Development of Imidazoline-2-Thiones Based Two-Photon Fluorescence Probes for Imaging Hypochlorite Generation in a Co-Culture System

A new FRET-based fluorescent probe: Colorimetric and ratiometric detection of hypochlorite and anti-counterfeiting applications

Hypochlorite (ClO-), as the main component of widely used disinfectants in daily life, comes into closer contact with the human body, which can lead to a number of diseases. The high-performance method is increasingly needed to detect ClO- in our daily life. In this report, we successfully synthesized a FRET ratiometric fluorescent probe (NDAC) containing benzoxadiazole moieties and coumarin moieties bound via ethylenediamine. As expected, NDAC has excellent selectivity and anti-interference ability toward ClO-, and the ratio of fluorescence intensity (I471 nm/I533 nm) has a very good linear relationship with the concentration of ClO-, with a wide linear range (2.5-1750 μM) and low detection limit (0.887 μM). Furthermore, we have successfully applied it for the quantitative detection of ClO- in water samples in daily life. At the same time, there is a very clear change in the fluorescence color after the reaction of the NDAC with ClO-. The blue/green value (B/G) of this color change also shows a very good linear relationship to ClO- (5.0-1000 μM). Therefore, the NDAC has also been successfully used for test strip detection and quantitative detection of ClO- in actual samples through smartphone-based fluorescence image analysis, and this method can provide faster, more convenient and more accessible detection. In addition, NDAC sensors also have potential applications in the field of information anti-counterfeiting.

Sulfur-based fluorescent probes for biological analysis: A review

The widespread adoption of small-molecule fluorescence detection methodologies in scientific research and industrial contexts can be ascribed to their inherent merits, including elevated sensitivity, exceptional selectivity, real-time detection capabilities, and non-destructive characteristics. In recent years, there has been a growing focus on small-molecule fluorescent probes engineered with sulfur elements, aiming to detect a diverse array of biologically active species. This review presents a comprehensive survey of sulfur-based fluorescent probes published from 2017 to 2023. The diverse repertoire of recognition sites, including but not limited to N, N-dimethylthiocarbamyl, disulfides, thioether, sulfonyls and sulfoxides, thiourea, thioester, thioacetal and thioketal, sulfhydryl, phenothiazine, thioamide, and others, inherent in these sulfur-based probes markedly amplifies their capacity for detecting a broad spectrum of analytes, such as metal ions, reactive oxygen species, reactive sulfur species, reactive nitrogen species, proteins, and beyond. Owing to the individual disparities in the molecular structures of the probes, analogous recognition units may be employed to discern diverse substrates. Subsequent to this classification, the review provides a concise summary and introduction to the design and biological applications of these probe molecules. Lastly, drawing upon a synthesis of published works, the review engages in a discussion regarding the merits and drawbacks of these fluorescent probes, offering guidance for future endeavors.

Fabrication of a paper-based facile and low-cost microfluidic device and digital imaging technique for point-of-need monitoring of hypochlorite

Lab-on-a-paper-based devices are promising alternatives to the existing arduous techniques for point-of-need monitoring. The present work reports an instant and facile method to produce a microfluidic paper-based analytical device (μPAD). The fabricated μPAD has been used to detect hypochlorite (OCl-) by incorporating newly synthesized chromo-fluorogenic ratiometric probes 1 and 2 into the sample reception zone. The probes showed high selectivity and fast response (<10 s) toward OCl- with an excellent linear relationship in the concentration range of 0-100 μM. The concentration-dependent fluorometric change driven by the reaction of 1@μPAD with OCl- has been monitored using gel-doc imaging systems, which is unprecedented. Digitizing the intensity of the colour solution with different mathematical models of colour has developed a straightforward method for monitoring OCl- without any interference from its competitors. 1@μPAD can detect OCl- at ∼10 times lower than the WHO recommended limit. The detection limit of 1@μPAD via a digital camera-based fluorescence technique was found to be better over digital camera-based cuvette assays. Therefore, 1@μPAD has been successfully utilized to monitor OCl- in actual environmental water samples with portability, ease of use, and sensitivity. The analytical RSD was found to be ≤3% based on fluorimetric detection using 1@μPAD. The chemodosimetric reaction between OCl- and the probe was evidenced by UV-vis and fluorescence spectroscopy, 1H NMR, and ESI-MS. The rapid response time, biocompatibility, low cytotoxicity, 100% aqueous solubility, ratiometric feature, and exclusive OCl- selectivity over other competitive ROS/RNS successfully lead to the application of the probes for bioimaging of exogenous as well as endogenous OCl- in normal cells (HEK293) and cancerous cells (HeLa).

