Coupling Nanoscale Precision with Multiscale Imaging: A Multifunctional Near-Infrared Dye for the Brain

Live imaging of primary neural cells is crucial for monitoring neuronal activity, especially multiscale and multifunctional imaging that offers excellent biocompatibility. Multiscale imaging can provide insights into cellular structure and function from the nanoscale to the millimeter scale. Multifunctional imaging can monitor different activities in the brain. However, this remains a challenge because of the lack of dyes with a high signal-to-background ratio, water solubility, and multiscale and multifunctional imaging capabilities. In this study, we present a neural dye with near-infrared (NIR) emissions (>700 nm) that enables ultrafast staining (in less than 1 min) for the imaging of primary neurons. This dye not only enables multiscale neural live-cell imaging from vesicles in neurites, neural membranes, and single neurons to the whole brain but also facilitates multifunctional imaging, such as the monitoring and quantifying of synaptic vesicles and the changes in membrane potential. We also explore the potential of this NIR neural dye for staining brain slices and live brains. The NIR neural dye exhibits superior binding with neural membranes compared to commercial dyes, thereby achieving multiscale and multifunctional brain neuroimaging. In conclusion, our findings introduce a significant breakthrough in neuroimaging dyes by developing a category of small molecular dyes.

Design and Application of Fluorescent Probes to Detect Cellular Physical Microenvironments

The microenvironment is indispensable for functionality of various biomacromolecules, subcellular compartments, living cells, and organisms. In particular, physical properties within the biological microenvironment could exert profound effects on both the cellular physiology and pathology, with parameters including the polarity, viscosity, pH, and other relevant factors. There is a significant demand to directly visualize and quantitatively measure the fluctuation in the cellular microenvironment with spatiotemporal resolution. To satisfy this need, analytical methods based on fluorescence probes offer great opportunities due to the facile, sensitive, and dynamic detection that these molecules could enable in varying biological settings from in vitro samples to live animal models. Herein, we focus on various types of small molecule fluorescent probes for the detection and measurement of physical parameters of the microenvironment, including pH, polarity, viscosity, mechanical force, temperature, and electron potential. For each parameter, we primarily describe the chemical mechanisms underlying how physical properties are correlated with changes of various fluorescent signals. This review provides both an overview and a perspective for the development of small molecule fluorescent probes to visualize the dynamic changes in the cellular environment, to expand the knowledge for biological process, and to enrich diagnostic tools for human diseases.

Development and photo-properties and intracellular behavior of visible-light-responsive molecule localizing to organelles of living cell

Visible-light-responsive azobenzene derivative in which a functional group having cell membrane permeability and a fluorophore were bonded was synthesized. This compound localized to the hydrophobic part in the lipid membrane of the liposome, and when the light corresponding to the transition absorption of azobenzene was irradiated, morphological change of the liposome was observed. When this compound was loaded into living cells, this molecule localized to the lysosome and when irradiated with light of the same wavelength caused cell death. These observed changes are thought to be due to photoisomerization of azobenzene derivatives.

Graphical abstract

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

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

Mitochondrial Targeting Long-Term Near-Infrared Imaging and Photodynamic Therapy Aggregation-Induced Emission Luminogens Manipulated by Thiophene

In recent years, the development and application of integrated probes for theranostics have attracted more and more attention. However, few biological probes can meet the needs of in vivo and in vitro long-term near-infrared imaging and photodynamic therapy, especially with a certain subcellular organelle targeting ability. Here, 2-chlorophenothiazine as a pharmacophore is linked to the mitochondrial targeting group pyridine cation through an alkyl chain, which is further linked to triphenylamine-based aggregation-induced emission groups to obtain two aggregation-induced emission luminogens (AIEgens). Only the presence or absence of thiophene causes two AIEgens to exhibit different structure-oriented characteristics. Although they are different with respec to mitochondrial targeting, cellular imaging, and cytotoxicity, they all have excellent in vivo and in vitro long-term near-infrared imaging and photodynamic therapy capabilities.

