Turn-on Fluorescent Probe for Exogenous and Endogenous Imaging of Hypochlorous Acid in Living Cells and Quantitative Application in Flow Cytometry

Large stokes shift and near-infrared fluorescent probe for bioimaging and evaluating the HClO in an rheumatoid arthritis mouse model

It is crucial to identify aberrant HClO levels in living things since they pose a major health risk and are a frequent reactive oxygen species (ROS) in living organisms. In order to detect HClO in various biological systems, we created and synthesized a near-infrared fluorescent probe with an oxime group (-C = N-OH) as a recognition unit. The probe DCMP1 has the advantages of fast response (10 min), near-infrared emission (660 nm), large Stokes shift (170 nm) and high selectivity. This probe DCMP1 not only detects endogenous HClO in living cells, but also enables further fluorescence detection of HClO in living zebrafish. More importantly, it can also be used for fluorescence imaging of HClO in an rheumatoid arthritis mouse model. This fluorescent probe DCMP1 is anticipated to be an effective tool for researching HClO.

A lantern-shaped fluorescent probe based on viologen/polyoxometalate for the detection of Ag+ in beverages and daily necessities

A lantern-shaped viologen/polyoxometalate (POM)-based compound [NiII(MSBP)2(H2O)2]·(β-Mo8O26)·H2O (Ni-POM) (MSBP = 1-(4-Methanesulfonyl-benzyl)-[4,4']bipyridinyl-1-ium) was successfully synthesized by a hydrothermal method for the efficient detection of Ag+. A strong affinity between Ag+ and SO in the viologen component of the Ni-POM structure made them interact, which led to blue fluorescence quenching. In the concentration range of 0.1-4 μM, a strong linear relationship was observed between the Ag+concentration and the fluorescence intensity ratio of Ni-POM, and the limit of detection (LOD) was 20.4 nM. Considering the widespread presence of Ag+ in various water sources, daily necessities and food preservatives, the utilization of Ni-POM for detecting the concentration of Ag+ in real samples (water, daily necessities and beverages) was proved to be highly effective. Moreover, a remarkable recovery rate ranging from 95.70 % to 103.60 % was achieved, indicating that the monitoring results of practical samples were satisfactory. A fluorescent ink based on Ni-POM was designed for the purpose of information confidentiality. More importantly, the hydrogel intelligent device for visual detection of Ag+ was developed, which could realize visual real-time on-site quantitative detection of Ag+ concentration in beverages and daily necessities. Therefore, Ni-POM provides an effective platform for the development of visually quantitative detection of Ag+ in food and daily necessities.

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

Advances in small-molecule fluorescent probes for the study of apoptosis

Apoptosis, as type I cell death, is an active death process strictly controlled by multiple genes, and plays a significant role in regulating various activities. Mounting research indicates that the unique modality of cell apoptosis is directly or indirectly related to different diseases including cancer, autoimmune diseases, viral diseases, neurodegenerative diseases, etc. However, the underlying mechanisms of cell apoptosis are complicated and not fully clarified yet, possibly due to the lack of effective chemical tools for the nondestructive and real-time visualization of apoptosis in complex biological systems. In the past 15 years, various small-molecule fluorescent probes (SMFPs) for imaging apoptosis in vitro and in vivo have attracted broad interest in related disease diagnostics and therapeutics. In this review, we aim to highlight the recent developments of SMFPs based on enzyme activity, plasma membranes, reactive oxygen species, reactive sulfur species, microenvironments and others during cell apoptosis. In particular, we generalize the mechanisms commonly used to design SMFPs for studying apoptosis. In addition, we discuss the limitations of reported probes, and emphasize the potential challenges and prospects in the future. We believe that this review will provide a comprehensive summary and challenging direction for the development of SMFPs in apoptosis related fields.

N-aminomorpholine-functionalized bromine-doped carbon dots for hypochlorous acid detection in foods and imaging in live cells

Hypochlorous acid (HClO) is used in food preservation. However, excessive HClO can deteriorate nutritional composition of food, compromise its quality, and potentially induce various diseases. Consequently, the development of multifunctional fluorescent probes for the sensitive and selective detection of HClO is highly anticipated for food safety. In this work, we designed a nanoprobe using N-aminomorpholine (AM)-functionalized bromine-doped carbon dots (Br-CDs-AM) for sensing HClO. This nanoprobe exhibits pH stability, strong resistance to photobleaching, superior long-term photostability (12 weeks), high sensitivity (19.3 nM), and an ultrarapid response (8 s) for detecting HClO residues in food matrices with percentage recovery (96.5 %-108 %) and RSDs less than 5.34 %. In addition, extremely low cytotoxicity and outstanding biocompatibility enable the nanoprobe to be used primarily for lysosome tracking and rapidly visualizing HClO in live cells. Thus, this study provides a new pathway to design unconventional nanoprobes for food safety assessment and subcellular organelle-specific imaging HClO.

