Synthesis and Characterization of Sulfonamide-Containing Naphthalimides as Fluorescent Probes

A tumor-targeting fluorescent probe has attracted increasing interest in fluorescent imaging for the noninvasive detection of cancers in recent years. Sulfonamide-containing naphthalimide derivatives (SN-2NI, SD-NI) were synthesized by the incorporation of N-butyl-4-ethyldiamino-1,8-naphthalene imide (NI) into sulfonamide (SN) and sulfadiazine (SD) as the tumor-targeting groups, respectively. These derivatives were further characterized by mass spectrometry (MS), nuclear magnetic resonance spectroscopy (1H NMR), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV), and a fluorescence assay. In vitro properties, including cell cytotoxicity and the cell uptake of tumor cells, were also evaluated. Sulfonamide-containing naphthalimide derivatives possessed low cell cytotoxicity to B16F10 melanoma cells. Moreover, SN-2NI and SD-NI can be taken up highly by B16F10 cells and then achieve good green fluorescent images in B16F10 cells. Therefore, sulfonamide-containing naphthalimide derivatives can be considered to be the potential probes used to target fluorescent imaging in tumors.

Supramolecular Control of the Temperature Responsiveness of Fluorescent Macrocyclic Molecular Rotamers

To fully harness the potential of molecular machines, it is crucial to develop methods by which to exert control over their speed of motion through the application of external stimuli. A conformationally strained macrocyclic fluorescent rotamer, CarROT, displays a reproducible and linear fluorescence decrease towards temperature over the physiological temperature range. Through the external addition of anions, cations or through deprotonation, the compound can access four discreet rotational speeds via supramolecular interactions (very slow, slow, fast and very fast) which in turn stop, reduce or enhance the thermoluminescent properties due to increasing or decreasing non-radiative decay processes, thereby providing a means to externally control the temperature sensitivity of the system. Through comparison with analogues with a higher degree of conformational freedom, the high thermosensitivity of CarROT over the physiological temperature range was determined to be due to conformational strain, which causes a high energy barrier to rotation over this range. Analogues with a higher degree of conformational freedom display lower sensitivities towards temperature over the same temperature range. This study provides an example of an information rich small molecule, in which programable rotational speed states can be observed with facile read-out.

Exquisite visualization of mitophagy and monitoring the increase of lysosomal micro-viscosity in mitophagy with an unusual pH-independent lysosomal rotor

BACKGROUND

Mitophagy plays indispensable roles in maintaining intracellular homeostasis in most eukaryotic cells by selectively eliminating superfluous components or damaged organelles. Thus, the co-operation of mitochondrial probes and lysosomal probes was presented to directly monitor mitophagy in dual colors. Nowadays, most of the lysosomal probes are composed of groups sensitive to pH, such as morpholine, amine and other weak bases. However, the pH in lysosomes would fluctuate in the process of mitophagy, leading to the optical interference. Thus, it is crucial to develop a pH-insensitive probe to overcome this tough problem to achieve exquisite visualization of mitophagy.

RESULTS

In this study, we rationally prepared a pH-independent lysosome probe to reduce the optical interference in mitophagy, and thus the process of mitophagy could be directly monitored in dual color through cooperation between IVDI and MTR, depending on Förster resonance energy transfer mechanism. IVDI shows remarkable fluorescence enhancement toward the increase of viscosity, and the fluorescence barely changes when pH varies. Due to the sensitivity to viscosity, the probe can visualize micro-viscosity alterations in lysosomes without washing procedures, and it showed better imaging properties than LTR. Thanks to the inertia of IVDI to pH, IVDI can exquisitely monitor mitophagy with MTR by FRET mechanism despite the changes of lysosomal pH in mitophagy, and the reduced fluorescence intensity ratio of green and red channels can indicate the occurrence of mitophagy. Based on the properties mentioned above, the real-time increase of micro-viscosity in lysosomes during mitophagy was exquisitely monitored through employing IVDI.

SIGNIFICANCE AND NOVELTY

Compared with the lysosomal fluorescent probes sensitive to pH, the pH-inert probe could reduce the influence of pH variation during mitophagy to achieve exquisite visualization of mitophagy in real-time. Besides, the probe could monitor the increase of lysosomal micro-viscosity in mitophagy. So, the probe possesses tremendous potential in the visualization of dynamic changes related to lysosomes in various physiological processes.

Advances in peptide-based drug delivery systems

Drug delivery systems (DDSs) are designed to deliver drugs to their specific targets to minimize their toxic effects and improve their susceptibility to clearance during targeted transport. Peptides have high affinity, low immunogenicity, simple amino acid composition, and adjustable molecular size; therefore, most peptides can be coupled to drugs via linkers to form peptide-drug conjugates (PDCs) and act as active pro-drugs. PDCs are widely thought to be promising DDSs, given their ability to improve drug bio-compatibility and physiological stability. Peptide-based DDSs are often used to deliver therapeutic substances such as anti-cancer drugs and nucleic acid-based drugs, which not only slow the degradation rate of drugs in vivo but also ensure the drug concentration at the targeted site and prolong the half-life of drugs in vivo. This article provides an profile of the advancements and future development in functional peptide-based DDSs both domestically and internationally in recent years, in the expectation of achieving targeted drug delivery incorporating functional peptides and taking full advantage of synergistic effects.

