Fluorescein-Based Chromogenic and Ratiometric Fluorescence Probe for Highly Selective Detection of Cysteine and Its Application in Bioimaging

Coumarin-Based Small-Molecule Fluorescent Chemosensors

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

Fluorescence turn-on sensing of L-cysteine based on FRET between Au-Ag nanoclusters and Au nanorods

The applications of metallic nanoclusters and nanoparticles in biological sensing have attracted special attention owing to their optical interaction based on fluorescence and surface plasmon resonance (SPR). In this work, we designed a fluorescent nanoprobe for the determination of L-cysteine (L-Cys) based on fluorescence resonance energy transfer (FRET) from gold‑silver bimetallic nanoclusters (Au-Ag NCs) to gold nanorods (AuNRs). Firstly, the negatively charged Au-Ag NCs protected by bovine serum albumin (BSA) are directly adsorbed on the surface of the positively charged AuNRs through electrostatic interaction, and the FRET effect leads to distinct fluorescence quenching of Au-Ag NCs at 615nm. The SPR wavelength of AuNRs is dependent on the aspect ratio, so the SPR of AuNRs could be tuned to have a better spectral overlap with fluorescence of Au-Ag NCs, which enhances the fluorescence quenching effect. Because the SH group of L-Cys has an affinity with gold, the addition of L-Cys can result in the release of Au-Ag NCs from the surface of AuNRs via forming AuS bonds. Thus, the introduction of L-Cys could effectively restore the fluorescence emission of the AuNRs/Au-Ag NCs system because of the restraint of FRET effect. Under the optimized conditions, the fluorescence recovery of AuNRs/Au-Ag NCs probe exhibits a linear response to L-Cys concentration ranging from 5 to 100μM, and the corresponding theoretical detection limit (LOD) is 1.73μM. Meanwhile, this method displays excellent sensitivity and selectivity for L-Cys over other amino acids, and it has been successfully applied to detect L-Cys in real samples.

A highly sensitive two-photon fluorescent probe for glutathione with near-infrared emission at 719 nm and intracellular glutathione imaging

A near-infrared turn-on two-photon fluorescent probe ST-BODIPY for glutathione-specific detection was designed and synthesized by attaching triphenylamine to BODIPY skeleton through the Knoevenagel condensation to prolong the maximum emission wavelength to the NIR region. And 2,4-dinitrobenzenesulfonyl group (DNBS), as the fluorescence quencher and thiol recognition moiety, was modified in 8 position of BODIPY. In the presence of GSH, the probe afforded an "off-on" signal response with a significant NIR fluorescence enhancement centered at 719 nm accompanying by quantum yield increased to 0.44, which was ascribed to the glutathione-induced S_NAr (aromatic substitution) reaction. Surprisingly, we found that the probe could discriminate GSH from other biothiols including Cys and Hcy upon the addition of intracellular concentrations of them. Time-dependence also demonstrated that the probe could distinguish GSH from Cys and Hcy under physiological environment. The limit of detection (LOD) for GSH was calculated as 25.46 nM from the titration experiments, which is lower than most previously reported GSH-selective probes. Under the Ti:sapphire pulsed laser's 800 nm irradiation, ST-BODIPY toward GSH generated an "off-on" signal response with a significant enhancement of fluorescence emission at 719 nm after treatment with GSH. Besides, the 2PA cross section value (σ₂) was calculated to be 410 GM, suggesting that it could not only function well as an excellent two-photon fluorescent probe for the detection of intracellular GSH, but also be applied for two-photon imaging with high sensitivity in living cells. Moreover, ST-BODIPY probe has been successfully employed for monitoring exogenous and endogenous GSH in MCF-7 cells with satisfying results, perhaps it was feasible for detecting abnormal contents of GSH in a biological system and accomplishing the goal of maintaining normal human activities.

