A de novo strategy to develop NIR precipitating fluorochrome for long-term in situ cell membrane bioimaging

Water solubility and folate receptor affinity-driven plasma membrane-targeted carbon dots for cancer cell imaging

Long-term labeling of the plasma membrane is crucial for visualizing membrane protein expression and morphological changes but is challenging due to the high fluidity of the plasma membrane, which can lead to probe diffusion or internalization of cells. Here, we precisely control the localization of carbon dots (M-CDs) on the plasma membrane without internalization after long-term observation under fluorescence microscopy. Adjusting the molar ratio of folic acid to o-phenylenediamine allowed fine-tuning of the water solubility and fluorescence emission of the carbon dots. Notably, carbon dots synthesized with a folic acid to o-phenylenediamine molar ratio of 1 : 10 (referred to as M-CD) exhibit excellent cell membrane targeting, likely due to the combination of suitable water-solubility and FA-FR affinity. The photostability of M-CDs is significantly superior to that of the commercial CellMask Crimson, allowing for specific recognition of folic acid receptor-positive cancer cells and minimal internalization over a period of up to 9 hours. This photostable, membrane-targeting M-CD provides a powerful tool for accurately, real-time, and non-invasively assessing the expression of folic acid receptors on cancer cell membranes and tumor metastasis.

Fluorescence-plane polarization for the real-time monitoring of transferase migration in living cells

Transferases are enzymes that exhibit multisite migration characteristics. Significantly, enzyme activity undergoes changes during this migration process, which inevitably impacts the physiological function of living organisms and can even lead to related malignant diseases. However, research in this field has been severely hindered by the lack of tools for the simultaneous and differential monitoring of site-specific transferase activity. Herein, we propose a novel strategy that integrates a fluorescence signal response with high sensitivity and an optical rotation signal response with superior spatial resolution. To validate the feasibility of this strategy, transferase γ-glutamyltransferase (GGT) was used as a model system to develop dual-mode chiral probes ACx-GGTB (AC17-GGTB and AC15-GGTB) using chiral amino acids as specific bifunctional recognition groups. The probes undergo structural changes under GGT, resulting in the release of bifunctional recognition groups (chiral amino acids) and simultaneously generate fluorescence signals and optical rotation signals. This dual-mode output exhibits high sensitivity and facilitates differentiation of sites. Furthermore, it enables simultaneous and differential detection of GGT activity at different sites during migration. We anticipate that probes developed based on this strategy will facilitate imaging-based monitoring of the activity for other transferases, thus providing an imaging platform suitable for the real-time tracking of transferase activity changes during migration.

Twisted Intramolecular Charge Transfer-Based Viscosity-Responsive Probe Reveals Lysosomal Degradation Process of Endocytosed Foreign Bodies

The optimization of nanomedicines requires a thorough understanding of nanocarrier attrition during lysosome-mediated biological processes. Real-time monitoring of endocytosis provides valuable insights into the lysosomal effects on nanocarriers and the release of nanodrugs. We report the development of a coresponsive probe that detects changes in the spatial viscosity of the intracellular domain caused by lysosomal degradation of foreign bodies. The probe, based on a benzofuro[2,3-d]pyrimidine structure, exhibits torsional intramolecular charge transfer (TICT) and responds to ambient viscosity changes with a sensitive fluorescence intensity. The antidiffused fluorescence transition of the probe in the spatially restricted domain serves as a key indicator for real-time monitoring. When encapsulated with diverse foreign bodies and emitted into macrophages by endocytosis, the probe forms nanoparticles. Lysosomes uptake these materials for intracellular digestion, causing alterations in the aggregation or depolymerization state of the nanoparticles, leading to viscosity changes manifested by the probe's fluorescence. By studying the spatial viscosity changes caused by lysosomal degradation of foreign bodies, our monitoring strategy contributes to understanding the digestion or escape capabilities of potential pharmaceutical-carrying nanocarriers, providing guidelines to design more effective nanocarriers that navigate lysosomal degradation to achieve precise drug payloads and release.

Fluorescent Nanocable as a Biomedical Tool: Intracellular Self-Assembly Formed by a Natural Product Interconnects and Synchronizes Mitochondria

Self-assembly processes commonly occur in various biological contexts to form functional biological structures. However, the self-assembly of nanofibers within cells by heterologous molecules showing a biological function is rare. In this work, we reported the intracellular formation of fluorescent nanofibers by a natural small molecule, lycobetaine (LBT), which facilitated the direct physical connection between mitochondria and synchronized their membrane potential oscillations. The luminescent properties of LBT enabled the real-time observation of nanofiber formation, while the semiconductive nature of the LBT nanofiber facilitated electrical signal transduction among the connected mitochondria. This study introduces an approach to modulate mitochondrial connectivity within cells using "nano-cables" which facilitate studies on synchronized mitochondrial operations and the underlying mechanisms of drug action.

A Near-Infrared-II Excitable Pyridinium Probe with 1000-Fold ON/OFF Ratio for γ-Glutamyltranspeptidase and Cancer Detection

Activity-based detection of γ-Glutamyltranspeptidase (GGT) using near-infrared (NIR) fluorescent probes is a promising strategy for early cancer diagnosis. Although NIR pyridinium probes show high performance in biochemical analysis, the aggregation of both the probes and parental fluorochromes in biological environments is prone to result in a low signal-to-noise ratio (SBR), thus affecting their clinical applications. Here, we develop a GGT-activatable aggregate probe called OTBP-G for two-photon fluorescence imaging in various biological environments under 1040 nm excitation. By rationally tunning the hydrophilicity and donor-acceptor strength, we enable a synergistic effect between twisted intramolecular charge transfer and intersystem crossing processes and realize a perfect dark state for OTBP-G before activation. After the enzymatic reaction, the parental fluorochrome exhibits bright aggregation-induced emission peaking at 670 nm. The fluorochrome-to-probe transformation can induce 1000-fold fluorescence ON/OFF ratio, realizing in vitro GGT detection with an SBR > 900. Activation of OTBP-G occurs within 1 min in vivo, showing an SBR > 400 in mouse ear blood vessels. OTBP-G can further enable the early detection of pulmonary metastasis in breast cancer by topically spraying, outperforming the clinical standard hematoxylin and eosin staining. We anticipate that the in-depth study of OTBP-G can prompt the development of early cancer diagnosis and tumor-related physiological research. Moreover, this work highlights the crucial role of hydrophilicity and donor-acceptor strength in maximizing the ON/OFF ratio of the TICT probes and showcases the potential of OTBP as a versatile platform for activity-based sensing.

