Sensitivity evaluation of NBD-SCN towards cysteine/homocysteine and its bioimaging applications

Concise Biothiol-Activatable HPQ-NBD Conjugate as a Targeted Theranostic Probe for Tumor Cells

Cancer, as a malignant tumor, seriously endangers human health. The study of cancer diagnosis and therapy has great practical significance. The development of theranostic agents has become a very important research topic. Nevertheless, some existing agents still have imperfections, such as complex structures and difficult syntheses. Therefore, it is urgent for researchers to develop simple novel theranostic agents. In this study, the precipitated fluorophore HAPQ was used as a simple drug molecule for the first time and combined with NBD-Cl to construct a simple and efficient theranostic probe (HAPQ-NBD). The theranostic probe can distinguish between tumor cells and normal cells based on the higher levels of biothiol in tumor cells. In addition, the probe can use biothiol as a control switch to release higher levels of precipitated fluorophore HAPQ in tumor cells, leading to selective high toxicity to tumor cells, thus achieving the goal of selectively killing tumor cells. The construction of probe HAPQ-NBD provides a practical tool for the diagnosis and therapy of cancer. It is expected that the development and utilization of precipitated fluorophore will provide a new method and strategy for cancer diagnosis and therapy.

Specific chiroptical sensing of cysteine via ultrasound-assisted formation of disulfide bonds in aqueous solution

Cysteine (Cys) can serve as a biomarker to indicate diseases or disorders, and its chiral sensing has attracted increasing attention. Herein, we established an ultrasound-facilitated chiral sensing method for Cys using 4-chloro-7-nitro-1,2,3-benzoxadiazole (NBD-Cl) and electronic circular dichroism (ECD) spectroscopy. The formation of chiral disulfide bonds induced degenerate exciton coupling between two NBD chromophores, resulting in intense Cotton effects (CEs) of the sensing product. The anisotropy factor (g) was linearly correlated with the enantiomeric excess of Cys across the visible region (400-500 nm), and other natural amino acids or biothiols did not interfere with the detection. This ultrasound-promoted efficient and specific chiral sensing method of Cys has potential for application in the diagnosis of related diseases.

A novel selective probe for detecting glutathione from other biothiols based on the concept of Fluorescence Fusion

Biological thiols importantly regulate the intracellular redox activity and metabolic level, but many of the developed probes for biothiols are facing difficulty in effectively distinguishing GSH from Cys/Hcy due to the similarity in mechanism. In this work, despite the previous pattern of "Logic Gate", we reported the concept of "Fluorescence Fusion" for the first time to achieve only one excitation-emission process. The exploited the probe, MZ-NBD, could quickly measure GSH in 10 min with a large Stokes shift (130 nm). Though the reacting mechanism was similar, only GSH could cause the "Fluorescence Fusion" with only one strong fluorescence response while Cys/Hcy caused two peaks. Adjusting the excitation wavelength could hardly split the fused peak into two. Though image recognition by artificial intelligence could easily distinguish the patterns of peaks, here we used the signal-treating method to realize the high selectivity towards GSH. Moreover, MZ-NBD could be utilized for rapid detection of GSH in living MCF-7 cells, which was more suitable for GSH than using the "Logic Gate" strategy. More than introducing a novel probe with the new concept, this work was meaningful as the linker of traditional reaction-based fluorescent probes and potential image recognition by artificial intelligence, thus led to various future researches in inter-disciplines.

NBD-based synthetic probes for sensing small molecules and proteins: design, sensing mechanisms and biological applications

Compounds with a nitrobenzoxadiazole (NBD) skeleton exhibit prominent useful properties including environmental sensitivity, high reactivity toward amines and biothiols (including H2S) accompanied by distinct colorimetric and fluorescent changes, fluorescence-quenching ability, and small size, all of which facilitate biomolecular sensing and self-assembly. Amines are important biological nucleophiles, and the unique activity of NBD ethers with amines has allowed for site-specific protein labelling and for the detection of enzyme activities. Both H2S and biothiols are involved in a wide range of physiological processes in mammals, and misregulation of these small molecules is associated with numerous diseases including cancers. In this review, we focus on NBD-based synthetic probes as advanced chemical tools for biomolecular sensing. Specifically, we discuss the sensing mechanisms and selectivity of the probes, the design strategies for multi-reactable multi-quenching probes, and the associated biological applications of these important constructs. We also highlight self-assembled NBD-based probes and outline future directions for NBD-based chemosensors. We hope that this comprehensive review will facilitate the development of future probes for investigating and understanding different biological processes and aid the development of potential theranostic agents.

