Response strategies and biological applications of organic fluorescent thermometry: cell- and mitochondrion-level detection

Temperature homeostasis is critical for cells to perform their physiological functions. Among the diverse methods for temperature detection, fluorescent temperature probes stand out as a proven and effective tool, especially for monitoring temperature in cells and suborganelles, with a specific emphasis on mitochondria. The utilization of these probes provides a new opportunity to enhance our understanding of the mechanisms and interconnections underlying various physiological activities related to temperature homeostasis. However, the complexity and variability of cells and suborganelles necessitate fluorescent temperature probes with high resolution and sensitivity. To meet the demanding requirements for intracellular/subcellular temperature detection, several strategies have been developed, offering a range of options to address this challenge. This review examines four fundamental temperature-response strategies employed by small molecule and polymer probes, including intramolecular rotation, polarity sensitivity, Förster resonance energy transfer, and structural changes. The primary emphasis was placed on elucidating molecular design and biological applications specific to each type of probe. Furthermore, this review provides an insightful discussion on factors that may affect fluorescent thermometry, providing valuable perspectives for future development in the field. Finally, the review concludes by presenting cutting-edge response strategies and research insights for mitigating biases in temperature sensing.

Thermochromic Afterglow from Benzene-1,4-Diboronic Acid-Doped Co-crystals

The accurate thermosensing requires a minimum impact of autofluorescence and light scattering from the samples. In this study, we discovered that commercially available benzene-1,4-diboronic acid (BDBA) doped co-crystals with trimethylolpropane (TMP) exhibit excellent thermochromic dual phosphorescence properties over a wide temperature range from -132 to 40 °C, despite its simple structure that does not have any donor-acceptor linkage. The dual phosphorescence was consisted of monomeric benzene-1,4-diboronate (BDBA ester) and aggregation-stabilized species. With an increase in temperature, the emission intensity from the monomeric state significantly decreased, whereas that originating from the aggregated state remained almost constant owing to the difference in their thermal stabilities. Further investigation revealed that molecular distortions in singlet excited states enable efficient intersystem crossing, causing efficient phosphorescence from the monomeric state of BDBA ester.

Thermal properties of nanofluids using hydrophilic and hydrophobic LiYF4:Yb/Er upconverting nanoparticles

Luminescent nanoparticles have shown great potential for thermal sensing in bio-applications. Nonetheless, these materials lack water dispersibility that can be overcome by modifying their surface properties with water dispersible molecules such as cysteine. Herein, we employ LiYF4:Er3+/Yb3+ upconverting nanoparticles (UCNPs) capped with oleate or modified with cysteine dispersed in cyclohexane or in water, respectively, as thermal probes. Upconversion emission was used to sense temperature with a relative thermal sensitivity of ∼1.24% K-1 (at 300 K) and a temperature uncertainty of 0.8 K for the oleate capped and of 0.5 K for cysteine modified NPs. To study the effect of the cysteine modification in the heat transfer processes, the thermal conductivity of the nanofluids was determined, yielding 0.123(6) W m-1 K-1 for the oleate capped UCNPs dispersed in cyclohexane and 0.50(7) W m-1 K-1 for the cysteine modified UCNPs dispersed in water. Moreover, through the heating curves, the nanofluids' thermal resistances were estimated, showing that the cysteine modification partially prevents heat transfer.

In vivo heat production dynamics during a contraction-relaxation cycle in rat single skeletal muscle fibers

Skeletal muscle generates heat via contraction-dependent (shivering) and independent (nonshivering) mechanisms. While this thermogenic capacity of skeletal muscle undoubtedly contributes to the body temperature homeostasis of animals and impacts various cellular functions, the intracellular temperature and its dynamics in skeletal muscle in vivo remain elusive. We aimed to determine the intracellular temperature and its changes within skeletal muscle in vivo during contraction and following relaxation. In addition, we tested the hypothesis that sarcoplasmic reticulum Ca2+ ATPase (SERCA) generates heat and increases the myocyte temperature during a transitory Ca2+-induced contraction-relaxation cycle. The intact spinotrapezius muscle of anesthetized adult male Wistar rats (n = 18) was exteriorized and loaded with the fluorescent probe Cellular Thermoprobe for Fluorescence Ratio (49.3 μM) by microinjection over 1 s. The fluorescence ratio (i.e., 580 nm/515 nm) was measured in vivo during 1) temperature increases induced by means of an external heater, and 2) Ca2+ injection (3.9 nL, 2.0 mM). The fluorescence ratio increased as a linear function of muscle surface temperature from 25 °C to 40 °C (r2 = 0.97, P < 0.01). Ca2+ injection (3.9 nL, 2.0 mM) significantly increased myocyte intracellular temperature: An effect that was suppressed by SERCA inhibition with cyclopiazonic acid (CPA, Ca2+: 38.3 ± 1.4 °C vs Ca2++CPA: 28.3 ± 2.8 °C, P < 0.01 at 1 min following injection). While muscle shortening occurred immediately after the Ca2+ injection, the increased muscle temperature was maintained during the relaxation phase. In this investigation, we demonstrated a novel model for measuring the intracellular temperature of skeletal muscle in vivo and further that heat generation occurs concomitant principally with SERCA functioning and muscle relaxation.