Dynamic Motions of Ligands around the Metal Centers Afford a Fidget Spinner-Type AIE Luminogen

A new type of aggregation-induced emission (AIE) luminogen containing a dimeric metal fragment and two or three phthalazine ligands is described, which shows dynamic motions of ligands around the metal centers in solution. Based on the variable-temperature and EXSY NMR spectroscopy data, X-ray crystallography structures, and computational results, three different pathways (i.e., reversible exchange with haptotropic shifts, circulation of ligands around the dimeric metal fragment, and walking on the spot of ligands on the metal centers) were considered for this dynamic behavior. Restriction of these dynamic processes in the aggregate forms of the compounds (in H2O/CH3CN solvent mixtures) contributes to their AIE. DFT calculations and NMR analysis showed that bright excited states for these molecules are not localized on isolated molecules, and the emission of them stemmed from π-dimers or π-oligomers. The morphologies and the mode of associations in the solvent mixtures were determined by using transmission electron microscopy (TEM) and concentration-dependent NMR spectroscopy. The computational results showed the presence of a conical intersection (CI) between the S0 and S1 excited state, which provides an accessible pathway for nonradiative decay in these systems.

Oxidized Low-Density Lipoprotein (Ox-LDL)-Triggered Double-Lock Probe for Spatiotemporal Lipoprotein Oxidation and Atherosclerotic Plaque Imaging

Low-density lipoprotein (LDL), especially oxidative modified LDL (Ox-LDL), is the key risk factor for plaque accumulation and the development of cardiovascular disease. Herein, a highly specific Ox-LDL-triggered fluorogenic-colorimetric probe Pro-P1 is developed for visualizing the oxidation and aggregation progress of lipoproteins and plaque. A series of green fluorescent protein chromophores with modified donor-acceptor structures, containing carbazole as an electron donor and various substituents including pyridine-vinyl (P1), phenol-vinyl (P2), N, N-dimethylaniline-vinyl (P3), and thiophene-vinyl (P4), have been synthesized and evaluated. Emission spectroscopy and theoretical calculations of P1-P4 indicate that P1 shows enhanced green fluorescence (λem = 560 nm) by inhibiting its twisted intramolecular charge transfer in the presence of Ox-LDL. This feature allows the selection of P1 as a sensitive probe to directly visualize ferroptosis and Cu2+ -mediated LDL oxidative aggregation via in situ formation of fluorophore-bound Ox-LDL in living cells. The red-emissive probe Pro-P1 (λem = 660 nm) is prepared via borate protection of P1, which can be cleaved into P1 under high expression of HOCl and Ox-LDL condition at the lesion site, resulting in enhanced green emission. The plaque area and size with clear boundaries can be delineated by colorimetric fluorescence imaging and fluorescence lifetime imaging with precise differentiation.

Evaluation of intracellular lipid droplets viscosity by a probe with high fluorescence quantum yield

BACKGROUND

Lipid droplets (LDs) are an important organelle as the main energy storage site in cells. LDs viscosity controls the material and energy exchange between it and other organelles. Furthermore, the LDs metabolic abnormalities, cell dysfunction, some diseases may be attributed to the singular LDs viscosity. Currently, the fluorescent probes for sensing the variations of LDs viscosity are still scarce and expose some drawbacks of low fluorescence quantum yield, low sensitivity and LDs polarity interference. Thus, the development of high performance probes is significant to detect LDs viscosity.

RESULTS

We hereby provide a lipophilic fluorescent probe (TPE-BET) with high fluorescence quantum yield (Φf, 0.91 in glycerol) for imaging LDs viscosity in living cells. With the increase of viscosity from 0.54 cp to 934 cp, the fluorescence at λex/λem = 405/520 nm and the fluorescence quantum yield of TPE-BET linearly increased by 64.9 and 128.5 folds, respectively. Meanwhile, the outstanding LDs staining capability of TPE-BET may provide a high spatial resolution for LDs imaging. The cell imaging of TPE-BET not only successfully observed the viscosity variations of LDs in cell stress models, e.g., ferroptosis, inflammation and mitophagy, but also revealed the increased viscosity and extracellular delivery of LDs in heavy metal cell injury models (Hg/As) for the first time, which may supply concrete evidence for understanding the structure and function of LDs.

SIGNIFICANCE

This represents a new fluorescent probe TPE-BET with high fluorescence quantum yield for imaging LDs viscosity, which may decrease the dose of probe and excitation light intensity along with the improvement on signal noise ratio (S/N). The imaging results of TPE-BET clarified that LDs viscosity may be an appraisal index on cell differentiation, state evaluation and drug screening.

