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.

Scaling-up of carbon dots hydrothermal synthesis from sugars in a continuous flow microreactor system for biomedical application as in vitro antimicrobial drug nanocarrier

Carbon dots (CDs) are a new class of nanomaterials exhibiting high biocompatibility, water solubility, functionality, and tunable fluorescence (FL) property. Due to the limitations of batch hydrothermal synthesis in terms of low CDs yield and long synthesis duration, this work aimed to increase its production capacity through a continuous flow reactor system. The influence of temperature and time was first studied in a batch reactor for glucose, xylose, sucrose and table sugar precursors. CDs synthesized from sucrose precursor exhibited the highest quantum yield (QY) (175.48%) and the average diameter less than 10 nm (~6.8 ± 1.1 nm) when synthesized at 220°C for 9 h. For a flow reactor system, the best condition for CDs production from sucrose was 1 mL min-1 flow rate at 280°C, and 0.2 MPa pressure yielding 53.03% QY and ~ 6.5 ± 0.6 nm average diameter (6.6 mg min-1 of CDs productivity). CDs were successfully used as ciprofloxacin (CP) nanocarrier for antimicrobial activity study. The cytotoxicity study showed that no effect of CDs on viability of L-929 fibroblast cells was detected until 1000 µg mL-1 CDs concentration. This finding demonstrates that CDs synthesized via a flow reactor system have a high zeta potential and suitable surface properties for nano-theranostic applications.

Towards the Use of Individual Fluorescent Nanoparticles as Ratiometric Sensors: Spectral Robustness of Ultrabright Nanoporous Silica Nanoparticles

Here we address an important roadblock that prevents the use of bright fluorescent nanoparticles as individual ratiometric sensors: the possible variation of fluorescence spectra between individual nanoparticles. Ratiometric measurements using florescent dyes have shown their utility in measuring the spatial distribution of temperature, acidity, and concentration of various ions. However, the dyes have a serious limitation in their use as sensors; namely, their fluorescent spectra can change due to interactions with the surrounding dye. Encapsulation of the d, e in a porous material can solve this issue. Recently, we demonstrated the use of ultrabright nanoporous silica nanoparticles (UNSNP) to measure temperature and acidity. The particles have at least two kinds of encapsulated dyes. Ultrahigh brightness of the particles allows measuring of the signal of interest at the single particle level. However, it raises the problem of spectral variation between particles, which is impossible to control at the nanoscale. Here, we study spectral variations between the UNSNP which have two different encapsulated dyes: rhodamine R6G and RB. The dyes can be used to measure temperature. We synthesized these particles using three different ratios of the dyes. We measured the spectra of individual nanoparticles and compared them with simulations. We observed a rather small variation of fluorescence spectra between individual UNSNP, and the spectra were in very good agreement with the results of our simulations. Thus, one can conclude that individual UNSNP can be used as effective ratiometric sensors.

Antibacterial Carbon Dots-Based Composites

The emergence and global spread of bacterial resistance to conventionally used antibiotics have highlighted the urgent need for new antimicrobial agents that might replace antibiotics. Currently, nanomaterials hold considerable promise as antimicrobial agents in anti-inflammatory therapy. Due to their distinctive functional physicochemical characteristics and exceptional biocompatibility, carbon dots (CDs)-based composites have attracted a lot of attention in the context of these antimicrobial nanomaterials. Here, a thorough assessment of current developments in the field of antimicrobial CDs-based composites is provided, starting with a brief explanation of the general synthesis procedures, categorization, and physicochemical characteristics of CDs-based composites. The many processes driving the antibacterial action of these composites are then thoroughly described, including physical destruction, oxidative stress, and the incorporation of antimicrobial agents. Finally, the obstacles that CDs-based composites now suffer in combating infectious diseases are outlined and investigated, along with the potential applications of antimicrobial CDs-based composites.