Warm Cells, Hot Mitochondria: Achievements and Problems of Ultralocal Thermometry

Temperature is a crucial regulator of the rate and direction of biochemical reactions and cell processes. The recent data indicating the presence of local thermal gradients associated with the sites of high-rate thermogenesis, on the one hand, demonstrate the possibility for the existence of "thermal signaling" in a cell and, on the other, are criticized on the basis of thermodynamic calculations and models. Here, we review the main thermometric techniques and sensors developed for the determination of temperature inside living cells and diverse intracellular compartments. A comparative analysis is conducted of the results obtained using these methods for the cytosol, nucleus, endo-/sarcoplasmic reticulum, and mitochondria, as well as their biological consistency. Special attention is given to the limitations, possible sources of errors and ambiguities of the sensor's responses. The issue of biological temperature limits in cells and organelles is considered. It is concluded that the elaboration of experimental protocols for ultralocal temperature measurements that take into account both the characteristics of biological systems, as well as the properties and limitations of each type of sensor is of critical importance for the generation of reliable results and further progress in this field.

A novel optical temperature sensor and energy transfer properties based on Tb3+/Sm3+ codoped SrY2(MoO4)4 phosphors

A series of SrY2(MoO4)4 phosphors doped and co-doped with Tb3+/Sm3+ ions was synthesized to develop new optical temperature sensor materials. The structures, morphologies, and luminescent characteristics of these phosphors were thoroughly investigated. Luminescence spectra of mono-doped SrY2(MoO4)4 phosphors were measured under the excitation at 375 and 403 nm corresponding to direct excitation of Tb3+ and Sm3+, respectively. The characteristic luminescence bands corresponding to electronic transitions of terbium and samarium ions were detected and investigated for different dopant concentrations. The emission spectrum of the Tb3+/Sm3+ co-doped sample exhibited a total of five distinct emission peaks, indicating an energy transfer from Tb3+ to Sm3+ ions. The energy transfer efficiency from Tb3+ ions to Sm3+ ions was investigated in detail. At elevated temperatures, Tb3+ and Sm3+ exhibited distinct thermal sensitivities in their emission and excitation spectra, leading to evident thermochromic behavior. The fluorescence intensity ratio (FIR) was utilized with dual center to evaluate the temperature sensitivity of SrY2(MoO4)4:Tb3+/Sm3+ phosphors. The temperature sensing mechanism relied on the emission band intensity ratios of the 4G5/2 → 6H5/2, 4G5/2 → 6H9/2, and 4G5/2 → 6H7/2 transitions of Sm3+ in conjunction with the 5D5/2 → 7F5/2 transitions of Tb3+. This approach demonstrated high thermal sensitivity values, reaching up to 0.9% K-1. The studied nanoparticles exhibited sub-degree thermal resolution, making them suitable candidates for precise temperature-sensing applications.

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.

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.

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.

Substantial impact of 3-iodothyronamine (T1AM) on the regulations of fluorescent thermoprobe-measured cellular temperature and natriuretic peptide expression in cardiomyocytes

There is growing interest in 3-iodothyronamine (T1AM), an active thyroid hormone metabolite, that induces negative inotropic and chronotropic actions in the heart and exerts systemic hypothermic action. We explored the direct impact of T1AM on cardiomyocytes with a focus on the regulation of the intracellular temperature and natriuretic peptide (NP) expression. A thermoprobe was successfully introduced into neonatal rat cardiomyocytes, and the temperature-dependent changes in the fluorescence intensity ratio were measured using a fluorescence microscope. After one-hour incubation with T1AM, the degree of change in the fluorescence intensity ratio was significantly lower in T1AM-treated cardiomyocytes than in equivalent solvent-treated controls (P < 0.01), indicating the direct hypothermic action of T1AM on cardiomyocytes. Furthermore, T1AM treatment upregulated B-type NP (BNP) gene expression comparable to treatment with endothelin-1 or phenylephrine. Of note, ERK phosphorylation was markedly increased after T1AM treatment, and inhibition of ERK phosphorylation by an MEK inhibitor completely cancelled both T1AM-induced decrease in thermoprobe-measured temperature and the increase in BNP expression. In summary, T1AM decreases fluorescent thermoprobe-measured temperatures (estimated intracellular temperatures) and increases BNP expression in cardiomyocytes by activating the MEK/ERK pathway. The present findings provide new insight into the direct myocardial cellular actions of T1AM in patients with severe heart failure.

