Oxidation Of L-cysteine Research Articles

Reprint of: l-Cysteine determination in embryo cell culture media using Co (II)-phthalocyanine modified disposable screen-printed electrodes

Thiol-containing compounds such as l-cysteine have been demonstrated to play an important role in metabolism and cellular growth, acting as powerful antioxidants. Consequently, their analytical determination in biological media has received a considerable amount of attention. In this work, an electrochemical sensor for the accurate electroanalytical determination of l-cysteine is proposed, based upon a Co(II)-phthalocyanine bulk modified disposable screen-printed graphite electrode (CoPc-SPE). Cyclic (CV) and Square Wave (SWV) voltammetry experiments have demonstrated an excellent electrocatalytic activity towards the electrochemical oxidation of l-cysteine using CoPc-SPEs within optimum neutral or basic pH. Moreover, the SWV response of l-cysteine is found to exhibit a linear range of 2.6–200μM, with a low limit of detection of 4μM (S/N=3) and a sensitivity of 0.78μAcm−2μM−1. Coefficient of variation for reproducibility and repeatability of l-cysteine determination using the CoPc-SPE are 3 and 0.4%, respectively. The effect of inherent interferences such as naturally occurring amino acids, cystine and cysteic acid has been also evaluated. Finally, the applicability of the l-cysteine electrochemical sensor based upon CoPc-SPEs has been successfully demonstrated for the first time for the assessment of l-cysteine in a complex embryo cell culture medium.

Electrochemical Studies of Zinc/Cysteine Interactions

Introduction The amino acid L-cysteine has been studied because of its role in several biochemical pathways (1). Electrochemical studies are possible due to the redox characteristics of the sulfhydryl group, so that the electrochemical characteristics of L-cysteine can be determined by voltammetric methods (2). In this study, the biochemical importance of zinc(II)/cysteine complexes in proteins (“zinc fingers”) has prompted an investigation of the interaction of ZnSO4with L-cysteine in various media. Experimental L-Cysteine was obtained from Sigma-Aldrich Company, and ZnSO4 .7 H2O was obtained from ThermoFisher Scientific. A Gamry Interface 1000 potentiostat with Gamry Framework software was used to carry out electrochemical experiments. Potentials are referenced against a Ag/AgCl reference electrode. Platinum and glassy carbon electrodes were purchased from BASi. Gold electrodes were obtained from eDAQ. Results and Discussion In a study of the complexation of Zn2+ by L-cysteine in pH 7.4 phosphate buffer, it was found that addition of ZnSO4 . 7 H2O to the buffer solution produced a cloudy appearance due to the low solubility of the resulting Zn3(PO4)2 . Upon addition of L-cysteine, the system was still cloudy at the 1:1 L-cysteine : Zn2+ point; however, at the 2:1 L-cysteine : Zn2+ point, the solution became clear. This behavior evidently occurs because the interaction of Zn2+ with L-cysteine is stronger than is its interaction with phosphate. These results are consistent with Zn(cys)2 complexation reported in the literature (3). The corresponding voltammograms at glassy carbon are shown in Figure 1. For ZnSO4 addition, the scan showed only a very small zinc stripping peak at the negative end of the potential range. At the 1:1 L-cysteine : Zn2+ point, processes are clearly evident for zinc deposition/stripping, and oxidation of complexed L-cysteine is observed at +0.7 V vs Ag/AgCl. This oxidation process is slightly altered compared to that observed for only L-cysteine (no added Zn2+) (2). Similar behavior for the oxidation processes was also observed at gold. Further additions of L-cysteine gave larger currents for the L-cysteine oxidation process. These results support the uptake of zinc ion into the solution as L-cysteine interacts with Zn2+, initially added as ZnSO4 . 7 H2O in pH 7.4 phosphate buffer. References C.K. Mathews, K.E. Van Holde, D.R. Appling, and S. J. Anthony-Cahill, Biochemistry, 4th Edition, Pearson Canada, Toronto, 2013. G. T. Cheek and M. A. Worosz, ECS Transactions, 2016, 72(27), 1-8. S. Foley and M. Enescu, Vibrational Spectroscopy, 2007, 44, 256-265. Figure 1. Cyclic voltammograms at glassy carbon (3 mm diameter) in aqueous pH 7.4 phosphate buffer, 100 mV/s scan rate. [ Dashed red line : ZnSO4 . 7 H2O, 2.1 mM nominal concentration ] [ Solid black line : Addition of 2.4 mM L-cysteine to previous solution ] Figure 1

