Latest Trends in Temperature Sensing by Molecular Probes

Temperature-dependent photoluminescence down to 77 K of organotin molecular rotors: eco-friendly synthesis, photophysical characterization, X-ray structures, and DFT studies

Fluorescent organotin compounds are useful in sensing, optoelectronic devices, and in vitro bioimaging. Although in vitro fluorescence bioimaging shows low resolution at room temperature, a better resolution is possible at cryotemperatures. Therefore, the search for new cryoluminescent materials with potential application in high-resolution fluorescence bioimaging remains a great challenge. Herein, we report the cryoluminescence properties of two fluorescent bis-organotin compounds, namely, BisNTHySnBu2 (5) and BisNTHySnPh2 (6), synthesized via microwave irradiation. All compounds were fully characterized using 1H, 13C, and 119Sn NMR spectroscopy, Raman spectroscopy, IR spectroscopy, and HR-MS. The 119Sn δ and 3J(1H,119Sn) of 5 and 6 indicate that two Sn-ligands are chemically and electronically equivalent, as confirmed by cyclic voltammetry. The crystal structure of 6 showed pentacoordinate tin atoms with skeleton ligands. The study of self-assembled monolayers of both Sn-complexes via STM microscopy revealed a similar supramolecular packing in lamella-like patterns, adopting a face-on arrangement, where molecules stay flat lying on HOPG in accordance with the height profile of closely packed monolayers on graphite of about 0.33 nm thickness. However, only the Sn complex 6, which bears phenyls, covers large surface areas. The photophysical properties of bis-organotin compounds were also investigated in solution (room and low temperatures) and in the solid state. Good luminescence properties in solutions with fluorescence quantum yields (Φ) of approximately 35% and 50% were found. Despite this, Φ is quenched in the solid state because of aggregation, as supported by solvent/non solvent fluorescence studies, which is in agreement with STM and AFM investigation.

Cell-resistant wavelength-shifting molecular beacons made of L-DNA and a clickable L-configured uridine

Wavelength-shifting molecular beacons were prepared from L-DNA. The clickable anchor for the two dyes, Cy3 and Cy5, was 2'-O-propargyl-L-uridine and was synthesized from L-ribose. Four clickable molecular beacons were prepared and double-modified with the azide dyes by a combination of click chemistry on a solid support for Cy3 during DNA synthesis and postsynthetic click chemistry for Cy5 in solution. Cy3 and Cy5 successfully formed a FRET pair in the beacons, and the closed form (red fluorescence) and the open form (green fluorescence) can be distinguished by the two-color fluorescence readout. Two molecular beacons were identified to show the greatest fluorescence contrast in temperature-dependent fluorescence measurements. The stability of the L-configured molecular beacons was demonstrated after several heating and cooling cycles as well as in the cell lysate. In comparison, D-configured molecular beacons showed a rapid decrease of fluorescence contrast in the cell lysate, which is caused by the opening of the beacons, probably due to degradation. This was confirmed in cell experiments using confocal microscopy. The L-configured molecular beacons are potential intracellular thermometers for future applications.

Molecular Engineering to Achieve AIE-active Fluorophore with Near-infrared (NIR) Emission and Temperature-sensitive Property

Near-infrared organic small molecule luminescent materials have the advantages of easy modification, high quantum efficiency, good biological affinity, and color adjustability; thus, have promising application prospects in the fields of photoelectric devices, sensitive detection, photodynamic therapy, and biomedical imaging. However, traditional organic luminescent molecules have the problems of short emission wavelength, aggregation-causing emission quenching, and low quantum yield. Herein, we successfully synthesized four D-π-A-D light-emitting molecules based on electron-withdrawing malonitrile group and different electron-donating arylamine groups. These compounds showed satisfactory solvatochromism, aggregation-induced emission, red and near-infrared fluorescence, high photoluminescence quantum efficiency and temperature response properties. This successful example of molecular engineering provides a valuable reference for the development of advanced NIR materials with AIE and temperature-sensitive properties.