Magnetically Actuated Biohybrid Microswimmers for Precise Photothermal Muscle Contraction

Various strategies have been designed for myotube contraction and skeletal muscle stimulation in recent years, aiming in the field of skeletal muscle tissue engineering and bionics. However, most of the current approaches lack controllability and adaptability for precise stimulation, especially at the microlevel. Herein, wireless and precise activation of muscle by using magnetic biohybrid microswimmers in combination with near-infrared (NIR) laser irradiation is successfully demonstrated. Biohybrid microswimmers are fabricated by dip-coating superparamagnetic Fe₃O₄ nanoparticles onto the chlorella microalgae, thus endowing robust navigation in various biological media due to magnetic actuation. Under the guidance of a rotating magnetic field, the engineered microswimmer can achieve precise motion toward a single C2C12-derived myotube. Upon NIR irradiation, the photothermal effect from the incorporated Fe₃O₄ nanoparticles results in local temperature increments of approximately 5 °C in the targeted myotube, which could efficiently trigger the contraction of myotube. The mechanism underlying this phenomenon is a Ca²⁺-independent case involving direct actin-myosin interactions. In vivo muscle fiber contraction and histological test further demonstrate the effectiveness and biosafety of our design. The as-developed biohybrid microswimmer-based strategy is possible to provide a renovation for tissue engineering and bionics.

Stimuli-responsive fluorescent nanogel: a nonconventional donor for ratiometric temperature and pH sensing

A reactive stimuli responsive fluorescent polyaminoamide nanogel (NANO-PAMAM) is synthesized via an aza-Michael polyaddition reaction in water and subsequently transformed to a ratiometric nanosensor via post-polymerization modification of the reactive NANO-PAMAM.

Fluorescence Thermometer: Intermediation of the Fontal Temperature and Light

The rapid advance of thermal materials and fluorescence spectroscopy has extensively promoted micro-scale fluorescence thermometry development in recent years. Based on the advantages of fast response, high sensitivity, simple operation,...

A Computable Gaussian Quantum Correlation for Continuous-Variable Systems

Generally speaking, it is difficult to compute the values of the Gaussian quantum discord and Gaussian geometric discord for Gaussian states, which limits their application. In the present paper, for any (n+m)-mode continuous-variable system, a computable Gaussian quantum correlation M is proposed. For any state ρAB of the system, M(ρAB) depends only on the covariant matrix of ρAB without any measurements performed on a subsystem or any optimization procedures, and thus is easily computed. Furthermore, M has the following attractive properties: (1) M is independent of the mean of states, is symmetric about the subsystems and has no ancilla problem; (2) M is locally Gaussian unitary invariant; (3) for a Gaussian state ρAB, M(ρAB)=0 if and only if ρAB is a product state; and (4) 0≤M((ΦA⊗ΦB)ρAB)≤M(ρAB) holds for any Gaussian state ρAB and any Gaussian channels ΦA and ΦB performed on the subsystem A and B, respectively. Therefore, M is a nice Gaussian correlation which describes the same Gaussian correlation as Gaussian quantum discord and Gaussian geometric discord when restricted on Gaussian states. As an application of M, a noninvasive quantum method for detecting intracellular temperature is proposed.

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.

Ultrabright fluorescent nanothermometers

Here we report on the first ultrabright fluorescent nanothermometers, ∼50 nm-size particles, capable of measuring temperature in 3D and down to the nanoscale. The temperature is measured through the recording of the ratio of fluorescence intensities of fluorescent dyes encapsulated inside the nanochannels of the silica matrix of each nanothermometer. The brightness of each particle excited at 488 nm is equivalent to the fluorescence coming from 150 molecules of rhodamine 6G and 1700 molecules of rhodamine B dyes. The fluorescence of both dyes is excited with a single wavelength due to the Förster resonance energy transfer (FRET). We demonstrate repeatable measurements of temperature with the uncertainty down to 0.4 K and a constant sensitivity of ∼1%/K in the range of 20-50 °C, which is of particular interest for biomedical applications. Due to the high fluorescence brightness, we demonstrate the possibility of measurement of accurate 3D temperature distributions in a hydrogel. The accuracy of the measurements is confirmed by numerical simulations. We further demonstrate the use of single nanothermometers to measure temperature. As an example, 5-8 nanothermometers are sufficient to measure temperature with an error of 2 K (with the measurement time of >0.7 s).

Thermal equilibria between conformers enable highly reliable single-fluorophore ratiometric thermometers

Fluorescent thermometers based on organic dyes play an important role in the visualization of dynamic temperature topography with high resolution. Many molecular thermometers contain two fluorophores with different temperature responses to achieve ratiometric temperature detection. However, the stability and reliability of such ratiometric thermometers are highly dependent on the susceptibility of the fluorophores towards photobleaching. Despite the use of single-fluorophore ratiometric thermometers in previous reports, the difficulty and complexity in their molecular design and synthesis severely hinder their widespread applicability. In this work, we have investigated the use of commercially available dyes (i.e., BD140 and LD688) with conformational isomers as ratiometric thermometers. Each of these dyes consists of a mixture of conformers with distinct UV-vis absorption and emission spectra. The thermal equilibrium of these conformers successfully enabled the development of ratiometric thermometers with good stability and reliability.