A New Quinone Based Fluorescent Probe for High Sensitive and Selective Detection of Biothiols and Its Application in Living Cell Imaging

In view of the vital role of biothiols in many physiological processes, the development of simple and efficient probe for the detection of biothiols is of great medical significance. In this work, we demonstrate the use of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), which respond rapidly to biothiols especially to glutathione, as a new fluorescent probe for the selective detection and bioimaging of biothiols. This new fluorescent probe can distinguish glutathione from cysteine and homocysteine easily under physiological concentration and detect glutathione quickly within three minutes. This probe exhibits high selectivity to biothiols and the detection limit was determined to be 3.08 × 10⁻⁹ M for glutathione, 8.55 × 10⁻⁸ M for cysteine, and 2.17 × 10⁻⁹ M for homocysteine, respectively. The sensing mechanism was further explored by density functional theory (DFT) and nuclear magnetic resonance (NMR) experiment; results showed that the interaction forces between the probe and biothiols were electrostatic interaction. In addition, the probe has been successfully applied to the detection of biothiols in Eca9706 cells by fluorescence confocal imaging technology.

Synthesis and spectral analysis of fluorescent probes for Ce4+ and OCl- ions based on fluorescein Schiff base with amino or hydrazine structure: Application in actual water samples and biological imaging

Two Schiff base fluorescein probes (FDA, FDH) based on fluorescein-aldehyde and nitroaniline derivatives were synthesized. The effects of amino and hydrazine substituents in fluorescein backbones were examined via fluorescence and absorbance spectra. In the presence of Ce⁴⁺, the fluorescence of FDA was quenched due to the ligand to metal charge transfer (LMCT). Hypochloric acid can react with the CN bond, and blocking the photo induced electron transfer (PET) of FDH leads to enhancement of the fluorescence. FDA showed detection limits for Ce⁴⁺ and OCl^- as low as 63 nM in concentration range of 0-4 μM. FDH showed detection limits for OCl^- as low as 0.8 μM in concentration rang 0-100 μM. Polyvinylidene fluoride (PVDF) membrane containing the probes was prepared for the real-time qualitative detection of Ce⁴⁺ and OCl^- in real water samples. The probes were successfully applied to biological imaging in vascular smooth muscle cells (VSMCs) and are expected to find applications in biosensing.

An enhanced fluorescence sensor for specific detection Cys over Hcy/GSH and its bioimaging in living cells

Cysteine (Cys) is not only the central matter of sulfur metabolism in cells but also the only amino acid with reduced thiol group in 20 kinds of natural amino acids. In animal cells, Cys is taking part in many important and essential biological functions including protein synthesis, detoxification and metabolism. The development and application of fluorescent probes for the detection of Cys have attracted more and more attention and interest. Herein, we report a new fluorescent probe NFA that utilized naphthyl carboxy fluorescein as fluorophore and acryloyl group as reaction site for Cys specific detection. The probe essentially has weak fluorescence. Cys addition to NFA containing system induced distinct enhanced fluorescence emission which was attributed to the nucleophilic reaction of cysteine and acryloyl to release the fluorophore. The signal fluorescent response detection system allows NFA to be a reliable tool for Cys detection with low detection limit (0.58 μM). And NFA has been successfully applied for Cys imaging specifically in live Hela cells, which promotes the probe as a potential tool to understand the pathology of Cys related diseases.

A ratiometric fluorescent BODIPY-based probe for rapid and highly sensitive detection of cysteine in human plasma

Biological thiols, especially low molecular weight thiols, including cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), play a pivotal role in physiological and pathological systems. Thus, the detection of biothiols is highly important for early diagnosis of diseases and evaluation of disease progression. Herein, we developed a highly selective and sensitive ratiometric fluorescent 8-Cl BODIPY-based probe with high fluorescence quantum yields. The probe displayed a sensitive response to Cys and Hcy over other biothiols, which can be visualized colorimetrically and/or fluorescently. The probe was successfully applied to detect Cys in human plasma, demonstrating its great value for practical application in biological sample analysis.

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˙⁻).

Fluorescein propiolate: a propiolate-decorated fluorescent probe with remarkable selectivity towards cysteine

A fluorescent probe decorated with an alkynyl ester unit (e.g. propiolate) displayed a selective turn-on type fluorescent response towards cysteine.

Functional synthetic probes for selective targeting and multi-analyte detection and imaging

In contrast to the classical design of a probe with one binding site to target one specific analyte, probes with multiple interaction sites or, alternatively, with single sites promoting tandem reactions to target one or multiple analytes, have been developed. They have been used in addressing the inherent challenges of selective targeting in the presence of structurally similar compounds and in complex matrices, as well as the visualization of the in vivo interaction or crosstalk between the analytes. Examples of analytes include reactive sulfur species, reactive oxygen species, nucleotides and enzymes. This review focuses on recent innovations in probe design, detection mechanisms and the investigation of biological processes. The vision is to promote the ongoing development of fluorescent probes to enable deeper insight into the physiology of bioactive analytes.

