Environment-sensitive behavior of fluorescent molecular rotors

Construction of a novel NIR-emissive rhodamine derivative for monitoring mitochondrial viscosity in ferroptosis

Ferroptosis, an iron-dependent programmed cell death mechanism, is mediated by distinct molecular pathways of lipid peroxidation caused by intracellular iron supplementation and glutathione synthesis inhibition that cause oxidative damage to the cell membrane. Monitoring viscosity changes of mitochondria is essential for a deeper understanding of ferroptosis, as mitochondria will be shrunk with increased membrane density and leading to drastic mitochondrial viscous changes during ferroptosis process. Thus, it is essential to explore novel and efficient fluorescent probes for monitoring viscosity in organisms. In this work, we designed and synthesized a mitochondria-targeting probe TJ-FRP for cellular viscosity measurement via fluorescence imaging method. To obtain this probe, we firstly developed a novel modifiable fluorescent π-extended xanthene dye TJ-FR by replacing the benzoic acid group with a strong electron-withdrawing perfluorobenzoic acid group at the 9-position of xanthene framework. The dye not only presents emission wavelength at 758 nm and a large stokes shift of 142 nm in water, but also the dye is low biotoxic, membrane permeable. By reaction with 4-aminobutyltriphenylphosphonium bromide, TJ-FR was converted to the mitochondria-targeting probe TJ-FRP. TJ-FRP was successfully applied for the imaging of viscosity in living cells. Especially, the probe can be applied for visualizing mitochondrial viscosity changes during various inducers-stimulated ferroptosis process in model cells. These findings suggest that this novel NIR fluorescent probe can serve as a powerful tool to monitor the viscosity in biological samples and may provide new insights for various diseases during ferroptosis.

Designing artificial fluorescent proteins and biosensors by genetically encoding molecular rotor-based amino acids

Fluorescent proteins are indispensable tools in biological and medical research. The fluorophores are typically encoded by the primary amino acid sequence, from which a fluorescent molecular rotor structure forms upon protein folding. Here, inspired by the fluorogenic property exhibited by native fluorophores, we designed a collection of fluorogenic non-canonical amino acids that feature this molecular rotor structure-termed fluorescent molecular rotor amino acids (FMR-AAs)-akin to native fluorescent protein fluorophores. By incorporating FMR-AAs into target proteins through an expanded genetic code, we use them as encoded fluorophore analogues within a confined protein microenvironment, thus readily transforming diverse non-fluorescent proteins into artificial fluorescent proteins. We also use FMR-AAs in selected proteins as sensitive fluorescent probes for monitoring protein-protein interactions and detecting protein conformational changes in vitro and in living cells. This approach enables the generation of artificial fluorescent proteins and the development of biosensors from potentially any protein of interest with minor modifications.

Electrostatically Engineered Tetraphenylethylene-Based Fluorescence Sensor for Protamine and Trypsin: Leveraging Aggregation-Induced Emission for Enhanced Sensitivity and Selectivity

The accurate detection of Protamine and Trypsin, two biomolecules with significant clinical and biological relevance, presents a substantial challenge because of their structural peculiarities, low abundance in physiological fluids, and potential interference from other substances. Protamine, a cationic protein, is crucial for counteracting heparin overdoses, whereas Trypsin, a serine protease, is integral to protein digestion and enzyme activation. This study introduces a novel fluorescence sensor based on a (4-(1,2,2-tris(4-phosphonophenyl)vinyl)phenyl)phosphonic acid octasodium salt (TPPE), leveraging aggregation-induced emission (AIE) characteristics and electrostatic interactions to achieve selective and sensitive detection of these biomolecules. Through comprehensive optical characterization, including ground-state absorption, steady-state, and time-resolved emission spectroscopy, the interaction mechanisms and aggregation dynamics of TPPE with Protamine and Trypsin were elucidated. The sensor exhibits very high sensitivity (LOD: 1.45 nM for Protamine and 32 pM for Trypsin), selectivity, and stability, successfully operating in complex biological matrices, such as human serum and urine. Importantly, this sensor design underscores the synergy between the AIE phenomena and biomolecular interactions, offering a promising alternative for analytical applications in biomedical research and clinical diagnostics. The principles outlined herein open new avenues for the development of other AIE-based sensors, expanding the toolkit available for detecting a wide range of biomolecules using similar design strategies.

Cy3-Based Nanoviscosity Determination of Mucus: Effect of Mucus Collection Methods and Antibiotics Treatment

The integrity of the protective mucus layer as a primary defense against pathogen invasion and microbial leakage into the intestinal epithelium can be compromised by the effects of antibiotics on the commensal microbiome. Changes in mucus integrity directly affect the solvent viscosity in the immediate vicinity of the mucin network, that is, the nanoviscosity, which in turn affects both biochemical reactions and selective transport. To assess mucus nanoviscosity, a reliable readout via the viscosity-dependent fluorescence lifetime of the molecular rotor dye cyanine 3 is established and nanoviscosities from porcine and murine ex vivo mucus are determined. To account for different mucin concentrations due to the removal of digestive residues during mucus collection, the power law dependence of mucin concentration on viscosity is used. The impact of antibiotics combinations (meropenem/vancomycin, gentamycin/ampicillin) on ex vivo intestinal mucus nanoviscosity is presented. The significant increase in viscosity of murine intestinal mucus after treatment suggests an effect of antibiotics on the microbiota that affects mucus integrity. This method will be a useful tool to assess how drugs, directly or indirectly, affect mucus integrity. Additionally, the method can be utilized to analyze the role of mucus nanoviscosity in health and disease, as well as in drug development.

Red-Emitting Pyrrolyl Squaraine Molecular Rotor Reports Variations of Plasma Membrane and Vesicular Viscosity in Fluorescence Lifetime Imaging

The viscosity that ensures the controlled diffusion of biomolecules in cells is a crucial biophysical parameter. Consequently, fluorescent probes capable of reporting viscosity variations are valuable tools in bioimaging. In this field, red-shifted probes are essential, as the widely used and gold standard probe remains green-emitting molecular rotors based on BODIPY. Here, we demonstrate that pyrrolyl squaraines, red-emissive fluorophores, exhibit high sensitivity over a wide viscosity range from 30 to 4890 mPa·s. Upon alkylation of the pyrrole moieties, the probes improve their sensitivity to viscosity through an enhanced twisted intramolecular charge transfer phenomenon. We utilized this scaffold to develop a plasma membrane probe, pSQ-PM, that efficiently stains the plasma membrane in a fluorogenic manner. Using fluorescence lifetime imaging, pSQ-PM enabled efficient sensing of viscosity variations in the plasma membrane under various conditions and in different cell lines (HeLa, U2OS, and NIH/3T3). Moreover, upon incubation, pSQ-PM stained the membrane of intracellular vesicles and suggested that the lysosomal membranes displayed enhanced fluidity.