Quinone Methide Based Self-Immobilizing Molecular Fluorescent Probes for In Situ Imaging of Enzymes

Enzymes play important roles not only in normal physiological processes but in the development of many diseases. In situ imaging of enzymes with high-resolution in living systems would helpful for clinical diagnosis and treatment. However, many molecular fluorescent probes suffer from the drawback of diffusing away from the reaction site of enzymes even out of the cells, losing the in situ information and resulting in poor imaging resolution. Quinone methide (QM) based self-immobilizing probes allow the fluorescent signal to be immobilized near the target for an extended period without deactivating the target enzymes, ensuring that it will provide amplified signals and in situ information of the target with high resolution. In this review, we summarized the recent progress of QM-based self-immobilizing probes including their design strategies, working mechanisms, classifications and applications in in situ enzyme imaging. This review calls for the development of more activatable QM-based probe with the advantages of high stability in the absence of the target but very high labeling efficiency after activation.

Highly-ordered assembled organic fluorescent materials for high-resolution bio-sensing: a review

Organic fluorescent materials (OFMs) play a crucial role in the development of biosensors, enabling the extraction of biochemical information within cells and organisms, extending to the human body. Concurrently, OFM biosensors contribute significantly to the progress of modern medical and biological research. However, the practical applications of OFM biosensors face challenges, including issues related to low resolution, dispersivity, and stability. To overcome these challenges, scientists have introduced interactive elements to enhance the order of OFMs. Highly-ordered assembled OFMs represent a novel material type applied to biosensors. In comparison to conventional fluorescent materials, highly-ordered assembled OFMs typically exhibit robust anti-diffusion properties, high imaging contrast, and excellent stability. This approach has emerged as a promising method for effectively tracking bio-signals, particularly in the non-invasive monitoring of chronic diseases. This review introduces several highly-ordered assembled OFMs used in biosensors and also discusses various interactions that are responsible for their assembly, such as hydrogen bonding, π-π interaction, dipole-dipole interaction, and ion electrostatic interaction. Furthermore, it delves into the various applications of these biosensors while addressing the drawbacks that currently limit their commercial application. This review aims to provide a theoretical foundation for designing high-performance, highly-ordered assembled OFM biosensors suitable for practical applications. Additionally, it sheds light on the evolving trends in OFM biosensors and their application fields, offering valuable insights into the future of this dynamic research area.

Tumor Microenvironment Activatable Nanoprodrug System for In Situ Fluorescence Imaging and Therapy of Liver Cancer

The development of new imaging and treatment nanoprodrug systems is highly demanded for diagnosis and therapy of liver cancer, a severe disease characterized by a high recurrence rate. Currently, available small molecule drugs are not possible for cancer diagnosis because of the fast diffusion of imaging agents and low efficacy in treatment due to poor water solubility and significant toxic side effects. In this study, we report the development of a tumor microenvironment activatable nanoprodrug system for the diagnosis and treatment of liver cancer. This nanoprodrug system can accumulate in the tumor site and be selectively activated by an excess of hydrogen peroxide (H2O2) in the tumor microenvironment, releasing near-infrared solid-state organic fluorescent probe (HPQCY-1) and phenylboronic acid-modified camptothecin (CPT) prodrug. Both HPQCY-1 and CPT prodrugs can be further activated in tumor sites for achieving more precise in situ near-infrared (NIR) fluorescence imaging and treatment while reducing the toxic effects of drugs on normal tissues. Additionally, the incorporation of hydrophilic multivalent chitosan as a carrier effectively improved the water solubility of the system. This research thus provides a practical new approach for the diagnosis and treatment of liver cancer.

Target-activated multicolor fluorescent dyes for 3D imaging of plasma membranes and tracking of apoptosis

Real-time tracking of dynamic changes in the three-dimensional morphology of the cell plasma membrane is of great importance for a deeper understanding of physiological processes related to the cell plasma membrane. However, there is a lack of imaging dyes that can specifically be used for a long term labelling of plasma membranes, especially for plant cells. Here, we have used molecular engineering strategies to develop a series of target-activated multicolour fluorescent dyes that can be used for long-term and three-dimensional imaging of plant cell plasma membranes. By combining different electron acceptors and donors, four molecular backbones with different emission colours from green to NIR have been obtained. In the designed styrene-based dyes, referred to as the SD dyes, several functional groups were introduced into the backbones to achieve the properties of target-activated fluorescence, rapid and wash-free staining, high plasma membrane targeting ability and long-term imaging function. Using onion epidermal cells as a platform, these dye molecules can provide high-quality imaging of the plasma membrane for up to 6 hours, providing a powerful tool for long-term monitoring of plasma membrane-related biological events. Calcium-mediated apoptosis of plant cells has been tracked for the first time by monitoring the morphological changes of the plasma membrane in real time using SD dyes. These dyes also exhibit excellent 3D imaging performance of the plasma membrane and were further used to track in real time the 3D morphological changes of the plasma membrane during plasmolysis of plant cells, providing a powerful imaging tool for three-dimensional (3D) biology. This work provides a set of multi-colour dye tools for long-term and three-dimensional imaging of plant cell plasma membranes, and also provides molecular design principles for guiding the transmembrane transport of small molecules.