Effects Induced by the Temperature and Chemical Environment on the Fluorescence of Water-Soluble Gold Nanoparticles Functionalized with a Perylene-Derivative Dye

We developed a fluorescent molecular probe based on gold nanoparticles functionalized with N,N'-bis(2-(1-piperazino)ethyl)-3,4,9,10-perylenetetracarboxylic acid diimide dihydrochloride, and these probes exhibit potential for applications in microscopic thermometry. The intensity of fluorescence was affected by changes in temperature. Chemical environments, such as different buffers with the same pH, also resulted in different fluorescence intensities. Due to the fluorescence intensity changes exhibited by modified gold nanoparticles, these materials are promising candidates for future technologies involving microscopic temperature measurements.

Molecular Engineering to Boost the Photo-Oxidase Activity of Molecular Rotors in Colorimetric Sensing of Temperatures

Some organic dyes and photosensitizers with strong visible absorption can behave as photo-responsive oxidase mimics. However, the relationship between the photo-oxidase activity and molecular structure remains unclear to date. In this work, a new type of photosensitizer with the characteristics of molecular rotors, namely DPPy, served as the molecular scaffold for further investigation. To adjust the photocatalytic oxidation ability, DAPy and CBPy were designed and synthesized based on the enhancement and diminishment of the intramolecular charge transfer (ICT) process, respectively. Kinetic studies revealed that DAPy and CBPy both exhibited highly efficient photo-activated oxidase-like activity with 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate, which were in good accordance with their molecular engineering to promote either type I or type II reactive oxygen species (ROS) generation. Impressively a colorimetric method based on the visible light induced oxidase-like activity of molecular rotors was developed to determine the environmental temperature for the first time. Both DAPy and CBPy showed distinct sensitivities toward temperature as compared with several molecular rotors based on the typical fluorimetric detection. This work provides a new strategy for the application of molecular rotors to overcome the non-emissive challenge in temperature sensing.

Probing organoid metabolism using fluorescence lifetime imaging microscopy (FLIM): The next frontier of drug discovery and disease understanding

Organoid models have been used to address important questions in developmental and cancer biology, tissue repair, advanced modelling of disease and therapies, among other bioengineering applications. Such 3D microenvironmental models can investigate the regulation of cell metabolism, and provide key insights into the mechanisms at the basis of cell growth, differentiation, communication, interactions with the environment and cell death. Their accessibility and complexity, based on 3D spatial and temporal heterogeneity, make organoids suitable for the application of novel, dynamic imaging microscopy methods, such as fluorescence lifetime imaging microscopy (FLIM) and related decay time-assessing readouts. Several biomarkers and assays have been proposed to study cell metabolism by FLIM in various organoid models. Herein, we present an expert-opinion discussion on the principles of FLIM and PLIM, instrumentation and data collection and analysis protocols, and general and emerging biosensor-based approaches, to highlight the pioneering work being performed in this field.

Organic Fluorescent Probes for Monitoring Micro-Environments in Living Cells and Tissues

As a vital parameter in living cells and tissues, the micro-environment is crucial for the living organisms. Significantly, organelles require proper micro-environment to achieve normal physiological processes, and the micro-environment in organelles can reflect the state of organelles in living cells. Moreover, some abnormal micro-environments in organelles are closely related to organelle dysfunction and disease development. So, visualizing and monitoring the variation of micro-environments in organelles is helpful for physiologists and pathologists to study the mechanisms of the relative diseases. Recently, a large variety of fluorescent probes was developed to study the micro-environments in living cells and tissues. However, the systematic and comprehensive reviews on the organelle micro-environment in living cells and tissues have rarely been published, which may hinder the research progress in the field of organic fluorescent probes. In this review, we will summarize the organic fluorescent probes for monitoring the microenvironment, such as viscosity, pH values, polarity, and temperature. Further, diverse organelles (mitochondria, lysosome, endoplasmic reticulum, cell membrane) about microenvironments will be displayed. In this process, the fluorescent probes about the "off-on" and ratiometric category (the diverse fluorescence emission) will be discussed. Moreover, the molecular designing, chemical synthesis, fluorescent mechanism, and the bio-applications of these organic fluorescent probes in cells and tissues will also be discussed. Significantly, the merits and defects of current microenvironment-sensitive probes are outlined and discussed, and the development tendency and challenges for this kind of probe are presented. In brief, this review mainly summarizes some typical examples and highlights the progress of organic fluorescent probes for monitoring micro-environments in living cells and tissues in recent research. We anticipate that this review will deepen the understanding of microenvironment in cells and tissues and facilitate the studies and development of physiology and pathology.