NBD-based fluorescent probes for separate detection of cysteine and biothiols via different reactivities

A NBD-based fluorescent probe is developed to seperately detect Cys and all biothiols via different reactivity.

A colorimetric and fluorescent chemosensor based on diarylethene for simultaneous detection and discrimination of biothiols

In this work, a novel probe D-HBT-NBD (1O) based on diarylethene to detect biothiols (including Cys, Hcy and GSH) was synthesized and the relative colorimetric and fluorescent properties were tested. The probe exhibited excellent photochromic properties and showed apparent colorimetric and fluorescent signals for Cys, Hcy and GSH. The probe can selectively detect Cys, Hcy and GSH by naked eyes for its open-ring isomer 1O and can discriminate Cys from Hcy/GSH by apparent color change from light orange to dark pink at the closed-ring state under the irradiation of UV light. At the excitation wavelength of 465 nm, the probe could be used to discriminate GSH from Cys/Hcy with no fluorescent emission at 570 nm. Taking advantage of the photochromic property of the diarylethene moiety and the different fluorescent properties of NBD derivatives of GSH and Cys/Hcy, 1O could be used as a novel probe to discriminate Cys, Hcy and GSH from each other simultaneously. Meanwhile, a logic gate was constructed based on the colorimetric and fluorescent properties of 1O.

POMOF/SWNT Nanocomposites with Prominent Peroxidase-Mimicking Activity for l-Cysteine "On-Off Switch" Colorimetric Biosensing

In order to explore novel colorimetric biosensors with high sensibility and selectivity, two new Keggin polyoxometalates (POMs)-based Cu-trz (1,2,4-triazole) metal-organic frameworks (MOFs) with suitable specific surface areas and multiple active sites were favorably fabricated; then single-walled carbon nanotubes (SWNTs) were merged with new POMOFs to construct POMOF/SWNT nanocomposites. Herein, POMOF/SWNT nanocomposites as peroxidase mimics were explored for the first time, and the peroxidase-mimicking activity of the prepared POMOF/SWNT nanocomposites is heavily dependent on the mass ratio of POMOFs and SWNTs, in which the maximum activity is achieved at the mass ratio of 2.5:1 (named PMNT-2). More importantly, PMNT-2 exhibits the lowest limit of detection (0.103 μM) among all reported materials to date and the assumable selectivity toward l-cysteine (l-Cys) detection. With these findings, a convenient, sensitive, and effective "on-off switch" colorimetric platform for l-Cys detection has been successfully developed, providing a promising prospect in the biosensors and clinical diagnosis fields.

FeCo nanoparticles-embedded carbon nanofibers as robust peroxidase mimics for sensitive colorimetric detection of l-cysteine

A simple and low cost detection of l-cysteine is essential in the fields of biosensors and medical diagnosis. In this study, we have developed a simple electrospinning, followed by calcination process to prepare FeCo nanoparticles embedded in carbon nanofibers (FeCo-CNFs) as an efficient peroxidase-like mimic for the detection of l-cysteine. FeCo nanoparticles are uniformly dispersed within CNFs, and their diameters are highly influenced by the calcination temperature. The calcination temperature also influences the peroxidase-like catalytic activity, and the maximum activity is achieved at a calcination temperature of 550 °C. Owing to the high catalytic activity of the as-prepared FeCo-CNFs, a colorimetric technique for the rapid and accurate determination of l-cysteine has been developed. The detection limit is about 0.15 μM with a wide linear range from 1 to 20 μM. In addition, a high selectivity for the detection of l-cysteine over other amino acids, glucose and common ions is achieved. This study provides a simple, rapid and sensitive sensing platform for the detection of l-cysteine, which is a promising candidate for potential applications in biosensing, medicine, environmental monitoring.

