Conjugate addition/cyclization sequence enables selective and simultaneous fluorescence detection of cysteine and homocysteine

A dihydronaphthalene based fluorescence probe for sensitive detection of cysteine and its application in bioimaging

A novel small molecule fluorescent probe NC-Cys for monitoring cysteine based on dihydronaphthalene was developed.

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.

Quinolone-based fluorescent probes for distinguished detection of Cys and GSH through different fluorescence channels

Dual-channel discrimination of Cys and GSH using a red fluorescent probe.

Molecular probe designviathe “covalent-assembly” principle

Fluorescent probes are useful molecular tools. We summarize the recent progress with the “covalent-assembly” design principle, which warrants high-performance fluorescence probes exhibiting a highly sensitive turn-on signal from the dark background.

BODIPY based realtime, reversible and targeted fluorescent probes for biothiol imaging in living cells

Real-time live cell imaging and quantification of biothiol dynamics are important for understanding pathophysiological processes.

Mutual correlation evaluation of Cys and Hcy in serum through reaction activity regulated fluorescence quantification

We report herein a strategy involving using a single fluorescent probe with the assistance of two pH conditions to simultaneously detect cysteine and homocysteine quantitatively.

A turn-on fluorescence probe for cysteine/homocysteine based on the nucleophilic-induced rearrangement of benzothiazole thioether

A fluorescent probe, 4-(benzothiazole-2-ylthio)-7-nitro-2,1,3-benzoxadiazole (TBT-NBD) was developed for cysteine (Cys) and homocysteine (Hcy). The reaction mechanism was based on the Cys/Hcy-induced nucleophilic substitution of benzothiazole thioether then Smiles rearrangement reaction to form corresponding amino-nitrobenzoxadiazole, which emitted yellow-green fluorescence and guaranteed the high selectivity for Cys/Hcy over glutathione (GSH). TBT-NBD could detect Cys/Hcy within 5 min in the presence of high concentration of GSH and other amino acids. Moreover, TBT-NBD had been exploited to identify intracellular Cys/Hcy in living cells in light of its low toxicity.

A lysosome-targetable fluorescent probe for real-time imaging cysteine under oxidative stress in living cells

As an effective lysosomal biomarker for oxidative stress status, cysteine (Cys) plays an important role in lysosomal proteolysis. Herein, we report the first lysosome-targetable fluorescence probe (MCAB) for Cys-selective detection based on nucleophilic addition reaction of sulfhydryl toward a α, β-unsaturated ketone and demonstrate its application to lysosomal-targetable imaging. MCAB is designed based on a α, β-unsaturated ethanoylcarbazole as the fluorophore and the thiols reaction site, and a methylcarbitol unit as a lysosome-targetable group. Upon reacting with Cys, this probe turns on highly specific fluorescence signals linearly proportional to Cys concentrations over the range of 0-300 μM. MCAB detects Cys with a rapid response time (within 12 min) and low limit of detection (0.38 μM). MCAB is highly selective to Cys over other similar biothiols including homocysteine (Hcy) and glutathione (GSH). Moreover, it also exhibits significant lysosomal-targetable ability, which is ideal for lysosomal Cys-selective imaging. Using MCAB, we have successfully visualized the fluctuation endogenous Cys in lysosomes under oxidative stress status in real-time.

A Novel ESIPT Phthalimide-Based Fluorescent Probe for Quantitative Detection of H2O2

Hydrogen peroxide (H₂O₂) is a majority reactive oxygen species (ROS) and acts as an essential role in pathological and physiological processes. Therefore, the development of quantitative detection of methods for H₂O₂ is necessary. Here, we constructed of a novel simple fluorescence probe for detection of H₂O₂ based on the excited-state intramolecular proton transfer process. The probe utilized a phthalimide derivative as the fluorophore and selected phenylboronic acid as the recognition site for H₂O₂. In response to H₂O₂, the probe exhibited 63-fold fluorescence intensity enhancement, a low detection limit (8.4 × 10^-8 M), and large Stokes shift (111 nm). In addition, the probe displayed high selectivity for H₂O₂ over other ROS. Moreover, the probe was successfully employed for imaging of H₂O₂ in living cells.