Visualization of the elevated levels of hypochlorous acid in Alzheimer's disease with a ruthenium(II) complex-based luminescence probe

Alzheimer's disease (AD) is an age-related neurodegenerative disorder that devastatingly affects people's lives. Accumulating evidence indicates that the pathological progression of AD is inseparably connected with hypochlorous acid (HClO). However, further exploring the biological function remains an open challenging due to a lack of effective tools to image HClO in AD brains. To this end, a ruthenium(II) luminescence probe, Ru-HClO, is developed for quantitative detection and visualization of HClO in nerve cells and AD brains. Ru-HClO shows quenched luminescence due to the PET process (excited electron transfer from Ru(II) center to diaminomaleonitrile) and the CN bond isomerization in the excited state. The HClO-triggered specific cleavage reaction with Ru-HClO cleaves the CN bond to form highly luminescent Ru-COOH. Ru-HClO shows rapid response speed, high sensitivity and selectivity, excellent biocompatibility, which makes the probe to be applied to semi-quantitative analysis of HClO in nerve cells and high-throughput screening of anti-AD drugs in the AD cell model. Moreover, using Ru-HClO as a probe, present work further validated that the elevated levels of HClO secretion were accompanied by the AD progressed. These findings may provide valuable results for figuring out the biological roles that HClO played in AD but also for accelerating anti-AD therapeutic discovery.

Designing a turn-on ultrasensitive fluorescent probe based on ICT-FRET for detection and bioimaging of Hypochlorous acid

Hypochlorous acid (HClO) plays an essential role in biological systems. The characteristics of potent oxidization and short lifetime make it challenging to detect specifically from other reactive oxygen species (ROS) at cellular levels. Therefore, its detection and imaging with high selectivity and sensitivity are of great significance. Herein a turn-on HClO fluorescent probe (named RNB-OCl) with boronate ester as the recognition site was designed and synthesized. The RNB-OCl displayed good selective and ultrasensitive to HClO with a low detection limit of 1.36 nM by the intramolecular charge transfer (ICT)-fluorescence resonance energy transfer (FRET) dual mechanism in reducing the fluorescence background and improving the sensitivity. In addition, the role of the ICT-FRET was further demonstrated by time-dependent density functional theory (TD-DFT) calculations. Furthermore, the probe RNB-OCl was successfully employed for imaging HClO in living cells.

A dual-site fluorescent probe for discriminately detecting low and high concentration of hypochlorite in living cells

Hypochlorite (ClO^-) is an important bioactive molecule of living system which plays essential roles in many physiological and pathological processes. There is no doubt that the biological roles of ClO^- depend highly on the concentration of ClO^-. Unfortunately, the relationship between the concentration of ClO^- and the biological process is unclear. Toward this purpose, in this work we addressed a core challenge for developing a powerful fluorescence tool for monitoring a wide concentration change (0-14 eq.) of ClO^- via two distinct detection manners. The probe displayed fluorescence variation (red to green) upon addition of ClO^- (0-4 eq.) and the color of test medium changed from red to colorless witnessed by the naked eyes. Surprisingly, in the presence of higher concentration of ClO^- (4-14 eq.), the probe displayed another fluorescent signal change from green to blue. After demonstrating the excellent sensing properties of the probe with ClO^- in vitro, it was successfully used to imaging different concentration of ClO^- in living cells. We expected that the probe could act as an exciting chemistry tool for imaging of ClO^- concentration-dependent oxidative stress event in biological system.

