A novel benzothiazole-based fluorescent probe for cysteine detection and its application on test paper and in living cells

Development of fluorescent probes with specific recognition moiety for hydrogen polysulfide

Hydrogen polysulfide (H2Sn, n > 1) is an important component of reactive sulfur species (RSS), which is an important substance for maintaining the redox balance in cells. However, limited recognition moieties are available for hydrogen polysulfide probe design. In this study, we have constructed a small library containing several organic molecules to explore a new specific recognition moiety for H2Sn fluorescent probe design. To validate the discovery, two fluorescent probes, 7 and BCC, were further developed based on coumarin and its derivative. The probes exhibited desirable specificity for H2Sn monitoring, which can be used for detecting H2Sn in solution and cells. The new specific recognition moiety for H2Sn fluorescent probe design discovered in this work has certain guiding significance for development of H2Sn probes exploring biological roles in the future.

Discovery of novel 2-(4-(benzyloxy)-5-(hydroxyl) phenyl) benzothiazole derivatives as multifunctional MAO-B inhibitors for the treatment of Parkinson's disease

To discover novel multifunctional agents for the treatment of Parkinson's disease, a series of 2-(4-(benzyloxy)-5-(hydroxyl) phenyl) benzothiazole derivatives was designed, synthesized and evaluated. The results revealed that representative compound 3h possessed potent and selective MAO-B inhibitory activity (IC₅₀ = 0.062 µM), and its inhibitory mode was competitive and reversible. Additionally, 3h also displayed excellent anti-oxidative effect (ORAC = 2.27 Trolox equivalent), significant metal chelating ability and appropriate BBB permeability. Moreover, 3h exhibited good neuroprotective effect and anti-neuroinflammtory ability. These results indicated that compound 3h was a promising candidate for further development against PD.

A highly chromogenic selective Rhodamine-chloride-based fluorescence probe activated by cysteine and application in living cells and zebrafish

Cysteine (Cys), one of the biological thiols, which plays critical roles in biological system regulating the balance of redox homeostasis. In order to monitor the level of Cys in the living cells and organisms, a chromogenic fluorescence probe Rhocl-Cys based on Rhodamine chloride exhibiting the preferable performance of fluorescence turn-on response reacting with Cys was presented. Rhocl-Cys responded rapidly to Cys within 20 min, and had stable fluorescence intensity within pH 6.0-10.0, high selectivity towards Cys and the anti-inference capability with a low detection limit of 0.80 μM. In particular, Rhocl-Cys could qualitatively and quantitatively monitor the level of endogenous and exogenous Cys in living cells and successfully apply to zebrafish detecting Cys. Therefore, these results might further provide the basis exploring the role of Cys in biological system and facilitate as clinical diagnostic molecular tools.

One-step synthesis of PY-NBD to distinguish Cys/Hcy and GSH in aqueous solutions and living cells by dual channels

An easy-to-synthesize fluorescent probe PY-NBD was developed to distinguish Cys/Hcy and GSH by two channels in aqueous solutions and living cells.

A near-infrared fluorescence turn-on probe based on Michael addition-intramolecular cyclization for specific detection of cysteine and its applications in environmental water and milk samples and living cells

Owing to its important biological functions in many physiological and pathological processes, it is necessary to develop efficient and appropriate detection methods for monitoring the levels of Cys in biological systems. Based on this, a novel rhodol-isophorone derivative (RHI) was designed and synthesized as a reaction-based fluorescence probe for specific detection of Cys with high sensitivity and large Stokes shift (155 nm). This probe was composed of an acrylate moiety (recognition group) and a rhodol-isophorone derivative (fluorophore). Probe RHI could react with Cys rapidly (15 min) with a 100-fold fluorescence enhancement. The limit of detection value was calculated to be 0.168 μM. When Cys was added, the color of the probe RHI solution turned from yellow to blue, indicating that Cys could be monitored by the naked eye. In addition, probe RHI was successfully utilized for detecting Cys in environmental water and milk samples. More importantly, the probe could be applied to imaging Cys in living cells with low cytotoxicity and good biocompatibility.

