Fluorescent Molecular Rotors for Viscosity Sensors

Characterization, Stability, and Antibrowning Effects of Oxyresveratrol Cyclodextrin Complexes Combined Use of Hydroxypropyl Methylcellulose

Oxyresveratrol (Oxy) has attracted much attention by employing it as an antibrowning agent in fruits and vegetables. In this study, the formation of cyclodextrin (CD) inclusion exhibited a certain protective effect on Oxy oxidative degradation, while hydroxypropyl-β-cyclodextrin (HP–β-CD) inclusion complex showed stronger stabilizing effects than those of β-cyclodextrin (β-CD). The combined use of CD and hydroxypropyl methylcellulose (HPMC) greatly improved the stability of Oxy–CD inclusion complexes, with approximately 70% of the trans-Oxy retained after 30 days of storage under light conditions at 25 °C. The results of the interaction between CD and Oxy determined by phase solubility studies and fluorescence spectroscopic analysis showed that the binding strength of CD and Oxy increased in the presence of HPMC. Moreover, Oxy combined with ascorbic acid and HPMC showed an excellent antibrowning effect on fresh-cut apple slices during the 48 h test period, indicating that adding HPMC as the third component will not influence the antibrowning activity of Oxy.

Lanthanide porphyrinoids as molecular theranostics

In recent years, lanthanide (Ln) porphyrinoids have received increasing attention as theranostics. Broadly speaking, the term 'theranostics' refers to agents designed to allow both disease diagnosis and therapeutic intervention. This Review summarises the history and the 'state-of-the-art' development of Ln porphyrinoids as theranostic agents. The emphasis is on the progress made within the past decade. Applications of Ln porphyrinoids in near-infrared (NIR, 650-1700 nm) fluorescence imaging (FL), magnetic resonance imaging (MRI), radiotherapy, and chemotherapy will be discussed. The use of Ln porphyrinoids as photo-activated agents ('phototheranostics') will also be highlighted in the context of three promising strategies for regulation of porphyrinic triplet energy dissipation pathways, namely: regioisomeric effects, metal regulation, and the use of expanded porphyrinoids. The goal of this Review is to showcase some of the ongoing efforts being made to optimise Ln porphyrinoids as theranostics and as phototheranostics, in order to provide a platform for understanding likely future developments in the area, including those associated with structure-based innovations, functional improvements, and emerging biological activation strategies.

Real-Time and In Situ Viscosity Monitoring in Industrial Adhesives Using Luminescent Cu(I) Phenanthroline Molecular Sensors

Monitoring the viscosity of polymers in real-time remains a challenge, especially in confined environments where traditional rheological measurements are hard to apply. In this study, we have utilized the luminescent complex [Cu(diptmp)2]+ (diptmp = 2,9-diisopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline) as an optical probe for real-time sensing of viscosity in various adhesives during the curing process (viscosity increases). The emission lifetime of the triplet metal-to-ligand charge transfer (3MLCT) state of [Cu(diptmp)2]+ in epoxy adhesive increased exponentially during curing, similar to viscosity values obtained from oscillatory rheology. The longer lifetime in higher viscosity materials was attributed to changes in the excited-state deactivation processes from a known Jahn-Teller distortion in the Cu(I) geometry from tetrahedral in the ground state to square planar in the excited state. The real-time viscosity was also monitored reversibly by emission lifetime during polymer swelling (viscosity and lifetime decrease) and unswelling (viscosity and lifetime increase). Monitoring emission lifetime, unlike measuring the excited-state lifetime via transient absorption measurements in our previous study, allowed us to measure viscosity in opaque samples which scatter light. The optical probe [Cu(diptmp)2]+ in Gorilla Glue adhesive showed a clear correlation of the emission intensity or lifetime to viscosity during the curing process. We have also compared these lifetime changes using [Ru(bpy)3]2+ (bpy = bipyridine) as a control. [Cu(diptmp)2]+ showed not only a higher emission lifetime but also more ubiquity as a real-time viscosity sensor.

Push–Pull Derivatives Based on 2,4′-Biphenylene Linker with Quinoxaline, [1,2,5]Oxadiazolo[3,4-B]Pyrazine and [1,2,5]Thiadiazolo[3,4-B]Pyrazine Electron Withdrawing Parts

A series of novel V-shaped quinoxaline, [1,2,5]oxadiazolo[3,4-b]pyrazine and [1,2,5]thiadiazolo[3,4-b]pyrazine push–pull derivatives with 2,4′-biphenylene linker were designed and their electrochemical, photophysical and nonlinear optical properties were investigated. [1,2,5]Oxadiazolo[3,4-b]pyrazine is the stronger electron-withdrawing fragment as shown by electrochemical, and photophysical data. All compounds are emissive in a solid-state (from the cyan to red region of the spectrum) and quinoxaline derivatives are emissions in DCM solution. It has been found that quinoxaline derivatives demonstrate important solvatochromism and extra-large Stokes shifts, characteristic of twisted intramolecular charge transfer excited state as well as aggregation induced emission. The experimental conclusions have been justified by theoretical (TD-)DFT calculations.

