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

A lysosome-targeted fluorescent probe for fluorescence imaging of hypochlorous acid in living cells and in vivo

Lysosomes, as crucial acidic organelles in cells, play a significant role in cellular functions. The levels and distribution of hypochlorous acid (HOCl) within lysosomes can profoundly impact their biological functionality. Hence, real-time monitoring of the concentration of HOCl in lysosomes holds paramount importance for further understanding various physiological and pathological processes associated with lysosomes. In this study, we developed a bodipy-based fluorescent probe derived from pyridine and phenyl selenide for the specific detection of HOCl in aqueous solutions. Leveraging the probe's sensitive photoinduced electron transfer effect from phenyl selenide to the fluorophore, the probe exhibited satisfactory high sensitivity (with a limit of detection of 5.2 nM and a response time of 15 s) to hypochlorous acid. Further biological experiments confirmed that the introduction of the pyridine moiety enabled the probe molecule to selectively target lysosomes. Moreover, the probe successfully facilitated real-time monitoring of HOCl in cell models stimulated by N-acetylcysteine (NAC) and lipopolysaccharide (LPS), as well as in a normal zebrafish model. This provides a universal method for dynamically sensing HOCl in lysosomes.

A G-quadruplex/thioflavin T-based label-free biosensor to detect ClO- in stress-induced hypertension

Hypochlorous acid (HClO), as an essential reactive oxygen species (ROS) in biological systems, plays a pivotal role in processes of physiology and pathology. Abnormal fluctuations in HClO concentration can lead to various diseases, such as inflammation, cardiovascular diseases, and neurodegeneration. Therefore, developing an approach to rapidly and sensitively quantify ClO- content is vital to biomedicine development and bioassays. Herein, we fabricated a novel "turn-on" label-free fluorescence DNA probe to specifically detect hypochlorite ion (ClO-) based on G-quadruplex formation. To this end, we designed a G-rich signal DNA sequence (S-DNA) and a block DNA sequence (B-DNA), followed by the introduction of ClO--responsive phosphorothioate (PS) into B-DNA. In the absence of ClO-, B-DNA hybridized with S-DNA, preventing G-quadruplex formation from S-DNA; this resulted in the relatively low fluorescence intensity of ThT. Once ClO- was added, the hydrolysis between PS and ClO- split the B-DNA into two fragments, resulting in B-DNA breaking away from S-DNA, allowing G-quadruplex formation from S-DNA and increasing the fluorescence intensity of ThT. Using this method, we can detect ClO- without the interference of additional reactive oxygen species. The detection limit of ClO- was as low as 10 nM. Furthermore, this method facilitates the detection of ClO- within the tissues of rats with stress-induced hypertension.

A novel thienothiophene-based "dual-responsive" probe for rapid, selective and sensitive detection of hypochlorite

BACKGROUND

Hypochlorite/hypochlorous acid (ClO-/HOCl) is a biologically crucial reactive oxygen species (ROS), produced in living organisms and has a critical role as an antimicrobial agent in the natural defense system. However, when ClO- is produced excessively, it can lead to the oxidative damage of biomolecules, resulting in organ damage and various diseases. Therefore, it is imperative to have a straightforward, quick and reliable method for over watching the minimum amount of ClO- in different environments.

RESULTS

Herein, a new probe TTM, containing thienothiophene and malononitrile units, was developed for exceptionally selective and sensitive hypochlorite (ClO-) detection. TTM demonstrated a rapid "turn-on" fluorescence response (<30 s), naked-eye detection (colorimetric), voltammetric read-out with anodic scan, low detection limit (LOD = 0.58 μM and 1.43 μM for optical and electrochemical methods, respectively) and applicability in detecting ClO- in real water samples and living cells.

SIGNIFICANCE AND NOVELTY

This study represents one of the rare examples of a small thienothiophene-based molecule for both optical and electrochemical detections of ClO- in an aqueous medium.

Novel NIR fluorescent probe for hypochlorite ion detection in biological systems

This study presents the synthesis and application of a novel fluorescent probe, NR-ClO, for the detection of hypochlorite ion (ClO-) in biological systems. The probe was synthesized through a nucleophilic substitution reaction between Nile red and dimethylcarbamothioic chloride. The synthesized probe had high sensitivity and selectivity towards ClO-, with a detection limit of 75 nM and a linear range of 0.1-200 μM. The probe's efficacy was validated through in vitro studies using HepG2 cells and in vivo experiments using a mouse model of rheumatoid arthritis. The findings demonstrate that the NR-ClO probe is a promisingly reliable tool for real-time monitoring of ClO- in complex biological environments.