Exploring cysteine regulation in cancer cell survival with a highly specific "Lock and Key" fluorescent probe for cysteine

Using a highly specific “lock and key” fluorescent Cys probe, we confirmed that targeting Cys metabolism to deplete intracellular Cys is a more potent strategy to sensitize cancer cells to chemotherapies.

To probe the regulatory roles of cysteine (Cys) in cancer cell survival, a highly selective and sensitive fluorescent Cys probe SiR was developed by employing a novel “lock and key” strategy, which allows Cys to be detected without any interference or probe consumption caused by the intracellular high concentration of glutathione (GSH). Using SiR, we confirmed that inhibiting cystine (Cys₂) transporter system x_c^– to deplete intracellular Cys is more efficient than inhibiting glutamate–cysteine ligase GCL to deplete intracellular GSH for sensitizing cancer cells to chemotherapy. Moreover, with the probe, a possible self-protection mechanism of cancer cells was indicated: when extracellular Cys sources are blocked, cancer cells could still survive by multidrug resistance protein transporter (Mrp1)-mediated export of intracellular GSH/GSSG as sources to supply intracellular Cys for resisting detrimental oxidative stress. Based on this finding, we further confirmed that abrogating the self-protection mechanism is an even more efficient strategy for sensitizing cancer cells to chemotherapy.

A 7-Hydroxybenzoxazinone-Containing Fluorescence Turn-On Probe for Biothiols and Its Bioimaging Applications

In this work, a novel 7-hydroxybenzoxazinone-based fluorescent probe (PBD) for the selective sensing of biothiols is reported. Upon treatment with biothiols, PBD shows a strong fluorescence enhancement (up to 70-fold) and a large Stokes shift (155 nm). Meanwhile, this probe exhibits high resistance to interference from other amino acids and competing species. PBD features good linearity ranges with a low detection limit of 14.5 nM for glutathione (GSH), 17.5 nM for cysteine (Cys), and 80.0 nM for homocysteine (Hcy), respectively. Finally, the potential utility of this probe for biothiol sensing in living HeLa cells is demonstrated.

A HBT-based bifunctional fluorescent probe for the ratiometric detection of fluoride and sulphite in real samples

Based on a core of 2-(benzo[d]thiazol-2-yl)phenol (HBT), a bifunctional ratiometric fluorescent probe, HBT-FS, was constructed for the discriminative detection of fluoride (F^-) and sulphite (SO₃^2-) with high sensitivity and selectivity. HBT-FS itself displayed a green fluorescence with λ_Em^max = 498 nm. The treatment of HBT-FS with F^- resulted in a red fluorescence (λ_Em^max = 634 nm) with a large Stokes shift and a 291-fold enhancement in the ratio of the fluorescence intensity (I_{634 nm}/I_{498 nm}). Upon the addition of SO₃^2-, HBT-FS exhibited a blue fluorescence (λ_Em^max = 371 nm) and the ratiometric fluorescence enhancement was remarkable (9445 folds for I_{371 nm}/I_{498 nm}). HBT-FS was successfully used to qualitatively and quantitatively determine F^- and SO₃^2- in a ratiometric manner in real samples.