Red fluorescent BODIPY molecular rotor for high microviscosity environments

Microviscosity has a strong impact for diffusion-controlled processes in biological environments. BODIPY molecular rotors are viscosity-sensitive fluorophores that provide a simple and non-invasive way to visualise microviscosity. Although green fluorescent probes are already well developed for imaging, thick biological samples require longer wavelengths for investigation. This work focuses on the examination of novelβ-substitutedmeso-phenyl-BODIPYs possessing a red emission. We report a new red fluorescent BODIPY-based probe BP-Vinyl-NO₂suitable for sensing microviscosity in rigid environments of over 100 000 cP viscosities. Furthermore, we demonstrate that changing the methyl position fromorthotometaon theβ-phenyl-substituted conjugate BP-PH-m2M-NO₂redshifts absorbance and fluorescence spectra while maintaining viscosity sensitivity. Finally, we show that nitro-substitution ofmeso-phenyl is a versatile approach to improve the sensitivity to viscosity while suppressing sensitivity to polarity and temperature of such derivatives. In summary, we present two nitro-substituted red fluorescent probes that could be used as lifetime-based microviscosity sensors.

Recent Advances of Fluorescence Probes for Imaging of Ferroptosis Process

Ferroptosis is an iron−dependent form of regulated cell death. It has attracted more and more research interests since it was found because of its potential physiological and pathological roles. In recent years, many efforts have been made for the developments and applications of selective fluorescence probes for real−time and in situ tracking of bioactive species during ferroptosis process, which is necessary and significant to further study the modulation mechanisms and pathological functions of ferroptosis. In this review, we will focus on summarizing the newly developed fluorescence probes that have been applied for ferroptosis imaging in the recent years, and comprehensively discussing their design strategies, including the probes for iron, reactive oxygen species, biothiols and intracellular microenvironmental factors.

Azobenzene photoisomerization probes cell membrane viscosity

The viscosity of cell membranes is a crucial parameter that affects the diffusion of small molecules both across and within the lipid membrane and that is related to several diseases. Therefore, the possibility to measure quantitatively membrane viscosity on the nanoscale is of great interest. Here, we report a complete investigation of the photophysics of an amphiphilic membrane-targeted azobenzene (ZIAPIN2) and we propose its use as a viscosity probe for cell membranes. We exploit ZIAPIN2 trans-cis photoisomerization to develop a molecular viscometer and to assess the viscosity of Escherichia coli bacteria membranes employing time-resolved fluorescence spectroscopy. Fluorescence lifetime measurements of ZIAPIN2 in E. coli bacteria suspensions correctly indicate that the membrane viscosity decreases as the temperature of the sample increases. Given the non-homogeneity and the anisotropy of cell membranes, as supported by the photophysical characterization of the probe within the lipid bilayer, we shed new light on the intricate membrane rheology.

Multifunctional Fluorescent Probe for Simultaneously Detecting Microviscosity, Micropolarity, and Carboxylesterases and Its Application in Bioimaging

Based on OR logic gate, we proposed a smart near-infrared (NIR) fluorescent probe, named VPCPP, for simultaneously monitoring local microviscosity, micropolarity, and carboxylesterases (CEs) in living cells through blue and red channels. This proposed probe was capable of distinguishing cancer cells from normal cells and had good potential for identifying living liver cell lines. Furthermore, the fluctuations of the three analytes of interest in different cell status was successfully explored. Particularly, facilitated with high-content analysis (HCA) and VPCPP, a simple and efficient high-throughput screening (HTS) platform was first constructed for screening antitumor drugs and studying their effect on the analytes. For the first time, we found that sorafenib-induced ferroptosis led to an increase in the microviscosity and up-regulation of CEs at the same time. Additionally, the procedure that aristolochic acid (AA) induced the overexpression of CEs was verified. Besides, VPCPP was utilized for imaging the variations of the two microenvironment parameters and CEs in the inflammation model. Finally, VPCPP was able to image the tumor ex vivo and in vivo through two channels and one channel separately, as well as to visualize the kidneys and liver ex vivo with dual emissions, which indicated that the probe had great potential for imaging applications such as medical diagnosis, preclinical research, and imaging-guided surgery.

