ESIPT-based fluorescence probe for the rapid detection of peroxynitrite 'AND' biological thiols

Nitroxyl donating and visualization with a coumarin-based fluorescence probe

Nitroxyl (HNO), the single-electron reduction product of nitric oxide (NO), has attracted great interest in the treatment of congestive heart failure in clinical trials. In this paper, we describe the first coumarin-based compound N-hydroxy-2-oxo-2H-chromene-6-sulfonamide (CD1) as a dualfunctional HNO donor, which can release both an HNO signaling molecule and a fluorescent reporter. Under physiological conditions (pH 7.4 and 37 °C), the CD1 HNO donor can readily decompose with a half-life of ∼90 min. The corresponding stoichiometry HNO from the CD1 donor was confirmed using both Vitamin B12 and phosphine compound traps. In addition to HNO releasing, specifically, the degradation product 2-oxo-2H-chromene-6-sulfinate (CS1) was generated as a fluorescent marker during the decomposition. Therefore, the HNO amount released in situ can be accurately monitored through fluorescence generation. As compared to the CD1 donor, the fluorescence intensity increased by about 4.9-fold. The concentration limit of detection of HNO releasing was determined to be ∼0.13 μM according to the fluorescence generation of CS1 at physiological conditions. Moreover, the bioimaging of the CD1 donor was demonstrated in the cell culture of HeLa cells, where the intracellular fluorescence signals were observed, inferring the site of HNO release. Finally, we anticipate that this novel coumarin-based CD1 donor opens a new platform for exploring the biology of HNO.

A Fluorescent Probe with Zwitterionic ESIPT Feature for Ratiometric Monitoring of Peroxynitrite In Vitro and In Vivo

Peroxynitrite (ONOO-), as a short-term reactive biological oxidant, could lead to a series of effects in various physiological and pathological processes due to its subtle concentration changes. In vivo monitoring of ONOO- and relevant physiological processes is urgently required. Herein, we describe a novel fluorescent probe termed HBT-Fl-BnB for the ratiometric detection of ONOO- in vitro and in vivo. The probe consists of an HBT core with Fl groups at the ortho and para positions responding to the zwitterionic excited-state intramolecular proton-transfer (zwitterionic ESIPT) process and a boronic acid pinacol ester with dual roles that block the zwitterionic ESIPT and recognize ONOO-. Thanks to the specificity as well as low cytotoxicity, success in imaging of endogenous and exogenous ONOO- in living cells by HBT-Fl-BnB was obtained. Additionally, the applicability of HBT-Fl-BnB to tracking the abnormal expression of ONOO- in vivo induced by inactivated Escherichia coli was also explored. This is the first report of a fluorescent probe for ONOO- sensing via a zwitterionic ESIPT mechanism.

Recent advances in fluorescent probes for dual-detecting ONOO- and analytes

Although peroxynitrite (ONOO-) plays an essential role in cellular redox homeostasis, its excess ONOO- will affect the normal physiological function of cells. Therefore, real-time monitoring of changes in local ONOO- will contribute to further revealing the biological functions. Reliable and accurate detection of biogenic ONOO- will definitely benefit for disentangling its complex functions in living systems. In the past few years, more fluorescent probes have been developed to help understand and reveal cellular ONOO- changes. However, there has been no comprehensive and critical review of multifunctional fluorescent probes for cellular ONOO- and other analytes. To highlight the recent advances, this review first summarized the recent progress of multifunctional fluorescent probes since 2018, focusing on molecular structures, response mechanisms, optical properties, and biological imaging in the detection and imaging of cellular ONOO- and analytes. We classified and discussed in detail the limitations of existing multifunctional probes, and proposed new ideas to overcome these limitations. Finally, the challenges and future development trends of ONOO- fluorescence probes were discussed. We hoped this review will provide new research directions for developing of multifunctional fluorescent probes and contribute to the early diagnosis and treatment of diseases.

Selective degradation of BRD4 suppresses lung cancer cell proliferation using GSH-responsive PROTAC precursors

BRD4,as a transcriptional and epigenetic regulator to mediate cellular functions, plays an important role in cancer development.Targeting BRD4 with conventional inhibitors in cancer therapy requires high doses, which often leads to off-target and adverse effects. BRD4-targeted proteolysis-targeting chimeras (PROTACs) can catalytically degrade BRD4 utilizing the endogenous proteasome system, and exhibit promising anti-tumor activity. However, most of the developed PROTACs are non-cancer specific and relatively toxic towards normal cells, limiting their practical applications in cancer treatment. By taking advantage of higher glutathione (GSH) levels in cancer cells than that in normal cells, we developed several GSH-responsive PROTAC precursors 1a-c via the attachment of a GSH-trigger unit on the hydroxyl group of the VHL (von Hippel-Lindau) ligand for the recruitment of E3 ligase. Among the precursors, 1a can be efficiently activated by the innately higher concentrations of GSH in lung cancer cells (A549 and H1299) to release active PROTAC 1, degrading intracellular BRD4 and resulting in cytotoxicity, which is confirmed by mechanistic investigation. On the other hand, 1a cannot be efficiently triggered in normal lung cells (WI38 and HULEC-5a) containing lower levels of GSH, therefore reducing the adverse effects on normal cells. This work provides an alternative proof of concept approach for developing stimuli-responsive PROTAC precursors, and affords a novel insight to improve the selectivity and minimize the adverse effects of current PROTACs, hence enhancing their clinical potential.

