Quantum dot nano thermometers reveal heterogeneous local thermogenesis in living cells

Nanoparticles based image-guided thermal therapy and temperature feedback

Nanoparticles have emerged as versatile tools in the realm of thermal therapy, offering precise control and feedback mechanisms for targeted treatments. This review explores the intersection of nanotechnology and thermal therapy, focusing on the utilization of nanoparticles for image-guided interventions and temperature monitoring. Starting with an exploration of local temperature dynamics compared to whole-body responses, we delve into the landscape of nanomaterials and their pivotal role in nanomedicine. Various physical stimuli employed in therapy and imaging are scrutinized, laying the foundation for nanothermal therapies and the accompanying challenges. A comprehensive analysis of nanomaterial architecture ensues, delineating the functionalities of magnetic, plasmonic, and luminescent nanomaterials within the context of thermal therapies. Nano-design intricacies, including core-shell structures and monodisperse properties, are dissected for their impact on therapeutic efficacy. Furthermore, considerations in designing in vivo nanomaterials, such as hydrodynamic radii and core sizes at sub-tissue levels, are elucidated. The review then delves into specific modalities of thermally induced therapy, including magnetically induced hyperthermia and luminescent-based thermal treatments. Magnetic hyperthermia treatment is explored alongside its imaging and relaxometric properties, emphasizing the implications of imaging formulations on biotransformation and biodistribution. This review also provides an overview of the magnetic hyperthermia treatment using magnetic nanoparticles to induce localized heat in tissues. Similarly, optical and thermal imaging techniques utilizing luminescent nanomaterials are discussed, highlighting their potential for light-induced thermal therapy and cellular-level temperature monitoring. Finally, the application landscape of diagnosis and photothermal therapy (PTT) is surveyed, encompassing diverse areas such as cancer treatment, drug delivery, antibacterial therapy, and immunotherapy. The utility of nanothermometers in elucidating thermal relaxation dynamics as a diagnostic tool is underscored, alongside discussions on PTT hyperthermia protocols. Moreover, the advancements in nanoparticle magnetic imaging and implications of imaging formulations especially in creating positive MRI contrast agents are also presented. This comprehensive review offers insights into the evolving landscape of nanoparticle-based image-guided thermal therapies, promising advancements in precision medicine and targeted interventions, underscoring the importance of continued research in optimization for the full potential of magnetic hyperthermia to improve its efficacy and clinical translation.

Temperature Dependence of Thermal Conductivity of Proteins: Contributions of Thermal Expansion and Grüneisen Parameter

The thermal conductivity of many materials depends on temperature due to several factors, including variation of heat capacity with temperature, changes in vibrational dynamics with temperature, and change in volume with temperature. For proteins some, but not all, of these influences on the variation of thermal conductivity with temperature have been investigated in the past. In this study, we examine the influence of change in volume, and corresponding changes in vibrational dynamics, on the temperature dependence of the thermal conductivity. Using a measured value for the coefficient of thermal expansion and recently computed values for the Grüneisen parameter of proteins we find that the thermal conductivity increases with increasing temperature due to change in volume with temperature. We compare the impact of thermal expansion on the variation of the thermal conductivity with temperature found in this study with contributions of heat capacity and anharmonic coupling examined previously. Using values of thermal transport coefficients computed for proteins we also model heating of water in a protein solution following photoexcitation.

A High-Precision Real-Time Temperature Acquisition Method Based on Magnetic Nanoparticles

The unique magnetothermal properties of magnetic nanoparticles enable the development of a high-precision, real-time, noninvasive temperature measurement method with significant potential in the biomedical field. Based on a low-frequency alternating magnetic field excitation model, we construct two additional magnetic field excitation models-alternating current-direct current superposition and dual-frequency superposition-to extract harmonic amplitude components from the magnetization response. To increase the accuracy of harmonic information acquisition, the effects of the truncation error, excitation magnetic field frequency, and amplitude are thoroughly analyzed, and optimal parameter values are selected to minimize the error. A single algorithm is designed for temperature inversion, and a joint algorithm is proposed to optimize the performance of the single algorithm. Under low-frequency alternating-current magnetic field excitation, the autonomous group particle swarm optimization method achieves superior real-time performance in terms of temperature inversion and running time. Compared with the opposition learning gray wolf optimizer and particle swarm optimization-gray wolf optimization, the proposed method achieves reductions of 52% and 68%, respectively. Additionally, under dual-frequency superimposed magnetic field excitation, a higher temperature inversion accuracy is achieved compared with that of the particle swarm optimization-gray wolf optimization algorithm, reducing the error from 0.237 K to 0.094 K.