Endoplasmic Reticulum Peroxynitrite Fluctuations in Hypoxia-Induced Endothelial Injury and Sepsis with a Two-Photon Fluorescence Probe

Sepsis is a serious systemic inflammatory disease that frequently results in death. Early diagnosis and timely targeted interventions could improve the therapeutic effect. Recent work has revealed that the reactive oxygen species (ROS) in the endoplasmic reticulum (ER) and hypoxia-induced endothelial injury play significant roles in sepsis. However, the relationship between the levels of peroxynitrite (ONOO^-) and hypoxia-induced endothelial injury as well as different states of sepsis remain unexplored. Herein, we developed a unique two-photon fluorescent probe (ER-ONOO^-) for detecting ONOO^- in aqueous solution that has high sensitivity, high selectivity, and ultrafast response time. In addition, ER-ONOO^- was successfully used to evaluate the levels of ONOO^- at the ER with three kinds of methods in a hypoxia-induced endothelial injury model. Furthermore, ER-ONOO^- is capable of monitoring the changes in organ fluorescence through ONOO^- variation in different stages of a cecum ligation and puncture (CLP) mouse model. Moreover, we also confirmed that the endoplasmic reticulum stress and oxidative stress participated in the CLP model. Consequently, this research can provide a reliable tool for studying ONOO^- fluctuation in sepsis and provide new insights into the pathogenic and therapeutic mechanisms involved.

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.

Two smart coumarin-based fluorescent probes with AIE effect for sensing ClO- and imaging in living cells

It's worth noting that detect effective methods for tracking ClO^- could help us uncover the function of ClO^- in living systems. Here, two coumarin-based probes, named (E)-3-(1-hydrazonoethyl)-2H-chromen-2-one (1A) and 3-((E)-1-(((E)-(2,3-dihydro-1H-imidazol-4-yl)methylene)-hydrazono)ethyl)- 2H-chromen-2-one (1B) with aggregation-induced emission (AIE) effect in Tris-HCl (pH = 7.2) buffer solution were synthesized and used for sensing ClO^- selectivity. 1A and 1B responded to ClO^- through the oxidation hydrolysis effect. The mechanism was further verified by HR-MS and DFT calculation. Cell imaging indicated that 1A and 1B were good membrane permeability with low toxicity to HEK293T, and expected to be used to detect ClO^- in cells.

A super large Stokes shift ratiometric fluorescent probe for highly selective sensing of ClO- in bio-imaging and real water samples

Based on the fluorescence resonance energy transfer (FRET), a ratiometric fluorescent probe (NQ) was successfully designed and synthesized, in which quinolinone moiety was selected as the energy donor and naphthalimide block as the energy acceptor. NQ has a super large Stokes shift (231 nm) and a big quantum yield (0.463). Compared with previously reported probes with similar recognition sites, NQ can high sensitively and selectively recognize ClO^- with a much low limit of detection (LOD = 21 nM) and extremely rapid response time (20 s). NQ has a strong anti-interference effect and a color change in the solution which can be seen by the "naked eye". Moreover, NQ can be applied to detect ClO^- in real water samples and living cells imaging.

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.

Simultaneous Monitoring of HOCl and Viscosity with Drug-Induced Pyroptosis in Live Cells and Acute Lung Injury

Pyroptosis is a newly identified form of cell death that is closely correlated with many diseases. Recent studies have indicated that the inflammation in pyroptosis would accelerate the generation of reactive oxygen species (ROS). In addition, intracellular viscosity is another key microenvironmental parameter that reflects many physiological and pathological states in the early stage, hypochlorous acid (HOCl), as an important ROS, also plays significant roles in a variety of pathologies. However, the fluctuation of viscosity and HOCl in the process of pyroptosis is still unknown. Herein, we present a dual-responsive fluorescent probe (Lyso-VH) for simultaneously detecting viscosity and HOCl. Lyso-VH was successfully used to image the fluctuation of HOCl and viscosity in the lysosome of three kinds of cells with dependent and independent channels. Moreover, Lyso-VH can be employed to investigate the changes of HOCl and viscosity during the process of pyroptosis in living cells and acute lung injury (ALI). Thus, this work can not only serve as a powerful tool to simultaneously visualize the fluctuation of HOCl and viscosity in lysosomes, but also provide a new insight into drug-induced pyroptosis in living cells and acute lung injury.

A dicyanoisophorone-based fluorescent probe for hypochlorite with a fast response and its applications in bioimaging

One kind of phenolic substituted dicyanoisophorone derivative (Is-OL) has been designed and successfully synthesized for the detection of hypochlorite in water samples, test strips and living HeLa cells. The probe Is-OL showed high sensitivity and selectivity to hypochlorite over other competitive ROS and metal ions. Moreover, Is-OL can react instantaneously with hypochlorite (<5 s) while exhibiting a significant color change from yellow to colorless, which makes "naked-eye" detection possible with a low detection limit (0.095 μM). The results based on water tests and living HeLa cell experiments showed that Is-OL could be applied as a potential candidate for the detection of hypochlorite.

