cellular thermometry technique by analyzing thermogenic reactions of different heat rates and time constants across length scales ranging from subcellular to tissues. Our work provides clarity on cellular heat diffusion modeling and on the required thermometry approach for probing thermogenic biochemical pathways.

Contextualizing the biological relevance of standardized high-resolution respirometry to assess mitochondrial function in permeabilized human skeletal muscle

Aim

This study sought to provide a statistically robust reference for measures of mitochondrial function from standardized high‐resolution respirometry with permeabilized human skeletal muscle (ex vivo), compare analogous values obtained via indirect calorimetry, arterial‐venous O₂ differences and ³¹P magnetic resonance spectroscopy (in vivo) and attempt to resolve differences across complementary methodologies as necessary.

Methods

Data derived from 831 study participants across research published throughout March 2009 to November 2019 were amassed to examine the biological relevance of ex vivo assessments under standard conditions, ie physiological temperatures of 37°C and respiratory chamber oxygen concentrations of ~250 to 500 μmol/L.

Results

Standard ex vivo‐derived measures are lower (Z ≥ 3.01, P ≤ .0258) en masse than corresponding in vivo‐derived values. Correcting respiratory values to account for mitochondrial temperatures 10°C higher than skeletal muscle temperatures at maximal exercise (~50°C): (i) transforms data to resemble (Z ≤ 0.8, P > .9999) analogous yet context‐specific in vivo measures, eg data collected during maximal 1‐leg knee extension exercise; and (ii) supports the position that maximal skeletal muscle respiratory rates exceed (Z ≥ 13.2, P < .0001) those achieved during maximal whole‐body exercise, e.g. maximal cycling efforts.

Conclusion

This study outlines and demonstrates necessary considerations when actualizing the biological relevance of human skeletal muscle respiratory control, metabolic flexibility and bioenergetics from standard ex vivo‐derived assessments using permeabilized human muscle. These findings detail how cross‐procedural comparisons of human skeletal muscle mitochondrial function may be collectively scrutinized in their relationship to human health and lifespan.

In situ measurements of intracellular thermal conductivity using heater-thermometer hybrid diamond nanosensors

Understanding heat dissipation processes at nanoscale during cellular thermogenesis is essential to clarify the relationships between the heat and biological processes in cells and organisms. A key parameter determining the heat flux inside a cell is the local thermal conductivity, a factor poorly investigated both experimentally and theoretically. Here, using a nanoheater/nanothermometer hybrid made of a polydopamine encapsulating a fluorescent nanodiamond, we measured the intracellular thermal conductivities of HeLa and MCF-7 cells with a spatial resolution of about 200 nm. The mean values determined in these two cell lines are both 0.11 ± 0.04 W m^-1 K^-1, which is significantly smaller than that of water. Bayesian analysis of the data suggests there is a variation of the thermal conductivity within a cell. These results make the biological impact of transient temperature spikes in a cell much more feasible, and suggest that cells may use heat flux for short-distance thermal signaling.

Thermal properties of lipid bilayers derived from the transient heating regime of upconverting nanoparticles

Heat transfer and thermal properties at the nanoscale can be challenging to obtain experimentally. These are potentially relevant for understanding thermoregulation in cells. Experimental data from the transient heating regime in conjunction with a model based on the energy conservation enable the determination of the specific heat capacities for all components of a nanoconstruct, namely an upconverting nanoparticle and its conformal lipid bilayer coating. This approach benefits from a very simple, cost-effective and non-invasive optical setup to measure the thermal parameters at the nanoscale. The time-dependent model developed herein lays the foundation to describe the dynamics of heat transfer at the nanoscale and were used to understand the heat dissipation by lipid bilayers.

Pitfalls in Monitoring Mitochondrial Temperature Using Charged Thermosensitive Fluorophores

Mitochondria are the source of internal heat which influences all cellular processes. Hence, monitoring mitochondrial temperature provides a unique insight into cell physiology. Using a thermosensitive fluorescent probe MitoThermo Yellow (MTY), we have shown recently that mitochondria within human cells are maintained at close to 50 °C when active, increasing their temperature locally by about 10 °C. Initially reported in the HEK293 cell line, we confirmed this finding in the HeLa cell line. Delving deeper, using MTY and MTX (MitoThermo X), a modified version of MTY, we unraveled some caveats related to the nature of these charged fluorophores. While enabling the assessment of mitochondrial temperature in HEK and HeLa cell lines, the reactivity of MTY to membrane potential variations in human primary skin fibroblasts precluded local temperature monitoring in these cells. Chemical modification of MTY into MTX did not result in a temperature probe unresponsive to membrane potential variations that could be universally used in any cell type to determine mitochondrial temperature. Thus, the cell-type dependence of MTY in measuring mitochondrial temperature, which is likely due to the variable binding of this dye to specific internal mitochondrial components, should imply cautiousness while using these nanothermometers for mitochondrial temperature analysis.