Michael Addition/S,N-Intramolecular Rearrangement Sequence Enables Selective Fluorescence Detection of Cysteine and Homocysteine

Acrylate has been widely used as the recognition unit for Cys fluorescent probes. Despite this widespread use, a potential drawback of this probe type is that the ester linkage between the fluorophore and acryloyl recognition unit is liable to be hydrolyzed by abundant esterase in the cytosol, thus affording a high background signal. To solve this problem, we herein put forward a new strategy to construct a selective fluorescent probe for cysteine (Cys)/homocysteine (Hcy) with propynamide as the recognition moiety. The free probe CPA displays weakly fluorescent emission in aqueous media because of the donor-excited photoinduced electron transfer (d-PET) process within the molecule. The Michael addition of Cys (or Hcy) thiols to the conjugated alkyne of CPA gives the expected β-sulfido-α,β-unsaturated amides (1a/1b), which subsequently undergo an intramolecular S,N rearrangement, yielding β-amino-α,β-unsaturated amides (2a/2b) as the final products. The above cascade reaction results in the blockage of d-PET within CPA, thus affording a dramatic fluorescence enhancement at 495 nm. The involvement of the sulfhydryl and the adjacent amino groups in the sensing process renders CPA high selectivity for Cys/Hcy over glutathione as well as other amino acids. The probe has been successfully applied to image Cys in different cell lines. Further, CPA shows two-photon fluorescence properties, and its ability to monitor Cys in deep tissues has been demonstrated by using two-photon microscopy.

Photoluminescence and Electrochemiluminescence Dual-Signaling Sensors for Selective Detection of Cysteine Based on Iridium(III) Complexes

Cysteine (Cys) is important in biosynthesis, detoxification, and metabolism. The selective detection of Cys over structurally similar homocysteine (Hcy) or glutathione (GSH) remains an immense challenge. Although there are many methods for detecting Cys, photoluminescence (PL) and electrochemiluminescence (ECL) techniques are well-suited for clinical diagnostics and analytical technology because of their high sensitivities. Herein, we report PL and ECL dual-channel sensors using cyclometalated iridium(III) complexes for the discrimination of Cys from Hcy and GSH. The sensors react with cysteine preferentially because of kinetic differences in intramolecular conjugate addition/cyclization, enabling phosphorescence enhancement and ECL decrease in the blue-shifted region. Sensor 1 shows ratiometric PL turn-on and ECL turn-off for Cys. In addition, unique ECL-enhancing behavior of sensor 1 toward GSH enables discrimination between Cys and GSH. Sensor 1 was successfully applied to the detection of Cys in human serum by the ECL method. We demonstrate the first case of a Cys-selective PL and ECL dual-channel chemodosimetric sensor based on cyclometalated iridium(III) complexes and expect that the rational design of efficient PL and ECL dual-channel sensors will be useful in diagnostic technology.

Biothiol-Activatable Bioluminescent Coelenterazine Derivative for Molecular Imaging in Vitro and in Vivo

There is a high demand for sensitive biothiol probes targeting cysteine, glutathione, and homocysteine. These biothiols are known as playing essential roles to maintain homeostasis and work as indicators of many diseases. This work presents a bioluminescent probe (named AMCM) to detect biothiols in live mammalian cells and in vivo with a limit of detection of 0.11 μM for cysteine in solution and high selectivity for biothiols, making it suitable for real-time biothiol detection in biological systems. Upon application to live cells, AMCM showed low cytotoxicity and sensitively reported bioluminescence in response to changes of biothiol levels. Furthermore, a bioluminescence resonance energy transfer system consisting of AMCM combined with the near-infrared fluorescent protein iRFP713 was applied to in vivo imaging, with emitted tissue-permeable luminescence in living mice. In summary, this work demonstrates that AMCM is of high practical value for the detection of biothiols in living cells and for deep tissue imaging in living animals.

Simultaneous recognition of cysteine and cytosine using thiophene-based organic nanoparticles decorated with Au NPs and bio-imaging of cells