Adaptive thermogenesis in brown adipose tissue involves activation of pannexin-1 channels

OBJECTIVE

Brown adipose tissue (BAT) is specialized in thermogenesis. The conversion of energy into heat in brown adipocytes proceeds via stimulation of β-adrenergic receptor (βAR)-dependent signaling and activation of mitochondrial uncoupling protein 1 (UCP1). We have previously demonstrated a functional role for pannexin-1 (Panx1) channels in white adipose tissue; however, it is not known whether Panx1 channels play a role in the regulation of brown adipocyte function. Here, we tested the hypothesis that Panx1 channels are involved in brown adipocyte activation and thermogenesis.

METHODS

In an immortalized brown pre-adipocytes cell line, Panx1 currents were measured using patch-clamp electrophysiology. Flow cytometry was used for assessment of dye uptake and luminescence assays for adenosine triphosphate (ATP) release, and cellular temperature measurement was performed using a ratiometric fluorescence thermometer. We used RNA interference and expression plasmids to manipulate expression of wild-type and mutant Panx1. We used previously described adipocyte-specific Panx1 knockout mice (Panx1Adip-/-) and generated brown adipocyte-specific Panx1 knockout mice (Panx1BAT-/-) to study pharmacological or cold-induced thermogenesis. Glucose uptake into brown adipose tissue was quantified by positron emission tomography (PET) analysis of 18F-fluorodeoxyglucose (18F-FDG) content. BAT temperature was measured using an implantable telemetric temperature probe.

RESULTS

In brown adipocytes, Panx1 channel activity was induced either by apoptosis-dependent caspase activation or by β3AR stimulation via a novel mechanism that involves Gβγ subunit binding to Panx1. Inactivation of Panx1 channels in cultured brown adipocytes resulted in inhibition of β3AR-induced lipolysis, UCP-1 expression, and cellular thermogenesis. In mice, adiponectin-Cre-dependent genetic deletion of Panx1 in all adipose tissue depots resulted in defective β3AR agonist- or cold-induced thermogenesis in BAT and suppressed beigeing of white adipose tissue. UCP1-Cre-dependent Panx1 deletion specifically in brown adipocytes reduced the capacity for adaptive thermogenesis without affecting beigeing of white adipose tissue and aggravated diet-induced obesity and insulin resistance.

CONCLUSIONS

These data demonstrate that Gβγ-dependent Panx1 channel activation is involved in β3AR-induced thermogenic regulation in brown adipocytes. Identification of Panx1 channels in BAT as novel thermo-regulatory elements downstream of β3AR activation may have therapeutic implications.

A high-precision thermometry microfluidic chip for real-time monitoring of the physiological process of live tumour cells

Temperature changes in cells are generally accompanied by physiological processes. Cellular temperature measurements can provide important information to fully understand cellular mechanisms. However, temperature measurements with conventional methods, such as fluorescent polymeric thermometers and thermocouples, have limitations of low sensitivity or cell state disturbance. We developed a microfluidic chip integrating a high-precision platinum (Pt) thermo-sensor that can culture cells and monitor the cellular temperature in situ. During detection, a constant temperature system with a stability of 0.015 °C was applied. The temperature coefficient of resistance of the Pt thermo-sensor was 2090 ppm/°C, giving a temperature resolution of the sensor of less than 0.008 °C. This microchip showed a good linear correlation between the temperature and resistance of the Pt sensor at 20-40 °C (R² = 0.999). Lung and liver cancer cells on the microchip grew normally and continuously. The maximum temperature fluctuation of H1975 (0.924 °C) was larger than that of HepG2 (0.250 °C). However, the temperature of adherent HepG2 cells changed over time, showing susceptibility to the environment most of the time compared to H1975. Moreover, the temperature increment of non-cancerous cells, such as hepatic stellate cells, was monitored in response to the stimulus of paraformaldehyde, showing the process of cell death. Therefore, this thermometric microchip integrated with cell culture could be a non-disposable and label-free tool for monitoring cellular temperature applied to the study of physiology and pathology.