A new benzo-bodipy based fluorescent probe for the highly sensitive detection of hypochlorous acid and its application in the living cells and zebrafish imaging

Due to the highly significant biological activity of hypochlorous acid, the monitoring of its concentration in vivo has received extensive attention. In this work, a photoinduced electron transfer (PeT) based benzo-bodipy fluorescent probe BBy-T has been developed for the rapid, sensitive, and selective detection of HClO in an aqueous solution. Based on the HClO-specific oxidation reaction, BBy-T exhibited a distinct fluorescence turn-on response to HClO with a remarkable Stokes shift (84 nm), immediate response (less than 20 s), and low detection limit (13.7 nM). In addition, the bioimaging results indicated that the probe BBy-T could be applied to real-time fluorescence imaging of living HeLa cells as well as living zebrafish.

A benzo BODIPY based fluorescent probe for selective visualization of hypochlorous acid in living cells and zebrafish

Hypochlorous acid (HOCl) is an essential endogenous reactive oxygen species in biological systems, playing a critical role in various physiological processes. Real-time monitoring of HOCl concentration in living organisms is essential for understanding its biological functions and pathological roles. In this study, we developed a novel fluorescent probe based on benzobodipy, BBDP, for rapid and sensitive detection of HOCl in aqueous solutions. The probe exhibited a significant fluorescence turn-on response to HOCl based on its specific oxidation reaction towards diphenylphosphine, with high selectivity, instantaneous response (less than 10 s), and low detection limit (21.6 nM). Furthermore, bioimaging results illustrated that the probe could be applied for real-time fluorescence imaging of HOCl in live cells and zebrafish. The development of BBDP may provide a new tool for exploring the biological functions of HOCl and its pathological roles in diseases.

Design and Synthesis of Novel Fluorescent Probe Based on Cyanobiphenyl and its Application in Detection of Hypochlorite

Hypochlorite is an important biological reactive oxygen species, which plays a pivotal role in various life activities. Excessive presence in the human body or excessive intake in life causes a series of diseases. To monitor the hypochlorite level in living cells, organisms and environment water samples, we herein designed and synthesized three organic small molecule fluorescent probes with different recognition sites based on nitrile biphenyl. Through performance comparison, it was found that probe A-HM exhibited the best detection performance for hypochlorite with a low detection limit of 2.47 × 10^-6 M. The introduction of hypochlorite will induce probe fluorescence A-HM to turn on, and the fluorescence colour will change from colourless to green. The application of A-HM in biological systems has been demonstrated by the imaging monitoring of hypochlorite in MCF-7, L929 cells and zebrafish. Furthermore, A-HM was also used for the accurate determination of the hypochlorite level in real water samples with high sensitivity and good recoveries.

Dual-Fluorophore and Dual-Site Multifunctional Fluorescence Sensor for Visualizing the Metabolic Process of GHS to SO2 and the SO2 Toxicological Mechanism by Two-Photon Imaging

As a momentous gas signal molecule, sulfur dioxide (SO₂) participates in diverse physiological activities. Excess SO₂ will cause an apparent decrease in the level of intracellular glutathione (GSH), thereby damaging the body's antioxidant defense system. In addition, endogenous SO₂ can be generated from GSH by reacting with thiosulfate (S₂O₃^2-) and enzymatically reduced to cysteine (Cys), a synthetic precursor of GSH. In view of their close correlation, a two-photon (TP) mitochondria-targeted multifunctional fluorescence sensor Mito-Na-BP was rationally designed and synthesized for detecting SO₂ and GSH simultaneously. Under single-wavelength excitation, the sensor responded to GSH-SO₂ and SO₂-GSH continuously with blue-shifted and green fluorescence-enhanced signal modes, respectively, not just to GSH (enhanced) and SO₂ (quenched) at 638 nm with a completely converse response tendency. Given its favorable spectral performance (high sensitivity, superior selectivity, and fast response rate) at physiological pH, Mito-Na-BP has been successfully applied in monitoring the level fluctuation of GSH affected from high-dose SO₂ and visualizing in real time the metabolic process of GSH to SO₂ by TP imaging. It is expected that this research will provide a convenient and efficient tool for elucidating intricate relationships of GSH and SO₂ and facilitate further exploration of their functions in biomedicine.