Enzyme-Activatable Near-Infrared Photosensitizer with High Enrichment in Tumor Cells Based on a Multi-Effect Design

Enzyme-activatable near-infrared (NIR) fluorescent probes and photosensitizers (PSs) have emerged as promising tools for molecular imaging and photodynamic therapy (PDT). However, in living organisms selective retention or even enrichment of these reagents after enzymatic activation at or near sites of interest remains a challenging task. Herein, we integrate non-covalent and covalent retention approaches to introduce a novel "1-to-3" multi-effect strategy-one enzymatic stimulus leads to three types of effects-for the design of an enzyme-activatable NIR probe or PS. Using this strategy, we have constructed an alkaline phosphatase (ALP)-activatable NIR fluorogenic probe and a NIR PS, which proved to be selectively activated by ALP to switch on NIR fluorescence or photosensitizing ability, respectively. Additionally, these reagents showed significant enrichment (over 2000-fold) in ALP-overexpressed tumor cells compared to the culture medium, accompanied by massive depletion of intracellular thiols, the major antioxidants in cells. The investigation of this ALP-activatable NIR PS in an in vivo PDT model resulted in complete suppression of HeLa tumors and full recovery of all tested mice. Encouragingly, even a single administration of this NIR PS was sufficient to completely suppress tumors in mice, demonstrating the high potential of this strategy in biomedical applications.

Activatable Near-Infrared Versatile Fluorescent and Chemiluminescent Dyes Based on the Dicyanomethylene-4H-pyran Scaffold: From Design to Imaging and Theranostics

Activatable fluorescent and chemiluminescent dyes with near-infrared emission have indispensable roles in the fields of bioimaging, molecular prodrugs, and phototheranostic agents. As one of the most popular fluorophore scaffolds, the dicyanomethylene-4H-pyran scaffold has been applied to fabricate a large number of versatile activatable optical dyes for analytes detection and diseases diagnosis and treatment by virtue of its high photostability, large Stokes shift, considerable two-photon absorption cross-section, and structural modifiability. This review discusses the molecular design strategies, recognition mechanisms, and both in vitro and in vivo bio-applications (especially for diagnosis and therapy of tumors) of activatable dicyanomethylene-4H-pyran dyes. The final section describes the current shortcomings and future development prospects of this topic.

Activatable organic probes for in situ imaging of biomolecules

Biomolecules are fundamental for various chemical and biological processes of living organisms. High-resolution in situ imaging of the dynamics and local distribution of biomolecules may facilitate better interpretation of diverse complex cell events in the biomedicine field. In different advanced imaging tools, fluorescence imaging-based activatable organic probes can be noninvasively and effortlessly internalized into cells and can be easily modified, which is essential for the in situ imaging of targets in living organisms. We here briefly summarize the existing general design strategies of activatable organic probes for retaining the fluorescence signal inside cells. We particularly describe the bioapplication of these probes for the in situ bioimaging. This review is expected to promote the development of new molecular tools for extending the application of these in situ imaging strategies to other biomolecules.

Prediction of Early Atherosclerotic Plaques Using a Sequence-Activated Fluorescence Probe for the Simultaneous Detection of γ-Glutamyl Transpeptidase and Hypobromous Acid

Atherosclerosis is a lipoprotein-driven disease, and there is no effective therapy to reverse atherosclerosis or existing plaques. Therefore, it is urgently necessary to create a noninvasive and reliable approach for early atherosclerosis detection to prevent initial plaque formation. Atherosclerosis is intimately associated with inflammation, which is accompanied by an excess of reactive oxygen species (ROS), leading to cells requiring more glutathione (GSH) to resist severe oxidative stress. Therefore, the GSH-hydrolyzed protein γ-glutamyl transpeptidase (GGT) and the ROS-hypobromous acid (HBrO) are potential biomarkers for predicting atherogenesis. Hence, to avoid false-positive diagnoses caused by a single biomarker, we constructed an ingenious sequence-activated double-locked TP fluorescent probe, C-HBrO-GGT, in which two sequential triggers of GGT and HBrO are meticulously designed to ensure that the probe fluoresces in response to HBrO only after GGT hydrolyzes the probe. By utilization of C-HBrO-GGT, the voltage-gated chloride channel (CLC-1)-HBrO-catalase (CAT)-GGT signaling pathway was confirmed in cellular level. Notably, the forthcoming atherosclerotic plaques were successfully predicted before the plaques could be observed via the naked eye or classical immunofluorescent staining. Collectively, this research proposed a powerful tool to indicate the precise position of mature plaques and provide early warning of atherosclerotic plaques.

De Novo Design of Near-Infrared Fluorescent Agents Activated by Peroxynitrite and Glutathione-Responsive Imaging for Diabetic Liver Disease

Diabetes and its complications, such as diabetes liver disease, is a major problem puzzling people's health. The detection of redox states in its pathological process can effectively help us gain a deeper understanding of the disease. The pair of oxidation-reduction substances peroxynitrite (ONOO- ) and glutathione (GSH) is considered to be closely related to their occurrence and development. Thus, direct visualization of ONOO- and GSH in diabetes liver disease is critical to evaluate the disease at the molecular level. Herein, two activatable agents NTCF-ONOO- and NTCF-GSH are prepared for selectively detecting ONOO- and GSH through protection and deprotection strategies based on hydroxyl and amino groups of near-infrared fluorophore. Fluorescence imaging of exogenous and endogenous ONOO- and GSH changes in living cells and in vivo is observed. The ONOO- and GSH level in the diabetes liver disease cellular model are visualized and the possible redox imbalance mechanism related to the oxidized (NAD+ ) and reduced (NADH) nicotinamide adenine dinucleotides is explored in this process. Moreover, these probes can sensitively recognize ONOO- and GSH in the process of oxidative stress resulting from streptozotocin and streptozotocin/acetaminophen-induced complex diabetic liver disease in vivo. In addition, they can be applied for monitoring the clinical serum sample related with diabetic patients.