Real-Time Temperature Sensing Using a Ratiometric Dual Fluorescent Protein Biosensor

Accurate temperature control within biological and chemical reaction samples and instrument calibration are essential to the diagnostic, pharmaceutical and chemical industries. This is particularly challenging for microlitre-scale reactions typically used in real-time PCR applications and differential scanning fluorometry. Here, we describe the development of a simple, inexpensive ratiometric dual fluorescent protein temperature biosensor (DFPTB). A combination of cycle three green fluorescent protein and a monomeric red fluorescent protein enabled the quantification of relative temperature changes and the identification of temperature discrepancies across a wide temperature range of 4-70 °C. The maximal sensitivity of 6.7% °C^-1 and precision of 0.1 °C were achieved in a biologically relevant temperature range of 25-42 °C in standard phosphate-buffered saline conditions at a pH of 7.2. Good temperature sensitivity was achieved in a variety of biological buffers and pH ranging from 4.8 to 9.1. The DFPTB can be used in either purified or mixed bacteria-encapsulated formats, paving the way for in vitro and in vivo applications for topologically precise temperature measurements.

Ultrasensitive Small-Molecule Fluorescent Thermometer Reveals Hot Mitochondria in Surgically Resected Human Tumors

The Warburg effect suggests that upregulated glycolysis arising from high glucose uptake in cancer cells might be accompanied with suppressed mitochondrial respiration. However, recent studies have shown that the mitochondrial temperature in cancer cells could be relatively higher than that in normal cells, suggesting hyperactive mitochondrial respiration in cancer cells. However, hot mitochondria have not been reported in patients with cancer. Here, near-infrared small-molecule fluorescent probes TRNs are rationally designed with two ethyl amino groups as the temperature-sensitive moiety. Afterward, a mitochondrial targeting group is installed via ether bonds on TRN-8 to build MTN. To the best of our knowledge, MTN is the near-infrared probe with the highest sensitivity for mitochondrial temperature. Moreover, it also displays high photostability, wide linearity, and high specificity. Using MTN, we can monitor the ups and downs of mitochondrial temperature in cancer cells upon the perturbations of mitochondrial respiration. Furthermore, we demonstrate that the mitochondrial temperature in surgically resected human tumors is relatively higher than that in paracancerous tissues. Our results indicate that relatively hot mitochondria may exist in tumors from patients. We envisage that our study provides critical evidence for revisiting the Warburg effect and cancer metabolism.

Metal-free FRET macrocycles of perylenediimide and aza-BODIPY for multifunctional sensing

Two multichromophoric FRET macrocycles M1 [1+1] and M2 [2+2] with red emission (λ_em ∼ 721 nm) composed of perylenediimide (PDI) as the energy donor and aza-BODIPY (ABDP) as the energy acceptor were synthesized by click reaction in a metal-free fashion. M1 and M2 exhibited distinct reversible ratiometric temperature responsive emission with temperature sensitivities of 0.09-0.14% °C^-1 and owing to the redox active chromophores, they showed solution phase redox responsive reversible colour changes.