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

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

Hybrid plasmonic nanodiamonds for thermometry and local photothermal therapy of melanoma: a comparative study

Hyperthermia plays a significant role in cancer treatment by inducing cell damage through temperature elevation, often used alongside other treatment modalities. During hyperthermia therapy, temperature control is crucial. Here, we report on a simple synthesis route of hybrid plasmonic nanodiamonds either completely wrapped with an Au shell (NV@Au) or densely covered with Au NPs (NV@SiO 2 @Au). Such integration of nanodiamonds with Au NPs is advantageous both for heating and precise thermometry at nanoscale. After structural and optical investigations, heating abilities of the obtained plasmonic nanodiamonds were thoroughly inspected on glass, in association with living cells, and in tissue slices ex vivo, revealing their effective heat generation under excitation with light using a single excitation source. The developed hybrid plasmonic nanodiamonds were finally applied for local photothermal therapy of melanoma in vivo, demonstrating their efficacy in eradicating cancer cells and monitoring temperature during the process.

Fluorescent Nanodiamonds for High-Resolution Thermometry in Biology

Optically active color centers in diamond and nanodiamonds can be utilized as quantum sensors for measuring various physical parameters, particularly magnetic and electric fields, as well as temperature. Due to their small size and possible surface functionalization, fluorescent nanodiamonds are extremely attractive systems for biological and medical applications since they can be used for intracellular experiments. This review focuses on fluorescent nanodiamonds for thermometry with high sensitivity and a nanoscale spatial resolution for the investigation of living systems. The current state of the art, possible further development, and potential limitations of fluorescent nanodiamonds as thermometers will be discussed here.

TRPM8 affects relative "cooling and heating" of subcellular organelles in microglia in a context-dependent manner

Thermoregulation and thermal homeostasis at the cellular and subcellular organelle level are poorly understood events. In this work, we used BV2, a microglial cell line, and a series of thermo-sensitive subcellular organelle-specific probes to analyze the relative changes in the spatio-temporal temperatures of different subcellular organelles, both qualitatively and quantitatively. These methodologies allowed us to understand the thermal relationship of different subcellular organelles also. We modulated BV2 cells by pharmacological application of activator or inhibitor of TRPM8 ion channel (a cold-sensitive ion channel) and/or by treating the cells with LPS, a molecule that induces pathogen-associated molecular patterns (PAMPs) signaling. We demonstrate that the temperatures of individual organelles remain variable within a physiological range, yet vary in different conditions. We also demonstrate that treating BV2 cells by TRPM8 modulators and/or LPS alters the organelle temperatures in a specific and context-dependent manner. We show that TRPM8 modulation and/or LPS can alter the relationship of mitochondrial membrane potential to mitochondrial temperature. Our work suggests that mitochondrial temperature positively influences ER temperature and negatively influences Golgi temperature. Golgi temperature positively influences membrane temperature. This understanding of thermal relationships may be crucial for dissecting cellular structures, function, and stress signaling and may be relevant for different diseases.

Non-Markovian enhancement of nonequilibrium quantum thermometry

Accurate measurement at low temperatures is essential for both gaining a fundamental understanding of physical processes and developing technological applications. In this paper, we propose a theoretical framework for quantum temperature sensing in a composite environment with non-Markovian dynamics. Our suggested system uses a single qubit as a temperature sensor to test its sensitivity in calculating the temperature of a composite environment. We show that the temperature sensor's sensitivity can saturate the quantum Cramér-Rao bound by measuring the σ[over ̂]_{z} observable of the probe qubit. Temperature sensing performance is measured using the quantum signal-to-noise ratio. We underline how non-Markovianity can enhance the performance of our thermometers. Furthermore, we emphasize that nonequilibrium conditions do not always result in the best sensitivities in temperature estimation.