Exploring the relationship between the "ON-OFF" mechanism of fluorescent probes and intramolecular charge transfer properties

In this study, the excited state charge distribution characteristics and fluorescence mechanism of HClO detection probes HN-ClO (weak fluorescence) and HN-ClO-F (strong fluorescence) probes were investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results of electrostatic potential (ESP) map and hole-electron analysis show that the HN-ClO and HN-ClO-F probes have obvious charge separation characteristics in the excited state. The excited state energy decomposition and Merz-Kollman charge analysis demonstrate the existence of distinct planar intramolecular charge transfer (PICT) features in HN-ClO and HN-ClO-F. Due to the strong charge coupling caused by the planar structure, the fluorescence of HN-ClO-F could occur. Furthermore, the weak fluorescence of HN-ClO is caused by inter-system crossing (ISC) between S₁ and T₁ state. Our result proves that the ICT process could exist in HN-ClO-F, but the PICT process does not cause fluorescence quenching, which have provided an excellent supplement to the mechanism of fluorescent probes. The conclusion is consistent with the fluorescence phenomenon observed in the experiment.

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.

Insights Into the Detection Selectivity of Redox and Non-redox Based Probes for the Superoxide Anion Using Coumarin and Chromone as the Fluorophores

In this study, we evaluated the applicability of various superoxide anion sensors which were designed based on either redox or non-redox mechanisms. Firstly, both redox- and non-redox-based superoxide anion probes were designed and synthesized using either coumarin or chromone as the fluorophores, and the photophysical properties of these probes were measured. Subsequently, the sensing preference of both types of probes toward various reactive oxygen species (ROS) was evaluated. We found that non-redox-based O2 •- probes exhibited broad sensing ability toward various ROS. By contrast, redox based O2 •- probes showed a clear reactivity hierarchy which was well correlated to the oxidizing strength of the ROS. Lastly, the detection selectivity of redox-based O2 •- recognizing probes was also observed when balancing various factors, such as reactant ROS concentrations, temperature, and changing reaction transformation rates. Herein, we concluded the selectivity advantage of redox-based O2 •- probes.

Deep-Red/Near-Infrared Turn-On Fluorescence Probes for Aldehyde Dehydrogenase 1A1 in Cancer Stem Cells

Accumulating evidence supports that cancer stem cells (CSCs) are responsible for cancer proliferation, metastasis, and therapy resistance; therefore, an effective strategy to identify and isolate CSCs is required urgently. Because of their low invasiveness and high signal/noise ratio, "turn-on" fluorescence probes working in the deep-red/near-infrared (DR/NIR) region are one of the most attractive yet undeveloped tools for CSC detection. Herein, we report DR/NIR turn-on fluorescence probes, CS5-A and CS7-A, targeted to aldehyde dehydrogenase 1A1 as an intracellular CSC marker. In contrast to the conventional "always-on" green-fluorescent ALDEFLUOR, we succeeded in generating high-contrast (signal/noise ratio > 8.3) and wash-free in vitro CSC imaging with the DR probe C5S-A. This probe can facilitate CSC isolation with minimal contamination by autofluorescence from other tissues through fluorescence-activated cell sorting. Furthermore, the NIR absorbance/emission and turn-on properties of C7S-A allow simple and rapid CSC detection in vivo within 15 min.

A TICT + AIE based fluorescent probe for ultrafast response of hypochlorite in living cells and mouse

Hypochlorite (HClO/ClO^-), an important reactive oxygen species (ROS), plays a significant role in the human immune system. Thus, developing a fast and efficient method for detecting ClO^- is quite necessary. Herein, we designed and synthesized a fluorescent probe TPB-CN based on twisted intramolecular charge transfer (TICT) and aggregation-induced emission (AIE) characteristics. The probe could respond to ClO^- with an ultrafast response velocity (<2 s). The detection limit was calculated to be 6.198 nM. In addition, probe TPB-CN was successfully applied for detecting ClO^- in living cells and mouse.

An effective fluorescent sensor for ClO- in aqueous media based on thiophene-cyanostilbene Schiff-base

Although some reports on sensing ClO^- had been presented, the ClO^- sensor with high selectivity and sensitivity in aqueous media was still expected. Herein, an effective fluorescent sensor for ClO^- in aqueous media was designed and synthesized by simple procedure based on cyanostilbene derivative (TCS). TCS exhibited strong fluorescence in aqueous media, which could be selectively quenched by ClO^- among various species. The detection limit was as low as 3.2 × 10^-8 M. The sensing mechanism of the oxidation of sulfur in thiophene unit was confirmed by the FT-IR spectrum, fluorescence Job's plot, ¹H NMR spectrum and MS spectrum. This sensor was successfully applied on detecting ClO^- in real sample and living-cell imaging, suggesting its potential application for sensing ClO^- in both vitro assay and vivo environment.