Simultaneous Detection of Hypochlorite and Singlet Oxygen by a Thiocoumarin-Based Ratiometric Fluorescent Probe

The development of fluorescent probes derived from thiocarbonyl compounds for reactive oxygen species has been actively pursued in recent years. However, a better understanding of the optical response behaviors of thiocarbonyl compounds toward reactive oxygen species remains a challenge. Along with this, further studies to overcome the limitation of a single emission channel and aggregation-caused quenching features of thiocarbonyl-based fluorescent probes are highly desirable. Due to the important role of hypochlorite and singlet oxygen in biological processes and their common coexistence in living systems with frequent intertransformations, the design of a fluorescent probe that can recognize both hypochlorite and singlet oxygen is of great interest. Herein, a thiocarbonyl-based ratiometric fluorescent probe (Fcoum-S) for simultaneous detection of hypochlorite and singlet oxygen in aqueous solution and living cells was designed and synthesized. Upon the addition of hypochlorite in Fcoum-S solution (phosphate-buffered saline, 10 mM, pH 7.4, 10% acetonitrile), a ratiometric fluorescence response was observed via a specific hypochlorite-promoted desulfurization reaction with a good linear relationship between the ratio of fluorescence intensities at 526 and 602 nm (I _526nm/I _602nm) and the hypochlorite concentrations (a low detection limit of 0.15 μM). Furthermore, upon green light irradiation, Fcoum-S was efficiently desulfurized to its oxo analogue (Fcoum-O) by in situ generated singlet oxygen, leading to a significant change in fluorescence. Fcoum-S could work well in an aqueous medium owing to the high reactivity of the thiocarbonyl group and the aggregation-induced emission characteristics. More importantly, Fcoum-S could target mitochondria and was successfully utilized for fluorescence imaging of mitochondrial hypochlorite/singlet oxygen in live cells. This work provides a molecular design guideline for further exploring thioketone derivatives as fluorescent probes.

A specifically triggered turn-on fluorescent probe platform and its visual imaging of HClO in cells, arthritis and tumors

Understanding disease-related processes at the molecular level is of great importance for the prevention and treatment of diseases. However, due to the lack of effective analytical tools, it is challenging to gain insight into the relationships between a specific bioactive molecule and the associated disease. Herein, a rapid turn-on resorufin-based fluorescent probe platform utilizing the HClO-specific oxidative cleavage of the amide was constructed, allowing the visualization of HClO in vitro and in vivo. These probes could quickly respond to HClO (< 50 s) with high selectivity and sensitivity (12-153 nM). The probe REClO-6 had the fastest response (30 s) and the highest sensitivity (12 nM), and was successfully used for the imaging of endogenous and exogenous HClO in cells and zebrafish. Notably, it was also successfully applied to the imaging of HClO in mouse arthritis and solid tumors. This study provided a rapid imaging analysis tool, which would be used to investigate the relationship between HClO and the disease-related physiological processes.

Surfactant-free synthesis of fluorescent platinum nanoclusters using HEPES buffer for hypochlorous acid sensing and imaging

A surfactant-free synthesis of noble-metal nanoclusters (NMNCs) with specific function has recently remained more attractive and superior in bio-applications. Herein, by employing the weak reducibility of non-toxic HEPES, we prepared novel water-soluble fluorescent HEPES@Pt NCs by a simple surfactant-free synthesis strategy for hypochlorous acid (HClO) sensing. The as-prepared Pt NCs featured ultra-small size (∼2 nm), bright blue fluorescence, high stability and biocompatibility, and the fluorescence of the Pt NC nanoprobe can be specifically quenched with hypochlorous acid by a static quenching process. Moreover, the surfactant-free Pt NC probe displays fascinating performances for HClO sensing, including fast response to HClO, high stability and specificity, and is further applied for imaging the fluctuations of the HClO concentration in living cells with satisfactory results for the first time. Thereby, we anticipate that it is a reliable and attractive approach to develop versatile NMNCs through the surfactant-free synthesis for further applications in biological research.

