An Easily Available Ratiometric Reaction-Based AIE Probe for Carbon Monoxide Light-up Imaging

Luminescence Probes in Bio-Applications: From Principle to Practice

Bioanalysis based on optical imaging has gained significant progress in the last few decades. Luminescence probes are capable of detecting, monitoring, and tracing particular biomolecules in complex biological systems to figure out the roles of these molecules in organisms. Considering the rapid development of luminescence probes for bio-applications and their promising future, we have attempted to explore the working principles and recent advances in bio-applications of luminescence probes, in the hope of helping readers gain a detailed understanding of luminescence probes developed in recent years. In this review, we first focus on the current widely used luminescence probes, including fluorescence probes, bioluminescence probes, chemiluminescence probes, afterglow probes, photoacoustic probes, and Cerenkov luminescence probes. The working principles for each type of luminescence probe are concisely described and the bio-application of the luminescence probes is summarized by category, including metal ions detection, secretion detection, imaging, and therapy.

Synthesis, optical properties, and application of novel chalcone skeleton as pH fluorescent probe: Based AIE + ESIPT strategy

A series of "turn off" pH fluorescence probes with chalcone skeleton for basic system have been developed. The molecules emitted bright yellow fluorescence under acidic condition, resulting AIE coupled ESIPT characteristic and ICT process. What's more, the compounds exhibited excellent sensitivity and selectivity for detecting pH as a facile "On-Off" fluorescence probe, and the fluorescence of them were quenched with the ESIPT process interrupted under alkaline condition. Theoretical calculation for the related compounds also performed to verify the electron effect on photophysical properties and confirm the rational speculation on the mechanism.

Aggregation induced emission and volatile acid vapour sensing in acridine appended poly (aryl ether) based low molecular weight organogelator

Considerable research attention has been devoted to the development of portable and rapid fluorescence sensors that can selectively detect volatile acids, due to the harmful effects of acid vapour on the environment and human health. Although various types of fluorophores have been reported for sensing volatile acid vapours, regulation of the sensory response using aggregation induced emissive (AIE) based gelators has rarely been reported. In this study, we present the design and synthesis of a novel organogelator that is capable of sensing volatile acids through AIE. An acridine-attached poly(aryl ether) dendron molecular system is synthesized through an aldimine coupling reaction, which self-assembles and forms a gel, exhibiting AIE behavior. The synthesized molecule and prepared gel were characterized using NMR, MASS, XRD, HRSEM and rheology techniques. The AIE property of APD was investigated using steady-state absorption and emission spectroscopic techniques. The sensory response of the APD gelator was tested with various analytes, and the results indicated that APD shows rapid response, particularly to acid vapours, where the detection limits (DL) of trifluoroacetic acid (TFA), hydrochloric acid (HCl) and nitric acid (HNO3) vapor were as low as 0.22, 0.9 and 0.30 ppm, respectively. An APD solid film in filter paper shows a visual color change from yellow to red in an aqueous acidic medium, and the effect is reversed in an alkaline medium. These findings suggest that an APD gelator could potentially be utilized to generate a portable acid vapor sensor kit due to its low detection limit and rapid response time, and it could be also be used as a substitute for existing acid indicators.

A long-wavelength mitochondria-targeted CO fluorescent probe for living cells and zebrafish imaging

Carbon monoxide (CO) not only causes damage to life and health as an environmental pollutant, but also undertakes many physiological functions in organisms. In particular, developing means that can be used for the determination of CO in organelles will provide insight into the vital role it plays. Studies have shown that mitochondrial respiration is closely related to CO concentrations, so it is critical to develop tools for CO detection in mitochondria. Here, we use a rhodamine derivative that can target mitochondria as fluorophores to construct a mitochondrial-labeled CO fluorescence probe (Rh-CO) with high sensitivity (detection limit: 9.4 nM), excellent water-solubility, and long emission (λem = 630 nm). Prominently, the probe has outstanding mitochondria-targeting capabilities. Moreover, we used transient glucose deprivation (TGD) and heme to stimulate endogenous CO production in living cells and zebrafish, respectively, and the probe exhibited excellent imaging capabilities. All in all, we expect this probe to contribute to a deeper understanding of the role played by CO in mitochondria.

Chemical Strategies Toward Prodrugs and Fluorescent Probes for Gasotransmitters

Three gaseous molecules are widely accepted as important gasotransmitters in mammalian cells, namely NO, CO and H2S. Due to the pharmacological effects observed in preclinical studies, these three gasotransmitters represent promising drug candidates for clinical translation. Fluorescent probes of the gasotransmitters are also in high demand; however, the mechanisms of actions or the roles played by gasotransmitters under both physiological and pathological conditions remain to be answered. In order to bring these challenges to the attention of both chemists and biologists working in this field, we herein summarize the chemical strategies used for the design of both probes and prodrugs of these three gasotransmitters.