Highly efficient red-emitting carbon dots as a "turn-on" temperature probe in living cells

Nanothermometers, which can precisely detect the intracellular temperature changes, have great potential to solve questions concerning the cellular processes. Thus, the temperature sensors that provide fluorescent "turn-on" signals in the biological transparency window are of highly desirable. To meet these criteria, this work reported a new "turn-on" carbon dot (CD)-based fluorescent nanothermometry device for sensing temperature in living cells. The CDs that emit bright red fluorescence (R-CDs; λ_max = 610 nm in water) were synthesized with o-phenylenediamine as carbon precursor via a facile solvothermal method. The R-CDs in water were almost nonfluorescent at 15 °C. As the temperature increased, the fluorescence intensity of R-CDs exhibited a gradual increase and the final enhancement factor was greater than 21-fold. The fluorescence intensity exhibited a linear response to temperature and a high-sensitive variation of ≈13.3 % °C^-1 was detected within a broad temperature range of 28-60 °C. Moreover, the R-CD thermal sensors also exhibited high storage stability, excellent response reversibility and superior photo- and thermo-stability. Due to its good biocompatibility and "intelligent" response to external temperature, the nanothermometer could be applied for sensing temperature changes in biological media.

The Role of N and S Doping on Photoluminescent Characteristics of Carbon Dots from Palm Bunches for Fluorimetric Sensing of Fe3+ Ion

This work aims to enhance the value of palm empty fruit bunches (EFBs), an abundant residue from the palm oil industry, as a precursor for the synthesis of luminescent carbon dots (CDs). The mechanism of fIuorimetric sensing using carbon dots for either enhancing or quenching photoluminescence properties when binding with analytes is useful for the detection of ultra-low amounts of analytes. This study revealed that EFB-derived CDs via hydrothermal synthesis exceptionally exhibited luminescence properties. In addition, surface modification for specific binding to a target molecule substantially augmented their PL characteristics. Among the different nitrogen and sulfur (N and S) doping agents used, including urea (U), sulfate (S), p-phenylenediamine (P), and sodium thiosulfate (TS), the results showed that PTS-CDs from the co-doping of p-phenylenediamine and sodium thiosulfate exhibited the highest PL properties. From this study on the fluorimetric sensing of several metal ions, PTS-CDs could effectively detect Fe3+ with the highest selectivity by fluorescence quenching to 79.1% at a limit of detection (LOD) of 0.1 µmol L−1. The PL quenching of PTS-CDs was linearly correlated with the wide range of Fe3+ concentration, ranging from 5 to 400 µmol L−1 (R2 = 0.9933).

A ratiometric sensor for selective detection of Hg2+ ions by combining second-order scattering and fluorescence signals of MIL-68(In)-NH2

Ratio fluorescence has attracted much attention because of its self-calibration properties. However, it is difficult to obtain suitable fluorescent materials with well-resolved signals simultaneously under one excitation. In this work, we report a different strategy, using MIL-68(In)-NH₂ as both the fluorescence element and the scattered light unit, and coupling the fluorescence and the scattered light to construct the fluorescence and scattered light ratio system. Based on the optical properties and the second-order scattering (SOS) of the material nanoparticles, the synthesized MIL-68(In)-NH₂ can be used to realize the ratio detection of Hg²⁺. Because the scattering intensity of small particle MIL-68(In)-NH₂ is weak, SOS is not obvious. When Hg²⁺ is introduced the coordination reaction between the amino nitrogen atoms of MIL-68(In)-NH₂ and Hg²⁺ make the particles larger, resulting in the decrease of fluorescence and the enhancement of SOS. As a result, a novel Hg²⁺ ratiometric detection method is developed by using the dual signal responses of the fluorescence and scattering. Under the optimal conditions (pH = 6, reaction time 5 min, room temperature, and the maximum excitation wavelength 365 nm), the linear range of the method is 0-100 μM, and the detection limit is 5.8 nM (Ksv = 9.89 × 10⁹ M^-1). In addition, the probe is successfully used to evaluate Hg²⁺ in actual water samples. Compared with the traditional method of recording only the fluorescence signal, the proposed fluorescence-scattering method provides a new strategy for the design of ratiometric sensors.

Carbon dot composites for bioapplications: a review

Recent advancements in the synthesis of carbon dot composites and their applications in biomedical fields (bioimaging, drug delivery and biosensing) have been carefully summarized. The current challenges and future trends of CD composites in this field have also been discussed.

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.