Targeted mitochondrial fluorescence probe with large stokes shift for detecting viscosity changes in vivo and in ferroptosis process

We created four fluorescent sensors in our work to determine the viscosity of mitochondria. Following screening, the probe Mito-3 was chosen because in contrast to the other three probes, it had a greater fluorescence enhancement, large Stokes shift (113 nm) and had a particular response to viscosity that was unaffected by polarity or biological species. As the viscosity increased from PBS to 90 % glycerol, the fluorescence intensity of probe at 586 nm increased 17-fold. Mito-3 has strong biocompatibility and is able to track changes in cell viscosity in response to nystatin and monensin stimulation. Furthermore, the probe has been successfully applied to detect changes in viscosity caused by nystatin and monensin in zebrafish. Above all, the probe can be applied to the increase in mitochondrial viscosity that accompanies the ferroptosis process. Mito-3 has the potential to help further study the relationship between viscosity and ferroptosis.

Modulation of fluorescence and phosphorescence of organoboron compounds from ortho-substituted phenolic Schiff bases by structural modification

Light emission from organoboron compounds of Schiff bases is found to depend strongly on their chemical structure. Two of these compounds (OB1 and OB2), which contain a benzene ring between the Schiff base moieties, exhibit weak fluorescence in methanol, with marked viscosity dependence. Fluorescence lifetimes of these compounds are in picosecond timescale, as determined by femtosecond optical gating (FOG). A significant enhancement in fluorescence intensity and lifetime is observed at 77 K, indicating the operation of an activated nonradiative process. Using fluorescence lifetime imaging microscopy (FLIM), OB1 and OB2 are shown to be potential membrane probes. The third (OB3), which is devoid of this benzene ring, exhibits relatively stronger fluorescence with nanosecond lifetimes at room temperature. No viscosity dependence is observed in this case. The emission spectrum at 77 K is markedly more intense and exhibits an additional red shifted structured feature, which persists for a few seconds. Hence, OB3 seems to have greater promise not only as fluorescent probe but also for light harvesting. The marked improvement of the light emission properties of OB3 compared with OB1 and OB2 is likely to serve as a pointer for the design of Schiff base-derived organoboron luminophores with diverse potential applications.

Fluorescent Molecular Rotors Quantify an Adjuvant-Induced Softening of Plant Wax

Epicuticular wax is the outmost layer of plant leaves that protects them from desiccation and penetration of harmful reagents. There is an intense industrial effort in the development of softening agents, adjuvants, that can adjust the permeability of the wax toward pesticides and, thus, play an important role in sustainable agriculture. However, mechanistic understanding of the structure and dynamic properties within the plant wax, particularly upon the application of adjuvants, is currently lacking. In this work, we demonstrate that fluorescence lifetime imaging microscopy (FLIM) combined with molecular rotors, fluorescent probes sensitive to viscosity, can directly probe the microviscosity of amorphous and crystalline phases of model plant wax layers. Moreover, this approach is able to quantify the changes in viscosity in both phases upon the addition of water and adjuvant solutions on top of the wax. We show that water permeation mostly perturbs the crystalline phase of the wax, while our chosen adjuvant, Plurafac LF431, mainly softens the amorphous phase of the wax. Our technique provides a facile and quantitative way to monitor dynamic properties within plant waxes with diffraction-limited resolution and reveals the effect of organic substances on wax structure and rigidity, crucial for designing next-generation agents to improve agricultural efficiency.

Fluorescence lifetime imaging microscopy of flexible and rigid dyes probes the biophysical properties of synthetic and biological membranes

Sensing of the biophysical properties of membranes using molecular reporters has recently regained widespread attention. This was elicited by the development of new probes of exquisite optical properties and increased performance, combined with developments in fluorescence detection. Here, we report on fluorescence lifetime imaging of various rigid and flexible fluorescent dyes to probe the biophysical properties of synthetic and biological membranes at steady state as well as upon the action of external membrane-modifying agents. We tested the solvatochromic dyes Nile red and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) (ammonium salt) (NBD), the viscosity sensor Bodipy C12, the flipper dye FliptR, as well as the dyes 3,3'-dioctadecyloxacarbocyanine perchlorate (DiO), Bodipy C16, lissamine-rhodamine, and Atto647, which are dyes with no previous reported environmental sensitivity. The performance of the fluorescent probes, many of which are commercially available, was benchmarked with well-known environmental reporters, with Nile red and Bodipy C12 being specific reporters of medium hydration and viscosity, respectively. We show that some widely used ordinary dyes with no previous report of sensing capabilities can exhibit competing performance compared to highly sensitive commercially available or custom-based solvatochromic dyes, molecular rotors, or flipper in a wide range of biophysics experiments. Compared to other methods, fluorescence lifetime imaging is a minimally invasive and nondestructive method with optical resolution. It enables biophysical mapping at steady state or assessment of the changes induced by membrane-active molecules at subcellular level in both synthetic and biological membranes when intensity measurements fail to do so. The results have important consequences for the specific choice of the sensor and take into consideration factors such as probe sensitivity, response to environmental changes, ease and speed of data analysis, and the probe's intracellular distribution, as well as potential side effects induced by labeling and imaging.