Unexpected luminescence of non-conjugated biomass-based polymers: new approach in photothermal imaging

Population growth, depletion of world resources and persistent toxic chemical production underline the need to seek new smart materials from inexpensive, biodegradable, and renewable feedstocks. Hence, "metal-free" ring-opening copolymerization to convert biomass carvone-based monomers into non-conventional luminescent biopolymers is considered a sustainable approach to achieve these goals. The non-conventional emission was studied in terms of steady-state and time-resolved spectroscopy in order to unravel the structure-properties for different carvone-based copolymers. The results highlighted the importance of the final copolymer folding structure as well as its environment in luminescent behavior (cluster-triggered emission). In all cases, their luminescent behavior is sensitive to small temperature fluctuations (where the minimum detected temperature is T_m ∼ 2 °C and relative sensitivity is S_r ∼ 6% °C) even at the microscopic scale, which endows these materials a great potential as thermosensitive smart polymers for photothermal imaging.

Thermally Controlled Exciplex Fluorescence in a Dynamic Homo[2]catenane

Motion-induced change in emission (MICE) is a phenomenon that can be employed to develop various types of probes, including temperature and viscosity sensors. Although MICE, arising from the conformational motion in particular compounds, has been studied extensively, this phenomenon has not been investigated in depth in mechanically interlocked molecules (MIMs) undergoing coconformational changes. Herein, we report the investigation of a thermoresponsive dynamic homo[2]catenane incorporating pyrene units and displaying relative circumrotational motions of its cyclophanes as evidenced by variable-temperature ¹H NMR spectroscopy and supported by its visualization through molecular dynamics simulations and quantum mechanics calculations. The relative coconformational motions induce a significant change in the fluorescence emission of the homo[2]catenane upon changes in temperature compared with its component cyclophanes. This variation in the exciplex emission of the homo[2]catenane is reversible as demonstrated by four complete cooling and heating cycles. This research opens up possibilities of using the coconformational changes in MIMs-based chromophores for probing fluctuations in temperature which could lead to applications in biomedicine or materials science.

Polymeric Nanoparticles with Embedded Eu(III) Complexes as Molecular Probes for Temperature Sensing

Three novel luminescent Eu(III) complexes, Eu1-Eu3, have been synthesized and characterized with CHN analysis, mass-spectrometry and ¹H NMR spectroscopy. The complexes display strong emission in dichloromethane solution upon excitation at 405 and 800 nm with a quantum yield from 18.3 to 31.6%, excited-state lifetimes in the range of 243-1016 ms at 20 °C, and lifetime temperature sensitivity of 0.9%/K (Eu1), 1.9%/K (Eu2), and 1.7%/K (Eu3). The chromophores were embedded into biocompatible latex nanoparticles (NPs_Eu1-NPs_Eu3) that prevented emission quenching and kept the photophysical characteristics of emitters unchanged with the highest temperature sensitivity of 1.3%/K (NPs_Eu2). For this probe cytotoxicity, internalization dynamics and localization in CHO-K1 cells were studied together with lifetime vs. temperature calibration in aqueous solution, phosphate buffer, and in a mixture of growth media and fetal bovine serum. The obtained data were then averaged to give the calibration curve, which was further used for temperature estimation in biological samples. The probe was stable in physiological media and displayed good reproducibility in cycling experiments between 20 and 40 °C. PLIM experiments with thermostated CHO-K1 cells incubated with NPs_Eu2 indicated that the probe could be used for temperature estimation in cells including the assessment of temperature variations upon chemical shock (sample treatment with mitochondrial uncoupling reagent).

