CdTe quantum dots as nanothermometers: towards highly sensitive thermal imaging

High-sensitivity luminescent temperature sensors: MFX:1%Sm2+ (M = Sr, Ba, X = Cl, Br)

The use of lanthanide luminescence has advanced the field of remote temperature sensing. Luminescence intensity ratio methods relying on emission from two thermally coupled energy levels are popular but suffer from a limited temperature range. Here, we present a versatile luminescent thermometer: Ba(Sr)FBr(Cl):Sm2+. The Sm2+ ion benefits from multiple thermally coupled excited states to extend the temperature range and has strong parity-allowed 4f6→4f55d1 absorption to increase brightness. We conduct a comparative analysis of the temperature sensing performance of Sm2+ in BaFBr, BaFCl, SrFBr, and SrFCl and address the role of concentration, host, and Boltzmann equilibration. Different thermal coupling schemes, 5D1-5D0 and 4f55d1-5D0, and temperature-dependent lifetimes enable accurate sensing between 350 and 800 kelvin. Differences in 4f55d1-5D0 energy gap allows optimization for a temperature range of interest. This type of Sm2+-based thermometer holds great potential for temperature monitoring in the wide and relevant range up to 500°C.

Plasmon-emitter coupling in cytosine-rich hairpin DNA-templated silver nanoclusters: Thermal reversibility, white light emission, and dynamics inside live cells

Bio-templated luminescent noble metal nanoclusters (NCs) have attracted great attention for their intriguing physicochemical properties. Continuous efforts are being made to prepare NCs with high fluorescence quantum yield (QY), good biocompatibility, and tunable emission properties for their widespread practical applications as new-generation environment-friendly photoluminescent materials in materials chemistry and biological systems. Herein, we explored the unique photophysical properties of silver nanoclusters (AgNCs) templated by cytosine-rich customized hairpin DNA. Our results indicate that a 36-nucleotide containing hairpin DNA with 20 cytosine (C20) in the loop can encapsulate photostable red-emitting AgNCs with an absolute QY of ∼24%. The luminescent properties in these DNA-templated AgNCs were found to be linked to the coupling between the surface plasmon and the emitter. These AgNCs exhibited excellent thermal sensitivity and were employed to produce high-quality white light emission with an impressive color rendering index of 90 in the presence of dansyl chloride. In addition, the as-prepared luminescent AgNCs possessing excellent biocompatibility can effectively mark the nuclear region of HeLa cells and can be employed as a luminescent probe to monitor the cellular dynamics at a single molecular resolution.

Intrinsic dual emissive insulin capped Au/Ag nanoclusters as single ratiometric nanoprobe for reversible detection of pH and temperature and cell imaging

pH and temperature are two important characteristics in cells and the environment. These, not only in the well-done regulation of cell functions but also in diagnosis and treatment, have a key role. Protein-protected bimetallic nanoclusters are abundantly used in the building of biosensors. However, insulin-stabilized Au-Ag nanoclusters with dual intrinsic emission have not been investigated yet. In this work, Dual emissive insulin templated Au-Ag nanocluster (Ins(Au/Ag)NCs) were first synthesized in a simple and green one-put manner. The two emission wavelengths of, as-prepared NCs centered at 410 and 630 nm, excited in one excitation wavelength (330 nm). These two emission peaks were assigned to the di-Tyrosine cross-linked formation and bimetallic nanoclusters respectively. Further analysis displayed that each emission band of Ins(Au/Ag)NCs responded to one variable whilst another peak remained constant; For blue and red emission wavelengths, pH dependency and thermo-responsibility were observed respectively. As-prepared nanoprobe with the intrinsic dual emissive feature was used for ratiometric determination of these parameters, each with a discrete response from another. The linear range of 6.0-9.0 for pH and 1 to 71 °C for temperature was obtained, which comprises the physiological range of pH and temperature and afforded intracellular sensing and imaging capability. As-prepared NCs probe show excellent biocompatibility and cell membrane permeability, and so were successfully applied as direct ratiometric pH and temperature probes in HeLa and HFF cells. More interestingly, this dual emissive nanoprobe is capable of distinguishing cancer cells from normal ones.