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

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

Multifunctional polysaccharide nanoprobes for biological imaging

Imaging and tracking biological targets or processes play an important role in revealing molecular mechanisms and disease states. Bioimaging via optical, nuclear, or magnetic resonance techniques enables high resolution, high sensitivity, and high depth imaging from the whole animal down to single cells via advanced functional nanoprobes. To overcome the limitations of single-modality imaging, multimodality nanoprobes have been engineered with a variety of imaging modalities and functionalities. Polysaccharides are sugar-containing bioactive polymers with superior biocompatibility, biodegradability, and solubility. The combination of polysaccharides with single or multiple contrast agents facilitates the development of novel nanoprobes with enhanced functions for biological imaging. Nanoprobes constructed with clinically applicable polysaccharides and contrast agents hold great potential for clinical translations. This review briefly introduces the basics of different imaging modalities and polysaccharides, then summarizes the recent progress of polysaccharide-based nanoprobes for biological imaging in various diseases, emphasizing bioimaging with optical, nuclear, and magnetic resonance techniques. The current issues and future directions regarding the development and applications of polysaccharide nanoprobes are further discussed.

Photoinduced charge transfer in push-pull pyrazoline-based chromophores - Relationship between molecular structure and photophysical, photovoltaic properties

The manuscript describes the effect of molecular structure on the photophysical and photovoltaic properties of the pyrazoline-based donor-branched-π-system-acceptor compounds decorated with two end groups: phenyl or thiophene. Although the absorption to the first singlet excited state is strongly allowed, the emission quantum yield is low in all studied solvents. This behaviour was explained by the existence of two non-radiative deactivation channels: the back electron transfer process, especially operated in polar solvents, and internal conversion realized as the rotation of flexible rotors (cyano, keto phenyl or thiophene). The feasibility of the photoinduced electron transfer process was corroborated by electrochemical, spectroelectrochemical measurements as well as DFT calculations. DFT calculations also support the existence of multiple conformations in the ground state, which differ from one another in terms of charge distribution and the values of ground state dipole moment. Finally, the mechanism of the singlet excited state deactivation of the studied compounds was determined by ultrafast pump-probe measurements. Our studies revealed that charge/electron transfer process may undergo over carbonyl bridge, included in branched π-system. Moreover, the thiophene decorated pyrazoline is characterized by a better photovoltaic power conversion efficiency, while the phenyl-ended pyrazoline can be applied as a viscosity sensor.

Recent Progress of Activity-Based Fluorescent Probes for Imaging Leucine Aminopeptidase

Leucine aminopeptidase (LAP) is an important protease that can specifically hydrolyze Leucine residues. LAP occurs in microorganisms, plants, animals, and humans and is involved in a variety of physiological processes in the human body. In the physiological system, abnormal levels of LAP are associated with a variety of diseases and pathological processes, such as cancer and drug-induced liver injury; thus, LAP was chosen as the early biochemical marker for many physiological processes, including cancer. Considering the importance of LAP in physiological and pathological processes, it is critical that high-efficiency and dependable technology be developed to monitor LAP levels. Herein, we summarize the organic small molecule fluorescence/chemiluminescence probes used for LAP detection in recent years, which can image LAP in cancer, drug-induced liver injury (DILI), and bacteria. It can also reveal the role of LAP in tumors and differentiate the serum of cirrhotic, drug-induced liver injury and normal models.

Monitoring amyloid aggregation via a twisted intramolecular charge transfer (TICT)-based fluorescent sensor array

Imaging amyloid-beta (Aβ) aggregation is critical for understanding the pathology and aiding the pre-symptomatic intervention of Alzheimer's disease (AD). Amyloid aggregation consists of multiple phases with increasing viscosities and demands probes with broad dynamic ranges and gradient sensitivities for continuous monitoring. Yet, existing probes designed based on the twisted intramolecular charge transfer (TICT) mechanism mainly focused on donor engineering, limiting the sensitivities and/or dynamic ranges of these fluorophores to a narrow window. Herein, using quantum chemical calculations, we investigated multiple factors affecting the TICT process of fluorophores. It includes the conjugation length, the net charge of the fluorophore scaffold, the donor strength, and the geometric pre-twisting. We have established an integrative framework for tuning TICT tendencies. Based on this framework, a platter of hemicyanines with varied sensitivities and dynamic ranges is synthesized, forming a sensor array and enabling the observation of various stages of Aβ aggregations. This approach will significantly facilitate the development of TICT-based fluorescent probes with tailored environmental sensitivities for numerous applications.