An efficient ruthenium(ii) tris(bipyridyl)-based chemosensor for the specific detection of cysteine and its luminescence imaging in living zebrafish

A water-soluble, red emissive and cysteine-specific probe has been achieved through 1,4-addition of cysteine to α,β-unsaturated ketones.

A coumarin-based dual optical probe for homocysteine with rapid response time, high sensitivity and selectivity

In this study, a new coumarin-based fluorescent and chromogenic dual channel probe (DC) was used for the selective detection of homocysteine (Hcy) over other amino acids, especially for cysteine (Cys) and glutathione (GSH). When Hcy is present in the solution, the remarkable fluorescence enhancement and obvious blue shift in UV-vis spectra can be observed. In addition, the color change from purple to yellow can be observed clearly by unaided eyes. This probe DC has fast response time, excellent sensitivity and selectivity to Hcy. A linear relationship exists between the ratio of emissions at 486 and 625 nm, and Hcy can be detected in a wide concentration range (0-200 μM). The signal-to-background ratio of fluorescence at 486 nm can reach 8.4, and the detection limit is calculated to be 3.5 µM. The response mechanism is proved to be the Michael addition reaction by Hcy. Preliminary results on cell imaging enable the practical application of Hcy tracing in living cells.

Hierarchical manganese dioxide nanoflowers enable accurate ratiometric fluorescence enzyme-linked immunosorbent assay

We first developed a green, mild and rapid method for the preparation of hierarchical manganese dioxide nanoflowers (MnO2 NFs) as nanozymes with intrinsic oxidase-like activity using citric acid for the reduction of potassium permanganate. The open structure of MnO2 NFs can lead to a larger specific surface area for improving the antibody loading amount and oxidase mimicking activity. In addition, carboxyl groups from the residual citric acid on the surface of MnO2 NFs afford a good affinity with the antibody through an amidation coupling reaction, which does not need complex surface modification anymore. Then, a reliable ratiometric fluorescence nanozyme-linked immunosorbent assay towards C-reactive protein (CRP) was designed successfully based on the MnO2 NFs. The high sensitivity and clinical feasibility of the presented methodology were demonstrated, which hold great promise in biomedical fields. Moreover, the mild and simple process for the synthesis of MnO2 NFs will make good contributions to many other applications.

A two-separated-emission fluorescent probe for simultaneous discrimination of Cys/Hcy and GSH upon excitation of two different wavelengths

It is challenging to simultaneously discriminate two or three biothiols from each other due to their structural similarities as well as reactions sites. The development of multiple-signal fluorescent probes would be a promising way to overcome this issue. Herein, a two-separated-emission fluorescent probe for biothiols was developed based on the combination of nitrobenzofurazan (NBD) and phenanthroimidazole fluorophores linked by a facile ether bond. In the presence of Cys and Hcy, the probe in DMF-H₂O demonstrates two separate fluorescence emissions at 480 and 550 nm upon excitation of two independent wavelengths. However, addition of GSH to the probe only leads to blue fluorescence at 480 nm. This difference can be reasonably ascribed to the fact that the NBD-GSH intermediate, unlike NBD-Cys/Hcy, cannot undergo an intramolecular cyclization-rearrangement reaction. The probe exhibits a rapid response with low limits of detection (14.7 nM for Cys, 14.4 nM for Hcy, and 13.4 nM for GSH) with large concentration ranges of 0-100 μM for Cys/Hcy and 0-200 μM for GSH. Furthermore, the probe is successfully applied to simultaneously distinguish endogenous Cys, Hcy, and GSH in living HeLa cells and zebrafish models.