Luminescence Thermometry Beyond the Biological Realm

As the field of luminescence thermometry has matured, practical applications of luminescence thermometry techniques have grown in both frequency and scope. Due to the biocompatibility of most luminescent thermometers, many of these applications fall within the realm of biology. However, luminescence thermometry is increasingly employed beyond the biological realm, with expanding applications in areas such as thermal characterization of microelectronics, catalysis, and plasmonics. Here, we review the motivations, methodologies, and advances linked to nonbiological applications of luminescence thermometry. We begin with a brief overview of luminescence thermometry probes and techniques, focusing on those most commonly used for nonbiological applications. We then address measurement capabilities that are particularly relevant for these applications and provide a detailed survey of results across various application categories. Throughout the review, we highlight measurement challenges and requirements that are distinct from those of biological applications. Finally, we discuss emerging areas and future directions that present opportunities for continued research.

Nanothermometry-Enabled Intelligent Laser Tissue Soldering

While often life-saving, surgical resectioning of diseased tissues puts patients at risk for post-operative complications. Sutures and staples are well-accepted and routinely used to reconnect tissues, however, their mechanical mismatch with biological soft tissue and invasiveness contribute to wound healing complications, infections, and post-operative fluid leakage. In principle, laser tissue soldering offers an attractive, minimally-invasive alternative for seamless soft tissue fusion. However, despite encouraging experimental observations, including accelerated healing and lowered infection risk, critical issues related to temperature monitoring and control during soldering and associated complications have prevented their clinical exploitation to date. Here, intelligent laser tissue soldering (iSoldering) with integrated nanothermometry is introduced as a promising yet unexplored approach to overcome the critical shortcomings of laser tissue soldering. It demonstrates that adding thermoplasmonic and nanothermometry nanoparticles to proteinaceous solders enables heat confinement and non-invasive temperature monitoring and control, offering a route to high-performance, leak-tight tissue sealing even at deep tissue sites. The resulting tissue seals exhibit excellent mechanical properties and resistance to chemically-aggressive digestive fluids, including gastrointestinal juice. The iSolder can be readily cut and shaped by surgeons to optimally fit the tissue defect and can even be applied using infrared light from a medically approved light source, hence fulfilling key prerequisites for application in the operating theatre. Overall, iSoldering enables reproducible and well-controlled high-performance tissue sealing, offering new prospects for its clinical exploitation in diverse fields ranging from cardiovascular to visceral and plastic surgery.

Nanoscale thermal control of a single living cell enabled by diamond heater-thermometer

We report a new approach to controllable thermal stimulation of a single living cell and its compartments. The technique is based on the use of a single polycrystalline diamond particle containing silicon-vacancy (SiV) color centers. Due to the presence of amorphous carbon at its intercrystalline boundaries, such a particle is an efficient light absorber and becomes a local heat source when illuminated by a laser. Furthermore, the temperature of such a local heater is tracked by the spectral shift of the zero-phonon line of SiV centers. Thus, the diamond particle acts simultaneously as a heater and a thermometer. In the current work, we demonstrate the ability of such a Diamond Heater-Thermometer (DHT) to locally alter the temperature, one of the numerous parameters that play a decisive role for the living organisms at the nanoscale. In particular, we show that the local heating of 11-12 °C relative to the ambient temperature (22 °C) next to individual HeLa cells and neurons, isolated from the mouse hippocampus, leads to a change in the intracellular distribution of the concentration of free calcium ions. For individual HeLa cells, a long-term (about 30 s) increase in the integral intensity of Fluo-4 NW fluorescence by about three times is observed, which characterizes an increase in the [Ca²⁺]_cyt concentration of free calcium in the cytoplasm. Heating near mouse hippocampal neurons also caused a calcium surge-an increase in the intensity of Fluo-4 NW fluorescence by 30% and a duration of ~ 0.4 ms.

Real-Time Ratiometric Optical Nanoscale Thermometry

All-optical nanothermometry has become a powerful, remote tool for measuring nanoscale temperatures in applications ranging from medicine to nano-optics and solid-state nanodevices. The key features of any candidate nanothermometer are brightness, sensitivity, and (signal, spatial, and temporal) resolution. Here, we demonstrate a real-time, diamond-based nanothermometry technique with excellent sensitivity (1.8% K^-1) and record-high resolution (5.8 × 10⁴ K Hz^-1/2 W cm^-2) based on codoped nanodiamonds. The distinct performance of our approach stems from two factors: (i) temperature sensors─nanodiamonds cohosting two group IV color centers─engineered to emit spectrally separated Stokes and anti-Stokes fluorescence signals under excitation by a single laser source and (ii) a parallel detection scheme based on filtering optics and high-sensitivity photon counters for fast readout. We demonstrate the performance of our method by monitoring temporal changes in the local temperature of a microcircuit and a MoTe₂ field-effect transistor. Our work advances a powerful, alternative strategy for time-resolved temperature monitoring and mapping of micro-/nanoscale devices such as microfluidic channels, nanophotonic circuits, and nanoelectronic devices, as well as complex biological environments such as tissues and cells.