Membraneless and membrane-bound organelles in an anhydrobiotic cell line are protected from desiccation-induced damage

Anhydrobiotic species can survive virtually complete water loss by entering a reversible ametabolic glassy state that may persist for years in ambient conditions. The Pv11 cell line was derived from the egg mass of the anhydrobiotic midge, Polypedilum vanderplanki, and is currently the only available anhydrobiotic cell line. Our results demonstrate that the necessary preconditioning for Pv11 cells to enter anhydrobiosis causes autophagy and reduces mitochondrial respiration by over 70%. We speculate that reorganizing cellular bioenergetics to create and conserve energy stores may be valuable to successfully recover after rehydration. Furthermore, mitochondria in preconditioned cells lose their membrane potential during desiccation but rapidly restore it within 30 min upon rehydration, demonstrating that the inner mitochondrial membrane integrity is well-preserved. Strikingly, the nucleolus remains visible immediately upon rehydration in preconditioned cells while absent in control cells. In contrast, a preconditioning-induced membraneless organelle reformed after rehydration, demonstrating that membraneless organelles in Pv11 cells can be either stabilized or recovered. Staining the endoplasmic reticulum and the Golgi apparatus revealed that these organelles fragment during preconditioning. We hypothesize that this process reduces sheering stress caused by rapid changes in cellular volume during desiccation and rehydration. Additionally, preconditioning was found to cause the filamentous-actin (F-actin) network to disassemble significantly and reduce the fusion of adjacent plasma membranes. This study offers several exciting avenues for future studies in the animal model and Pv11 cell line that will further our understanding of anhydrobiosis and may lead to advancements in storing sensitive biologics at ambient temperatures for months or years.

The role of memory-dependent friction and solvent viscosity in isomerization kinetics in viscogenic media

Molecular isomerization kinetics in liquid solvent depends on a complex interplay between the solvent friction acting on the molecule, internal dissipation effects (also known as internal friction), the viscosity of the solvent, and the dihedral free energy profile. Due to the absence of accurate techniques to directly evaluate isomerization friction, it has not been possible to explore these relationships in full. By combining extensive molecular dynamics simulations with friction memory-kernel extraction techniques we consider a variety of small, isomerising molecules under a range of different viscogenic conditions and directly evaluate the viscosity dependence of the friction acting on a rotating dihedral. We reveal that the influence of different viscogenic media on isomerization kinetics can be dramatically different, even when measured at the same viscosity. This is due to the dynamic solute-solvent coupling, mediated by time-dependent friction memory kernels. We also show that deviations from the linear dependence of isomerization rates on solvent viscosity, which are often simply attributed to internal friction effects, are due to the simultaneous violation of two fundamental relationships: the Stokes-Einstein relation and the overdamped Kramers prediction for the barrier-crossing rate, both of which require explicit knowledge of friction.

Dual Functional Microcapsule based on Monodisperse Short PEG Amphiphile for Drug Encapsulation and Protein Affinity Controlled Release

A short monodisperse poly(ethylene glycol) (PEG) and a neutral organic rotamer conjugate TEG-BTA-2 amphiphile was designed for the construction of a stimuli-responsive switchable self-assembled structure for drug encapsulation by noncovalent interaction and targeted controlled delivery. A short PEG, tetraethylene glycol (TEG) was covalently attached with a neutral organic rotamer benzothiazole dye (BTA-2) affording the neutral TEG-BTA-2 (<500 D). The TEG-BTA-2 is self-assembled into a microsphere in an aqueous medium, but remarkably undergoes morphology change switching to a rice-like microcapsule for curcumin encapsulation. Curcumin-loaded microcapsules were stable in an aqueous solution, however, were noticed disintegrating upon the addition of BSA protein. This is possibly due to an interaction with BSA protein leading to a protein affinity-controlled curcumin release in a neutral PBS buffer. Moreover, cell internalization of the neutral amphiphile TEG-BTA-2 into A549 cells was observed by fluorescence microscopy, providing an opportunity for application as a molecular vehicle for targeted drug delivery and monitoring.

Excited State Rotational Freedom Impacts Viscosity Sensitivity in Arylcyanoamide Fluorescent Molecular Rotor Dyes

The microviscosity of intracellular environments plays an important role in monitoring cellular function. Thus, the capability of detecting changes in viscosity can be utilized for the detection of different disease states. Viscosity-sensitive fluorescent molecular rotors are potentially excellent probes for these applications; however, the predictable relationships between chemical structural features and viscosity sensitivity are poorly understood. Here, we investigate a set of arylcyanoamide-based fluorescent probes and the effect of small aliphatic substituents on their viscosity sensitivity. We found that the location of the substituents and the type of π-network of the fluorophore can significantly affect the viscosity sensitivity of these fluorophores. Computational analysis supported the notion that the excited state rotational energy barrier plays a dominant role in the relative viscosity sensitivity of these fluorophores. These findings provide valuable insight into the design of molecular rotor-based fluorophores for viscosity measurement.

High-Fidelity Assay Based on Turn-Off Fluorescence to Detect the Perturbations of Cellular Proteostasis

The persistence of neurodegenerative diseases has necessitated the development of new strategies to monitor protein homeostasis (proteostasis). Previous efforts in our laboratory have focused on the development of fluorogenic strategies to observe the onset and progression of proteostatic stress. These works utilized solvatochromic and viscosity sensitive fluorophores to sense protein folded states, enabling stressor screening with an increase in the emission intensity upon aggregation. In this work, we present a novel, high-fidelity assay to detect perturbations of cellular proteostasis, where the fluorescence intensity decreases with the onset of proteostatic stress. Utilizing a fluorogenic, hydroxymethyl silicon-rhodamine probe to differentiate between protein folded states, we establish the validity of this technology in living cells by demonstrating a two-fold difference in fluorescence intensity between unstressed and stressed conditions.

Synthesis and Enzymatic Incorporation of a Dual-App Nucleotide Probe That Reports Antibiotics-Induced Conformational Change in the Bacterial Ribosomal Decoding Site RNA

Natural nucleosides are nonfluorescent and do not have intrinsic labels that can be readily utilized for analyzing nucleic acid structure and recognition. In this regard, researchers typically use the so-called "one-label, one-technique" approach to study nucleic acids. However, we envisioned that a responsive dual-app nucleoside system that harnesses the power of two complementing biophysical techniques namely, fluorescence and 19F NMR, will allow the investigation of nucleic acid conformations more comprehensively than before. We recently introduced a nucleoside analogue by tagging trifluoromethyl-benzofuran at the C5 position of 2'-deoxyuridine, which serves as an excellent fluorescent and 19F NMR probe to study G-quadruplex and i-motif structures. Taking forward, here, we report the development of a ribonucleotide version of the dual-app probe to monitor antibiotics-induced conformational changes in RNA. The ribonucleotide analog is derived by conjugating trifluoromethyl-benzofuran at the C5 position of uridine (TFBF-UTP). The analog is efficiently incorporated by T7 RNA polymerase to produce functionalized RNA transcripts. Detailed photophysical and 19F NMR of the nucleoside and nucleotide incorporated into RNA oligonucleotides revealed that the analog is structurally minimally invasive and can be used for probing RNA conformations by fluorescence and 19F NMR techniques. Using the probe, we monitored and estimated aminoglycoside antibiotics binding to the bacterial ribosomal decoding site RNA (A-site, a very important RNA target). While 2-aminopurine, a famous fluorescent nucleic acid probe, fails to detect structurally similar aminoglycoside antibiotics binding to the A-site, our probe reports the binding of different aminoglycosides to the A-site. Taken together, our results demonstrate that TFBF-UTP is a very useful addition to the nucleic acid analysis toolbox and could be used to devise discovery platforms to identify new RNA binders of therapeutic potential.