A Two-Photon Excited Near-Infrared Iridium(III) Complex for Multi-signal Detection and Multimodal Imaging of Hypochlorite

Hypochlorite (ClO^-), as a type of reactive oxygen species (ROS), plays a crucial role in the process of oxidative stress and is closely related to many diseases. Thus, developing a method for detecting and imaging of ClO^- with high sensitivity and selectivity is of great significance. However, the applications of most luminescent probes are limited to the fact that the excitation and emission wavelengths of them are in the visible light region rather than in the near-infrared (NIR) region. Hence, an NIR iridium(III) complex (Mul-NIRIr) with two-photon excitation is designed for the detecting and imaging of ClO^-. In the presence of ClO^-, the luminescent intensity and lifetime of Mul-NIRIr are remarkably enhanced. Interestingly, Mul-NIRIr also exhibits excellent electrochemiluminescence (ECL) properties, and the ECL signal is significantly enhanced with the addition of ClO^-. What is more, Mul-NIRIr is also suitable for the detection and analysis ClO^- by flow cytometry. Therefore, Mul-NIRIr is developed to detect multiple signals and is successfully applied to detect exogenous and endogenous ClO^- in living cells with one-photon, two-photon, and phosphorescence lifetime image microscopy (PLIM). In addition, Mul-NIRIr was successfully used for imaging of ClO^- in tissues and inflammatory mouse models. All of the above results indicate that Mul-NIRIr is highly effective in detecting ClO^- in living systems.

Recent development of synthetic probes for detection of hypochlorous acid/hypochlorite

Hypochlorous acid/hypochlorite (HOCl/OCl^-), as one of the most important reactive oxygen species (ROS), plays an important role in various physiological and pathological processes. Nonproperly located or abnormal concentration of OCl^-, however, is associated with many diseases. Thus, developing the fluorescent probe for detecting OCl^- is of great significance. To this end, in last decade, many fluorescent probes have been developed and applied for detecting HOCl/OCl^- in vitro and in vivo. Despite a great progress has achieved, the development and application of near-infrared fluorescent HOCl/OCl^- probe still have some challenges. For example, highly specific and sensitive NIR fluorescent HOCl/OCl^- probes applied in endogenous OCl^- detection and subcellular organelle bioimaging. In this review, we summarized the representative cases of HOCl/OCl^- probes with properties that mentioned above. The discussion contains design strategies, detection mechanisms, as well as applications in bioimaging.

Taking advantage of the aromatisation of 7-diethylamino-4-methyl-3,4-dihydrocoumarin in the fluorescence sensing of superoxide anion

The aromatisation of 7-diethylamino-3,4-dihydrocoumarin provides an alternative fluorescent probing technique to selectively detect the concentration of superoxide anion in solution. In addition, we reported the advantage of evaluating O₂˙^- sensing probes in anhydrous DMSO instead of in aqueous buffers when using KO₂ as the surrogate of O₂˙^-.

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^-.

Off/On Amino-Functionalized Polyhedral Oligomeric Silsesquioxane-Perylene Diimides Based Hydrophilic Luminescent Polymer for Aqueous Fluoride Ion Detection

Fluoride ion detection in water focuses much attention due to the serious healthy impact in human pathologies. For fluoride recognition, the chemical affinity between fluoride and silicon has been developed on the basis of the degradation mechanism. However, most fluorescent probes are the "turn off" type due to the aggregation of the degradational products. Herein, we first developed an "off-on" hydrophilic luminescent polymer composed of amino-functionalized polyhedral oligomeric silsesquioxane (AE-POSS) and perylene diimides (PDIs) for fluoride ion in water. The AE-PDI polymer was "turned off" because of the photoinduced electron transfer (PET) between PDI and AE-POSS, and then after reaction with F^-, the fluorescent emission could "turn on" obviously because the PET was blocked by the degradation of the cage. The PET from amino-POSS to PDI was proved by FL spectrum and energies of HOMO and LUMO orbitals. ²⁹Si, ¹⁹F NMR, and ¹H NMR titration, XRD, FTIR, size analysis, and ion chromatography were applied to demonstrate the degradation mechanism. These results indicated that the higher quantum yield could be obtained by introducing the amide group in the PDI and the products of AE-PDI polymer might exist in the form of complex compounds with partial condensation of organosiloxane. With high selectivity and sensitivity (detection limit of 16.2 ppb), this probe was successfully applied for F^- detection in actual water samples.