Real-Time Visualization and Monitoring of Physiological Dynamics by Aggregation-Induced Emission Luminogens (AIEgens)

Physiological dynamics in living cells and tissues are crucial for maintenance and regulation of their normal activities and functionalities. Tiny fluctuations in physiological microenvironments can leverage significant influences on cell growth, metabolism, differentiation, and apoptosis as well as disease evolution. Fluorescence imaging based on aggregation-induced emission luminogens (AIEgens) exhibits superior advantages in real-time sensing and monitoring of the physiological dynamics in living systems, including its unique properties such as high sensitivity and rapid response, flexible molecular design, and versatile nano- to mesostructural fabrication. The introduction of canonic AIEgens with long-wavelength, near-infrared, or microwave emission, persistent luminescence, and diversified excitation source (e.g., chemo- or bioluminescence) offers researchers a tool to evaluate the resulting molecules with excellent performance in response to subtle fluctuations in bioactivities with broader dimensionalities and deeper hierarchies.

Fluorescent Amphiphilic Quaternized β-Chitin: Antibacterial Mechanism and Cell Imaging

Fluorescent materials span multiple applications from biological probes and chemical sensing to optoelectronic systems. Although great efforts have been made toward developing classes of fluorescent materials, 100,000+ traditional fluorescent dyes still suffer from the obstacle of aggregation-caused quenching (ACQ). Thus, designing fluorescent materials with excellent optical performance from ACQ luminogens remains challenging. In this work, we prepared fluorescent amphiphilic quaternized β-chitin (QC-F) via nucleophilic addition between the amino groups of QC and isothiocyanate groups of fluorescein isothiocyanate (FITC). Due to the covalent anchoring of the QC backbone, steric hindrance of the bulky acetamido groups, electrostatic repulsion of the quaternary ammonium groups, and homogeneous distribution of FITC units, the FITC units were spatially and electronically isolated, and the QC-F series exhibited unique fluorescent behaviors. The QC-F series could be used to observe their interactions with microbial cells through fluorescence imaging to gain insights into the QC antibacterial mechanism. Moreover, with their favorable cytocompatibility, the QC-F series are also suitable for cell imaging. Thus, the present work will broaden the applications of chitin and conventional ACQ luminogens.

Multifunction in One Molecule: Mitochondrial Imaging and Photothermal & Photodynamic Cytotoxicity of Fast-Response Near-Infrared Fluorescent Probes with Aggregation-Induced Emission Characteristics

HJS and DHJS, two near-infrared emissive and mitochondria-targeted therapy probes, have been designed. They exhibited photothermal & photodynamic cytotoxicity and aggregation-induced emission (AIE) characteristics. Interestingly, we could receive fluorescence immediately after adding the probes without washing in 1 min. They could quickly enter cancer cells and selectively localized to the mitochondria firstly. When the concentration of probes was low (<5 μM), they could respond sensitively to the mitochondrial membrane potential and would selectively enter the mitochondria with red fluorescence. However, when the concentration was high (≥5 μM), they would preferentially enter the mitochondria and have the property of dual-channel fluorescence imaging (red and near-infrared) even after 24 h. What's more, they increased the intracellular reactive oxygen species (ROS) levels, decreased the mitochondrial membrane potentials, and then induced apoptosis, which were proved by confocal imaging and flow cytometry experiments. In addition, the results of photothermal experiment and cytotoxicity test showed that the probes had good photothermal and photodynamic toxicity to cancer cells. In vitro and in vivo experiments also proved the excellent near-infrared (NIR) imaging ability, good biocompatibility and certain inhibition of tumor growth ability of DHJS.

Mitochondria-Targeted Sensor Array with Aggregation-Induced Emission Luminogens for Identification of Various Cells

Accurate discrimination of cancerous cells is a good solution for early diagnosis of tumors. The mitochondrion plays an important role in cells. Herein, the five aggregation-induced emission luminogens (AIEgens) with various double positive charges are synthesized to image mitochondria. Tetraphenylethylene (TPE) molecules are modified by methoxy groups, conjugated donor-acceptor, and different positive charges to achieve multicolor emission. The five AIEgens form the PTx-Sa (positive mitochondria-target molecular sensor array) to perform cross-fluorescence response based on the mitochondria-targeted imaging to achieve the discrimination of various cells. Principal component analysis of the cross-response fluorescence data of PTx-Sa shows that 100% accurate identification of various cells, including cancer cells and normal cells, digestive tract cancer cells, gastric cancer cells, and mixed gastric cancer cells. By support vector machine to show the predictive ability of PTx-Sa to unknown cells by using blind samples. This is the first time to apply mitochondria-targeted sensor array to identification of various cells.

