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

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

A Facile Fabrication of Lysosome-Targeting pH Fluorescent Nanosensor Based on PEGylated Polyester Block Copolymer

A novel lysosome-targeting PEGylated polyester-based fluorescent pH nanosensor is fabricated by the combination of ring-opening copolymerization (ROCOP), side-group modification and subsequent self-assembly. First, a key target amphiphilic copolymer carrier for rhodamine (Rh) pH indicator is synthesized in a facile manner by the ROCOP of phthalic anhydride with allyl glycidyl ether using mPEG-OH and t-BuP1/Et3B as the macroinitiator and binary catalyst, respectively. Subsequently, Rh moieties are covalently attached on the polymer chain with controllable grafting degree via an efficient thiol-ene click reaction. Concurrently, the effect of catalyst systems and reaction conditions on the catalytic copolymerization performance is presented, and the quantitative introduction of Rh is described in detail. Owing to its amphiphilic characteristics, the rhodamine-functionalized polyester-based block copolymer can self-assemble into micelles. With the covalent incorporation of Rh moieties, the as-formed micelles exhibit excellent absorption and fluorescence-responsive sensitivity and selectivity towards H+ in the presence of various metal cations. Moreover, the as-prepared micelles with favorable water dispersibility, good pH sensitivity and excellent biocompatibility also display appreciable cell-membrane permeability, staining ability and pH detection capability for lysosomes in living cells. This work provides a new strategy for the facile synthesis of novel biocompatible polymeric fluorescent pH nanosensors for the fluorescence imaging of lysosomal pH changes.

Thermal Response and Thermochromism of Methyl Red-Based Copolymer Systems - Coupled Responsiveness in Critical Solution Behaviour and Optical Absorption Properties

Until now, only limited experimental knowledge and sparse theoretical treatment about the mechanisms of thermochromism of azo dyes in solution has been available. Especially the coupling of thermoresponsiveness of polymers...

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,...

Combined fluorophore and phosphor conjugation: a new design concept for simultaneous and spatially localized dual lifetime intracellular sensing of oxygen and pH

In this communication, we propose a new strategy for double-parametric biosensing and present a dual pH/O₂ lifetime sensor based on the covalent conjugation of fluorescein (pH sensor) and an orthometalated iridium complex (O₂ sensor) to human serum albumin (HSA). The resulting conjugate demonstrates biocompatibility, low toxicity, and fast cellular uptake, and displays independent lifetime responses towards variations in media acidity and oxygen concentration that makes it suitable for application as an effective pH/O₂ probe in luminescence microscopy using the FLIM/PLIM detection mode. The concept applicability has been exemplified using the dual spatially and temporally localized intracellular sensing of pH and O₂ concentration in living cells.

Active-Targeting Polymeric Dual Nanosensor for Ratiometrically Measuring Proton and Oxygen Concentrations in Mitochondria

Dysfunction of mitochondria is closely related to neurodegenerative diseases, heart diseases, cancers, and so on. Because both proton and oxygen participate in vital biochemical reactions occurring in mitochondria such as adenosine triphosphate (ATP) generation, measuring proton and oxygen concentrations in mitochondria is therefore crucial for monitoring mitochondria activities and understanding cellular behavior. For this purpose, we developed a ratiometric fluorescent nanosensor for simultaneously sensing and imaging O₂ and pH in mitochondria. The steps are as follows: (1) Styrene was copolymerized with 2-aminoethyl methacrylate hydrochloride to produce amino-functionalized polymer nanoparticles. (2) The reference dye rhodamine B isothiocyanate (RBITC) and oxygen-sensitive dye platinum(II) octaethylporphyrin (PtOEP) were encapsulated into a polymer sphere during polymerization, while the pH indicator fluorescein isothiocyanate (FITC) and mitochondrial-targeting molecule (3-carboxypropyl)triphenylphosphonium bromide (TPP) were further modified on the surface of the nanoparticles. The developed nanosensor shows a narrow distribution of particle size, high sensitivity toward O₂ and pH, excellent stability, and low cytotoxicity. These remarkable features of the dual nanosensor render them capable of real-time sensing and imaging of O₂ and pH in mitochondria with high spatial resolution. Applying the mitochondrial-targeted dual nanosensor in HeLa cells, we quantitatively measured and imaged mitochondrial proton and oxygen concentration variations after carbonyl cyanide m-chlorophenylhydrazone (CCCP) treatment.