Evaluation of a novel pyridinium cation-linked styryl-based boronate probe for the detection of selected inflammation-related oxidants

Reactive oxygen and nitrogen species (RONS) are a range of chemical individuals produced by living cells that contribute to the proper functioning of organisms. Cells under oxidative and nitrative stress show excessive production of RONS (including hydrogen peroxide, H2O2, hypochlorous acid, HOCl, and peroxynitrite, ONOO-) which may result in a damage proteins, lipids, and genetic material. Thus, the development of probes for in vivo detection of such oxidants is an active area of research, focusing on molecular redox sensors, including boronate-caged fluorophores. Here, we report a boronate-based styryl probe with a cationic pyridinium moiety (BANEP+) for the fluorescent detection of selected biological oxidants in vitro and in vivo. We compare the chemical reactivity of the BANEP+ probe toward H2O2, HOCl, and ONOO- and examine the influence of the major intracellular non-enzymatic antioxidant molecule, glutathione (GSH). We demonstrate that, at the physiologically relevant GSH concentration, the BANEP+ probe is efficiently oxidized by peroxynitrite, forming its phenolic derivative HNEP+. GSH does not affect the fluorescence properties of the BANEP+ and HNEP+ dyes. Finally, we report the identification of a novel type of molecular marker, with the boronate moiety replaced by the iodine atom, formed from the probe in the presence of HOCl and iodide anion. We conclude that the reported chemical reactivity and structural features of the BANEP+ probe may be a basis for the development of new red fluorescent probes for in vitro and in vivo detection of ONOO-.

Development of an activatable far-red fluorescent probe for rapid visualization of hypochlorous acid in live cells and mice with neuroinflammation

Recent investigations have suggested that abnormally elevated levels of HOCl may be tightly related to the severity of neuroinflammation. Although some successes have been achieved, fluorescent probes with far-red fluorescence emission and capable of detecting HOCl with high specificity in pure aqueous solution are still urgently needed. Herein, a responsive far-red fluorescent probe, DCI-H, has been constructed to monitor HOCl activity in vivo and in vitro. DCI-H could rapidly respond to HOCl within 120 s and had a low detection limit for HOCl of 1.5 nM. Importantly, physiologically common interfering species, except for HOCl, did not cause a change in the fluorescence intensity of DCI-HOCl at 655 nm. The results of confocal imaging demonstrated the ability of DCI-H to visualize endogenous HOCl produced by MPO-catalyzed H2O2/Cl- and LPS stimulation. With the assistance of DCI-H, upregulation of HOCl levels was observed in the mice model of LPS-induced neuroinflammation. Thus, we believed that DCI-H provided a valuable tool for HOCl detection and diagnosis of inflammation-related diseases.

Recent progress of near-infrared fluorescent probes in the determination of reactive oxygen species for disease diagnosis

Reactive oxygen species (ROS), a chemically defined group of reactive molecules derived from molecular oxygen, are involved in a variety of physiological and pathological processes, including immune defense, cellular metabolism, and other physiological processes. To access their detailed function in these processes, it is critical to establish rapid, accurate and in situ assays for these species in vivo. Among the potential assays, fluorescent probes are considered as the most promising candidate to monitor the biological ROS in vivo with great spatial and temporal resolution and are extensively used as an excellent tool in modern redox biology discovery. Recently, abundant fluorescent probes have been successively developed for in vitro or intracellular detection of ROS, but most of them could not be used for in vivo imaging due to their intrinsic shortcomings such as short emission wavelengths, phototoxicity and poor tissue penetration. Recent development of fluorescent ROS probes with near-infrared emission aim to address these concerns to develop practical assays. Herein, we review recent developments of ROS-sensitive near-infrared fluorescent probes, with an emphasis on the design, synthesis, characteristics of fluorescent probes, as well as their applications. We hope this review will aid the development of a new generation of efficient, sensitive and biocompatible fluorescent probes for in vivo ROS detection.

Fluorescence-enhanced detection of hypochlorite based on in situ synthesis of functionalization-free carbon spheres

Hypochlorite (ClO-) exposure has been confirmed to be associated with many serious diseases. Although abundant organic molecule-based probes have demonstrated high sensitivity and selectivity for ClO- response, they often suffer from limitations including tedious preparation steps, poor water solubility, and the use of toxic solvent. In this work, a novel fluorescent sensor based on carbon spheres (CS) synthesized by solvothermal method was presented for ClO- detection. In the presence of ClO-, the obtained micro-size CS that initially displayed very weak fluorescence experienced a significant fluorescence enhancement in the blue channel, and a linear response range of 2-110 μM with detection limit of 10.7 nM could be achieved. In addition to proposed mechanism verification, a field visualization platform based on smartphone was designed to monitor hypochlorite in real environmental water samples to demonstrate its potential in portable detection.