High-precision detection and navigation surgery of colorectal cancer micrometastases

Surgical resection is an effective treatment for colorectal cancer (CRC) patients, whereas occult metastases hinder the curative effect. Currently, there is no effective method to achieve intraoperatively diagnosis of tumor-positive lymph nodes (LNs). Herein, we adopt a near-infrared-II (NIR-II) organic donor-pi-acceptor-pi-donor probe FE-2PEG, which exhibits bright fluorescence over 1100 nm, excellent photostability, blood circulation time, and biocompatibility, to achieve high-performance bioimaging with improved temporal and spatial resolution. Importantly, the FE-2PEG shows efficient passive enrichment in orthotopic CRC, metastatic mesenteric LNs, and peritoneal metastases by enhanced permeability and retention effect. Under NIR-II fluorescence-guided surgery (FGS), the peritoneal micrometastases were resected with a sensitivity of 94.51%, specificity of 86.59%, positive predictive value (PPV) of 96.57%, and negative predictive value of 79.78%. The PPV still achieves 96.07% even for micrometastases less than 3 mm. Pathological staining and NIR-II microscopy imaging proved that FE-2PEG could successfully delineate the boundary between the tumor and normal tissues. Dual-color NIR-II imaging strategy with FE-2PEG (1100 ~ 1300 nm) and PbS@CdS quantum dots (> 1500 nm) successfully protects both blood supply and normal tissues during surgery. The NIR-II-based FGS provides a promising prospect for precise intraoperative diagnosis and minimally invasive surgery of CRC.

Recent progress of self-immobilizing and self-precipitating molecular fluorescent probes for higher-spatial-resolution imaging

Flourished in the past two decades, fluorescent probe technology provides researchers with accurate and efficient tools for in situ imaging of biomarkers in living cells and tissues and may play a significant role in clinical diagnosis and treatment such as biomarker detection, fluorescence imaging-guided surgery, and photothermal/photodynamic therapy. In situ imaging of biomarkers depends on the spatial resolution of molecular probes. Nevertheless, the majority of currently available molecular fluorescent probes suffer from the drawback of diffusing from the target region. This leads to a rapid attenuation of the fluorescent signal over time and a reduction in spatial resolution. Consequently, the diffused fluorescent signal cannot accurately reflect the in situ information of the target. Self-immobilizing and self-precipitating molecular fluorescent probes can be used to overcome this problem. These probes ensure that the fluorescent signal remains at the location where the signal is generated for a long time. In this review, we introduce the development history of the two types of probes and classify them in detail according to different design strategies. In addition, we compare their advantages and disadvantages, summarize some representative studies conducted in recent years, and propose prospects for this field.

Orderly Self-Assembly of Organic Fluorophores for Sensing and Imaging

Fluorescence imaging utilizing traditional organic fluorophores is extensively applied in both cellular and in vivo studies. However, it faces significant obstacles, such as low signal-to-background ratio (SBR) and spurious positive/negative signals, primarily due to the facile diffusion of these fluorophores. To cope with this challenge, orderly self-assembled functionalized organic fluorophores have gained significant attention in the past decades. These fluorophores can create nanoaggregates via a well-ordered self-assembly process, thus prolonging their residency time within cells and in vivo settings. The development of self-assembled-based fluorophores is an emerging field, and as such, in this review, we present a summary of the progress and challenges of self-assembly fluorophores, focusing on their development history, self-assembly mechanisms, and biomedical applications. We hope that the insights provided herein will assist scientists in further developing functionalized organic fluorophores for in situ imaging, sensing, and therapy.

Construction of HPQ-based activatable fluorescent probe for peroxynitrite and its application in ferroptosis and mice model of LPS-induced inflammation

As one of the important members of reactive oxygen species, ONOO- plays a crucial role in signal transduction, immune response, and other physiological activities. Aberrant changes in ONOO- levels in the living organism are usually associated with many diseases. Therefore, it is important to establish a highly selective and sensitive method for the determination of ONOO- in vivo. Herein, we designed a novel ratio near-infrared fluorescent probe for ONOO- by directly conjugating dicyanoisophorone (DCI) to hydroxyphenyl-quinazolinone (HPQ). Surprisingly, HPQD was unaffected by environmental viscosity and responded rapidly to ONOO- within 40 s. The linear range of ONOO- detection was from 0 μM to 35 μM. Impressively, HPQD did not react with reactive oxygen species and was sensitive to exogenous/endogenous ONOO- in live cells. We also investigated the relationship between ONOO- and ferroptosis and achieved in vivo diagnosis and efficacy evaluation of mice model of LPS-induced inflammation, which showed promising prospects of HPQD in ONOO--related studies.

Enzyme-Instructed Aggregation/Dispersion of Fluorophores for Near-Infrared Fluorescence Imaging In Vivo

Near-infrared (NIR) fluorescence is a noninvasive, highly sensitive, and high-resolution modality with great potential for in vivo imaging. Compared with "Always-On" probes, activatable NIR fluorescent probes with "Turn-Off/On" or "Ratiometric" fluorescent signals at target sites exhibit better signal-to-noise ratio (SNR), wherein enzymes are one of the ideal triggers for probe activation, which play vital roles in a variety of biological processes. In this review, we provide an overview of enzyme-activatable NIR fluorescent probes and concentrate on the design strategies and sensing mechanisms. We focus on the aggregation/dispersion state of fluorophores after the interaction of probes and enzymes and finally discuss the current challenges and provide some perspective ideas for the construction of enzyme-activatable NIR fluorescent probes.