Biophysical Reviews: 2020-looking back, going forward

After first describing the issue contents (Biophysical Reviews-Volume 12 Issue 6), this Editorial goes on to provide a short round-up of the activities of the journal in 2020. Directly following this Editorial are two obituaries marking the recent deaths of Prof. Fumio Oosawa (Japan) and Dr. Herbert Tabor (USA)-two major figures in Biophysical/Biochemical science from the last 100 years.

The conundrum of hot mitochondria

The mitochondrion is often referred as the cellular powerhouse because the organelle oxidizes organic acids and NADH derived from nutriments, converting around 40% of the Gibbs free energy change of these reactions into ATP, the major energy currency of cell metabolism. Mitochondria are thus microscopic furnaces that inevitably release heat as a by-product of these reactions, and this contributes to body warming, especially in endotherms like birds and mammals. Over the last decade, the idea has emerged that mitochondria could be warmer than the cytosol, because of their intense energy metabolism. It has even been suggested that our own mitochondria could operate under normal conditions at a temperature close to 50 °C, something difficult to reconcile with the laws of thermal physics. Here, using our combined expertise in biology and physics, we exhaustively review the reports that led to the concept of a hot mitochondrion, which is essentially based on the development and use of a variety of molecular thermosensors whose intrinsic fluorescence is modified by temperature. Then, we discuss the physical concepts of heat diffusion, including mechanisms like phonons scattering, which occur in the nanoscale range. Although most of approaches with thermosensors studies present relatively sparse data and lack absolute temperature calibration, overall, they do support the hypothesis of hot mitochondria. However, there is no convincing physical explanation that would allow the organelle to maintain a higher temperature than its surroundings. We nevertheless proposed some research directions, mainly biological, that might help throw light on this intriguing conundrum.

Advances and challenges for fluorescence nanothermometry

Fluorescent nanothermometers can probe changes in local temperature in living cells and in vivo and reveal fundamental insights into biological properties. This field has attracted global efforts in developing both temperature-responsive materials and detection procedures to achieve sub-degree temperature resolution in biosystems. Recent generations of nanothermometers show superior performance to earlier ones and also offer multifunctionality, enabling state-of-the-art functional imaging with improved spatial, temporal and temperature resolutions for monitoring the metabolism of intracellular organelles and internal organs. Although progress in this field has been rapid, it has not been without controversy, as recent studies have shown possible biased sensing during fluorescence-based detection. Here, we introduce the design principles and advances in fluorescence nanothermometry, highlight application achievements, discuss scenarios that may lead to biased sensing, analyze the challenges ahead in terms of both fundamental issues and practical implementations, and point to new directions for improving this interdisciplinary field.

Real-Time Intracellular Temperature Imaging Using Lanthanide-Bearing Polymeric Micelles

Measurement of thermogenesis in individual cells is a remarkable challenge due to the complexity of the biochemical environment (such as pH and ionic strength) and to the rapid and yet not well-understood heat transfer mechanisms throughout the cell. Here, we present a unique system for intracellular temperature mapping in a fluorescence microscope (uncertainty of 0.2 K) using rationally designed luminescent Ln³⁺-bearing polymeric micellar probes (Ln = Sm, Eu) incubated in breast cancer MDA-MB468 cells. Two-dimensional (2D) thermal images recorded increasing the temperature of the cells culture medium between 296 and 304 K shows inhomogeneous intracellular temperature progressions up to ∼20 degrees and subcellular gradients of ∼5 degrees between the nucleolus and the rest of the cell, illustrating the thermogenic activity of the different organelles and highlighting the potential of this tool to study intracellular processes.

Biophysical Reviews' national biophysical society partnership program

This Special Issue is focused on the Biophysical Society of Japan. It represents the first in a series tasked with introducing an individual national biophysical society to the wider biophysical community. In this Editorial for Volume 12 Issue 2, I first outline the nature and goals of this program before going on to describe the contents of the Special Issue that relate to the activities organized by the Biophysical Society of Japan and the scope of the research performed by its members.