Electrochemical detection of thiols with a coenzyme pyrroloquinoline quinone modified electrode

Gold Nanoparticles-Based Colorimetric Immunoassay of Carcinoembryonic Antigen with Metal-Organic Framework to Load Quinones for Catalytic Oxidation of Cysteine

This work reported gold nanoparticles (AuNPs)-based colorimetric immunoassay with the Cu-based metal-organic framework (MOF) to load pyrroloquinoline quinone (PQQ) for the catalytic oxidation of cysteine. In this method, both Cu2+ and PQQ in the MOF could promote the oxidation of inducer cysteine by redox cycling, thus limiting the cysteine-induced aggregation of AuNPs and achieving dual signal amplification. Specifically, the recombinant carcinoembryonic antigen (CEA) targets were anchored on the MOF through the metal coordination interactions between the hexahistidine (His6) tag in CEA and the unsaturated Cu2+ sites in MOF. The CEA/PQQ-loaded MOF could be captured by the antibody-coated ELISA plate to catalyze the oxidation of cysteine. However, once the target CEA in the samples bound to the antibody immobilized on the plate surface, the attachment of CEA/PQQ-loaded MOF would be limited. Cysteine remaining in the solution would trigger the aggregation of AuNPs and cause a color change from red to blue. The target concentration was positively related to the aggregation and color change of AuNPs. The signal-on competitive plasmonic immunoassay exhibited a low detection limit with a linear range of 0.01-1 ng/mL. Note that most of the proteins in commercial ELISA kits are recombinant with a His6 tag in the N- or C-terminal, so the work could provide a sensitive plasmonic platform for the detection of biomarkers.

Prediction of Antioxidant Capacity of Thiolate-Disulfide Systems Using Species-Specific Basicity Values

The principal reactions that maintain redox homeostasis in living systems are the deprotonation of thiols, followed by the oxidative conversion of the produced thiolates into disulfides, which thus reduce the harmful oxidizing agents. The various biological thiols have different molecule-specific propensities to carry on the co-dependent deprotonation and redox processes. This study utilizes the known correlation between thiolate basicities and oxidizabilities, to quantify antioxidant or reducing capacities and pH-dependences of thiol-disulfide antioxidant systems, as a tool to find adequate molecules against oxidative stress.

Analytical developments in the synergism of copper particles and cysteine: a review

Cysteine, a sulfur-containing amino acid, is a vital candidate for physiology. Coinage metal particles (both clusters and nanoparticles) are highly interesting for their spectacular plasmonic properties. In this case, copper is the most important candidate for its cost-effectiveness and abundance. However, rapid oxidation destroys the stability of copper particles, warranting the necessity of suitable capping agents and experimental conditions. Cysteine can efficiently carry out such a role. On the contrary, cysteine sensing is a vital step for biomedical science. This review article is based on a comparative account of copper particles with cysteine passivation and copper particles for cysteine sensing. For the deep understanding of readers, we discuss nanoparticles and nanoclusters, properties of cysteine, and importance of capping agents, along with various synthetic protocols and applications (sensing and bioimaging) of cysteine-capped copper particles (cysteine-capped copper nanoparticles and cysteine-capped copper nanoclusters). We also include copper nanoparticles and copper nanoclusters for cysteine sensing. As copper is a plasmonic material, fluorometric and colorimetric methods are mostly used for sensing. Real sample analysis for both copper particles with cysteine and copper particles for cysteine sensing are also incorporated in this review to demonstrate their practical applications. Both cysteine-capped copper particles and copper particles for cysteine sensing are the main essence of this review. The aspect of the synergism of copper and cysteine (unlike other amino acids) is quite promising for future researchers.

Optical Bioassays Based on the Signal Amplification of Redox Cycling

Optical bioassays are challenged by the growing requirements of sensitivity and simplicity. Recent developments in the combination of redox cycling with different optical methods for signal amplification have proven to have tremendous potential for improving analytical performances. In this review, we summarized the advances in optical bioassays based on the signal amplification of redox cycling, including colorimetry, fluorescence, surface-enhanced Raman scattering, chemiluminescence, and electrochemiluminescence. Furthermore, this review highlighted the general principles to effectively couple redox cycling with optical bioassays, and particular attention was focused on current challenges and future opportunities.