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.

Ultrasensitive Ratiometric Fluorescent Nanothermometer with Reverse Signal Changes for Intracellular Temperature Mapping

Sensing temperature at the subcellular level is pivotal for gaining essential thermal insights into diverse biological processes. However, achieving sensitive and accurate sensing of the intracellular temperature remains a challenge. Herein, we develop a ratiometric organic fluorescent nanothermometer with reverse signal changes for the ultrasensitive mapping of intracellular temperature. The nanothermometer is fabricated from a binary mixture of saturated fatty acids with a noneutectic composition, a red-emissive aggregation-caused quenching luminogen, and a green-emissive aggregation-induced emission luminogen using a modified nanoprecipitation method. Different from the eutectic mixture with a single phase-transition point, the noneutectic mixture possesses two solid-liquid phase transitions, which not only allows for reversible regulation of the aggregation states of the encapsulated luminogens but also effectively broadens the temperature sensing range (25-48 °C) across the physiological temperature range. Remarkably, the nanothermometer exhibits reverse and sensitive signal changes, demonstrating maximum relative thermal sensitivities of up to 63.66% °C-1 in aqueous systems and 44.01% °C-1 in the intracellular environment, respectively. Taking advantage of these outstanding thermometric performances, the nanothermometer is further employed to intracellularly monitor minute temperature variations upon chemical stimulation. This study provides a powerful tool for the exploration of dynamic cellular thermal activities, holding great promise in unveiling intricate physiological processes.

Coherence-Enhanced Single-Qubit Thermometry out of Equilibrium

The metrological limits of thermometry operated in nonequilibrium dynamical regimes are analyzed. We consider a finite-dimensional quantum system, employed as a quantum thermometer, in contact with a thermal bath inducing Markovian thermalization dynamics. The quantum thermometer is initialized in a generic quantum state, possibly including quantum coherence with respect to the Hamiltonian basis. We prove that the precision of the thermometer, quantified by the Quantum Fisher Information, is enhanced by the quantum coherence in its initial state. We analytically show this in the specific case of qubit thermometers for which the maximization of the Quantum Fisher Information occurs at a finite time during the transient thermalization dynamics. Such a finite-time precision enhancement can be better than the precision that is achieved asymptotically.

Ca2+ Overload Decreased Cellular Viability in Magnetic Hyperthermia without a Macroscopic Temperature Rise

Magnetic hyperthermia is a crucial medical engineering technique for treating diseases, which usually uses alternating magnetic fields (AMF) to interplay with magnetic substances to generate heat. Recently, it has been found that in some cases, there is no detectable temperature increment after applying an AMF, which caused corresponding effects surprisingly. The mechanisms involved in this phenomenon are not yet fully understood. In this study, we aimed to explore the role of Ca2+ overload in the magnetic hyperthermia effect without a perceptible temperature rise. A cellular system expressing the fusion proteins TRPV1 and ferritin was prepared. The application of an AMF (518 kHz, 16 kA/m) could induce the fusion protein to release a large amount of iron ions, which then participates in the production of massive reactive oxygen radicals (ROS). Both ROS and its induced lipid oxidation enticed the opening of ion channels, causing intracellular Ca2+ overload, which further led to decreased cellular viability. Taken together, Ca2+ overload triggered by elevated ROS and the induced oxidation of lipids contributes to the magnetic hyperthermia effect without a perceptible temperature rise. These findings would be beneficial for expanding the application of temperature-free magnetic hyperthermia, such as in cellular and neural regulation, design of new cancer treatment methods.

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

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.