Aminoindole and naphthalimide based charge transfer fluorescent probes for pH sensing and live cell imaging

Fluorescent probes are essential for imaging of cancer cells and for tracking organelles inside cells. We have synthesized three molecular rotors AIN, AINP and F-AINP based on 1-aminoindole (AI) as an electron donor and naphthalimide as an electron acceptor. All compounds showed charge transfer (CT) character, aggregation induced emission (AIE) and emission responsiveness towards temperature variation and solvent viscosity. AINP was most sensitive towards viscosity among all molecules with a viscosity sensitivity of ∼0.37. AIN, AINP and F-AINP showed negative temperature coefficients in chloroform with internal sensitivities of -0.04% °C^-1, -0.08% °C^-1 and -0.1% °C^-1, respectively. Furthermore, all the rotors were sensitive towards the pH of the solvent environment as revealed by acid titration and base back-titration and served as colorimetric pH sensors with intriguing photophysical characteristics. Additionally, AINP and F-AINP were used to image the live cancer cell line A549 and the fibroblast cell line L929, and the imaging studies revealed the incorporation of dyes in the cytoplasmic space of the cells except for the nuclei.

Nanothermometer with Temperature Induced Reversible Emission for Evaluation of Intracellular Thermal Dynamics

Temperature dynamics reflect the physiological state of cells, and accurate measurement of intracellular temperature helps to understand the biological processes. Herein, we report a novel nanothermometer by conjugating a fluorescent probe 3-ethyl-2-[4-(1,2,2-triphenylvinyl)styryl]benzothiazol-3-ium iodide (TPEBT) with a thermoresponsive polymer poly(N-isopropylacrylamide-co-tetrabutylphosphonium styrenesulfonate) [P(NIPAM-co-TPSS)]. The derived nanoprobe TPEBT-P(NIPAM-co-TPSS) self-assembles into micelles with TPEBT as hydrophobic core and PNIPAM as hydrophilic shell. It exhibits aggregation-induced emission (AIE) at λ_ex/λ_em = 420/640 nm in aqueous medium with a quantum yield of Φ_F 11.9%. The rise in temperature transforms PNIPAM chains from linear to compact spheres to serve as the core of micelles, and meanwhile converts TPEBT from the state of aggregation to dispersion and redistributes in the micellar shell. Temperature-driven phase transition of P(NIPAM-co-TPSS) mediates the reversible aggregation and disaggregation of TPEBT and endows the nanothermometer with temperature-dependent AIE features and favorable sensitivity for temperature sensing in 32-40 °C. TPEBT-P(NIPAM-co-TPSS) is taken up by HeLa cells to distribute mainly in lysosomes. It enables quantitative visualization of in situ thermal dynamics in response to stimuli from carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, oligomycin, genipin, and lipopolysaccharide. The real-time monitoring of photothermal-induced intracellular temperature variation is further conducted.

Intracellular Physical Properties with Small Organic Fluorescent Probes: Recent Advances and Future Perspectives

The intracellular physical parameters i. e., polarity, viscosity, fluidity, tension, potential, and temperature of a live cell are the hallmark of cellular health and have garnered immense interest over the past decade. In this context, small molecule organic fluorophores exhibit prominent useful properties including easy functionalizability, environmental sensitivity, biocompatibility, and fast yet efficient cellular uptakability which has made them a popular tool to understand intra-cellular micro-environmental properties. Throughout this discussion, we have outlined the basic design strategies of small molecules for specific organelle targeting and quantification of physical properties. The values of these parameters are indicative of cellular homeostasis and subtle alteration may be considered as the onset of disease. We believe this comprehensive review will facilitate the development of potential future probes for superior insight into the physical parameters that are yet to be quantified.

Fluorescence Switching for Temperature Sensing in Water

A water-soluble thermochromic molecular switch with spectrally resolved fluorescence in its two interconvertible states can be assembled in three synthetic steps by integrating a fluorescent coumarin chromophore, a hydrophilic oligo(ethylene glycol) chain, and a switchable oxazole heterocycle in the same covalent skeleton. Measurements of its two emissions in separate detection channels of a fluorescence microscope permit the noninvasive and ratiometric sensing of temperature at the micrometer level with millisecond response in aqueous solutions and within hydrogel matrices. The ratiometric optical output of this fluorescent molecular switch overcomes the limitations of single-wavelength fluorescent probes and enables noninvasive temperature mapping at length scales that are not accessible to conventional thermometers based on physical contact.