Dual Fluorimetric Sensor for Tandem Detection of Cadmium and Cysteine: An Approach for Designing a Molecular Keypad Lock System

A fluorimetric sensor for dual and sensitive detection of Cd2+ ion and Cysteine (based on 2-picolylamine platform) was developed.The sensor was designed and synthesized by simple condensation method and characterized by using common spectroscopic methods. The observations made from the kinetics of absorption and emission profile shows that probe Pdac behaves as ''ON-OFF'' fluorescent quenching sensor for cadmium ions. The probe exhibit selectivity in fluorescence quenching behaviour over other competitive metal ions, and also the Pdac-Cd2+ ensemble behave as an efficient ''OFF-ON'' type sensor for an essential amino acid Cysteine. Moreover, this dual sensing nature of the sensor makes it successfully applied for the designing of a molecular keypad lock system.

Photostable yellow emissive carbon dots for iron-mediated reversible sensing of biothiols and cellular imaging

The different levels of biothiols in cells can not only screen cancer cells but also play a selective role in killing cancer cells. Therefore, accurate monitoring of biothiol in cancer cells is of great research significance. Herein, novel yellow emission CDs (Y-CDs) were prepared by a simple hydrothermal method using 2, 5-dihydroxyterephthalic acid (DHTA) as precursors. The Y-CDs as a highly efficient dual-mode sensor could detect Fe3+ and biothiols by colorimetric and fluorescence signals. Especially, with the addition of L-Cysteine, the quenched fluorescence could be quickly restored within 2 min and the detection limit was as low as 31.65 nM. Additionally, this sensor was utilized to sense biothiols in actual samples and living cells due to its eminent biocompatibility. Finally, the Y-CDs were successfully applied not only as fluorescent ink for message encryption but also as a portable solid hydrogels sensor for the detection of Fe3+ and biothiols. Therefore, these results suggested that Y-CDs could serve as a promising sensor for Fe3+ and biothiol detection in early cancer screening.

Electrochemical Biosensors for Whole Blood Analysis: Recent Progress, Challenges, and Future Perspectives

Whole blood, as one of the most significant biological fluids, provides critical information for health management and disease monitoring. Over the past 10 years, advances in nanotechnology, microfluidics, and biomarker research have spurred the development of powerful miniaturized diagnostic systems for whole blood testing toward the goal of disease monitoring and treatment. Among the techniques employed for whole-blood diagnostics, electrochemical biosensors, as known to be rapid, sensitive, capable of miniaturization, reagentless and washing free, become a class of emerging technology to achieve the target detection specifically and directly in complex media, e.g., whole blood or even in the living body. Here we are aiming to provide a comprehensive review to summarize advances over the past decade in the development of electrochemical sensors for whole blood analysis. Further, we address the remaining challenges and opportunities to integrate electrochemical sensing platforms.

Triazine based fluorescent sensor for sequential detection of Hg2+ and L-Cysteine in real samples and application in logic Gate: A combination of Extensive experimental and theoretical analysis

Triazine based fluorescent sensor TBT was rationally designed and synthesized to achieve sequential detection of Hg²⁺ and L-cysteine based on the presence of sulfur moiety and suitable cavity in the molecule. Sensor TBT exhibited excellent sensing potential for the selective detection of Hg²⁺ ions and L-cysteine (Cys) in real samples. Upon addition of Hg²⁺ to sensor TBT, enhancement in emission intensity of sensor TBT was observed which was accredited to the presence of sulfur moiety and size of cavity in the sensor. Upon interaction with Hg²⁺ blockage of intramolecular charge transfer (ICT) along with chelation-enhanced fluorescence (CHEF) resulted in the increase in fluorescence emission intensity of sensor TBT. Further, TBT-Hg²⁺ complex was employed for the selective detection of Cys through fluorescence quenching mechanism. This was attributed to the significantly stronger interaction of Cys with Hg²⁺, which resulted in the formation of Cys-Hg²⁺ complex and subsequently sensor TBT was released from TBT-Hg²⁺ complex. The nature of interaction between TBT-Hg²⁺ and Cys-Hg²⁺ complex was evaluated through ¹H NMR titration experimentations. Extensive DFT studies were also carried out which include thermodynamic stability, frontier molecular orbitals (FMO), density of states (DOS), non-covalent interaction (NCI), quantum theory of atom in molecule (QTAIM), electron density differences (EDD) and natural bond orbital (NBO) analyses. All the studies supported the non-covalent type of interaction between analytes and sensor TBT. The limit of detection for Hg²⁺ ions was found to be as low as 61.9 nM. Sensor TBT was also employed for the quantitative detection of Hg²⁺ and Cys in real samples. Additionally, logic gate was fabricated by using sequential detection strategy.