A coumarin-based reversible two-photon fluorescence probe for imaging glutathione near N-methyl-D-aspartate (NMDA) receptors

Glutathione (GSH) is known to play a key role in the modulation of the redox environment in N-methyl-d-aspartate (NMDA) receptors. Coumarin derivative 1 bearing cyanoacrylamide and ifenprodil moieties was synthesized and reported to monitor GSH near NMDA receptors. The cyanoacrylamide moiety allows probe 1 to monitor GSH reversibly at pH 7.4 and the ifenprodil group acts as a directing group for NMDA receptors. Two-photon fluorescence microscopy allows probe 1 to successfully sense endogenous GSH in neuronal cells and hippocampal tissues with excitation at 750 nm. Furthermore, the addition of H₂O₂ and GSH induced a decrease and an increase in fluorescence emission. Probe 1 can serve as a potential practical imaging tool to get important information on GSH in the brain.

3D two-photon brain imaging reveals dihydroartemisinin exerts antiepileptic effects by modulating iron homeostasis

Imbalanced iron homeostasis plays a crucial role in neurological diseases, yet direct imaging evidence revealing the distribution of active ferrous iron (Fe²⁺) in the living brain remains scarce. Here, we present a near-infrared excited two-photon fluorescent probe (FeP) for imaging changes of Fe²⁺ flux in the living epileptic mouse brain. In vivo 3D two-photon brain imaging with FeP directly revealed abnormal elevation of Fe²⁺ in the epileptic mouse brain. Moreover, we found that dihydroartemisinin (DHA), a lead compound discovered through probe-based high-throughput screening, plays a critical role in modulating iron homeostasis. In addition, we revealed that DHA might exert its antiepileptic effects by modulating iron homeostasis in the brain and finally inhibiting ferroptosis. This work provides a reliable chemical tool for assessing the status of ferrous iron in the living epileptic mouse brain and may aid the rapid discovery of antiepileptic drug candidates.

Two-photon ratiometric fluorescent probe for imaging of hypochlorous acid in acute lung injury and its remediation effect

Acute lung injury (ALI) is a pulmonary inflammatory disease with high morbidity and mortality rates. However, owing to the unknown etiology and rapid progression of the disease, the diagnosis of ALI is full of challenges with no effective treatment. Since the inflammatory response and oxidative stress played vital roles in the development of ALI, we herein developed the largest emission cross-shift (△λ = 145 nm) two-photon ratiometric fluorescent probe of TPRS-HOCl with high selectivity and short response time toward hypochlorous acid (HOCl) for exploring the relevance between the degree of ALI and HOCl concentration in the development process of the disease. In addition, the inhibition effect of HOCl during different treatment periods was also evaluated. Moreover, the tendency of imaging results was basically in accordance with that of hematoxylin and eosin (H&E) staining and the treatment effect became better in the early stage when using N-acetylcysteine (NAC), demonstrating the sensitivity of TPRS-HOCl toward ALI response. Thus, TPRS-HOCl has great potential to diagnose ALI in the early stage and guide for effective treatment.

A Fast-Responsive OFF-ON Near-Infrared-II Fluorescent Probe for In Vivo Detection of Hypochlorous Acid in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a common chronic autoimmune inflammatory disease, and its etiology is closely related to the overproduction of hypochlorous acid (HClO). However, early detection of RA using an activatable near-infrared-II (NIR-II, 1000-1700 nm) fluorescent probe remains challenging. Herein, we first report an "OFF-ON" NIR-II fluorescent probe named PTA (phenothiazine triphenylamine) for imaging HClO in deep-seated early RA. Electron-rich phenothiazine in the core of PTA was utilized as both an HClO-recognition moiety and a precursor of electron acceptors, displaying a typical donor-acceptor-donor structure with excellent NIR-II emission at 936/1237 nm once reacted with HClO. The probe PTA exhibited good water solubility, high photostability, and rapid response capability toward HClO within 30 s. Moreover, it was able to sensitively and specifically detect exogenous and endogenous HClO in living cells in both visible and NIR-II windows. Notably, PTA enabled the sensitive and rapid visualization of HClO generation in an inflammatory RA mouse model, showing a 4.3-fold higher NIR-II fluorescence intensity than that in normal hindlimb joints. These results demonstrate that PTA holds great promise as a robust platform for diagnosis of HOCl-mediated inflammatory disorders.