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.

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

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

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

Abnormal temperature-dependent photoluminescence characteristics of ReS2nanowalls

Two samples with [001] orientated rhenium disulfide (ReS₂) nanowalls (NWs) grown above and in front of precursor (NH₄ReO₄) by chemical vapor deposition were investigated. The temperature-dependent photoluminescence (PL) indicated that the PL peak exhibited linear blue-shift at a rate of ∼0.24 meV K^-1with increasing the temperature from 10 to 300 K, while the linewidth monotonically increased due to the exciton-phonon interaction. This abnormal blue-shift of PL emission energy, which is explained by a competition between the band gap shrinkage and the energy level degeneracy with respect to the increase of temperature and lattice constant, enables ReS₂NWs to possess great potential for development of thermal sensors. In addition, exciton localization effect in the ReS₂NWs from abundant edges and weak interlayer interaction was also observed to be related to the height and density of ReS₂NWs. These results not only enrich the understanding for exciton dynamics in ReS₂NWs, but also help to exploit ReS₂NWs for device applications.

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

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

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.

Advances and challenges for fluorescence nanothermometry

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

Carbon Dots as New Generation Materials for Nanothermometer: Review

Highly sensitive non-contact mode temperature sensing is substantial for studying fundamental chemical reactions, biological processes, and applications in medical diagnostics. Nanoscale-based thermometers are guaranteeing non-invasive probes for sensitive and precise temperature sensing with subcellular resolution. Fluorescence-based temperature sensors have shown great capacity since they operate as "non-contact" mode and offer the dual functions of cellular imaging and sensing the temperature at the molecular level. Advancements in nanomaterials and nanotechnology have led to the development of novel sensors, such as nanothermometers (novel temperature-sensing materials with a high spatial resolution at the nanoscale). Such nanothermometers have been developed using different platforms such as fluorescent proteins, organic compounds, metal nanoparticles, rare-earth-doped nanoparticles, and semiconductor quantum dots. Carbon dots (CDs) have attracted interest in many research fields because of outstanding properties such as strong fluorescence, photobleaching resistance, chemical stability, low-cost precursors, low toxicity, and biocompatibility. Recent reports showed the thermal-sensing behavior of some CDs that make them an alternative to other nanomaterials-based thermometers. This kind of luminescent-based thermometer is promising for nanocavity temperature sensing and thermal mapping to grasp a better understanding of biological processes. With CDs still in its early stages as nanoscale-based material for thermal sensing, in this review, we provide a comprehensive understanding of this novel nanothermometer, methods of functionalization to enhance thermal sensitivity and resolution, and mechanism of the thermal sensing behavior.

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.

Tailoring the Heterostructure of Colloidal Quantum Dots for Ratiometric Optical Nanothermometry

Colloidal quantum dots (QDs) are a fascinating class of semiconducting nanocrystals, thanks to their optical properties tunable through size and composition, and simple synthesis methods. Recently, colloidal double-emission QDs have been successfully applied as competitive optical temperature sensors, since they exhibit structure-tunable double emission, temperature-dependent photoluminescence, high quantum yield, and excellent photostability. Until now, QDs have been used as nanothermometers for in vivo biological thermal imaging, and thermal mapping in complex environments at the sub-microscale to nanoscale range. In this Review, recent progress for QD-based nanothermometers is highlighted and perspectives for future work are described.