An AND-gate bioluminescent probe for precise tumor imaging

Sensitivity and specificity are two indispensable requirements to ensure diagnostic accuracy. Dual-locked probes with "AND-gate" logic theory have emerged as a powerful tool to enhance imaging specificity, avoid "false positive" results, and realize correlation analysis. In addition, bioluminescence imaging (BLI) is an excitation-free optical modality with high sensitivity and low background and can thus be combined with a dual-locked strategy for precise disease imaging. Here, we developed a novel AND-gate bioluminescent probe, FK-Luc-BH, which is capable of responding to two different tumor biomarkers (cathepsin L and ClO^-). The good specificity of FK-Luc-BH was proven, as an obvious BL signal could only be observed in the solution containing both cathepsin L (CTSL) and ClO^-. 4T1-fLuc cells and tumors treated with FK-Luc-BH exhibited significantly higher BL signals than those treated with unresponsive control compound Ac-Luc-EA or cotreated with FK-Luc-BH and a ClO^- scavenger/cathepsin inhibitor, demonstrating the ability of FK-Luc-BH to precisely recognize tumors in which CTSL and ClO^- coexist.

Recent progress in the development of fluorescent probes for imaging pathological oxidative stress

Oxidative stress is closely related to the physiopathology of numerous diseases. Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) are direct participants and important biomarkers of oxidative stress. A comprehensive understanding of their changes can help us evaluate disease pathogenesis and progression and facilitate early diagnosis and drug development. In recent years, fluorescent probes have been developed for real-time monitoring of ROS, RNS and RSS levels in vitro and in vivo. In this review, conventional design strategies of fluorescent probes for ROS, RNS, and RSS detection are discussed from three aspects: fluorophores, linkers, and recognition groups. We introduce representative fluorescent probes for ROS, RNS, and RSS detection in cells, physiological/pathological processes (e.g., Inflammation, Drug Induced Organ Injury and Ischemia/Reperfusion Injury etc.), and specific diseases (e.g., neurodegenerative diseases, epilepsy, depression, diabetes and cancer, etc.). We then highlight the achievements, current challenges, and prospects for fluorescent probes in the pathophysiology of oxidative stress-related diseases.

TDDFT study on the simultaneous sensing mechanism for peroxynitrite and glutathione by a bifunctional fluorescent probe

Using time-dependent density functional theory (TDDFT) method, the response mechanism of a reported bifunctional fluorescent probe for simultaneous recognition of peroxynitrite and glutathione (Chem. Commun. 2018, 54, 11336) was theoretically studied. Calculated vertical excitation energies based on the ground-state and excited-state geometries were consistent with the corresponding experimental ultraviolet-visible and fluorescence spectra. In the ground state, electron delocalization in the probe was limited because its geometry was restrained by steric hindrance. Frontier molecular orbital analysis has shown that the probe should undergo photoinduced electron transfer (PET) from the benzothiazole moiety to the maleimide moiety after excitation. The nonplanar structure together with PET led to fluorescence quenching of the probe. The probe could be dealkylated by peroxynitrite anion. The resulting intramolecular hydrogen bond increasesd the planarity of the molecule, while also gave rise to an excited-state proton-transfer process. Moreover, the addition reaction between the probe and glutathione inhibited the PET process. These two analytes together contributed to the fluorescence enhancement of the final product. This theoretical sensing mechanism for peroxynitrite and glutathione may potentially be important for the design and enhancement of novel probes.

Excited-state intramolecular proton transfer (ESIPT)-based fluorescent probes for biomarker detection: design, mechanism, and application

Biomarkers are essential in biology, physiology, and pharmacology; thus, their detection is of extensive importance. Fluorescent probes provide effective tools for detecting biomarkers exactly. Excited state intramolecular proton transfer (ESIPT), one of the significant photophysical processes that possesses specific photoisomerization between Keto and Enol forms, can effectively avoid annoying interference from the background with a large Stokes shift. Hence, ESIPT is an excellent choice for biomarker monitoring. Based on the ESIPT process, abundant probes were designed and synthesized using three major design methods. In this review, we conclude probes for 14 kinds of biomarkers based on ESIPT explored in the past five years, summarize these general design methods, and highlight their application for biomarker detection in vitro or in vivo.

Fluorescent "AND" logic gates for simultaneous detection of thiols and protons: photophysical properties, mechanism and bioimaging of living cells

Five fluorescent probes TP1-5 were demonstrated as two-input "AND" molecular logic gates for the detection of thiols and protons. The molecules were designed based on "thiol receptor-spacer₁-fluorophore-spacer₂-proton receptor" mode. The logic gates were constructed by employing maleimide, naphthalimide and morpholine (TP1-3)/N-methyl piperazine (TP4-5) as the thiol receptor, fluorophore and proton receptor, respectively. All probes show significant fluorescence enhancements upon addition of both protons and thiols. However, much weaker spectral responses were observed with the addition of only one single analyte. The fluorescence outputs, based on photoinduced electron transfer (PET) and (twisted) intramolecular charge transfer (TICT/ICT), were modulated by the proton receptor and linker. The length of spacer₁ affects the responses toward thiols, whereas spacer₂ influences the sensing performance toward protons. The difference between the pK_a values of morpholine (∼5.80) and N-methyl piperazine (∼7.10) enables us to detect thiols in divergent pH circumstances. TP1-3 exhibit an excellent "AND" logic function for simultaneous detection of protons and thiols as well as bioimaging thiols in weakly acidic living cells. However, TP4 and TP5 are not good candidates for executing "AND" logic operation possibly due to the stronger electron donating properties and steric effect of N-methyl piperazine.