N-Acetylcysteine Interference with a Glucose Dehydrogenase Linked Glucose Meter

BACKGROUND

Our objective was to determine the effect of therapeutic concentrations of N-acetylcysteine, following intravenous infusion, on the measurement of blood glucose using a Roche Diagnostics glucose dehydrogenase-linked glucose meter compared to hospital laboratory methods.

METHODS

N-acetylcysteine was added to aliquots of blood, with glucose promptly measured by the glucose meter, blood gas analyzer (glucose oxidase comparative method) and following centrifugation, plasma glucose measured with a hexokinase spectrophotometric comparative method. Glucose results were evaluated with linear regression and Bland Altman plots.

RESULTS

In the presence of NAC, at concentrations greater than 5 mg/dL (0.31 mmol/L), positively biased glucose meter results were compared to the clinical laboratory results. Multivariate linear regression revealed that NAC-mediated meter results are influenced by NAC and glucose concentrations.

CONCLUSIONS

The addition of therapeutic concentrations of NAC to blood produces statistically significant positive biases when measured with the glucose dehydrogenase linked glucose meter device.

Synthesis of a Novel Electrochemical Probe for the Sensitive and Selective Detection of Biothiols and Its Clinical Applications

The ability to estimate and quantify biothiols in biological fluids is very significant for attaining a detailed understanding of biothiols-related pathological diseases. Most of the developed methods for biothiols detection are not suitable for this purpose owing to their low sensitivity, poor selectivity, and long experimental procedures. In this study, a novel and simple structure electrochemical probe has been synthesized for the first time for the selective determination of biothiols. The developed probe is based on using 2,4-dinitrobenzenesulfonyl moiety (DNBS) as a selective recognition moiety for biothiols. The electrochemical probe was successfully fabricated through a facile one-step reaction between 2,4-dinitrobenzenesulfonyl chloride (DNBS-Cl) and p-aminophenol. The successful synthesis of the probe was confirmed by using different characterization techniques such as an NMR spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, and mass spectrometry. Biothiols can selectively cleave the DNBS moiety through an aromatic nucleophilic substitution (ANS) reaction within 10 min to release p-aminophenol, which is a highly electrochemical active molecule that can be selectively detected easily by cyclic voltammetry at low potential. The probe has been employed for the quantification of cysteine, glutathione, and homocysteine with a LOD of 1.50, 3.48, and 4.67 μM, respectively. Excellent recoveries have been achieved in the range of 95.44-98.71% for the determination of the total biothiols in the human plasma sample.

Highly sensitive voltammetric determination of NADH based on N-CQDs decorated SnO2/ionic liquid/carbon paste electrode

A highly sensitive and selective modified electrode was successfully developed for the monitoring of nicotinamide adenine dinucleotide (NADH) in the presence of folic acid. In this regard, a carbon paste electrode (CPE) was functionalized by the nitrogen-doped carbon quantum dots/tin oxide (N-CQDs/SnO₂) nanocomposite and 1-butyl-2,3-dimethyl imidazolium hexafluorophosphate ([C₄DMIM][PF₆]) ionic liquid (IL). The structure and surface morphology of the nanocomposite were characterized by various methods, including field emission scanning electron microscopy, energy dispersive spectroscopy (EDS), high-resolution transmission electron microscopy (HR-TEM), and x-ray diffraction (XRD). The modified electrode displayed powerful and long-lasting electron mediating activity, with well-separated NADH and folic acid oxidation peaks. The sensing response of the developed [C₄DMIM][PF₆]/N-CQDs/SnO₂/CPE platform was evaluated by determining NADH via the voltammetric technique under the optimized operating conditions. The current peaks of the square wave voltammograms of NADH and folic acid increased linearly with enhancing its concentrations within the ranges of 0.003-275μM NADH and 0.4-380μM folic acid. The detection limits for NADH and folic acid were obtained at 0.8 nM and 0.1μM, respectively. Interference species such as glucose, urea, tryptophan, glycine, methionine, and vitamin B₁₂had no influence on the ability of the fabricated modified electrode to detect the target species. The low detection limit, high sensitivity, excellent selectivity, superior stability, and cost-effectiveness made it suitable for the quantification of NADH in the real biological samples with the recovery percent values in the range of 97.5%-103%.