Rational Design of MMP-Independent Near-Infrared Fluorescent Probes for Accurately Monitoring Mitochondrial Viscosity

Mitochondrial viscosity affects metabolite diffusion and mitochondrial metabolism and is associated with many diseases. However, the accuracy of mitochondria-targeting fluorescent probes in measuring viscosity is unsatisfactory because these probes can diffuse from mitochondria during mitophagy with a decreased mitochondrial membrane potential (MMP). To avoid this problem, by incorporating different alkyl side chains into dihydroxanthene fluorophores (denoted as DHX), we developed six near-infrared (NIR) probes for the accurate detection of mitochondrial viscosity, and the sensitivity to viscosity and the mitochondrial targeting and anchoring capability of these probes increased by increasing the alkyl chain length. Among them, DHX-V-C₁₂ had a highly selective response to viscosity variations with minimum interference from polarity, pH, and other biologically relevant species. Furthermore, DHX-V-C₁₂ was used to monitor the mitochondrial viscosity changes of HeLa cells treated by ionophores (nystatin, monensin) or under starvation conditions. We hope that this mitochondrial targeting and anchoring strategy based on increasing the alkyl chain length will be a general strategy for the accurate detection of mitochondrial analytes, enabling the accurate study of mitochondrial functions.

Fluorescent probes for ferroptosis bioimaging: advances, challenges, and prospects

Ferroptosis is a form of regulatory cell death distinct from caspase-dependent apoptosis and plays an important role in life entities. Since ferroptosis involves a variety of complex regulatory factors, the levels of certain biological species and microenvironments would change during this process. Thus, the investigation of the level fluctuation of key target analytes during ferroptosis is of great significance for disease treatment and drug design. Toward this aim, multiple organic fluorescent probes with simple preparation and non-destructive detection have been developed, and research over the past decade has uncovered a vast array of homeostasis and other physiological characteristics of ferroptosis. However, this significant and cutting-edge topic has not yet been reviewed. In this work, we aim to highlight the latest breakthrough results of fluorescent probes for monitoring various bio-related molecules and microenvironments during ferroptosis at the cellular, tissue and in vivo levels. Accordingly, this tutorial review has been organized according to the target molecules identified by the probes including ionic species, reactive sulfur species, reactive oxygen species, biomacromolecules, microenvironment, and others. In addition to providing new insights into the findings of each fluorescent probe in ferroptosis studies, we also discuss the defects and limitations of the probes developed, and highlight the potential challenges and further prospects in this domain. We anticipate that this review will convey profound implications for designing powerful fluorescent probes to decrypt changes in key molecules and microenvironments during ferroptosis.

Hexagon AgNCs/PVP Crystallization Induced Cathode Electrochemiluminescence Enhancement for miRNA221 Biosensing

Aggredation-induced electrochemiluminescence (AIECL) promises an efficient strategy for synthesize highly luminescent emitter and co-reactant for ECL analysis, however, rational control of electrogenerated emission intensity is still challenging. The low electroconductivity and amorphous molecular configuration are intrinsic bottleneck. This work reveals the impact of polyvinyl pyrrolidone backbone regulated silver nanocrystallines (AgNCs/PVP) on the cathode AIECL properties in near infrared region, by employing the Box-Behnken designed response surface computation model to modulate crystal aggregates. Electron paramagnetic resonance spectroscopy discovered hydrogen radical (HO^• ) dominant reductive-oxidative (R-O) ECL mechanism with AgNCs acting as the co-reaction accelerator in graphene oxide/persulfate system (GO/S₂ O₈ ^2- ). Both theoretical calculation and experimental measurement testified that the ECL of AgNCs in GO/S₂ O₈ ^2- dependent on the concentration of in situ electrochemical oxidized Ag⁺ . The high efficiency of crystallization-induced enhanced ECL (CIECL) originates from 1) the effective electron transfer of Ag⁺ accelerated HO^• produce to notable promote radioactive transition, and 2) twisted intramolecular charge transfer from the electron-rich donor of PVP to electron-deficient receptor of Ag⁰ to restrict nonradioactive transition. The AgNCs/PVP with CIECL effect are applied to construct an ultrasensitive platform for miR-221 assay with a lower detection limit of 7.47 × 10³  copies mL^-1 than typical qPCR method.

Dihydroxanthene-Based Near-infrared Fluorescent Probes for Monitoring Mitochondrial Viscosity in Living Cells and Mice

Aberrant mitochondrial viscosity is closely associated with many diseases and cellular malfunctions. Thus, the development of reliable methods for monitoring mitochondrial viscosity variations has attracted considerable attention. Herein, through stepwise structural modulation of the dihydroxanthene fluorophore (DHX), we developed three NIR fluorescent probes, named DHX-V-1-3, for detecting mitochondrial viscosity. Among them, DHX-V-3 displayed the highest signal-to-noise ratio (67-fold) for viscosity with outstanding selectivity and showed excellent mitochondria targeting and immobilization ability. At the cellular level, the DHX-V-3 probe was successfully applied to image the mitochondrial viscosity in live cells upon treatment with lipopolysaccharide (LPS) or nystatin. Moreover, benefiting from its NIR emission and the increased depth of tissue imaging, DHX-V-3 demonstrated the ability to visualize the increased viscosity in LPS-treated mice.

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.

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

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

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.