Ratiometric Detection of Hypochlorous Acid in Brain Tissues of Neuroinflammation and Maternal Immune Activation Models with a Deep-Red/Near-Infrared Emitting Probe

Reactive oxygen species (ROS) produced by an inflammatory response in the brain are associated with various neurological disorders. To investigate ROS-associated neuroinflammatory diseases, fluorescent probes with practicality are in demand. We have investigated hypochlorous acid, an important ROS, in the brain tissues of neuroinflammation and maternal immune activation (MIA) model mice, using a new fluorescent probe. The probe has outstanding features over many known probes, such as providing two bright ratio signals in cells and tissues in deep-red/near-infrared wavelength regions with a large spectral separation, in addition to being strongly fluorescent, photo- and chemo-stable, highly selective and sensitive, fast responding, and biocompatible. We have found that the level of hypochlorous acid in the brain tissue of a neuroinflammatory mouse model was higher (2.7-4.0-fold) compared with that in normal brain tissue. Furthermore, the level of hypochlorous acid in the brain tissue of a MIA mouse model was higher (1.2-1.3-fold) compared with that in the normal brain tissue. The "robust" probe provides a practical tool for studying ROS-associated neurological disorders.

AND-Logic Strategy for Accurate Analysis of Alzheimer's Disease via Fluorescent Probe Lighted Up by Two Specific Biomarkers

Alzheimer's disease (AD) has become a global threat to the elderly health with a short survival time after diagnosis. Due to the asymptomatic stage during the early development, patients are usually diagnosed at the middle or late stage. Therefore, an efficient tool for AD early diagnosis deserves considerable attention, which could make a significant contribution to the treatment intervention. A fluorescent probe has been widely applied for detecting and visualizing species of interest in vitro and in vivo, and the proper reaction between the probe and analytes is responsible for the fluorescence change to provide a lighting-on or ratiometric responsive pattern with satisfactory sensing behavior. In this work, we report the first attempt to build up an AND-logic probe P2 for AD accuracy diagnosis taking butyrylcholinesterase (BChE) and reactive oxygen species (ROSs) as dual targets. Upon the co-stimulation by these two factors through enzymatic hydrolysis and redox reaction, the NIR emission could be readily turned on. This AND sensing pattern avoided the false-positive response effectively, and other diseases sharing one biomarker could hardly induce a NIR fluorescence response. The sensing assay has also been confirmed to be feasible in vitro and in vivo with good sensibility and selectivity. It is worth mentioning that the probe structure has been optimized in terms of the linkage length. This study shows that probe P2 with a connecting arm of medium length (one methylene, n = 1) has superior sensing performance, promising to provide a reference for the relative structure design.

Hypochlorite-Activated Fluorescence Emission and Antibacterial Activities of Imidazole Derivatives for Biological Applications

The ability to detect hypochlorite (HOCl/ClO-) in vivo is of great importance to identify and visualize infection. Here, we report the use of imidazoline-2-thione (R 1 SR 2 ) probes, which act to both sense ClO- and kill bacteria. The N2C=S moieties can recognize ClO- among various typical reactive oxygen species (ROS) and turn into imidazolium moieties (R 1 IR 2 ) via desulfurization. This was observed through UV-vis absorption and fluorescence emission spectroscopy, with a high fluorescence emission quantum yield (ՓF = 43-99%) and large Stokes shift (∆v∼115 nm). Furthermore, the DIM probe, which was prepared by treating the DSM probe with ClO-, also displayed antibacterial efficacy toward not only Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) but also methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum ß-lactamase-producing Escherichia coli (ESBL-EC), that is, antibiotic-resistant bacteria. These results suggest that the DSM probe has great potential to carry out the dual roles of a fluorogenic probe and killer of bacteria.

Cyanine-Dyad Molecular Probe for the Simultaneous Profiling of the Evolution of Multiple Radical Species During Bacterial Infections

Real-time monitoring of the evolution of bacterial infection-associated multiple radical species is critical to accurately profile the pathogenesis and host-defense mechanisms. Here, we present a unique dual wavelength near-infrared (NIR) cyanine-dyad molecular probe (HCy5-Cy7) for simultaneous monitoring of reactive oxygen and nitrogen species (RONS) variations both in vitro and in vivo. HCy5-Cy7 specifically turns on its fluorescence at 660 nm via superoxide or hydroxyl radical (O₂ ^.- , ^. OH)-mediated oxidation of reduced HCy5 moiety to Cy5, while peroxynitrite or hypochlorous species (ONOO^- , ClO^- )-induced Cy7 structural degradation causes the emission turn-off at 800 nm. Such multispectral but reverse signal responses allow multiplex manifestation of in situ oxidative and nitrosative stress events during the pathogenic and defensive processes in both bacteria-infected macrophage cells and living mice. Most importantly, this study may also provide new perspectives for understanding the bacterial pathogenesis and advancing the precision medicine against infectious diseases.