Execution of aggregation-induced emission as nano-sensors for hypochlorite detection and application for bioimaging in living cells and zebrafish

Hypochlorite (ClO^-) could be used as a diagnostic marker for inflammation and related diseases. Although there have been many reports on probes for ClO^- imaging, there was still a lack of specificity and anti-interference ability. Herein, carbazole (NEC) and tetraphenylethylene (TPE) equipped with thiobarbituric acid (TBA), NEC-TBA and TPE-TBA, were synthesized and used as a fluorescence biosensor for monitoring ClO^- with aggregation-induced emission (AIE) effect. we identified that TPE-TBA, with formed nanoparticles in the mean grain size at 76 nm (5 μM), was a superior probe to target ClO^- over other analytes with fluorescence "turn off" strategy. Subsequently, to explore the bioimaging application, TPE-TBA was able to sense exogenous ClO^- in living HeLa cells through fluorescence imaging. In zebrafish model, TPE-TBA effectively captured exogenous ClO^- in the entire organization of zebrafish. Overall, these AIE-based probes merit further development as organism targeting ClO^- sensors.

A fluorescein-carbazole-based fluorescent probe for imaging of endogenous hypochlorite in living cells and zebrafish

In this article, a fluorescent probe FCZ with fluorescein-carbazole as the basic skeleton was designed and synthesized. In contrast to the presences of other coexisting anions, active oxygen and organic thiols, the probe could be "turn-on", exhibiting specific fluorescence performance towards hypochlorite (ClO-). Comprehensive analyses by electrospray ionization mass spectrometry (ESI-MS), thin-layer chromatography (TLC), nuclear magnetic resonance (NMR), and density functional theory/time-dependent density functional theory (DFT/TDDFT) revealed that ClO^- could react with the imine bond of the probe, forming fluorescein, resulting in a significant increase of fluorescence emission intensity. The probe has a detection limit for ClO^- in water of 0.056 μmol/L. In addition, the probe was successfully applied to detect ClO^- in water samples, as well as in the imaging of ClO^- in RAW264.7 cells and zebrafish.

Microfluidic Device for the Determination of Water Chlorination Levels Combining a Fluorescent meso-Enamine Boron Dipyrromethene Probe and a Microhydrocyclone for Gas Bubble Separation

Chlorination procedures are commonly applied in swimming pool water and wastewater treatment, yet also in food, pharmaceutical, and paper production. The amount of chlorine in water needs to be strictly controlled to ensure efficient killing of pathogens but avoid the induction of negative health effects. Miniaturized microfluidic fluorescence sensors are an appealing approach here when aiming at online or at-site measurements. Two meso-enamine-substituted boron dipyrromethene (BODIPY) dyes were found to exhibit favorable indication properties, their reaction with hypochlorite leading to strong fluorescence enhancement. Real-time assays became possible after integration of these fluorescent probes with designed two-dimensional (2D) and three-dimensional (3D) microfluidic chips, incorporating a passive sinusoidal mixer and a microhydrocyclone, respectively. A comparison of the two microfluidic systems, including their abilities to prevent accumulation or circulation of microbubbles produced by the chemical indication reaction, showed excellent fluidic behavior for the microhydrocyclone-based device. After coupling to a miniaturized optical reader for fluorescence detection, the 2D microfluidic system showed a promising detection range of 0.04-0.5 mg L^-1 while still being prone to bubble-induced fluctuations and suffering from considerably low signal gain. The microhydrocyclone-based system was distinctly more robust against gas bubbles, showed a higher signal gain, and allowed us to halve the limit of detection to 0.02 mg L^-1. The use of the 3D system to quantify the chlorine content of swimming pool water samples for sensitive and quantitative chlorine monitoring was demonstrated.