A dual-responsive fluorescent probe based on cyanine and naphthalimide units for detecting HClO and H2S in living cells

Hypochlorous acid (HClO) and hydrogen sulfide (H2S) play very important roles in both physiological and pathological processes, however, the methods for simultaneously detecting HClO and H2S were rarely reported. Here, a dual-responsive fluorescent probe (CyNa-N3) based on cyanine and naphthalimide dyes was synthesized and investigated. The fluorescence probe showed better sensitivity, high selectivity response to HClO and H2S by red emission and green emission bands, and the limits of detection were 0.17 µM and 0.15 µM respectively. MS (Mass Spectrum) and 1H NMR (Nuclear Magnetic Resonance) confirmed the sensing mechanism of CyNa-N3 detected HClO and H2S, the calculation of density functional theory (DFT) further explained the internal mechanism of spectral change of CyNa-N3. Moreover, CyNa-N3 was successfully applied to image HClO and H2S in living cells, which is beneficial for more efficient application in biological imaging.

A dual-lock-controlled mitochondria-targeted ratiometric fluorescence probe for simultaneous detection of atherosclerosis-related HClO and viscosity

Precise detection of inflammatory microenvironment-related viscosity and hypochlorous acid (HClO) contributes to illuminating the pathogenesis and further diagnosing of atherosclerosis (AS). Herein, a dual-lock-controlled mitochondria-targeted fluorescence probe (NS) for simultaneous imaging of HClO and viscosity in AS-related foam cells is presented. NS performs linear increase in green-fluorescence along with increased viscosity (excited at 425 nm), permitting "off-on" fluorescence imaging of viscosity. Meanwhile, upon HClO activation, NS exhibits red-shifted and enhanced fluorescence in orange, thus leading to ratiometric fluorescence quantification of HClO (excited at 465 nm). Such dual-lock-controlled effect makes NS realize simultaneous imaging of viscosity and HClO with high sensitivity and selectivity via "off-on" and ratiometric fluorescence readouts, respectively. Besides, endowed with mitochondria-targeting capacity, NS achieves in situ imaging of mitochondria viscosity and HClO in living RAW264.7 cells. Importantly, for the first time, NS realizes simultaneous imaging of mitochondria viscosity and HClO in macrophage-derived foam cells, revealing the close association between HClO level and viscosity change in mitochondria during foaming translation of macrophages in atherogenesis. This work not only provides a novel strategy and tool to image organelle-located viscosity and HClO in living systems, but also holds great potential in early diagnosis of AS.

A turn-on fluorescence strategy for hypochlorous acid detection based on DNAzyme-assisted cyclic signal amplification

Hypochlorous acid (HClO) is a crucial active oxygen component and one of the innate immunity's barrier substances in the body. Abnormal fluctuations in HClO concentration can lead to increased oxidative stress, cellular dysfunction, and the onset of various diseases. Thus, developing convenient, affordable, efficient, and sensitive methods to monitor HClO concentration in healthcare and pathophysiology research is highly significant. In this study, we developed a novel fluorescence strategy for HClO detection based on nucleic acid oxidative cleavage and Pb2+-dependent DNAzyme. By introducing a phosphorothioate site in the hairpin-structured nucleic acid sequence, the nucleic acid probe specifically recognized HClO and underwent oxidative cleavage. Upon cleavage, the enzyme strand is liberated, forming DNAzyme. This DNAzyme then cleaves the substrate strand, liberating the initially quenched fluorescent dyes and generating a turn-on fluorescent signal. The enzyme strand produced by the oxidative cleavage of HClO can be repeatedly utilized, thus realizing the cyclic signal amplification to reduce background noise. We verified the detection mechanism of this strategy through stepwise fluorescence spectroscopy analysis and electrophoresis. Under optimal experimental conditions, the method achieved a detection limit of 5.38 nM and a linear range of 1 nM-800 nM. This method demonstrated exceptional performance in actual biological sample testing and presented an exciting opportunity for expanded utilization in clinical diagnosis and medical research.

HClO imaging in vivo and drug-damaged liver tissues by a large Stokes shift fluorescent probe

Drug-induced liver injury (DILI), as a classic acute inflammation, has attracted widespread concern due to its unpredictability and severity. Among the various reactive oxygen species, HClO has been used as a marker for the detection of DILI process. Thus, we designed and synthesized a "turn-on" fluorescent probe FBC-DS by modifying 3'-formyl-4'-hydroxy-[1,1'-biphenyl]-4-carbonitrile (FBC-OH) with N, N-dimethylthiocarbamate group for sensitively sensing HClO. Probe FBC-DS showed a low detection limit (65 nM), fast response time (30 s), an enormous Stokes shift (183 nm) and 85-fold fluorescence enhancement at 508 nm in the detection of HClO. Probe FBC-DS could monitor exogenous and endogenous HClO in living HeLa cells, HepG2 cells and zebrafish. In addition, probe FBC-DS has been successfully utilized in biological vectors for imaging acetaminophen (APAP)-induced endogenous HClO. Moreover, DILI caused by APAP is evaluated by probe FBC-DS through imaging over-expression of endogenous HClO in the mice liver injury models. All in all, we have every reason to believe that probe FBC-DS can be a potential tool to study the complex biological relationship between HClO and drug-induced liver injury.