Electrochemical flow injection analysis of the interaction between pyrroloquinoline quinone (PQQ) and α-synuclein peptides related to Parkinson's disease

α-Synuclein (α-syn) is a hallmark protein of Parkinson's disease (PD). The aggregation process of α-syn has been heavily associated with the pathogenesis of PD. With the exponentially growing number of potential therapeutic compounds that can inhibit the aggregation of α-syn, there is now a significant demand for a high-throughput analysis system. Herein, a novel flow injection analysis system with an electrochemical biosensor as the detector was developed to study the interaction of a well-described antioxidant and amyloid inhibitor, pyrroloquinoline quinone (PQQ) with α-synuclein peptides. Screen-printed gold electrodes (SPEs) were modified using heptapeptides from α-syn wild-type (WT) and mutants such as lysine knock-out (ETEE) and E46K. Affinity binding events between these peptides and PQQ were analyzed by electrochemical impedance spectroscopy (EIS) and further confirmed by high-performance liquid chromatography (HPLC), liquid chromatography/mass spectrometry (LC/MS), and nuclear magnetic resonance (NMR) spectroscopy. HPLC and LC/MS results revealed that PQQ formed a stable complex with α-syn. NMR results confirmed that the α-syn-PQQ complex was formed via a Schiff base formation-like process. In addition, results showed that lysine residues influenced the binding event, in which the presence of an extra lysine stabilized the α-syn-PQQ complex, and the absence of a lysine significantly decreased the interaction of α-syn with PQQ. Therefore, we concluded that EIS is a promising technique for the evaluation of the interaction between PQQ-based amyloid inhibitors and α-syn. The electrochemical flow injection analysis assembly provided a rapid and low-cost drug discovery platform for the evaluation of small molecule-protein interactions.

A Turn-on Fluorescent Probe for the Discrimination of Cys/Hcy and GSH With Dual Emission Signals

In this paper, we successfully synthesized a simple and versatile fluorescent probe. This probe was not only easily prepared with a high yield, but also showed rapid selective and sensitive responses for Cys/Hcy and GSH. The probe can be used as a naked-eye detector for Cys/Hcy and GSH from other analytes. As a fluorescent sensor, it can be used to simultaneously detect and discriminate Cys/Hcy from GSH with two fluorescent emission signals without spectral crosstalk.

Development of manganese dioxide-based nanoprobes for fluorescence detection and imaging of glutathione

A manganese dioxide-based nanoprobe is developed for fluorescence detection and imaging of glutathione (GSH) in yeast cells and onion tissues.

TiO2 nanotube array-modified electrodes for L-cysteine biosensing: experimental and density-functional theory study

We report a non-enzymatic facile method for the detection of L-cysteine (L-Cyst) using free-standing TiO₂ nanotube (TNT) array-modified glassy carbon electrodes (GCEs). Self-organized, highly ordered, and vertically oriented TNT arrays were fabricated by anodization of titanium sheets in ethylene glycol-based electrolyte. Detailed electrochemical measurements were performed and it was found that modified GCE exhibited high current compared to the pristine counterpart. The high current of the modified electrode was attributed to the high surface area and enhanced electrocatalytic activities of the TNTs toward the L-Cyst oxidation. Under the optimum conditions, the modified electrode exhibited a high sensitivity of ∼1.68 µA mM^-1 cm^-2 with a low detection limit of ∼0.1 mM. The fabricated electrode was found to be sensitive to pH and electrolyte temperature. The real sample analysis of the proposed method showed a decent recovery toward L-Cyst addition in human blood serum. Furthermore, the density-funcational theory (DFT) analysis revealed that TNTs have greater affinity toward L-Cyst, having stronger binding distance after its adsorption. The higher negative E _ads values suggested a stable and chemisorption nature. The density of states results show that the E _gap of TNTs is significantly reduced after L-Cyst adsorption. The modified GCE showed excellent selectivity, enhanced stability, and fast response, which make TNTs a promising candidate for the enzyme-free detection of other biological analytes.