Simultaneous detection of lysosomal SO2 and viscosity using a hemicyanine-based fluorescent probe

The changes in sulfur dioxide and viscosity of lysosomes are significant indicators in physiological processes and the cell microenvironment. This study aimed to synthesize a hemicyanine-based probe for simultaneous detection of SO₂ and viscosity. The probe could not only rationally detect sulfur dioxide in a semi-aqueous solution with high sensitivity (limit of detection = 0.78 μM) and fast response (within 30 s) but also monitor viscosity via fluorescence emission enhancement at 580 nm. Further, the dual-response probe was successfully used to image SO₂ and viscosity in the lysosomes of living cells.

Highly Efficient Luminescence from Charge-Transfer Gold Nanoclusters Enabled by Lewis Acid

Understanding the complicated intramolecular charge transfer (ICT) behaviors of nanomaterials is crucial to the development of high-quality nanoluminophores for various applications. However, the ICT process in molecule-like metal nanoclusters has been rarely explored. Herein, a proton binding-induced enhanced ICT state is discovered in 6-aza-2-thiothymine-protected gold nanoclusters (ATT-AuNCs). Such an excited-state electron transfer process gives rise to the weakened and red-shifted photoluminescence of these nanoclusters. By the joint use of this newfound ICT mechanism and a restriction of intramolecular motion (RIM) strategy, a red shift in the emission maxima of 30 nm with 27.5-fold higher fluorescence quantum efficiency is achieved after introducing rare-earth scandium ion (Sc³⁺) into ATT-AuNCs. Furthermore, it is found that upon the addition of Sc³⁺, the photoinduced electron transfer (PET) rate from ATT-AuNCs to minocycline is largely accelerated by forming a donor-bridge-acceptor structure. This paper offers a simple method to modulate the luminescent properties of metal nanoclusters for the rational design of next-generation sensing platforms.

Novel Meso-Benzothiazole-Substituted BODIPY-Based AIE Fluorescent Rotor for Imaging Lysosomal Viscosity and Monitoring Autophagy

Meso-substituted boron dipyrromethenes (BODIPYs) provide a potential and innovative strategy for the synergistic construction of aggregation-induced emission (AIE) probes and fluorescent rotors for monitoring cellular viscosity changes, which play critical roles in understanding the function of viscosity in its closely associated diseases. Therefore, for the first time, a BODIPY-based fluorescent probe (1) with a rotatable meso-benzothiazole group was rationally designed and synthesized, showing both good viscosity-responsive and AIE properties. Probe 1 through direct linkage with the thiazole group, showed nearly no emission in low viscous solvents; however, a strong emission at 534 nm appeared and increased gradually with the increase in viscosity, attributing to the efficient restriction of the rotatable meso-benzothiazole group. The intensity (log I₅₃₄) displayed a good linear relationship with viscosity (log η) in the viscous range of 0.59-945 cP in methanol/glycerol mixtures. Interestingly, 1 showed enhanced emission at 534 nm in 70% water compared to pure acetonitrile due to the aggregation-induced inhibited rotations. Cellular imaging suggested that 1 could successfully sense lysosomal viscosity changes induced by lipopolysaccharide, nystatin, low temperature, and dexamethasone in living cells, which could be further applied in autophagy monitoring by tracing viscosity changes. As a comparison, its analogue 2 directly linking with the phenyl group showed no viscosity-responsive or AIE properties. Therefore, for the first time, we reported a meso-benzothiazole-BODIPY-based fluorescent rotor with AIE and lysosomal viscosity-responsive properties in nervous cells, which was further applied in monitoring autophagy, and this work thus could provide an innovative strategy for the design of potential AIE and viscosity-responsive probes.

Dextran Fluorescent Probes Containing Sulfadiazine and Rhodamine B Groups

Fluorescent imaging has been expanded, as a non-invasive diagnostic modality for cancers, in recent years. Fluorescent probes in the near-infrared window can provide high sensitivity, resolution, and signal-to-noise ratio, without the use of ionizing radiation. Some fluorescent compounds with low molecular weight, such as rhodamine B (RhB) and indocyanine green (ICG), have been used in fluorescent imaging to improve imaging contrast and sensitivity; however, since these probes are excreted from the body quickly, they possess significant restrictions for imaging. To find a potential solution to this, this work investigated the synthesis and properties of novel macromolecular fluorescent compounds. Herein, water-soluble dextran fluorescent compounds (SD-Dextran-RhB) were prepared by the attachment of RhB and sulfadiazine (SD) derivatives to dextran carrier. These fluorescent compounds were then characterized through IR, ¹H NMR, ¹³C NMR, UV, GPC, and other methods. Assays of their cellular uptake and cell cytotoxicity and fluorescent imaging were also performed. Through this study, it was found that SD-Dextran-RhB is sensitive to acidic conditions and possesses low cell cytotoxicities compared to normal 293 cells and HepG2 and HeLa tumor cells. Moreover, SD-Dextran-RhB demonstrated good fluorescent imaging in HepG2 and HeLa cells. Therefore, SD-Dextran-RhB is suitable to be potentially applied as a probe in the fluorescent imaging of tumors.