A Commercially Available 2-aminoanthracene Fluorescent Probe for Rapid and Sensitive Detection of Hypochlorite in 100% Buffer Solution and its Application in Complex Water Samples

Hypochlorite (ClO-), a crucial chemical in the living organism, engages in various physiological activities. However, high amounts of ClO- result in oxidative damage. In this work, a commercially available 2-aminoanthracene (AA) was used to detect ClO-. AA demonstrated distinct properties such as superior selectivity and rapid response (< 30 s) with a low detection limit (140 nM) towards ClO- in 100% buffer solution. Furthermore, the probe exhibited a notable achievement by effectively identifying the presence of ClO- in complicated water samples. In conclusion, AA offers an easy-to-use and accurate method for quantifying ClO- in complex water samples.

An Anthracene Carboxamide-Based Fluorescent Probe for Rapid and Sensitive Detection of Mitochondrial Hypochlorite in Living Cells

Mitochondrial hypochlorite (ClO-) plays important and often contradictory roles in maintaining the redox balance of mitochondria. Abnormal ClO- levels can induce mitochondrial inactivation and further cause cell apoptosis. Herein, we have developed an anthracene carboxyimide-based fluorescent probe mito-ACS for imaging mitochondrial ClO- in living cells. This probe exhibits some distinctive features as excellent resistance to photobleaching, high selectivity and sensitivity, as well as good water solubility. Mito-ACS showed a noticeable fluorescence response toward ClO- with a fast response (within 6 s) and a low detection limit (23 nM). Moreover, the introduction of triphenylphosphonium makes the probe soluble in water and selectively localizes to mitochondria. Furthermore, mito-ACS was successfully applied to image mitochondria ClO- in living cells with low toxicity. Remarkably. the less used fluorophore anthracene carboxyimide exhibiting excellent photostability and desirable optical properties provides a promising application prospect in biological systems.

Hypochlorous acid-activated near-infrared fluorescent probe for in vivo/exogenous detection and dairy toxicity evaluation

Oxidative stress has been reported to be closely associated with many diseases, and an excessive overdose of hypochlorite (ClO^-) can also induce stress-related diseases. In this study, we designed and synthesized a NIR probe, named W-1a based on computational analysis of DCM (4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran) derivatives for specific detection of ClO^-. The probe exhibited dual fluorescence and colorimetric sensing with a response time of <1 min and a detection limit of 0.15 μM. Additionally, the probe was successfully applied for fluorescence imaging of ClO^- at the cellular level and ebrafish endogenous/exogenous ClO^- assay and dairy toxicity assessment. Thus, we present a potential method for developing an efficient and reliable detection of ClO^- in early stage using near-infrared dyes.

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 Simple ICT-Based Fluorescent Probe for HOCl and Bioimaging Applications

Over the past few decades, drug-induced liver damage (DILI) has become a serious public health problem due to drug abuse. Among multifarious reactive oxygen species, mounting evidence attests that ClO^- has been used as a potential biomarker in DILI. In this work, a new "turn-on" fluorescent probe 1 was designed and synthesized by modifying 4'-hydroxybiphenyl-4-carbonitrile (dye 2) with N, N-dimethylthiocarbamate as a response site for detecting ClO^-. Probe 1 displayed a low detection limit (72 nM), fast response time (30 s), wide pH operating range (6-8), great tissue penetration, large Stokes shift (125 nm) and 291-fold fluorescence enhancement at 475 nm in the mapping of ClO^-. Probe 1 could trace amounts of exogenous and endogenous ClO^- with high sensitivity in MCF-7 cells and HeLa cells. Expectantly, the fluoxetine-induced liver injury model is successfully established, and probe 1 has been used for detecting the fluctuation of ClO^- levels in the mouse model of fluoxetine-induced liver injury. All in all, probe 1 with its high specificity, good biological compatibility and liver tissue penetration ability is expected to assist with the early diagnosis of DILI and the clinical screening of various new drugs. We expect that probe 1 could be efficiently used as a powerful molecular tool to predict clinical DILI and explore molecular mechanisms between molecules and disease.

Monitoring of Hypochlorite Level in Fruits, Vegetables, and Dairy Products: A BODIPY-Based Fluorescent Probe for the Rapid and Highly Selective Detection of Hypochlorite

Hypochlorite/hypochlorous acid (ClO^-/HOCl), among the diverse reactive oxygen species, plays a vital role in various biological processes. Besides, ClO^- is widely known as a sanitizer for fruits, vegetables, and fresh-cut produce, killing bacteria and pathogens. However, excessive level of ClO^- can lead to the oxidation of biomolecules such as DNA, RNA, and proteins, threatening vital organs. Therefore, reliable and effective methods are of utmost importance to monitor trace amounts of ClO^-. In this work, a novel BODIPY-based fluorescent probe bearing thiophene and a malononitrile moiety (BOD-CN) was designed and constructed to efficiently detect ClO^-, which exhibited distinct features such as excellent selectivity, sensitivity (LOD = 83.3 nM), and rapid response (<30 s). Importantly, the probe successfully detected ClO^- in various spiked water, milk, vegetable, and fruit samples. In all, BOD-CN offers a clearly promising approach to describe the quality of ClO^--added dairy products, water, fresh vegetables, and fruits.