Molecular Rotors: Fluorescent Sensors for Microviscosity and Conformation of Biomolecules

The viscosity and crowding of biological environment are considered vital for the correct cellular function, and alterations in these parameters are known to underly a number of pathologies including diabetes, malaria, cancer and neurodegenerative diseases, to name a few. Over the last decades, fluorescent molecular probes termed molecular rotors proved extremely useful for exploring viscosity, crowding, and underlying molecular interactions in biologically relevant settings. In this review, we will discuss the basic principles underpinning the functionality of these probes and will review advances in their use as sensors for lipid order, protein crowding and conformation, temperature and non-canonical nucleic acid structures in live cells and other relevant biological settings.

Light-Triggered Enhancement of Fluorescence Efficiency in Organic Cages

The fluorescence efficiency of excited molecules can be enhanced by many external factors. Here, we showcase a surprising phenomenon whereby light is used as a gating source to increase the fluorescence efficiency of organic cages composed of biphenyl subunits. We show that the enhancement of fluorescence is not due to structural changes or ground-state events. Cryo-fluorescence measurements and kinetic studies suggest a restriction of the phenyl-based structures in the excited state, leading to increased fluorescence, which is also supported by time-resolved measurements. Through computational calculations, we propose that the planarization of the biphenyl units within the cages contributes to emission enhancement. This phenomenon offers insights into the design of optoelectronic structures with improved fluorescence properties.

Dual role far red fluorescent molecular rotor for decoding the plasma membrane and mitochondrial viscosity

The dysfunctions in the mitochondria are associated with various pathological conditions like neurodegeneration, metabolic disorder, and cancer, leading to dysregulated cell death. Here, we have designed and synthesized a julolidine-based molecular rotor (JMT) to target mitochondria with far-red emission accounting for mitochondrial dysfunction. JMT showed viscosity sensitivity with 160-fold enhancement in fluorescence intensity. The origin of the dark state in a lower viscous environment was investigated through density functional calculations. We have employed JMT to monitor mitochondrial dysfunction induced by nystatin using confocal and fluorescence lifetime imaging microscopy. Further, we investigated mitochondrial abnormalities under inflammatory conditions triggered by lipopolysaccharide in live HeLa cells. The cellular uptake mechanisms of JMT were studied using various endocytosis inhibitors. Moreover, we reported tracking small fluorescent molecule switching from mitochondria to the plasma membrane upon introducing mitochondrial depolarizer in cells. On treating the mitochondria potential uncoupler, JMT relocates to the cell membrane and can be utilized for understanding the interplay between mitochondria and cell membranes. Moreover, JMT was applied to stain the RBC plasma membrane isolated from human blood.

Viscosity-Induced Emission of 5-(Diarylmethylene)imidazolone with Extended Conjugation via Attachment of N-Methylpyrrole at the 2-Position

We have developed a series of 2-monoaryl-5-diarylmethylene analogs of the green fluorescent protein chromophore to study their viscosity-induced emission (VIE) properties. The analogs were synthesized by a condensation with methyl imidate and N-(diarylmethylene)glycinate. Among the analogs, the N-methylpyrrol-2-yl-substituted analog 1h induced the most remarkable VIE behavior in triglyceride and lipid bilayers probably due to the high π-electron-rich property of the pyrrole ring. The pyrrole substituent in imidazolone analogs can be expected to become a common template for introducing VIE behavior.

Fluorescence lifetime Hong-Ou-Mandel sensing

Fluorescence Lifetime Imaging Microscopy in the time domain is typically performed by recording the arrival time of photons either by using electronic time tagging or a gated detector. As such the temporal resolution is limited by the performance of the electronics to 100's of picoseconds. Here, we demonstrate a fluorescence lifetime measurement technique based on photon-bunching statistics with a resolution that is only dependent on the duration of the reference photon or laser pulse, which can readily reach the 1-0.1 picosecond timescale. A range of fluorescent dyes having lifetimes spanning from 1.6 to 7 picoseconds have been here measured with only ~1 s measurement duration. We corroborate the effectiveness of the technique by measuring the Newtonian viscosity of glycerol/water mixtures by means of a molecular rotor having over an order of magnitude variability in lifetime, thus introducing a new method for contact-free nanorheology. Accessing fluorescence lifetime information at such high temporal resolution opens a doorway for a wide range of fluorescent markers to be adopted for studying yet unexplored fast biological processes, as well as fundamental interactions such as lifetime shortening in resonant plasmonic devices.

Fluorescent molecular rotors as versatile in situ sensors for protein quantitation

Accurate protein quantitation is essential for many cellular mechanistic studies. Existing technology relies on extrinsic sample evaluation that requires significant volumes of sample as well as addition of assay-specific reagents and importantly, is a terminal analysis. This study exploits the unique chemical features of a fluorescent molecular rotor that fluctuates between twisted-to-untwisted states, with a subsequent intensity increase in fluorescence depending on environmental conditions (e.g., viscosity). Here we report the development of a rapid, sensitive in situ protein quantitation method using ARCAM-1, a representative fluorescent molecular rotor that can be employed in both non-terminal and terminal assays.