Zinc Donor–Acceptor Schiff Base Complexes as Thermally Activated Delayed Fluorescence Emitters

Four new zinc(II) Schiff base complexes with carbazole electron donor units and either a 2,3-pyrazinedicarbonitrile or a phthalonitrile acceptor unit were synthesized. The donor units are equipped with two bulky 2-ethylhexyl alkyl chains to increase the solubility of the complexes in organic solvents. Furthermore, the effect of an additional phenyl linker between donor and acceptor unit on the photophysical properties was investigated. Apart from prompt fluorescence, the Schiff base complexes show thermally activated delayed fluorescence (TADF) with quantum yields up to 47%. The dyes bearing a phthalonitrile acceptor emit in the green–yellow part of the electromagnetic spectrum and those with the stronger 2,3-pyrazinedicarbonitrile acceptor—in the orange–red part of the spectrum. The emission quantum yields decrease upon substitution of phthalonitrile with 2,3-pyrazinedicarbonitrile and upon introduction of the phenyl spacer. The TADF decay times vary between 130 µs and 3.5 ms at ambient temperature. The weaker phthalonitrile acceptor and the additional phenyl linker favor longer TADF decay times. All the complexes show highly temperature-dependent TADF decay time (temperature coefficients above −3%/K at ambient conditions) which makes them potentially suitable for application as molecular thermometers. Immobilized into cell-penetrating RL-100 nanoparticles, the best representative shows temperature coefficients of −5.4%/K at 25 °C that makes the material interesting for further application in intracellular imaging.

Thermo-active Smart Electrospun Nanofibers

The recent burst of research on smart materials is a clear evidence of the growing interest of the scientific community, industry, and society in the field. The exploitation of the great potential of stimuli-responsive materials for sensing, actuation, logic, and control applications is favored and supported by new manufacturing technologies, such as electrospinning, that allows to endow smart materials with micro- and nano-structuration, thus opening up additional and unprecedented prospects. In this wide and lively scenario, this article systematically reviews the current advances in the development of thermo-active electrospun fibers and textiles, sorting them, according to their response to the thermal stimulus. Hence, several platforms including thermo-responsive systems, shape memory polymers, thermo-optically responsive systems, phase change materials, thermoelectric materials, and pyroelectric materials, have been described and critically discussed. The difference in active species and outputs of the aforementioned categories has been highlighted, evidencing the transversal nature of temperature stimulus. Moreover, the potential of novel thermo-active materials has been pointed out, revealing how their development could take to utmost interesting achievements. This article is protected by copyright. All rights reserved.

Give or Take: Effects of Electron-Accepting/-Withdrawing Groups in Red-Fluorescent BODIPY Molecular Rotors

Mapping microviscosity, temperature, and polarity in biosystems is an important capability that can aid in disease detection. This can be achieved using fluorescent sensors based on a green-emitting BODIPY group. However, red fluorescent sensors are desired for convenient imaging of biological samples. It is known that phenyl substituents in the β position of the BODIPY core can shift the fluorescence spectra to longer wavelengths. In this research, we report how electron-withdrawing (EWG) and -donating (EDG) groups can change the spectral and sensory properties of β-phenyl-substituted BODIPYs. We present a trifluoromethyl-substituted (EWG) conjugate with moderate temperature sensing properties and a methoxy-substituted (EDG) molecule that could be used as a lifetime-based polarity probe. In this study, we utilise experimental results of steady-state and time-resolved fluorescence, as well as quantum chemical calculations using density functional theory (DFT). We also explain how the energy barrier height (Ea) for non-radiative relaxation affects the probe’s sensitivity to temperature and viscosity and provide appropriate Ea ranges for the best possible sensitivity to viscosity and temperature.