In vivo monitoring of tissue regeneration using a ratiometric lysosomal AIE probe

Tissue regeneration is a crucial self-renewal capability involving many complex biological processes. Although transgenic techniques and fluorescence immunohistochemical staining have promoted our understanding of tissue regeneration, simultaneous quantification and visualization of tissue regeneration processes is not easy to achieve. Herein, we developed a simple and quantitative method for the real-time and non-invasive observation of the process of tissue regeneration. The synthesized ratiometric aggregation-induced-emission (AIE) probe exhibits high selectivity and reversibility for pH responses, good ability to map lysosomal pH both in vitro and in vivo, good biocompatibility and excellent photostability. The caudal fin regeneration of a fish model (medaka larvae) was monitored by tracking the lysosomal pH change. It was found that the mean lysosomal pH is reduced during 24-48 hpa to promote the autophagic activity for cell debris degradation. Our research can quantify the changes in mean lysosomal pH and also exhibit its distribution during the caudal fin regeneration. We believe that the AIE-active lysosomal pH probe can also be potentially used for long-term tracking of various lysosome-involved biological processes, such as tracking the stress responses of tissue, tracking the inflammatory responses, and so on.

Mitochondria-Targeted Polydopamine Nanocomposite with AIE Photosensitizer for Image-Guided Photodynamic and Photothermal Tumor Ablation

Photodynamic therapy (PDT) and photothermal therapy (PTT) are two kinds of treatment for tumors. Herein, a new aggregation-induced emission (AIE)gen (MeO-TPE-indo, MTi) is synthesized with a D-π-A conjugated structure. MTi, which has an electron donor and an acceptor on a tetraphenylethene (TPE) conjugated skeleton, can induce the effective generation of reactive oxygen species (ROS) for PDT. With the guide of the indolium group, MTi can target and image mitochondrion selectively. In order to get good dispersion in water and long-time retention in tumors, MTi is modified on the surface of polydopamine nanoparticles (PDA NPs) to form the nanocomposite (PDA-MeO-TPE-indo, PMTi) by π-π and hydrogen interactions. PMTi is a nanoscale composite for imaging-guided PDT and PTT in tumor treatment, which is constructed with AIEgens and PDA for the first time. The organic functional molecules are combined with nanomaterials for building a multifunctional diagnosis and treatment platform by utilizing the advantages of both sides.

A turn-on near-infrared fluorescence probe with aggregation-induced emission based on dibenzo[a,c]phenazine for detection of superoxide anions and its application in cell imaging

A new turn-on near-infrared fluorescence probe (BDP) based on dibenzo[a,c]phenazine for superoxide anion detection with aggregation-induced emission properties as well as a desirable large Stokes shift was designed and synthesized. After BDP reacted with superoxide, the initial diphenyl-phosphinyl groups of BDP were cleaved, resulting in the production of the pyridinium modified fluorophore (BD) with near-infrared emission. The fluorescent sensor BDP has a high selectivity for superoxide anions over some other intracellular ROSs, reductants, metal ions and amino acids. When HepG2 cells undergo apoptosis and inflammation, BDP is a good probe to keep track of the endogenous superoxide anion level by confocal laser scanning microscopic imaging.

A red mitochondria-targeted AIEgen for visualizing H2S in living cells and tumours

A red mitochondria-targeted AIEgen with greater conjugate and more positive charges for visualizing H₂S in cells and tumours.