Al3+-Responsive Ratiometric Fluorescent Sensor for Creatinine Detection: Thioflavin-T and Sulfated-β-Cyclodextrin Synergy

Kidney disorders are a rising global health issue, necessitating early diagnosis for effective treatment. Creatinine, a metabolic waste product from muscles, serves as an ideal biomarker for kidney damage. The existing optical methods for creatinine detection often involve labor-intensive synthesis processes and present challenges with the aqueous solubility and sensitivity to experimental variations. In this study, we introduce a straightforward fluorescence "turn-on" ratiometric sensor system for creatinine detection in aqueous media with a limit of detection of 0.5 μM. The sensor is based on sulfated-β-cyclodextrin (SCD)-templated H-aggregate of a commercially available, ultrafast rotor dye thioflavin-T (ThT). The Al3+ ion-induced dissociation of ThT-SCD aggregates, followed by reassociation upon creatinine addition, generates a detectable signal. The modulation of monomer/aggregate equilibrium due to the disassembly/reassembly of the ThT-SCD system under Al3+/creatinine influence serves as the optimal strategy for ratiometric creatinine detection in aqueous media. Our sensor framework offers several advantages: utilization of the readily available dye ThT, which eliminates the need for a laborious synthesis of custom fluorescent probes; ratiometric sensing, which improves quantitative analysis accuracy; and compatibility with complex aqueous media. The sensor's practical utility has been successfully demonstrated in artificial urine samples. In summary, our sensor system represents a significant advancement in the rapid, selective, and sensitive detection of the clinically crucial bioanalyte creatinine, offering potential benefits for the early diagnosis and management of kidney disorders.

Julolidine-based small molecular probes for fluorescence imaging of RNA in live cells

Intracellular RNA imaging with organic small molecular probes has been an intense topic, although the number of such reported dyes, particularly dyes with high quantum yields and long wavelength excitation/emission, is quite limited. The present work reports the design and synthesis of three cationic julolidine-azolium conjugates (OX-JLD, BTZ-JLD and SEZ-JLD) as turn-on fluorescent probes with appreciably high quantum yields and brightness upon interaction with RNA. A structure-efficiency relationship has been established for their potential for the interaction and imaging of intracellular RNA. Given their chemical structure, the free rotation between the donor and the acceptor gets restricted when the probes bind with RNA resulting in strong fluorescence emission towards a higher wavelength upon photoexcitation. A detailed investigation revealed that the photophysical properties and the optical responses of two probes, viz. BTZ-JLD and SEZ-JLD, towards RNA are very promising and qualify them to be suitable candidates for biological studies, particularly for cellular imaging applications. The probes allow imaging of intracellular RNA with prominent staining of nucleoli in live cells under a range of physiological conditions. The results of the cellular digest test established the appreciable RNA selectivity of BTZ-JLD and SEZ-JLD inside the cellular environment. Moreover, a comparison between the relative intensity profile of SEZ-JLD before and after the RNA-digestion test inside the cellular environment indicated that the interference of cellular viscosity in fluorescence enhancement is insignificant, and hence, SEZ-JLD can be used as a cell membrane permeable cationic molecular probe for deep-red imaging of intracellular RNA with a good degree of selectivity.

Specific Fluorescent Probes for Imaging DNA in Cell-Free Solution and in Mitochondria in Living Cells

New styryl dyes consisting of N-methylpyridine or N-methylquinoline scaffolds were synthesized, and their binding affinities for DNA in cell-free solution were studied. The replacement of heterocyclic residue from the pyridine to quinoline group as well as variation in the phenyl part strongly influenced their binding modes, binding affinities, and spectroscopic responses. Biological experiments showed the low toxicity of the obtained dyes and their applicability as selective dyes for mitochondria in living cells.

Comprehensive monitoring of mitochondrial viscosity variation during different cell death processes by a NIR mitochondria-targeting fluorescence probe

Cell death is a fundamental feature of multicellular organisms, in which mitochondria play crucial roles. Therefore, revealing and monitoring the microenvironment of mitochondria are significant to investigate cell death process. Herein, the mitochondrial viscosity variation behaviors of a series of different cell death processes were monitored with a NIR mitochondria-targeting fluorescence probe FLV. FLV was designed based on a rotatable flavylocyanine fluorophore that presented selective and sensitive NIR fluorescence enhancement response with the increase of environmental viscosity. Fluorescence imaging experiments of living cells incubated with nystatin or under different temperature indicated that FLV was capable of imaging the change of mitochondrial viscosity. Finally, FLV was applied for monitoring the mitochondrial viscosity variation during different cell death processes. It was found that there were obvious mitochondrial viscosity increases during apoptosis, necrosis and autophagy; however, no detectable mitochondrial viscosity variation was observed in ferroptosis process incubated with ferroptosis inducer erastin or RSL3 for 6 h. These results demonstrated that FLV is a viable tool for monitoring the mitochondrial viscosity variation and is likely to be used in the diagnosis of the mitochondrial viscosity-associated cell processes and diseases.

Designing a green-emitting viscosity-sensitive 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) probe for plasma membrane viscosity imaging

Viscosity is a key characteristic of lipid membranes - it governs the passive diffusion of solutes and affects the lipid raft formation and membrane fluidity. Precise determination of viscosity values in biological systems is of great interest and viscosity-sensitive fluorescent probes offer a convenient solution for this task. In this work we present a novel membrane-targeting and water-soluble viscosity probe BODIPY-PM, which is based on one of the most frequently used probes BODIPY-C10. Despite its regular use, BODIPY-C10 suffers from poor integration into liquid-ordered lipid phases and lack of water solubility. Here, we investigate the photophysical characteristics of BODIPY-PM and demonstrate that solvent polarity only slightly affects the viscosity-sensing qualities of BODIPY-PM. In addition, with fluorescence lifetime imaging microscopy (FLIM), we imaged microviscosity in complex biological systems - large unilamellar vesicles (LUVs), tethered bilayer membranes (tBLMs) and live lung cancer cells. Our study showcases that BODIPY-PM preferentially stains the plasma membranes of live cells, equally well partitions into both liquid-ordered and liquid-disordered phases and reliably distinguishes lipid phase separation in tBLMs and LUVs.