Dual-targetable fluorescent probe for mapping the fluctuation of peroxynitrite in drug-induced liver injury model

Drug-induced liver injury (DILI) is one of the most common and serious adverse drug reactions which can cause acute liver failure or even death in severe cases. With the incidence rate increasing over the years, DILI has became a frequent clinical liver disease and a global public health problem. As a biomarker of DILI, the detection of peroxynitrite (ONOO^-) has became a powerful tool for the early diagnosis of liver injury. Here, we synthesized five mitochondria-targetable probes, 1-5, for detecting endogenous ONOO^-. Through dye-screening, probe 5 was stood out by its excellent performance. In the presence of ONOO^-, the fluorescence signal of probe 5 reduced 40-fold in 19 s with a low detection limit (9.36 nM). At the same time, the transformation can be observed with the naked eye under sunlight or UV lamp without being affected by the other reactive species. Even better, with low toxicity and high biocompatibility, probe 5 could successfully detect endogenous ONOO^- in the mitochondrion of cells. Finally, probe 5 could specifically target the liver, and can be employed for monitoring the therapeutic effect of hepatoprotective medicine after drug-induced hepatotoxicity in vivo. In brief, probe 5 has the practical application capability for diagnosing the severity of the liver injury and researching the therapeutic effect of antidote in complex bio-systems.

Boronates as hydrogen peroxide-reactive warheads in the design of detection probes, prodrugs, and nanomedicines used in tumors and other diseases

Hydrogen peroxide (H₂O₂) has always been a topic of great interests attributed to its vital role in biological process. H₂O₂ is known as a major reactive oxygen species (ROS) which is involve in numerous physiological processes such as cell proliferation, signal transduction, differentiation, and even pathogenesis. A plenty of diseases development such as chronic disease, inflammatory disease, and organ dysfunction are found to be relevant to abnormality of H₂O₂ production. Thus, imminent and feasible strategies to modulate and detect H₂O₂ level in vitro and in vivo have gained great importance. To date, the boronate-based chemical structure probes have been widely used to address the problems from the above aspects because of the rearranged chemical bonding which can detect and quantify ROS including hydrogen peroxide (H₂O₂) and peroxynitrite (ONOO^-). This present article discusses boronate-based probes based on the chemical structure difference as well as reactivities to H₂O₂ and ONOO^-. In this review, we also focus on the application of boronate-based probes in the field of cell imaging, prodrugs nanoplatform, nanomedicines, and electrochemical biosensors for disease diagnosis and treatment. In a nutshell, we outline the recent application of boronate-based probes and represent the prospective potentiality in biomedical domain in the future.

Unveiling the effects of atomic electronegativity on the ESIPT mechanism and luminescence property of new coumarin benzothiazole Fluorophore: A TD-DFT exploration

The excited-state intramolecular proton transfer (ESIPT) mechanism, photophysical properties of 8-(benzo[D] thiazole-2-yl)-7-hydroxy-2H-benzopyran-2-one (L-HKS) and the effect of O/Se atomic substitution on L-HKS have been studied in detail based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The S atom in the thiazole ring of L-HKS has been replaced by O/Se atom (denoted to L-HKO/L-HKSe) to analyze the effects of atomic electronegativity on the intramolecular H-bond, absorption/emission spectrum and ESIPT process. Through the analysis of series of calculated results, it can be found that the intramolecular H-bonds at normal form and tautomer form are enhanced and weakened in the S₁ state, respectively, which is favorable to ESIPT process. The potential energy curves revealed that the ESIPT process is much easier to occur gradually from L-HKO to L-HKS and L-HKSe, as the electron-withdrawing ability of atom (from O to S and Se) is weakened. The atomic substitution also has an effect on the photophysical properties. From L-HKO to L-HKS, the emission peak at tautomer form red-shifts 70 nm. The energy gaps of the three compounds follow the decreased order of L-HKO (4.866 eV) > L-HKS (4.753 eV) > L-HKSe (4.371 eV) with the weakened electron-withdrawing ability of atom (from O to S and Se), which leads to the gradual red-shift of the absorption spectra from L-HKO to L-HKS and L-HKSe.

Fluorescent probes and functional materials for biomedical applications

Due to their simplicity in preparation, sensitivity and selectivity, fluorescent probes have become the analytical tool of choice in a wide range of research and industrial fields, facilitating the rapid detection of chemical substances of interest as well as the study of important physiological and pathological processes at the cellular level. In addition, many long-wavelength fluorescent probes developed have also proven applicable for in vivo biomedical applications including fluorescence-guided disease diagnosis and theranostics (e.g., fluorogenic prodrugs). Impressive progresses have been made in the development of sensing agents and materials for the detection of ions, organic small molecules, and biomacromolecules including enzymes, DNAs/RNAs, lipids, and carbohydrates that play crucial roles in biological and disease-relevant events. Here, we highlight examples of fluorescent probes and functional materials for biological applications selected from the special issues “Fluorescent Probes” and “Molecular Sensors and Logic Gates” recently published in this journal, offering insights into the future development of powerful fluorescence-based chemical tools for basic biological studies and clinical translation.