Probe with large Stokes shift for effective cysteine imaging in living cells

A new fluorescence probe L, which featured with a large Stokes shift (216 nm), was designed for sensitive detection of cysteine (Cys) and a potential sensing mechanism derived from excited state intramolecular proton transfer (ESIPT) was proposed. More importantly, probe L exhibits higher selective to Cys than other amino acid due to its specific cyclization between acrylate group and Cys. Meanwhile, the probe L shows a low detection limit of 8.82 × 10^-8 M, which is enough for detecting Cys in organisms. Furthermore, this probe displays high biocompatibility and can image Cys in living cells.

Using a 3-hydroxyflavone derivative as a fluorescent probe for the indirect determination of aminothiols separated by ion-pair HPLC

Homocysteine, cysteine, cysteinyl-glycine, and glutathione are significant biological aminothiols (ATs) that are marker-molecules in Down syndrome, Alzheimer's disease, or have been implicated as risk factors in atherosclerosis and other vascular diseases, and therefore rapid determination of these molecules is desirable. After reduction of the disulfides, a widely used method utilizes derivatization with ammonium 7-fluorobenzo-2-oxa-1,3-diazole-4-sulfonate (SBD-F) as a fluorogenic probe prior to reversed-phase HPLC separation followed by fluorescence detection. The traditional HPLC determination of ATs is time consuming and economically expensive. We have developed an ion-pair HPLC method coupled with indirect fluorescence detection after post-column reaction with a 2,4-dinitrobenzenesulfonate derivative of a 3-hydroxyflavone. The accuracy, precision, post-column temperature and residence time, and limit-of-detection were evaluated. Sample throughput and reduced sample preparation time of over an hour for the existing methods to less than 20 minutes for the new method is also demonstrated. No statistical differences in HCy, Cys, or Cys-Gly determinations in plasma samples were observed between our method and the traditional HPLC method.

A coumarin-based fluorescence sensor for rapid discrimination of cysteine/homocysteine and glutathione under dual excitation wavelengths

Biological thiols (Cys, Hcy and GSH) are crucial biomolecules in living cells and play indispensable roles in maintaining the redox homeostasis of organisms. But due to their similar molecular structure, the development of effective tools for distinguishing two or three of them remains a great difficulty. Herein, we constructed a sensitive sensor (CB) by connecting the bifunctional fluorescent reagent with coumarin derivatives for simultaneous recognition of these three thiols through different pathways. Free CB had no fluorescence; however, with gradual addition of thiols, the chlorine unit was replaced by sulfhydryl. Furthermore, the intramolecular rearrangement occurred between the amino and sulfhydryl groups of Cys/Hcy and yellow fluorescence was observed at 570 nm. However, GSH with a large structure could not undergo intramolecular rearrangement, and green fluorescence was excited at 505 nm. In this way, Cys/Hcy and GSH can be detected distinctively. Under dual excitation wavelengths, CB exhibited high selectivity and fast response to the three thiols. Furthermore, CB was successfully applied to imaging endogenous and exogenous thiols in living cells and zebrafish, providing us with a reliable tool for thiols recognition.

The mechanisms of a bifunctional fluorescent probe for detecting fluoride and sulfite based on excited-state intramolecular proton transfer and intramolecular charge transfer