Intracellular Thermometry at the Micro-/Nanoscale and its Potential Application to Study Protein Aggregation Related to Neurodegenerative Diseases

Temperature is a fundamental physical parameter that influences biological processes in living cells. Hence, intracellular temperature mapping can be used to derive useful information reflective of thermodynamic properties and cellular behaviour. Herein, existing publications on different thermometry systems, focusing on those that employ fluorescence-based techniques, are reviewed. From developments based on fluorescent proteins and inorganic molecules to metal nanoclusters and fluorescent polymers, the general findings of intracellular measurements from different research groups are discussed. Furthermore, the contradiction of mitochondrial thermogenesis and nuclear-cytoplasmic temperature differences to current thermodynamic understanding are highlighted. Lastly, intracellular thermometry is proposed as a tool to quantify the energy flow and cost associated with amyloid-β42 (Aβ42) aggregation, a hallmark of Alzheimer's disease.

Implementing Defects for Ratiometric Luminescence Thermometry

In luminescence thermometry enabling temperature reading at a distance, an important challenge is to propose new solutions that open measuring and material possibilities. Responding to these needs, in the nanocrystalline phosphors of yttrium oxide Y₂O₃ and lutetium oxide Lu₂O₃, temperature-dependent emission of trivalent terbium Tb³⁺ dopant ions was recorded at the excitation wavelength 266 nm. The signal of intensity decreasing with temperature was monitored in the range corresponding to the ⁵D₄ → ⁷F₆ emission band. On the other hand, defect emission intensity obtained upon 543 nm excitation increases significantly at elevated temperatures. The opposite thermal monotonicity of these two signals in the same spectral range enabled development of the single band ratiometric luminescent thermometer of as high a relative sensitivity as 4.92%/°C and 2%/°C for Y₂O₃:Tb³⁺ and Lu₂O₃:Tb³⁺ nanocrystals, respectively. This study presents the first report on luminescent thermometry using defect emission in inorganic phosphors.

The challenge of intracellular temperature

This short review begins with a brief introductory summary of luminescence nanothermometry. Current applications of luminescence nanothermometry are introduced in biological contexts. Then, theoretical bases of the “temperature” that luminescence nanothermometry determines are discussed. This argument is followed by the 10⁵ gap issue between simple calculation and the measurements reported in literatures. The gap issue is challenged by recent literatures reporting single-cell thermometry using non-luminescent probes, as well as a report that determines the thermal conductivity of a single lipid bilayer using luminescence nanothermometry. In the end, we argue if we can be optimistic about the solution of the 10⁵ gap issue.

808 nm-Light-Excited Near-Infrared Luminescent Lanthanide Metal-Organic Frameworks for Highly Sensitive Physiological Temperature Sensing

Ongoing demand for accurate self-calibrated noninvasive thermometers for micro-/nano-scale applications, particular biomedical diagnosis, is driving the development of temperature sensors. Here a new type of lanthanide metal-organic framework having near-infrared absorption and near-infrared emission features is presented, and it is based on efficient Nd³⁺ -to-Yb³⁺ energy transfer in 808 nm photoexcitation. The results show that the ratiometric parameter of Nd_0.5 Yb_0.5 TPTC (TPTC= 1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic acid) can deliver good exponential-type luminescence response to temperature in the physiological regime (293-328 K) with high relative sensitivity and accurate temperature resolution, as well as good biocompatibility and chemical stability. Such lanthanide-based materials are especially useful in biomedical applications.

AIE-active non-conjugated poly(N-vinylcaprolactam) as a fluorescent thermometer for intracellular temperature imaging

Since temperature is one of the most significant physiological parameters that dictate the cellular status of living organisms, accurate intracellular temperature measurement is crucial and a valuable biomarker for the diagnosis and treatment of diseases. Herein, we introduce the foremost example of a non-conjugated polymer as a next generation fluorescent thermometer which is capable of addressing the key shortcomings including toxicity and thermal-induced fluorescence quenching associated with π-π conjugated system-based thermometers developed so far. We revealed, for the first time, the unique photophysical and aggregation-induced emission (AIE) characteristics of well-known thermoresponsive poly(N-vinylcaprolactam) (PNVCL) devoid of any classical fluorophore entity. PNVCL underwent a coil to globular conformational transition in an aqueous medium and appeared to be fluorescent above its lower critical solution temperature (LCST) near body temperature (38 °C). Eventually, this intriguing aspect enabled higher cellular uptake of PNVCL at the LCST boundary. By virtue of the AIE effect, the thermo-induced aggregation phenomenon has been ingeniously utilized to apply PNVCL as a novel fluorescent thermometer for intracellular temperature determination.

Water-soluble poly(N-isopropylacrylamide) nanoparticles grafted to trivalent lanthanide complexes as highly sensitive ratiometric nanothermometers

A new water-soluble, luminescent ratiometric nanothermometer with a tunable temperature-responsive range, high thermal sensitivity and good repeatability was designed and synthesized by grafting lanthanide complexes to pNIPAM polymeric nanoparticles.