Highly efficient ratiometric nanothermometers based on colloidal carbon quantum dots

Optical nanothermometers have attracted much attention due to their non-contact and precise measurement with high spatial resolution at the micro- and nanoscales. They can be applied in various fields such as micro-opto-electronics, photonics, and biomedical thermal and pH sensing, while most thermal sensors reported so far contain heavy metals or have low sensitivity. Herein, we demonstrate a highly sensitive ratiometric thermal sensor based on colloidal C-dots. C-dots exhibit dual emission originating from the band gap emission and surface-dominant emission, which show a different temperature-dependent photoluminescence (PL) response. Among different surface-functionalized C-dots, C-dots@OH exhibit an absolute thermal sensitivity of -0.082 °C-1, which is the highest among various types of ratiometric thermosensors, making it a very promising candidate for high-sensitivity, self-calibrated nanoscale thermometry. As a proof-of-concept, C-dots@OH were employed to monitor the intracellular temperature (32-42 °C), showing a clear trend for temperature variation in a single cell, indicating that C-dots could offer a powerful tool for a potential precise measurement of the intracellular temperature. They could also be used as thermal sensors for nano-electronic and optoelectronic devices.

Luminescent PMMA Films and PMMA@SiO2 Nanoparticles with Embedded Ln3+ Complexes for Highly Sensitive Optical Thermometers in the Physiological Temperature Range*

In recent years, luminescent materials doped with Ln³⁺ ions have attracted much attention for their application as optical thermometers based on both downshifting and upconversion processes. This study presents research done on the development of highly sensitive optical thermometers in the physiological temperature range based on poly(methyl methacrylate) (PMMA) films doped with two series of visible Ln³⁺ complexes (Ln³⁺ =Tb³⁺ , Eu³⁺ , and Sm³⁺ ) and SiO₂ nanoparticles (NPs) coated with these PMMA films. The best performing PMMA film doped with Tb³⁺ and Eu³⁺ complexes was the PMMA[TbEuL₁ tppo]1 film (L₁ =4,4,4-trifluoro-1-phenyl-1,3-butadionate; tppo=triphenylphosphine oxide), which showed good temperature sensing of S_r =4.21 % K^-1 at 313 K, whereas for the PMMA films doped with Tb³⁺ and Sm³⁺ complexes the best performing was the PMMA[TbSmL₂ tppo]3 film (L₂ =4,4,4-trifluoro-1-(4-chlorophenyl)-1,3-butadionate), with S_r =3.64 % K^-1 at 313 K. Additionally, SiO₂ NPs coated with the best performing films from each of the series of PMMA films (Tb-Eu and Tb-Sm) and their temperature-sensing properties were studied in water, showing excellent performance in the physiological temperature range (PMMA[TbEuL₁ tppo]1@SiO₂ : S_r =3.84 % °C at 20 °C; PMMA[TbSmL₂ tppo]3@SiO₂ : S_r =3.27 % °C at 20 °C) and the toxicity of these nanoparticles on human cells was studied, showing that they were nontoxic.

Mitochondria-Anchored Molecular Thermometer Quantitatively Monitoring Cellular Inflammations

Temperature in mitochondria can be a critical indicator of cell metabolism. Given the highly dynamic and inhomogeneous nature of mitochondria, it remains a big challenge to quantitatively monitor the local temperature changes during different cellular processes. To implement this task, we extend our strategy on mitochondria-anchored thermometers from "on-off" probe Mito-TEM to a ratiometric probe Mito-TEM 2.0 based on the Förster resonance energy transfer mechanism. Mito-TEM 2.0 exhibits not only a sensitive response to temperature through the ratiometric changes of dual emissions but also the specific immobilization in mitochondria via covalent bonds. Both characters support accurate and reliable detection of local temperature for a long time, even in malfunctioning mitochondria. By applying Mito-TEM 2.0 in fluorescence ratiometric imaging of cells and zebrafishes, we make a breakthrough in the quantitative visualization of mitochondrial temperature rises in different inflammation states.