Organic Semiconducting Nanoparticles for Biosensor: A Review

Highly bio-compatible organic semiconductors are widely used as biosensors, but their long-term stability can be compromised due to photo-degradation and structural instability. To address this issue, scientists have developed organic semiconductor nanoparticles (OSNs) by incorporating organic semiconductors into a stable framework or self-assembled structure. OSNs have shown excellent performance and can be used as high-resolution biosensors in modern medical and biological research. They have been used for a wide range of applications, such as detecting small biological molecules, nucleic acids, and enzyme levels, as well as vascular imaging, tumor localization, and more. In particular, OSNs can simulate fine particulate matters (PM_2.5, indicating particulate matter with an aerodynamic diameter less than or equal to 2.5 μm) and can be used to study the biodistribution, clearance pathways, and health effects of such particles. However, there are still some problems that need to be solved, such as toxicity, metabolic mechanism, and fluorescence intensity. In this review, based on the structure and design strategies of OSNs, we introduce various types of OSNs-based biosensors with functional groups used as biosensors and discuss their applications in both in vitro and in vivo tracking. Finally, we also discuss the design strategies and potential future trends of OSNs-based biosensors. This review provides a theoretical scaffold for the design of high-performance OSNs-based biosensors and highlights important trends and future directions for their development and application.

In situ orderly self-assembly strategy affording NIR-II-J-aggregates for in vivo imaging and surgical navigation

J-aggregation, an effective strategy to extend wavelength, has been considered as a promising method for constructing NIR-II fluorophores. However, due to weak intermolecular interactions, conventional J-aggregates are easily decomposed into monomers in the biological environment. Although adding external carriers could help conventional J-aggregates stabilize, such methods still suffer from high-concentration dependence and are unsuitable for activatable probes design. Besides, these carriers-assisted nanoparticles are risky of disassembly in lipophilic environment. Herein, by fusing the precipitated dye (HPQ) which has orderly self-assembly structure, onto simple hemi-cyanine conjugated system, we construct a series of activatable, high-stability NIR-II-J-aggregates which overcome conventional J-aggregates carrier's dependence and could in situ self-assembly in vivo. Further, we employ the NIR-II-J-aggregates probe HPQ-Zzh-B to achieve the long-term in situ imaging of tumor and precise tumor resection by NIR-II imaging navigation for reducing lung metastasis. We believe this strategy will advance the development of controllable NIR-II-J-aggregates and precise bioimaging in vivo.

Long-term spatiotemporal and highly specific imaging of the plasma membrane of diverse plant cells using a near-infrared AIE probe

Fluorescent probes are valuable tools to visualize plasma membranes intuitively and clearly and their related physiological processes in a spatiotemporal manner. However, most existing probes have only realized the specific staining of the plasma membranes of animal/human cells within a very short time period, while almost no fluorescent probes have been developed for the long-term imaging of the plasma membranes of plant cells. Herein, we designed an AIE-active probe with NIR emission to achieve four-dimensional spatiotemporal imaging of the plasma membranes of plant cells based on a collaboration approach involving multiple strategies, demonstrated long-term real-time monitoring of morphological changes of plasma membranes for the first time, and further proved its wide applicability to plant cells of different types and diverse plant species. In the design concept, three effective strategies including the similarity and intermiscibility principle, antipermeability strategy and strong electrostatic interactions were combined to allow the probe to specifically target and anchor the plasma membrane for an ultralong amount of time on the premise of guaranteeing its sufficiently high aqueous solubility. The designed APMem-1 can quickly penetrate cell walls to specifically stain the plasma membranes of all plant cells in a very short time with advanced features (ultrafast staining, wash-free, and desirable biocompatibility) and the probe shows excellent plasma membrane specificity without staining other areas of the cell in comparison to commercial FM dyes. The longest imaging time of APMem-1 can be up to 10 h with comparable performance in both imaging contrast and imaging integrity. The validation experiments on different types of plant cells and diverse plants convincingly proved the universality of APMem-1. The development of plasma membrane probes with four-dimensional spatial and ultralong-term imaging ability provides a valuable tool to monitor the dynamic processes of plasma membrane-related events in an intuitive and real-time manner.

Molecular Engineering of Near-Infrared Fluorescent Probes for Cell Membrane Imaging

Cell membrane (CM) is a phospholipid bilayer that maintains integrity of a whole cell and relates to many physiological and pathological processes. Developing CM imaging tools is a feasible method for visualizing membrane-related events. In recent decades, small-molecular fluorescent probes in the near-infrared (NIR) region have been pursued extensively for CM staining to investigate its functions and related events. In this review, we summarize development of such probes from the aspect of design principles, CM-targeting mechanisms and biological applications. Moreover, at the end of this review, the challenges and future research directions in designing NIR CM-targeting probes are discussed. This review indicates that more efforts are required to design activatable NIR CM-targeting probes, easily prepared and biocompatible probes with long retention time regarding CM, super-resolution imaging probes for monitoring CM nanoscale organization and multifunctional probes with imaging and phototherapy effects.

Enhancing the Selectivity of Leucine Aminopeptidase Near-Infrared Fluorescent Probes for Assisting in Surgical Tumor Resection

Selective fluorescence imaging of analytes is a challenge for monitoring diseases as homologues interfere with the imaging agents. Leucine aminopeptidase (LAP), a kind of protease, is related to tumor pathogenesis. The known LAP fluorescent probes based on leucine recognition have limited selectivity. Herein, a selective t-butyl-alanine recognition unit for LAP through the ligand regulation strategy is prepared as a new near-infrared (NIR) fluorescent probe (DCM-LAP) having a large Stokes shift of 214 nm and a high sensitivity with a detection limit of 168 mU/L. DCM-LAP has an enhanced response toward LAP with NIR fluorescence at 656 nm based on intramolecular charge transfer. The probe is selective without being interfered with by biological enzymes including the aminopeptidase N (APN). DCM-LAP can image LAP activity in living cells. It can also visualize the cell invasion and migration processes. DCM-LAP is employed in the real-time imaging of LAP in tumor-bearing nude mice and guides in the accurate resection of breast tumors. It also distinguishes tumor tissues from normal with a high tumor-to-normal ratio (9.8). The DCM-LAP probe can thus assist in the investigations of LAP-associated clinical disease.