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.

Construction of HClO activated near-infrared fluorescent probe for imaging hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common malignancies in the liver with poor prognosis. In order to improve the prognosis and overall survival of patients with HCC, it is important to identify it at early stage and resect it precisely. Cell microenvironment, active compounds, and enzymes may change during the cancerization of hepatocytes. Hypochlorous acid (HClO), one of the most significant signal molecules in the cellular signaling pathway, plays an important role in many cellular processes. To detect and treat liver cancers, it is imperative to study how HClO levels change in hepatocytes. However, developing fluorescent probes specific to liver cells to detect HClO still challenging. Herein, we designed and synthesized a NIR hepatocyte-specific fluorescent probe (MBH-MT) that displayed excellent optical properties for detecting HClO in biological samples. Cell imaging experiment conducted with the unique probe MBH-MT, showed that the biocompatible sensor is capable of monitoring HClO and distinguishing normal cells from cancer cells (e.g., HepG2, HUVEC, RAW264.7, L02 and HK-2 cells). An organ imaging experiment with the probe MBH-MT demonstrated its effectiveness in diagnosing and imaging hepatocellular carcinoma in vivo. MBH-MT's in situ imaging also demonstrated that it can target and image mouse hepatocellular carcinomas. Furthermore, MBH-MT has also successfully been used to diagnose and guide liver cancer surgery early. In the future, we expect that this powerful tool may be help in the detection and imaging of hepatocellular carcinoma, which may affect a large number of people.

A NIR BODIPY-based ratiometric fluorescent probe for HClO detection with high selectivity and sensitivity in real water samples and living zebrafish

Hypochlorous acid (HClO) plays an important role in many physiological and pathological activities. In this work, a novel BODIPY-based Near-infrared (NIR) ratiometric fluorescent probe BODIPY-Hyp was designed for the rapid detection of HClO. The probe BODIPY-Hyp was highly selective and sensitive for HClO with a low detection limit of 16.74 nM and short response time of less than 60 s. The probe BODIPY-Hyp in response to HClO exhibited a significant blue-shifted fluorescence emission from 700 nm to 530 nm, and its fluorescence intensity ratio (I_{530 nm}/I_{700 nm}) increased about 1200 times before and after adding HClO. Moreover, the reaction mechanism of BODIPY-Hyp with HClO was verified by HRMS analysis, ¹H NMR titration and DFT calculations. Furthermore, BODIPY-Hyp was successfully processed into a portable test strip-based device for the detection of HClO. In addition, the probe BODIPY-Hyp could be used in real time to monitor the levels of HClO in living zebrafish larvae. In conclusion, BODIPY-Hyp has great application potential in the life and environmental sciences.

A fluorescent probe based on phenothiazine for detection of ClO− with naked-eye color change properties

Hypochlorite (ClO^-) plays a key role in life systems and it is necessary to develop an effective detection method. In view of the significant advantages of the fluorescent probe, we have synthesized a naked-eye recognition fluorescent probe NNCF for the detection of ClO^- based on phenothiazine and naphthalimide. The probe NNCF is sensitive (LOD = 9.5 nM) and fast for ClO^- (within 30 s), and its Stokes shift is as large as 161 nm. In addition, the probe NNCF has been successfully used for imaging detection of exogenous ClO^- in MCF-7 cells with low toxicity.

A novel near-infrared ratiometric fluorescent probe targeting lysosomes for imaging HOCl in vitro and in vivo

Hypochlorous acid (HOCl), as an important biological reactive oxygen species (ROS), plays an important role in microbial immune defense and inflammatory response. Abnormal levels of HOCl in lysosomes can cause lysosomal membrane rupture and release of various hydrolases, leading to a variety of diseases, including cancer. In order to better monitor the level of HOCl in lysosomes, phenothiazine was chosen as fluorophore to construct a NIR fluorescent probe PMM with intramolecular change transfer process (ICT). PMM is a colorimetric and ratiometric fluorescent probe, which has high sensitivity with a low detection limit (20 nM), high selectivity and anti-interference. PMM has good stability in the weakly acidic environment of pH 4.0-5.5. PPM has good localization ability for lysosomes and has been successfully used for fluorescence imaging of exogenous and endogenous HOCl in HepG2 cells. Moreover, nude mouse imaging also demonstrated that PMM could be used to detect HOCl in vivo.