8-Furylimidazolo-2′-deoxycytidine: crystal structure, packing, atropisomerism and fluorescence

8-Furylimidazolo-2′-deoxycytidine (furImidC), C14H14N4O5, is a fluorescent analogue of 2′-deoxycytidine, also displaying the same recognition face. As a constituent of DNA, furImidC forms extraordinarily strong silver-mediated self-pairs. Crystal structure determination revealed that furImidC adopts two types of disordered residues: the sugar unit and the furyl moiety. The disorder of the sugar residue amounts to an 87:13 split. The disorder of the furyl ring results from axial chirality at the C8—C2′′ bond connecting the nucleobase to the heterocycle. The two atropisomers are present in unequal proportions [occupancies of 0.69 (2) and 0.31 (2)], and the nucleobase and the furyl moiety are coplanar. Considering the atomic sites with predominant occupancy, an anti conformation with χ = − 147.2 (7)° was found at the glycosylic bond and the 2′-deoxyribosyl moiety shows a C2′-endo (S, 2 T 1) conformation, with P = 160.0°. A 1H NMR-based conformational analysis of the furanose puckering revealed that the S conformation predominates also in solution. In the solid state, two neighbouring furImidC molecules are arranged in a head-to-tail fashion, but with a notable tilt of the molecules with respect to each other. Consequently, one N—H...N hydrogen bond is found for neighbouring molecules within one layer, while a second N—H...N hydrogen bond is formed to a molecule of an adjacent layer. In addition, hydrogen bonding is observed between the nucleobase and the sugar residue. A Hirshfeld surface analysis was performed to visualize the intermolecular interactions observed in the X-ray study. In addition, the fluorescence spectra of furImidC were measured in solvents of different polarity and viscosity. furImidC responds to microenvironmental changes (polarity and viscosity), which is explained by a hindered rotation of the furyl residue in solvents of high viscosity.

Novel lysosome-targeted fluorescent molecular rotors based on a cyanine-like modular system and their application in living cells

Two novel fluorescence molecular rotors DpIn and NaIn were designed and synthesized involving of indolium units linked with meta-diphenol or ortha-naphthalenediol moiety, respectively. They underwent intramolecular charge transfer to form a cyanine-like modular system at a physiological pH. In glycerol aqueous solutions, the probe DpIn exhibited NIR strong emission (3-fold) at ca. 700 nm, while the probe NaIn displayed a turn-on emission (8-fold) with a larger Stokes shift (⊿λ ≈ 97 nm). The HeLa cell imaging experiments indicated probe DpIn and NaIn both exhibited excellent selectivity for staining intracellular lysosomes instead of mitochondria. ¹H NMR spectra revealed that more electrons were accumulated around benzene ring of indolium groups, which could be the evidence for its basic character leading to the lysosomes targeted staining. Furthermore, the probe NaIn proved to be an ideal lysosome-targeting tracer for monitor the changes of viscosity caused by stimuli in living cells.

Effects of ms-aryl substitution on the structure and spectral properties of new CH(Ar)-bis(BODIPY) luminophores

In this article, we present synthesis, spectral characteristics, and results of DFT calculations of new CH(R)-bis(BODIPY) 1-3. They are characterized by the conformational mobility and sensitivity of fluorescence to polarity, proton-, electron donor ability and viscosity of the solvation environment. It is shown that fluorescence intensity of 1-3 increases in the homologous series of alcohols (ethanol, 1-propanol, 1-butanol, 1-octanol, 1-decanol) mainly due to decrease of medium acidic properties. The viscosity of the medium effects on the 1-3 fluorescence in a lesser degree. Compared to 1 and 2, the 3 is the most sensitive towards viscosity both in low-viscosity homologous alcohols and in high-viscosity ethanol-glycerol mixtures. In this regard, the sensitivity of fluorescence of CH(MeOPh)-bis(BODIPY) (compound 3) to the viscosity was studied in binary mixtures of polar DMF and low-polarity toluene with castor and vaseline oils, as well as to the macroviscosity of the solvate environment in mixtures of toluene with polystyrene. Prospects of the practical application of CH(R)-bis(BODIPY)s are proposed for the analysis of polarity, proton-donor properties and viscosity of the medium.

Study of a carbazole–bromobenzothiadiazole derived fluorescent molecular rotor: crystal structure, redox activity, and solvatofluorochromic effects

A carbazole–benzothiadiazole based molecular rotor was systematically examined by experimental and modelling studies. The results disclosed interesting amphoteric redox activity and TICT emission that is sensitive to solvent polarity and viscosity.

A porphyrin-triazatruxene dyad for ratiometric two-photon fluorescent sensing of intracellular viscosity

By combining an electron-rich triazatruxene unit (TAT) to an electron-deficient zinc porphyrin fluorophore (ZnPor) via an ethynyl bridge, a new two-photon fluorescent viscosity rotor (TAT-ZnPor) with typical donor-π-acceptor (D-π-A) electronic...