Rapid SERS Detection of Thiol-Containing Natural Products in Culturing Complex

Thiol-containing natural products possess a wide range of bioactivities. The burst of synthetic biology technology facilitates the discovery of new thiol-containing active ingredients. Herein, we report a sensitive, quick, and robust surface-enhanced Raman scattering technology for specific and multiplex detection of thiol-containing compounds without purification requirements and also indicating the thiols with different chemical environments. Using this platform, we successfully demonstrated the simultaneous detection of thiol-containing compounds from as low as 1 μM of analytes spiked in complex culture matrices.

Fluorescence "Off-On" Probe for L-Cysteine Detection Based on Nitrogen Doped Carbon Dots

Herein, a simple and efficient fluorescence analysis method for L-Cysteine (L-Cys) was established. The method was based on the fluorescent "off-on" mode of nitrogen doped carbon dots (NCDs). The NCDs were prepared via a facile one-step solvothermal method. In the process of exploring the bio-functional application of these newly synthesized NCDs, we found these NCDs with rich functional groups exhibited excellent optical properties. In addition, these newly synthesized NCDs showed an excitation-dependent emissions photolumine-scent (PL) property and exhibited good performance in the detection of Fe³⁺ ions by quenching the blue emission fluorescence. Interestingly, the quenched fluorescence of NCDs was recovered with the addition of L-Cys, which provided a novel approach for L-Cys detection. The NCDs-based fluorescent "off-on" sensor has a wide linear detection range (0-100 μM), and a relatively low detection limits (0.35 μM) for L-Cys. This simple fluorescent "off-on" approach is, very sensitive and selective for L-Cys detection, which also provides a new insight on NCDs biosensor application.

A Computational Investigation of the Binding of the Selenium Analogues of Ergothioneine and Ovothiol to Cu(I) and Cu(II) and the Effect of Binding on the Redox Potential of the Cu(II)/Cu(I) Redox Couple

The complexes formed from the binding of ovoselenol (OSeH) and ergoseloneine (ESeH) to Cu(II) and Cu(I) have been investigated with DFT methods. From the calculated thermodynamics, the binding of OSeH and ESeH to Cu(II) and Cu(I) ions increases the reduction potential for the Cu(II)/Cu(I) redox couple. The calculated reduction potentials for the Cu(II)(OSe)2/Cu(I)(OSeH)3+ and Cu(II)(ESe)2/Cu(I)(ESeH)3+ redox couples were found to be 1.15 V and 1.24 V in a dilute aqueous solution. By combining the half reactions for the oxidation of OSeH to the diselenide OSeSeO with the reduction of Cu(II)(OSe)2 to Cu(I)(OSeH)3+, the calculated EMF was 0.90 V. For the oxidation of ESeH to the diselenide ESeSeE with the concomitant reduction of Cu(II)(ESe)2 to Cu(I)(ESeH)3+, the calculated EMF was 0.67 V. Thus, for both systems, the reduction of Cu(II) to Cu(I) with concomitant formation of either diselenide is thermodynamically favourable, and it is expected that both OSeH and ESeH are suitable for the protection against copper induced oxidative damage. As a result, the inhibition of the recycling of Cu(I) to Cu(II) is thermodynamically favourable in the presence of OSeH and ESeH.

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.

Fluorine Meets Amine: Reducing Microenvironment-Induced Amino-Activatable Nanoprobes for 19F-Magnetic Resonance Imaging of Biothiols

¹⁹F-magnetic resonance imaging (MRI) is of great significance for noninvasive imaging and detection of various diseases. However, the main obstacle in the application of ¹⁹F-MRI agents stems from the unmet signal sensitivity due to the poor water solubility and restricted mobility of segments with high number of fluorine atoms. Herein, we report a kind of intracellular reducing microenvironment-induced amino-activatable ¹⁹F-MRI nanoprobe, which can be used for specific imaging of biothiols. In principle, the nanoprobe has an initial architecture of hydrophobic core, where the trifluoromethyl-containing segments are compactly packed and ¹⁹F NMR/MRI signals are quenched ("OFF" state). Upon encountering sulfydryl, the strong electron-withdrawing 2,4-dinitrobenzenesulfonyl groups are excised to recover secondary amino groups, whose p K_a is proved to be 7.21. As a consequence, the molecular weight loss of the hydrophobic segment and the protonation of amino groups induce significant disturbance of hydrophilic/hydrophobic balance, leading to the disassembly of the nanoprobes and regain of spin-spin relaxation and ¹⁹F NMR/MRI signals ("ON" state, T₂ up to 296 ± 5.3 ms). This nanoprobe shows high sensitivity and selectivity to biothiols, enabling intracellular and intratumoral imaging of glutathione. Our study not only provides a new nanoprobe candidate for biothiols imaging in vivo but also a promising strategy for the molecular design of real water-soluble and highly sensitive ¹⁹F-MRI nanoprobes.