An ESIPT-based reversible ratiometric fluorescent sensor for detecting HClO/H2S redox cycle in living cells

HClO and H₂S, as two kinds of crucial small biomolecules, are endowed various roles in biological organisms. The redox balance between HClO and H₂S is closely related to the physiological and pathological processes. Thus, it is significant to monitor the redox process between HClO and H₂S. Inspired by the advantages of ratiometric fluorescent probes, we firstly developed a reversible ratiometric fluorescent probe (BT-Se) for HClO and H₂S via combination of phenyl selenide as the response group and 2-(2'-hydroxyphenyl)benzothiazole dye as the fluorophore. The proposed probe BT-Se could detect HClO with well-separated dual emission (110 nm), fast response, good selectivity and sensitivity owing to the oxidation reaction of the Se atom induced by HClO. Moreover, only H₂S could effectively recover the fluorescence of the detection system to the original state via H₂S induced-reduction of selenoxide. Cell imaging studies demonstrated that the probe BT-Se was capable of ratiometric monitoring the changes of intracellular HClO/H₂S, which suggested that it has great potential for researching the biological functions of HClO and H₂S.

Oxidative Stress in Age-Related Neurodegenerative Diseases: An Overview of Recent Tools and Findings

Reactive oxygen species (ROS) have been described to induce a broad range of redox-dependent signaling reactions in physiological conditions. Nevertheless, an excessive accumulation of ROS leads to oxidative stress, which was traditionally considered as detrimental for cells and organisms, due to the oxidative damage they cause to biomolecules. During ageing, elevated ROS levels result in the accumulation of damaged proteins, which may exhibit altered enzymatic function or physical properties (e.g., aggregation propensity). Emerging evidence also highlights the relationship between oxidative stress and age-related pathologies, such as protein misfolding-based neurodegenerative diseases (e.g., Parkinson's (PD), Alzheimer's (AD) and Huntington's (HD) diseases). In this review we aim to introduce the role of oxidative stress in physiology and pathology and then focus on the state-of-the-art techniques available to detect and quantify ROS and oxidized proteins in live cells and in vivo, providing a guide to those aiming to characterize the role of oxidative stress in ageing and neurodegenerative diseases. Lastly, we discuss recently published data on the role of oxidative stress in neurological disorders.

A red-emissive benzothiazole-based luminophore with ESIPT and AIE natures and its application for detecting and imaging hypochlorous acid

A new "ESIPT + AIE" based dye of benzothiazole with red emission and a large Stokes shift was constructed by combining 2-(2'-hydroxyphenyl)benzothiazole as the ESIPT unit and α-cyanostilbene as the AIE unit. The compound BACN was found to be a ideal HClO chemosensor, and presented palpable fluorescence and colorimetric responses toward HClO via the HClO-trigged oxidation cleavage of the ethylene bridge activated by the electron withdrawing cyano group. BACN was capable of recognizing HClO rapidly (12 s) and sensitively under physiological conditions, with good selectivity over other biologically pertinent substances. Thanks to strong red emission (λ_em = 606 nm) and large Stokes shift (213 nm) resulted from the combination of ESIPT and AIE effects, it was successfully utilized for the recognition of exogenous and endogenous HClO in living HeLa cells.

Lysosome-targetable brightly green fluorescence carbon dots for real-time monitoring in cell and highly efficient removal in environment of hypochlorite

Due to the lacks of lysosome localization group and reaction/interaction site for hypochlorite (ClO^-) on the surface of the carbon dots (C-dots), no C-dots-based lysosome-targeted fluorescence probes have, so far, been reported for real-time monitoring intracellular ClO^-. In this work, 1,3,6-trinitropyrene (TNP) was used as a precursor to prepare C-dots with maximum excitation and emission wavelengths at 485 and 532 nm, respectively, and quantum yield ∼ 27% by a hydrothermal approach at 196 °C for 6 h under a reductive atmosphere. The brightly green C-dots can sensitively and quickly respond to ClO^- in aqueous solution through surface chemical reaction, showing a linear relationship in the range of 0.5-120 μΜ ClO^- with 0.27 μΜ of limit of detection (LOD). Most significantly, the C-dots can localize at intracellular lysosome to image ClO^- in lysosomes. Also, the magnetic nanocomposites (C-dots@Fe₃O₄ MNCs) were fabricated via a simple electrostatic self-assembly between Fe₃O₄ magnetic nanoparticles (Fe₃O₄ MNPs) and C-dots for highly efficient removal of ClO^- in real samples. Therefore, lysosome-targetable C-dots-based probes for real-time monitoring ClO^- were successfully constructed, opening up a promising door to investigate the biological functions and pathological roles of ClO^- at organelle levels.