A Ratiometric Two-Photon Fluorescent Cysteine Probe with Well-Resolved Dual Emissions Based on Intramolecular Charge Transfer-Mediated Two-Photon-FRET Integration Mechanism

The development of an efficient ratiometric two-photon fluorescence imaging probe is crucial for in situ monitoring of biothiol cysteine (Cys) in biosystems, but the current reported intramolecular charge transfer (ICT)-based one suffers from serious overlap between the shifted emission bands. To address this issue, we herein for the first time constructed an ICT-mediated two-photon excited fluorescence resonance energy transfer (TP-FRET) system consisting of a two-photon fluorogen benzo[ h]chromene and a Cys-responsive benzoxadiazole-analogue dye. Different from a previous mechanism that utilized single two-photon fluorogen to acquire a ratiometric signal, ICT was used to switch on the TP-FRET process of the energy transfer dyad by eliciting an absorption shift of benzoxadiazole with Cys to modulate the spectral overlap level between benzo[ h]chromene emission and benzoxadiazole absorption, resulting in two well-separated emission signal changes with large emission wavelength shift (120 nm), fixed two-photon excitation maximum (750 nm), and significant variation in fluorescence ratio (over 36-fold). Therefore, it can be successfully employed to ratiometrically visualize Cys in HeLa cells and liver tissues. Importantly, this new ICT-mediated TP-FRET integration mechanism would be convenient for designing ratiometric two-photon fluorescent probes with two well-resolved emission spectra suitable for high resolution two-photon fluorescence bioimaging.

A series of BODIPY-based probes for the detection of cysteine and homocysteine in living cells

Biothiols, such as glutathione (GSH), homocysteine (Hcy) and cysteine (Cys), are important biomarkers and play crucial roles in many physiological processes. Thus, the detection of biothiols is highly important for early diagnosis of diseases and evaluation of disease progression. Herein, new types of BODIPY-based fluorescent probes (probe 1, probe 2 and probe 3) capable of cysteine (Cys)/homocysteine (Hcy) sensing with high selectivity over other amino acids were developed. In addition, we further studied the influence of different electronegativity substituents on these probes to sensing Cys/Hcy. Ultimately, we concluded that the electron withdrawing group on probe 1 can accelerate the probe response to Cys/Hcy, and probe 1 was successfully applied for selective imaging Cys/Hcy in living cells.

A two-photon ratiometric fluorescent probe for highly selective sensing of mitochondrial cysteine in live cells

We report herein a two-photon ratiometric fluorescent probe (DNEPI) for mitochondrial cysteine (Cys) detection on the basis of a merocyanine (compound 1) as the two-photon fluorophore and a 2,4-dinitrobenzensulfonyl (DNBS) unit as the biothiol reaction site. Upon reaction with Cys in DMSO/PBS (1/1, v/v), DNEPI showed a distinct ratiometric fluorescence emission characteristic (F_{583 nm}/F_{485 nm}) linearly proportional to Cys concentrations over the range of 2-10 μM, which was attribute to the enhanced intramolecular charge transfer (ICT) effect by cleavage of the sulfonic acid ester bond of DNEPI to release compound 1. More importantly, the probe could detect Cys with a fast response time (within 2 min) and the detection limit was quantitatively calculated as 0.29 μM. Furthermore, DNEPI not only exhibited high selectivity toward Cys over other similar biothiols, including homocysteine (Hcy) and glutathione (GSH), but also displayed significant mitochondrial-targeting ability, which were favorable for mitochondrial Cys-selective imaging. Subsequently, application of DNEPI to Cys imaging in live cells was successfully achieved by two-photon fluorescence microscopy, suggesting that the probe proposed here could be used to monitor mitochondrial Cys concentration changes in live cells with negligible interference from other biological thiols.

Coumarin/fluorescein-fused fluorescent dyes for rapidly monitoring mitochondrial pH changes in living cells

On base of the good optical properties of coumarin and fluorescein, we designed and synthesized two coumarin/fluorescein-fused fluorescent dyes (CF dyes), which enlarged the emission wavelength and increased the Stokes shift of fluorescein moiety. The corresponding optical properties of CF dyes were investigated in detail. CF dyes could easily introduce other groups to design different functional molecules. CF dyes also exhibited rapid and sensitive responses to pH values in the range of 4.0-7.4 through the characterization of absorption and fluorescence spectra in buffer solution. More importantly, CF ethyl ester dye (CFE dye) not only showed good cell membrane permeability and low cytotoxicity, but also had the ability to rapidly monitor mitochondrial pH changes in living cells.