A molecular rotor FLIM probe reveals dynamic coupling between mitochondrial inner membrane fluidity and cellular respiration

The inner mitochondrial membrane (IMM), housing components of the electron transport chain (ETC), is the site for respiration. The ETC relies on mobile carriers; therefore, it has long been argued that the fluidity of the densely packed IMM can potentially influence ETC flux and cell physiology. However, it is unclear if cells temporally modulate IMM fluidity upon metabolic or other stimulation. Using a photostable, red-shifted, cell-permeable molecular-rotor, Mitorotor-1, we present a multiplexed approach for quantitatively mapping IMM fluidity in living cells. This reveals IMM fluidity to be linked to cellular-respiration and responsive to stimuli. Multiple approaches combining in vitro experiments and live-cell fluorescence (FLIM) lifetime imaging microscopy (FLIM) show Mitorotor-1 to robustly report IMM 'microviscosity'/fluidity through changes in molecular free volume. Interestingly, external osmotic stimuli cause controlled swelling/compaction of mitochondria, thereby revealing a graded Mitorotor-1 response to IMM microviscosity. Lateral diffusion measurements of IMM correlate with microviscosity reported via Mitorotor-1 FLIM-lifetime, showing convergence of independent approaches for measuring IMM local-order. Mitorotor-1 FLIM reveals mitochondrial heterogeneity in IMM fluidity; between-and-within cells and across single mitochondrion. Multiplexed FLIM lifetime imaging of Mitorotor-1 and NADH autofluorescence reveals that IMM fluidity positively correlates with respiration, across individual cells. Remarkably, we find that stimulating respiration, through nutrient deprivation or chemically, also leads to increase in IMM fluidity. These data suggest that modulating IMM fluidity supports enhanced respiratory flux. Our study presents a robust method for measuring IMM fluidity and suggests a dynamic regulatory paradigm of modulating IMM local order on changing metabolic demand.

Synthesis and Evaluation of Diguanosine Cap Analogs Modified at the C8-Position by Suzuki-Miyaura Cross-Coupling: Discovery of 7-Methylguanosine-Based Molecular Rotors

Chemical modifications of the mRNA cap structure can enhance the stability, translational properties, and half-life of mRNAs, thereby altering the therapeutic properties of synthetic mRNA. However, cap structure modification is challenging because of the instability of the 5'-5'-triphosphate bridge and N7-methylguanosine. The Suzuki-Miyaura cross-coupling reaction between boronic acid and halogen compound is a mild, convenient, and potentially applicable approach for modifying biomolecules. Herein, we describe two methods to synthesize C8-modified cap structures using the Suzuki-Miyaura cross-coupling reaction. Both methods employed phosphorimidazolide chemistry to form the 5',5'-triphosphate bridge. However, in the first method, the introduction of the modification via the Suzuki-Miyaura cross-coupling reaction at the C8 position occurs postsynthetically, at the dinucleotide level, whereas in the second method, the modification was introduced at the level of the nucleoside 5'-monophosphate, and later, the triphosphate bridge was formed. Both methods were successfully applied to incorporate six different groups (methyl, cyclopropyl, phenyl, 4-dimethylaminophenyl, 4-cyanophenyl, and 1-pyrene) into either the m⁷G or G moieties of the cap structure. Aromatic substituents at the C8-position of guanosine form a push-pull system that exhibits environment-sensitive fluorescence. We demonstrated that this phenomenon can be harnessed to study the interaction with cap-binding proteins, e.g., eIF4E, DcpS, Nudt16, and snurportin.

Dense and Acidic Organelle-Targeted Visualization in Living Cells: Application of Viscosity-Responsive Fluorescence Utilizing Restricted Access to Minimum Energy Conical Intersection

Cell-imaging methods with functional fluorescent probes are an indispensable technique to evaluate physical parameters in cellular microenvironments. In particular, molecular rotors, which take advantage of the twisted intramolecular charge transfer (TICT) process, have helped evaluate microviscosity. However, the involvement of charge-separated species in the fluorescence process potentially limits the quantitative evaluation of viscosity. Herein, we developed viscosity-responsive fluorescent probes for cell imaging that are not dependent on the TICT process. We synthesized AnP₂-H and AnP₂-OEG, both of which contain 9,10-di(piperazinyl)anthracene, based on 9,10-bis(N,N-dialkylamino)anthracene that adopts a nonflat geometry at minimum energy conical intersection. AnP₂-H and AnP₂-OEG exhibited enhanced fluorescence as the viscosity increased, with sensitivities comparable to those of conventional molecular rotors. In living cell systems, AnP₂-OEG showed low cytotoxicity and, reflecting its viscosity-responsive property, allowed specific visualization of dense and acidic organelles such as lysosomes, secretory granules, and melanosomes under washout-free conditions. These results provide a new direction for developing functional fluorescent probes targeting dense organelles.

A dual-emission fluorescence probe for the detection of viscosity and hydrazine in environmental and biological samples

The microenvironments of biological systems are associated with the pathology of organisms. This study, aimed to construct a hemicyanine-based probe (1), which can respond to mitochondrial viscosity and hydrazine (N₂H₄), for imaging application in living cells and zebrafish. The probe showed no fluorescence due to the intramolecular rotation in the solution; however, it exhibited a strong emission at 730 nm when the molecules were restricted to a high-viscosity environment. The addition of N₂H₄ caused an elimination reaction of the N-substituted group in the pyridinium part and further broke the CC bond to produce a highly fluorescent hydrazone. Also, the probe could selectively and quantitatively detect N₂H₄ via the fluorescence enhancement at 510 nm in a concentration range of 0 μM-140μM, with the limit of detection being 0.0485 μM. This probe may be used to study diseases related to N₂H₄ and viscosity changes in biological systems. Furthermore, the analysis methods based on probe 1 for N₂H₄ detection in soil, water, and air samples were successfully established.

Thiophene-Extended Fluorescent Nucleosides as Molecular Rotor-Type Fluorogenic Sensors for Biomolecular Interactions

Fluorescent molecular rotors are versatile tools for the investigation of biomolecular interactions and the monitoring of microenvironmental changes in biological systems. They can transform invisible information into a fluorescence signal as a straightforward response. Their utility is synergistically amplified when they are merged with biomolecules. Despite the tremendous significance and superior programmability of nucleic acids, there are very few reports on the development of molecular rotor-type isomorphic nucleosides. Here, we report the synthesis and characterization of a highly emissive molecular rotor-containing thymine nucleoside (^ThexT) and its 2'-O-methyluridine analogue (2'-OMe-^ThexU) as fluorogenic microenvironment-sensitive sensors that emit vivid fluorescence via an interaction with the target proteins. ^ThexT and 2'-OMe-^ThexU may potentially serve as robust probes for a broad range of applications, such as fluorescence mapping, to monitor viscosity changes and specific protein-binding interactions in biological systems.