Comparing the abundance of HClO in cancer/normal cells and visualizing in vivo using a mitochondria-targeted ultra-fast fluorescent probe

Organisms are operating and evolving with a highly sophisticated and intelligent defense mechanism to resist bacterial and viral infections. This process involves a variety of reactive oxygen species (ROS), and they coordinate with each other to support different physiological activities. Due to its strong oxidizing properties, hypochlorous acid (HClO), a part of ROS, is a powerful antimicrobial agent in living organisms and exerts a crucial role in the immune system. However, the excessive production of HClO can cause cell damage and even cell death. Herein, we combined benzene-conjugated benzopyrylium as the fluorophore and dimethylthiocarbamoyl chloride as the recognition site to rationally design a probe (BBD). The fluorescence of the probe was quenched based on an effective PET molecular mechanism. Surprisingly, BBD exhibited a turn-on red fluorescence signal for HClO with ultra-fast response (5 s) and high selectivity. Moreover, BBD located mitochondria well and it was found that the abundance of HClO is higher in HeLa cells compared to that in normal cells. Finally, BBD was successfully applied to the visualization of HClO in zebrafish and nude mice.

Molecular Design toward Heavy-Atom-free Photosensitizers Based on the C═S Bond and their Dual Functions in Hypoxia Photodynamic Cancer Therapy and ClO- Detection

In this article, we designed and synthesized the thionated NpImidazole derivatives BS and NS, new heavy-atom-free photosensitizers, which efficiently generate a triplet excited state with high singlet oxygen quantum yield. The introduction of the C═S bond to the NpImidazole core is essential for increasing spin-orbit coupling (SOC). The fluorescence emission of BS and NS was quenched at standard ambient temperature, accompanied with the increase in the ISC process from the singlet states to triplet excited states via thionation. BS and NS showed negligible dark cytotoxicity against HeLa cells in working concentration. In contrast, BS and NS rapidly induced cell death under blue light irradiation both under normoxia and hypoxia conditions. Our current study demonstrates that the C═S group can play an important role in type I ROS generation of PSs, which are unprecedented in the previous reports. Finally, the photophysical changes were assigned to the oxidative desulfurization of the C═S group of BS and NS to the C═O group of the corresponding BO and NO via hypochlorite. The combined results demonstrated the dual function of BS and NS as a fluorescent imaging agent for ClO^- and an anti-cancer therapeutic by PDT that showed the potential strategy for "one-for-all" and multifunctional agents.

A lysosome-targetable two-photon excited near-infrared fluorescent probe for visualizing hypochlorous acid-involved arthritis and its treatment

Lysosome of phagocyte is the main site of hypochlorous acid (HClO) production, and HClO can be employed as the biomarker for the diagnosis and treatment evaluation of arthritis. In recent years, developing fluorescent probes for lysosomal HClO has attracted considerable attention, but most of them still have some defects, such as autofluorescence, phototoxicity and photobleaching because of their excitation and emission located in short-wavelength region. Due to the advantages of two-photon fluorescent probes with near-infrared emissions, a lysosome-targetable two-photon fluorescent probe (Lyso-TP-HClO) with a near-infrared emission was reported in this paper. Lyso-TP-HClO has a high selectivity and a high sensitivity to HClO in the linear range (10.0 × 10^-8 to 5.0 × 10^-6 M), with a detection limit of 3.0 × 10^-8 M. Due to the two-photon excited near-infrared emission, Lyso-TP-HClO has excellent imaging performances, such as small autofluorescence, excellent photostability, and large imaging depth. Furthermore, Lyso-TP-HClO was successfully employed for visualizing lysosomal HClO in bacteria-infected cells. At last, we have successfully used Lyso-TP-HClO to image the arthritis and evaluate the treatment of arthritis in mice. All the results confirm that Lyso-TP-HClO is a useful chemical tool for imaging of lysosomal HClO, the diagnosis of arthritis, and treatment evaluation of arthritis.

Imidazoline synthesis: mechanistic investigations show that Fe catalysts promote a new multicomponent redox reaction

Multicomponent reactions are attracting strong interest because they contribute to develop more efficient synthetic chemistry. Understanding their mechanism at the molecular level is thus an important issue to optimize their operation. The development of integrated experimental and theoretical approaches has very recently emerged as most powerful to achieve this goal. In the wake of our recent investigation of amidine synthesis, we used this approach to explore how an Fe-catalyzed aziridination can lead to an imidazoline when run in acetonitrile. We report that the synthesis of imidazoline by combination of styrene, acetonitrile, an iron catalyst and a nitrene precursor occurs along a new kind of multicomponent reaction. The formation of imidazoline results from acetonitrile interception of a benzyl radical styrene aziridination intermediate within Fe coordination sphere, as opposed to classical nucleophilic opening of the aziridine by a Lewis acid. Comparison of this mechanism to that of amidine formation allows a rationalization of the modes of intermediates trapping by acetonitrile according to the oxidation state Fe active species. The molecular understanding of these processes may help to design other multicomponent reactions.