Highly Photostable Fluorescent Tracker with pH-Insensitivity for Long-Term Imaging of Lysosomal Dynamics in Live Cells

Visualizing and tracking lysosomal dynamic changes is crucially important in the fields of physiology and pathology. Most currently used pH-dependent small-molecule lysotrackers and sensors usually fail to visualize and track the changes due to (1) their leakage from lysosomes when the lysosomal pH increases and (2) their low photostability. Therefore, it is of significant interest to develop lysosomal probes for visualizing and tracking lysosomal dynamics independent of pH fluctuations and with high photostability. Herein, we found that the popular dicyanomethylene-4H-pyran (DCM) derivative DCM-NH₂ can selectively target and label lysosomes with bright red fluorescence regardless of pH changes. The fluorescence enhancement in lysosomes has probably resulted from their microenvironment of polarity and viscosity. Compared with the commonly used commercial lysosomal molecular probes (LysoTracker Deep Red (LTDR) and LysoTracker Red DND-99), DCM-NH₂ was demonstrated to exhibit a much stronger tolerance in lysosomes against various treatments and microenvironmental changes, and lysosomal membrane permeability could not cause DCM-NH₂ to lose imaging of their targets as well. Moreover, DCM-NH₂ exhibited a superior anti-photobleaching ability and low (photo-) cytotoxicity, which, along with pH-insensitivity, ensured its capability of long-term visualizing and tracking lysosomal dynamics. Lysosomal dynamic events such as the kiss-and-run process, fusion-fission, and mitophagy were successfully recorded with DCM-NH₂. Our study thus confirms that DCM-NH₂ is highly competitive for lysosomal imaging by overcoming the limitations of the commercial LysoTrackers and highlights the unexplored application of DCM-NH₂ in bioimaging.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Dual-Mode Carbon Aerogel/Iron Rubber Sensor

Nowadays, the integration of easy production, simple structure, high sensitivity, and multifunctionality is the developing tendency for flexible sensors. Herein we report a facile manufacture of a highly flexible, sensitive, and multifunctional dual-mode sensor with an ultrasimple structure by directly attaching magnetic iron rubber (IR) onto the surface of carbon aerogel (CA) derived from melamine foam. The dual-mode CA/IR sensor exhibits high sensitivities of 5.6 kPa^-1 and 1.6·10^-3 Oe^-1, respectively, toward pressure and magnetic field in a wide frequency ranging from 0.1 to 10 Hz, which are higher than those of the existing flexible pressure/magnetism sensors. The multifunctionality of the dual-mode CA/IR sensor is demonstrated by monitoring blood pulse, human breath, balloon volume, and thoracic volume via pressure and magnetism sensing or their combination. Due to its simple structure and high sensitivities, the dual-mode sensor is employed as the building block to create a direction-recognizable sensor for identifying the directions of pressure and magnetic field for the awareness of surrounding barriers that are of practical importance in sophisticated situations such as autonomous artificial intelligence, autodriving and robotics, and so on.

A silica supported tricarbocyanine based pH nanosensor with a large Stokes shift and a near infrared fluorescence response: performance in vitro and in live cells

We report the synthesis of a near-infrared (NIR) fluorescent pH probe with a remarkable Stokes shift (∼135 nm) based on a tricarbocyanine (Cy-PIP).