Er3+/Yb3+ co-doped nanocrystals modified with 6-aminocaproic acid for temperature sensing in biomedicine

We report β-PbF2:Er3+/Yb3+ nanocrystals (NCs) modified with 6-aminocaproic acid (6AA) via wet chemical etching of glass ceramics (GCs). NCs body-doped with trivalent rare-earth (RE3+) ions were released from the GCs by etching of the glass matrix and modified with bifunctional 6AA ligands to enhance their water solubility. They have good stability in water with an average diameter of 56 nm and display efficient green (521, 550 nm) and red (660 nm) emission under the excitation of a 940 nm laser. High absolute sensitivity (S A) and relative sensitivity (S R) (0.0027 K-1 and 1.18% K-1, respectively) and high resolution (0.25 K) were achieved for temperature sensing in the biological temperature range, using the fluorescence intensity ratio (FIR) technique. All of the experimental results indicate that the Er3+/Yb3+ co-doped NCs modified with 6AA may potentially be useful as fluorescent biological temperature sensors.

Cytoplasmic convection currents and intracellular temperature gradients

Intracellular thermometry has recently demonstrated temperatures in the nucleus, mitochondria, and centrosome to be significantly higher than those of the cytoplasm and cell membrane. This local thermogenesis and the resulting temperature gradient could facilitate the development of persistent, self-organizing convection currents in the cytoplasm of large eukaryotes. Using 3-dimensional computational simulations of intracellular fluid motion, we quantify the convective velocities that could result from the temperature differences observed experimentally. Based on these velocities, we identify the conditions necessary for this temperature-driven bulk flow to dominate over random thermal diffusive motion at the scale of a single eukaryotic cell. With temperature gradients of the order 1°C and diffusion coefficients comparable to those described in the literature, Péclet numbers ≥ 1 are feasible and permit comparable or greater effects of convection than diffusion in determining intracellular mass flux. In addition to the temperature gradient, the resulting flow patterns would also depend on the spatial localization of the heat source, the shape of the cell membrane, and the complex intracellular structure including the cytoskeleton. While this intracellular convection would be highly context-dependent, in certain settings, convective motion could provide a previously unrecognized mechanism for directed, bulk transport within eukaryotic cells.

A Personal Journey across Fluorescent Sensing and Logic Associated with Polymers of Various Kinds

Our experiences concerning fluorescent molecular sensing and logic devices and their intersections with polymer science are the foci of this brief review. Proton-, metal ion- and polarity-responsive cases of these devices are placed in polymeric micro- or nano-environments, some of which involve phase separation. This leads to mapping of chemical species on the nanoscale. These devices also take advantage of thermal properties of some polymers in water in order to reincarnate themselves as thermometers. When the phase separation leads to particles, the latter can be labelled with identification tags based on molecular logic. Such particles also give rise to reusable sensors, although molecular-scale resolution is sacrificed in the process. Polymeric nano-environments also help to organize rather complex molecular logic systems from their simple components. Overall, our little experiences suggest that researchers in sensing and logic would benefit if they assimilate polymer concepts.

Cationic Fluorescent Nanogel Thermometers based on Thermoresponsive Poly(N-isopropylacrylamide) and Environment-Sensitive Benzofurazan

Cationic nanogels of N-isopropylacrylamide (NIPAM), including NIPAM-based cationic fluorescent nanogel thermometers, were synthesized with a cationic radical initiator previously developed in our laboratory. These cationic nanogels were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta potential measurements and fluorescence spectroscopy, as summarized in the temperature-dependent fluorescence response based on the structural change in polyNIPAM units in aqueous solutions. Cellular experiments using HeLa (human epithelial carcinoma) cells demonstrated that NIPAM-based cationic fluorescent nanogel thermometers can spontaneously enter the cells under mild conditions (at 25 °C for 20 min) and can show significant fluorescence enhancement without cytotoxicity with increasing culture medium temperature. The combination of the ability to enter cells and non-cytotoxicity is the most important advantage of cationic fluorescent nanogel thermometers compared with other types of fluorescent polymeric thermometers, i.e., anionic nanogel thermometers and cationic/anionic linear polymeric thermometers.

A reversible fluorescent probe based on C[double bond, length as m-dash]N isomerization for the selective detection of formaldehyde in living cells and in vivo

Formaldehyde (FA) is an endogenously produced reactive carbonyl species (RCS) through biological metabolic processes whose concentration is closely related to human health and disease. Noninvasive and real-time detection of FA concentration in organisms is very important for revealing the physiological and pathological functions of FA. Herein, we design and synthesize a reversible fluorescent probe BOD-NH₂ for the detection of FA in living cells and in vivo. The probe is composed of two moieties: the BODIPY fluorophore and the primary amino group response unit. The probe undergoes an intracellular aldimine condensation reaction with FA and forms imine (C[double bond, length as m-dash]N) which will result in C[double bond, length as m-dash]N isomerization and rotation to turn-off the fluorescence of the probe. It is important that the probe can show a reversible response to FA. The probe BOD-NH₂ has been successfully applied for detecting and imaging FA in the cytoplasm of living cells. BOD-NH₂ is capable of detecting fluctuations in the levels of endogenous and exogenous FA in different types of living cells. The probe can be used to visualize the FA concentration in fresh hippocampus and the probe can further qualitatively evaluate the FA concentrations in ex vivo-dissected organs. Moreover, BOD-NH₂ can also be used for imaging in mice. The above applications make our new probe a potential chemical tool for the study of physiological and pathological functions of FA in cells and in vivo.