A magnetic CoFe2O4/SiO2 nanocomposite fabricated by the sol-gel method for electrocatalytic oxidation and determination of L-cysteine

Magnetic CoFe2O4/SiO2 spinel-type nanocomposites have been fabricated by a sol-gel method in the presence of various acids. Their structural, morphological and magnetic properties were characterized by XRD, SEM, TEM, FTIR, VSM and EDX which revealed that they are formed in the presence of all precursors. TEM analysis indicates homogeneous and porous spherical morphology with nanosize grains 10–20 nm in diameter in the presence of salicylic acid. Electrochemical sensor application of nanocrystalline CoFe2O4/SiO2 synthesized by salicylic acid for determination of L-cysteine was investigated using differential pulse voltammetry (DPV) and cyclic voltammetry (CV). DPV indicates that the sensor shows remarkable sensitivity for the determination of L-Cys. The response of a glassy carbon electrode modified with CoFe2O4/SiO2 is linear in the 0.02–425 μM L-Cys concentration range, with a 0.20 μM detection limit (at an S/N ratio of 3). The electrode produces a negligible current response for tryptophan, glutamic acid and citric acid at the working potential applied (+0.748 V vs Ag/AgCl). The electrode is reliable, simple, rapidly prepared, precise, and the method does not require extensive sample treatment.

L- Cysteine Based Polymer Matrix Decorated with Au-Nanoparticles: As a Sensing Platform for Simultaneous Determination of Hydroquinone and Catechol

In the present study reported, fully electrochemical methodology was used to prepare a new nanocomposite on a glassy carbon electrode. Cysteic acid was formed by electrochemical oxidation of L-cysteine on the surface of a glassy carbon electrode (GCE) and used as a proper polymeric framework for deposition of Au nanoparticles (AuNPs). Field emission scanning electron microscopy (FESEM) was used for the surface characterization. It was shown that this composite electrode not only separates the voltammetric signals of hydroquinone (HQ) and catechol (CC), but also shows higher oxidation current for these molecules. Under the optimized condition, a linear dynamic range of 0.09 μmol dm−3 to 39.2 μmol dm−3 range for hydroquinone with the detection limit of 20 nM and from 0.09 μmol dm−3 to 39.2 μmol dm−3 for catechol with the detection limit of 60 nM were obtained. The proposed method was evaluated by determination of HQ and CC in rain and tap water samples with satisfactory results (recovery > 96%).

A glassy carbon electrode modified with carbon quantum dots and polyalizarin yellow R dyes for enhanced electrocatalytic oxidation and nanomolar detection of l-cysteine

A novel electrochemical sensor for the detection of l-cysteine was proposed based on immobilizing poly (alizarin yellow R)/carbon quantum dots film on glassy carbon electrode. Hydrothermal treatment was used to prepare carbon quantum dots. Transmission electron microscopy and Fourier transform infrared spectroscopy were used for characterization of carbon quantum dots. Electrochemical impedance spectroscopy, cyclic voltammetry and amperommetry were utilized to confirm the successful stepwise assembly procedure of the sensor. The electrocatalytic behaviors of the sensor were also investigated by cyclic voltammetry and amperommetry. Results showed that poly (alizarin yellow R)/carbon dots exhibited a remarkable electrocatalytic activity for the oxidation of l-cysteine under optimal conditions. The electrocatalytic response of the sensor was proportional to the l-cysteine concentration in the range of (0.3 to 3.6μM) and (3.9 to 7.2μM) with a limit of detection and sensitivity of 90nM and 0.482μA/μM, respectively. The modified electrode demonstrated many advantages such as simple preparation, high sensitivity, low detection of limit, excellent catalytic activity at physiological pH values, short response time, and remarkable antifouling property toward l-cysteine and its oxidation product.