Encapsulation of Dual Emitting Giant Quantum Dots in Silica Nanoparticles for Optical Ratiometric Temperature Nanosensors

Accurate temperature measurements with a high spatial resolution for application in the biomedical fields demand novel nanosized thermometers with new advanced properties. Here, a water dispersible ratiometric temperature sensor is fabricated by encapsulating in silica nanoparticles, organic capped PbS@CdS@CdS “giant” quantum dots (GQDs), characterized by dual emission in the visible and near infrared spectral range, already assessed as efficient fluorescent nanothermometers. The chemical stability, easy surface functionalization, limited toxicity and transparency of the silica coating represent advantageous features for the realization of a nanoscale heterostructure suitable for temperature sensing. However, the strong dependence of the optical properties on the morphology of the final core–shell nanoparticle requires an accurate control of the encapsulation process. We carried out a systematic investigation of the synthetic conditions to achieve, by the microemulsion method, uniform and single core silica coated GQD (GQD@SiO2) nanoparticles and subsequently recorded temperature-dependent fluorescent spectra in the 281-313 K temperature range, suited for biological systems. The ratiometric response—the ratio between the two integrated PbS and CdS emission bands—is found to monotonically decrease with the temperature, showing a sensitivity comparable to bare GQDs, and thus confirming the effectiveness of the functionalization strategy and the potential of GQD@SiO2 in future biomedical applications.

A simple yet effective AIE-based fluorescent nano-thermometer for temperature mapping in living cells using fluorescence lifetime imaging microscopy

We designed and synthesized a novel nano-thermometer using aggregation-induced-emission (AIE) dye as the reporter and household butter as the matrix. This temperature nanosensor showed decreased fluorescence intensities (∼2%/°C) and shorter fluorescence lifetimes (∼0.11 ns/°C) upon increasing the environmental temperature in the physiological temperature range. Such fluorescence responses were reversible and independent of the environmental pH and ionic strength. The application of these nano-thermometers in temperature sensing in living cells using fluorescence lifetime imaging microscopy (FLIM) was also demonstrated. To the best of our knowledge, this is the first example of AIE-based nano-thermometer for temperature sensing in living cells. This work also provides us with a simple and low-cost method for rapid fabrication of an effective nanosensor based on AIE mechanism.

GFP fluorescence peak fraction analysis based nanothermometer for the assessment of exothermal mitochondria activity in live cells

Nanothermometry methods with intracellular sensitivities have the potential to make important contributions to fundamental cell biology and medical fields, as temperature is a relevant physical parameter for molecular reactions to occur inside the cells and changes of local temperature are well identified therapeutic strategies. Here we show how the GFP can be used to assess temperature-based on a novel fluorescence peak fraction method. Further, we use standard GFP transfection reagents to assess temperature intracellularly in HeLa cells expressing GFP in the mitochondria. High thermal resolution and sensitivity of around 0.26% °C-1 and 2.5% °C-1, were achieved for wt-GFP in solution and emGFP-Mito within the cell, respectively. We demonstrate that the GFP-based nanothermometer is suited to directly follow the temperature changes induced by a chemical uncoupler reagent that acts on the mitochondria. The spatial resolution allows distinguishing local heating variations within the different cellular compartments. Our discovery may lead to establishing intracellular nanothermometry as a standard method applicable to the wide range of live cells able to express GFP.

Green synthesis of up- and down-conversion photoluminescent carbon dots from coffee beans for Fe3+ detection and cell imaging

CDs with up- and down-conversion photoluminescence have been synthesized by one-step hydrothermal and used for bioimaging and intracellular Fe³⁺ detection.

Accurate intracellular and in vivo temperature sensing based on CuInS2/ZnS QD micelles

We found that core–shell CuInS₂/ZnS QDs have obvious temperature dependence and they can be used for accurate intracellular and in vivo temperature sensing after being encapsulated by micelles, which exhibit high intracellular and in vivo thermal sensitivity.

Optical differential temperature measurement with beat frequency phase fluorometry

We present, to the best of our knowledge, a new method for differential temperature measurement based on thermal sensitivity of the fluorescence lifetime of thermographic phosphors. Pairs of thermographic phosphors are excited with intensity-modulated light at frequencies ω and ω+Δω. The phase shift Δθ of the summary fluorescence intensity beat signal envelope is measured. A prototype of a fluorometric differential temperature sensor is developed, and feasibility of the method is experimentally demonstrated with a Sm²⁺:SrFCl crystal and the D₁5->F7₀ transition for high thermal sensitivity. The observed linear dependence between envelope phase shift Δθ and temperature difference ΔT agrees with the theoretical prediction. Sensitivity of S=-0.97°/°C was achieved. This method could also be applied to differential measurements of any parameter affecting fluorescence lifetime.