Dual-modal polypeptide-containing contrast agents for magnetic resonance/fluorescence imaging

Dual-modal magnetic resonance/fluorescent imaging (MRI/FI) attracts moreandmoreattentions in diagnosis of tumors. A corresponding dual-modal imaging agent with sufficient tumor sensitivity and specificity should be matched to improve imaging quality. Tripeptide (RGD) and pentapeptide (YIGSR) were selected as the tumor-targeting groups and attached to gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) and rhodamine B (RhB), and then make two novel polypeptide-based derivatives (RGD-Gd-DTPA-RhB and YIGSR-Gd-DTPA-RhB), respectively. These derivatives were further characterized and their properties, such as cell uptake, cell cytotoxicity, MRI and FI assay, were measured. YIGSR-Gd-DTPA-RhB and RGD-Gd-DTPA-RhB had high relaxivity, good tumor-targeting property, low cell cytotoxicity and good red FI in B16F10 melanoma cells. Moreover, YIGSR-Gd-DTPA-RhB and RGD-Gd-DTPA-RhB possessed high uptake to B16F10 melanoma, and then achieve highly enhanced FI and MRI of tumors in mice for a prolonged time. Therefore, YIGSR-Gd-DTPA-RhB and RGD-Gd-DTPA-RhB can be applied as the potential agents for tumor targeted MRI/FI in vivo.

Activatable molecular rotor based on bithiophene quinolinium toward viscosity detection in liquids

The development of non-invasive and effective viscosity inspection methods during the liquid deterioration process is urgently needed since viscosity is one of the most important physical parameters of liquids. Methods featuring rapid detection, high sensitivity, cheap equipment, and fast result output are greatly desired. In this study, a viscosity-sensitive molecular rotor (BTPEQ) with a large Stokes shift (187 nm), and long emission wavelength (648 nm) has been developed. The rotor is comprised of a bithiophene donor and quinolinium acceptor, and displays a typical twisted intramolecular-charge transfer (TICT) feature, with good photostability, selectivity, and universality in various commercial liquids. With the aid of BTPEQ, the thickening effects of liquid thickeners can be determined. More importantly, BTPEQ was explored to visualize the viscosity variations in liquids at different metamorphic stages, and it was found that the viscosity level in microenvironments is highly dependent on the liquid food metamorphic period. It is worth noting that this approach can facilitate the continued perfection of fluorescent analytical methods for food quality and safety inspection.

Local detection of pH-induced disaggregation of biocompatible micelles by fluorescence switch ON

Fluorogenic nanoparticles (NPs) able to sense different physiological environments and respond with disaggregation and fluorescence switching OFF/ON are powerful tools in nanomedicine as they can combine diagnostics with therapeutic action. pH-responsive NPs are particularly interesting as they can differentiate cancer tissues from healthy ones, they can drive selective intracellular drug release and they can act as pH biosensors. Controlled polymerization techniques are the basis of such materials as they provide solid routes towards the synthesis of pH-responsive block copolymers that are able to assemble/disassemble following protonation/deprotonation. Ring opening metathesis polymerization (ROMP), in particular, has been recently exploited for the development of experimental nanomedicines owing to the efficient direct polymerization of both natural and synthetic functionalities. Here, we capitalize on these features and provide synthetic routes for the design of pH-responsive fluorogenic micelles via the assembly of ROMP block-copolymers. While detailed photophysical characterization validates the pH response, a proof of concept experiment in a model cancer cell line confirmed the activity of the biocompatible micelles in relevant biological environments, therefore pointing out the potential of this approach in the development of novel nano-theranostic agents.

A Near-Infrared Probe for Specific Imaging of Lipid Droplets in Living Cells

Lipid droplets (LDs) are involved in various physiological processes and associated with cancer development, and are regarded as a potential tumor marker for cancer diagnosis. Monitoring LDs is of prior importance to understand their involvement in biological mechanisms and the early detection of cancers. Highly sensitive and specific noninvasive fluorescent probes are particularly desirable for imaging LDs and cancer diagnosis. Herein, according to the high-viscosity and low-polarity microenvironment in LDs, we developed four easily prepared LDs-specific probes based on noncharged merocyanines. Among them, LD-1 absorbs and emits in the near-infrared (NIR) region with a large Stokes shift. Importantly, LD-1 displayed high sensitivity to high viscosity and low polarity, which allowed it to show high LDs-targeting ability. In cell imaging, LD-1 successfully probed the changes in LDs in the presence of oleic acid or during ferroptosis and was used to distinguish cancer cells from normal cells.

A Fluorogenic Far Red-Emitting Molecular Viscometer for Ascertaining Lysosomal Stress in Live Cells and Caenorhabditis elegans

The cellular physiochemical properties such as polarity, viscosity, and pH play a critical role in cellular homeostasis. The dynamic change of lysosomal viscosity in live cells associated with different environmental stress remains enigmatic and needs to be explored. We have developed a new class of Julolidine-based molecular viscometers with an extended π-conjugation to probe the lysosomal viscosity in live cells. High biocompatibility, pH tolerance, and the fluorogenic response with far red-emission (>600 nm) properties make these molecular viscometers suitable for live-cell fluorescence imaging in Caenorhabditis elegans. Among these probes, JIND-Mor is specifically designed to target lysosomes via simple modification. The real-time monitoring of lysosomal viscosity change under cellular stress was achieved. We believe that such a class of molecule viscometers has the potential to monitor lysosomal health in pathogenic conditions.