Fluorescence Modulation by Ultrafast Chromophore Twisting Events: Developing a Powerful Toolset for Fluorescent-Protein-Based Imaging

The advancement of modern life sciences has benefited tremendously from the discovery and development of fluorescent proteins (FPs), widely expressed in live cells to track a myriad of cellular events. The chromophores of various FPs can undergo many ultrafast photophysical and/or photochemical processes in the electronic excited state and emit fluorescence with different colors. However, the chromophore becomes essentially nonfluorescent in solution environment due to its intrinsic twisting capability upon photoexcitation. To study "microscopic" torsional events and their effects on "macroscopic" fluorescence, we have developed an integrated ultrafast characterization platform involving femtosecond transient absorption (fs-TA) and wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS). A wide range of naturally occurring, circularly permuted, non-canonical amino-acid-decorated FPs and FP-based optical highlighters with photochromicity, photoconversion, and/or photoswitching capabilities have been recently investigated in great detail. Twisting conformational motions were elucidated to exist in all of these systems but to various extents. The associated different ultrafast pathways can be monitored via frequency changes of characteristic Raman bands during primary events and functional processes. The mapped electronic and structural dynamics information is crucial and has shown great potential and initial success for the rational design of proteins and other photoreceptors with novel functions and fluorescence properties.

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.

Fluorescence-based sensing of the bioenergetic and physicochemical status of the cell

Fluorescence-based sensors play a fundamental role in biological research. These sensors can be based on fluorescent proteins, fluorescent probes or they can be hybrid systems. The availability of a very large dataset of fluorescent molecules, both genetically encoded and synthetically produced, together with the structural insights on many sensing domains, allowed to rationally design a high variety of sensors, capable of monitoring both molecular and global changes in living cells or in in vitro systems. The advancements in the fluorescence-imaging field helped researchers to obtain a deeper understanding of how and where specific changes occur in a cell or in vitro by combining the readout of the fluorescent sensors with the spatial information provided by fluorescent microscopy techniques. In this review we give an overview of the state of the art in the field of fluorescent biosensors and fluorescence imaging techniques, and eventually guide the reader through the choice of the best combination of fluorescent tools and techniques to answer specific biological questions. We particularly focus on sensors for probing the bioenergetics and physicochemical status of the cell.

Visual detection of viscosity through activatable molecular rotor with aggregation-induced emission

Food safety has become one of the urgent affairs in the global public health studies, and irregular viscosity is closely associated with the food spoilage extent. In this study, one kind of activatable molecular rotor (TPA-PBZ) based on triphenylamine derivates has been synthesized via the Schiff base condensation reaction. This rotor is comprised by donor-accepter conjugated structure, with aggregation induced-emission feature and a large Stokes shift of 160 nm in water. The rotation of aromatic rings in TPA-PBZ is restricted in high-viscosity microenvironment, with the gradually increasing fluorescence emission signal at 568 nm. Significantly, this rotor TPA-PBZ has successfully been applied not only in the determination of thickening effects of food gum, but also in the detection of viscosity enhancement during the liquid food spoilage process. This molecular rotor can be utilized as an intelligent monitor platform for food quality and safety inspection in viscosity-related conditions.

Aggregation induced emission (AIE) molecules for measurement of intracellular temperature, pH, and viscosity sensing

This book chapter presents insightful growth and progress in the field of sensing especially, temperature, pH, and viscosity sensing. We focus more on aggregation-induced emission (AIE)-active materials for measuring intracellular pH, viscosity, and temperature by means of fluorescence and absorption study. A special emphasis is given on AIE active fluorescent molecules, molecular rotors, polymeric nanomaterials which are considered as the important aspects of sense. It also gives the fundamental and brief understanding between these different AIE active material and its application in biological systems.

Molecular rotors as reporters for viscosity of solutions of collagen like peptides

We have studied the suitability of using a molecular rotor-based steady-state fluorometric assay for evaluating changes in both the conformation and the viscosity of collagen-like peptide solutions. Our results indicate that a positive charge incorporated on the hydrophobic tail of the BODIPY molecular rotor favours the dye specificity as a reporter for viscosity of these solutions.

Designing a Red-Emitting Viscosity-Sensitive BODIPY Fluorophore for Intracellular Viscosity Imaging

Viscosity imaging at a microscopic scale can provide important information about biosystems, including the development of serious illnesses. Microviscosity imaging is achievable with viscosity-sensitive fluorophores, the most popular of which are based on the BODIPY group. However, most of the BODIPY probes fluoresce green light, whereas the red luminescence is desired for the imaging of biological samples. Designing a new viscosity probe with suitable spectroscopic properties is a challenging task because it is difficult to preserve viscosity sensitivity after modifying the molecular structure. Here we describe how we developed a new red-emitting, viscosity-sensitive, BODIPY fluorophore BP-PH-2M-NO₂ that is suitable for reliable intracellular viscosity imaging of lipid droplets in MCF-7 breast cancer cells. The design of BP-PH-2M-NO₂ was aided by DFT calculations that allowed a successful prediction of the viscosity sensitivity of fluorophores before synthesis. In summary, we report a new red viscosity probe possessing monoexponential fluorescence decay that makes it attractive for lifetime-based viscosity imaging.