X-Ray Nanothermometry of Nanoparticles in Tumor-Mimicking Tissues under Photothermia

Temperature plays a critical role in regulating body mechanisms and indicating inflammatory processes. Local temperature increments above 42 °C are shown to kill cancer cells in tumorous tissue, leading to the development of nanoparticle-mediated thermo-therapeutic strategies for fighting oncological diseases. Remarkably, these therapeutic effects can occur without macroscopic temperature rise, suggesting localized nanoparticle heating, and minimizing side effects on healthy tissues. Nanothermometry has received considerable attention as a means of developing nanothermosensing approaches to monitor the temperature at the core of nanoparticle atoms inside cells. In this study, a label-free, direct, and universal nanoscale thermometry is proposed to monitor the thermal processes of nanoparticles under photoexcitation in the tumor environment. Gold-iron oxide nanohybrids are utilized as multifunctional photothermal agents internalized in a 3D tumor model of glioblastoma that mimics the in vivo scenario. The local temperature under near-infrared photo-excitation is monitored by X-ray absorption spectroscopy (XAS) at the Au L3 -edge (11 919 eV) to obtain their temperature in cells, deepening the knowledge of nanothermal tumor treatments. This nanothermometric approach demonstrates its potential in detecting high nanothermal changes in tumor-mimicking tissues. It offers a notable advantage by enabling thermal sensing of any element, effectively transforming any material into a nanothermometer within biological environments.

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.

Thermoelectric Nanofluidics Probing Thermal Heterogeneity inside Single Cells

Accurate temperature measurement in one living cell is of great significance for understanding biological functions and regulation. Here, a nanopipet electric thermometer (NET) is established for real-time intracellular temperature measurement. Based on the temperature-controlled ion migration, the temperature change in solution results in altered ion mobilities and ion distributions, which can be converted to the thermoelectric responses of NET in a galvanostatic configuration. The exponential relationship between the voltage and the temperature promises highly sensitive thermoelectric responses up to 11.1 mV K-1, which is over an order of magnitude higher than previous thermoelectric thermometry. Moreover, the NET exhibits superior thermal resolution of 25 mK and spatiotemporal resolution of 100 nm and 0.9 ms as well as excellent stability and reproducibility. Benefiting from these unique features, both thermal fluctuations in steady-state cells and heat generation and dissipation upon drug administration can be successfully monitored, which are hardly achieved by current methods. By using NET, thermal heterogeneities of single cancer cells during immunotherapy were reported first in this work, in which the increased intracellular temperature was demonstrated to be associated with the survival benefit and resistance of cancer cells in immunotherapy. This work not only provides a reliable method for microscopic temperature monitoring but also gains new insights to elucidate the mechanism of immune evasion and therapeutic resistance.

Microbial thermogenesis is dependent on ATP concentrations and the protein kinases ArcB, GlnL, and YccC

Organisms necessarily release heat energy in their pursuit of survival. This process is known as cellular thermogenesis and is implicated in many processes from cancer metabolism to spontaneous farm fires. However, the molecular basis for this fundamental phenomenon is yet to be elucidated. Here, we show that the major players involved in the cellular thermogenesis of Escherichia coli are the protein kinases ArcB, GlnL, and YccC. We also reveal the substrate-level control of adenosine triphosphate (ATP)-driven autophosphorylation that governs cellular thermogenesis. Specifically, through live cell microcalorimetry, we find these regulatory proteins, when knocked out in a model E. coli strain, dysregulate cellular thermogenesis. This dysregulation can be seen in an average 25% or greater increase in heat output by these cells. We also discover that both heat output and intracellular ATP levels are maximal during the late log phase of growth. Additionally, we show that microbial thermogenesis can be engineered through overexpressing glnL. Our results demonstrate a correlation between ATP concentrations in the cell and a cell's ability to generate excess heat. We expect this work to be the foundation for engineering thermogenically tuned organisms for a variety of applications.

Photothermia at the nanoscale induces ferroptosis via nanoparticle degradation

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

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

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

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

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

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.