Reactivity descriptors for iron porphyrins and iron phthalocyanines as catalysts for the electrooxidation of reduced glutathione

We have examined the electrocatalytic activity for the oxidation of glutathione of a wide variety of molecular catalysts: FeN4 complexes (MN4 = metal porphyrins and metal phthalocyanines) confined on the surface of graphite electrodes in order to establish reactivity descriptors for these catalysts for this reaction. We have conducted these studies mainly in alkaline media (pH = 13). The reaction order in OH− is 1 for pH values lower than 8. For higher pH values, the reaction becomes pH independent. The reaction order in glutathione is close to 1 in the concentration range examined (10−3–10−2 M). The activity of the surface-confined MN4 complexes is related to the Fe(II)/(I) and the Fe(III)/(II) redox transitions of the immobilized FeN4 complexes. The catalysts are active only in the potential range where the Fe(II) state predominates. The activity as (log j) E versus the Fe(II)/(I) formal potential varies in a non-linear fashion giving a volcano correlation as previously observed for the oxidation of L-cysteine and many other reactions catalyzed by MN4 complexes. A plot of (E)logj versus the Fe(II)/(I) formal potential gives also an asymmetrical volcano, with one of the branches with a slope close to unity. These volcano correlations clearly shows that the Fe(II)/(I) redox potential needs to be tuned to a certain potential to obtain a maximum activity for the oxidation of glutathione. Most Fe porphyrins show low activity because the metal center Fe(III) is in the wrong oxidation state in the potential range studied.

L-Cysteine determination in embryo cell culture media using Co (II)-phthalocyanine modified disposable screen-printed electrodes

Thiol-containing compounds such as l-cysteine have been demonstrated to play an important role in metabolism and cellular growth, acting as powerful antioxidants. Consequently, their analytical determination in biological media has received a considerable amount of attention. In this work, an electrochemical sensor for the accurate electroanalytical determination of l-cysteine is proposed, based upon a Co(II)-phthalocyanine bulk modified disposable screen-printed graphite electrode (CoPc-SPE). Cyclic (CV) and Square Wave (SWV) voltammetry experiments have demonstrated an excellent electrocatalytic activity towards the electrochemical oxidation of l-cysteine using CoPc-SPEs within optimum neutral or basic pH. Moreover, the SWV response of l-cysteine is found to exhibit a linear range of 2.6–200μM, with a low limit of detection of 4μM (S/N=3) and a sensitivity of 0.78μAcm−2μM−1. Coefficient of variation for reproducibility and repeatability of l-cysteine determination using the CoPc-SPE are 3 and 0.4%, respectively. The effect of inherent interferences such as naturally occurring amino acids, cystine and cysteic acid has been also evaluated. Finally, the applicability of the l-cysteine electrochemical sensor based upon CoPc-SPEs has been successfully demonstrated for the first time for the assessment of l-cysteine in a complex embryo cell culture medium.

Electrochemical Studies of Cysteine

Introduction L-Cysteine is an important amino acid because of its role in various biochemical pathways (1). The sulfhydryl functional group gives L-cysteine significant redox characteristics, prompting an investigation of its electrochemical behavior in various media and electrode surfaces. The interaction of protein cysteine residues with Zn2+ (“zinc fingers”) also suggested a voltammetric study of the effect of Zn2+ on L-cysteine electrochemistry. Experimental L-Cysteine was obtained from Sigma-Aldrich. Electrochemical experiments were carried out using a Gamry Interface 1000 potentiostat and Gamry Framework software. Potentials are referenced against a Ag/AgCl reference electrode. Platinum and glassy carbon electrodes were obtained from BASi. Gold electrodes were purchased from eDAQ. Results and Discussion The voltammetric behavior of 5 mM Zn2+ in 0.10 M KNO3 is shown in the figure. Zinc deposition at -1.0 V was not very distinct, and there was no well-defined stripping peak. Addition of equimolar L-cysteine produced a definite stripping peak at -0.8 V, evidently an effect of complexation of Zn2+ by L-cysteine. The L-cysteine voltammetry was also affected by the presence of Zn2+. At platinum and gold with no addition of Zn2+, oxidation of L-cysteine occurred as merged peaks at +0.8 and +1.0 V (2), with no oxidation feature at +0.6 V. Even with Zn2+ added to L-cysteine n solution, an initial positive-going scan gave no +0.6 V peak, and scanning through the zinc reduction/stripping region was necessary to produce the +0.6 V feature on the second sweep to more positive potentials as shown in the figure. The stripping of the zinc layer from the gold surface may produce a renewed gold surface which allows strong adsorption of L-cysteine on gold. In general, L-cysteine is known to interact strongly with gold surfaces (3). References C. K. Mathews, K. E. Van Holde, D. R. Appling, and S. J. Anthony-Cahill, Biochemistry, 4th Edition, Pearson Canada, Toronto, 2013. J. Koryta and J. Pradac, J. Electroanal. Chem., 1968, 17, 185-189. A. J. Tudos, P. J. Vandeberg, and D. C. Johnson, Analytical Chem., 1995, 67, 552-556. Figure 1. Cyclic voltammograms at gold (1 mm diameter) in aqueous 0.10 M KNO3. 100 mV/s. [ Dashed line : 5.0 mM ZnSO4 ] [ Solid line : 5.0 mM ZnSO4 + 5.3 mM L-cysteine ] Figure 1