A Hybrid Coumarin-Semifluorescein-Based Fluorescent Probe for the Detection of Cysteine

A new type of turn-on fluorescent probe CF-AC for the detection of Cys was firstly reported. The probe exhibited an excellent response to Cys with high selectively and sensitivity. In the presence of Cys, two fluorescence emission peaks at 525 nm and 650 nm appeared accompanied by the fluorescence color change from blue to red. Morever, the probe had good biocompatibility and could be successfully used for fluorescence imaging of Cys in MCF-7 cells.

A novel NBD-based fluorescent turn-on probe for the detection of cysteine and homocysteine in living cells

Biothiols, such as cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), are involved in a number of biological processes and play crucial roles in biological systems. Thus, the detection of biothiols is highly important for early diagnosis of diseases and evaluation of disease progression. Herein, we developed a new turn-on fluorescent probe 1 based on 7-nitro-2,1,3-benzoxadiazole (NBD) with high selectivity and sensitivity for Cys/Hcy on account of nucleophilic substitution and Smiles rearrangement reaction. The probe could sense Cys/Hcy rapidly, the intensity of fluorescence increased immediately within 1min. Furthermore, the probe is low toxic and has been successfully applied to detect intracellular Cys/Hcy by cell fluorescence imaging in living normal and cancer cells.

A NBD-S-rhodamine dyad for dual-color discriminative imaging of biothiols and Cys/Hcy

A fluorescent probe based on fast thiolysis of NBD thioether is developed for dual-color discriminative imaging of Cys and GSH.

Green and Red Fluorescent Dyes for Translational Applications in Imaging and Sensing Analytes: A Dual-Color Flag

Red and green are two of the most-preferred colors from the entire chromatic spectrum, and red and green dyes are widely used in biochemistry, immunohistochemistry, immune-staining, and nanochemistry applications. Selective dyes with green and red excitable chromophores can be used in biological environments, such as tissues and cells, and can be irradiated with visible light without cell damage. This critical review, covering a period of five years, provides an overview of the most-relevant results on the use of red and green fluorescent dyes in the fields of bio-, chemo- and nanoscience. The review focuses on fluorescent dyes containing chromophores such as fluorescein, rhodamine, cyanine, boron-dipyrromethene (BODIPY), 7-nitobenz-2-oxa-1,3-diazole-4-yl, naphthalimide, acridine orange, perylene diimides, coumarins, rosamine, Nile red, naphthalene diimide, distyrylpyridinium, benzophosphole P-oxide, benzoresorufins, and tetrapyrrolic macrocycles. Metal complexes and nanomaterials with these dyes are also discussed.

A highly sensitive fluorescent probe for selective detection of cysteine/homocysteine from glutathione and its application in living cells and tissues

A fluorescent probe NIPY-NBD was designed for detecting Cys/Hcy against GSH and applied in cell/tumor tissue imaging.

A Fluorescent Coumarin-Based Probe for the Fast Detection of Cysteine with Live Cell Application

A new coumarin-based fluorescent probe, containing an allylic esters group, has been designed and synthesized for sensing cysteine in physiological pH. In this fluorescent probe, the coumarin was applied as the fluorophore and an allylic esters group was combined as both a fluorescence quencher and a recognition unit. The probe can selectively and sensitively detect cysteine (Cys) over homocysteine, glutathione, and other amino acids, and has a rapid response time of 30 min and a low detection limit of 47.7 nM. In addition, the probe could be applied for cell imaging with low cytotoxicity.

Thiolysis of NBD-based dyes for colorimetric and fluorescence detection of H2S and biothiols: design and biological applications

H₂S-specific fluorescent/colorimetric probes based on the thiolysis of NBD dyes are summarized.

An aryl-thioether substituted nitrobenzothiadiazole probe for the selective detection of cysteine and homocysteine

An aryl-thioether substituted nitrobenzothiadiazole probe was synthesized and employed to detect cysteine and homocysteine selectively in living cells. Interestingly, both cysteine (Cys) and homocysteine (Hcy) promote an enhancement of the fluorescence intensity of the probe at pH 7.4 while only Cys gives rise to this enhancement under weakly acidic conditions (pH 6.0).