Biomolecules like cysteine and cytosine play a significant role in many physiological processes, and their unusual level in biological systems can lead to many diseases including cancer. Indeed, the need for selective detection of these moieties by a fluorescence probe is imperative. Thus, thiophene based Schiff N,N'-bis(thiophene-2-ylmethylene)thiophenemethane (BMTM) was synthesized and then characterized using several analytical techniques before converting it into organic nanoparticles (ONPs). Then, fluorescent organic inorganic nanohybrids (FONs) were obtained after decorating ONPs with AuNPs to yield BMTM-Au-ONPs (FONPs). The morphology of the particles, analyzed using a Transmission Electron Microscope (TEM), shows that AuNPs were embedded with low density organic matter (ONPs). FONPs were employed to recognize cysteine and cytosine simultaneously. No interference was observed from other moieties such as guanine, uracyl, NADH, NAD, ATP, and adenine during the detection. It means that the intensity of the fluorescence signal was significantly changed (enhanced for cytosine and quenched for cysteine). So, FONPs were used to detect cysteine and cytosine in real samples, like Saccharomyces cerevisiae cells. As expected, no considerable fluorescence signal for cysteine was observed, while for cytosine, strong fluorescence signals were detected in the cells. DFT was used to explain the interaction of FONPs with cysteine or cytosine.

Development of a new fluorescent probe for cysteine detection in processed food samples

Cysteine is a crucial amino acid, found in a huge amount in protein-rich foods. We focused our research to determine the amount of free cysteine consumed highly in foods such as pork, beef, poultry, eggs, dairy, red peppers, soybeans, broccoli, brussels sprouts, oats, and wheat germs. A newly designed carbazole-pyridine-based fluorescent probe (CPI) has been introduced for quantitative estimation of cysteine (Cys) with a "turn on" fluorescence in some popular processed food samples chosen from our daily diet. CPI shows both naked eye and UV-visible color changes upon interaction with cysteine. The binding approach between CPI and Cys at biological pH has been thoroughly explored by UV-visible and fluorescence spectroscopy. From Job's plot analysis, 1:1 stoichiometric reaction between CPI and Cys is observed with a detection limit of 3.8 μM. NMR, ESI mass spectrometry, and time-dependent density functional theory (TD-DFT) study enlightens the formation of more stable product CPI-Cys. The "turn on" response of the probe CPI occurs due to the interruption of intra-molecular charge transfer (ICT) process upon reacting with cysteine. Moreover, CPI is a very stable, cost-effective compound and exhibits excellent real-time selectivity towards Cys over all other comparative biorelevant analytes. Interestingly, our proposed method is much advantageous as it is able to estimate cysteine predominantly by screening out other comparative biocomponents found in different protein-rich foods.

Simply Structured Conjugated Compounds with Cyanoacrylate or Acrylonitrile Groups for Sensing of p-Toluenethiol

Simply structured conjugated compounds with cyanoacrylate (CA 1 - 4) and acrylonitrile (AN 1 - 4) terminal groups were synthesized by a Knoevenagel condensation reaction in one step and investigated for their recognition properties to p-toluenethiol by UV-vis, fluorescence spectra, and FT-IR measurements. When p-toluenethiol was added to CA 1, the FT-IR spectra revealed a cleavage of alkene caused by the addition reaction between p-toluenethiol and CA 1. An increase in p-toluenethiol concentration, a blue-shifted absorption band, and a decrease in the fluorescence intensity of CA 1 were observed because of the decrease in its effective conjugated length. Therefore, the most simply structured CA 1 was found to be the most effective and the most sensitive chemical sensor for p-toluenethiol.

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.

A novel 2,5-bis(benzo[d]thiazol-2-yl)phenol scaffold-based ratiometric fluorescent probe for sensing cysteine in aqueous solution and serum

An efficient and novel 2,5-bis(benzo[d]thiazol-2-yl)phenol scaffold-based ratiometric fluorescent probe BTP-Cys for the sensing of cysteine has been developed. The probe BTP-Cys with acrylates moiety, as recognition site, has been successfully constructed on account of the excited state intramolecular proton transfer (ESIPT) mechanism. Upon the treatment with Cys (0-250 μM), this probe BTP-Cys exhibits a dramatic fluorescent intensity ratios enhancement (from 0.03 to 18.3) and a large emission shift (113 nm). The detection limit of this probe is as low as 3.8 × 10^-7 M. Importantly, the concentration and time dependent of Cys in bovine serum albumin (BSA) has also been measured, indicating that BTP-Cys could be a biocompatible and rapid probe for Cys in vitro.