Advances in the integration of quantum dots with various nanomaterials for biomedical and environmental applications

Quantum dots (QDs) are semiconductor nanocrystals with distinct characteristics of high brightness, large Stokes shift and broad absorption spectra, large molar extinction coefficients, high quantum yield, good photostability and long fluorescence lifetime. The QDs have replaced the conventional fluorophores with wide applications in immunoassays, microarrays, fluorescence imaging, targeted drug delivery and therapy. The integration of QDs with various nanomaterials such as noble metal nanoparticles, carbon allotropes, upconversion nanoparticles (UCNPs), metal oxides and metal-organic frameworks (MOFs) brings new opportunities and possibilities in nanoscience and nanotechnology. In this review, we summarize the recent advances in the integration of QDs with various nanomaterials for biomedical and environmental applications including sensing, bioimaging, theranostics and cancer therapy. We highlight the involved interactions such as fluorescence resonance energy transfer (FRET), plasmon enhanced fluorescence (PEF), and nanometal surface energy transfer (NSET) as well as the synergistic effect resulting from the integration of QDs with nanomaterials. In addition, we discuss the sensing and imaging mechanisms of different strategies and give new insight into the challenges and future direction as well.

Tuning the sensitivity of lanthanide-activated NIR nanothermometers in the biological windows

Lanthanide-activated SrF₂ nanoparticles with a multishell architecture were investigated as optical thermometers in the biological windows. A ratiometric approach based on the relative changes in the intensities of different lanthanide (Nd³⁺ and Yb³⁺) NIR emissions was applied to investigate the thermometric properties of the nanoparticles. It was found that an appropriate doping with Er³⁺ ions can increase the thermometric properties of the Nd³⁺-Yb³⁺ coupled systems. In addition, a core containing Yb³⁺ and Tm³⁺ can generate light in the visible and UV regions upon near-infrared (NIR) laser excitation at 980 nm. The multishell structure combined with the rational choice of dopants proves to be particularly important to control and enhance the performance of nanoparticles as NIR nanothermometers.

Quantum dots to probe temperature and pressure in highly confined liquids

A new in situ technique for temperature and pressure measurement within dynamic thin-film flows of liquids is presented. The technique is based on the fluorescence emission sensitivity of CdSe/CdS/ZnS quantum dots to temperature and pressure variations. In this respect, the quantum dots were dispersed in squalane, and their emission energy dependence on temperature and pressure was calibrated under static conditions. Temperature calibration was established between 295 K and 393 K showing a temperature sensitivity of 0.32 meV K⁻¹. Pressure calibration was, in turn, conducted up to 1.1 GPa using a diamond anvil cell, yielding a pressure sensitivity of 33.2 meV GPa⁻¹. The potential of CdSe/CdS/ZnS quantum dots as sensors to probe temperature and pressure was proven by applying the in situ technique to thin films of liquids undergoing dynamic conditions. Namely, temperature rises have been measured in liquid films subjected to shear heating between two parallel plates in an optical rheometer. In addition, pressure rises have been measured in a lubricated point contact under pure rolling and isothermal conditions. In both cases, the measured values have been successfully compared with theoretical or numerical predictions. These comparisons allowed the validation of the new in situ technique and demonstrated the potential of the quantum dots for further mapping application in more complex and/or severe conditions.

A new in situ technique using CdSe/CdS/ZnS quantum dots fluorescence to probe pressure and temperature within highly confined flows of liquids.

Luminescence properties of ZnGa2O4:Cr3+,Bi3+ nanophosphors for thermometry applications

Luminescence properties of chromium(iii) and bismuth(iii) co-doped ZnGa₂O₄ nanoparticles are investigated for thermometry applications.

YVO4:Nd3+ nanophosphors as NIR-to-NIR thermal sensors in wide temperature range

We report on the potential application of NIR-to-NIR Nd3+-doped yttrium vanadate nanoparticles with both emission and excitation operating within biological windows as thermal sensors in 123-873 K temperature range. It was demonstrated that thermal sensing could be based on three temperature dependent luminescence parameters: the luminescence intensity ratio, the spectral line position and the line bandwidth. Advantages and limitations of each sensing parameter as well as thermal sensitivity and thermal uncertainty were calculated and discussed. The influence of Nd3+ doping concentration on the sensitivity of luminescent thermometers was also studied.