Ultrasensitive fluorescent detection of pesticides in real sample by using green carbon dots

Pesticides, widely used in modern agriculture, could potentially cause environmental pollution and affect human lives. Hence, the development of a highly sensitive sensing element to detect pesticide residues is crucial for food safety and ecosystem protection. Optical methods based on fluorescence properties provide an ideal approach for screening and quantification of these compounds in different medias including water, plant, and nutritional products. The development of fluorescence emitting carbon dot-based sensors for monitoring pesticides has attracted great attention in recent years. In comparison to other fluorophores, carbon dots have more promising optical features, higher quantum yields and better biocompatibility. This article aims to present a novel fluorescent sensing method of diazinon, glyphosate, and amicarbazone using plant-based carbon dots. A comprehensive characterization of carbon dots obtained from cauliflower was performed by methods including UV-visible, FTIR spectroscopy, fluorometry, AFM, DLS, and zeta sizer. Following this step, carbon dots were used to detect pesticides. The fluorescence quenching property of carbon dots has been utilized to identify detection limit of 0.25, 0.5, and 2 ng ml-1 for diazinon, amicarbazone, and glyphosate, respectively. Also, real sample study revealed that the detection of pesticides accompanied by our developed nano-sensor is repeatable and accurate. According to carbon dots specificity determination, the prepared nano sensor does not have the potential to identify "bromacil" and "dialen super" pesticides but the other three mentioned pesticides are detectable. The results confirm that synthesized green carbon dots are well qualified for application in food safety and environmental monitoring.

Aggregation-Induced Room-Temperature Phosphorescence Obtained from Water-Dispersible Carbon Dot-Based Composite Materials

Room-temperature phosphorescence (RTP) materials are desirable in chemical sensing because of their long emission lifetime and they are free from background autofluorescence. Nevertheless, the achievement of RTP in aqueous solution is still a highly challenging task. Herein, a molten salt method to prepare carbon dot (CD)-based RTP materials is presented by direct calcination of carbon sources in the presence of inorganic salts. The resultant CD composites (CDs@MP) exhibit bright RTP with a quantum yield of 26.4% and a lifetime of 1.28 s, which lasts for about 6 s to the naked eye. Importantly, their aqueous dispersion also has good RTP characteristics. This is the first time that the long-lived CDs@MP with RTP are achieved in aqueous solution owing to the synergistic effect of crystalline confinement and aggregation-induced phosphorescence. Further investigations reveal that three key processes may be responsible for the observed RTP of the composite materials: (1) The rigid crystalline salt shell can preserve the triplet states of CDs@MP in water and suppress the nonradiative deactivation; (2) The addition of high-charge-density metal ions Mg(II) and phosphorus element in the composite facilitates the singlet-to-triplet intersystem crossing process and enhances the RTP emission; (3) The aggregation of CDs@MP nanocomposites enables the matrix shell to self-assemble into a network, which further improves the rigidity of the shell and prevents the intermolecular motions, hence prolonging the RTP lifetime. The unique RTP feature and good water dispersibility allow the CD-based composite materials to be applicable in detection of temperature and pH in the aqueous phase. Our approach for producing long-lived RTP CDs@MP is effective, simple, and low-cost, which opens a new route to develop RTP materials that are applicable in aqueous solution.

A new dual-recognition strategy for hybrid ratiometric and ratiometric sensing perfluorooctane sulfonic acid based on high fluorescent carbon dots with ethidium bromide

In this work, a novel strategy for perfluorooctane sulfonic acid (PFOS) detection was established by using a ratiometric nanosensor with the combination of fluorescence and second-order scattering (SOS). The practical ratiometric nanosensor was synthesized simply by mixing fluorescent dye ethidium bromide (EB) and nitrogen doped carbon dots (N-CDs) which was synthesized by a one-step hydrothermal method with Victoria Blue B, possessing three emission peaks at 472 nm, 560 nm and 600 nm under a single wavelength excitation of 280 nm, respectively. The EB served as the reference signal label, and the N-CDs, having response to the analytes, acted as the response signal label. To achieve ratiometric detection, the fluorescence emission of the N-CDs was turned off and the SOS emission was turned on with the addition of the target PFOS. To achieve colorimetric detection, with the help of EB, the fluorescence of the system changed from green to orange. Under the optimal conditions, the difference of F₄₇₂/I₅₆₈ of the nanosensor had good linearity against the concentrations of PFOS within a linear range of 0-2.0 μM. The limit of detection was as low as 27.8 nM, which was low enough for the detection of PFOS in water samples. The proposed method has been successfully applied to the detection of PFOS with RSD <1.67%. The results show that the as-prepared N-CDs/EB ratiometric nanosensor has potential application for the detection of PFOS in environmental monitoring.