Deciphering the ultrafast dynamics of a new tetraphenylethylene derivative in solutions: charge separation, phenyl ring rotation and CC bond twisting

Tetraphenylethylene (TPE) derivatives are one of the fundamental units for developing aggregation induced emission (AIE) scaffolds. However, the underlying mechanisms implicated in the relaxation of the excited TPE remain a topic of ongoing discussion, while the effect of bulky substituents on its photobehaviour is still under scrutiny. Here, we report a detailed study of the photophysical properties of a new symmetrical and bulky TPE derivative with terphenyl groups (TTECOOBu) in solvents of different polarities and viscosities. Using femto- to nanosecond (fs-ns) time-resolved absorption and emission techniques, we elucidated the role of the phenyl group rotations and core ethylene bond twisting in its behaviour. We demonstrate that TTECOOBu in DCM solutions undergoes a 600 fs charge separation along the ethylene bond leading to a resonance structure with a lifetime of ∼1 ns. The latter relaxes via two consecutive events: a twisting of the ethylene bond (∼ 9 ps) and a rotation of the phenyl rings (∼ 30 ps) leading to conformationally-relaxed species with a largely Stokes-shifted emission (∼ 12 500 cm^-1). The formation of the red-emitting species clearly depends on the solvent viscosity and rigidity of the medium. Contrary to the photobehavior in the highly viscous triacetin or rigid polymer matrix of PMMA, a reversible mechanism was observed in DCM and DMF solutions. These results provide new findings on the ultrafast mechanisms of excited TPE derivatives and should help in the development of new molecular rotors with interesting AIE properties for photonic applications.

Directly imaging emergence of phase separation in peroxidized lipid membranes

Lipid peroxidation is a process which is key in cell signaling and disease, it is exploited in cancer therapy in the form of photodynamic therapy. The appearance of hydrophilic moieties within the bilayer's hydrocarbon core will dramatically alter the structure and mechanical behavior of membranes. Here, we combine viscosity sensitive fluorophores, advanced microscopy, and X-ray diffraction and molecular simulations to directly and quantitatively measure the bilayer's structural and viscoelastic properties, and correlate these with atomistic molecular modelling. Our results indicate an increase in microviscosity and a decrease in the bending rigidity upon peroxidation of the membranes, contrary to the trend observed with non-oxidized lipids. Fluorescence lifetime imaging microscopy and MD simulations give evidence for the presence of membrane regions of different local order in the oxidized membranes. We hypothesize that oxidation promotes stronger lipid-lipid interactions, which lead to an increase in the lateral heterogeneity within the bilayer and the creation of lipid clusters of higher order.

A Far-Red Molecular Rotor Fluorogenic Trehalose Probe for Live Mycobacteria Detection and Drug-Susceptibility Testing

Increasing the speed, specificity, sensitivity, and accessibility of mycobacteria detection tools are important challenges for tuberculosis (TB) research and diagnosis. In this regard, previously reported fluorogenic trehalose analogues have shown potential, but their green-emitting dyes may limit sensitivity and applications in complex settings. Here, we describe a trehalose-based fluorogenic probe featuring a molecular rotor turn-on fluorophore with bright far-red emission (RMR-Tre). RMR-Tre, which exploits the unique biosynthetic enzymes and environment of the mycobacterial outer membrane to achieve fluorescence activation, enables fast, no-wash, low-background fluorescence detection of live mycobacteria. Aided by the red-shifted molecular rotor fluorophore, RMR-Tre exhibited up to a 100-fold enhancement in M. tuberculosis labeling compared to existing fluorogenic trehalose probes. We show that RMR-Tre reports on M. tuberculosis drug resistance in a facile assay, demonstrating its potential as a TB diagnostic tool.

Adsorption of a water-soluble molecular rotor fluorescent probe on hydrophobic surfaces

Environmentally sensitive molecular rotors are widely used to probe the local molecular environment in e.g. polymer solutions, polymer glasses, and biological systems. These applications make it important to understand its fluorescence properties in the vicinity of a solid surface, since fluorescence microscopy generically employs cover slides, and measurements are often done in its immediate vicinity. Here, we use a confocal microscope to investigate the fluorescence of (4-DASPI) in glycerol/water solutions close to the interface using hydrophilic or hydrophobic cover slips. Despite the dye's high solubility in water, the observed lengthening of the fluorescence lifetime close to the hydrophobic surface, implies a surprising affinity of the dye with the surface. Because the homogeneous solution and the refractive index mismatch reduces the optical sectioning power of the microscope, we quantify the affinity with the help of a simple model of the signal vs. depth of focus, exhibiting surface and bulk contributions. The model reduces artefacts due to refractive index mismatch, as supported by Monte Carlo ray tracing simulations.

Unraveling the Mechanistic Pathway for the Dual Fluorescence in Green Fluorescent Protein (GFP) Chromophore Analogue: A Detailed Theoretical Investigation

The photophysical properties of the para-sulfonamide (p-TsABDI) analogue of the green fluorescent protein (GFP) chromophore with both proton donating and accepting sites have been exploited in polar solvents to understand the origin of the unusual dual fluorescence nature of the chromophore. In the polar solvents, the compound undergoes structural rearrangement upon photoexcitation, leading to the ultrafast excited-state intermolecular proton transfer (ESIPT) phenomenon at the S₁ surface. In this work, we employed both the static electronic structure calculations and on-the-fly molecular dynamics simulation to unravel the underlying reason for this peculiar behavior of the p-TsABDI analogue in polar solvents. To represent this adequately and provide extensive information on the ESIPT mechanism mediated by the solvent molecules, we considered explicit solvent molecules using the integral equation formalism variant of polarizable continuum (IEFPCM) model. From the static calculation analysis, we can conclude that the dual emissive behavior of the compound is ascribed to the proton transfer (PT) phenomena in the excited-state. However, based on the static calculation exclusively, it is hard to ascertain the mechanistic pathway of the PT phenomena. Hence, to investigate the dynamics and reaction mechanism for the ESIPT process, we performed the on-the-fly dynamics simulation for p-TsABDI in solvent clusters. Dynamics simulation results reveal that, based on the time lag between all the proton transfer processes, the ESIPT mechanism occurs in a stepwise manner from the benzylidene moiety of the chromophore to its imidazolinone moiety. However, the nonexistence of crossings between the S₁- and S₀-states confirms the PT characteristics of the reactions.