Highly efficient upconversion of Er3+ in Yb3+ codoped non-cytotoxic strontium lanthanum aluminate phosphor for low temperature sensors

Er³⁺ and Er³⁺/Yb³⁺ melilite-based SrLaAl₃O₇ (SLA) phosphors were synthesized by a facile Pechine method. The differences in emission intensities of ⁴I_13/2 → ⁴I_15/2 transition in NIR region when excited with Ar⁺ and 980 nm lasers were explained in terms of energy transfer mechanisms. Temperature and power dependence of upconversion bands in the visible region centered at 528, 548 and 660 nm pertaining to ²H_11/2, ⁴S_3/2 and ⁴F_9/2 → ⁴I_15/2 transitions were investigated. Fluorescence intensity ratio (FIR) technique was used to explore temperature sensing behaviour of the thermally coupled levels ²H_11/2/⁴S_3/2 of Er³⁺ ions in the phosphors within the temperature range 14–300 K and the results were extrapolated up to 600 K. Anomalous intensity trend observed in Er³⁺ doped SLA phosphor was discussed using energy level structure. Cytotoxicity of phosphors has been evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in Bluegill sunfish cells (BF-2). The non-cytotoxic nature and high sensitivity of the present phosphors pay a way for their use in vitro studies and provide potential interest as a thermo graphic phosphor at the contact of biological products.

Ratiometric near-infrared fluorescence nanothermometry in the OTN-NIR (NIR II/III) biological window based on rare-earth doped β-NaYF4 nanoparticles

A novel nanothermometer based on over-1000 nm (OTN) near-infrared (NIR) emission of rare-earth doped ceramic nanophosphors (RED-CNPs) was developed for temperature measurement in deep tissue. Hexagonal-phase β-NaYF₄ nanoparticles co-doped with Yb³⁺, Ho³⁺, and Er³⁺ (NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs) were synthesized and used as a nanothermometer. The NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs displayed two OTN-NIR emission peaks in the second (NIR-II) (at 1150 nm of Ho³⁺) and third (NIR-III) (at 1550 nm of Er³⁺) biological window regions under NIR (980 nm) excitation in the first (NIR-I) biological window region. Oleic acid (OA) capped NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs were dispersed in non-polar media, i.e., cyclohexane, and showed a temperature-dependent intensity ratio of the emission peaks of Ho³⁺ and Er³⁺ (I_Ho/I_Er). The temperature-dependent I_Ho/I_Er of the OA-NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs was also evident through imitation tissue. The surfaces of the NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs were modified with a poly(ethylene glycol) (PEG)-based block copolymer. The PEGylated NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs were dispersed in water and emitted strong NIR-II and III emissions under NIR-I excitation. The PEGylated NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs were injected into mice via the tail vein, and the OTN-NIR emissions of the PEGylated NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs from the mouse blood vessels were clearly observed using an OTN-NIR fluorescence in vivo imaging system. In a polar media, water, the I_Ho/I_Er of PEGylated NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs was inversely related to the temperature. In both non-polar and polar media, the I_Ho/I_Er values of the NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs were almost linearly dependent on the temperature. The obtained NaYF₄:Yb³⁺,Ho³⁺,Er³⁺ NPs are promising as a novel fluorescent nanothermometer for deep tissue.

Nanothermometry Measure of Muscle Efficiency

Despite recent advances in thermometry, determination of temperature at the nanometer scale in single molecules to live cells remains a challenge that holds great promise in disease detection among others. In the present study, we use a new approach to nanometer scale thermometry with a spatial and thermal resolution of 80 nm and 1 mK respectively, by directly associating 2 nm cadmium telluride quantum dots (CdTe QDs) to the subject under study. The 2 nm CdTe QDs physically adhered to bovine cardiac and rabbit skeletal muscle myosin, enabling the determination of heat released when ATP is hydrolyzed by both myosin motors. Greater heat loss reflects less work performed by the motor, hence decreased efficiency. Surprisingly, we found rabbit skeletal myosin to be more efficient than bovine cardiac. We have further extended this approach to demonstrate the gain in efficiency of Drosophila melanogaster skeletal muscle overexpressing the PGC-1α homologue spargel, a known mediator of improved exercise performance in humans. Our results establish a novel approach to determine muscle efficiency with promise for early diagnosis and treatment of various metabolic disorders including cancer.