Electrochemical Molecular Switch for the Selective Profiling of Cysteine in Live Cells and Whole Blood and for the Quantification of Aminoacylase-1

A first-of-a-kind latent electrochemical redox probe, ferrocene carbamate phenyl acrylate (FCPA), was developed for the selective detection of cysteine (Cys) and aminoacylase (ACY-1). The electrochemical signal generated by this probe was shown to be highly specific to Cys and insensitive to other amino acids and biological redox reactants. The FCPA-incorporated electrochemical sensor exhibited a broad dynamic range of 0.25-100 μM toward Cys. This probe also proficiently monitored the ACY-1-catalyzed biochemical transformation of N-acetylcysteine (NAC) into Cys, and this proficiency was used to develop an electrochemical assay for quantifying active ACY-1, which it did so in a dynamic range of 10-200 pM (0.1-2 mU/cm³) with a detection limit of 1 pM (0.01 mU/cm³). Furthermore, the probe was utilized in real-time tracking and quantification of cellular Cys production, specifically in Escherichia coli W3110, along with a whole blood assay to determine levels of Cys and spiked ACY-1 in blood with a reliable analytical performance.

BODIPY-based turn-on fluorescent probes for cysteine and homocysteine

Cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are interconnected and play essential roles in many biological processes. It is significant importance to detect these thiols for investigating their functions in cells and disease diagnosis. In this work, we have designed and synthesized two novel BODIPY-based turn-on fluorescent probes (BDP-Ph and BDP-R-Ph) carrying 4-methoxythiophenol moiety at meso position as good leaving group for highly selective detection of Cys and Hcy. Furthermore, the probes have been successfully applied to detect intracellular Cys and Hcy by fluorescent imaging in living cells.

A near-infrared fluorescent probe for direct and selective detection of cysteine over homocysteine and glutathione

In this work, we have designed and synthesized the fluorescent probe 1, which showed a highly selective and sensitive response to Cys over Hcy/GSH in the test. Moreover, the color of probe solution has changed dramatically from colorless to pink with the addition of Cys within 10 min. Meanwhile, the fluorescence intensity exhibited perfectly positive correlation with concentration of Cys from 0 to 200 μM, which offered the important condition for quantitative analysis. Finally, the bioimaging and fluorescence response of probe 1 for fetal calf serum are a powerful safeguard for practical detection of Cys. Therefore, this near-infrared probe will be of great benefit for detecting Cys in the biological systems.

Fluorescent probes for detecting cysteine

Cysteine plays a crucial role in physiological processes. Therefore, it is necessary to develop a method for detecting cysteine. Fluorimetry has the advantages of convenient detection, short response time, high sensitivity and good selectivity. In this review, fluorescent probes that detect cysteine over the past three years are summarized based on structural features of fluorophores such as coumarin, BODIPY, rhodamine, fluorescein, CDs, QDs, etc and reaction groups including acrylate, aldehyde, halogen, 7-nitrobenzofurazan, etc. Then, effects of different combinations between fluorophores and response groups on probe properties and detection performances are discussed.

Protein Fibril-Templated Biomimetic Synthesis of Highly Fluorescent Gold Nanoclusters and Their Applications in Cysteine Sensing

Biomimetic synthesis of multifunctional fluorescent gold nanoclusters (Au NCs) is of great demand because of their ever-increasing applications. In this study, we have used self-assembled bovine serum albumin (BSA) amyloid-like nanofibers as the bioinspired scaffold for the synthesis of Au NCs. The amyloid fibril stabilized gold nanocluster (Fib-Au NC) has been found to have appreciable enhancement of fluorescence emission and a large 25 nm red shift in its emission maxima when compared to its monomeric protein counterpart (BSA-Au NC). The underlying mechanism accountable for the fluorescence behavior and its spectral shift has been thoroughly investigated by a combined use of spectroscopic and microscopic techniques. We have subsequently demonstrated the use of Fib-Au NCs for cysteine (Cys) sensing both in vitro and inside live cells. Additionally, cellular uptake and postpermeation effect of Fib-Au NCs have also been ascertained by detailed flow cytometry analysis, viability assay, and real-time apoptotic gene expression profiling.