Catalytic Single-Particle Nano-Impacts: Theory and Experiment. Poly(vinylferrocene)-Modified Graphene Nanoplatelet Mediated l-Cysteine Oxidation

The theory of catalytic nano-impacts is developed specifically for the case of a single particle in contact with an electrode for a short period of time in which it mediates electron transfer to or from a species in homogeneous solution. The theory is applied to impacts of chemically modified graphene nanoplatelets in which the immobilized ferrocene/ferrocenium couple is used to mediate the oxidation of l-cysteine in aqueous solution. Theory and experiment are in good agreement, and the catalytic rate constant is found to be (2.4 ± 1.2) × 103 M–1 s–1.

Molybdenum nitride/nitrogen-doped multi-walled carbon nanotubes hybrid nanocomposites as novel electrochemical sensor for detection l-cysteine

In this work, we reported a novel, facile and effective strategy by using hydrothermal reaction and following high-temperature pyrolysis under the ammonia flow to fabricate the molybdenum nitride nanosheets/N-doped multi-walled carbon nanotubes (MoN/N-MWNTs) hybrid nanocomposite. MoN/N-MWNTs nanocomposite was characterized via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and electrochemical methods. This nanocomposite exhibits excellent electrocatalytic oxidation of l-cysteine (l-CySH). An amperometric l-CySH sensor was constructed based on the MoN/N-MWNTs modified glassy carbon electrode. The as-prepared MoN/N-MWNTs nanocomposite has favourable conductivity, large surface area due to the MWNTs as supporter and highly exposed active sites of MoN. Moreover, the influence of different loading amounts of MoN was researched. The optimized MoN/N-MWNTs catalyst showed a wide detection range that was composed by two line segments (5μM– 0.79mM and 0.79mM–12.60mM), a sensitivity of 198.59nAμM−1cm−2 in the low concentration range, a rapidly response within 1.5s as well as good reproducibility and stability.

Dealing with the sulfur part of cysteine: four enzymatic steps degrade l-cysteine to pyruvate and thiosulfate in Arabidopsis mitochondria.

Amino acid catabolism is essential for adjusting pool sizes of free amino acids and takes part in energy production as well as nutrient remobilization. The carbon skeletons are generally converted to precursors or intermediates of the tricarboxylic acid cycle. In the case of cysteine, the reduced sulfur derived from the thiol group also has to be oxidized in order to prevent accumulation to toxic concentrations. Here we present a mitochondrial sulfur catabolic pathway catalyzing the complete oxidation of l-cysteine to pyruvate and thiosulfate. After transamination to 3-mercaptopyruvate, the sulfhydryl group from l-cysteine is transferred to glutathione by sulfurtransferase 1 and oxidized to sulfite by the sulfur dioxygenase ETHE1. Sulfite is then converted to thiosulfate by addition of a second persulfide group by sulfurtransferase 1. This pathway is most relevant during early embryo development and for vegetative growth under light-limiting conditions. Characterization of a double mutant produced from Arabidopsis thaliana T-DNA insertion lines for ETHE1 and sulfurtransferase 1 revealed that an intermediate of the ETHE1 dependent pathway, most likely a persulfide, interferes with amino acid catabolism and induces early senescence.