Development of a "double reaction" type-based fluorescent probe for the imaging of superoxide anion in living cells

Superoxide anion (O2•-) is an important ROS in living systems, and rapid and in situ detection of O2•- is critical for the in-depth study of its roles in the closely related diseases. Herein, we present a "double reaction" type-based fluorescent probe (BZT) for the imaging of O2•- in living cells. BZT employed a triflate group as a recognition site for O2•-. In response to O2•-, the probe BZT underwent double chemical reactions, including the nucleophilic reaction between O2•- and triflate, and the cyclization reaction through the other nucleophilic reaction between hydroxyl and cyano group. BZT could show high sensitivity and selectivity to O2•-. Biological imaging experiments demonstrated that the probe BZT could be successfully applied to detect the exogenous and endogenous O2•- in living cells, and the results suggested that rutin could efficiently scavenge the endogenous O2•- induced by rotenone. We expected that the developed probe could provide a valuable tool to investigate the pathological roles of O2•- in relevant diseases.

A lipid droplet-targeting fluorescent probe for specific H2S imaging in biosamples and development of smartphone platform

Hydrogen sulfide (H2S), a significant gas signal molecule, is closely related to various physiological/pathological processes. The monitoring of H2S is crucial in understanding the occurrence and development of diseases such as cancers. Emerging evidence suggests that abnormal regulation of Lipid droplets (LDs) is associated with many human diseases. For example, cancer cells are characterized by the abnormal accumulation of LDs. Therefore, understanding the relationship between LDs and cancer is of great significance for developing therapies against cancer. To address this challenge, we designed and developed a LD-targeting and H2S-activated probe (BTDA-DNB) by engineering a 2,4-dinitrophenyl ether (DNBE) as the H2S reactive site. In the presence of H2S, a strongly fluorescent emitter, 3-(benzo[d]thiazol-2-yl)-N,N-diethyl-2-imino-2H-chromen-7-amine (BTDA) was obtained with the leaving of DNBE group. BTDA-DNB displayed favorable sensitivity, selectivity and functioning well at physiological pH. The probe features excellent LD-targeting specificity and low cellular toxicity. The practical applications of LD-targeting probe BTDA-DNB as H2S probe in living cells, cancer tissues and Arabidopsis seedling have been evaluated. The excellent imaging performance demonstrates a potential ability for cancer diagnosis. Benefitted from the excellent performance on visual recognition H2S, a robust smartphone-integrated platform for H2S analysis was also successfully established.

In situ imaging of signaling molecule carbon monoxide in plants with a fluorescent probe

Carbon monoxide (CO) is a recently discovered gasotransmitter. In animals, it has been found that endogenously produced CO participates in the regulation of various metabolic processes. Recent research has indicated that CO, acting as a signaling molecule, plays a crucial regulatory role in plant development and their response to abiotic stress. In this work, we developed a fluorescent probe, named COP (carbonic oxide Probe), for the in situ imaging of CO in Arabidopsis thaliana plant tissues. The probe was designed by combining malononitrile-naphthalene as the fluorophore and a typical palladium-mediated reaction mechanism. When reacted with the released CO, COP showed an obvious fluorescence enhancement at 575 nm, which could be observed in naked-eye conditions. With a linear range of 0-10 μM, the limit of detection of COP was determined as 0.38 μM. The detection system based on COP indicated several advantages including relatively rapid response within 20 min, steadiness in a wide pH range of 5.0-10.0, high selectivity, and applicative anti-interference. Moreover, with a penetration depth of 30 μm, COP enabled 3D imaging of CO dynamics in plant samples, whether it was caused by agent release, heavy metal stress, or inner oxidation. This work provides a fluorescent probe for monitoring CO levels in plant samples, and it expands the application field of CO-detection technology, assisting researchers in understanding the dynamic changes in plant physiological processes, making it an important tool for studying plant physiology and biological processes.

The emergence and advancement of Tsuji-Trost reaction triggered carbon monoxide recognition and bioimaging

Considering that carbon monoxide is both a vital gasotransmitter and an obnoxious gas, tremendous efforts have been dedicated toward its recognition through various methods. However, the fluorescent light-up approach through the exploration of optical markers remains one of the most convenient methods owing to its several advantages. Amongst the different approaches towards the development of CO responsive optically active molecular markers, the Tsuji-Trost reaction-based CO recognition strategy has remained one of the most significant areas of interest across researchers working in this field. However, there have been no attempts to exclusively summarize the commendable work done in this area yet. The current review, therefore, attempts to summarize the developments of various optical probes following this reaction strategy until the year 2022. This review provides detailed mechanistic insights into the Tsuji-Trost mediated CO detection strategy. Besides, discussions on the strategic development and employment of probes based on various allyl derivatives - allyl carbamate/carbonate/ethers - will provide a thorough understanding of the detection method. The significant advancements of the Tsuji-Trost reaction as an interesting strategy that is accepted and extensively explored for monitoring CO in various media including air, aqueous solutions and living systems have been elaborately discussed. Various potential applications and utilization of these developed fluorogenic probes for tracing CO in different living systems have been examined systematically. Moreover, monitoring of exogenous/endogenous CO levels, modulation of intracellular CO concentration under various induced conditions and bioimaging of CO in in vivo models have also been detailed here. Briefly, this review summarizes the current prospects of this detection method and the future directions in related fields.