Screening of dicyanoisophorone-based probes for highly sensitive detection of viscosity changes in living cells and zebrafish

Herein, seven viscosity-sensitive probes were developed via simple structural modification of dicyanoisophorone (DCO)-derived dyes. Among them, DCO-5 significantly enhances (180-fold) the response signal in highly viscous aqueous media while showing insensitivity to polarity changes or pH variations, and enables the successful detection of viscosity changes in nystatin-treated HepG2 cells, PC 12 cells and zebrafish.

A prostate-specific membrane antigen activated molecular rotor for real-time fluorescence imaging

Surgery is an efficient way to treat localized prostate cancer (PCa), however, it is challenging to demarcate rapidly and accurately the tumor boundary intraoperatively, as existing tumor detection methods are seldom performed in real-time. To overcome those limitations, we develop a fluorescent molecular rotor that specifically targets the prostate-specific membrane antigen (PSMA), an established marker for PCa. The probes have picomolar affinity (IC50 = 63-118 pM) for PSMA and generate virtually instantaneous onset of robust fluorescent signal proportional to the concentration of the PSMA-probe complex. In vitro and ex vivo experiments using PCa cell lines and clinical samples, respectively, indicate the utility of the probe for biomedical applications, including real-time monitoring of endocytosis and tumor staging. Experiments performed in a PCa xenograft model reveal suitability of the probe for imaging applications in vivo.

Rigidochromism by imide functionalisation of an aminomaleimide fluorophore

Fluorescent dyes that exhibit high solid state quantum yields and sensitivity to the mechanical properties of their local environment are useful for a wide variety of applications, but are limited in chemical diversity. We report a trityl-functionalised maleimide that displays rigidochromic behaviour, becoming highly fluorescent when immobilised in a solid matrix, while displaying negligible fluorescence in solution. Furthermore, the dye's quantum yield is shown to be sensitive to the nature of the surrounding matrix. Computational studies reveal that this behaviour arises from the precise tuning of inter- and intramolecular noncovalent interactions. This work expands the diversity of molecules exhibiting solid state environment sensitivity, and provides important fundamental insights into their design.

Liquid-liquid phase separation of Tau by self and complex coacervation

The liquid-liquid phase separation (LLPS) of Tau has been postulated to play a role in modulating the aggregation property of Tau, a process known to be critically associated with the pathology of a broad range of neurodegenerative diseases including Alzheimer's Disease. Tau can undergo LLPS by homotypic interaction through self-coacervation (SC) or by heterotypic association through complex-coacervation (CC) between Tau and binding partners such as RNA. What is unclear is in what way the formation mechanisms for self and complex coacervation of Tau are similar or different, and the addition of a binding partner to Tau alters the properties of LLPS and Tau. A combination of in vitro experimental and computational study reveals that the primary driving force for both Tau CC and SC is electrostatic interactions between Tau-RNA or Tau-Tau macromolecules. The liquid condensates formed by the complex coacervation of Tau and RNA have distinctly higher micro-viscosity and greater thermal stability than that formed by the SC of Tau. Our study shows that subtle changes in solution conditions, including molecular crowding and the presence of binding partners, can lead to the formation of different types of Tau condensates with distinct micro-viscosity that can coexist as persistent and immiscible entities in solution. We speculate that the formation, rheological properties and stability of Tau droplets can be readily tuned by cellular factors, and that liquid condensation of Tau can alter the conformational equilibrium of Tau.

Monitoring of the decreased mitochondrial viscosity during heat stroke with a mitochondrial AIE probe

Heat stroke is a fatal condition which usually results in central nervous system dysfunction, organism damage and even death. The relationship between heat stroke and mitochondria is still relatively unknown due to a lack of suitable tools. Herein, an aggregation-induced emission (AIE) probe CSP, by introducing a pyridinium cation as the mitochondria-targeted group to an AIE active core cyanostilbene skeleton, is highly sensitive to viscosity changes due to the restriction of intramolecular motion (RIM) and inhibition of twisted intramolecular charge transfer (TICT) in high-viscosity systems. As expected, with the viscosity increasing from 0.903 cP (0% glycerol) to 965 cP (99% glycerol), CSP exhibited a significant enhancement (more than 117-fold) in fluorescence intensity at 625 nm, with an excellent linear relationship between log I _{625 nm} and log η (R² = 0.9869, slope as high as 0.6727). More importantly, using CSP we have successfully monitored the decreased mitochondrial viscosity during heat stroke for the first time. All these features render the probe a promising candidate for further understanding the mechanism underlying mitochondria-associated heat stroke.