Formation, characterization and electrochemical properties of novel tetrasubstituted cobalt phthalocyanines bearing tetrahydropyran, furan and coumarin moieties

Cobalt phthalocyanines (CoPcs) bearing peripherally tetrasubstituted tetrahydropyran (thp) or furan (fur) moieties were formed and spectroscopically characterized. Structural elucidations of 4-(tetrahydropyran-2-methoxy)phthalonitrile (1) and 4-(furan-2-methylthio)phthalonitrile (2) were confirmed via single crystal X-ray analysis. The redox properties of CoPc-thp (3) and CoPc-fur (4) were investigated via cyclic and squarewave voltammetry as well as UV–Vis spectroelectrochemistry. Glassy carbon electrodes (GCEs) modified with 3, 4 and a previously reported coumarin (cou) substituted CoPc (CoPc-cou, 5), were tested for their electrocatalytic activities toward l-cysteine. While the bare GCE and 4-GCE showed no peaks for l-cysteine oxidation in the 0.0–0.70V potential window; 3-GCE and 5-GCE showed peaks at 0.42V and 0.52V, respectively. Kinetic parameters were determined by chronoamperometry studies. l-Cysteine oxidation using 3-GCE was found to proceed at a faster rate than 5-GCE.

Electrochemical Studies of L-Cysteine

Introduction L-Cysteine is an important amino acid because of its role in various biochemical pathways (1). Considering that L-cysteine is one of the few amino acids with significant redox characteristics, it was considered worthwhile to investigate its electrochemical behavior in various media and electrode surfaces. Experimental L-Cysteine was obtained from Sigma-Aldrich. Electrochemical experiments were carried out using a Gamry Interface 1000 potentiostat and Gamry Framework software. Potentials are referenced against a Ag/AgCl reference electrode. Platinum and glassy carbon electrodes were obtained from BASi. Results and Discussion At a platinum electrode, L-cysteine in aqueous 0.10 M KNO3 produced a cyclic voltammogram with two merged oxidation processes (Figure 1), which were much more clearly resolved using differential pulse voltammetry. Preparative electrolysis at the potential of the first oxidation process (+0.80 V) resulted in the precipitation of cystine, which is the coupling product formed as a result of one-electron L-cysteine oxidation. The current for the reduction peak at -0.60 V (dotted line in Figure 1) increased during the electrolysis, suggesting that this reduction process is related to cystine formation. Similar results, including cystine formation, were also obtained in aqueous phosphate buffer at pH 7. It is likely that the platinum oxide layer on the electrode surface plays a role in L-cysteine oxidation under these conditions (2). The results of additional studies of L-cysteine oxidation in other media, particularly dimethylsulfoxide (DMSO), are also planned for presentation. References C. K. Mathews, K. E. Van Holde, D. R. Appling, and S. J. Anthony-Cahill, Biochemistry, 4th Edition, Pearson Canada, Toronto, 2013.J. Koryta and J. Pradac, J. Electroanal. Chem., 17, 185-189 (1968). Figure 1. Cyclic voltammogram of 10 mM L-cysteine at platinum in aqueous 0.10 M KNO3. 100 mV/s. First sweep (solid line), second sweep (dotted line). Figure 1

Photoelectrochemical properties of bare fluorine doped tin oxide and its electrocatalysis and photoelectrocatalysis toward cysteine oxidation

We first revealed that the bare fluorine doped tin oxide (FTO) under the cathodic polarization over −0.7V (vs.SCE) shows very sensitive to the irradiating light in a wide wavelength region 850–400nm in the aqueous solution free of a redox couple, and its band gap of energy Eg is determined to be 1.38eV via the photoelectrochemical method. The bare FTO can effectively catalyze electrochemically L-cysteine (CySH) oxidation and especially shows the photocatalytic ability toward CySH oxidation. Thus the bare FTO electrode can be directly used for determination of CySH concentration using cyclic voltammetry in both the dark and light illumination and it can be used to recognize CySH among 20 α-amino acids found in proteins, based on the low oxidation peak potential and unique photoelectric response. The rate-determining step for the photocatalytic oxidation of CySH on the bare FTO electrode is controlled by supply of charge inside the FTO film to the electrode surface, which exhibits the typical characteristics of semiconductors.