Novel Quinolizine AIE System: Visualization of Molecular Motion and Elaborate Tailoring for Biological Application

Molecular motions are ubiquitous in nature and they immutably play intrinsic roles in all actions. However, exploring appropriate models to decipher molecular motions is an extremely important but very challenging task for researchers. Considering aggregation-induced emission (AIE) luminogens possess their unique merits to visualize molecular motions, it is particularly fascinating to construct new AIE systems as models to study molecular motion. Herein, a novel quinolizine (QLZ) AIE system was constructed based on the restriction intramolecular vibration (RIV) mechanism. It was demonstrated that QLZ could act as an ideal model to visualize single-molecule motion and macroscopic molecular motion via fluorescence change. Additionally, further elaborate tailoring of this impressive core achieved highly efficient reactive oxygen species production and realized fluorescence imaging-guided photodynamic therapy applications, which confirms the great application potential of this new AIE-active QLZ core. Therefore, this work not only provides an ideal model to visualize molecular motion but also opens a new way for the application of AIEgens.

Activity-based fluorescent probes for sensing and imaging of Reactive Carbonyl species (RCSs)

This review explains various strategies for developing fluorescent probes to detect reactive carbonyl species (RCS). There are sevaral number of mono and diacarbonyls among 30 varieties of reactive carbonyl species (RCSs) so far discovered, which play pivotal roles in pathological processes such as cancer, neurodegenerative diseases, cardiovascular disease, renal failure, and diabetes mellitus. These RCSs play essential roles in maintaining ion channels regulation, cellular signaling pathways, and metabolisms. Among RCSs, Carbon moxide (CO) is also utilized for its cardioprotective, anti-inflammatory, and anti-apoptotic effects. Fluorescence-based non-invasive optical tools have come out as one of the promising methods for analyzing the concentrations and co-localizations of these small metabolites. There has been a tremendous eruption in developing fluorescent probes for selective detection of specific RCSs within cellular and aqueous environments due to its high sensitivity, high spatial and temporal resolution of fluorescence imaging. Fluorescence-based sensing mechanisms such as intramolecular charge transfer (ICT), photoinduced electron transfer (PeT), excited-state intramolecular proton transfer (ESIPT), and fluorescence resonance energy transfer (FRET) are described. In particular, probes for dicarbonyls such as methylglyoxal (MGO), malondialdehyde (MDA), along with monocarbonyls that include formaldehyde (FA), carbon monoxide (CO) and phosgene are discussed. One of the most exciting advances in this review is the summary of fluorescent probes of dicarbonyl compounds.

Fluorescent probe strategy for live cell distinction

Live cell discrimination is the first and essential step to understand complex biosystems. Conventional cell discrimination involving various antibodies relies on selective surface biomarkers. Compared to antibodies, the fluorescent probe strategy allows the utilisation of intracellular biomarkers, providing broader options with unique chemical principles to achieve the live cell distinction. In general, fluorescent probes can be retained in cells by interacting with biomolecules, accumulating via transporters, and participating in metabolism. Based on the target difference, fluorescent probe strategy can be divided into several categories: protein-oriented live cell distinction (POLD), carbohydrate-oriented live cell distinction (COLD), DNA-oriented live cell distinction (DOLD), gating-oriented live cell distinction (GOLD), metabolism-oriented live cell distinction (MOLD) and lipid-oriented live cell distinction (LOLD). In this review, we will outline the concepts and mechanisms of different strategies, introduce their applications in cell-type discrimination, and discuss their advantages and challenges in this area. We expect this tutorial will provide a new perspective on the mechanisms of fluorescent probe strategy and facilitate the development of cell-type-specific probes.

Novel cationic meso-CF3 BODIPY-based AIE fluorescent rotors for imaging viscosity in mitochondria

Two novel meso-CF₃ BODIPY-based fluorescent rotors have been rationally prepared and found to sensitively respond to viscosity in living cells with a fluorescence "turn-on" effect, attributed to the special restricted rotation of meso-CF₃ group in viscous environments. Interestingly, a monostyryl probe with one cationic group exhibits good mitochondrial localization and AIE property.

Recent advances in fluorescent probes for lipid droplets

Lipid droplets (LDs) have been known as a non-negligible cellular organelle for lipid storage and metabolism. Fluorescent probes for imaging LDs would be paramount for depicting their functions in cells. Although commercially available Nile Red and BODIPY^tm 493/503 have been widely used for labelling LDs, they exhibit unsatisfactory specificity and spectroscopic properties. This feature article reviews the recent advances in organic fluorescent probes for imaging LDs. We first introduce the key points for probe design, including regulating hydrophobicity and enhancing fluorescence quantum yield in LDs. Then, we summarize the structural features and biological applications of some representative LD probes classified by their frameworks. In addition, the current challenges and future research trends for the fluorescent probes of LDs are discussed as well.