Fluorescent pH-Sensing Probe Based on Biorefinery Wood Lignosulfonate and Its Application in Human Cancer Cell Bioimaging

A water-soluble, ratiometric fluorescent pH probe, L-SRhB, was synthesized via grafting spirolactam Rhodamine B (SRhB) to lignosulfonate (LS). As the ring-opening product of L-SRhB, FL-SRhB was also prepared. The pH-response experiment indicated that L-SRhB showed a rapid response to pH changes from 4.60 to 6.20 with a pK_a of 5.35, which indicated that L-SRhB has the potential for pH detection of acidic organelle. In addition, the two probes were internalized successfully by living cells through the endocytosis pathway and could distinguish normal cells from cancer cells by different cell staining rates. In addition, L-SRhB showed obvious cytotoxicity to cancer cells, whereas it was nontoxic to normal cells in the same condition. L-SRhB might have potential in cancer therapy. L-SRhB might be a promising ratiometric fluorescent pH sensor and bioimaging dye for the recognition of cancer cells. The results also provided a new perspective to the high-value utilization of lignin.

A protein-dye hybrid system as a narrow range tunable intracellular pH sensor

Accurate monitoring of pH variations inside cells is important for the early diagnosis of diseases such as cancer. Even though a variety of different pH sensors are available, construction of a custom-made sensor array for measuring minute variations in a narrow biological pH window, using easily available constituents, is a challenge. Here we report two-component hybrid sensors derived from a protein and organic dye nanoparticles whose sensitivity range can be tuned by choosing different ratios of the components, to monitor the minute pH variations in a given system. The dye interacts noncovalently with the protein at lower pH and covalently at higher pH, triggering two distinguishable fluorescent signals at 700 and 480 nm, respectively. The pH sensitivity region of the probe can be tuned for every unit of the pH window resulting in custom-made pH sensors. These narrow range tunable pH sensors have been used to monitor pH variations in HeLa cells using the fluorescence imaging technique.

Multiresponsive Behavior of Functional Poly(p-phenylene vinylene)s in Water

The multiresponsive behavior of functionalized water-soluble conjugated polymers (CPs) is presented with potential applications for sensors. In this study, we investigated the aqueous solubility behavior of water-soluble CPs with high photoluminescence and with a particular focus on their pH and temperature responsiveness. For this purpose, two poly(phenylene vinylene)s (PPVs)-namely 2,5-substituted PPVs bearing both carboxylic acid and methoxyoligoethylene glycol units-were investigated, with different amount of carboxylic acid units. Changes in the pH and temperature of polymer solutions led to a response in the fluorescence intensity in a pH range from 3 to 10 and for temperatures ranging from 10 to 85 °C. Additionally, it is demonstrated that the polymer with the largest number of carboxylic acid groups displays upper critical solution temperature (UCST)-like thermoresponsive behavior in the presence of a divalent ion like Ca2+. The sensing capability of these water-soluble PPVs could be utilized to design smart materials with multiresponsive behavior in biomedicine and soft materials.

The Influence of Virus Infection on the Extracellular pH of the Host Cell Detected on Cell Membrane

Influenza virus infection can result in changes in the cellular ion levels at 2-3 h post-infection. More H(+) is produced by glycolysis, and the viral M2 proton channel also plays a role in the capture and release of H(+) during both viral entry and egress. Then the cells might regulate the intracellular pH by increasing the export of H(+) from the intracellular compartment. Increased H(+) export could lead indirectly to increased extracellular acidity. To detect changes in extracellular pH of both virus-infected and uninfected cells, pH sensors were synthesized using polystyrene beads (ϕ1 μm) containing Rhodamine B and Fluorescein isothiocyanate (FITC). The fluorescence intensity of FITC can respond to both pH and temperature. So Rhodamine B was also introduced in the sensor for temperature compensation. Then the pH can be measured after temperature compensation. The sensor was adhered to cell membrane for extracellular pH measurement. The results showed that the multiplication of influenza virus in host cell decreased extracellular pH of the host cell by 0.5-0.6 in 4 h after the virus bound to the cell membrane, compared to that in uninfected cells. Immunostaining revealed the presence of viral PB1 protein in the nucleus of virus-bound cells that exhibited extracellular pH changes, but no PB1 protein are detected in virus-unbound cells where the extracellular pH remained constant.