A computational study on H2S release and amide formation from thionoesters and cysteine

The recognition of the biological activity of H2S has drawn much attention to the development of biocompatible H2S release reactions. Thiol-, particularly cysteine-triggered systems which mimic the enzymatic conversion of cysteine or homocysteine to H2S have been intensively reported recently. Herein, a density functional theory (DFT) study was performed to address the reaction mechanism of H2S release and potential amide bond formation from thionoesters and cysteine to gain deeper mechanistic insights. Three possible mechanisms were considered and we found that the one starting from the nucleophilic addition of the ionized mercapto of cysteine on thionoester to generate a dithioester intermediate (Path A) is kinetically favored over the others starting from the nucleophilic addition of the amine of cysteine to generate thionoamide intermediates (Paths B and C). Dithioester then undergoes intramolecular nucleophilic addition of an amine group and the rate-limiting water-assisted proton transfer to generate a cyclic thiol intermediate, and finally affords H2S and dihydrothiazole via water-assisted elimination. The hydrolysis of thionoamide or dihydrothiazole to produce amide is highly difficult under neutral conditions but is operative under strong basic conditions, which explains the experimental observation that dihydrothiazole rather than amide is the major product. Meanwhile, the ring opening reaction of the cyclic thiol intermediate to form the more stable thionoamide is detrimental to H2S release and becomes competitive under basic conditions.

Highly specific monitoring and imaging of endogenous and exogenous cysteine in living cells

Cys is one of the important biothiols and its abnormal concentration may pose a threat to human health. Therefore, the monitoring of Cys in organisms is of great significance. GSH and Hcy, as the other two biothiols, have similar chemical structures and active sites to Cys. Consequently, developing fluorescent probes to independently detect Cys has become a challenging problem. Keeping this in mind, α-β unsaturated ketone as a recognition group was integrated into the coumarin group skeleton to synthesize a fluorescent probe SC. After the nucleophilic addition reaction of Cys with SC, the conjugated system of SC was blocked and the fluorescent enhanced obviously. SC was able to detect Cys specifically under the same excitation with a low detection limit (11.1 nM). SC showed a rapid respond to Cys (120 s) and good fluorescent stability over a wide pH range. In addition, it achieved extracorporeal circulation in the presence of H₂O₂ or NEM. In the end, SC could be applied to detecting endogenous and exogenous Cys under biological condition due to its slight cytotoxicity and good biocompatibility. This provided a powerful tool for studying the physiological function of Cys exclusively.

A novel probe for colorimetric and near-infrared fluorescence detection of cysteine in aqueous solution, cells and zebrafish

Cysteine(Cys) is tightly related to physiological and pathological of human, and the imbalance of concentration of cysteine in the intracellular are associated with many diseases. Here, a novel NIR fluorescent probe TCF-Cys was designed and synthesized, and both the optimal excitation and emission wavelength of them were between 650 and 900 nm, that within the "optical window" of biological tissues. In aqueous solution, TCF-Cys, which with an acrylate extremity as a recognizing unit, exhibited excellent "turn-on" fluorescence response for Cys superior to other amino acids and thiols with a limit of detection of 0.1323 μM. Moreover, as an excellent naked-eye colorimetric indicator, TCF-Cys could effectively distinguishing the Cys, Hcy and GSH in aqueous solution through color change. Then, the response mechanism of TCF-Cys for Cys was revealed by TLC, ¹H NMR, HPLC, HRMS and DFT calculation. Finally, TCF-Cys was successfully employed to fluorescence specifically map of exogenous and endogenous Cys in living cells and zebrafish with low toxicity.