Sonochemical synthesis of silver nanoparticles anchored reduced graphene oxide nanosheets for selective and sensitive detection of glutathione

Developed here an eco-friendly, one-pot approach toward rapid synthesis of silver nanoparticles anchored reduced graphene oxide (AgNPs_(TMSPED)-rGO) nanosheets via sonochemical irradiation method, using an aqueous solution mixture of GO and AgNO₃ in the presence of N-[3(trimethoxysilyl)propyl] ethylenediamine (TMSPED) without any reducing agent. As synthesized decorated nanosheets was thoroughly characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Pristine AgNPs_(TMSPED), pristine rGO_(TMSPED) and as prepared AgNPs_(TMSPED)-rGO materials were employed to modify the glassy carbon (GC) electrode and demonstrated its excellent electrocatalytic activities towards glutathione (GSH). Voltammetry and amperometry measurements were utilized to assess the electrochemical properties towards the glutathione detection. When the Ag nanoparticles were anchored onto the rGO surface, the observed results illustrated that the electrocatalytic properties of rGO might be enhanced. The resulting sensor exhibits excellent repeatability and long-term stability. Furthermore, AgNPs_(TMSPED)-rGO/GC electrode able to be employed for the selective determination of GSH in amperometric analysis in the presence of ascorbic acid (AA), dopamine (DA), uric acid (UA) and glucose. Finally, this modified electrode was effectively applied to determine glutathione in real samples with good recoveries.

A colorimetric sensor of cysteine based on self-assembly nanostructures of Fe3+-H2O2/Tetramethylbenzidine system with "On-Off" switching function

Many strategies have been explored for selectively and sensitively detecting cysteine in different samples. Here, a novel colorimetric sensor based on self-assembly nanostructures of Fe³⁺-H₂O₂/Tetramethylbenzidine system with dual-level logic gate function and colorimetric determination of cysteine were firstly explored. The proposed Fe³⁺-H₂O₂-TMB system provides a sensitive optical signal due to the selectively reductive ability of cysteine to the oxidized TMB and thus could be successfully applied to the construction of instant on-site visual detection method with a paper based test strip for cysteine determination in a sample solution as well as for a dual-level logic gate fabrication.

Electropolymerized Pyrrole-Based Conductive Polymeric Ionic Liquids and Their Application for Solid-Phase Microextraction

Pyrrole was covalently bonded to 1-methyl and 1-benzylimidazolium ionic liquids (ILs) via an N-substituted alkyl linkage to prepare electropolymerizable IL monomers with excellent thermal stability. The methylimidazolium IL, [pyrrole-C₆MIm]⁺, was then electropolymerized on macro- and microelectrode materials to form conductive polymeric IL (CPIL)-modified surfaces. Electrochemical characterization of a 1.6 mm diameter Pt disk electrode modified with poly[pyrrole-C₆MIm]⁺ demonstrated a selective uptake for an anionic redox probe while rejecting a cationic redox probe. Furthermore, electropolymerization of [pyrrole-C₆MIm]⁺ doped with single-walled carbon nanotubes (SWNT) on 125 μm platinum wires produced 42 μm thick poly[pyrrole-C₆MIm]⁺/SWNT films compared to 17 μm in the absence of SWNT and 5 μm for the previously reported poly[thiophene-C₆MIm]⁺ coatings. The poly[pyrrole-C₆MIm]⁺/SWNT films were prepared with reproducible thicknesses as well as thermal properties sufficient for high-temperature applications, such as solid-phase microextraction (SPME) with gas chromatographic analysis. The utilization of the CPIL sorbent materials in SPME experiments provided excellent extraction efficiencies and selectivity toward organic aromatic analytes. The CPIL sorbent coatings also yielded outstanding fiber-to-fiber reproducibility on the basis of extraction efficiencies and improved response for a range of analytes relative to commercial 100 μm poly(dimethylsiloxane) fibers when normalized for differences in film thickness. Poly[pyrrole-C₆MIm]⁺ CPIL coatings doped with SWNT are therefore promising new sorbent materials for SPME analyses.