Rapid and visual detection for hypochlorite of an AIE enhanced fluorescence probe

In this paper, a new 'turn-on' fluorescence probe for the rapid, sensitive, and visual detection of hypochlorite is reported. The push-pull type trianiline-tricyanofuran-based fluorescent probe was prepared using a condensation reaction between tricyanofuran and the thiophene-trianiline derivative that had high quantum yields and showed aggregation-induced emission enhanced properties. Upon exposure to hypochlorite, prominent fluorescence enhancement of the probe was observed via the release of the fluorophore from the probe. The probe showed a ratiometric absorption change at 315 nm and 575 nm. Importantly, the probe showed an excellent detection limit for hypochlorite at 1.2 × 10^-7  M in solution and it was successfully applied for monitoring hypochlorite in waste water by test strip. This work reports a new fluorescence analytical sensing method for hypochlorite that has potential practical value in environmental monitoring and biological discrimination.

Two coumarin-based turn-on fluorescent probes based on for hypochlorous acid detection and imaging in living cells

This work, two turn-on fluorescent probes (3-acetyl-2H-chromen-2-one (ACO) & (1E)-1-(1-(2-oxo-2H-chromen-3-yl)ethylidene)thiosemicarbazide (CETC)) based on coumarin have been designed and synthesized, which could selectively and sensitively recognize ClO^- with fast response time. ACO &CETC were almost non fluorescent possibly due to both the lacton form of coumarin and unbridged CN bonds which can undergo a nonradiative decay process in the excited state. Upon the addition of ClO^-, ACO &CETC were oxidized to ring - opened by cleavage the CO and CN and the fluorescence intensity were increased considerably. Fluorescence titration experiments showed that the detection limit ACO &CETC is as low as 22 nm and 51 nm respectively. In particular, some relevant reactive species, including OH, ¹O₂, H₂O₂, KO₂, some anions and cations cannot be interference with the test. In live cell experiments, ACO &CETC were successfully applied to image exogenous ClO^- in HepG2 cells. Therefore, ACO &CETC not only could image ClO^- in living cells but also proved that CO and CN can be cleavage by ClO^-.

A highly specific and ultrasensitive probe for the imaging of inflammation-induced endogenous hypochlorous acid

A two-photon fluorescent probe with high sensitivity and selectivity was constructed and applied to visualize endogenous HOCl in lipopolysaccharide-induced inflammation.

A novel highly specific and ultrasensitive fluorescent probe for monitoring hypochlorous acid and its application in live cells

Based on the important role of hypochlorous acid (HOCl) in the immune system and numerous physiological processes, the detection of intracellular basal HOCl is of significant interest. In this work, we present a simple thiocarbamate-protected fluorescein fluorescent probe, TCFL, for imaging basal HOCl in live cells. Surprisingly, probe TCFL could determine HOCl quantitatively in a large concentration range with a detection limit of 0.65 nM. In addition, probe TCFL showed excellent specificity for HOCl in the presence of other higher concentration analytes (1 mM). Moreover, probe TCFL exhibited a rapid response (within 3 s) to HOCl and thus could provide a tool for real-time monitoring of HOCl. Importantly, probe TCFL with outstanding response features could be applied for monitoring basal HOCl in live cells.

A highly specific, ultrasensitive, thiocarbamate-caged fluorescein probe was developed for real-time detection of HOCl in live cells.

Overriding Phthalate Decomposition When Exploring Mycophenolic Acid Intermediates as Selenium-Based ROS Biological Probes

Hypochlorous (OCl^-) acid is the most well-known bacterial oxidant to be produced by neutrophils. Excess amounts of OCl^- can cause various disorders in living systems. Herein, we have designed, synthesized, and characterized two novel organoselenium-based target molecules (Probe-1 and Probe-OCl) based on a synthetic intermediate of mycophenolic acid for the aqueous detection of OCl^-. Probe 1 has been recently reported (Org. Lett. 2018, 20, 3557-3561); both probes show immediate "turn-on" fluorescence (<1 s) upon the addition of OCl^-, display an increase in the fluorescence quantum yield (3.7-fold in Probe-1 and 11.6-fold in Probe-OCl), and are completely soluble in aqueous media without the help of any cosolvent. However, a decrease in the "turn-on" intensity with the oxidized version of Probe-1 in cell assays due to the anhydride/phthalate functionality suggests that probe degradation occurs based on hydrolytic action (a probe degradation half-life of ∼1500 s at 15 μM Probe-1 and 150 μM OCl). Thus, the change of "anhydride" to "methylamide" begets Probe-OCl, which possesses more stability without sacrificing its water solubility properties and responses at short times. Further studies suggest that Probe-OCl is highly stable within physiological pH (pH = 7.4). Surprisingly, in live cell experiments involving U-2 OS cells and HeLa cells, Probe-OCl accumulated and aggregated in lipid droplets and gives a "turn-on" fluorescence response. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays confirmed that Probe-OCl is not toxic. Cuvette aggregation studies were also performed (tetrahydrofuran/H₂O) to demonstrate aggregation-induced fluorescence at longer times. Our current hypothesis is that the "turn-on" fluorescence effect is caused by the aggregation-induced emission mechanism available for Probe-OCl. In this case, in tandem, we reanalyzed the Mes-BOD-SePh derivative to compare and contrast cell localization as imaged by confocal microscopy; fluorescence emission occurs in the absence of, or prior to, Se oxidation.