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

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

A palette of site-specific organelle fluorescent thermometers

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

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

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

Passive dual-probe near-field microscopy

Accurate and simultaneous multiposition near-field measurements are essential to study the time-dependent local dynamics, including heat and carrier transfer. The existing passive long-wavelength infrared (LWIR) scattering-type scanning near-field optical microscopy (s-SNOM) systems with a single probe cannot perform precise near-field measurements of the heat or carrier transporting process at the nanoscale level. Therefore, in this study, we developed a passive LWIR s-SNOM system with two probes. To test the effectiveness of the proposed passive LWIR dual-probe s-SNOM system, each probe was precisely controlled using a shear-force feedback system, and the mechanical interference between the probes was used to monitor the distance between the probes. We achieved simultaneous near-field measurements at two different positions 500 nm apart using the proposed passive LWIR dual-probe s-SNOM system. The simultaneously detected near-field signals from two different points were extracted individually, making this technique an effective nanoscale analysis tool for local carrier dynamics.

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.

A Ratiometric Organic Fluorescent Nanogel Thermometer for Highly Sensitive Temperature Sensing

Sensing temperature in biological systems is of great importance, as it is constructive to understanding various physiological and pathological processes. However, the realization of highly sensitive temperature sensing with organic fluorescent nanothermometers remains challenging. In this study, we report a ratiometric fluorescent nanogel thermometer and study its application in the determination of bactericidal temperature. The nanogel is composed of a polarity-sensitive aggregation-induced emission luminogen with dual emissions, a thermoresponsive polymer with a phase transition function, and an ionic surface with net positive charges. During temperature-induced phase transition, the nanogel exhibits a reversible and sensitive spectral change between a red-emissive state and a blue-emissive state by responding to the hydrophilic-to-hydrophobic change in the local environment. The correlation between the emission intensity ratio of the two states and the external temperature is delicately established, and the maximum relative thermal sensitivities of the optimal nanogel are determined to be 128.42 and 68.39% °C⁻¹ in water and a simulated physiological environment, respectively. The nanogel is further applied to indicate the bactericidal temperature in both visual and ratiometric ways, holding great promise in the rapid prediction of photothermal antibacterial effects and other temperature-related biological events.

Quantitative Comparison of the Light-to-Heat Conversion Efficiency in Nanomaterials Suitable for Photothermal Therapy

Functional colloidal nanoparticles capable of converting between various energy types are finding an increasing number of applications. One of the relevant examples concerns light-to-heat-converting colloidal nanoparticles that may be useful for localized photothermal therapy of cancers. Unfortunately, quantitative comparison and ranking of nanoheaters are not straightforward as materials of different compositions and structures have different photophysical and chemical properties and may interact differently with the biological environment. In terms of photophysical properties, the most relevant information to rank these nanoheaters is the light-to-heat conversion efficiency, which, along with information on the absorption capacity of the material, can be used to directly compare materials. In this work, we evaluate the light-to-heat conversion properties of 17 different nanoheaters belonging to different groups (plasmonic, semiconductor, lanthanide-doped nanocrystals, carbon nanocrystals, and metal oxides). We conclude that the light-to-heat conversion efficiency alone is not meaningful enough as many materials have similar conversion efficiencies─in the range of 80-99%─while they significantly differ in their extinction coefficient. We therefore constructed their qualitative ranking based on the external conversion efficiency, which takes into account the conventionally defined light-to-heat conversion efficiency and its absorption capacity. This ranking demonstrated the differences between the samples more meaningfully. Among the studied systems, the top-ranking materials were black porous silicon and CuS nanocrystals. These results allow us to select the most favorable materials for photo-based theranostics and set a new standard in the characterization of nanoheaters.