A simple ratiometric fluorescent probe for two-photon imaging of carbon monoxide in living cells and zebrafish

Carbon monoxide (CO) is an important gas signaling molecule and has been widely involved in regulating important life processes. Effective monitoring of CO in living systems is critical. Combined with the accuracy of ratio detection and the advantages of two-photon imaging, a simple ratiometric two-photon fluorescent probe RTFP was rationally designed and synthesized using 7-(diethylamino)-4-hydroxycoumarin as a two-photon fluorophore and allyl carbonate as the reactive unit. Probe RTFP exhibited excellent selectivity and sensitivity towards CO, and was successfully applied to image endogenous CO in living cells and zebrafish.

Light activated simultaneous release and recognition of biological signaling molecule carbon monoxide (CO)

The therapeutic action of carbon monoxide (CO) is very well known and has been studied on various types of tissues and animals. However, real-time spatial and temporal tracking and release of CO is still a challenging task. This paper reported an amphiphilic CO sensing probe NP and phospholipid 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) based nanoscale vesicular sensing system Ves-NP consisting of NP. The liposomal sensing system (Ves-NP) showed good selectivity and sensitivity for CO without any interference from other relevant biological analytes. Detection of CO is monitored by fluorescence OFF-ON signal. Ves-NP displayed LOD of 5.94 µM for CO detection with a response time of 5 min. Further, in a novel attempt, Ves-NP is co-embedded with the amphiphilic CO-releasing molecule 1-Mn(CO)₃ to make an analyte replacement probe Ves-NP-CO. Having a both CO releasing and sensing moiety at the surface of the same liposomal system Ves-NP-CO play a dual role. Ves-NP-CO is used for the simultaneous release and recognition of CO that can be controlled by light. Thus, in this novel approach, for the first time we have attached both the release and recognition units of CO in the vesicular surface, both release and recognition simultaneously monitored by the change in fluorescent OFF-ON signal.

A Review for In Vitro and In Vivo Detection and Imaging of Gaseous Signal Molecule Carbon Monoxide by Fluorescent Probes

Carbon monoxide (CO) is a vital endogenous gaseous transmitter molecule involved in the regulation of various physiological and pathological processes in living biosystems. In order to investigate the biological function of CO, many technologies have been developed to monitor the level of endogenous CO in biosystems. Among them, the fluorescence detection technology based on the fluorescent probe has the advantages of high sensitivity, excellent selectivity, simple operation, especially non-invasive damage to biological samples, and the possibility of real-time in situ detection, etc., which is considered to be one of the most effective and applicable detection techniques. Therefore, in the last few years, a lot of work has been carried out on the design, synthesis and in vivo fluorescence imaging studies of CO fluorescent probes. Furthermore, using fluorescent probes to detect the changes in CO concentrations in living cells and tissues as well as in organisms has been one of the hot research topics in recent years. However, it is still a challenge to rationally design CO fluorescent probe with excellent optical performance, structural stability, low background interference, good biocompatibility, and excellent water solubility. Therefore, this review focuses on the research progress of CO fluorescent probes in the detection mechanism and biological applications in recent years. However, this popular and leading topic has rarely been summarized comprehensively to date. Thus, the research progress of CO fluorescent probes in recent years is reviewed in terms of their design concept, detection mechanism, and their biological applications. In addition, the relationship between the structure and performance of the probes was also discussed. More significantly, we hope that more excellent optical properties fluorescent probes for gaseous transmitter molecule CO detection and imaging will overcome the current problems of high biotoxicity and limited water solubility in future.

Deep-Depth Imaging of Hepatic Ischemia/Reperfusion Injury Using a Carbon Monoxide-Activated Upconversion Luminescence Nanosystem

Exploring a chemical imaging tool for visualizing the endogenous CO biosignaling molecule is of great importance in understanding the pathophysiological functions of CO in complex biological systems. Most of the existing CO fluorescent probes show excitation and emission in the region of ultraviolet and visible light, which are not suitable for application in in vivo deep-depth imaging of CO. Herein, a new near-infrared (NIR) to NIR upconversion luminescence (UCL) nanosystem for in vivo visualization of CO was developed, which possesses the merits of high selectivity and sensitivity, a deep tissue penetration depth, and a high signal-to-noise ratio. In this design, upon interaction with CO, the maxima absorption peak of the nanosystem showed a significant blue shift from 795 nm to 621 nm and triggered a remarkable turn-on NIR UCL signal due to the luminescence resonance energy transfer process. Leveraging this nanosystem, we achieved an NIR UCL visualization of the generation of CO biosignals caused by hypoxic, acute inflammation, or ischemic injury in living cells, zebrafish, and mice. Moreover, the protective effect of CO in zebrafish models of oxygen and glucose deprivation/reperfusion (OGD/R) and mice models of lipopolysaccharide-induced oxidative stress (LOS) and hepatic ischemia/reperfusion (HI/R) was also further verified. Therefore, this work discloses that the nanosystem not only serves as a promising nanoplatform to study biological signaling pathways of CO in pathophysiological events, but may also provide a powerful tool for HI/R injury diagnosis.