A near-infrared fluorescent probe based on photostable Si-rhodamine for imaging hypochlorous acid during lysosome-involved inflammatory response

Hypochloric acid (HClO) is mainly distributed in acidic lysosomes of phagocytes and closely associated with numerous physiological and pathological processes, especially inflammatory response. Fluorescent probe has become an important tool for imaging HClO in lysosomes, but suffered from interference from autofluorescence in vivo, phototoxicity to biosamples and photobleaching phenomenon due to their short-wavelength excitation and emission. Unfortunately, up to now, no near-infrared (NIR) lysosome-targetable fluorescent probe has been reported for imaging HClO. In this paper, a near-infrared fluorescent probe Lyso-NIR-HClO for imaging lysosomal HClO was reported for the first time. Lyso-NIR-HClO based on Si-rhodamine is consisted of a morpholine unit as a lysosome-targetable group and a HClO-mediated cyclization reaction site as a response group, which was applied for highly selective and sensitive detection and imaging for endogenous and exogenous HClO in lysosomes, with a linear range from 5.0 × 10^-8 to 1.0 × 10^-5 M and a detection limit of 2.0 × 10^-8 M in vitro. Attributed to NIR emission and excellent photostability of Si-rhodamine, Lyso-NIR-HClO exhibits excellent performances in vivo, such as low interference from intracellular autofluorescence, stable and persistent fluorescence signal and good tissue penetration, which are in favor of accurate, time-lapse and long-term imaging for HClO. Finally, we applied the probe Lyso-NIR-HClO to visualize endogenous HClO during lysosome-involved inflammatory response including bacteria-infected cells and inflamed mouse model with satisfactory results. The above results proved that Lyso-NIR-HClO would be a potentially useful tool for the study of biological functions and pathological roles of HClO in lysosomes, especially role of lysosome in the inflammatory response.

Rapid Response Fluorescence Probe Enabled In Vivo Diagnosis and Assessing Treatment Response of Hypochlorous Acid-Mediated Rheumatoid Arthritis

Diagnosis and early assessment of the treatment response of rheumatoid arthritis (RA) necessitates a reliable bioanalytical method for rapid, sensitive, and specific detection of the hypochlorous acid (HOCl) biomarker in inflammatory diseases. Herein, two fluorescence probes, Probe-1 and Probe-2 are developed for quantitative monitoring and visualization of inflammatory response-related HOCl levels in vitro and in vivo. In the presence of HOCl, fluorescence "OFF-ON" response is obtained for both the probes as a result of specific HOCl-triggered C=N bond cleavage reaction. Probe-1 and Probe-2 feature rapid response (<4 s), a high degree of sensitivity and selectivity toward HOCl, which allow them to be used for quantification of HOCl in a simulated physiological condition. Using Probe-2 as the probe, fluorescence imaging and flow cytometry analysis of HOCl levels in lysosome of inflammatory mimic cells, visualization of HOCl generation in endotoxin-induced inflammation of adult zebrafish and RA of mice are possible. Probe-2 exhibits high effectiveness for early assessment of the treatment response of HOCl-mediated RA in mice with an antiarthritic drug, methotrexate (MTX). The results demonstrate that Probe-2 is a powerful tool for future studies on diagnosis and monitoring treatment efficiency in a broad range of inflammatory diseases, including RA.

A benzothiazole-based fluorescent probe for hypochlorous acid detection and imaging in living cells

A benzothiazole-based turn-on fluorescent probe with a large Stokes shift (190nm) has been developed for hypochlorous acid detection. The probe displays prompt fluorescence response for HClO with excellent selectivity over other reactive oxygen species as well as a low detection limit of 0.08μM. The sensing mechanism involves the HClO-induced specific oxidation of oxime moiety of the probe to nitrile oxide, which was confirmed by HPLC-MS technique. Furthermore, imaging studies demonstrated that the probe is cell permeable and can be applied to detect HClO in living cells.