Graphene quantum dot functionalized by beta-cyclodextrin: a novel nanocomposite toward amplification of l-cysteine electro-oxidation signals

A novel nano-composite of graphene quantum dot-β-cyclodextrin was fabricated on the surface of glassy carbon electrode (GCE) using one step and green electrodeposition method. The redox of l-Cys behavior prepared nano-composite-modified of GCE was then characterized by cyclic voltammetry, chronoamperometry, and differential pulse voltammetry. Voltammograms of the modified electrode are recorded in physiological pH (phosphate buffer solution, pH 7.4), the kinetics of charge transfer and mass transport processes across the nano-composite/solution interface were studied. The modified electrode showed an efficient electrocatalytic activity toward the oxidation of l-Cysteine (l-Cys) through a surface-mediated electron transfer. The catalytic rate constant and the l-Cys diffusion coefficient were reported. A sensitive and time-saving method (differential pulse voltammetry) was developed for the analysis of l-Cys. The proposed voltammetric method was also applied to the determination of l-Cys using graphene quantum dot-β-cyclodextrin-GCE.

The "Gln-Type" Thiol Dioxygenase from Azotobacter vinelandii is a 3-Mercaptopropionic Acid Dioxygenase.

Cysteine dioxygenase (CDO) is a non-heme iron enzyme that catalyzes the O2-dependent oxidation of l-cysteine to produce cysteinesulfinic acid. Bacterial CDOs have been subdivided as either "Arg-type" or "Gln-type" on the basis of the identity of conserved active site residues. To date, "Gln-type" enzymes remain largely uncharacterized. It was recently noted that the "Gln-type" enzymes are more homologous with another thiol dioxygenase [3-mercaptopropionate dioxygenase (MDO)] identified in Variovorax paradoxus, suggesting that enzymes of the "Gln-type" subclass are in fact MDOs. In this work, a putative "Gln-type" thiol dioxygenase from Azotobacter vinelandii (Av) was purified to homogeneity and characterized. Steady-state assays were performed using three substrates [3-mercaptopropionic acid (3mpa), l-cysteine (cys), and cysteamine (ca)]. Despite comparable maximal velocities, the "Gln-type" Av enzyme exhibited a specificity for 3mpa (kcat/KM = 72000 M(-1) s(-1)) nearly 2 orders of magnitude greater than those for cys (110 M(-1) s(-1)) and ca (11 M(-1) s(-1)). Supporting X-band electron paramagnetic resonance (EPR) studies were performed using nitric oxide (NO) as a surrogate for O2 binding to confirm obligate-ordered addition of substrate prior to NO. Stoichimetric addition of NO to solutions of 3mpa-bound enzyme quantitatively yields an iron-nitrosyl species (Av ES-NO) with EPR features consistent with a mononuclear (S = (3)/2) {FeNO}(7) site. Conversely, two distinct substrate-bound conformations were observed in Av ES-NO samples prepared with cys and ca, suggesting heterogeneous binding within the enzymatic active site. Analytical EPR simulations are provided to establish the relative binding affinity for each substrate (3map > cys > ca). Both kinetic and spectroscopic results presented here are consistent with 3mpa being the preferred substrate for this enzyme.

Electrocatalytic Determination of Cysteine Using a Carbon Ionic Liquid Electrode Modified with Terpyridine Copper(II) Complex

ABSTRACTThe oxidation of L-cysteine was studied on a carbon ionic liquid electrode modified with terpyridine copper(II) complex (4′-phenyl-2,2′:6’,2″-terpyridine copper(II)). Cysteine provided an oxidation peak at the relatively low overpotential value of approximately 40 mV at the modified electrode. The oxidation current significantly increased, suggesting high electrocatalytic activity of the terpyridine copper(II) complex toward cysteine. The oxidation current was proportional to cysteine concentration from 0.1 to 40 µM with a limit of detection 0.01 µM. The response of the modified electrode to cysteine was studied in the presence of ascorbic acid, tyrosine, glucose, glycine, uric acid, and dopamine. No interferences were observed. The method was employed for the determination of cysteine in serum.