Fluorescent probe for sensitive discrimination of Hcy and Cys/GSH in living cells via dual-emission

Abnormal levels of Cys, Hcy and GSH are associated with various diseases, thus monitoring biothiols is of great significance. In this work, a dual-emission responsive near-infrared fluorescent probe NIR-NBD for detecting Hcy and Cys/GSH was developed based on the conjugation of a dicyanoisophorone based fluorophore (NIR-OH) and 7-nitrobenzofurazan (NBD). To our surprise, the addition of Hcy induced significant fluorescence enhancement at both 549 and 697 nm; while Cys/GSH resulted in major fluorescence emission at 697 nm. The detection limit was determined to be 33.2 nM for Cys, 33.5 nM for Hcy, and 34.4 nM for GSH. Therefore, the probe can be used for discriminative detection of Hcy and Cys/GSH. Moreover, fluorescence imaging of HeLa cells indicated that the probe was cell membrane permeable and could be used for visualizing Hcy and Cys/GSH in living cells.

Fluorometric determination and intracellular imaging of cysteine by using glutathione capped gold nanoclusters and cerium(III) induced aggregation

A turn-on fluorometric method is described for selective and sensitive detection of cysteine (Cys). Gold nanoclusters (Au NCs) capped with glutathione (GSH) are used as a fluorescent probe. If Ce³⁺ ion are present, they will bind to the carboxy groups of the GSH-capped Au NC. This results in aggregation-induced emission enhancement (AIEE), best measured at excitation/emission wavelengths of 360/575 nm. On addition of Cys, which has less steric hindrance compared with GSH and higher affinity for Ce³⁺, it will bind to Ce³⁺ through the carboxyl group and link with Au NCs via Au-S bond. Hence, the AIEE is increased and Cys can be quantified via this effect with a linear response in the 0.4-120 μmol L^-1 Cys concentration range and a detection limit of 0.15 μmol L^-1. Graphical abstract Schematic presentation of cysteine detection via the Ce³⁺-triggered aggregation of glutathione capped gold nanoclusters which leads to increased yellow fluorescence.

Thiol-ene click reaction-induced fluorescence enhancement by altering the radiative rate for assaying butyrylcholinesterase activity

Butyrylcholinesterase (BChE) generally acts as an important plasma biomarker for clinical diagnosis due to its major contribution to human plasma cholinesterase levels, but its current fluorometric assay relying on fluorogenic substrates frequently suffers from the lack of sufficiently fast response time and specific recognition of substrates relative to the traditional Ellman's method. In this work, we report a fluorescent molecular probe for assaying BChE activity based on thiol-triggered fluorescence enhancement via thiol-ene click reactions. A low-temperature experiment and theoretical analysis exclude the possibility of weak fluorescence of the probe caused by an intramolecular photoinduced electron transfer process and support the main cause of an ultraslow radiative rate due to the introduction of two acrylyl groups. This probe has sensitive fluorescence responses to thiols via thiol-ene click chemistry, and it can distinguish between glutathione and cysteine or homocysteine in different emission colors. The rapid reaction kinetics of this probe enables it to monitor hydrolysis reactions catalyzed by butyrylcholinesterase (BChE) in a real-time manner. This probe is used to develop the first fluorometric assay of BChE activity based on fluorescence enhancement triggered by thiol-ene click chemistry using butyrylthiocholine as the substrate. The established BChE assay shows excellent sensitivity, and is capable of avoiding the interference from glutathione and acetylcholinesterase (AChE) in a complex matrix. The inhibition test of tacrine on BChE with this assay substantiates its feasibility in screening potential inhibitors of BChE. This work demonstrates a design strategy of fluorescent probes lighted up by thiol-ene click reactions, reveals the main cause of thiol-triggered fluorescence enhancement by altering the radiative rate, and provides the first fluorometric assay of BChE based on rapid thiol-ene click reactions.

A human vision inspired adaptive platform for one-on-multiple recognition

The fluorescence of a coordinative molecule DCM displaying an intramolecular charge transfer (ICT) effect is regulated by several metal ions. These DCM-metal complexes were adopted to recognize different chemicals, including the recognition of triethylenetetramine, thiol-containing amino acids, and H2S upon binding DCM with Zn2+, Ag+, and Pb2+, respectively. This is in analogy to the general mode of human trichromatic color vision.