Photo- and Radiofrequency-Induced Heating of Photoluminescent Colloidal Carbon Dots

Nitrogen- and oxygen-containing carbon nanoparticles (O, N-CDs) were prepared by a facile one-step solvothermal method using urea and citric acid precursors. This method is cost-effective and easily scalable, and the resulting O, N-CDs can be used without additional functionalization and sample pretreatment. The structure of O, N-CDs was characterized by TEM, AFM, Raman, UV-vis, and FTIR spectroscopies. The obtained O, N-CDs with a mean diameter of 4.4 nm can be easily dispersed in aqueous solutions. The colloidal aqueous solutions of O, N-CDs show significant photothermal responses under red-IR and radiofrequency (RF) irradiations. The as-prepared O, N-CDs have a bright temperature-dependent photoluminescence (PL). PL/PLE spectral maps were shown to be used for temperature evaluation purposes in the range of 30-50 °C. In such a way, the O, N-CDs could be used for biomedicine-related applications such as hyperthermia with simultaneous temperature estimation with PL imaging.

Luminescent upconversion nanoparticles evaluating temperature-induced stress experienced by aquatic organisms owing to environmental variations

Growing anthropogenic activities are significantly influencing the environment and especially aquatic ecosystems. Therefore, there is an increasing demand to develop techniques for monitoring and assessing freshwater habitat changes so that interventions can prevent irrevocable damage. We explore an approach for screening the temperature-induced stress experienced by aquatic organisms owing to environmental variations. Luminescent spectra of upconversion [Y2O3: Yb, Er] particles embedded within Caridina multidentata shrimps are measured, while ambient temperature gradient is inducing stress conditions. The inverse linear dependence of the logarithmic ratio of the luminescence intensity provides an effective means for temperature evaluation inside aquatic species in vivo. The measured luminescence shows high photostability on the background of the complete absence of biotissues’ autofluorescence, as well as no obscuration of the luminescence signal from upconversion particles. Current approach of hybrid sensing has a great potential for monitoring variations in aquatic ecosystems driven by climate changes and pollution.

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Luminescence spectra induced by upconversion particles are embedded into aquatic animals

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Real-time quantitative assessment of temperature inside aquatic species in vivo

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Evaluation of stress handled by water organisms owing to environmental variations

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Hybrid sensing approach for monitoring environmental variations driven by climate change

Repetitive afterglow in zirconia by pulsed near-infrared irradiation toward biological temperature sensing

Photoluminescence provides information about the surrounding environment. In this study, aiming to develop a non-invasive deep body-temperature sensing method, we investigated photoluminescence properties of afterglow zirconia (ZrO2) by pulsed near-infrared (NIR) light irradiation based on the biological temperature. Pulsed light irradiation produced optically stimulated luminescence, followed by afterglow, with the property of repeating 100 times or more. Furthermore, the basic principle of temperature measurement was demonstrated through afterglow decay curve measurements. The use of harmless ZrO2 as a sensing probe and NIR light, which is relatively permeable to living tissues, is expected to realize temperature measurements in the brain and may also facilitate optogenetic treatment.

Microfluidics Temperature Compensating and Monitoring Based on Liquid Metal Heat Transfer

Microfluidic devices offer excellent heat transfer, enabling the biochemical reactions to be more efficient. However, the precision of temperature sensing and control of microfluids is limited by the size effect. Here in this work, the relationship between the microfluids and the glass substrate of a typical microfluidic device is investigated. With an intelligent structure design and liquid metal, we demonstrated that a millimeter-scale industrial temperature sensor could be utilized for temperature sensing of micro-scale fluids. We proposed a heat transfer model based on this design, where the local correlations between the macro-scale temperature sensor and the micro-scale fluids were investigated. As a demonstration, a set of temperature-sensitive nucleic acid amplification tests were taken to show the precision of temperature control for micro-scale reagents. Comparations of theoretical and experimental data further verify the effectiveness of our heat transfer model. With the presented compensation approach, the slight fluorescent intensity changes caused by isothermal amplification polymerase chain reaction (PCR) temperature could be distinguished. For instance, the probability distribution plots of fluorescent intensity are significant from each other, even if the amplification temperature has a difference of 1 °C. Thus, this method may serve as a universal approach for micro–macro interface sensing and is helpful beyond microfluidic applications.