Conjugated Aggregation-Induced Fluorescent Materials for Biofluorescent Probes: A Review

The common fluorescent conjugated materials present weak or quenching luminescent phenomena in the solid or aggregate state (ACQ), which limits their applications in medicine and biology. In the last two decades, certain materials, named aggregation-induced emission (AIE) fluorescent materials, have exhibited strong luminescent properties in the aggregate state, which can overcome the ACQ phenomenon. Due to their intrinsic properties, the AIE materials have been successfully used in biolabeling, where they can not only detect the species of ions and their concentrations in organisms, but can also monitor the organisms' physiological activity. In addition, these kinds of materials often present non-biological toxicity. Thus, AIE materials have become some of the most popular biofluorescent probe materials and are attracting more and more attention. This field is still in its early infancy, and several open challenges urgently need to be addressed, such as the materials' biocompatibility, metabolism, and so on. Designing a high-performance AIE material for biofluorescent probes is still challenging. In this review, based on the molecular design concept, various AIE materials with functional groups in the biofluorescent probes are introduced, including tetrastyrene materials, distilbene anthracene materials, triphenylamine materials, and hexaphenylsilole materials. In addition, according to the molecular system design strategy, the donor-acceptor (D-A) system and hydrogen-bonding AIE materials used as biofluorescent probes are reviewed. Finally, the biofluorescent probe design concept and potential evolution trends are discussed. The final goal is to outline a theoretical scaffold for the design of high-performance AIE biofluorescent probes that can at the same time further the development of the applications of AIE-based biofluorescent probes.

γ-Glutamyltransferase-Activatable Fluoro-Photoacoustic Reporter for Highly Sensitive Diagnosis of Acute Liver Injury and Tumor

γ-Glutamyltransferase (GGT) has been recognized as an important clinical biomarker that is closely related to many diseases. Visualizing the GGT fluctuation facilitates early disease-related diagnosis and therapy. Herein, an activated probe (NIR-GGT) for the imaging of GGT activity was prepared. The probe consists of a stable NIR fluorophore with the tunable amino group decorated with the γ-glutamate group as a GGT-sensing unit linked by a self-elimination group. NIR-GGT can sensitively recognize GGT and cause a strong turn-on fluorescent and photoacoustic signal. The up-regulation of the GGT expression in acetaminophen-induced acute liver injury was imaged using NIR-GGT. The probe can track changes in the GGT level in the early stages of drug-induced acute liver injury (DIALI) and its remedy process by fluorescent and photoacoustic dual-modality imaging with a high temporal-spatial resolution. NIR-GGT can also be used to differentiate between tumor and para-carcinowa tissues in vivo. The probe may be a potential tool for the diagnosis of early-stage DIALI and accurate tumor resection in the clinical field.

Molecularly engineered AIEgens with enhanced quantum and singlet-oxygen yield for mitochondria-targeted imaging and photodynamic therapy

Luminogens characteristic of aggregation-induced emission (AIEgens) have been extensively exploited for the development of imaging-guided photodynamic therapeutic (PDT) agents. However, intramolecular rotation of donor-acceptor (D-A) type AIEgens favors non-radiative decay of photonic energy which results in unsatisfactory fluorescence quantum and singlet oxygen yields. To address this issue, we developed several molecularly engineered AIEgens with partially "locked" molecular structures enhancing both fluorescence emission and the production of triplet excitons. A triphenylphosphine group was introduced to form a D-A conjugate, improving water solubility and the capacity for mitochondrial localization of the resulting probes. Experimental and theoretical analyses suggest that the much higher quantum and singlet oxygen yield of a structurally "significantly-locked" probe (LOCK-2) than its "partially locked" (LOCK-1) and "unlocked" equivalent (LOCK-0) is a result of suppressed AIE and twisted intramolecular charge transfer. LOCK-2 was also used for the mitochondrial-targeting, fluorescence image-guided PDT of liver cancer cells.

High-fidelity imaging of lysosomal enzyme through in situ ordered assembly of small molecular fluorescent probes

As an organelle in cells, lysosomes play an important role in the degradation of biological macromolecules and pathogens. To elucidate the function of lysosomes in normal or disease states, recently, various fluorescent probes have been reported for imaging lysosomal analytes. However, because of the particularity of the lysosomal environment, most of the reported lysosomal fluorescent probes suffered from a series of practical issues such as easy diffusion, low detection signal-to-background ratio and false signal. To address these issues, based on an optimized in situ ordered assembly solid-state fluorophore HDPQ, we herein put forward a new strategy for the design of lysosomal enzymes probes. As a proof concept, we synthesized a fluorescent probe HDPQ-GLU for lysosomal enzyme β-glucuronidase (GLU). Experiment results displayed that compared with general lysosomal probe, the novel lysosomal probe not only exhibited excellent anti-pH interference ability and high signal-to-noise ratio in aqueous solution, but also has excellent long-term in situ imaging ability in the living system. Using this probe, we have realized high-fidelity and long-term in situ tracking GLU in lysosomes of living cells and evaluated the dynamic changes of GLU during the growth period of zebrafish. We anticipate that the new strategy based on the novel in situ ordered assembly solid-state fluorophore HDPQ may be a potential platform for developing fluorescent probes for high-fidelity imaging of lysosomal enzymes.

A fast responsive and cell membrane-targetable near-infrared H2S fluorescent probe for drug resistance bioassays in chemotherapy

A cell membrane-targeted near-infrared fluorescent probe, CMCu-H₂S, was fabricated through employing hydrophobic chains and cyclen-Cu²⁺ as targeting and recognition groups, respectively. NIR fluorescence of CMCu-H₂S can significantly increase after reacting with H₂S by removing the quenchable Cu²⁺. This probe exhibited high selectivity and an extremely fast response rate. Cell imaging results demonstrated that there was a close relationship between the overexpression of NFS1 and drug resistance and inhibition of NFS1 was beneficial for improving the efficacy of chemotherapy.