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.

Molecular rotors in haemoglobin and bovine serum albumin proteins

Molecular rotors are fluorescent viscosity probes and their response in simple fluids is known to be a Förster-Hoffman power law, allowing the viscosity of the medium to be quantified by its fluorescence intensity. They are attractive probes in biological media, usually consisting of proteins, but how does a molecular rotor behave in a protein solution? The response of the DASPI molecular rotor is compared in two globular protein solutions of similar size, haemoglobin (Hb) and bovine serum albumin, one absorbent, the other not. In absorbent Hb, a model validated by experiments in triangular geometry allows one to correct the absorbing effect and to compare the rotor response in both proteins. With concomitant microrheology measurements, we investigate the relation between the DASPI fluorescence intensity and solution viscosity. In protein solutions, we show that viscosity is no longer the parameter determining the rotor response in contrast to simple fluids. Varying the viscosity by concentration or temperature is not equivalent, and the Förster-Hoffmann power laws do not apply when the solution concentration varies. We show that the concentration regime of the protein solution, semi-dilute or concentrated, determines the sensitivity of the rotor to its environment.

E/Z photoisomerization pathway in pristine and fluorinated di(3-furyl)ethenes

We report an XMCQDPT2 study of the E/Z photoisomerization in a series of fluorinated di(3-furyl)ethenes (3DFEs). Upon excitation, pristine and low-fluorinated 3DFE show conventional behavior of many diarylethenes: unhindered twisting motion toward the pyramidalized zwitterionic state where relaxation to the ground state occurs. However, deep fluorination of 3DFEs can hamper E-to-Z isomerization by giving rise to an alternative excited-state relaxation pathway: an out-of-plane motion of a ring fluorine atom. Importantly, the case of fluorinated 3DFEs reveals serious deficiencies of the popular TDDFT approach. With some commonly used exchange-correlation functionals, the alternative relaxation pathway is not reproduced and, moreover, an irrelevant ring rotation coordinate is predicted instead. Nevertheless, TDDFT remains qualitatively adequate for the E-to-Z twisting coordinate taken alone.

Synthesis and Strong Solvatochromism of Push-Pull Thienylthiazole Boron Complexes

The solvatochromic behavior of two donor-π bridge-acceptor (D-π-A) compounds based on the 2-(3-boryl-2-thienyl)thiazole π-linker and indandione acceptor moiety are investigated. DFT/TD-DFT calculations were performed in combination with steady-state absorption and emission measurements, along with electrochemical studies, to elucidate the effect of two different strongly electron-donating hydrazonyl units on the solvatochromic and fluorescence behavior of these compounds. The Lippert–Mataga equation was used to estimate the change in dipole moments (Δµ) between ground and excited states based on the measured spectroscopic properties in solvents of varying polarity with the data being supported by theoretical studies. The two asymmetrical D-π-A molecules feature strong solvatochromic shifts in fluorescence of up to ~4300 cm⁻¹ and a concomitant change of the emission color from yellow to red. These changes were accompanied by an increase in Stokes shift to reach values as large as ~5700–5800 cm⁻¹. Quantum yields of ca. 0.75 could be observed for the N,N-dimethylhydrazonyl derivative in nonpolar solvents, which gradually decreased along with increasing solvent polarity, as opposed to the consistently reduced values obtained for the N,N-diphenylhydrazonyl derivative of up to ca. 0.20 in nonpolar solvents. These two push–pull molecules are contrasted with a structurally similar acceptor-π bridge-acceptor (A-π-A) compound.

Phototoxicity of BODIPY in long-term imaging can be reduced by intramolecular motion

For long-term live-cell fluorescence imaging and biosensing, it is crucial to work with a dye that has high fluorescence quantum yield and photostability without being detrimental to the cells. In this paper, we demonstrate that neutral boron-dipyrromethene (BODIPY)-based molecular rotors have great properties for high-light-dosage demanding live-cell fluorescence imaging applications that require repetitive illuminations. In molecular rotors, an intramolecular rotation (IMR) allows an alternative route for the decay of the singlet excited state (S₁) via the formation of an intramolecular charge transfer state (CT). The occurrence of IMR reduces the probability of the formation of a triplet state (T₁) which could further react with molecular oxygen (³O₂) to form cytotoxic reactive oxygen species, e.g., singlet oxygen (¹O₂). We demonstrate that the oxygen-related nature of the phototoxicity for BODIPY derivatives can be significantly reduced if a neutral molecular rotor is used as a probe. The studied neutral molecular rotor probe shows remarkably lower phototoxicity when compared with both the non-rotating BODIPY derivative and the cationic BODIPY-based molecular rotor in different light dosages and dye concentrations. It is also evident that the charge and localization of the fluorescent probe are as significant as the IMR in terms of the phototoxicity in a long-term live-cell imaging.

Environmentally sensitive fluorescent nucleoside analogues as probes for nucleic acid - protein interactions: molecular design and biosensing applications

Fluorescent nucleoside analogues (FNAs) are indispensable in studying the interactions of nucleic acids with nucleic acid-binding proteins. By replacing one of the poorly emissive natural nucleosides, FNAs enable real-time optical monitoring of the binding interactions in solutions, under physiologically relevant conditions, with high sensitivity. Besides that, FNAs are widely used to probe conformational dynamics of biomolecular complexes using time-resolved fluorescence methods. Because of that, FNAs are tools of high utility for fundamental biological research, with potential applications in molecular diagnostics and drug discovery. Here I review the structural and physical factors that can be used for the conversion of the molecular binding events into a detectable fluorescence output. Typical environmentally sensitive FNAs, their properties and applications, and future challenges in the field are discussed.

A Novel fluorescent probe with large Stokes shift for the detection of viscosity changes and its imaging in living cells

As an important cellular micro environmental parameter, viscosity could reflect the status of living cells. Small molecular fluorescent probe is a vital tool to measure the change of viscosity in living cells. A novel fluorescence probe ZL-1 with a large Stokes shift (in methanol it reached to 153 nm and in glycerol it reached to 125 nm) and excellent sensitivity toward viscosity was developed. The sharp enhancement of the emission intensity for the probe ZL-1 from low viscous methanol to high viscous glycerol indicated the probe ZL-1 could be respond to the viscosity variations. Moreover, the probe ZL-1 has been successfully utilized to detect of viscosity variations in living cells.