Dual-Channel Fluorescent Probe for the Simultaneous Monitoring of Peroxynitrite and Adenosine-5'-triphosphate in Cellular Applications

Changes in adenosine triphosphate (ATP) and peroxynitrite (ONOO-) concentrations have been correlated in a number of diseases including ischemia-reperfusion injury and drug-induced liver injury. Herein, we report the development of a fluorescent probe ATP-LW, which enables the simultaneous detection of ONOO- and ATP. ONOO- selectively oxidizes the boronate pinacol ester of ATP-LW to afford the fluorescent 4-hydroxy-1,8-naphthalimide product NA-OH (λex = 450 nm, λem = 562 nm or λex = 488 nm, λem = 568 nm). In contrast, the binding of ATP to ATP-LW induces the spirolactam ring opening of rhodamine to afford a highly emissive product (λex = 520 nm, λem = 587 nm). Due to the differences in emission between the ONOO- and ATP products, ATP-LW allows ONOO- levels to be monitored in the green channel (λex = 488 nm, λem = 500-575 nm) and ATP concentrations in the red channel (λex = 514 nm, λem = 575-650 nm). The use of ATP-LW as a combined ONOO- and ATP probe was demonstrated using hepatocytes (HL-7702 cells) in cellular imaging experiments. Treatment of HL-7702 cells with oligomycin A (an inhibitor of ATP synthase) resulted in a reduction of signal intensity in the red channel and an increase in that of the green channel as expected for a reduction in ATP concentrations. Similar fluorescence changes were seen in the presence of SIN-1 (an exogenous ONOO- donor).

A thiocarbonate-caged fluorescent probe for specific visualization of peroxynitrite in living cells and zebrafish

Peroxynitrite (ONOO^-), a highly reactive oxygen species (ROS), is implicated with many physiological and pathological processes including cancer, neurodegenerative diseases and inflammation. In this regard, developing effective tools for highly selective tracking of ONOO^- is urgently needed. Herein, we constructed a concise and specific fluorescent probe NA-ONOO for sensing ONOO^- by conjugating an ONOO^--specific recognition group ((4-methoxyphenylthio)carbonyl, a thiocarbonate derivative) with a naphthalene fluorophore. The probe, NA-ONOO, was in a dark state because the high electrophilicity of (4-methoxyphenylthio)carbonyl disturbs the intramolecular charge transfer (ICT) in the fluorophore. Upon treatment with ONOO^-, the fluorescent emission was sharply boosted (quantum yield Φ: 3% to 56.6%) owing to an ONOO^- triggered release of (4-methoxyphenylthio)carbonyl from NA-ONOO. Optical analyses showed that NA-ONOO presented high selectivity and sensitivity toward ONOO^-. With good cell permeability and biocompatibility, the NA-ONOO probe was successfully applied to imaging and tracing exogenous and endogenous ONOO^- in living cells and zebrafish. The probe NA-ONOO presents a new recognition group and a promising method for further investigating ONOO^- in living systems.

A novel p-dimethylaminophenylether-based fluorescent probe for the detection of native ONOO- in cells and zebrafish

Peroxynitrite (ONOO^-) is a highly reactive substance, and plays an essential part in maintaining cellular homeostasis. It is crucial to monitor the ONOO^- level in cells in normal and abnormal states. We introduced a p-dimethylaminophenylether-based fluorescent probe PDPE-PN, which could be synthesized readily. The new probe had prominent sensitivity and specificity, and a fast response towards ONOO^-. The spectral performance of probe PDPE-PN was outstanding and the limit of detection was 69 nM. Probe PDPE-PN with low toxicity was applied to detect endogenous/exogenous ONOO^- in RAW 264.7 macrophages and zebrafish. Importantly, successful application of the new receptor opens up new ideas for the design of ONOO^- probes.

A butterfly-shaped ESIPT molecule with solid-state fluorescence for the detection of latent fingerprints and exogenous and endogenous ONOO- by caging of the phenol donor

The latent fingerprints (LFPs) at the crime scene are unique and stable, which are considered as an important clue in criminal justice and forensic identification. Herein, a butterfly-shaped molecule DPTS with solid fluorescence plus excited-state intramolecular proton transfer (ESIPT) properties was used to develop for enhancing the visualization of the LFPs. Considering the solid fluorescence of DPTS, the color and efficiency of DPTS with a large Stokes shift (216 nm) can be tuned by changing the morphology of its aggregates, and gradually red-shifted (green-yellow-red) with increasing water content. Furthermore, its effectiveness for the detection of LFPs was demonstrated on various different substrates including paper box, tinfoil and weighting paper. The emissive fingerprint of DPTS obtained gave good fluorescence images with high contrast and resolution such as the core, delta, bifurcation, ridge termination, independent ridge and pores. Caging of the phenol donor of DPTS with a sensitive biomarker group provided DPTS-ONOO^-, which had high sensitive with detection limit of 5 nM and the quantification limit of 21 nM toward ONOO^-. Modularly derived DPTS-ONOO^- was synthesized and demonstrated specific fluorescence imaging of exogenous and endogenous peroxynitrite (ONOO^-) in living macrophage cells.