Glutathione-driven Cu(i)-O2 chemistry: a new light-up fluorescent assay for intracellular glutathione

Besides its widely known role as an endogenous antioxidant in scavenging free radicals, glutathione (GSH) can also play the role of prooxidant and promote CuO-induced formation of hydroxyl radicals to light up a fluorescent signal through Cu(i)-O2 chemistry without requiring additional H2O2. This approach is independent of the mechanisms of enzyme mimics, such as the well-known oxidase and peroxidase mimetics, providing a new method to simply and effectively analyze intracellular GSH.

Detection of analytes in mitochondria without interference from other sites based on an innovative ratiometric fluorophore

Mitochondria are vital organelles that not only produce cellular energy but also participate in many biological processes. Recently, various fluorescent probes have been developed for mitochondrial imaging. However, due to the lack of suitable dyes or strategies, it is difficult for most reported mitochondrial targeting probes to prove whether the analytes they detected are from mitochondria. In addition, positive charge on mitochondrial probes can seriously affect the mitochondrial environment. To address these issues, we herein put forward a novel strategy for probe design based on a smart NIR dye (HDFL) for mitochondrial targeting detection. Compared to general mitochondrial targeting probes that are modified with a target site and a reaction site, the new strategy is to combine the two sites together for a mitochondrial probe that would provide accurate detection of analytes in mitochondria without interference. As a proof of concept, we synthesized a mitochondrial-targetable probe HDFL-Cys for cysteine. Bioimaging studies have shown that the new type of probe HDFL-Cys can first accumulate in mitochondria and then react with the analyte (cysteine) accompanied by the departure of the targeting group (lipophilic cation moieties). Thus, it can specifically detect the analyte in mitochondria without interference from extra-mitochondrial analytes. We anticipate that the new strategy based on the novel NIR dye HDFL may be a potential platform for developing desirable ratiometric fluorescent probes for mitochondrial imaging.

Highly selective and ratiometric fluorescent nanoprobe for the detection of cysteine and its application in test strips

Cysteine (Cys) is a bithiol that plays a vital role in many physiological processes. However, it is difficult to discriminate Cys from homocysteine (Hcy) and glutathione (GSH), due to their similar chemical structures and reactivity. Herein, we have developed a polymeric nanoprobe, nanoHFA, for ratiometric, highly selective, and sensitive detection of Cys based on 7-hydroxycoumarin-3-carboxylic acid (HC) and fluorescein isothiocyanate (FITC)-acrylate (FITC-A) group-functionalized lipopolymer DSPE-PEG. The probe nanoHFA showed a strong fluorescence emission peak centered at 450 nm attributed to HC and a weak fluorescence emission peak centered at 520 nm due to the photoinduced electron transfer (PET) process of FITC induced by acrylate group. In the presence of Cys, the fluorescence signal at 520 nm could be lit up and the ratio of F_520nm/F_450nm showed a good linear relationship in the range of 5-60 μM with a low detection limit of 0.37 μM. The probe also displayed excellent water solubility and high selectivity to Cys over other biothiols such as Hcy and GSH. Moreover, we further used probe nanoHFA to detect Cu²⁺ ions in the range of 100-550 nM with a detection limit of 77 nM. The nanoprobe was successfully applied for the quantitative detection of Cys in fetal bovine serum, and fluorescent strips were developed for facile and visual detection of Cys and Cu²⁺ ions. Graphical abstract ᅟ.

Dually emitting gold-silver nanoclusters as viable ratiometric fluorescent probes for cysteine and arginine

Glutathione coated gold and silver nanoclusters (GSH-Au/AgNCs) were synthesized by one-pot reduction methods and are found to be viable fluorescent nanoprobes for cysteine (Cys) and arginine (Arg), with good selectivity over other amino acids. The GSH-Au/AgNCs have two emissions at 616 nm and 412 nm when excited at 360 nm. With the increased concentration of Cys, the ratio of the emission intensities (I₆₁₆/I₄₁₂) linearly decreases with Cys in concentration ranging from 0.05 to 10 μM and from 10 to 50 μM, respectively. With increased concentrations of Arg, the ratio of I₆₁₆/I₄₁₂ linearly decreases with Arg concentration ranging from 0 to 50 μM and from 50 to 100 μM, respectively. The probe was applied to the determination of Cys and Arg in spiked samples of serum and urine where it gave good recoveries. Graphical abstract Glutathione-coated gold and silver nanoclusters (GSH-Au/AgNCs) were synthesized by one-pot reduction and are found to be viable fluorescent nanoprobes for cysteine (Cys) and arginine (Arg).