Two-photon small-molecule fluorescence-based agents for sensing, imaging, and therapy within biological systems

In this tutorial review, we will explore recent advances for the design, construction and application of two-photon excited fluorescence (TPEF)-based small-molecule probes.

K. M, Biswas S. Fluorogenic naked eye “turn-on” sensing of hypochlorous acid by a Zr-based metal organic framework

A (dimethylthiocarbamoyl)oxy functionalized Zr-based UiO-66 MOF was utilized for the first time as a fluorogenic turn-on detector for the sensitive and specific sensing of HOCl in an aqueous medium.

A visible-light-excitable mitochondria-targeted europium complex probe for hypochlorous acid and its application to time-gated luminescence bioimaging

Development of fluorescent/luminescent probes for rapid, selective and sensitive detection of reactive oxygen species (ROS) is of great significance for understanding the roles of ROS in pathophysiological processes. In the present research, a visible-light-excitable Eu³⁺ complex-based probe, Eu(L)₃(DPBT), is designed and synthesized for the time-gated luminescence (TGL) determination of hypochlorous acid (HClO) in vitro and in vivo. The proposed probe exhibits a rapid, selective and sensitive TGL response to HClO, and excellent localization of mitochondria in living cells with low cytotoxicity. These features allow the probe to be used for the TGL sensing and imaging of HClO formation in mimic inflammatory cells at a subcellular level, as well as in endotoxin-induced liver injury and rheumatoid arthritis in live mice. In addition, by immobilizing the probe in the PEG hydrogel, the smart sensor films with rapid response to HClO were prepared, and successfully used for the real-time monitoring of HClO generation in mouse wounds, in order to distinguish the infected wounds from acute ones. Overall, this study provides a useful tool for the clinical monitoring and treatment of wound diseases.

Revisiting imidazolium receptors for the recognition of anions: highlighted research during 2010-2019

Imidazolium based receptors selectively recognize anions, and have received more and more attention. In 2006 and 2010, we reviewed the mechanism and progress of imidazolium salt recognition of anions, respectively. In the past ten years, new developments have emerged in this area, including some new imidazolium motifs and the identification of a wider variety of biological anions. In this review, we discuss the progress of imidazolium receptors for the recognition of anions in the period of 2010-2019 and highlight the trends in this area. We first classify receptors based on motifs, including some newly emerging receptors, as well as new advances in existing receptor types at this stage. Then we discuss separately according to the types of anions, including ATP, GTP, DNA and RNA.

Novel near-infrared fluorescent probe with a large Stokes shift for sensing hypochlorous acid in mitochondria

Hypochlorous acid (HOCl) plays a crucial role in various physiological and pathological processes. However, it is still a challenge to design a xanthene-based near-infrared (NIR) fluorescent probe with a large Stokes shift for sensing HOCl. In this work, a novel mitochondria-targeted fluorescent probe, MXS, with a large Stokes shift based on a xanthene-hemicyanine dyad structure, has been successfully designed and synthesized for the specific detection of HOCl. Gratifyingly, the peak-to-peak Stokes shift of MXS was found to be 130 nm, which was obviously larger than those of conventional rhodamine dyes and most reported xanthene-based hypochlorous acid probes. As expected, MXS exhibited high selectivity, high sensitivity, and fast response time (30 s) for the detection of HOCl via a specific HOCl-promoted intramolecular charge transfer process. The detection limit of MXS for HOCl is calculated to be as low as 72 nM, enabling its use within the physiological concentration range of HOCl (5-25 μM). Importantly, MXS is able to permeate cell membranes and accumulate in the mitochondria, which is convenient for monitoring the variation of hypochlorous acid concentration in the mitochondria of living cells.