A portable luminescent thermometer based on green up-conversion emission of Er3+/Yb3+ co-doped tellurite glass

The determination of temperature is essential in many applications in the biomedical, technological, and industrial fields. Optical thermometry appears to be an excellent alternative for conventional electric temperature sensors because it is a non-contact method that offers a fast response, electromagnetic passivity, and high temperature sensitivity. In this paper, we propose an optical thermometer probe comprising an Er³⁺/Yb³⁺ co-doped tellurite glass attached to the tip of an optical fibre and optically coupled to a laser source and a portable USB spectrometer. The ratio of the up-conversion green emission integrated peak areas when excited at 980 nm was temperature dependent, and it was used to calibrate the thermometer. The thermometer was operated in the range of 5-50 °C and 50-200 °C, and it revealed excellent linearity (r² > 0.99), suitable accuracy, and precisions of ±0.5 and ±1.1 °C, respectively. By optimizing Er³⁺ concentration, we could obtain the high green emission intensity, and in turn, high thermal sensitivity for the probe. The probe fabricated in the study exhibited suitable properties for its application as a temperature sensor and superior performance compared to other Er³⁺ -based optical thermometers in terms of thermal sensitivity.

Ultrahigh-sensitive optical temperature sensing based on quasi-thermalized green emissions from Er:ZnO

Green emitting Er:ZnO microrods for ultra-high sensitive optical ratiometric temperature sensing, by following the fluorescence intensity ratio technique.

Optical trapping for biosensing: materials and applications

Optical trapping has been evidence as a very powerful tool for the manipulation and study of biological entities. This review explains the main concepts regarding the use of optical trapping for biosensing, focusing its attention to those applications involving the manipulation of particles which are used as handles, force transducers and sensors.

Thermal sensing with CdTe/CdS/ZnS quantum dots in human umbilical vein endothelial cells

An experimental methodology is presented to measure the temperature variation in cells with the usage of CdTe/CdS/ZnS core/shell/shell quantum dots as nanothermometers.

Performance of Nano-Submicron-Stripe Pd Thin-Film Temperature Sensors

Dozens of small dual-beam thin-film temperature sensors with a total width down to 430 nm were fabricated and tested. The sensors were all made from 90-nm-thick Pd thin films, where the width of the narrow stripes was 70-100 nm and that of the wide ones was 210-800 nm. Two different calibration methods showed consistent and repeatable sensitivities of 0.7-1.2 μV/K for the sensors, confirming that the sensitivity mainly depended on the width configuration of each sensor. By integrating arrays of such sensors on a practical testing platform using hybrid e-beam lithography and photolithography techniques, we demonstrated that these sensors were capable of detecting a weak surface temperature difference of 0.1-0.2 K at microscale, and they could be scaled up as built-in temperature sensors in many practical devices.

Brain heating induced by near-infrared lasers during multiphoton microscopy

Two-photon imaging and optogenetic stimulation rely on high illumination powers, particularly for state-of-the-art applications that target deeper structures, achieve faster measurements, or probe larger brain areas. However, little information is available on heating and resulting damage induced by high-power illumination in the brain. In the current study we used thermocouple probes and quantum dot nanothermometers to measure temperature changes induced by two-photon microscopy in the neocortex of awake and anaesthetized mice. We characterized heating as a function of wavelength, exposure time, and distance from the center of illumination. Although total power is highest near the surface of the brain, heating was most severe hundreds of micrometers below the focal plane, due to heat dissipation through the cranial window. Continuous illumination of a 1-mm(2) area produced a peak temperature increase of ∼1.8°C/100 mW. Continuous illumination with powers above 250 mW induced lasting damage, detected with immunohistochemistry against Iba1, glial fibrillary acidic protein, heat shock proteins, and activated caspase-3. Higher powers were usable in experiments with limited duty ratios, suggesting an approach to mitigate damage in high-power microscopy experiments.

Detection of Temperature Difference in Neuronal Cells

For a better understanding of the mechanisms behind cellular functions, quantification of the heterogeneity in an organism or cells is essential. Recently, the importance of quantifying temperature has been highlighted, as it correlates with biochemical reaction rates. Several methods for detecting intracellular temperature have recently been established. Here we develop a novel method for sensing temperature in living cells based on the imaging technique of fluorescence of quantum dots. We apply the method to quantify the temperature difference in a human derived neuronal cell line, SH-SY5Y. Our results show that temperatures in the cell body and neurites are different and thus suggest that inhomogeneous heat production and dissipation happen in a cell. We estimate that heterogeneous heat dissipation results from the characteristic shape of neuronal cells, which consist of several compartments formed with different surface-volume ratios. Inhomogeneous heat production is attributable to the localization of specific organelles as the heat source.