Cell surface-localized imaging and sensing

Systematically dissecting the molecular basis of the cell surface as well as its related biological activities is considered as one of the most cutting-edge fields in fundamental sciences. The advent of various advanced cell imaging techniques allows us to gain a glimpse of how the cell surface is structured and coordinated with other cellular components to respond to intracellular signals and environmental stimuli. Nowadays, cell surface-related studies have entered a new era featured by a redirected aim of not just understanding but artificially manipulating/remodeling the cell surface properties. To meet this goal, biologists and chemists are intensely engaged in developing more maneuverable cell surface labeling strategies by exploiting the cell's intrinsic biosynthetic machinery or direct chemical/physical binding methods for imaging, sensing, and biomedical applications. In this review, we summarize the recent advances that focus on the visualization of various cell surface structures/dynamics and accurate monitoring of the microenvironment of the cell surface. Future challenges and opportunities in these fields are discussed, and the importance of cell surface-based studies is highlighted.

Construction of fluorescent rotors with multiple intramolecular rotation sites for visualization of cellular viscous compartments with elevated fidelity

Viscosity-sensitive fluorescent dyes are widely utilized to image viscous intracellular compartments with high fidelity. However, the sensitivity of many fluorescent rotors needs improvement for bioimaging applications. Herein, we proposed to construct a fluorescent rotor with multiple intramolecular rotation sites to elevate its sensitivity to environmental viscosity. The fabricated fluorescent rotor showed evidently increased sensitivity to viscosity and had the potential to image intracellular viscous compartments with improved fidelity. By decorating the rotor with sidechains of different lengths, we successfully fabricated two fluorescent probes, TAPI-6 and TAPI-16, to visualize the mitochondria and plasma membrane, respectively, with high fidelity. The two probes were also successfully utilized to clearly visualize the mitochondria and plasma membranes in skeletal muscle tissue, cardiac muscle tissue, and liver tissue, demonstrating the potential of the fluorescent rotor for bioimaging applications. We believe that the strategy of increasing the sensitivity to viscosity using multiple rotation sites is valuable for the construction of fluorescent rotors, and the presented fluorescent probes in this work can serve as powerful tools for biological research.

A Xanthene Dye-based Sensor for Viscosity and Cell Imaging

A new xanthene dye, namely ImX, has been facilely prepared by reaction of 4-(1H-Imidazol-1-yl)benzaldehyde with N, N-diethyl-3-aminophenol in concentrated propionic acid, and then treated by p-chloranil. ImX presents the maximum absorption and emission band centered at 562 nm and 583 nm in water, respectively. Fluorescent spectra investigations demonstrate that ImX shows viscosity-selective fluorescent response and emission enhancement when the solvent viscosity increases from 1.1 cp. (water) to 1248 cp. (98 % glycerol). In addition, this viscosity-selective fluorescence response covers a wide pH range from 2.5 to 10.0. More significantly, ImX demonstrates low cytotoxicity and can be employed as tracer for the detection of Monensin-triggered viscosity enhancement by cell imaging.

Real-Time Monitoring Mitochondrial Viscosity during Mitophagy Using a Mitochondria-Immobilized Near-Infrared Aggregation-Induced Emission Probe

Mitophagy plays a crucial role in maintaining intracellular homeostasis through the removal of dysfunctional mitochondria and recycling their constituents in a lysosome-degradative pathway, which leads to microenvironmental changes within mitochondria, such as the pH, viscosity, and polarity. However, most of the mitochondrial fluorescence viscosity probes only rely on electrostatic attraction and readily leak out from the mitochondria during mitophagy with a decreased membrane potential, thus easily leading to an inaccurate detection of viscosity changes. In this work, we report a mitochondria-immobilized NIR-emissive aggregation-induced emission (AIE) probe CS-Py-BC, which allows for an off-on fluorescence response to viscosity, thus enabling the real-time monitoring viscosity variation during mitophagy. This system consists of a cyanostilbene skeleton as the AIE active core and viscosity-sensitive unit, a pyridinium cation for the mitochondria-targeting group, and a benzyl chloride subunit that induces mitochondrial immobilization. As the viscosity increased from 0.903 cP (0% glycerol) to 965 cP (99% glycerol), CS-Py-BC exhibited an about 92-fold increase in fluorescence intensity at 650 nm, which might be attributed to the restriction of rotation and inhibition of twisted intramolecular charge transfer in a high viscosity system. We also revealed that CS-Py-BC could be well immobilized onto mitochondria, regardless of the mitochondrial membrane potential fluctuation. Most importantly, using CS-Py-BC, we have successfully visualized the increased mitochondrial viscosity during starvation or rapamycin-induced mitophagy in real time. All these features render CS-Py-BC a promising candidate to investigate mitophagy-associated dynamic physiological and pathological processes.