A visual and reversible assay for temperature using thioflavin T-doped lanthanide/nucleotide coordination polymers

In this work, we prepared a type of thioflavin T (ThT)-doped lanthanide/nucleotide coordination polymer by the self-assembly of ThT, europium ions (Eu(3+)) and nucleotides (guanosine monophosphate, GMP) in aqueous solution (i.e. ThT/Eu/GMP). The Eu/GMP coordination polymers show excellent adaptive inclusion properties for ThT in a convenient one-step approach, which can readily enhance the fluorescence of ThT via the restricted effect. Moreover, the as-prepared hydrophilic ThT/Eu/GMP coordination polymers have the capability to act as a temperature-sensitive, visual and reversible sensor in aqueous solution under the irradiation of visible light. Our proposed design is cost-effective and simple to prepare without chemical modification or fluorescence labeling.

Selective Imaging of Late Endosomes with a pH-Sensitive Diazaoxatriangulene Fluorescent Probe

Late endosomes are a major trafficking hub in the cell at the crossroads between endocytosis, autophagy, and degradation in lysosomes. Herein is disclosed the first small molecule allowing their selective imaging and monitoring in the form of a diazaoxatriangulene fluorophore, 1a (hexadecyl side chain). The compound is prepared in three steps from a simple carbenium precursor. In nanospheres, this pH-sensitive (pKa = 7.3), photochemically stable dye fluoresces in the red part of visible light (601 and 578 nm, acid and basic forms, respectively) with a quantum yield between 14 and 16% and an excited-state lifetime of 7.7-7.8 ns. Importantly, the protonated form 1a·H(+) provokes a specific staining of late endosome compartments (pH 5.0-5.5) after 5 h of incubation with HeLa cells. Not surprisingly, this late endosome marking depends on the intra-organelle pH, and changing the nature of the lipophilic chain provokes a loss of selectivity. Interestingly, fixation of the fluorophore is readily achieved with paraformaldehyde, giving the possibility to image both live and fixed cells.

Fluorescent probes and bioimaging: alkali metals, alkaline earth metals and pH

All living species and life forms have an absolute requirement for bio-functional metals and acid-base equilibrium chemistry owing to the critical roles they play in biological processes. Hence, a great need exists for efficient methods to detect and monitor biometals and acids. In the last few years, great attention has been paid to the development of organic molecule based fluorescent chemosensors. The availability of new synthetic fluorescent probes has made fluorescence microscopy an indispensable tool for tracing biologically important molecules and in the area of clinical diagnostics. This review highlights the recent advances that have been made in the design and bioimaging applications of fluorescent probes for alkali metals and alkaline earth metal cations, including lithium, sodium and potassium, magnesium and calcium, and for pH determination within biological systems.

Design and synthesis of a rhodol isomer and its derivatives with high selectivity and sensitivity for sensing Hg2+ and F− in aqueous media

A fluorescent rhodol isomer has developed as a versatile fluorophore for the molecular design of chemosensors and fluorescent tracers.

Conventional fluorophore-free dual pH- and thermo-responsive luminescent alternating copolymer

Herein, we have designed and synthesized a novel traditional fluorophore-free water-soluble fluorescent copolymer based on a poly(maleimide-alt-styrene) skeleton, which responds to both pH and temperature in aqueous medium.

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.

Temperature sensing using red fluorescent protein

Genetically encoded fluorescent proteins are extensively utilized for labeling and imaging proteins, organelles, cell tissues, and whole organisms. In this study, we explored the feasibility of mRFP1 and its variants for measuring intracellular temperature. A linear relationship was observed between the temperature and fluorescence intensity of mRFP1 and its variants. Temperature sensitivities of E. coli expressing mRFP1, mRFP-P63A and mRFP-P63A[(4R)-FP] were -1.27%, -1.26% and -0.77%/°C, respectively. Finally, we demonstrated the potentiality of mRFP1 and its variants as an in vivo temperature sensor.