Mitochondria-Targeting and Reversible Near-Infrared Emissive Iridium(III) Probe for in vivo ONOO-/GSH Redox Cycles Monitoring

Peroxynitrite (ONOO^-) and glutathione (GSH), two unique reactive species, play an essential regulating role in the oxidation and antioxidation in the living body and are closely associated with various physiological and pathological processes, like cancer, cardiovascular disorders, diabetes, inflammation, Alzheimer's disease, and hepatotoxicity. Thus, it is crucial to study mitochondria ONOO^-/GSH redox cycles by an effective molecular tool. In this work, a mitochondria-targeting and redox-reversible near-infrared (NIR) phosphorescent iridium complex, Ir-diol, has been synthesized and used for the detection and imaging of a cellular redox state by visualizing endogenous ONOO^-/GSH content. Ir-diol shows excellent photophysical properties, including NIR emission (the maximum emissive wavelength for 704 nm, approximately) and high phosphorescent quantum yield (Φ = 0.136) and exhibits high sensitivity and selectivity toward ONOO^-/GSH redox cycles in aqueous solution and living cells. Therefore, these features, combined with low cytotoxicity and excellent cell permeability, enable probe Ir-diol to monitor the changes of the intracellular ONOO^-/GSH level induced by drug both in vitro and in vivo.

Dual-locked spectroscopic probes for sensing and therapy

Optical imaging probes allow us to detect and uncover the physiological and pathological functions of an analyte of interest at the molecular level in a non-invasive, longitudinal manner. By virtue of simplicity, low cost, high sensitivity, adaptation to automated analysis, capacity for spatially resolved imaging and diverse signal output modes, optical imaging probes have been widely applied in biology, physiology, pharmacology and medicine. To build a reliable and practically/clinically relevant probe, the design process often encompasses multidisciplinary themes, including chemistry, biology and medicine. Within the repertoire of probes, dual-locked systems are particularly interesting as a result of their ability to offer enhanced specificity and multiplex detection. In addition, chemiluminescence is a low-background, excitation-free optical modality and, thus, can be integrated into dual-locked systems, permitting crosstalk-free fluorescent and chemiluminescent detection of two distinct biomarkers. For many researchers, these dual-locked systems remain a 'black box'. Therefore, this Review aims to offer a 'beginner's guide' to such dual-locked systems, providing simple explanations on how they work, what they can do and where they have been applied, in order to help readers develop a deeper understanding of this rich area of research.

A novel borate fluorescent probe for rapid selective intracellular peroxynitrite imaging

Peroxynitrite (ONOO^-) is one of the species of reactive nitrogen (RNS), which plays an important role in antibacterial activity and signal transduction and other physiological and pathological processes. In this paper, based on the benzyl borate group, a new fluorescent probe capable of detecting ONOO^- with high selectivity and sensitivity is designed, and the possible mechanism of the interaction between probe and ONOO^- is proposed. The probe shows high fluorescence response to ONOO^- in a wide pH range (7.0-11.5). Moreover, the probe exhibit good permeability, and the content of ONOO^- in cancer cells and normal cells was successfully monitored.

Excited-State Intramolecular Proton Transfer: A Short Introductory Review

In this short review, we attempt to unfold various aspects of excited-state intramolecular proton transfer (ESIPT) from the studies that are available up to date. Since Weller’s discovery of ESIPT in salicylic acid (SA) and its derivative methyl salicylate (MS), numerous studies have emerged on the topic and it has become an attractive field of research because of its manifold applications. Here, we discuss some critical aspects of ESIPT and tautomerization from the mechanistic viewpoint. We address excitation wavelength dependence, anti-Kasha ESIPT, fast and slow ESIPT, reversibility and irreversibility of ESIPT, hydrogen bonding and geometrical factors, excited-state double proton transfer (ESDPT), concerted and stepwise ESDPT.

Designing an ESIPT-based fluorescent probe for imaging of hydrogen peroxide during the ferroptosis process

Hydrogen peroxide (H₂O₂), depending on its levels, plays a crucial role in either modulating various biological processes as a signal molecule, or mediating oxidative damage as a toxin. Therefore, monitoring intracellular H₂O₂ levels is pivotal for exploring its physiological and pathological roles. Using a modified 2-(2'-hydroxyphenyl) benzothiazole (HBT) as the fluorophore, and a pinacol phenylborate ester as the responsive group, herein we developed an excited-state intramolecular proton transfer (ESIPT)-based probe BTFMB. The probe exhibited turn-on fluorescence response, large Stokes shift (162 nm) and low detection limit (109 nM) toward H₂O₂, and was successfully applied for monitoring exogenous and endogenous production of H₂O₂, and identifying accumulation of H₂O₂ during the ferroptosis process.

Boronate-Based Probes for Biological Oxidants: A Novel Class of Molecular Tools for Redox Biology

Boronate-based molecular probes are emerging as one of the most effective tools for detection and quantitation of peroxynitrite and hydroperoxides. This review discusses the chemical reactivity of boronate compounds in the context of their use for detection of biological oxidants, and presents examples of the practical use of those probes in selected chemical, enzymatic, and biological systems. The particular reactivity of boronates toward nucleophilic oxidants makes them a distinct class of probes for redox biology studies. We focus on the recent progress in the design and application of boronate-based probes in redox studies and perspectives for further developments.