Fluorescent Method for the Detection of Biothiols Using an Ag⁺-Mediated Conformational Switch

In this work, a novel, simple, and time-saving fluorescence approach for the detection of biothiols (glutathione and cysteine) was developed by employing a DNA probe labeled with 2-aminopurine. As an adenine analogue, 2-aminopurine exhibits high fluorescence intensity that can be rapidly quenched in the presence of DNA. In the presence of Ag⁺, the fluorescence increased significantly, which was a result of the formation of cytosine–Ag⁺–cytosine base pairs and the release of 2-aminopurine. Upon addition of either glutathione or cysteine, the structure of cytosine–Ag⁺–cytosine was disrupted, a product of the stronger affinity between biothiols and Ag⁺. As a result, the 2-aminopurine-labeled DNA probe returned to its former structure, and the fluorescence signal was quenched accordingly. The detection limit for glutathione and cysteine was 3 nM and 5 nM, respectively. Furthermore, the determination of biothiols in human blood serum provided a potential application for the probe as a diagnostic tool in clinical practice.

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 specific fluorescent probe for fast detection and cellular imaging of cysteine based on a water-soluble conjugated polymer combined with copper(II)

In pure water system, the specific and rapid detection of cysteine (Cys) is very important and challenging. Herein, a new optical probe was developed for the purpose based on the complex of cupric ion (Cu²⁺) with a water-soluble conjugated polymer, poly[3-(3-N,N-diacetateaminopropoxy)-4-methyl thiophene disodium salts] (PTCO₂). The fluorescence of PTCO₂ in 100% aqueous solution can almost completely extinguished by Cu²⁺ ions due to its intrinsic paramagnetic properties. Among various amino acids, only Cys causes immediately the efficient recovery of the Cu²⁺-quenched fluorescence of PTCO₂ with ~31-folds fluorescence enhancement because of the stronger affinity of Cys to Cu²⁺ leading to the formation of Cu²⁺-Cys complex through Cu-S bond and separation of Cu²⁺ from weak-fluorescent PTCO₂-Cu(II) ensemble and thereby restoring the free PTCO₂ fluorescence. In tris-HCl buffer solution (2 mM, pH 7.4), the intensity of the restored fluorescence is linear with the concentration of Cys, ranging from 0 to 120 μM and the estimated detection limit of Cys is 3.3 × 10^-7 M with the correlation coefficient R = 0.9981. In addition, the PTCO₂-Cu(II) ensemble probe exhibits low cytotoxicity and good membrane penetration, and its application in living cell imaging of Cys has also been explored.

A Near-Infrared Ratiometric Fluorescent Probe for Highly Selective Recognition and Bioimaging of Cysteine

A benzothiazole-based near-infrared (NIR) ratiometric fluorescent probe (HBT-Cys) was developed for discriminating cysteine (Cys) from homocysteine (Hcy) and glutathione (GSH). The probe was designed by masking phenol group in the conjugated benzothiazole derivative with methacrylate group that could be selectively removed by Cys, and therefore an intramolecular charge transfer (ICT) fluorescence was switched on in the NIR region. In the absence of Cys, the probe exhibited a strong blue fluorescence emission at 431 nm, whereas a NIR fluorescence emission at 710 nm was significantly enhanced accompanied by a decrease of emission at 431 nm in the presence of Cys, allowing a ratiometric fluorescence detection of Cys. The fluorescence intensity ratio (I710nm/I431nm) showed a good linear relationship with Cys concentration of 1-40 μM with the detection limit of 0.5 μM. The sensing mechanism was explored based on MS experimental analysis and DFT theoretical calculation. Moreover, the fluorescent probe was successfully used for fluorescence bioimaging of Cys in living cells.

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.