The mechanisms of 2-(Benzo[d]thiazol-2-yl)phenol-based bifunctional probe (HBT-FS) for detecting fluoride (F^-) and sulfite (SO₃ ^2-) based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) have been theoretically studied. Laplacian bond order of HBT-FS indicates that the F^- ion cleaves the Si-O bond and then forms Compound 2 possessing a six-membered ring with a hydrogen bond. Potential energy curves and dynamic simulations confirm that ESIPT in Compound 2 occurs along with this hydrogen bond and forms a keto structure with an emission at 623 nm, which agrees with the observed experimental value (634 nm) after adding F^-. Therefore, the fluorescence red-shift (from 498  to 634 nm) of HBT-FS observed in experiment after adding F^- is caused by ESIPT. The SO₃ ^2- ion is added to the C₅ site of HBT-FS, which is confirmed by orbital-weighted dual descriptor, and then forms Compound 3 with fluorescence located at 404 nm. The experimentally measured fluorescence at 371 nm after adding SO₃ ^2- is assigned to Compound 3. Charge transfer analyses indicate that the ICT extent of Compound 3 is relatively weak compared with that of HBT-FS because of the destruction of the conjugated structure by the addition reaction of SO₃ ^2-, which induces the blue-shift of the fluorescence of HBT-FS from 498 to 371 nm. The different fluorescence responses make HBT-FS a fluorescent probe to discriminatorily detect F^- and SO₃ ^2-.

A mitochondria-targeted fluorescent dye naphthalimide-thioether-cyanine for NIR-activated photodynamic treatment of cancer cells

In this work, an NIR-activated fluorescent dye naphthalimide-thioether-cyanine (NPSCY) was developed for the photodynamic treatment of cancer cells. In this dye, naphthalimide and cyanine were selected as the two fluorophores, which were linked by the thioether group. Under 660 nm irradiation, NPSCY could produce 1O2 rapidly, suggesting the potential for photodynamic therapy. Cys can be considered as one of the markers of cancer cells and NPSCY could distinguish Cys from three channels (433 nm, 475 nm, 733 nm) due to the bilateral recognition of the thioether group, which was helpful for accurately locating cancer cells. Fortunately, NPSCY could also produce 1O2 after being reacted with the intracellular biological thiols, which also avoided the inactivation of the photosensitizer in cancer cells. The co-localization coefficient of 0.873 indicated that the cyanine group promoted the aggregation of NPSCY in mitochondria. This photosensitizer showed low dark toxicity and high phototoxicity. Meanwhile, the half-maximal inhibitory concentration (IC50) was calculated to be 3.7 μM. NPSCY could inhibit cell migration after irradiation at 660 nm.

Design and synthesis of a novel "turn-on" long range measuring fluorescent probe for monitoring endogenous cysteine in living cells and Caenorhabditis elegans

Cysteine (Cys) is an indispensable small organic molecule containing sulfhydryl groups, which has essential regulatory effects on the physiological process of human body. In this work, a red emission fluorescent probe TCFQ-Cys was designed and exploited based on 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile-derivatives. The probe could effectively monitor Cys through the typical acrylate cleavage. The detecting system showed a red emission at 633 nm and the fluorescence was stable within the pH range of 6-9. The detection could be completed in 30 min. TCFQ-Cys presented high sensitivity with a detection limit of 0.133 μM and high selectivity towards Cys from other biological mercaptans. The most important feature was that the system had a wide linear range of 0-300 μM, which covered the physiological requirements of Cys detection. Subsequently, we conducted the biological imaging of Cys in MCF-7 cells and Caenorhabditis elegans (C. elegans). Therefore, TCFQ-Cys had a practical application prospect for further investigating the physiological function of Cys.

A curcumin-analogous fluorescent sensor for cysteine detection with a bilateral-response click-like mechanism

A novel curcumin-analogous fluorescent sensor, DNP, was developed for cysteine detection with a bilateral-response click-like mechanism. DNP indicated high selectivity and practical sensitivity. It could recognize Cys from other biologically relevant molecules, especially, from GSH and Hcy. The most interesting point was that, with typical azide groups for sensing, DNP indicated a covalent binding procedure with Cys instead of a presupposed simple reduction for reductive sulfide. Moreover, the recognition occurred at both sides of the sensor. DNP could be utilized into the detection of endogenous and exogenous Cys in living cells. Though the specific optical performances of DNP still need optimization, this work supplied novel information for broadening the vision on fluorophores and mechanisms, for the monitoring of Cys and even other sulfur-containing species.