: a synthetic platform for the development of fluorescent probes for diagnostic and theranostic applications

Reaction-based fluorescent-probes have proven successful for the visualisation of biological species in various cellular processes. Unfortunately, in order to tailor the design of a fluorescent probe to a specific application (i.e. organelle targeting, material and theranostic applications) often requires extensive synthetic efforts and the synthetic screening of a range of fluorophores to match the required synthetic needs. In this work, we have identified Pinkment-OH as a unique “plug-and-play” synthetic platform that can be used to develop a range of ONOO⁻ responsive fluorescent probes for a variety of applications. These include theranostic-based applications and potential material-based/bioconjugation applications. The as prepared probes displayed an excellent sensitivity and selectivity for ONOO⁻ over other ROS. In vitro studies using HeLa cells and RAW 264.7 macrophages demonstrated their ability to detect exogenously and endogenously produced ONOO⁻. Evaluation in an LPS-induced inflammation mouse model illustrated the ability to monitor ONOO⁻ production in acute inflammation. Lastly, theranostic-based probes enabled the simultaneous evaluation of indomethacin-based therapeutic effects combined with the visualisation of an inflammation biomarker in RAW 264.7 cells.

Pinkment, a resorufin based ONOO⁻ selective and sensitive ‘plug and play’ fluorescence-based platform for in vitro and in vivo use, enables facile functionalisation for various imaging and theranostic applications.

Protein Encapsulation: A Nanocarrier Approach to the Fluorescence Imaging of an Enzyme-Based Biomarker

Here, we report a new pentafluoropropanamido rhodamine fluorescent probe (ACS-HNE) that allows for the selective detection of neutrophil elastase (NE). ACS-HNE displayed high sensitivity, with a low limit of detection (<5.3 nM), and excellent selectivity toward elastase over other relevant biological analytes and enzymes. The comparatively poor solubility and cell permeability of neat ACS-HNE was improved by creating an ACS-HNE-albumin complex; this approach allowed for improvements in the in situ visualization of elastase activity in RAW 264.7 cells relative to ACS-HNE alone. The present study thus serves to demonstrate a simple universal strategy that may be used to overcome cell impermeability and solubility limitations, and to prepare probes suitable for the cellular imaging of enzymatic activity in vitro.

Design and Development of an HBT-Based Ratiometric Fluorescent Probe to Monitor Stress-Induced Premature Senescence

Stress-induced premature senescence (SIPS) can be induced in tumor cells by reactive oxygen species (ROS) or oncogenes. The antineoplastic drugs cause apoptosis and senescence by damaging the DNA. Although the detection of cellular senescence is important to monitor drug response during anticancer therapy, only a few probes have been studied for imaging SIPS. In this study, we developed 2-(2'-hydroxyphenyl)benzothiazole (HBT)-based fluorescent probes to determine SIPS by monitoring the oxidative stress and β-galactosidase activity. HBT is a commonly used fluorophore because of its luminescence mechanism via excited-state intramolecular proton transfer, and it has attractive properties, such as a four-level photochemical process and large Stokes shift (151 nm). A novel fluorescent probe, (2-(benzo[d]thiazol-2-yl)phenyl)boronic acid, was prepared for the detection of ROS, including H₂O₂, via the oxidation reaction of arylboronic acids to form the fluorescent phenol, HBT. In addition, to determine the enzymatic activity of β-galactosidase, a 2-(4'-chloro-2'-hydroxyphenyl)benzothiazole (CBT)-based enzymatic turn-on probe (CBT-β-Gal) was designed and synthesized. β-Galactosidase catalyzed the hydrolysis of β-galactopyranoside from CBT-β-Gal to release the fluorescent CBT. These probes were capable of ratiometric imaging the accumulation of H₂O₂ and the degree of β-galatosidase activity in contrast to H₂O₂-untreated and H₂O₂-treated HeLa cells. Furthermore, these probes were successfully employed for imaging the increased levels of ROS and β-galactosidase activity in the doxorubicin-treated HeLa cells.

In Situ Fluorescence Imaging of the Levels of Glycosylation and Phosphorylation by a MOF-Based Nanoprobe in Depressed Mice

Elucidating the relationship between glycosylation and phosphorylation of protein post-translational modifications is of great significance for understanding most diseases. Mass spectrometry has been widely used in research of protein phosphorylation and glycosylation. However, to realize in situ and dynamic analysis of the levels of phosphorylation and glycosylation in cells and in vivo, mass spectrometry still has certain difficulties. Herein, a nano-MOF-based fluorescent probe with Zr(IV) and boric acid as the active center was designed and prepared. Fluorescence detection and imaging of phosphate is achieved through the specific interaction of Zr(IV) and phosphate. With aim to achieve specific recognition of glycosylation sites, the boronic acid group was modified in the MOF structure, and the fluorescence of the MOFs was regulated by the alizarin red. Thus, the glycosylation sites were recognized by the competition between alizarin red and glycosyl. Finally, the nanoprobe was successfully applied for in situ fluorescence imaging of the levels of glycosylation and phosphorylation in depressed mice.