Fixable Molecular Thermometer for Real-Time Visualization and Quantification of Mitochondrial Temperature

A change of mitochondrial temperature can be an important indicator of mitochondrial metabolism that generates considerable heat. For this reason, development of fluorescent probes to detect mitochondrial temperature has become an attractive topic. Previous efforts have successfully addressed the major issues, such as temperature sensitivity and mitochondrial targetability. However, there remains a key obstacle to practical applications. Considering the highly dynamic features of mitochondria, especially the variation of the inner-membrane potential, it is quite necessary to permanently immobilize a temperature probe in mitochondria in order to avoid unstable intracellular localization along with the changes of mitochondrial status. Herein, we report Mito-TEM, the first fixable, fluorescent molecular thermometer. Mito-TEM is based on a positively charged rhodamine B fluorophore that has the tendency of being attracted to mitochondria, which have negative potential. This fluorophore containing rotatable substituents also contributes to the temperature-responsive fluorescence property. Most importantly, a benzaldehyde is introduced in Mito-TEM as an anchoring unit that condenses with aminos of the protein and thus immobilizes the probe in mitochondria. The specific immobilization of Mito-TEM in mitochondria is unambiguously demonstrated in colocalization imaging. By using Mito-TEM, a method of visualizing and quantifying a temperature distribution through grayscale imaging of mitochondria is established and further applied to monitor the temperature changes of live cells under light heating and PMA stimulation.

Silver Nanowire-Based Fluorescence Thermometer for a Single Cell

A fluorescence thermometer based on silver nanowires (AgNWs) is realized by assembling Texas Red (TR)-marked thermal-sensitive DNA stem-loops (TR-DNA stem-loop) on the surface of AgNWs. Temperature configures the structure of the TR-DNA stem-loop and resultantly adjusts the energy transfer between TR and the AgNWs, which could sensitively control the fluorescence intensity of the thermometer. The thermometer is sensitive to the temperature ranging from 30 to 40 °C with the sensitivity of 2.6%/°C. Under the assistance of laser confocal microscopy, a temperature change within a single cell was observed by the monofilament AgNW-based thermometer.

Thermal sensing in fluid at the micro-nano-scales

Temperature is one of the most fundamental parameters for the characterization of a physical system. With rapid development of lab-on-a-chip and biology at single cell level, a great demand has risen for the temperature sensors with high spatial, temporal, and thermal resolution. Nevertheless, measuring temperature in liquid environment is always a technical challenge. Various factors may affect the sensing results, such as the fabrication parameters of built-in sensors, thermal property of electrical insulating layer, and stability of fluorescent thermometers in liquid environment. In this review, we focused on different kinds of micro/nano-thermometers applied in the thermal sensing for microfluidic systems and cultured cells. We discussed the advantages and limitations of these thermometers in specific applications and the challenges and possible solutions for more accurate temperature measurements in further studies.

Intracellular thermometry with fluorescent sensors for thermal biology

Temperature influences the activities of living organisms at various levels. Cells not only detect environmental temperature changes through their unique temperature-sensitive molecular machineries but also muster an appropriate response to the temperature change to maintain their inherent functions. Despite the fundamental involvement of temperature in physiological phenomena, the mechanism by which cells produce and use heat is largely unknown. Recently, fluorescent thermosensors that function as thermometers in live cells have attracted much attention in biology. These new tools, made of various temperature-sensitive molecules, have allowed for intracellular thermometry at the single-cell level. Intriguing spatiotemporal temperature variations, including organelle-specific thermogenesis, have been revealed with these fluorescent thermosensors, which suggest an intrinsic connection between temperature and cell functions. Moreover, fluorescent thermosensors have shown that intracellular temperature changes at the microscopic level are largely different from those assumed for a water environment at the macroscopic level. Thus, the employment of fluorescent thermosensors will uncover novel mechanisms of intracellular temperature-assisted physiological functions.