An AIE based fluorescent chemosensor for ratiometric detection of hypochlorous acid and its application

Detecting and imaging intracellular hypochlorous acid (HClO) is of great importance owning to its prominent role in numerous pathological and physiological processes. In this contribution, a novel AIE-based fluorescent chemosensor has been developed by employing a benzothiazole derivative. The synthesized probe displayed remarkable ratiometric fluorescent response to HClO with a large emission shift (139 nm), resulting in naked-eye fluorescence changes from red to blue. Under the optimal conditions, this probe was capable of quantitatively detecting HClO within 10 s, and possessed good sensitivity and high selectivity toward HClO over other biologically relevant species. Moreover, it has been successfully utilized to image the exogenous and endogenous HClO in living cells through dual channels, and conveniently detect hypochlorous acid solution on test strips with better accuracy, demonstrating its potential for monitoring HClO in biological and environment fields.

Mn3+/Mn4+ ion-doped carbon dots as fenton-like catalysts for fluorescence dual-signal detection of dopamine

Carbon dots (CDs), a new zero-dimensional material, have ignited a revolution in the fields of sensing, bioimaging, and biomedicine. However, the difficulty of preparing CDs with Fenton-like catalytic properties has seriously hindered their application in the diagnosis of oxidation/reduction biomolecules or metal ions. Here, an innovative method was successfully established to synthesize Mn3+/Mn4+ ion-doped blue-green fluorescent CDs with Fenton-like catalytic properties using manganese acetate as the manganese source. Specifically, the CDs prepared here were equipped with functional groups of -COOH, NH2, C=O, and Mn-O, offering the possibility to function as a fluorescence sensor. More importantly, the introduction of manganese acetate resulted in the preparation of CDs with Fenton-like catalytic properties, and the dual-signal fluorescence detection of dopamine (DA) was realized with linear ranges of 100–275 nM and 325–525 nM, and the detection limits were 3 and 12 nM, respectively. In addition, due to the Fenton-like catalytic activity of Mn3+/Mn4+ ion-doped CDs, the material has broad application prospects in the detection of oxidation/reduction biomolecules or metal ions related to disease diagnosis and prevention.

Reversible AIE-active fluorescent probe with a large emission peak shift for ratiometric detection of food freshness indicator H2S

It is crucial to on-site monitor H₂S for addressing the concerns associated with food safety. We rationally prepared an AIE-active fluorescent probe (CLBZ) with the aggregated state conversion for sensing H₂S in a ratiometric response manner. CLBZ displayed ratiometric response, fast response time (5 s), well-resolved emission peak shift (147 nm) and high selectivity towards H₂S, and it can be used as a reversible and reusable probe. The probe-based test strip was also developed to conveniently detect H₂S generated during food spoilage in the absence of laboratory instruments. It achieved the consistent results and sensitivity with that determined by the colony forming unit (CFU) assay. These results paved a successful way to develop an effective analytical method for food quality and safety.

Reactive Species-Activatable AIEgens for Biomedical Applications

Precision medicine requires highly sensitive and specific diagnostic strategies with high spatiotemporal resolution. Accurate detection and monitoring of endogenously generated biomarkers at the very early disease stage is of extensive importance for precise diagnosis and treatment. Aggregation-induced emission luminogens (AIEgens) have emerged as a new type of excellent optical agents, which show great promise for numerous biomedical applications. In this review, we highlight the recent advances of AIE-based probes for detecting reactive species (including reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive sulfur species (RSS), and reactive carbonyl species (RCS)) and related biomedical applications. The molecular design strategies for increasing the sensitivity, tuning the response wavelength, and realizing afterglow imaging are summarized, and theranostic applications in reactive species-related major diseases such as cancer, inflammation, and vascular diseases are reviewed. The challenges and outlooks for the reactive species-activatable AIE systems for disease diagnostics and therapeutics are also discussed. This review aims to offer guidance for designing AIE-based specifically activatable optical agents for biomedical applications, as well as providing a comprehensive understanding about the structure-property application relationships. We hope it will inspire more interesting researches about reactive species-activatable probes and advance clinical translations.

A Fast-Response AIE-Active Ratiometric Fluorescent Probe for the Detection of Carboxylesterase

Hepatocellular carcinoma (HCC) is associated with a high mortality rate worldwide. The therapeutic outcomes can be significantly improved if diagnosis and treatment are initiated earlier in the disease process. Recently, the carboxylesterase (CaE) activity/level in human plasma was reported to be a novel serological biomarker candidate for HCC. In this article, we fabricated a new fluorescent probe with AIE characteristics for the rapid detection of CaE with a more reliable ratiometric response mode. The TCFISE probe showed high sensitivity (LOD: 93.0 μU/mL) and selectivity toward CaE. Furthermore, the good pH stability, superior resistance against photobleaching, and low cytotoxicity highlight the high potential of the TCFISE probe for application in the monitoring of CaE activity in complex biological samples and in live cells, tissues, and animals.