A General Method to Develop Highly Environmentally Sensitive Fluorescent Probes and AIEgens

Environmentally sensitive fluorescent probes (including AIEgens) play pivotal roles in numerous biological studies. Many of these functional materials are developed based on the twisted intramolecular charge transfer (TICT) mechanism. However, the TICT tendency of dialkylated amino groups in biocompatible main-stream fluorophores (i.e., coumarins and rhodamines) is weak, limiting their sensitivities. Herein, by replacing dialkylated amino donors with an N-methylpyrrole group to enhance TICT, a simple and general method to engineer highly environmentally sensitive fluorescent probes is reported. This method yields a platter of colorful fluorescent probes that demonstrates outstanding polarity and viscosity sensitivity with large turn-on ratios (up to 191 times for polarity and 14 times for viscosity), as well as distinct aggregation-induced emission (AIE) characteristics. The utilities of these probes in both wash-free bioimaging and protein detections are also successfully demonstrated. It is expected that this molecular design strategy will inspire the creation of many environmentally sensitive probes.

A Dual-Control Strategy by Phosphate Ions and Local Microviscosity for Tracking Adenosine Triphosphate Metabolism in Mitochondria and Cellular Activity Dynamically

Adenosine triphosphate (ATP) acts as the main energy source for growth and development in organisms, and the disorder reflects the mitochondrial damage to a large extent. Therefore, an efficient tool for the evaluation of the ATP metabolic level is important to track mitochondrial health, providing an additional perspective for an in-depth long-term study on living activities. Herein, a twisted intramolecular charge transfer (TICT) framework is utilized to build up a sensitive receptor, Mito-VP, with a negligible background to target mitochondrial ATP metabolism by monitoring the phosphate ion (Pi) level upon ATP hydrolysis under the overall consideration of the structural and functional features of mitochondria. The responsive fluorescence could be lighted on under the dual control of Pi and local microviscosity, and the two steps of ATP hydrolysis could be captured through fluorescence. In addition to the well-behaved mitochondrial targeting, the energy metabolism at cellular and organism levels has been clarified via mitosis and zebrafish development, respectively.

Development of Near-Infrared Nucleic Acid Mimics of Fluorescent Proteins for In Vivo Imaging of Viral RNA with Turn-On Fluorescence

GFP-like fluorescent proteins and their molecular mimics have revolutionized bioimaging research, but their emissions are largely limited in the visible to far-red region, hampering the in vivo applications in intact animals. Herein, we structurally modulate GFP-like chromophores using a donor-acceptor-acceptor (D-A-A') molecular configuration to discover a set of novel fluorogenic derivatives with infrared-shifted spectra. These chromophores can be fluorescently elicited by their specific interaction with G-quadruplex (G4), a unique noncanonical nucleic acid secondary structure, via inhibition of the chromophores' twisted-intramolecular charge transfer. This feature allows us to create, for the first time, FP mimics with tunable emission in the near-infrared (NIR) region (Em_max = 664-705 nm), namely, infrared G-quadruplex mimics of FPs (igMFP). Compared with their FP counterparts, igMFPs exhibit remarkably higher quantum yields, larger Stokes shift, and better photostability. In a proof-of-concept application using pathogen-related G4s as the target, we exploited igMFPs to directly visualize native hepatitis C virus (HCV) RNA genome in living cells via their in situ formation by the chromophore-bound viral G4 structure in the HCV core gene. Furthermore, igMFPs are capable of high contrast HCV RNA imaging in living mice bearing a HCV RNA-presenting mini-organ, providing the first application of FP mimics in whole-animal imaging.

Twisted intramolecular charge transfer (TICT) and twists beyond TICT: from mechanisms to rational designs of bright and sensitive fluorophores

The twisted intramolecular charge transfer (TICT) mechanism has guided the development of numerous bright and sensitive fluorophores. This review briefly overviews the history of establishing the TICT mechanism, and systematically summarizes the molecular design strategies in modulating the TICT tendency of various organic fluorophores towards different applications, along with key milestone studies and representative examples. Additionally, we also succinctly review the twisted intramolecular charge shuttle (TICS) and twists during photoinduced electron transfer (PET), and compare their similarities and differences with TICT, with emphasis on understanding the structure-property relationships between the twisted geometries and how they can directly affect the fluorescence of the molecules. Such structure-property relationships presented herein will greatly aid the rational development of fluorophores that involve molecular twisting in the excited state.

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.

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.

Excimer-monomer fluorescence changes by supramolecular disassembly for protein sensing and quantification

A protein binding-induced supramolecular dissociation strategy is developed with the ratio of monomer and excimer fluorescence as the tool for protein sensing and quantification. Due to the "lock-and-key" strategy based on specific ligand-protein binding, the probe exhibits excellent selectivity and quantification accuracy to the protein of interest. The ratiometric approach is immune to interference from extrinsic quenchers, while preserving the opportunity to be protein specific.

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.