Copper inorganic-organic hybrid coordination compound as a novel L-cysteine electrochemical sensor: Synthesis, characterization, spectroscopy and crystal structure

Dinuclear coordination compound of Cu(II), namely, [Cu2(pydc)2(pz)(H2O)2] ⋅2H2O, where pydc = pyridine-2,6-dicarboxylic acid (dipicolinic acid) and pz = pyrazine has been synthesized and characterized by elemental analysis, spectra (IR, UV-Vis), thermal (TG/DTG) analysis, magnetic measurements and single crystal X-ray diffraction. In the dimeric structure, the planar tridentate pyridine-2,6-dicarboxylic acid dianion coordinates to a Cu(II) ion in a meridional fashion and defines the basal plane of the complex. The fourth equatorial coordination site is then occupied by a pyrazine molecule that functions as a linear bidentate ligand bridging two Cu(II) complexes to form a dimer. The axial positions of each Cu(II) complex are occupied by one water molecule to form a distorted square pyramidal geometry. The complicated hydrogen bonding network accompanied with C–O ⋯π and C–H ⋯π stacking interactions assemble the crystal structure of 1 into a fascinating supramolecular architecture. Electrochemical behavior of [Cu2(pydc)2(pz)(H2O)2] (Cu-PDAP) on the surface of carbon nanotube (CNTs) glassy carbon electrode (GCE) is described. Oxidation of cysteine on the surface of modified electrode was investigated with cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The results show that the Cu-PDAP/CNTs film displays excellent electrochemical catalytic activities towards L-cysteine oxidation. Dinuclear coordination compound of Cu(II), namely, [Cu2(pydc)2(pz)(H2O)2].2H2O, has been synthesized and characterized by elemental analysis, spectral (IR, UV-Vis), thermal (TG/DTG) analysis as well as single crystal X-ray diffraction. Electrochemical behavior of [Cu2(pydc)2(pz)(H2O)2] on the surface of carbon nanotube (CNTs) glassy carbon electrode (GCE) was described and oxidation of cysteine on the surface of modified electrode was investigated.

A novel electrochemical sensor based on Fe2O3 nanoparticles/N-doped graphene for electrocatalytic oxidation of L-cysteine

A novel electrochemical sensor based on Fe2O3 nanoparticles supported on N-doped graphene (Fe2O3NPs/N–GR) has been fabricated for electrocatalytic oxidation and detection of L-cysteine. The Fe2O3NPs/N–GR was characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and electrochemical methods. The electrochemical behavior of L-cysteine at Fe2O3NPs/N–GR was investigated by voltammetry. The Fe2O3NPs/N–GR showed a very efficient electrocatalytic activity for the oxidation of L-cysteine in 0.1 M phosphate buffer solution (PBS, pH 7.0). The oxidation peak currents increased dramatically at Fe2O3NPs/N–GR-modified electrode. The potential utility of the sensor was demonstrated by applying it to the analytical determination of L-cysteine. Experimental results showed that the electrocatalytic current increased linearly with L-cysteine concentration in the range of 0.2–400 μM with a lowest detectable concentration of 0.1 μM.

Fabrication and electrochemical determination of l-cysteine of a composite film based on V-substituted polyoxometalates and Au@2Ag core–shell nanoparticles

In this work, a novel composite film containing H7P2Mo17V1O62 (P2Mo17V) and Au-Ag bimetallic core–shell nanoparticles (Au@Ag NPs) has been fabricated on quartz slides, silicon wafers and ITO electrodes by the layer-by-layer self-assembly technique. The composite film was characterized by UV–vis spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) measurements. Due to the synergistic effect between Au and Ag, the composite film modified electrode showed excellent electrocatalytic activity toward the oxidation of l-cysteine with a broad linear range of 2.5×10−8–7.625×10−6M, a low detection limit of 2.76×10−8M (S/N=3), a high sensitivity of 1.7946μA/μM and fast response time (<2s). In addition, the electrode exhibited excellent reproducibility and long-term stability as well as negligible interference from l-tryptophan, l(+)-glutamic acid, citric acid, lactic acid and glucose. Furthermore, the applicability of this sensor was successfully employed to detect l-cysteine in milk samples.