A Hemicyanine-Assembled Upconversion Nanosystem for NIR-Excited Visualization of Carbon Monoxide Bio-Signaling In Vivo

Carbon monoxide (CO) is considered as the second gasotransmitter involved in a series of physiological and pathological processes. Although a number of organic fluorescent probes have been developed for imaging CO, these probes display excitation within the ultraviolet or visible range, which restrict their applications in the complex biosystems. In the present work, a strategy is developed to construct an upconversion nanoparticles-based nanosystem for upconversion luminescent (UCL) sensing CO. This nanosystem exhibits a fast response to CO with high sensitivity and selectivity in aqueous solution by a near-infrared-excited ratiometric UCL detection method. Meanwhile, laser scanning upconversion luminescence microscope experiments demonstrate that this nanosystem can visualize the endogenous CO bio-signaling in living cells, deep tissues, zebrafish, and living mice by ratiometric UCL imaging. In particular, this nanosystem has been successfully employed in visualization of the endogenous CO bio-signaling through up-regulation of heme oxygenase-1 (HO-1) in the progression of hypoxia, acute inflammation, or ischemic injury. This work demonstrates that the outstanding performance of the nanosystem not only can provide an effective tool for further understanding the role of CO in the physiological and pathological environment, but also may have great potential ability for tracking the expression of HO-1 in living systems.

A TCF-Based Carbon Monoxide NIR-Probe without the Interference of BSA and Its Application in Living Cells

As toxic gaseous pollution, carbon monoxide (CO) plays an essential role in many pathological and physiological processes, well-known as the third gasotransmitter. Owning to the reducibility of CO, the Pd⁰-mediated Tsuji-Trost reaction has drawn much attention in CO detection in vitro and in vivo, using allyl ester and allyl ether caged fluorophores as probes and PdCl₂ as co-probes. Because of its higher decaging reactivity than allyl ether in the Pd⁰-mediated Tsuji-Trost reaction, the allyl ester group is more popular in CO probe design. However, during the application of allyl ester caged probes, it was found that bovine serum albumin (BSA) in the fetal bovine serum (FBS), an irreplaceable nutrient in cell culture media, could hydrolyze the allyl ester bond, and thus give erroneous imaging results. In this work, dicyanomethylenedihydrofuran (TCF) and dicyanoisophorone (DCI) were selected as electron acceptors for constructing near-infrared-emission fluorophores with electron donor phenolic OH. An allyl ester and allyl ether group were installed onto TCF-OH and DCI-OH, constructing four potential CO fluorescent probes, TCF-ester, TCF-ether, DCI-ester, and DCI-ether. Our data revealed that ester bonds of TCF-ester and DCI-ester could completely hydrolyze in 20 min, but ether bonds in TCF-ether and DCI-ether tolerate the hydrolysis of BSA and no released fluorescence was observed even up to 2 h. Moreover, passing through the screen, it was concluded that TCF-ether is superior to DCI-ether due to its higher reactivity in a Pd⁰-mediated Tsuji-Trost reaction. Also, the large stokes shift of TCF-OH, absorption and emission at 408 nm and 618 nm respectively, make TCF-ether desirable for fluorescent imaging because of differentiating signals from the excitation light source. Lastly, TCF-ether has been successfully applied to the detection of CO in H9C2 cells.

Ratiometric imaging of butyrylcholinesterase activity in mice with nonalcoholic fatty liver using an AIE-based fluorescent probe

Butyrylcholinesterase (BChE) is an essential human biomarker which is related to liver and neurodegenerative diseases. It is of great significance to develop a fluorescent probe that can image BChE in vitro and in vivo. Unfortunately, most fluorescent probes that are based on a single change in fluorescence intensity are susceptible to environmental interference. Therefore, we reported an easily available ratiometric fluorescent probe, TB-BChE, with aggregation-induced emission (AIE) characteristics for ratiometric imaging of BChE. TB-BChE demonstrated excellent sensitivity (LOD = 39.24 ng mL^-1) and specificity for BChE. Moreover, we have successfully studied the ratiometric imaging of TB-BChE to BChE in a nonalcoholic fatty liver disease model. These results indicated that TB-BChE is expected to become a powerful analysis tool for butyrylcholinesterase research in basic medicine and clinical applications.

Detection and imaging of Carbon monoxide releasing Molecule-2 in HeLa cells and zebrafish using a Metal-Free Near-Infrared fluorescent off-on probe

By incorporating 4-nitrobenzene as the recognition unit into a stable hemicyanine skeleton based on IR 780, a designed near-infrared fluorescent off-on probe DNXI shows excellent analytical performance in real-time monitoring of carbon monoxide releasing molecule-2 (CORM-2) in living HeLa cells and zebrafish without transition metals. The response mechanism of CORM-2 with DNXI may reduce the nitro group to an amino group, followed by 1,6-rearrangement elimination reaction, resulting in the recovery of both color and fluorescence signal. As a result, DNXI shows very low background signal, which is rather desired for achieving sensitive detection of CORM-2. Compared with the existing CORM-2 probes, DNXI shows excellent optical performance in vitro and in vivo, high selectivity and sensitivity to CORM-2, as well as near-infrared fluorescence emission 712 nm, with a low detection limit of 103 nM. More importantly, DNXI is low cytotoxic, cell membrane permeable, and its applicability has been demonstrated for monitoring CORM-2 in living HeLa cells and zebrafish. These superior properties of the probe enable it have great potential to be used in biological systems or in vivo related studies.