Rational Design for Multicolor Flavone-Based Fluorophores with Aggregation-Induced Emission Enhancement Characteristics and Applications in Mitochondria-Imaging

Fluorophores with aggregation-induced emission enhancement (AIEE) properties have attracted more attention in recent years. In order to realise more valuable applications, the different kinds of AIEE molecules are in serious need of further development. Therefore, a novel flavone-based AIEE system derived from restriction of intramolecular rotation (RIR) was designed and synthesized in this work. The results revealed that six of the compounds showed typical AIEE characteristics, with fluorescence emissions from purple, blue, cyan to green, tunable by changing substituent groups. This flavone-based AIEE system has never been reported before. The AIEE characteristics were investigated by optical spectroscopy, fluorescence photographs, scanning electron microscopy (SEM), fluorescence quantum yields (Ф_F) and fluorescence lifetime in the CH₃OH/H₂O mixed solution. Moreover, benefiting from the simple structures and small molecular weight, they could permeate cells faster than current high-molecular-weight AIEE molecules. Furthermore, to examine possible biomedical applications, fluorescence imaging in living A549 lung cells and cell viabilities were examined, and the results displayed that these fluorophores showed good cellular uptake and low cytotoxicity within the experimental concentration range. In addition, these AIEE compounds possessed excellent specificity for mitochondrial targeting and mitochondrial morphological change tracking, besides, they displayed superior photostability, which indicated they are potential candidates for mitochondrial imaging.

Zincke's Salt-Substituted Tetraphenylethylenes for Fluorometric Turn-On Detection of Glutathione and Fluorescence Imaging of Cancer Cells

In this paper, we report Zincke's salt-substituted tetraphenylethylenes 1a and 1b with Cl^- and PF₆^- as counteranions, respectively. The crystal structure of 1b was determined. Both 1a and 1b are almost nonemissive even in the aggregated states. This is attributed to the photoinduced electron transfer from 2,2-bis(4-methoxyphenyl)-1-phenylvinyl-phenyl unit to 1-(2,4-dinitrophenyl) pyridinium unit within 1a and 1b. The results demonstrate that the emissions of 1a and 1b in aqueous solution can be switched on upon either reaction with GSH or light irradiation. On the basis of the reaction between 1a and GSH, 1a can be utilized for the fluorescence turn-on detection of GSH selectively, and GSH with concentration as low as 36.9 nM can be detected. The transformation of 1b into 2 under light irradiation results in the fluorescence imaging of Hela and U2OS cells and phototoxicity toward Hela and U2OS cells after the protonation of pyridine unit in 2 because of the acidic environment of tumor cells. Aggregates of 1b can be up-taken by Hela and U2OS cells and fluorescence imaging has been successfully recorded with CLSM. Moreover, the protonated form of 2 can function as photosensitizer and 1b shows phototoxicity toward tumor cells such as Hela and U2OS cells.

Aggregation-Induced Fluorescence Probe for Monitoring Membrane Potential Changes in Mitochondria

Fluorescent probe 2 was designed for selectively determining mitochondria membrane potential changes. The probe selectively detects changes in the mitochondria membrane potential in a manner that is more sensitive than that of the commercially available indicator, Rodamine 123. As a result, the probe 2 is ideal for future studies designed to assess the functions of mitochondria in cells.

Mitochondrial Imaging with Combined Fluorescence and Stimulated Raman Scattering Microscopy Using a Probe of the Aggregation-Induced Emission Characteristic

In vivo quantitative measurement of biodistribution plays a critical role in the drug/probe development and diagnosis/treatment process monitoring. In this work, we report a probe, named AIE-SRS-Mito, for imaging mitochondria in live cells via fluorescence (FL) and stimulated Raman scattering (SRS) imaging. The probe features an aggregation-induced emission (AIE) characteristic and possesses an enhanced alkyne Raman peak at 2223 cm^-1. The dual-mode imaging of AIE-SRS-Mito for selective mitochondrion-targeting was examined on a homemade FL-SRS microscope system. The detection limit of the probe in the SRS imaging was estimated to be 8.5 μM. Due to the linear concentration dependence of SRS and inertness of the alkyne Raman signal to environmental changes, the intracellular distribution of the probe was studied, showing a local concentration of >2.0 mM in the mitochondria matrix, which was >100-fold higher than the incubation concentration. To the best of our knowledge, this is the first time that the local concentration of AIE molecules inside cells has been measured noninvasively and directly. Also, the nonquenching effect of such AIE molecules in cell imaging has been verified by the positive correlation of FL and SRS signals. Our work will encourage the utilization of SRS microscopy for quantitative characterization of FL probes or other nonfluorescent compounds in living biological systems and the development of FL-SRS dual-mode probes for specific biotargets.