BODIPY-Based Fluorescent Probes for Selective Visualization of Endogenous Hypochlorous Acid in Living Cells via Triazolopyridine Formation

In this work, the two pyridylhydrazone-tethered BODIPY compounds (2 and 3) were synthesized. These compounds aimed to detect hypochlorous acid (HOCl) species via cyclic triazolopyridine formation. The open forms and the resulting cyclic forms of BODIPYs (2, 3, 4, and 5) were fully characterized by nuclear magnetic resonance, mass spectrometry, infrared spectroscopy, and single-crystal X-ray diffraction. These two probes can selectively detect HOCl through a fluorescence turn-on mechanism with the limit of detections of 0.21 µM and 0.77 µM for compounds 2 and 3, respectively. This fluorescence enhancement phenomenon could be the effect from C = N isomerization inhibition due to HOCl-triggered triazolopyridine formation. In cell imaging experiments, these compounds showed excellent biocompatibility toward RAW 264.7 murine live macrophage cells and greatly visualized endogenous HOCl in living cells stimulated with lipopolysaccharide.

A novel TCF-aza-BODIPY-based near-infrared fluorescent probe for highly selective detection of hypochlorous acid in living cells

Hypochlorous acid/hypochlorite (HOCl/ClO^-) plays important roles in killing bacterial and causing damage to living tissues, and its abnormal levels could lead to many diseases. Although great efforts have been devoted, fluorescent probes for HOCl/ClO^- with near-infrared fluorescence, good selectivity/sensitivity, and low background are still important and urgent. In this work, a novel double-bond-linked TCF-aza-BODIPY-based near-infrared fluorescent probe (3) was rationally designed, successfully prepared, and applied for sensing HOCl/ClO^- in both solutions and living RAW264.7 cells, showing good selectivity and fluorescence "turn-on" phenomenon at 670 nm with low background. The limit of detection towards ClO^- was determined to be 0.36 μM through the linear fluorescence changes at 670 nm in a broad ClO^--concentration range of 0-150 μM. Furthermore, the sensing mechanism was investigated by mass spectrometry and compared with 1, suggesting that the remarkable spectroscopic changes could be ascribed to the oxidization of the double bond to the aldehyde group, accompanied with the leaving of the TCF group. Confocal imaging experiments also confirmed the remarkable intracellular fluorescence enhancements through incubation of ClO^- and phorbol ester 12-myristate 13-acetate (PMA) in RAW264.7 cells. Therefore, for the first time, we reported a near-infrared TCF-aza-BODIPY-based fluorescent probe for highly sensitive and fluorescence "turn-on" detection of both exogenous and endogenous HOCl in living RAW264.7 cells through the quick oxidation of a conjugated double bond.

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.

Defining roles of specific reactive oxygen species (ROS) in cell biology and physiology

'Reactive oxygen species' (ROS) is a generic term that defines a wide variety of oxidant molecules with vastly different properties and biological functions that range from signalling to causing cell damage. Consequently, the description of oxidants needs to be chemically precise to translate research on their biological effects into therapeutic benefit in redox medicine. This Expert Recommendation article pinpoints key issues associated with identifying the physiological roles of oxidants, focusing on H₂O₂ and O₂^.-. The generic term ROS should not be used to describe specific molecular agents. We also advocate for greater precision in measurement of H₂O₂, O₂^.- and other oxidants, along with more specific identification of their signalling targets. Future work should also consider inter-organellar communication and the interactions of redox-sensitive signalling targets within organs and whole organisms, including the contribution of environmental exposures. To achieve these goals, development of tools that enable site-specific and real-time detection and quantification of individual oxidants in cells and model organisms are needed. We also stress that physiological O₂ levels should be maintained in cell culture to better mimic in vivo redox reactions associated with specific cell types. Use of precise definitions and analytical tools will help harmonize research among the many scientific disciplines working on the common goal of understanding redox biology.

A NIR fluorescent probe for the specific detection of hypochlorite and its application in vitro and in vivo

It is of great necessity to exploit a real-time, highly selective and sensitive method for hypochlorite (ClO^-) detection in both the environment and living systems because of the complex influence of ClO^- on health. In this paper, based on the intramolecular charge transfer (ICT) effect, a NIR fluorescent probe (probe DAB) was designed for the accurate detection of ClO^-, which produced a fluorescence response to ClO^- with high selectivity and rapid response (within 1 min). The probe DAB could determine ClO^- over the linear range of 0-80 μM with a low detection limit of 1.46 μM. And the sensing mechanism between the probe and ClO^- was verified using HPLC and MS. To further prove its practicability, the probe was applied for detecting ClO^- in actual water samples. In addition, owing to its good sensing properties and low cytotoxicity, probe DAB could be expediently applied to visualize ClO^- in living cells and zebrafish, and it is expected to be a useful tool for investigating the detailed functions and mechanisms of ClO^- in living systems.