A novel benzo[a]phenazin-based fluorescence probe for selective detection of cysteine with anti-cancer potency

Among the physiological and pathological sulfur-containing species, cysteine (Cys) is the most typical one which is an important component of the REDOX system in vivo. Monitoring the level of Cys from other competing species seems quite important in pre-clinical diagnosis and therapeutic evaluation. Herein, we developed a selective fluorescent probe, BPCys, for Cys from the benzo[a]phenazin backbone which had the potential of anti-cancer potency. BPCys suggested advantages including high specificity (40 fold over other species), high sensitivity (detection limit: 18 nM), wide pH adaptability (6.0-11.0) and in particular, the anti-cancer effect. Biological assays and in silico simulation hinted the potency of the detecting product on Topoisomerase I/II. In brief, this study raised a practical strategy for monitoring the Cys level in living cells, especially in cancer models with its anti-cancer potential, thus opened the mind of exploring more specific tool for specific applications.

Introducing ortho-methoxyl group as a fluorescence-enhancing and bathochromic-shift bi-functional strategy for typical cysteine sensors

A practical strategy of introducing ortho-methoxyl group was explored to achieve the fluorescence-enhancing and bathochromic-shift bi-functional optimization. It was tested in the Cys sensing ISOPH-X series, thus the successful case, ISOPH-2, was obtained. It realized the optimization in a simple and compatible way. The corresponding strategy was basically established during the confirmation of checkpoints including applicable steadiness (over 24 h), wide pH range (7.0-9.0), rapid response (20 min), good biocompatibility, high sensitivity (LOD = 0.072 nm), high selectivity and biological monitoring of Cys in living cells as well as C. elegans. In this work, the o-methoxyl introduction strategy led to a 15 nm red shift and a near 4-fold fluorescence enhancement. This strategy could be combined with the double bond-introducing approach. Compared with reported strategies, by breaking the dilemma between red shift and strong fluorescent intensity, this strategy might offer beneficial information for exploiting better sensors with more fluorophores and mechanisms for their targets.

A diazabenzoperylene derivative as ratiometric fluorescent probe for cysteine with super large Stokes shift

Biothiols, including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play key roles in biological processes, and detecting such thiols selectively is critical for understanding functions of biothiols. In this work, a pyridazine annelated perylene-based fluorescent probe PAPC is synthesized for highly selective detection of Cys. PAPC exhibits strong blue emission in PBS, while the red emission at 605 nm can be observed in the presence of Cys. The probe PAPC shows ratiometric fluorescence (I₆₀₅/I₄₆₀) detection of Cys with wide linear range of 1-120 μM and low detection limit of 0.19 μM. Super large Stokes shift (170 nm) and ratiometric fluorescence endow the probe low background signal. The discrimination of Cys over Hcy and GSH can be achieved through the difference of the ratiometric fluorescence. In addition, the probe has been proven to track Cys in real samples such as urine and HeLa cells. Therefore, PAPC probe is a promising candidate for detecting Cys in practical application. Graphical abstract.

Benzoxazine-based fluorescent probes with different auxochrome groups for cysteine detection

Three 5H-benzo[a]phenoxazin-5-one-based (benzoresorufin and nile-red) Cysteine (Cys) detection probes have been comparatively designed and synthesized in this paper. The optical experiments exhibit probe 1b with a crotonoyl group has no response toward Cys; while probes 1a and 1c have the same reaction site (acryloyl group), their optical responses to Cys are quite different. The benzoresorufin-based-probe 1a shows a turn-on fluorescence response (118-fold) to Cys at 631 nm and affords a very low detection limit (DL = 19.8 nM). Compared with probe 1a, the nile-red-based probe 1c displays gradually diminishing fluorescence intensity with increased Cys concentration at 665 nm. And the notable different fluorescence response mechanisms of probes 1a and 1c toward Cys can be interpreted by HRMS and time-dependent density functional theorety (TDDFT) calculations. Furthermore, both of the two probes indicate high sensitivity and selectivity toward Cys over other similar structured amino acids including homocysteine (Hcy) and glutathione (GSH). Further cellular applications of the two probes have been successfully performed in HeLa cells.