Laminar shear flow increases hydrogen sulfide and activates a nitric oxide producing signaling cascade in endothelial cells

Laminar shear flow triggers a signaling cascade that maintains the integrity of endothelial cells (ECs). Hydrogen sulfide (H2S), a new gasotransmitter is regarded as an upstream regulator of nitric oxide (NO). Whether the H2S-generating enzymes are correlated to the enzymes involved in NO production under shear flow conditions remains unclear as yet. In the present study, the cultured ECs were subjected to a constant shear flow (12 dyn/cm(2)) in a parallel flow chamber system. We investigated the expression of three key enzymes for H2S biosynthesis, cystathionine-γ-lyase (CSE), cystathionine-β-synthase (CBS), and 3-mercapto-sulfurtransferase (3-MST). Shear flow markedly increased the level of 3-MST. Shear flow enhanced the production of H2S was determined by NBD-SCN reagent that can bind to cysteine/homocystein. Exogenous treatment of NaHS that can release gaseous H2S, ECs showed an increase of phosphorylation in Akt(S473), ERK(T202/Y204) and eNOS(S1177). This indicated that H2S can trigger the NO-production signaling cascade. Silencing of CSE, CBS and 3-MST genes by siRNA separately attenuated the phosphorylation levels of Akt(S473) and eNOS(S1177) under shear flow conditions. The particular mode of shear flow increased H2S production. The interplay between H2S and NO-generating enzymes were discussed in the present study.

Ratiometric measurement of hydrogen sulfide and cysteine/homocysteine ratios using a dual-fluorophore fragmentation strategy

Hydrogen sulfide (H₂S) is an integral signaling molecule in biology with complex generation, translocation, and metabolism processes that are intertwined with cellular thiols. Differentiating the complex interplay between H₂S and biological thiols, however, remains challenging due to the difficulty of monitoring H₂S and thiol levels simultaneously in complex redox environments. As a step toward unraveling the complexities of H₂S and thiols in sulfur redox homeostasis, we present a dual-fluorophore fragmentation strategy that allows for the ratiometric determination of relative H₂S and cysteine (Cys) or homocysteine (Hcy) concentrations, two important metabolites in H₂S biosynthesis. The key design principle is based on a nitrobenzofurazan-coumarin (NBD-Coum) construct, which fragments into spectroscopically differentiable products upon nucleophilic aromatic substitution with either H₂S or Cys/Hcy. Measurement of the ratio of fluorescence intensities from coumarin and the NBD-Cys or NBD-Hcy adducts generates a sigmoidal response with a dynamic range of 3 orders of magnitude. The developed scaffold displays a rapid response (<1 min) and is selective for sulfhydryl-containing nucleophiles over other reactive sulfur, oxygen, and nitrogen species, including alcohol- and amine-functionalized amino acids, polyatomic anionic sulfur species, NO, and HNO. Additionally, NBD-Coum is demonstrated to differentiate and report on different oxidative stress stimuli in simulated sulfur pools containing H₂S, Cys, and cystine.

Hydrogen sulfide increases nitric oxide production and subsequent S-nitrosylation in endothelial cells

Hydrogen sulfide (H₂S) and nitric oxide (NO), two endogenous gaseous molecules in endothelial cells, got increased attention with respect to their protective roles in the cardiovascular system. However, the details of the signaling pathways between H₂S and NO in endothelia cells remain unclear. In this study, a treatment with NaHS profoundly increased the expression and the activity of endothelial nitric oxide synthase. Elevated gaseous NO levels were observed by a novel and specific fluorescent probe, 5-amino-2-(6-hydroxy-3-oxo-3H-xanthen-9-yl)benzoic acid methyl ester (FA-OMe), and quantified by flow cytometry. Further study indicated an increase of upstream regulator for eNOS activation, AMP-activated protein kinase (AMPK), and protein kinase B (Akt). By using a biotin switch, the level of NO-mediated protein S-nitrosylation was also enhanced. However, with the addition of the NO donor, NOC-18, the expressions of cystathionine-γ-lyase, cystathionine-β-synthase, and 3-mercaptopyruvate sulfurtransferase were not changed. The level of H₂S was also monitored by a new designed fluorescent probe, 4-nitro-7-thiocyanatobenz-2-oxa-1,3-diazole (NBD-SCN) with high specificity. Therefore, NO did not reciprocally increase the expression of H₂S-generating enzymes and the H₂S level. The present study provides an integrated insight of cellular responses to H₂S and NO from protein expression to gaseous molecule generation, which indicates the upstream role of H₂S in modulating NO production and protein S-nitrosylation.