Twisted Schiff-base macrocycle showing excited-state intramolecular proton-transfer (ESIPT): assembly and sensing properties

A Schiff-base macrocyclic host showing ESIPT-based AIEE and nanoparticle assembly.

Coumarin-based fluorescent probe for the rapid detection of peroxynitrite ‘AND’ biological thiols

A coumarin-based novel ‘AND’ logic fluorescent probe ROS-AHC has been developed for the simultaneous detection of ONOO⁻ and biological thiols. ROS-AHC was shown to exhibit only a very small fluorescence response upon addition of a single GSH or ONOO⁻ analyte. Exposure to both analytes, however, resulted in a significant fluorescence enhancement.

A coumarin-based novel ‘AND’ logic fluorescent probe ROS-AHC has been developed for the simultaneous detection of ONOO⁻ and biological thiols.

Protein encapsulation: a new approach for improving the capability of small-molecule fluorogenic probes

Herein, we report a protein-based hybridization strategy that exploits the host-guest chemistry of HSA (human serum albumin) to solubilize the otherwise cell impermeable ONOO^- fluorescent probe Pinkment-OAc. Formation of a HSA/Pinkment-OAc supramolecular hybrid was confirmed by SAXS and solution-state analyses. This HSA/Pinkment-OAc hybrid provided an enhanced fluorescence response towards ONOO^- versus Pinkment-OAc alone, as determined by in vitro experiments. The HSA/Pinkment-OAc hybrid was also evaluated in RAW 264.7 macrophages and HeLa cancer cell lines, which displayed an enhanced cell permeability enabling the detection of SIN-1 and LPS generated ONOO^- and the in vivo imaging of acute inflammation in LPS-treated mice. A remarkable 5.6 fold (RAW 264.7), 8.7-fold (HeLa) and 2.7-fold increased response was seen relative to Pinkment-OAc alone at the cellular level and in vivo, respectively. We anticipate that HSA/fluorescent probe hybrids will soon become ubiquitous and routinely applied to overcome solubility issues associated with hydrophobic fluorescent imaging agents designed to detect disease related biomarkers.

Reaction-Based Fluorescent Probes for the Detection and Imaging of Reactive Oxygen, Nitrogen, and Sulfur Species

This Account describes a range of strategies for the development of fluorescent probes for detecting reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive (redox-active) sulfur species (RSS). Many ROS/RNS have been implicated in pathological processes such as Alzheimer's disease, cancer, diabetes mellitus, cardiovascular disease, and aging, while many RSS play important roles in maintaining redox homeostasis, serving as antioxidants and acting as free radical scavengers. Fluorescence-based systems have emerged as one of the best ways to monitor the concentrations and locations of these often very short lived species. Because of the high levels of sensitivity and in particular their ability to be used for temporal and spatial sampling for in vivo imaging applications. As a direct result, there has been a huge surge in the development of fluorescent probes for sensitive and selective detection of ROS, RNS, and RSS within cellular environments. However, cellular environments are extremely complex, often with more than one species involved in a given biochemical process. As a result, there has been a rise in the development of dual-responsive fluorescent probes (AND-logic probes) that can monitor the presence of more than one species in a biological environment. Our aim with this Account is to introduce the fluorescent probes that we have developed for in vitro and in vivo measurement of ROS, RNS, and RSS. Fluorescence-based sensing mechanisms used in the construction of the probes include photoinduced electron transfer, intramolecular charge transfer, excited-state intramolecular proton transfer (ESIPT), and fluorescence resonance energy transfer. In particular, probes for hydrogen peroxide, hypochlorous acid, superoxide, peroxynitrite, glutathione, cysteine, homocysteine, and hydrogen sulfide are discussed. In addition, we describe the development of AND-logic-based systems capable of detecting two species, such as peroxynitrite and glutathione. One of the most interesting advances contained in this Account is our extension of indicator displacement assays (IDAs) to reaction-based indicator displacement assays (RIAs). In an IDA system, an indicator is allowed to bind reversibly to a receptor. Then a competitive analyte is introduced into the system, resulting in displacement of the indicator from the host, which in turn modulates the optical signal. With an RIA-based system, the indicator is cleaved from a preformed receptor-indicator complex rather than being displaced by the analyte. Nevertheless, without a doubt the most significant result contained in this Account is the use of an ESIPT-based probe for the simultaneous sensing of fibrous proteins/peptides AND environmental ROS/RNS.

Detection and quantification of nitric oxide-derived oxidants in biological systems

The free radical nitric oxide (NO^•) exerts biological effects through the direct and reversible interaction with specific targets (e.g. soluble guanylate cyclase) or through the generation of secondary species, many of which can oxidize, nitrosate or nitrate biomolecules. The NO^•-derived reactive species are typically short-lived, and their preferential fates depend on kinetic and compartmentalization aspects. Their detection and quantification are technically challenging. In general, the strategies employed are based either on the detection of relatively stable end products or on the use of synthetic probes, and they are not always selective for a particular species. In this study, we describe the biologically relevant characteristics of the reactive species formed downstream from NO^•, and we discuss the approaches currently available for the analysis of NO^•, nitrogen dioxide (NO₂ ^•), dinitrogen trioxide (N₂O₃), nitroxyl (HNO), and peroxynitrite (ONOO^-/ONOOH), as well as peroxynitrite-derived hydroxyl (HO^•) and carbonate anion (CO₃ ^•-) radicals. We also discuss the biological origins of and analytical tools for detecting nitrite (NO₂ ^-), nitrate (NO₃ ^-), nitrosyl-metal complexes, S-nitrosothiols, and 3-nitrotyrosine. Moreover, we highlight state-of-the-art methods, alert readers to caveats of widely used techniques, and encourage retirement of approaches that have been supplanted by more reliable and selective tools for detecting and measuring NO^•-derived oxidants. We emphasize that the use of appropriate analytical methods needs to be strongly grounded in a chemical and biochemical understanding of the species and mechanistic pathways involved.