Computer-Aided Design of DNA Self-Limited Assembly for Relative Quantification of Membrane Proteins

Immunofluorescence imaging of cells plays a vital role in biomedical research and clinical diagnosis. However, when it is applied to relative quantification of proteins, it suffers from insufficient fluorescence intensity or partial overexposure, resulting in inaccurate relative quantification. Herein, we report a computer-aided design of DNA self-limited assembly (CAD-SLA) technology and apply it for relative quantification of membrane proteins, a concept proposed for the first time. CAD-SLA can achieve exponential cascade signal amplification in one pot and terminate at any desired level. By conjugating CAD-SLA with immunofluorescence, in situ imaging of cell membrane proteins is achieved with a controllable amplification level. Besides, comprehensive fluorescence intensity information from fluorescent images can be obtained, accurately showing relative quantitative information. Slight protein expression differences previously indistinguishable by immunofluorescence or Western blotting can now be discriminated, making fluorescence imaging-based relative quantification a promising tool for membrane protein analysis. From the perspectives of both DNA self-assembly technology and immunofluorescence technology, this work has solved difficult problems and provided important reference for future development.

Concise Biothiol-Activatable HPQ-NBD Conjugate as a Targeted Theranostic Probe for Tumor Cells

Cancer, as a malignant tumor, seriously endangers human health. The study of cancer diagnosis and therapy has great practical significance. The development of theranostic agents has become a very important research topic. Nevertheless, some existing agents still have imperfections, such as complex structures and difficult syntheses. Therefore, it is urgent for researchers to develop simple novel theranostic agents. In this study, the precipitated fluorophore HAPQ was used as a simple drug molecule for the first time and combined with NBD-Cl to construct a simple and efficient theranostic probe (HAPQ-NBD). The theranostic probe can distinguish between tumor cells and normal cells based on the higher levels of biothiol in tumor cells. In addition, the probe can use biothiol as a control switch to release higher levels of precipitated fluorophore HAPQ in tumor cells, leading to selective high toxicity to tumor cells, thus achieving the goal of selectively killing tumor cells. The construction of probe HAPQ-NBD provides a practical tool for the diagnosis and therapy of cancer. It is expected that the development and utilization of precipitated fluorophore will provide a new method and strategy for cancer diagnosis and therapy.

Conjugated Oligoelectrolytes for Long-Term Tumor Tracking with Incremental NIR-II Emission

The design and synthesis of the near-infrared (NIR)-II emissive conjugated oligoelectrolyte COE-BBT are reported. COE-BBT has a solubility in aqueous media greater than 50 mg mL^-1 , low toxicity, and a propensity to intercalate lipid bilayers, wherein it exhibits a higher emission quantum yield relative to aqueous media. Addition of COE-BBT to cells provides two emission channels, at ≈500 and ≈1020 nm, depending on the excitation wavelength, which facilitates in vitro confocal microscopy and in vivo animal imaging. The NIR-II emission of COE-BBT is used to track intracranial and subcutaneous tumor progression in mice. Of relevance is that the total NIR-II intensity increases over time. This phenomenon is attributed to a progressive attenuation of a COE-BBT self-quenching effect within the cells due to the expected dye dilution per cell as the tumor proliferates.

Recent Progress of Cyanine Fluorophores for NIR-II Sensing and Imaging

The cyanine fluorophores, a kind of classic organic fluorophores, are famous for their high extinction coefficient, simple synthetic route, and relatively long absorption and emission wavelengths. Moreover, the excellent biocompatibility and low toxicity in biological samples make cyanine fluorophores show excellent application value in the biomedical field, especially in Near-Infrared II (NIR-II) sensing and imaging. In this review, we briefly outline the history, characteristics, and current state of development of cyanine fluorophores. In particular, we described the application of cyanine fluorophores in NIR-II sensing and imaging. We hope this review can help researchers grab the latest information in the fast-growing field of cyanine fluorophores for NIR-II sensing and imaging.

Antipermeability Strategy to Achieve Extremely High Specificity and Ultralong Imaging of Diverse Cell Membranes Based on Restriction-Induced Emission of AIEgens

Long-term in situ cell membrane-targeted bioimaging is of great significance for studying specific biological processes and functions, but currently developed membrane probes are rarely simultaneously used to image the plasma membrane of animal and plant cells, and these probes lack sufficiently high long-term targeting ability. Herein, we proposed an antipermeability strategy to achieve highly specific and long-term imaging of plasma membranes of both human and plant cells using the steric hindrance effect and restriction-induced emission of AIE-active probes based on an updated membrane model. A certain degree of rigidity of plasma membrane containing a large ratio of rigid cholesterol molecules in the updated membrane model provides a promising opportunity to design antipermeable probes by introducing a rigid steric hindrance group in the probe. The designed antipermeable probes can anchor inside plasma membrane for a long term relying on the combination of the steric hindrance effect and the electrostatic and hydrophobic interactions between the probe and the membrane, as well as light up the membrane via the restriction-induced emission mechanism. The excellent performance in imaging completeness and specificity for both human cells and plant cells clearly shows that these designed probes possess outstanding antipermeability to achieve long-term specific imaging of membrane. These probes also show some advanced features such as ultrafast staining, wash-free merit, favorable biocompatibility, good photostability, and effective resistance to viscosity and pH alteration. This work also provides a valuable design principle for membrane probes of plant cells that the designed probes require a suitable molecular size favoring the penetration of small pores of cell walls.