A Benzothiazole-Based Fluorescent Probe for Ratiometric Detection of Al3+ and Its Application in Water Samples and Cell Imaging

An easily prepared benzothiazole-based probe (BHM) was prepared and characterized by general spectra, including ¹H NMR, ¹³C NMR, HRMS, and single-crystal X-ray diffraction. Based on the synergistic mechanism of the inhabitation of intramolecular charge transfer (ICT), the BHM displayed high selectivity and sensitivity for Al³⁺ in DMF/H₂O (v/v, 1/1) through an obvious blue-shift in the fluorescent spectrum and significant color change detected by the naked eye, respectively. The binding ratio of BHM with Al³⁺ was 1:1, as determined by the Job plot, and the binding details were investigated using FT-IR, ¹H NMR titration, and ESI-MS analysis. Furthermore, the BHM was successfully applied in the detection of Al³⁺ in the Songhua River and on a test stripe. Fluorescence imaging experiments confirmed that the BHM could be used to monitor Al³⁺ in human stromal cells (HSC).

Synthesis of green fluorescent carbon materials using byproducts of the sulfite-pulping procedure residue for live cell imaging and Ag+ ion determination

A simple synthesis strategy was designed and applied to synthesize nitrogen and sulfur co-doped aminated ligninsulfonate/graphene quantum dots (ASL/GQDs) composites using citric acid monohydrate and byproducts of the sulfite-pulping procedure (sodium lignosulfonate). The combination of these two materials improves surface chemical activities and electronic characteristics. As a result，the combination offers excellent photoluminescence properties and sensitivity. The fluorescence intensity of the as-prepared ASL/GQDs composites is more than three times that of the free GQDs. ASL/GQDs based fluorescent probe was applied to sensitively determine Ag⁺ with a good linearity in a range from 0.005 to 500 μM with a correlation coefficient of 0.99. The method was also used successfully to determine the amount of Ag⁺ in environmental water samples. Using an MTT assay, the ASL/GQDs have low toxicity and are biocompatible with A549 cells, and may be successfully used to image A549 cells.

Chalcone-analogue fluorescent probes for detecting thiophenols in seawater samples

Two efficient chalcone fluorescent probes (probe-KCN1 and probe-KCN2) were developed for the detection of thiophenols. Upon gradual addition of thiophenols to the fluorescent probes, the fluorescence intensity of the emission band at 550 nm is enhanced about 40-fold, with a large Stokes shift (130 nm). Probe-KCN1 responds to thiophenols with a good range of linearity and a detection limit of 79 nΜ (R² = 0.9915), and Probe-KCN2 responds selectively to thiophenols over other amino acids, common metal ions and other potential interferents with a detection limit of 96 nM (R² = 0.9978). The low-toxicity probe has been successfully used to detect thiophenols in samples of seawater. These results demonstrate that Probe-KCN is a class of specific probes that might provide a simple way to monitor changes in thiophenols at low concentrations in seawater samples.

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.

A dual-selective fluorescent probe for discriminating glutathione and homocysteine simultaneously

Homocysteine (Hcy) and glutathione (GSH) play important roles in a variety of physiological and pathological processes. Abnormal levels of Hcy and GSH are related to various diseases. Fluorescent probes for detecting them with sensitive and selective are highly desirable. However, efficient discrimination of Hcy and GSH is still a challenge for their similar molecular structures and chemical properties. Herein, we report a naphthalimide and sulfonyl benzoxadiazole (SBD) based dual-selective fluorescent probe for Hcy and GSH over other amino acids. The probe exhibited weak fluorescence (Φ = 0.075, in DMSO) at 490 nm and fluorescence enhancement upon addition of GSH (1-20 μM) with a detection limit of 0.8 μM. The probe also exhibited ratiometric fluorescence responses for Hcy (fluorescence at 490 nm decreased and fluorescence at 552 nm increased). The fluorescence intensity ratio (I₅₅₂/I₄₉₀) showed a good linear correlation with the Hcy concentrations in range of 3-20 μM and the detection limit was 0.1 μM. Moreover, this probe was successfully utilized for monitoring Hcy and GSH in living cells.