Comparison of glutathione levels measured using optimized monochlorobimane assay with those from ortho-phthalaldehyde assay in intact cells

Fluorometric glutathione assays have been generally preferred for their high specificity and sensitivity. An additional advantage offered by fluorescent bimane dyes is their ability to penetrate inside the cell. Their ability to react with glutathione within intact cells is frequently useful in flow cytometry and microscopy. Hence, the aims of our study were to use monochlorobimane for optimizing a spectrofluorometric glutathione assay in cells and then to compare that assay with the frequently used ortho-phthalaldehyde assay. We used glutathione-depleting agents (e.g., cisplatin and diethylmalonate) to induce cell impairment. For glutathione assessment, monochlorobimane (40μM) was added to cells and fluorescence was detected at 394/490nm. In addition to the regularly used calculation of glutathione levels from fluorescence change after 60min, we used an optimized calculation from the linear part of the fluorescence curve after 10min of measurement. We found that 10min treatment of cells with monochlorobimane is sufficient for evaluating cellular glutathione concentration and provides results entirely comparable with those from the standard ortho-phthalaldehyde assay. In contrast, the results obtained by the standardly used evaluation after 60min of monochlorobimane treatment provided higher glutathione values. We conclude that measuring glutathione using monochlorobimane with the here-described optimized evaluation of fluorescence signal could be a simple and useful method for routine and rapid assessment of glutathione within intact cells in large numbers of samples.

Glassy carbon electrode modified with 7,7,8,8-tetracyanoquinodimethane and graphene oxide triggered a synergistic effect: Low-potential amperometric detection of reduced glutathione

A sensitive electrochemical sensor based on the synergistic effect of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and graphene oxide (GO) for low-potential amperometric detection of reduced glutathione (GSH) in pH 7.2 phosphate buffer solution (PBS) has been reported. This is the first time that the combination of GO and TCNQ have been successfully employed to construct an electrochemical sensor for the detection of glutathione. The surface of the glassy carbon electrode (GCE) was modified by a drop casting using TCNQ and GO. Cyclic voltammetric measurements showed that TCNQ and GO triggered a synergistic effect and exhibited an unexpected electrocatalytic activity towards GSH oxidation, compared to GCE modified with only GO, TCNQ or TCNQ/electrochemically reduced GO. Three oxidation waves for GSH were found at -0.05, 0.1 and 0.5V, respectively. Amperometric techniques were employed to detect GSH sensitively using a GCE modified with TCNQ/GO at -0.05V. The electrochemical sensor showed a wide linear range from 0.25 to 124.3μM and 124.3μM to 1.67mM with a limit of detection of 0.15μM. The electroanalytical sensor was successfully applied towards the detection of GSH in an eye drop solution.

An excited state intramolecular proton transfer dye based fluorescence turn-on probe for fast detection of thiols and its applications in bioimaging

In this study, a new fluorescent probe 2-(2'-hydroxy-5'-N-maleimide phenyl)-benzothiazole (probe 1), was designed and synthesized by linking the excited state intramolecular proton transfer (ESIPT) fluorophore to the maleimide group for selective detection of thiols in aqueous solution. The fluorescence of probe 1 is strongly quenched by maleimide group through the photo-induced electron transfer (PET) mechanism, but after reaction with thiol, the fluorescence of ESIPT fluorophore is restored, affording a large Stokes shifts. Upon addition of cysteine (Cys), probe 1 exhibited a fast response time (complete within 30s) and a high signal-to-noise ratio (up to 23-fold). It showed a high selectivity and excellent sensitivity to thiols over other relevant biological species, with a detection limit of 3.78×10^-8M (S/N=3). Moreover, the probe was successfully applied to the imaging of thiols in living cells.