Dual-emission ratiometric nanoprobe for visual detection of Cu(II) and intracellular fluorescence imaging

Copper is an essential mineral nutrient for the human body. However, excessive levels of copper accumulated in the body can cause some diseases. Therefore, it is great significant to establish a sensitive bioprobe to recognize copper ions (Cu²⁺) in vivo. In our work, nitrogen-doped carbon dots (N-CDs) and gold nanoclusters (Au NCs) are selected as luminescent nanomaterials and the Au NCs/N- CDs nanohybrids is successfully synthesized by coupling method. The Au NCs/N-CDs exhibited characteristic dual-emission peaks at 450 and 620 nm when excited by a single-wavelength of 380 nm. When different amounts of Cu²⁺ are introduced, the fluorescence intensity of the Au NCs is gradually weakened and fluorescence intensity of the N-CDs is almost unchanged, which can facilitate the visual detection of Cu²⁺. The Au NCs/N-CDs nanohybrid possesses good selectivity to Cu²⁺ with a limit of detection (LOD) is 3.5 μM and linear detection range of 10-150 μM. Visualization detection of Cu²⁺ is implemented by using nanoprobe in water samples. Furthermore, the ratiometric nanoprobe is utilized to the toxicity test of liver cancer cells, indicating excellent biocompatibility and low toxicity. This nanoprobe has been used to the intracellular fluorescence imaging. Moreover, this method is expected to be used to monitor the changes of Cu²⁺ concentration in hepatocytes.

Effect of carbonization degree of carbon dots on cytotoxicity and photo-induced toxicity to cells

BACKGROUND

Pristine carbon dots (CDs) derived from citric acid pyrolysis are used in a variety of biomedical research such as imaging and drug delivery. However, potential cytotoxic effects of pyrolysis temperature on cells is underexplored. To address this need, we studied toxicity of the CDs to breast cancer cells using MTT and LDH assays. In addition, we investigated photo-induced cytotoxicity of the synthesized CDs in a wide concentration range under white light.

RESULTS

Our results suggest little cytotoxicity of the CDs after 24 h exposure of cells. Only the high quantum yield CDs caused a significant toxicity to cells at the highest concentrations of 2.0 and 1.5 mg/ml compared to other CDs at similar concentrations. The synthesized CDs entered the cells without any significant cytotoxicity. The CDs also caused a concentration- and irradiation time-dependent photo-induced cytotoxicity.

CONCLUSION

The optimization of synthesis conditions from this study may help develop safe and efficient CDs for imaging and drug delivery.

Retrosynthesis of Tunable Fluorescent Carbon Dots for Precise Long-Term Mitochondrial Tracking

Mitochondria play a significant role in many cellular processes. Precise long-term tracking of mitochondrial status and behavior is very important for regulating cell fate and treating mitochondrial diseases. However, developing probes with photostability, long-term tracking capability, and tunable long-wavelength fluorescence has been a challenge in mitochondrial targeting. Carbon dots (CDs) as new fluorescent nanomaterials with low toxicity and high stability show increasing advantages in bioimaging. Herein, the mitochondria tracking CDs (MitoTCD) with intrinsic mitochondrial imaging capability and tunable long-wavelength fluorescence from green to red are synthesized where the lipophilic cation of rhodamine is served as the luminescent center of CDs. Due to the excellent photostability, superior fluorescence properties and favorable biocompatibility, these MitoTCD are successfully used for mitochondrial targeting imaging of HeLa cells in vitro and can be tracked as long as six passages, which is suitable for long-term cell imaging. Moreover, these MitoTCD can also be used for zebrafish imaging in vivo.

Ratiometric fluorescent nanoprobes for visual detection: Design principles and recent advances - A review

Signal generation techniques for visual detection of analytes have received a great deal of attention in various sensing fields. These approaches are considered to be advantageous when instrumentation cannot be employed, such as for on-site assays, point-of-care tests, and he althcare diagnostics in resource-constrained areas. Amongst various visual detection approaches explored for non-invasive quantitative measurements, ratiometric fluorescence sensing has received particular attention as a potential method to overcome the limitations of intensity-based probes. This technique relies on changes in the intensity of two or more emission bands (induced by an analyte), resulting in an effective internal referencing which improves the sensitivity of the detection. The self-calibration, together with the unique optophysical properties of nanoparticles (NPs) have made the ratiometric fluorescent nanoprobes more sensitive and reliable, which in turn, can result in more precise visual detection of the analytes. Over the past few years, a vast number of ratiometric sensing probes using nanostructured fluorophores have been designed and reported for a wide variety of sensing, imaging, and biomedical applications. In this work, a review on the NP-based ratiometric fluorescent sensors has been presented to meticulously elucidate their development, advances and challenges. With a special emphasis on visual detection, the most important steps in the design of fluorescent ratiometric nanoprobes have been given and based on different classes of analytes, recent applications of fluorescent ratiometric nanoprobes have been summarized. The challenges for the future use of the technique investigated in this review have been also discussed.