Exchange Speed of Four-Component Nanorotors Correlates with Hammett Substituent Constants

Three distinct four-component supramolecular nanorotors were prepared, using, for the first time, bipyridine instead of phenanthroline stations in the stator. Following our established self-sorting protocol to multicomponent nanodevices, the nanorotors were self-assembled by mixing the stator, rotators with various pyridine head groups, copper(I) ions and 1,4-diazabicyclo[2.2.2]octane (DABCO). Whereas the exchange of a phenanthroline vs. a bipyridine station did not entail significant changes in the rotational exchange frequency, the para-substituents at the pyridine head group of the rotator had drastic consequences on the speed: 4-OMe (k298 = 35 kHz), 4-H (k298 = 77 kHz) and 4-NO2 (k298 = 843 kHz). The exchange frequency (log k) showed an excellent linear correlation with both the Hammett substituent constants and log K of the copper(I)–ligand interaction, proving that rotator–copper(I) bond cleavage is the key determining factor in the rate-determining step.

Liquid-Liquid Phase Separation of Tau Driven by Hydrophobic Interaction Facilitates Fibrillization of Tau

Amyloid aggregation of tau protein is implicated in neurodegenerative diseases, yet its facilitating factors are poorly understood. Recently, tau has been shown to undergo liquid liquid phase separation (LLPS) both in vivo and in vitro. LLPS was shown to facilitate tau amyloid aggregation in certain cases, while being independent of aggregation in other cases. It is therefore important to understand the differentiating properties that resolve this apparent conflict. We report on a model system of hydrophobically driven LLPS induced by high salt concentration (LLPS-HS), and compare it to electrostatically driven LLPS represented by tau-RNA/heparin complex coacervation (LLPS-ED). We show that LLPS-HS promotes tau protein dehydration, undergoes maturation and directly leads to canonical tau fibrils, while LLPS-ED is reversible, remains hydrated and does not promote amyloid aggregation. We show that the nature of the interaction driving tau condensation is a differentiating factor between aggregation-prone and aggregation-independent LLPS.

Photophysics of Fluorescent Contact Sensors Based on the Dicyanodihydrofuran Motif

Fluorescent molecular rotors have been used for measurements of local mobility on molecular length scales, for example to determine viscosity, and for the visualization of contact between two surfaces. In the present work, we deepen our insight into the excited-state deactivation kinetics and mechanics of dicyanodihydrofuran-based molecular rotors. We extend the scope of the use of this class of rotors for contact sensing with a red-shifted member of the family. This allows for contact detection with a range of excitation wavelengths up to ∼600 nm. Steady-state fluorescence shows that the fluorescence quantum yield of these rotors depends not only on the rigidity of their environment, but - under certain conditions - also on its polarity. While excited state decay via rotation about the exocyclic double bond is rapid in nonpolar solvents and twisting of a single bond allows for fast decay in polar solvents, the barriers for both processes are significant in solvents of intermediate polarity. This effect may also occur in other molecular rotors, and it should be considered when applying such molecules as local mobility probes.

Two-photon fluorescent probes for detecting the viscosity of lipid droplets and its application in living cells

Lipid droplets (LDs) are storage organelles at the centre of lipid and energy homeostasis, which act as vital hubs of cellular metabolism and the key to maintaining lipid and energy homeostasis. We synthesized a new two-photon fluorescent probe (CIV) that could detect the viscosity of lipid droplets. The probe is constructed via the typical ICT system of D–π–A using carbazole as the donor and imidazole as the acceptor. With the increase in viscosity from PBS to 99% glycerol, the fluorescence intensity of CIV increased by 13-fold, showing sensitivity and specificity towards viscosity. In addition, CIV showed low toxicity and excellent biocompatibility in cytotoxicity tests, and was successfully used for living cell LD imaging. Taken together, the results widen the way for the development of novel fluorescent probe-based the visualization LDs and detection in solutions, physiology and pathology.

A novel two-photon fluorescence probe (CIV) can detect the viscosity and locate lipid droplets in living cells.

Molecular Rotors with Aggregation-Induced Emission (AIE) as Fluorescent Probes for the Control of Polyurethane Synthesis

In this work, the use of fluorescent molecular rotors such as 9-(2,2-dicyanovinyl)julolidine (DCVJ) and 2,3-bis(4-(phenyl(4-(1,2,2-triphenylvinyl) phenyl)amino)phenyl)fumaronitrile (TPETPAFN) was proposed for the real-time monitoring of polyurethane (PU) formation in a solution of dimethylacetamide starting with 4,4′-methylenediphenyl diisocyanate (MDI) and different polyethylene glycols (PEG400 and PEG600) as diols. Notably, relative viscosity variations were compared with fluorescence changes, recorded as a function of the polymerization progress. The agreement between these two parameters suggested the innovative use of a low-cost fluorescence detection system based on a LED/photodiode assembly directly mountable on the reaction apparatus. The general validity of the proposed experiments enabled the monitoring of polyurethane polymerization and suggested its effective applications to a variety of industrial polymers, showing viscosity enhancement during polymerization.