Species-Specific Standard Redox Potential of Thiol-Disulfide Systems: A Key Parameter to Develop Agents against Oxidative Stress

Microscopic standard redox potential, a new physico-chemical parameter was introduced and determined to quantify thiol-disulfide equilibria of biological significance. The highly composite, codependent acid-base and redox equilibria of thiols could so far be converted into pH-dependent, apparent redox potentials (E’°) only. Since the formation of stable metal-thiolate complexes precludes the direct thiol-disulfide redox potential measurements by usual electrochemical techniques, an indirect method had to be elaborated. In this work, the species-specific, pH-independent standard redox potentials of glutathione were determined primarily by comparing it to 1-methylnicotinamide, the simplest NAD⁺ analogue. Secondarily, the species-specific standard redox potentials of the two-electron redox transitions of cysteamine, cysteine, homocysteine, penicillamine, and ovothiol were determined using their microscopic redox equilibrium constants with glutathione. The 30 different, microscopic standard redox potential values show close correlation with the respective thiolate basicities and provide sound means for the development of potent agents against oxidative stress.

Real time monitoring of aminothiol level in blood using a near-infrared dye assisted deep tissue fluorescence and photoacoustic bimodal imaging

The development of molecular probes for the detection and imaging of biological thiols is a major step forward diagnosing various types of diseases. Previously reported thiol imaging strategies were mainly based on a single mode of imaging with a limited application in vivo. In this work, we introduced an unsymmetrical near-infrared (NIR) squaraine dye (USq) as an exogenous contrast agent for photoacoustic and fluorescence bimodal imaging of thiol variations in live animals. USq exhibits a narrow absorption band at 680 nm that generates a photoacoustic signal and a strong NIR emission at 700 nm (ΦF = 0.27), which is applicable for deep tissue optical imaging. Both photoacoustic and fluorescence signals could selectively disappear in the presence of different thiols. Through in vitro and in vivo imaging studies, unique imaging capability of USq was demonstrated, and the effect of food uptake on the increased level of aminothiols in blood was confirmed.

Identification of lactate dehydrogenase as a mammalian pyrroloquinoline quinone (PQQ)-binding protein

Pyrroloquinoline quinone (PQQ), a redox-active o-quinone, is an important nutrient involved in numerous physiological and biochemical processes in mammals. Despite such beneficial functions, the underlying molecular mechanisms remain to be established. In the present study, using PQQ-immobilized Sepharose beads as a probe, we examined the presence of protein(s) that are capable of binding PQQ in mouse NIH/3T3 fibroblasts and identified five cellular proteins, including l-lactate dehydrogenase (LDH) A chain, as potential mammalian PQQ-binding proteins. In vitro studies using a purified rabbit muscle LDH show that PQQ inhibits the formation of lactate from pyruvate in the presence of NADH (forward reaction), whereas it enhances the conversion of lactate to pyruvate in the presence of NAD⁺ (reverse reaction). The molecular mechanism underlying PQQ-mediated regulation of LDH activity is attributed to the oxidation of NADH to NAD⁺ by PQQ. Indeed, the PQQ-bound LDH oxidizes NADH, generating NAD⁺, and significantly catalyzes the conversion of lactate to pyruvate. Furthermore, PQQ attenuates cellular lactate release and increases intracellular ATP levels in the NIH/3T3 fibroblasts. Our results suggest that PQQ, modulating LDH activity to facilitate pyruvate formation through its redox-cycling activity, may be involved in the enhanced energy production via mitochondrial TCA cycle and oxidative phosphorylation.

Synergistic effect of pyrroloquinoline quinone and graphene nano-interface for facile fabrication of sensitive NADH biosensor

A self-assembly composite of graphene-pyrroloquinoline quinone (PQQ) was fabricated and modified on glassy carbon electrode (GCE) for sensitive detection of nicotinamide adenine dinucleotide (NADH). Chitosan (CTS) was applied to disperse graphene to form a stable robust film on GCE. A synergistic effect between PQQ and graphene was observed during the electrocatalytic oxidation of NADH, with about 260mV reduction in the oxidation potential and 2.5-fold increase in the oxidation current compared with those on the bare GCE. The electrochemical sensors based on the modified electrodes allowed the detection of NADH with a good linear dependence from 0.32 to 220µM with a high sensitivity of 0.421µAµM^-1cm^-2 and a low detection limit of 0.16µM (S/N=3). It could also eliminate the interference of electroactive substances like ascorbic acid (AA), uric acid, and dopamine and its derivatives. The outstanding performances of graphene-PQQ/CTS composite capable of improving the electrical conductivity and accelerating the electron transport suggested its promising applications for design of different graphene based composites used in electrochemical sensing and energy fields.