Activated charcoal significantly improved the reliability of methods for quantitative analysis of endogenous substances in biological specimens: Glutathione and cysteine as cases

When developing a quantitative assay for exogenous or endogenous compounds, guidelines for method validation normally recommend that the biological specimens should be prepared in corresponding authentic matrices, yet "analyte-free authentic matrices" is in general not available. It is generally known that GSH and CYS are endogenous compounds and present in both prokaryotes and eukaryotes. Herein, an efficient approach for the quantitative analysis of endogenous substances in biological specimens was developed, and glutathione (GSH) & cysteine (CYS) were chosen as model endogenous substances. Activated carbon (AC), a common adsorbent for the adsorption of environmental pollutants, was used to remove the endogenous GSH and CYS and prepare "GSH&CYS-free biological matrix". The endogenous GSH and CYS in mouse plasma, blood and liver homogenate were found can be almost removed via incubating with 100 mg of AC for 2 h. After optimizing the derivatization reagents, internal standard and analytical parameters, a reliable quantitative assay of GSH and CYS in mouse plasma, blood and liver homogenate was developed and validated on LC-ESI-MS/MS using corresponding AC-adsorbed mouse biological matrices. The validation results indicated that the developed method provided suitable accuracy, sensitivity, specificity and high throughput for the analysis of GSH and CYS. Finally, the developed LC-ESI-MS/MS assay was successfully applied to measure the concentrations of GSH and CYS in liver injury mice. The presently developed methodology could be widely applied in the quantitative analysis of endogenous compounds in various complex mixtures such as biological, herbal and environmental samples.

Hydroxyapatite nanoparticle based fluorometric determination and imaging of cysteine and homocysteine in living cells

Fluorescent hydroxyapatite nanoparticles (HAP-NPs) were prepared by reacting calcium ion with phosphate in the presence of Eu(III) ion. The HAP-NPs display large Stokes' shift and two strong fluorescence emissions with peaks at 590 nm and 615 nm when excited at 250 nm. The HAP-NPs also have good photostability and water solubility. The HAP-NPs combined with Cu(II) were applied to fluorometric determination of cysteine and homocysteine in biological samples and in living cells. In this detection scheme, the fluorescence of HAP-NPs is initially quenched by Cu(II). The addition of biothiols results in the formation of Cu(II)-thiol complexes and leads to fluorescence recovery. The assay allows cysteine to be detected with a 110 nM detection limit, and homocysteine with a 160 nM detection limit. The assay was successfully applied to the analysis of cysteine in spiked human serum samples and to imaging of cysteine in HeLa cells, and this demonstrates its potential for clinical testing and in biomedical research. Graphical abstract Fluorescent hydroxyapatite nanoparticles were synthesized and combined with Cu²⁺ for fluorescence sensing of biothiols (cysteine and homocysteine) in complex biological samples and in living cells.

A novel dual-emission fluorescent nanohybrid containing silica nanoparticles and gold nanoclusters for ratiometric determination of cysteine based on turn-on fluorescence strategy

A novel fluorescence sensor SiO₂NPs/AuNCs nanohybrid has been used developed for ratiometric visual detection of Cys.

A NIR rhodamine fluorescent chemodosimeter specific for glutathione: Knoevenagel condensation, detection of intracellular glutathione and living cell imaging

A new near-infrared probe for detecting glutathione based on conjugate addition and intramolecular amino induced spirolactam opening named RhAN was designed and synthesized. Its emission intensity enhance more than 90-fold upon addition of GSH. In addition, it also has high sensitivity with low detection limit of 0.1 μM.