A water-soluble BODIPY-based fluorescent probe for rapid and selective detection of hypochlorous acid in living cells

We designed and synthesized 4,4-di-(4'-methylmercaptophenoxy)-8-(N-methylpyridinium-2-yl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (probe 1) as a water-soluble BODIPY derivative for rapid and selective detection of hypochlorous acid. The pyridinium-2-yl linked at the meso position of BODIPY core was used to maintain highly fluorescent nature and to increase water solubility. Methylmercaptophenoxy was selected as responsive site installed on the boron atom (to replace the fluorine atom) and induced the photoinduced electron-transfer (PeT) effect to quench the fluorescence of BODIPY. The probe exhibited 83.9 μg mL^-1 solubility in PBS (10 mM, pH 7.4), and possessed very low fluorescence (Φ_f = 0.0013). Upon addition of HClO, the probe could display a distinct response in 1 min and generate marked fluorescence enhancement by 100-fold due to the oxidation of thioether into sulfoxide to terminate PeT process. A limit of detection of 53 nM was calculated for HClO in the linear response range from 0 μM to 10 μM, and the probe was successfully applied to image HClO in living cells.

Reductive stress imaging in the endoplasmic reticulum by using living cells and zebrafish

Excessive accumulation of reducing agents in the ER leads to a constitutively high UPR. And the co-function of GSH, Cys and HOCl in biological processes is not well understood. To address this, a TP probe, NPCC, was developed for monitoring reductive stress in the ER. It can also distinguish cancer cells from normal cells.

Dual-biomarker-triggered fluorescence probes for differentiating cancer cells and revealing synergistic antioxidant effects under oxidative stress

Hydrogen sulfide (H2S) and human NAD(P)H:quinine oxidoreductase 1 (hNQO1) are potential cancer biomarkers and also vital participants in cellular redox homeostasis. Simultaneous detection of these two biomarkers would benefit the diagnostic precision of related cancers and could also help to investigate their crosstalk in response to oxidative stress. Despite this importance, fluorescent probes that can be activated by the dual action of H2S detection and hNQO1 activity have not been investigated. To this end, dual-biomarker-triggered fluorescent probes 1 and 2 were rationally constructed by installing two chemoselective triggering groups into one fluorophore. Probe 1 provides a small turn-on fluorescence response toward H2S but a much larger response to both H2S and hNQO1 in tandem. By contrast, fluorescence probe 2 is activated only in the presence of both H2S and hNQO1. Probe 2 exhibits a large fluorescence turn-on (>400 fold), high sensitivity, excellent selectivity as well as good biocompatibility, enabling the detection of both endogenous H2S and hNQO1 activity in living cells. Bioimaging results indicated that probe 2 could differentiate HT29 and HepG2 cancer cells from HCT116, FHC and HeLa cells owing to the existence of relatively high endogenous levels of both biomarkers. Expanded investigations using 2 revealed that cells could generate more endogenous H2S and hNQO1 upon exposure to exogenous hydrogen peroxide (H2O2), implying the synergistic antioxidant effects under conditions of cellular oxidative stress.

ESIPT-based fluorescence probe for the ratiometric detection of superoxide

A simple ESIPT-based fluorescence probe (HMBT-LW) was developed for the detection of superoxide (O₂˙⁻).

A smart sensor for rapid detection of lethal hydrazine in human blood and drinking water

A newly designed and synthesized probe showed good cell permeability, low cytotoxicity, fast fluorogenic recognition, and “naked-eye” detection of a lethal health hazard, hydrazine, even at concentrations significantly below the TLV levels present in living cells, drinking water and industrial effluent.

In situ visualization of peroxisomal peroxynitrite in the livers of mice with acute liver injury induced by carbon tetrachloride using a new two-photon fluorescent probe

In situ visualization of peroxisomal peroxynitrite in the livers of mice with acute liver injury using a new two-photon fluorescent probe.

Coumarin-based fluorescent ‘AND’ logic gate probes for the detection of homocysteine and a chosen biological analyte

With this research we set out to develop a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys. Probe JEG-CAB was constructed by attaching the ONOO⁻ reactive unit, benzyl boronate ester, to a HCys/Cys reactive fluorescent probe, CAH. Similarly, the core unit CAH was functionalised with the nitroreductase (NTR) reactive p-nitrobenzyl unit to produce probe JEG-CAN. Both, JEG-CAB and JEG-CAN exhibited a significant fluorescence increase when exposed to either HCys and ONOO⁻ (JEG-CAB) or HCys and NTR (JEG-CAN) thus demonstrating their effectiveness to function as AND logic gates for HCys and a chosen analyte.

With this research we set out to develop of a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys.