Fast-Specific Fluorescent Probes to Visualize Norepinephrine Signaling Pathways and Its Flux in the Epileptic Mice Brain

Norepinephrine (NE) is synthesized in the locus coeruleus and widely projected throughout the brain and spinal cord. It regulates various actions and consciousness linked to a variety of neurological diseases. A "hunting-shooting" strategy was proposed in this work to improve the specificity and response rate of an NE fluorescent probe: 2-(cyclohex-2-en-1-ylidene)malononitrile derivatives were chosen as a fluorophore. To create a dual-site probe, an aldehyde group was added to the ortho of the ester group (or benzene sulfonate). Because of its excellent electrophilic activity, the aldehyde group could rapidly "hunt" the amino group and then form an intramolecular five-membered ring via the nucleophilic reaction with the β-hydroxyl group. The -NH- in the five-membered ring "shoots" the adjacent ester group, releasing the fluorophore and allowing for rapid and specific NE detection. The NE release and reuptake ″emetic″-″swallow″ transient process is captured and visualized under the action of the primary NE receptor drug. Furthermore, by introducing halogen into the fluorophore to lengthen the absorption wavelength, improve lipid solubility, and adjust the pK_a appropriately, the probe successfully penetrated the blood-brain barrier (BBB). In situ synchronous probe imaging was used to detect the NE level in the brains of epileptic and normal mice, and abnormal expression of NE in the brain was discovered during epilepsy. Brain anatomy was used to examine the distribution and level changes of NE in various brain regions before and after epilepsy. This research provides useful tools and a theoretical foundation for diagnosing and treating central nervous system diseases early.

A novel strategy for the functionalization and design of 4-methylene-4H-pyran merocyanines via enamination and 1,8-conjugate addition

4-Methylene-4H-pyrans are popular merocyanine dyes, but their functionalization is limited by the Knoevenagel condensation with aromatic aldehydes. In this work, we developed a novel approach for the construction of a new class of pyran fluorophores based on enamination and subsequent nucleophilic substitution of the dimethylamino group via 1,8-conjugate addition/elimination. This methodology includes selective transformations leading to previously unknown symmetrical and asymmetrical structures. The dimethylaminovinyl-substituted pyrans are reactive intermediates and can be considered as a convenient synthetic tool for the construction of new merocyanines with tunable fluorescence (417-628 nm). The main strategies for the modification of the pyran moiety have been determined for the construction and targeted design of fluorophores. Pyrans bearing two enamine moieties demonstrate significant light extinction coefficients (up to 116 000 M^-1 cm^-1), high quantum yields (up to 69%) and large Stokes shifts (up to 152 nm) because of their strong push-pull nature. Density Functional Theory (DFT) calculations were performed for the explanation of the structural and photophysical features of the prepared merocyanines. The developed approach can be considered as a useful platform for further application of 4-methylene-4H-pyrans as promising fluorophores for sensors and solar cells, and in bioimaging.

A Near InfraRed Emissive Chemosensor for Zn2+ and Phosphate Derivatives Based on a Di-(2-picolyl)amine-styrylflavylium Push-Pull Fluorophore

A new Near InfraRed (NIR) fluorescent chemosensor for metal ions and anions is herein presented. The fluorophore is based on a styrylflavylium dye, a synthetic analogue of the natural anthocyanin family, with a di-(2-picolyl)amine (DPA) moiety as the metal chelating unit. The substitution pattern of the styrylflavylium core (with tertiary amines on positions 7 and 4') shifts the optical properties of the dye towards the NIR region of the electronic spectra, due to a strong push-pull character over the π-conjugated system. The NIR chemosensor is highly sensitive to the presence of Zn²⁺, which induces a strong CHelation Enhanced Fluorescence (CHEF) effect upon binding to the DPA unit (2.7 fold increase). The strongest competing ion is Cu²⁺, with a complete fluorescence quenching, while other metals induce lower responses on the optical properties of the chemosensor. Subsequent anion screening of the Zn²⁺-chemosensor coordination compound has demonstrated a distinct selectivity towards adenosine 5'-triphosphate (ATP) and adenosine 5'-diphosphate (ADP), with high association constants (K ~ 10⁶ M^-1) and a strong CHEF effect (2.4 and 2.9 fold fluorescence increase for ATP and ADP, respectively). Intracellular studies with the Zn²⁺-complexed sensor showed strong luminescence in the cellular membrane of Gram^- bacteria (E. coli) and mitochondrial membrane of mammalian cells (A659), which highlights its possible application for intracellular labelling.

Progress of Dicyanomethylene-4H-Pyran Derivatives in Biological Sensing Based on ICT Effect

As one of the typical fluorescent cores, dicyanomethylene-4H-pyran (DCM) derivatives exhibit excellent photophysical and photochemical properties, such as large Stokes shift, excellent light stability, and tunable near-infrared (NIR) emission. The luminescence mechanism of DCM probes mainly depends on the intramolecular charge transfer (ICT). Hence, by regulating the ICT process, the probes can specifically act on the target molecule. Accordingly, a series of NIR DCM probes have been constructed to detect the ions, reactive oxygen species (ROS), and biological macromolecules in cells. However, there is no relevant review to summarize it at present. This minireview mainly summarizes the NIR DCM probes based on ICT effect and their applications in biosensors and biological imaging in recent years. This will be beneficial to innovatively construct new DCM probes and actively promote their application in the future.

Reasonable design of NIR AIEgens for fluorescence imaging and effective photothermal/photodynamic cancer therapy

The development of a multifunctional single molecule phototherapeutic agent with excellent fluorescence imaging, photothermal therapy and photodynamic therapy at the same time is still a challenging task, which mainly arises from the low absorbance of the molecule, and the complexity of energy dissipation and molecular design. Herein, four donor-acceptor (D-A) compounds were synthesized by linking triphenylamine (TPA), thiophene/thieno[3,2-b]thiophene and different cyano acceptor structures. In this design, we propose a molecular design strategy to redshift absorption and increase the molar extinction coefficient (ε) by enhancing electron-withdrawing acceptors and enlarging the π-conjugation plane unit. Due to the twisted structure of TPA, these compounds exhibit aggregation-induced emission (AIE) characteristics. Notably, these AIEgens have long emission wavelengths, excellent photostability, biocompatibility, photothermal stability and singlet oxygen (¹O₂) generation performance. Among them, the photothermal conversion efficiency of a compound (named TCF-SS-TPA NPs) can reach 84.5%. Cellular internalization and therapy showed that TCF-SS-TPA NPs have good biocompatibility, excellent cell bioimaging and cancer phototherapy capabilities in vitro. This study will stimulate the molecular design of multifunctional phototherapeutics to realize effective synergistic cancer therapy.

A Solvatochromic Fluorescent Probe Reveals Polarity Heterogeneity upon Protein Aggregation in Cells

We report a crystallization-induced emission fluorophore to quantitatively interrogate the polarity of aggregated proteins. This solvatochromic probe, namely "AggRetina" probe, inherently binds to aggregated proteins and exhibits both a polarity-dependent fluorescence emission wavelength shift and a viscosity-dependent fluorescence intensity increase. Regulation of its polarity sensitivity was achieved by extending the conjugation length. Different proteins bear diverse polarity upon aggregation, leading to different resistance to proteolysis. Polarity primarily decreases during protein misfolding but viscosity mainly increases upon the formation of insoluble aggregates. We quantified the polarity of aggregated protein-of-interest in live cells via HaloTag bioorthogonal labeling, revealing polarity heterogeneity within cellular aggregates. The enriched micro-environment details inside misfolded and aggregated proteins may correlate to their bio-chemical properties and pathogenicity.

A New Phenolate-Ion-Type Two-Photon Near Infrared Fluorophore-Based Biosensor for High-Performance Detection of HNO

Although (E)-4-(2-(4-(dicyanomethylene)-4H-chromen-2-yl)vinyl)phenolate anion (DCPO^- ) has recently emerged as a potential near infrared (NIR) biosensor signaling unit, the pK_a value of its conjugate acid is relatively high (∼9); this will lead to relatively low concentrations of DCPO^- under physiological conditions and, hence, unsatisfactory sensitivity of DCPO^- -based bio-probes. By difluoro-substitution on DCPO^- , we have exploited a new fluorophore of o-FDCPO^- whose conjugate acid has a much lower pK_a value of 7.42. Meanwhile, o-FDCPO^- is NIR emissive with λ_em =693 nm and has a 0.76-fold higher fluorescence efficiency than DCPO^- . The significant superiority of o-FDCPO^- over DCPO^- in sensitivity for NIR biosensor applications was confirmed by comparative studies on two HNO probes, namely o-FDCPO-P and DCPO-P, which bear signaling units of o-FDCPO^- and DCPO^- , respectively. Moreover, o-FDCPO-P has been demonstrated to be a high-performance HNO probe with high selectivity, high sensitivity (detection limit: 50 nm), and a rapid response, together with a two-photon NIR-excitation imaging capability.

Tuning Emission Wavelength of Polymorphous Crystal via Controllable Alkyl Chain Stacking and Its Vapor- and Thermo-Responsive Fluorescence

Tuning fluorescence colour of solid-state materials has become a topic of increasing interest for both fundamental mechanism study and practical applications such as sensors, optical recording and security printing. In this work, a fluorescent colour tuneable molecule BA-C16 is rationally designed and facilely synthesized by attaching flexible long alkyl chains to 2-hydroxybenzophenone azine (BA), which shows both aggregation-induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) characteristics. Compared to BA, the simple introduction of long alkyl chains in BA-C16 leads to an emission wavelength redshift from 542 to 558 nm. This strategy of extending emission wavelength is rarely reported, and is ascribed to the enlarged through-space π-conjugation between interplanar molecules in the aggregate of BA-C16. Three crystals of BA-C16 are obtained with green, yellowish green and yellow emission. According to characterization by X-ray crystallography, X-ray powder diffraction and differential scanning calorimetry, alkyl chains play an important role in inducing different stacking modes of the three crystals, which further leads to polymorph-dependent fluorescence colour. BA-C16 exhibits tuneable solid-state fluorescence upon vapor fumigation, or annealing based on a transition between a "near-monomer" crystalline state and a "dimer" crystalline state. BA-C16 is further applied for rewritable fluorescence printing tuned by vapor- and thermal-treatment.

A fluorescent probe based on tetrahydro[5]helicene derivative with large Stokes shift for rapid and highly selective recognition of hydrogen sulfide

In this work, we have designed and synthesized a dinitrobenzene-sulfonate tetrahydro[5]helicene (H-DNP) as an effective fluorescent probe for detection of hydrogen sulfide (H₂S). Upon the addition of H₂S, a significant fluorescence enhancement (75-fold) at 495 nm can be observed with a distinct color change from colorless to yellow. Additionally, H-DNP shows low background spectroscopic signal, large Stokes Shift up to ~140 nm, good sensitivity, rapid response time less than 2 min, low detection limit (48 nM) and high selectivity toward common bio-thiols (Cysteine, Homocysteine and Glutathione). Compared with the previous dinitrophenoxy tetrahydro[5]helicene, this probe has shorter response time and lower detection limit. Most importantly, this probe H-DNP has low toxicity to cells and excellent cell permeability, which can be applied to visualize H₂S in living cells.

Excited State Intramolecular Proton Transfer Plus Aggregation-Induced Emission-Based Diketopyrrolopyrrole Luminogen: Photophysical Properties and Simultaneously Discriminative Detection of Trace Water in Three Organic Solvents

Developing solid state near-IR (NIR) emitters and simultaneously discriminative detection of trace water in organic solvents has long been a significant challenge. In this work, a novel diketopyrrolopyrrole-based luminogen (DPP1) with excited state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) characteristics has been designed and synthesized. Its amorphous and crystal solids show red and NIR-emissive fluorescence at 625 and 675 nm, respectively. When DPP1 reacts with fluoride anion, the resulting system (DPP1·F) can discriminatively detect the water content in aprotic solvents with colorimetric and fluorescent dual modes. Distinct fluorescent responses of "turn-on", "ratiometric turn-off", and "ratiometric turn-on" and low limits of detection of 0.0064, 0.042, and 0. The water-induced sensitive and fast change in THF was applied to the determination of water in foodstuffs in practical solid state indicator paper strips.

A novel near-infrared fluorescent light-up probe for tumor imaging and drug-induced liver injury detection

A novel near-infrared fluorescent light-up probe with a tumor-homing pentapeptide, CREKA (Cys-Arg-Glu-Lys-Ala), specifically binds to fibrin-fibronectin complexes was rationally designed and developed for biomedical imaging. Its superior practical applications in tumor imaging and drug-induced liver injury detection are well demonstrated for the first time.

Nanomized tumor-microenvironment-active NIR fluorescent prodrug for ensuring synchronous occurrences of drug release and fluorescence tracing

Improving the bioavailability and tumor-targeting ability of a prodrug, as well as monitoring its active ingredient release in vivo, is still a challenge in cancer diagnosis and therapy. Herein, a specific nanomized tumor-microenvironment-active near-infrared (NIR) fluorescent DCM-S-GEM/PEG prodrug was developed as a potent monitoring platform, wherein we conjugated antitumor drug gemcitabine (GEM) and NIR fluorescent chromophore dicyanomethylene-4H-pyran (DCM) via glutathione (GSH)-activatable disulfide linker and encapsulated DCM-S-GEM into an amphiphilic polymer DSPE-mPEG by self-assembly. The nanomized DCM-S-GEM/PEG prodrug exhibits excellent photostability and high biocompatibility, significantly improving the therapeutic efficacy toward lung tumor cells with fewer side-effects toward normal cells. Furthermore, when compared with the standalone DCM-S-GEM prodrug, the micellization with diblock DSPE-mPEG avoids fast metabolism, facilitates the accumulation of drugs in lung tumor tissues, displays longer tumor retention, and realizes precise drug release in lung tumors. The nanomized DCM-S-GEM/PEG prodrug can be developed as a promising tool to monitor prodrug delivery and activation processes in vivo.

Functionalized AIE nanoparticles with efficient deep-red emission, mitochondrial specificity, cancer cell selectivity and multiphoton susceptibility

Multiphoton microscopy is an exciting tool for biomedical research because it can be used to image single cells in vivo due to its greater penetration depth, lower phototoxicity and higher resolution when compared to confocal laser scanning microscopy. This helps researchers understand how certain cells change over time and evaluate the efficacy of different therapies. Herein, we report a new AIE luminogen (AIEgen), abbreviated as TPE-TETRAD, with a favorable absorption and efficient deep-red emission in the solid state. TPE-TETRAD possesses a high two-photon absorption cross-section (313 MG at 830 nm) and a rich array of non-linear optical properties including aggregation-induced three-photon luminescence. Biotinylated TPE-TETRAD nanoparticles are also fabricated and applied to stain mitochondria in live cancer cells with high specificity. The purpose of this study is to characterize a novel deep-red AIEgen and fabricate biotinylated nanoparticles for applications as (1) biocompatible and photostable AIE probes for specific mitochondria imaging and (2) multiphoton imaging probes suitable for two/three-photon fluorescence microscopy.

Peptide Receptor-Targeted Fluorescent Probe: Visualization and Discrimination between Chronic and Acute Ulcerative Colitis

The inflammatory activity of ulcerative colitis plays an important role in the medical treatment. However, accurate and real-time monitoring of the colitis activity with noninvasive bioimaging method is still challenging, especially in distinguishing between chronic and acute colitis. As a good receptor, the oligopeptide transporter (PepT1) is overexpressed in the colonic epithelial cells of chronic ulcerative colitis, which can deliver tripeptide KPV (Lys-Pro-Val, the C-terminal sequence of α-MSH) into cytosol in the intestine. Herein, we report a PepT1 peptide receptor-targeted fluorescent probe, dicyanomethylene-4H-pyran (DCM)-KPV, with the strategy of conjugating the KPV into the DCM chromophore. The diagnostic fluorescent probe bestows a specific receptor-targeted interaction with PepT1 through the KPV moiety, possessing several beneficial characteristics, such as efficient long emission, low photobleaching, negligible cytotoxicity, and high cytocompatibility in living cells. We build the overexpressed PepT1 on the cytomembrane of ulcerative colitis model Caco-2 cell as the efficient receptor to accumulate the targeted tripeptide KPV in the cytoplasm and nucleus. With the co-localization of DCM-KPV and the DNA-specific fluorophore, DAPI, the specifically long emission from chromophore DCM and efficient receptor-targeted peptide KPV, the fluorescent probe of DCM-KPV makes a breakthrough to the direct noninvasive observation of the accumulation in colon inflammation regions via intestinal mucosa, even successfully distinguishing the chronic, acute ulcerative colitis and normal groups. Compared with the traditional unenhanced magnetic resonance imaging and hematoxylin and eosin (H&E) staining, we make full use of exploiting the specific target-receptor interaction between the tripeptide unit, KPV, and the oligopeptide transporter, PepT1, for sensing selectivity. The desirable diagnostic ability of DCM-KPV can guarantee the real-time tracking and visualization of the role of intracellular KPV on ulcerative colitis, which provides an alternative to replace the time-consuming and tissue sampling-invasive H&E staining diagnosis.

One-pot fabrication of FRET-based fluorescent probe for detecting copper ion and sulfide anion in 100% aqueous media

The design of effective tools for detecting copper ion (Cu²⁺) and sulfide anion (S^2-) is of great importance due to the abnormal level of Cu²⁺ and S^2- has been associated with an increase in risk of many diseases. Herein, we report on the fabrication of fluorescence resonance energy transfer (FRET) based fluorescent probe PF (PEI-FITC) for detecting Cu²⁺ and S^2- in 100% aqueous media via a facile one-pot method by covalent linking fluorescein isothiocyanate (FITC) with branched-polyethylenimine (b-PEI). PF could selectively coordinate with Cu²⁺ among 10 metal ions to form PF-Cu²⁺ complex, resulting in fluorescence quenching through FRET mechanism. Furthermore, the in situ generated PF-Cu²⁺ complex can be used to selectively detect S^2- based on the displacement approach, resulting in an off-on type sensing. There is no obvious interference from other anions, such as Cl^-, NO₃^-, ClO₄^-, SO₄^2-, HCO₃^-, CO₃^2-, Br^-, HPO₄^2-, F^- and S₂O₃^2-. In addition, PF was successfully used to determine Cu²⁺ and S^2- in human serum and tap water samples. Therefore, the FRET-based probe PF may provide a new method for selective detection of multifarious analysts in biological and environmental applications, and even hold promise for application in more complicated systems.

In Situ Ratiometric Quantitative Tracing of Intracellular Leucine Aminopeptidase Activity via an Activatable Near-Infrared Fluorescent Probe

Leucine aminopeptidase (LAP), one of the important proteolytic enzymes, is intertwined with the progress of many pathological disorders as a well-defined biomarker. To explore fluorescent aminopeptidase probe for quantitative detection of LAP distribution and dynamic changes, herein we report a LAP-targeting near-infrared (NIR) fluorescent probe (DCM-Leu) for ratiometric quantitative trapping of LAP activity in different kinds of living cells. DCM-Leu is composed of a NIR-emitting fluorophore (DCM) as a reporter and l-leucine as a triggered moiety, which are linked together by an amide bond specific for LAP cleavage. High contrast on the ratiometric NIR fluorescence signal can be achieved in response to LAP activity, thus enabling quantification of endogenous LAP with "build-in calibration" as well as minimal background interference. Its ratiometric NIR signal can be blocked in a dose-dependent manner by bestatin, an LAP inhibitor, indicating that the alteration of endogenous LAP activity results in these obviously fluorescent signal responses. It is worth noting that DCM-Leu features striking characteristics such as a large Stokes shift (∼205 nm), superior selectivity, and strong photostability responding to LAP. Impressively, not only did we successfully exemplify DCM-Leu in situ ratiometric trapping and quantification of endogenous LAP activity in various types of living cells, but also, with the aid of three-dimensional confocal imaging, the intracellular LAP distribution is clearly observed from different perspectives for the first time, owing to the high signal-to-noise of ratiometric NIR fluorescent response. Collectively, these results demonstrate preclinical potential value of DCM-Leu serving as a useful NIR fluorescent probe for early detection of LAP-associated disease and screening inhibitor.