Electrochemical and Solvent-Mediated Visible-to-Near-Infrared Spectroscopic Switching of Benzoselenadiazole Fluorophores

A series of electronic push-pull, pull-pull, and push fluorophores has been prepared from a benzoselenadiazole core so that their spectroscopic, electrochemical, spectro-electrochemical, and spectro-electrofluorescence properties could be examined. The emission wavelengths and fluorescence quantum yields (Φ_fl ) of the N,N-dimethyl fluorophores were contingent on the solvent polarity and they ranged from 615 to 850 nm in aprotic solvents. The positive solvatochromism and the quenched Φ_fl in polar solvents were consistent with an intramolecular charge-transfer state (ICT). Meanwhile, a locally excited state (LE) was assigned in nonpolar solvents from the blue-shifted emission and high Φ_fl . The N,N-dimethylamine fluorophores examined could be both electrochemically oxidized and reduced, whereas the symmetric dinitro pull-pull derivative could be only reversibly reduced. Courtesy of their electrochemical reversibility, the fluorophores could reversibly change color from yellow to blue with an applied potential in addition to switching off their emission. The absorption of the electrochemically generated intermediates of the N,N-dimethyl derivatives spanned 500 nm over the visible and the NIR regions. The colors could be switched for upwards of two hours with applied potential, illustrating their potential use as electroactive materials in electrochromic devices.

Molecular rotor as a structural probe of glucan polymers: Amylopectin, phytoglycogen, and their β-limit dextrins as models

Fluorescence emissions of molecular rotors (MRs) are affected by local restrictions to molecular motion, and therefore it was considered that MRs can be used as structural probes of biopolymers. In this study, 9-(2-carboxy-2-cyanovinyl)-julolidine (CCVJ), a hydrophilic MR, was used to differentiate branched α-D-glucans, including amylopectin, phytoglycogen, and their β-limit dextrins. CCVJ emissions of glucan dispersions were correlated with dispersion viscosities and glucan branch structures. In diluted glucan dispersions, CCVJ emission showed essentially linear correlation with glucan content. In concentrated glucan dispersions, CCVJ emission correlated with viscosity in a double-logarithmic linear pattern, with phytoglycogen showing much greater sensitivities than amylopectin. In the plots of CCVJ emission vs. molar amount of branch, phytoglycogen materials showed greater slopes than their amylopectin counterparts, suggesting evident effects of branch structure on the restrictions to CCVJ molecules. Overall, CCVJ has demonstrated its fluorescent sensitivity with glucans, showing strong potentials as a structural probe of biopolymers.

A fluoran-based viscosity probe with high-performance for lysosome-targeted fluorescence imaging

A new fluorescent probe Lyso-Fl has been facilely prepared by an esterification reaction of spironolactone fluoran dye Rdi with ethanol, which shows viscosity-selective response by fluorescence. The new probe delivers obvious fluorescence signal enhancement when environmental viscosity changes from 1.01 cP (water) to 1256 cP (98% glycerol). And, both the emission intensity (575 nm) and fluorescence lifetime of Lyso-Fl exhibit individually good linear relationships with the solution viscosity. Besides, Lyso-Fl gives a selective response to viscosity among various biological species and exhibits pH-independent (1-10) fluorescent signals towards viscosity. More importantly, Lyso-Fl shows low cytotoxicity and can be utilized for monitoring of dexamethasone-stimulated viscosity enhancement by cell imaging with excellent lysosome-targeted performance, promoting it a promising fluorescent probe for lysosomal viscosity detection.

Development of alkyne-BODIPYs as viscosity sensitive fluorescent probes for enumeration of bacterial cells

We report a series of alkyne-functionalized meso-aryl boron dipyrrin (BODIPY) molecular rotors sensitive to viscosity. The planar and twisted conformation within the molecular structure decides the viscosity-dependent behavior. The variations in fluorescence lifetime and intensity were appreciable to the local viscosity. Hence, the dye has been successfully employed in the enumeration of microbes by considering the proportionate fluorescence intensity of the BODIPYs as an index of the number of cells per mL. With increasing cells per mL, the viscosity of the bacterial solution is increased. Consequently, the fluorescence intensity of the sample containing BODIPY tends to increase due to the restricted rotation in the viscous medium. The BODIPY probe offers high sensitivity and is easier than other conventional techniques of colony-forming unit (CFU) determination. The theoretical studies indicate that intramolecular charge transfer is responsible for the enhanced fluorescence intensity in a highly viscous solvent.