Temperature estimation of a pair of trapped ions

We apply estimation theory to a system formed by two interacting trapped ions. By using the Fisher matrix formalism, we introduce a simple scheme for estimation of the temperature of the longitudinal vibrational modes of the ions. We use the ions interaction to effectively infer the temperature of the individual ions, by optimising the interaction time evolution and by measuring only over one of the ions. We also investigate the effect of a non-thermal reservoir over the inference approach. The non-classicality of one of the ions vibrational modes, introduced due to a squeezed thermal reservoir, does not directly affect the inference of the individual temperatures, although allowing the modes to be entangled. To check actual experimental conditions, we analyze the temperature inference under heating due to surface-electrode noise.

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.

Reliable and Remote Monitoring of Absolute Temperature during Liver Inflammation via Luminescence-Lifetime-Based Nanothermometry

Temperature of tissues and organs is one of the first parameters affected by physiological and pathological processes, such as metabolic activity, acute trauma, or infection-induced inflammation. Therefore, the onset and development of these processes can be detected by monitoring deviations from basal temperature. To accomplish this, minimally invasive, reliable, and accurate measurement of the absolute temperature of internal organs is required. Luminescence nanothermometry is the ideal technology for meeting these requirements. Although this technique has lately undergone remarkable developments, its reliability is being questioned due to spectral distortions caused by biological tissues. In this work, how the use of bright Ag₂ S nanoparticles featuring temperature-dependent fluorescence lifetime enables reliable and accurate measurement of the absolute temperature of the liver in mice subjected to lipopolysaccharide-induced inflammation is demonstrated. Beyond the remarkable thermal sensitivity (≈ 3% °C^-1 around 37 °C) and thermal resolution obtained (smaller than 0.3 °C), the results included in this work set a blueprint for the development of new diagnostic procedures based on the use of intracorporeal temperature as a physiological indicator.

Intracellular Thermal Probing Using Aggregated Fluorescent Nanodiamonds

Intracellular thermometry provides important information about the physiological activity of single cells and has been implemented using diverse temperature-sensitive materials as nanoprobes. However, measuring the temperature of specific organelles or subcellular structures is challenging because it requires precise positioning of the nanoprobes. Here, it is shown that dispersed fluorescent nanodiamonds (FNDs) endocytosed in living cells can be aggregated into microspheres using optical forces and used as intracellular temperature probes. The aggregation of the FNDs and electromagnetic resonance between individual nanodiamonds in the microspheres lead to a sevenfold intensity enhancement of 546-nm laser excitation. With the assistance of a scanning optical tweezing system, the FND microspheres can be precisely patterned and positioned within the cells. By measuring the fluorescence spectra of the microspheres, the temperatures at different locations within the cells are detected. The method provides an approach to the constructing and positioning of nanoprobes in an intracellular manner, which has potential applications in high-precision and flexible single-cell analysis.

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,...

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.

Targeted Nanoscale 3D Thermal Imaging of Tumor Cell Surface with Functionalized Quantum Dots

Measuring the changes in tumor cell surface temperature can provide insights into cellular metabolism and pathological features, which is significant for targeted chemotherapy and hyperthermic therapy. However, conventional micro-nano scale methods are invasive and can only measure the temperature of cells across a single plane, which excludes specific organelles. In this study, fluorescence quantum dots (QDs) are functionalized with the membrane transport protein transferrin (Tf) as a thermo-sensor specific for tumor cell membrane. The covalent conjugation is optimized to maintain the relative fluorescence intensity of the Tf-QDs to >90%. In addition, the Tf-QDs undergo changes in the fluorescence spectra as a function of temperature, underscoring its thermo-sensor function. Double helix point spread function imaging optical path is designed to locate the probe at nanoscale, and 3D thermal imaging technology is proposed to measure the local temperature distribution and direction of heat flux on the tumor cell surface. This novel targeted nanoscale 3D thermometry method can be a highly promising tool for measuring the local and global temperature distribution across intracellular organelles.

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.