Luminescent probes for hypochlorous acid in vitro and in vivo

HClO/ClO^- is the most effective antibacterial active oxygen in neutrophils. However, its excessive existence often leads to the destruction of human physiological mechanisms. In recent years, the developed luminescent probes for the detection of HClO/ClO^- are not only conducive to improve the sensitivity and selectivity of HClO/ClO^- detection, but also play a crucial role in understanding the biological functions of HClO/ClO^-. In addition, luminescent probe-based biological imaging for HClO/ClO^- at sub-cellular resolution has become a powerful tool for biopathology and medical diagnostic research. This article reviews a variety of luminescent probes for the detection of HClO/ClO^-in vitro and in vivo with different design principles and mechanisms, including fluorescence, phosphorescence, and chemiluminescence. The photophysical/chemical properties and biological applications of these luminescent probes were outlined. Finally, we summarized the merits and demerits of the developed luminescent probes and discussed their challenges and future development trends. It is hoped that this review can provide some inspiration for the development of luminescent probe-based strategies and to promote the further research of biomedical luminescent probes for HClO/ClO^-.

Epileptic brain fluorescent imaging reveals apigenin can relieve the myeloperoxidase-mediated oxidative stress and inhibit ferroptosis

Myeloperoxidase (MPO)-mediated oxidative stress has been suggested to play an important role in the pathological dysfunction of epileptic brains. However, there is currently no robust brain-imaging tool to detect real-time endogenous hypochlorite (HClO) generation by MPO or a fluorescent probe for rapid high-throughput screening of antiepileptic agents that control the MPO-mediated chlorination stress. Herein, we report an efficient two-photon fluorescence probe (named HCP) for the real-time detection of endogenous HClO signals generated by MPO in the brain of kainic acid (KA)-induced epileptic mice, where HClO-dependent chlorination of quinolone fluorophore gives the enhanced fluorescence response. With this probe, we visualized directly the endogenous HClO fluxes generated by the overexpression of MPO activity in vivo and ex vivo in mouse brains with epileptic behaviors. Notably, by using HCP, we have also constructed a high-throughput screening approach to rapidly screen the potential antiepileptic agents to control MPO-mediated oxidative stress. Moreover, from this screen, we identified that the flavonoid compound apigenin can relieve the MPO-mediated oxidative stress and inhibit the ferroptosis of neuronal cells. Overall, this work provides a versatile fluorescence tool for elucidating the role of HClO generation by MPO in the pathology of epileptic seizures and for rapidly discovering additional antiepileptic agents to prevent and treat epilepsy.

An endoplasmic reticulum-targeted two-photon fluorescent probe for bioimaging of HClO generated during sleep deprivation

With the development of social society, sleep deprivation has become a serious and common issue. Previous studies documented that there is a correlation between sleep deprivation and oxidative stress. However, the information of sleep deprivation related ROS has rarely been obtained. Also, it has been demonstrated that sleep deprivation can induce endoplasmic reticulum (ER) stress. As such, for a better understanding of sleep deprivation as well as its related diseases, it is important to develop probes with ER-targeting ability for detecting ROS generated in this process. Herein, a novel two-photon fluorescent molecular probe, JX-1, was designed for sensing HClO in live cells and zebrafish. The investigation data showed that in addition to real-time response (about 150 s), the probe also exhibited high sensitivity and selectivity. Moreover, the probe JX-1 demonstrated two-photon fluorescence, low cytotoxicity and ER targeting ability. These prominent properties enabled the utilization of the probe for monitoring exogenous and endogenous HClO in both live cells and zebrafish. Using this useful tool, it was found that sleep deprivation can induce the generation of HClO in zebrafish.

An Activatable AIEgen Probe for High-Fidelity Monitoring of Overexpressed Tumor Enzyme Activity and Its Application to Surgical Tumor Excision

Monitoring fluctuations in enzyme overexpression facilitates early tumor detection and excision. An AIEgen probe (DQM-ALP) for the imaging of alkaline phosphatase (ALP) activity was synthesized. The probe consists of a quinoline-malononitrile (QM) core decorated with hydrophilic phosphate groups as ALP-recognition units. The rapid liberation of DQM-OH aggregates in the presence of ALP resulted in aggregation-induced fluorescence. The up-regulation of ALP expression in tumor cells was imaged using DQM-ALP. The probe permeated into 3D cervical and liver tumor spheroids for imaging spatially heterogeneous ALP activity with high spatial resolution on a two-photon microscopy platform, providing the fluorescence-guided recognition of sub-millimeter tumorigenesis. DQM-ALP enabled differentiation between tumor and normal tissue ex vivo and in vivo, suggesting that the probe may serve as a powerful tool to assist surgeons during tumor resection.