Guidelines for measuring reactive oxygen species and oxidative damage in cells and in vivo

Multiple roles of reactive oxygen species (ROS) and their consequences for health and disease are emerging throughout biological sciences. This development has led researchers unfamiliar with the complexities of ROS and their reactions to employ commercial kits and probes to measure ROS and oxidative damage inappropriately, treating ROS (a generic abbreviation) as if it were a discrete molecular entity. Unfortunately, the application and interpretation of these measurements are fraught with challenges and limitations. This can lead to misleading claims entering the literature and impeding progress, despite a well-established body of knowledge on how best to assess individual ROS, their reactions, role as signalling molecules and the oxidative damage that they can cause. In this consensus statement we illuminate problems that can arise with many commonly used approaches for measurement of ROS and oxidative damage, and propose guidelines for best practice. We hope that these strategies will be useful to those who find their research requiring assessment of ROS, oxidative damage and redox signalling in cells and in vivo.

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.

Rhodol-Based Fluorescent Probes Used for Fast Response toward ClO– and Delayed Determination of H2O2 in Living Cells

Reactive oxygen species (ROS), a class of reactive oxidants, play critical roles in signal transduction, cell metabolism, immune defense, and other physiological processes. Abnormally excessive levels of ROS can cause diseases and thus, investigations into the relevant biology and medicine are significant. The behavior of ROS in inflammation has been rarely elucidated. In this work, two ROS fluorescent probes, FS-ROS1 and FS-ROS2 have been designed and synthesized. FS-ROS1 responds rapidly (~1 min) to ClO– and gradually (~30 min) to H2O2 with an increase in fluorescence at ~656 nm and 640 nm of more than 100-fold in vitro. At a concentration of 10 μM, FS-ROS1 labels the L929 cell and Raw264.7 cell wells in 30 min with excellent biocompatibility and without washing. After labelling, FS-ROS1 exhibited a rational fluorescence increase upon the addition of 1, 10, 100, and 200 μM of H2O2. Based on these results, inflammatory cells, stimulated with 800 nM dexamethasone and polyIC, showed a higher increase in fluorescence than the control cells. These results suggest that H2O2 and ClO– might be important signaling molecules during inflammations.

A Highly Selective Fluorescent Probe for Hypochlorous Acid in Living Cells Based on a Naphthalene Derivative

Hypochlorous acid (HOCl) was crucial for maintaining the homeostasis in cells and plays vital roles in many physiological and pathological processes. In this work, a highly selective fluorescent probe for hypochlorous acid in living cells was constructed and prepared based on a naphthalene derivative. A naphthalene derivative was utilized as the fluorescent group, and N,N-dimethylthiocarbamate was applied as the selective recognition site for HOCl. Before adding HOCl, the fluorescent probe exhibited weak fluorescence. Upon adding HOCl, the fluorescent probe displayed remarkable fluorescence enhancement. The fluorescence intensity at 502 nm showed a linear response to the concentration of HOCl from 3.0 × 10−7 to 1.0 × 10−5 mol·L−1. The detection limit was estimated to be 1.5 × 10−7 mol·L−1 for HOCl. The fluorescent probe showed fast response and outstanding selectivity toward HOCl. It owned good biocompatibility and had also been successfully applied in the confocal imaging of exogenous and endogenous HOCl in living cells.

Tb(III)-DO3A and BODIPY dyad as multimode responsive hypochlorite probe

A multimode responsive hypochlorite probe 1, based on Terbium (III)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (Tb(III)-DO3A) and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY), is described. We have shown that probe 1 can detect ClO^- by absorption, fluorescence, and phosphorescence simultaneously. The multimode response makes probe 1 a versatile ClO^- probe for practical applications. We have found that probe 1 can be used in naked-eye colorimetric and fluorogenic detection of NaOCl in solution. Also, we have constructed a colorimetric test paper for visual sensing of NaOCl. Furthermore, fluorescence imaging studies indicated that probe 1 was a versatile tool for in vitro imaging of NaOCl in living cells. Thus, to the best of our knowledge, probe 1 represents one of the rare examples of multimode responsive ClO^- probes.

A deep-red lysosome-targetable fluorescent probe for detection of hypochlorous acid in pure water and its imaging application in living cells and zebrafish

Hypochlorite plays a significant role in physiological processes, particularly regulation of lysosomal functions, and is involved in various diseases. Thus, it is crucial to develop highly sensitive and selective molecule tools to detect HClO in lysosomes. Herein, a novel 2H-benzo[h]chromene-pyridine derivative (1) was synthesized through condensation reaction, which exhibited a notable deep-red emission at 640 nm in pure water. This deep-red emission was specifically quenched by adding ClO^-. The response of probe 1 toward ClO^- was rapid (within 10 s), sensitive (detection limit of 0.012 μM), and effective over a wide range of pH (1.0-12.0). Due to the existence of morpholine as the lysosome-targeting unit, the probe was successfully utilized to monitor lysosomal ClO^-. Moreover, the probe 1 was also applied to detecting ClO^- in zebrafish.

A benzocoumarin-based fluorescent probe for ultra-sensitive and fast detection of endogenous/exogenous hypochlorous acid and its applications

Hypochlorous acid (HOCl) is widely used in daily production and life because of its green and strongly oxidizing properties.