Submicrometer-Sized Thermometer Particles Exploiting Selective Nucleic Acid Stability

Encapsulated nucleic acid selective damage quantification by real-time polymerase chain reaction is used as sensing mechanism to build a novel class of submicrometer size thermometer. Thanks to the high thermal and chemical stability, and the capability of storing the read accumulated thermal history, the sensor overcomes some of current limitations in small scale thermometry.

Temperature-Dependent Exciton and Trap-Related Photoluminescence of CdTe Quantum Dots Embedded in a NaCl Matrix: Implication in Thermometry

Temperature-dependent optical studies of semiconductor quantum dots (QDs) are fundamentally important for a variety of sensing and imaging applications. The steady-state and time-resolved photoluminescence properties of CdTe QDs in the size range from 2.3 to 3.1 nm embedded into a protective matrix of NaCl are studied as a function of temperature from 80 to 360 K. The temperature coefficient is found to be strongly dependent on QD size, with the highest sensitivity obtained for the smallest size of QDs. The emission from solid-state CdTe QD-based powders is maintained with high color purity over a wide range of temperatures. Photoluminescence lifetime data suggest that temperature dependence of the intrinsic radiative lifetime in CdTe QDs is rather weak, and it is mostly the temperature-dependent nonradiative decay of CdTe QDs which is responsible for the thermal quenching of photoluminescence intensity. By virtue of the temperature-dependent photoluminescence behavior, high color purity, photostability, and high photoluminescence quantum yield (26%-37% in the solid state), CdTe QDs embedded in NaCl matrices are useful solid-state probes for thermal imaging and sensing over a wide range of temperatures within a number of detection schemes and outstanding sensitivity, such as luminescence thermochromic imaging, ratiometric luminescence, and luminescence lifetime thermal sensing.

Temperature-Dependent Photoluminescence of g-C3N4: Implication for Temperature Sensing

We report the temperature-dependent photoluminescence (PL) properties of polymeric graphite-like carbon nitride (g-C3N4) and a methodology for the determination of quantum efficiency along with the activation energy. The PL is shown to originate from three different pathways of transitions: σ*-LP, π*-LP, and π*-π, respectively. The overall activation energy is found to be ∼73.58 meV which is much lower than the exciton binding energy reported theoretically but ideal for highly sensitive wide-range temperature sensing. The quantum yield derived from the PL data is 23.3%, whereas the absolute quantum yield is 5.3%. We propose that the temperature-dependent PL can be exploited for the evaluation of the temperature dependency of quantum yield as well as for temperature sensing. Our analysis further indicates that g-C3N4 is well-suited for wide-range temperature sensing.

Hybrid nanostructures for high-sensitivity luminescence nanothermometry in the second biological window

Hybrid nanostructures containing neodymium-doped nanoparticles and infrared-emitting quantum dots constitute highly sensitive luminescent thermometers operating in the second biological window. They demonstrate that accurate subtissue fluorescence thermal sensing is possible.

Multicolor fluorescent graphene quantum dots colorimetrically responsive to all-pH and a wide temperature range

Smart functional nanomaterials colorimetrically responsive to all-pH and a wide temperature range are urgently needed due to their widespread applications in biotechnology, drug delivery, diagnosis and optical sensing. Although graphene quantum dots possess remarkable advantages in biological applications, they are only stable in neutral or weak acidic solutions, and strong acidic or alkaline conditions invariably suppress or diminish the fluorescence intensity. Herein, we report a new type of water-soluble, multicolor fluorescent graphene quantum dot which is responsive to all-pH from 1 to 14 with the naked eye. The synthesis was accomplished by electrolysis of the graphite rod, followed by refluxing in a concentrated nitric and sulfuric acid mixed solution. We demonstrate the novel red fluorescence of quinone structures transformed from the lactone structures under strong alkaline conditions. The fluorescence of the resulting graphene quantum dots was also found to be responsive to the temperature changes, demonstrating their great potential as a dual probe of pH and temperature in complicated environments such as biological media.

Quantum dots: bright and versatile in vitro and in vivo fluorescence imaging biosensors

Colourful cells and tissues: semiconductor quantum dots and their versatile applications in multiplexed bioimaging research.

Quantifying the photothermal efficiency of gold nanoparticles using tryptophan as an in situ fluorescent thermometer

The photothermal efficiencies, denoting the efficiency of transducing incident light to heat, of gold nanoparticles of different diameters (∅ = 22–86 nm) were quantified upon exposure at 532 nm.