Organic nanostructure-based probes for two-photon imaging of mitochondria and microbes with emission between 430 nm and 640 nm

Multi-photon excitation and versatile fluorescent probes are in high need for biological imaging, since one probe can satisfy many needs as a biosensor. Herein we synthesize a series of two-photon excited probes based on tetraphenylethene (TPE) structures (TPE-Acr, TPE-Py, and TPE-Quino), which can image both mammalian cells and bacteria based on aggregation-induced emission (AIE) without washing them. Because of cationic moieties, the fluorescent molecules can aggregate into nanoscale fluorescent organic nanoscale dots to image mitochondria and bacteria with tunable emissions using both one-photon and two-photon excitation. Our research demonstrates that these AIE-dots expand the functions of luminescent organic dots to construct efficient fluorescent sensors applicable to both one-photon and two-photon excitation for bio-imaging of bacteria and mammalian cells.

Introductory lecture: recent research progress on aggregation-induced emission

Since the discovery of the aggregation-induced emission (AIE) phenomenon in 2001, research on AIE molecules has drawn much attention, and this area has been expanding tremendously. This brief review will focus on recent advances in the science and application of AIE molecules, including new mechanistic understanding, new AIE molecules for sensing and imaging, stimuli-responsive AIE molecules and applications of AIE molecules for OLEDs. Moreover, this review will give a perspective on the possible opportunities and challenges that exist in the future for this area.

Emissive nanoparticles from pyridinium-substituted tetraphenylethylene salts: imaging and selective cytotoxicity towards cancer cells in vitro and in vivo by varying counter anions

Chemotherapeutics specifically targeting cancer cells without damaging healthy cells is the long-awaited goal of cancer treatment. In this paper, a series of nanoparticles (NanoTPES 1-4) assembled from pyridinium-substituted tetraphenylethylene salts were synthesized and investigated both in vitro and in vivo for this purpose. By changing the counter anions, NanoTPES 1-4 exhibit tunable emission colors, sizes and surface charges. NanoTPES 2 and 3 with tetraphenyl borate and tetra(4-chlorophenyl) borate as the respective anions selectively imaged and targeted mitochondria in cancer cells. Accordingly, these two nanoparticles specifically kill cancer cells with minimal effect on normal cells. Such selective cytotoxicity was attributed to the change of membrane potential and inhibition of ATP synthesis in the mitochondria of cancer cells. Furthermore, both NanoTPES 2 and 3 exhibited efficient tumor accumulation and tumor growth inhibition in vivo, with negligible systemic toxicity.

A membrane-activatable near-infrared fluorescent probe with ultra-photostability for mitochondrial membrane potentials

Dark for light: A fluorescence quencher was turned into a near-infrared probe for mitochondrial membrane potential in living cells and mice.

Fluorescent probes for the selective detection of chemical species inside mitochondria

This feature article systematically summarizes the development of fluorescent probes for the selective detection of chemical species inside mitochondria.

Studies on the structures, cytotoxicity and apoptosis mechanism of 8-hydroxylquinoline rhodium(iii) complexes in T-24 cells

Two rhodium(iii) complexes showed good cytotoxicity. The underlying investigation of the apoptosis mechanism suggested that the mitochondrial apoptotic pathway was involved.

Organelle-specific bioprobes based on fluorogens with aggregation-induced emission (AIE) characteristics

A number of aggregation-induced emission (AIE) probes with high photostability and specificity have been developed for organelle imaging and image-guided cancer cell ablation.

Fluorescence turn-on for the highly selective detection of nitric oxide in vitro and in living cells

A water-soluble, biocompatible, small molecular fluorescent turn-on probe was developed for the highly selective and sensitive detection of NO in vitro and in living cells.