Sensitive fluorescence on-off probes for the fast detection of a chemical warfare agent mimic

Two highly sensitive probes bearing a nucleophilic imine moiety have been utilized for the selective detection of chemical warfare agent (CWA) mimics. Diethyl chlorophosphate (DCP) was used as mimic CWAs. Both iminocoumarin-benzothiazole-based probes not only demonstrated a remarkable fluorescence ON-OFF response and good recognition, but also exhibited fast response times (10s) along with color changes upon addition of DCP. Limits of detection for the two sensors 1 and 2 were calculated as 0.065μM and 0.21μM, respectively, which are much lower than most other reported probes. These two probes not only show high sensitivity and selectivity in solution, but can also be applied for the recognition of DCP in the gas state, with significant color changes easily observed by the naked eye.

N-Doped Graphene Quantum Dots-Decorated V2O5 Nanosheet for Fluorescence Turn Off-On Detection of Cysteine

The development of a fast-response sensing technique for detection of cysteine can provide an analytical platform for prescreening of disease. Herein, we have developed a fluorescence turn off-on fluorescence sensing platform by combining nitrogen-doped graphene quantum dots (N-GQDs) with V₂O₅ nanosheets for the sensitive and selective detection of cysteine in human serum samples. V₂O₅ nanosheets with 2-4 layers are successfully synthesized via a simple and scalable liquid exfoliation method and then deposited with 2-8 nm of N-GQDs as the fluorescence turn off-on nanoprobe for effective detection of cysteine in human serum samples. The V₂O₅ nanosheets serve as both fluorescence quencher and cysteine recognizer in the sensing platform. The fluorescence intensity of N-GQDs with quantum yield of 0.34 can be quenched after attachment onto V₂O₅ nanosheets. The addition of cysteine triggers the reduction of V₂O₅ to V⁴⁺ as well as the release of N-GQDs within 4 min, resulting in the recovery of fluorescence intensity for the turn off-on detection of cysteine. The sensing platform exhibits a two-stage linear response to cysteine in the concentration range of 0.1-15 and 15-125 μM at pH 6.5, and the limit of detection is 50 nM. The fluorescence response of N-GQD@V₂O₅ exhibits high selectivity toward cysteine over other 22 electrolytes and biomolecules. Moreover, this promising platform is successfully applied in detection of cysteine in human serum samples with excellent recovery of (95 ± 3.8) - (108 ± 2.4)%. These results clearly demonstrate a newly developed redox reaction-based nanosensing platform using N-GQD@V₂O₅ nanocomposites as the sensing probe for cysteine-associated disease monitoring and diagnosis in biomedical applications, which can open an avenue for the development of high performance and robust sensing probes to detect organic metabolites.

Metal-assisted selective recognition of biothiols by a synthetic receptor array

A synergistic combination of a deep cavitand host, fluorophore guests and transition metal ions can be used to sense small molecule thiols of biological interest with good efficiency and selectivity in complex aqueous media.

Development of a two-photon turn-on fluorescent probe for cysteine and its bio-imaging applications in living cells, tissues, and zebrafish

A two-photon fluorescent probe Co-Cys for detecting cysteine has been designed to monitor cysteine in cells, tissues and zebrafish.

A NIR facile, cell-compatible fluorescent sensor for glutathione based on Michael addition induced cascade spirolactam opening and its application in hepatocellular carcinoma

A NIR fluorescence probe with NIR emission wavelength at 746 nm and high quantum yield of 0.36 was designed and synthesized to selectively detect GSH over Hcy and Cys in living systems.

Competition between two cysteines in covalent binding of biliverdin to phytochrome domains

In this work, we disclose a mechanism of competing chemical reactions of protein assembly for a bacterial phytochrome using modern methods of molecular modeling.

A Michael addition–cyclization-based switch-on fluorescent chemodosimeter for cysteine and its application in live cell imaging

We designed and synthesized a fast response fluorescent probe, BTAC (benzothiazol-azacoumarin), for detection of cysteine (Cys).