Rhenium-guanidine complex as photosensitizer: trigger HeLa cell apoptosis through death receptor-mediated, mitochondria-mediated and cell cycle arrest pathways

During the last decades, growing evidence indicates that the photodynamic antitumor activity of transition metal complexes, and Re(I) compounds are potential candidates for photodynamic therapy (PDT). This study reports the synthesis, characterization, and anti-tumor activity of three new Re(I)-guadinium complexes. Cytotoxicity tests reveal that complex Re1 increased cytotoxicity by 145-fold from IC50 > 180 μM in the dark to 1.3 ± 0.7 μM following 10 min of light irradiation (425 nm) in HeLa cells. Further, the mechanism by which Re1 induces apoptosis in the presence or absence of light irradiation was investigated, and results indicate that cell death was caused through different pathways. Upon irradiation, Re1 first accumulates on the cell membrane and interacts with death receptors to activate the extrinsic death receptor-mediated signaling pathway, then is transported into the cell cytoplasm. Most of the intracellular Re1 locates within mitochondria, improving the ROS level, and decreasing MMP and ATP levels, and inducing the activation of caspase-9 and, thus, apoptosis. Subsequently, the residual Re1 can translocate into the cell nucleus, and activates the p53 pathway, causing cell-cycle arrest and eventually cell death.

Glycopolymer Engineering of the Cell Surface Changes the Single Cell Migratory Direction and Inhibits the Collective Migration of Cancer Cells

Cancer cell migration is one of the most important processes in cancer metastasis. Metastasis is the major cause of death from most solid tumors; therefore, suppressing cancer cell migration is an important means of reducing cancer mortality. Cell surface engineering can alter the interactions between cells and their microenvironment, thereby offering an effective method of controlling the migration of the cells. This paper reports that modification of the mouse melanoma (B16) cancer cell surface with glycopolymers affects the migration of the cells. Changes in cell morphology, migratory trajectories, and velocity were investigated by time-lapse cell tracking. The data showed that the migration direction is altered and diffusion slows down for modified B16 cells compared to unmodified B16 cells. When modified and unmodified B16 cells were mixed, wound-healing experiments and particle image velocimetry (PIV) analysis showed that the collective migration of unmodified B16 cells was suppressed because of vortexlike motions induced by the modified cells. The work demonstrates the important role of surface properties/modification in cancer cell migration, thereby providing new insights relative to the treatment of cancer metastasis.

Progress and Perspective of Solid-State Organic Fluorophores for Biomedical Applications

Fluorescent organic dyes have been extensively used as raw materials for the development of versatile imaging tools in the field of biomedicine. Particularly, the development of solid-state organic fluorophores (SSOFs) in the past 20 years has exhibited an upward trend. In recent years, studies on SSOFs have focused on the development of advanced tools, such as optical contrast agents and phototherapy agents, for biomedical applications. However, the practical application of these tools has been hindered owing to several limitations. Thus, in this Perspective, we have provided insights that could aid researchers to further develop these tools and overcome the limitations such as limited aqueous dispersibility, low biocompatibility, and uncontrolled emission. First, we described the inherent photophysical properties and fluorescence mechanisms of conventional, aggregation-induced emissive, and precipitating SSOFs with respect to their biomedical applications. Subsequently, we highlighted the recent development of functionalized SSOFs for bioimaging, biosensing, and theranostics. Finally, we elucidated the potential prospects and limitations of current SSOF-based tools associated with biomedical applications.

Molecular engineering of organic-based agents for in situ bioimaging and phototherapeutics

In situ monitoring of the location and transportation of bioactive molecules is essential for deciphering diverse biological events in the field of biomedicine. In addition, obtaining the in situ information of lesions will provide a clear perspective for surgeons to perform precise resection in clinical surgery. Notably, delivering drugs or operating photodynamic therapy/photothermal therapy in situ by labeling the lesion regions of interest can improve treatment and reduce side effects in vivo. In various advanced imaging and therapy modalities, optical theranostic agents based on organic small molecules can be conveniently modified as needed and can be easily internalized into cells/lesions in a non-invasive manner, which are prerequisites for in situ bioimaging and precision treatment. In this tutorial review, we first summarize the in situ molecular immobilization strategies to retain small-molecule agents inside cells/lesions to prevent their diffusion in living organisms. Emphasis will be focused on introducing the application of these strategies for in situ imaging of biomolecules and precision treatment, particularly pertaining to why targeting therapy in situ is required.

Precipitated Fluorophore-Based Molecular Probe for In Situ Imaging of Aminopeptidase N in Living Cells and Tumors

Aminopeptidase N (APN) is capable of cleaving N-terminal amino acids from peptides with alanine in the N-terminal position and plays a key role in the growth, migration, and metastasis of cancer. However, reliable in situ information is hard to be obtained with the current APN-responsive molecular probes because the released fluorophores are cytoplasmic soluble and thus rapidly depart from the enzymatic reaction sites and spread out all over the cytoplasm. Here, we report a de novo precipitated fluorophore, HBPQ, which is completely insoluble in water and shows strong yellow solid emission when excited with a 405 nm laser. Owing to the controllable solid fluorescence of HBPQ by the protection-deprotection of phenolic hydroxyl, we further utilized HBPQ to design an APN-responsive fluorogenic probe (HBPQ-A) for the imaging of intracellular APN. Importantly, HBPQ-A can not only perform in situ imaging of APN in different organelles (e.g., lysosomes, mitochondria, endoplasmic reticula, and so forth) but also display a stable and indiffusible fluorescent signal for reliable mapping of the distribution of APN in living cells. In addition, through real-time imaging of APN in 4T1 tumors, we found that the fluorescent signal with high fidelity generated by HBPQ-A could remain constant even after 12 h, which further confirmed its diffusion-resistant ability and long-term reliable imaging ability. We believe that the precipitated fluorophore may have great potential for long-term in situ imaging.

Rhodium(iii)-catalyzed annulation of 3-arylquinazolinones with alkynes via double C–H activation: an efficient route for quinolino[2,1-b]quinazolinones

An effective method for the synthesis of quinolino[2,1-b]quinazolinones has been described.