A lipid droplet-targetable and biothiol-sensitive fluorescent probe for the diagnosis of cancer cells/tissues

Lipid droplets (LDs) have recently been reported as an attractive target for cancer diagnosis and treatment, owing to their special structure or microenvironment changes in cancer development and resistance. However, the relationship between the biothiol level of LDs and cancer is still poorly understood, partially owing to the absence of effective molecular tools. Herein, we developed a LD-targetable and biothiol-sensitive fluorescent probe, BTDA-RSS, by introducing 2,4-dinitrobenzenesulfonyl (DNBS) as the biothiol reaction group into a benzothiazolyl derivative. BTDA-RSS displayed a marked and rapid fluorescence turn-on response toward biothiols, due to the biothiol-triggered cleavage of DNBS to yield the highly fluorescent benzothiazolyl iminocoumarin BTDA. In addition, the probe shows significant LD-targetable ability, and has been applied for imaging endogenous/exogenous biothiol changes in LDs. Importantly, BTDA-RSS has successfully been utilized to distinguish cancerous cells/tissues from normal cells/tissues with excellent contrast. Surprisingly, we demonstrated for the first time the visualization of LD biothiols in surgical specimens from cancer patients, thereby holding great potential for the application of BTDA-RSS in the clinical diagnosis of human cancers.

Construction of a unique two-photon fluorescent probe and the application for endogenous CO detection in live organisms

Carbon monoxide (CO) is one of the most significant signal molecules and plays an important role in regulating human physiological and pathological processes. In this study, a novel Pd-based complex (Pd-BNP-OH) was developed for endogenous CO detection. The structure and morphology of Pd-BNP-OH was characterized by SEM, XPS, and NMR analyses. When Pd-BNP-OH was reacted with CO, a strong fluorescence enhancement at 510 nm was observed. In addition, Pd-BNP-OH exhibited high stability and selectivity toward CO in PBS buffer. In biological experiments, Pd-BNP-OH exhibited little cytotoxicity in cellular environment, and a bright fluorescence turn on was observed in the presence of exogenous CO and endogenous generated CO. The probe was then applied for CO detection in live zebrafish by both one-photon and two-photon excitation. Significantly, Pd-BNP-OH has excellent two-photon property, controllable structure and high biocompatibility. These features enable the probe to detect endogenously generated carbon monoxide in live organisms successfully.

Golgi-Targeting Fluorescent Probe for Monitoring CO-Releasing Molecule-3 In Vitro and In Vivo

CO-releasing molecule-3 (CORM-3), mainly metal carbonyl compounds, is widely used as experimental tools to deliver CO, a biological "gasotransmitter", in mammalian systems. CORM-3 is also proposed as a potential new antimicrobial agent, which kills bacteria effectively and rapidly in vitro and in animal models. Organelle-targeting therapy, as a highly effective therapeutic strategy with little toxic and side effects, has important research significance and development prospects. Therefore, the development of effective methods for detecting and tracking CORM-3 at the subcellular level has important implications. In this paper, an easily available Golgi-targetable fluorescent probe (Golgi-Nap-CORM-3) was proposed for CORM-3 detection. In the probe Golgi-Nap-CORM-3, the phenyl sulfonamide group was selected as the Golgi-targetable unit, naphthalimide dye was chosen as a fluorophore, and the nitro group was selected as a CORM-3-responsive unit. Golgi-Nap-CORM-3 shows a CORM-3-reponsive increase of fluorescence emission at 520 nm. Using the excellent probe, the change of CORM-3 in HeLa cells, HepG2 cells, and zebrafish is successfully monitored. This study demonstrates very important information for the study of CORM-3 in vivo systems.

A Critical Review on Organic Small Fluorescent Probes for Monitoring Carbon Monoxide in Biology

Endogenous carbon monoxide (CO) is an important intracellular gas messenger that is intimately involved in many physiological and pathological processes. The abnormal concentration of CO in living organisms can cause many diseases. Therefore, it is of great significance to monitor CO in biological samples. Fluorescent probe technology provides an effective and convenient method for CO monitoring, with the advantages of high selectivity and sensitivity, fast response time and in situ fluorescence imaging in biological tissues, which is favored by the majority of researchers. In this paper, the research progress of CO fluorescent probes since 2018 is reviewed, and the design, detection mechanism and biological application of the related fluorescent probes are summarized. And the relationship between the structure and performance of the probes is discussed. Furthermore, the development trend and application prospect of CO fluorescent probes are prospected.

Donor–acceptor strategy to construct near infrared AIEgens for cell imaging

A donor–acceptor strategy was applied to construct NIR AIEgens. Six new AIEgens were obtained and among them, DMNIC exhibited the longest emission maximum at 694 nm and was successfully applied for NIR cell imaging.

A novel highly sensitive fluorescent probe for imaging endogenous CO

A highly sensitive and selective fluorescent probe was constructed to detect carbon monoxide in living cells and zebrafish.