Fluorescent Carbon Dots an Effective Nano-Thermometer in Vitro Applications

Fluorescent sensing of temperature in nanoscale regions has many advantages and applications in the biological field. Herein, blue emitting carbon dots (CDs) are designed and successfully developed using a one step hydrothermal method. As synthesized CDs exhibit temperature dependent photoluminescent (PL) intensity and PL decay lifetime over the physiological temperature ranging from room temperature (RT) to 70 °C. The PL intensity and PL decay lifetime of the obtained CDs correlate linearly to temperature (RT-70 °C) with correlation coefficient of 0.997 and 0.996, respectively. Additionally, dual mode thermal sensing (PL intensity/lifetime) make these CDs a promising optical nanothermometer over alternative semiconductors quantum dots and CD-based quantum dots. Moreover, the resultant aqueous CDs demonstrate excitation-independent blue emission, and the PL quantum yield (QY) is reached at 44.5%. The obtained CDs illustrate stable performance to high ionic environments and photobleaching even after keeping them for 2 h under continues UV irradiation. Furthermore, blue emitting CDs have low cytotoxicity for T-ca. cells and illuminate deep blue fluorescence under the excitation of 406 nm. As a result, high thermal sensitivity of these fluorescent CDs has potential to detect temperature in living cells in the range of 25-40 °C.

Cyclopropyl Substituents Transform the Viscosity-Sensitive BODIPY Molecular Rotor into a Temperature Sensor

A quantitative fluorescent probe that responds to changes in temperature is highly desirable for studies of biological environments, particularly in cellulo. Here, we report new cell-permeable fluorescence probes based on the BODIPY moiety that respond to environmental temperature. The new probes were developed on the basis of a well-established BODIPY-based viscosity probe by functionalization with cyclopropyl substituents at α and β positions of the BODIPY core. In contrast to the parent BODIPY fluorophore, α-cyclopropyl-substituted fluorophore displays temperature-dependent time-resolved fluorescence decays showing greatly diminished viscosity dependence, making it an attractive sensor to be used with fluorescence lifetime imaging microscopy (FLIM). We performed theoretical calculations that help rationalize the effect of the cyclopropyl substituents on the photophysical behavior of the new BODIPYs. In summary, we designed an attractive new quantitative FLIM-based temperature probe that can be used for temperature sensing in live cells.

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.

Nanodiamond Theranostic for Light-Controlled Intracellular Heating and Nanoscale Temperature Sensing

Temperature is an essential parameter in all biological systems, but information about the actual temperature in living cells is limited. Especially, in photothermal therapy, local intracellular temperature changes induce cell death but the local temperature gradients are not known. Highly sensitive nanothermometers would be required to measure and report local temperature changes independent of the intracellular environment, including pH or ions. Fluorescent nanodiamonds (ND) enable temperature sensing at the nanoscale independent of external conditions. Herein, we prepare ND nanothermometers coated with a nanogel shell and the photothermal agent indocyanine green serves as a heat generator and sensor. Upon irradiation, programmed cell death was induced in cancer cells with high spatial control. In parallel, the increase in local temperature was recorded by the ND nanothermometers. This approach represents a great step forward to record local temperature changes in different cellular environments inside cells and correlate these with thermal biology.

Two-photon GCaMP6f imaging of infrared neural stimulation evoked calcium signals in mouse cortical neurons in vivo

Infrared neural stimulation is a promising tool for stimulating the brain because it can be used to excite with high spatial precision without the need of delivering or inserting any exogenous agent into the tissue. Very few studies have explored its use in the brain, as most investigations have focused on sensory or motor nerve stimulation. Using intravital calcium imaging with the genetically encoded calcium indicator GCaMP6f, here we show that the application of infrared neural stimulation induces intracellular calcium signals in Layer 2/3 neurons in mouse cortex in vivo. The number of neurons exhibiting infrared-induced calcium response as well as the amplitude of those signals are shown to be both increasing with the energy density applied. By studying as well the spatial extent of the stimulation, we show that reproducibility of the stimulation is achieved mainly in the central part of the infrared beam path. Stimulating in vivo at such a degree of precision and without any exogenous chromophores enables multiple applications, from mapping the brain's connectome to applications in systems neuroscience and the development of new therapeutic tools for investigating the pathological brain.