Overview of N-Rich Antennae Investigated in Lanthanide-Based Temperature Sensing

The market share of noncontact temperature sensors is expending due to fast technological and medical evolutions. In the wide variety of noncontact sensors, lanthanide-based temperature sensors stand out. They benefit from high photostability, relatively long decay times and high quantum yields. To circumvent their low molar light absorption, the incorporation of a light-harvesting antenna is required. This Review provides an overview of the nitrogen-rich antennae in lanthanide-based temperature sensors, emitting in the visible light spectrum, and discusses their temperature sensor ability. The N-rich ligands are incorporated in many different platforms. The investigation of different antennae is required to develop temperature sensors with diverse optical properties and to create a diverse offer for the multiple application fields. Molecular probes, consisting of small molecules, are first discussed. Furthermore, the thermometer properties of ratiometric temperature sensors, based on di- and polynuclear complexes, metal-organic frameworks, periodic mesoporous organosilicas and porous organic polymers, are summarized. The antenna mainly determines the application potential of the ratiometric thermometer. It can be observed that molecular probes are operational in the broad physiological range, metal-organic frameworks are generally very useful in the cryogenic region, periodic mesoporous organosilica show temperature dependency in the physiological range, and porous organic polymers are operative in the cryogenic-to-medium temperature range.

Luminescence lifetime imaging of three-dimensional biological objects

A major focus of current biological studies is to fill the knowledge gaps between cell, tissue and organism scales. To this end, a wide array of contemporary optical analytical tools enable multiparameter quantitative imaging of live and fixed cells, three-dimensional (3D) systems, tissues, organs and organisms in the context of their complex spatiotemporal biological and molecular features. In particular, the modalities of luminescence lifetime imaging, comprising fluorescence lifetime imaging (FLI) and phosphorescence lifetime imaging microscopy (PLIM), in synergy with Förster resonance energy transfer (FRET) assays, provide a wealth of information. On the application side, the luminescence lifetime of endogenous molecules inside cells and tissues, overexpressed fluorescent protein fusion biosensor constructs or probes delivered externally provide molecular insights at multiple scales into protein-protein interaction networks, cellular metabolism, dynamics of molecular oxygen and hypoxia, physiologically important ions, and other physical and physiological parameters. Luminescence lifetime imaging offers a unique window into the physiological and structural environment of cells and tissues, enabling a new level of functional and molecular analysis in addition to providing 3D spatially resolved and longitudinal measurements that can range from microscopic to macroscopic scale. We provide an overview of luminescence lifetime imaging and summarize key biological applications from cells and tissues to organisms.

Metal-Free Organic Luminophores that Exhibit Dual Fluorescence and Phosphorescence Emission at Room Temperature

Dual-fluorescent-phosphorescent compounds have attracted increasing attention in various fields, such as bio-imaging, data protection/encryption, ratiometric luminescence sensing, and white-light emission. Conventional dual-emissive compounds contain a phosphorescent organometallic complex of a precious metal, such as iridium or platinum. However, the use of precious metals in organic materials has several drawbacks. This Minireview focuses on precious-metal-free organic light-emitting materials that exhibit dual fluorescence and phosphorescence emission in the solid state at room temperature to produce bimodal steady-state emission spectra. The dual emitters presented herein are categorized into the following six compound classes: (1) difluoroboron diaroylmethanes, (2) diarylketones, (3) diarylsulfones, (4) triazines and pyrimidines, (5) fused phenazines, and (6) N-arylcarbazoles.

Effects of transition metal cations and temperature on the luminescence of a 3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrole-2-olate anion

The luminescence intensity of a 3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olate anion (HA−) drops to zero upon complexation with transition metal cations, and reversibly drops by 6–7 times upon heating from 27 up to 123 °C.

Thermo-responsive light-emitting metal complexes and related materials

This review discusses the fundamentals and design strategies for the development of thermo-responsive metal–ligand coordination materials and the applications of these materials in temperature sensing, bioimaging, information security, etc.