Local pH tracking in living cells

Continuous and simultaneous 3D single-particle movement and local pH detection in HeLa cells were demonstrated for the first time by combining fluorescent mesoporous silica nanoparticles (FMSNs) and a single-particle tracking (SPT) technique with a precision of ∼10 nm. FMSNs, synthesized by the co-condensation of both pH-sensitive and reference dyes with a silica/surfactant source, allow long-term reliable ratiometric pH measurements with a precision better than 0.3 pH unit because of their excellent brightness and stability. pH variation in the surrounding area of FMSNs during endocytosis was monitored in real-time. Acidification and low mobility of FMSNs were observed at the early endocytic stage, whereas basification and high mobility of FMSNs were observed at the late stage. Our results indicate that it is possible to monitor local pH changes in the environments surrounding nanoparticles during the cellular uptake process of FMSNs, which provides much needed information for designing an efficient drug delivery nanosystem.

A quinoline based Schiff-base compound as pH sensor

A new Schiff-base compound, 1,4-bis-(quinolin-6-ylimino methyl)benzene (BQB), has been explored as reversible ratiometric fluorescence pH sensor.

Novel biocompatible fluorescent polymeric micelles based on 1,8-naphthalimide derivatives for cell imaging

A new type of fluorescent polymeric micelle was developed by the aqueous self-assembly from amphiphilic block copolymers via click chemistry and showed good potential for application in cell imaging with good biocompatibility. Notably, P1 can readily serve as an excellent intracellular pH sensor in whole living cells based on PET inhibition

Thermoresponsive polymeric temperature sensors with broad sensing regimes

Polymeric temperature sensors with a broad sensing regime in aqueous solution have been developed.

Wide-range thermochromic luminescence of organoboronium complexes

Alkyl-substituted tetra-coordinate organoboronium bisdiketone complexes exhibit dramatic luminescence thermochromism in organic solvents.

Metal-organic framework based highly selective fluorescence turn-on probe for hydrogen sulphide

Hydrogen sulphide (H₂S) is known to play a vital role in human physiology and pathology which stimulated interest in understanding complex behaviour of H₂S. Discerning the pathways of H₂S production and its mode of action is still a challenge owing to its volatile and reactive nature. Herein we report azide functionalized metal-organic framework (MOF) as a selective turn-on fluorescent probe for H₂S detection. The MOF shows highly selective and fast response towards H₂S even in presence of other relevant biomolecules. Low cytotoxicity and H₂S detection in live cells, demonstrate the potential of MOF towards monitoring H₂S chemistry in biological system. To the best of our knowledge this is the first example of MOF that exhibit fast and highly selective fluorescence turn-on response towards H₂S under physiological conditions.

pH luminescence switching, dihydrogen phosphate sensing, and cellular uptake of a heterobimetallic ruthenium(II)-rhenium(I) complex

A new heterobimetallic ruthenium(II)-rhenium(I) complex of [Ru(bpy)2(HL)Re(CO)3Cl](ClO4)2·6H2O (RuHLRe) {bpy = 2,2'-bipyridine and HL = 2-(4-(2,6-di(pyridin-2-yl)pyridin-4-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline} was synthesised and characterised by elemental analysis, proton nuclear magnetic resonance spectroscopy, and mass spectrometry. The ground- and excited-state acid-base properties of RuHLRe were studied using UV-Vis absorption spectrophotometric and spectrofluorimetric titrations in a 100 : 1 (v/v) Britton-Robinson buffer-CH3CN solution combined with luminescence lifetime measurements. The complex exhibited two-step separate protonation-deprotonation processes in both the ground and excited states. The complex acted as pH-induced "off-on-off" luminescence switches (I(on)/I(off) = 31.0 and 14.6), with one of the switching actions being driven by pH variations over the physiological pH range (5.3-8.0). Importantly, cellular imaging and cytotoxicity experiments demonstrated that RuHLRe rapidly and selectively illuminated the membrane of HeLa cells over fixed cells and exhibited reduced cytotoxicity at the imaging concentration compared to the Re(I)-free parent Ru(II) complex. In addition, RuHLRe acted as an efficient "turn on" emission sensor for H2PO4(-) and "turn off" emission sensor for F(-) and OAc(-).