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.

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

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

The morphology and surface charge-dependent cellular uptake efficiency of upconversion nanostructures revealed by single-particle optical microscopy

The cellular uptake efficiency of nanostructures has been demonstrated to be highly dependent on the surface charge, size and shape although the cellular internalization process is still far from being well-understood. In this work, a series of NaYF4:Yb3+, Er3+ upconversion nanoparticles (UCNPs) with different morphologies and surface coatings were prepared to explore the influence of surface charge and morphology on the cellular internalization process with single-particle fluorescence microscopy. It is found that the higher the surface charge and larger the surface-to-volume ratio of the nanoparticles, the more efficient the cellular uptake will be. Particularly, the surface charge is demonstrated to be the primary influence factor for small sized nanoparticles on the cellular uptake process. By blocking the endocytosis routes with temperature modulation (from 37 to 4 °C) or introduction of chemical inhibitors (dynasore and genistein), multiplexed mechanisms are found to be involved in the cellular uptake process, including clathrin- and caveolae-mediated endocytosis, physical adhesion and penetration, and so on. Moreover, in the aspect of size effect, an energy-dependent endocytosis process plays a more important role for larger size particles. In short, this study presents a pattern of cellular internalization pathway for the nanoparticles with different morphologies and surface charges, which would provide useful information for the development of robust drug delivery systems.

Ratiometric optical nanoprobes enable accurate molecular detection and imaging

Exploring and understanding biological and pathological changes are of great significance for early diagnosis and therapy of diseases. Optical sensing and imaging approaches have experienced major progress in this field. Particularly, an emergence of various functional optical nanoprobes has provided enhanced sensitivity, specificity, targeting ability, as well as multiplexing and multimodal capabilities due to improvements in their intrinsic physicochemical and optical properties. However, one of the biggest challenges of conventional optical nanoprobes is their absolute intensity-dependent signal readout, which causes inaccurate sensing and imaging results due to the presence of various analyte-independent factors that can cause fluctuations in their absolute signal intensity. Ratiometric measurements provide built-in self-calibration for signal correction, enabling more sensitive and reliable detection. Optimizing nanoprobe designs with ratiometric strategies can surmount many of the limitations encountered by traditional optical nanoprobes. This review first elaborates upon existing optical nanoprobes that exploit ratiometric measurements for improved sensing and imaging, including fluorescence, surface enhanced Raman scattering (SERS), and photoacoustic nanoprobes. Next, a thorough discussion is provided on design strategies for these nanoprobes, and their potential biomedical applications for targeting specific biomolecule populations (e.g. cancer biomarkers and small molecules with physiological relevance), for imaging the tumor microenvironment (e.g. pH, reactive oxygen species, hypoxia, enzyme and metal ions), as well as for intraoperative image guidance of tumor-resection procedures.

Microcirculation-mediated preconditioning and intracellular hypothermia

Microcirculation is a network of perfused capillaries that connects macrocirculation with the cells. Although research has provided insight into microcirculatory blood flow, our knowledge remains limited. In this article, we propose a new role of microcirculation in physiological and shock states. In healthy individuals, microcirculation maintains cellular homeostasis via preconditioning. When blood volume decreases, the ensuing microcirculatory changes result in heterogeneity of perfusion and tissue oxygenation. Initially, this is partly compensated by the preserved autoregulation and the increase in the metabolism rate of cells, but at later stages, the loss of autoregulation activates the cascade of intracellular hypothermia.

A new two-dimensional layered germanate with in situ embedded carbon dots for optical temperature sensing

A carbon dots@germanate composite has been synthesized by embedding carbon dots into a double-layered germanate in situ, which exhibits excitation-dependent and temperature-responsive photoluminescence.