Ratiometric Imaging of MMP-2 Activity Facilitates Tumor Detection Using Activatable Near-Infrared Fluorescent Semiconducting Polymer Nanoparticles

Enzyme-activatable ratiometric near-infrared (NIR) fluorescent probes enabling noninvasive imaging of enzyme activity in vivo are promising for biomedical research; however, such probes with ratiometric fluorescence emissions both in NIR window under a single NIR light excitation are largely unexplored. Here, a quenched NIR fluorophore of Cy5.5 is integrated with NIR fluorescent poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT)-based semiconducting polymer nanoparticles (SPNs), and an α_v β₃ integrin-targeting and matrix metalloproteinase-2 (MMP-2)-activatable ratiometric fluorescent probe (SPN-MMP-RGD) is developed. Under excitation at 660 nm, SPN-MMP-RGD shows "always-on" fluorescence of PCPDTBT (830 nm) and activatable fluorescence of Cy5.5 (690 nm) toward MMP-2, affording a remarkable ≈176-fold enhancement in fluorescence intensity ratio between 690 and 830 nm (I₆₉₀ /I₈₃₀ ) for sensitive detection of MMP-2 activity in vitro and in tumor cells. By virtue of ratiometric fluorescence imaging independently of probe's concentration, SPN-MMP-RGD can not only accurately report on MMP-2 levels regarding different tumor sizes, but also noninvasively delineate MMP-2-positive tiny gastric tumors metastasis in vivo. The authors' study reveals the potential of SPN-MMP-RGD for ratiometric fluorescence imaging of MMP-2 activity via combining two independent NIR fluorophores, which can be amenable for the design of other enzyme-activatable ratiometric NIR fluorescent probes for reliable in vivo imaging.

Optically superior fluorescent probes for selective imaging of cells, tumors, and reactive chemical species

Fluorescent chemical probes have become powerful tools to study biological events in living cells. They provide a great opportunity to quantitatively and qualitatively analyze the physiological and biochemical properties of living cells in real time. The ability of researchers to manipulate these probes for a desired specific purpose has turned many heads in the scientific community. Despite a slow start, fluorescent probe research has seen exponential growth over the last decade in the world. This change required some adventurous and creative scientists from different fields-like biology, medicine, and chemistry-to come together to facilitate the constant expansion of this field. This review article introduces some fundamental concepts related to fluorescent probe designing and development. It also summarizes various fluorescent probes with superior optical properties used in fields like cell biology, cellular imaging, medical research, and cancer diagnosis. It is hoped that this article will encourage more young and creative scientists to contribute their talents to this field.

Two-phase activated colorimetric and ratiometric fluorescent sensor for visual detection of phosgene via AIE coupled TICT processes

In this paper, we specifically designed and synthesized an excellent colorimetric and ratiometric fluorescent sensor DPA-CI for rapid and convenient detection of the highly toxic phosgene. DPA-CI was developed by incorporated a diphenylamine (DPA) and a 2-imine-3-benzo[d]imidazole as the enhanced push-pull electronic structure into the coumarin fluorophore matrix. The sensor DPA-CI towards phosgene sensing exhibited both visible colorimetric and ratiometric fluorescent color change in solution and in gaseous conditions with TICT and AIE mechanism respectively, which can be easily distinguished by using the naked eye. Also, the sensor DPA-CI showed splendid sensing performance such as excellent selectivity, rapid response (less than 8 s in THF and 2 min in gaseous condition), and fair sensitivity (limit of detection less than 0.11 ppm in gaseous condition and 0.27 μM in solution). The design strategy based on enhanced push-pull electronic structure with AIE and TICT properties will be helpful to construct a solid optical sensor with excellent potential application prospects for portable and visual sensing of gaseous phosgene through distinct color and ratiometric fluorescence change by the naked eyes.

A metal-free coumarin-based fluorescent probe for the turn-on monitoring of carbon monoxide in an aqueous solution and living cells

Endogenous carbon monoxide (CO) is an essential cell signaling molecule, which is closely related to numerous physiological and pathological processes. Therefore, it is of great significance to monitor CO in living samples. Fluorescent probe technique provides an effective and convenient method for monitoring CO. Although many fluorescent probes of CO have been reported, most of them require the introduction of heavy metal ions (Pd2+), which is not conducive to the practical application of these probes. Herein, a metal-free coumarin-based fluorescent probe was developed for monitoring CO. For this probe, coumarin was selected as the fluorophore, the aromatic nitro group was used as the reactive site, and the carboxyl moiety acted as the water-soluble unit. As a result, this probe has been proved to be able to monitor CO with good specificity and excellent sensitivity in water medium. After interacting with CO, the aromatic group was reduced to an aryl amino group; accordingly, the emission intensity of the peak corresponding to the probe at 450 nm significantly increased. Moreover, it successfully realized the detection of CO in living cells by a fluorescence signal.

Twisted intramolecular motion arrested in aggregated state emission and the nonlinear optical properties of pyrene pyrazoline derivatives

Heterocyclic pyrene pyrazoline moieties containing similar structures but with differences in thiophene (PPT), furan (PPF) and pyridine (PPP) substitutions at the terminal molecules were synthesized. Their aggregation behaviour in THF-water mixtures was investigated and results demonstrated that PPT and PPP exhibited aggregation-induced emission (AIE), whereas PPF exhibited aggregation-induced blue-shifted emission (AIBSE). PPT and PPP provided red-shifted emission, while PPF had observed blue-shifted emission at high water fractions of 70-90%, confirming that aggregation effects played a major role in the molecular structure. Two emission peaks from locally excited and twisted intramolecular charge transfer confirmed the twisted nature from the dihedral angle values of the free reorganized molecules that were completely restricted in high water fractions due to molecular aggregation. This was further confirmed from colour Commission Internationale de l'Eclairage values as well as dynamic light scattering analysis. Third-order nonlinear optical properties were studied using a Nd:Yag laser beam Z-scan technique at 532 nm. The open aperture Z-scan revealed that PPT and PPF towards the peak point endured strong saturable absorption, whereas PPP indicated a strong reverse saturable absorption process. The AIE and AIBSE mechanisms from undergoing restricted twisting intramolecular motion in the aggregated luminogens provide great insight into new developments in AIEgen materials for these optoelectronic materials.

Fluorescent small organic probes for biosensing

Small-molecule based fluorescent probes are increasingly important for the detection and imaging of biological signaling molecules due to their simplicity, high selectivity and sensitivity, whilst being non-invasive, and suitable for real-time analysis of living systems. With this perspective we highlight sensing mechanisms including Förster resonance energy transfer (FRET), intramolecular charge transfer (ICT), photoinduced electron transfer (PeT), excited state intramolecular proton transfer (ESIPT), aggregation induced emission (AIE) and multiple modality fluorescence approaches including dual/triple sensing mechanisms (DSM or TSM). Throughout the perspective we highlight the remaining challenges and suggest potential directions for development towards improved small-molecule fluorescent probes suitable for biosensing.

A triphenylamine derived fluorescent probe for efficient detection of H2S based on aggregation-induced emission

The probe TPA-HS can quickly identify H₂S (20 minutes) and release TPA-CHO with aggregation-induced emission properties.

Lighting up forensic science by aggregation-induced emission: A review

Forensic science requires a fast, sensitive, and anti-interfering imaging tool for on-site investigation and bio-analysis. The aggregation-induced emission (AIE) phenomenon exhibits remarkable luminescence properties (large Stokes shift, diverse molecular structures, and high photo-stability), which can provide a viable solution for on-site analysis, while at the same time overcoming the problem of aggregation-caused quenching (ACQ). Based on the outstanding performance in chemical analysis and bio-sensing, AIE materials have great prospects in the field of forensic science. Therefore, the application of AIE in forensic science has been summarized for the first time in this article. After a brief introduction to the concept and development of AIE, its applications in the determination of toxic or hazardous substances, based on data on poisoning deaths, has been summarized. Subsequently, besides the bio-imaging function, other applications of AIE in analyzing markers related to forensic genetics, forensic pathology, (focusing on the corpse) and clinical forensics (focusing on the living) have been discussed. In addition, applications of AIE molecules in criminal investigations, including recognition of fingerprints and blood stains, detection of explosives and chemical warfare agents, and anti-counterfeiting have also been presented. It is hoped that this review will light up the future of forensic science by stimulating more research work on the suitability of AIE materials in advancing forensic science.

Construction of a Smart Nanofluidic Sensor through a Redox Reaction Strategy for High-Performance Carbon Monoxide Sensing

Carbon monoxide (CO), an important gas signaling molecule, demonstrated various physiological and pathological functions by regulating the ion flux of biological channels. Herein, inspired by the CO-regulated K⁺ channel in vivo, we propose a smart CO-responsive nanosensor through the redox reaction strategy. Such nanosensor demonstrated an outstanding CO specificity and selectivity with high ion rectification (∼9) as well as excellent stability and recyclability. Therefore, these results will provide a new direction for the design of nanochannel-based sensors for future practical and biological applications.

An AIE probe for imaging mitochondrial SO2-induced stress and SO2 levels during heat stroke

A probe, MITO-TPE, was developed for imaging mitochondrial SO₂ with good selectivity, high sensitivity, and a fast response time. Cell imaging indicated that SO₂-induced oxidative stress may cause damage to cells through O₂˙^- bursting. MITO-TPE has here been used to image the misregulation of SO₂ levels in mitochondria during heat stroke for the first time.

Recent Advances in Fluorescence Light-Up Endogenous and Exogenous Carbon Monoxide Detection in Biology

Considerable attention has been paid by the scientific community to detect toxic carbon monoxide (CO) in sub-cellular organelles like mitochondria, lysosomes, nuclei, etc. due to their generation and accumulation through numerous biological processes and their role as signal transducer, therapeutics, etc. Various methods are also available for detection of CO, but fluorescence light-up detection is considered the best due to its easy and accurate sensing capability. As of now, no review is available in the literature dedicated to fluorescent detection of only CO both in vitro and in vivo, but considering the huge amount of work reporting every year, it is necessary to have an account of all the recent significant works devoted to it. This review will give special attention to the most noteworthy development of fluorescent light-up probes for the detection of cellular and sub-cellular targetable CO starting from 2012 and emphasizing also the mechanism of action and the applications.