The thermogenic actions of natriuretic peptide in brown adipocytes: The direct measurement of the intracellular temperature using a fluorescent thermoprobe

In addition to the various effects of natriuretic peptides (NPs) on cardiovascular systems, increasing attention is being paid to the possibility that NPs induce adipose tissue browning and activate thermogenic program. We herein established a direct intracellular temperature measurement system using a fluorescent thermoprobe and investigated the thermogenic effects of A-type NP (ANP) on brown adipocytes. The thermoprobe was successfully introduced into rat brown adipocytes, and the temperature dependent change in fluorescence intensity ratio was measured using a fluorescence microscope. After one-hour incubation with ANP, the degree of the change in fluorescence intensity ratio was significantly higher in ANP-treated (P < 0.01) adipocytes compared to untreated controls. The ANP treatment increased uncoupling protein-1 (UCP1) mRNA levels, which is one of the markers of thermogenesis in adipocytes, while the intracellular ATP content was not changed, indicating mitochondrial uncoupled respiration. Intriguingly, these thermogenic actions of ANP were more prominent when brown adipocytes were incubated at 35 °C than at 37 °C. Moreover, the increase in the intracellular temperature and the expression of UCP1 induced by ANP were cancelled by p38MAPK inhibition. Taken together, this study directly demonstrated the thermogenic actions of ANP in brown adipocytes through the use of a novel method of intracellular temperature measurement.

Difference in intracellular temperature rise between matured and precursor brown adipocytes in response to uncoupler and β-adrenergic agonist stimuli

Brown adipocytes function to maintain body temperature by heat production. However, direct measurement of heat production at a single cell level remains difficult. Here we developed a method to measure the temperature within primary cultured brown adipocytes using a cationic fluorescent polymeric thermometer. Placement of the thermometer within a matured brown adipocyte and a precursor cell enabled the detection of heat production following uncoupler treatment. The increase in the intracellular temperature due to stimulation with a mitochondrial uncoupler was higher in matured brown adipocytes than in precursor cells. Stimulation with a β-adrenergic receptor (β-AR) agonist, norepinephrine, raised the intracellular temperature of matured brown adipocytes to a level comparable to that observed after stimulation with a β3-AR-specific agonist, CL316.243. In contrast, neither β-AR agonist induced an intracellular temperature increase in precursor cells. Further, pretreatment of brown adipocytes with a β3-AR antagonist inhibited the norepinephrine-stimulated elevation of temperature. These results demonstrate that our novel method successfully determined the difference in intracellular temperature increase between matured brown adipocytes and precursor cells in response to stimulation by an uncoupler and β-AR agonists.

Measurement of local temperature increments induced by cultured HepG2 cells with micro-thermocouples in a thermally stabilized system

To monitor the temperature distribution of a cell and its changes under varied conditions is currently a technical challenge. A variety of non-contact methods used for measuring cellular temperature have been developed, where changes of local temperature at cell-level and sub-cell-level are indirectly calculated through the changes in intensity, band-shape, bandwidth, lifetime or polarization anisotropy of the fluorescence spectra recorded from the nano-sized fluorescent materials pre-injected into the target cell. Unfortunately, the optical properties of the fluorescent nano-materials may be affected by complicated intracellular environment, leading to unexpected measurement errors and controversial arguments. Here, we attempted to offer an alternative approach for measuring the absolute increments of local temperature in micro-Testing Zones induced by live cells. In this method, built-in high-performance micro-thermocouple arrays and double-stabilized system with a stability of 10 mK were applied. Increments of local temperature close to adherent human hepatoblastoma (HepG2) cells were continuously recorded for days without stimulus, showing frequent fluctuations within 60 mK and a maximum increment by 285 mK. This method may open a door for real-time recording of the absolute local temperature increments of individual cells, therefore offering valuable information for cell biology and clinical therapy in the field of cancer research.

Genetically encoded ratiometric fluorescent thermometer with wide range and rapid response

Temperature is a fundamental physical parameter that plays an important role in biological reactions and events. Although thermometers developed previously have been used to investigate several important phenomena, such as heterogeneous temperature distribution in a single living cell and heat generation in mitochondria, the development of a thermometer with a sensitivity over a wide temperature range and rapid response is still desired to quantify temperature change in not only homeotherms but also poikilotherms from the cellular level to in vivo. To overcome the weaknesses of the conventional thermometers, such as a limitation of applicable species and a low temporal resolution, owing to the narrow temperature range of sensitivity and the thermometry method, respectively, we developed a genetically encoded ratiometric fluorescent temperature indicator, gTEMP, by using two fluorescent proteins with different temperature sensitivities. Our thermometric method enabled a fast tracking of the temperature change with a time resolution of 50 ms. We used this method to observe the spatiotemporal temperature change between the cytoplasm and nucleus in cells, and quantified thermogenesis from the mitochondria matrix in a single living cell after stimulation with carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, which was an uncoupler of oxidative phosphorylation. Moreover, exploiting the wide temperature range of sensitivity from 5°C to 50°C of gTEMP, we monitored the temperature in a living medaka embryo for 15 hours and showed the feasibility of in vivo thermometry in various living species.

Intracellular temperature measurements with fluorescent polymeric thermometers

Intracellular temperature can be measured using fluorescent polymeric thermometersviatheir temperature-dependent fluorescence signals.