Capability of a carbon-polyvinylchloride composite electrode for the detection of dopamine, ascorbic acid and uric acid

In this work, the composite carbon-polyvinylchloride (C-PVC) was used as an electrode for the detection of dopamine, ascorbic acid, uric acid and their mixtures by differential pulse voltammetry (DPV). The results showed that the untreated C-PVC electrode was selective and stable for the oxidation of dopamine in a mixture containing uric acid and an excess of ascorbic acid in acidic medium. The pre-treated C-PVC electrode in a neutral medium exhibited good resolution of the mixture components in the micro molar concentration range of DA. The ageing of the C-PVC electrode during longer time periods did not affect the peak potential and the detection of dopamine, uric acid and ascorbic acid in 0.1 M H₂SO₄. The practical analytical utility of the C-PVC electrode was demonstrated by the measurement of uric acid in human urine and serum samples without any preliminary pre-treatment.     

Highly sensitive and selective electrochemical determination of dopamine and ascorbic acid at Ag/Ag2S modified electrode

A biosensor electrode possessing highly sensitive and selective determination of dopamine (DA) is fabricated. This electrode, a silver (Ag) thin film on indium-tin-oxide glass, is treated with a silver sulfide (Ag₂S) film using electrochemical deposition. Active Ag ion is easier to form on Ag₂S than on pristine Ag, which prefers to attract ascorbic acid (AA). The Ag₂S layer reduces the oxidation potential of AA due to the electrostatic interaction, which results in well-separation of mixed oxidation responses to both of DA and AA. Besides, the Ag₂S-modified electrode exhibits dramatic electrocatalytic effect on the oxidation of DA in the presence of AA. In 0.1 M phosphate buffer solution at pH ∼ 7.0, the differential pulse voltammetric peak intensity linearly correlates with DA concentration in two regions, viz. 1.0–10, and 10–100 μM, with correlation coefficient of 0.998 and 0.995, respectively. The lowest concentration limit of 1.0 μM DA can be detected. The interference of AA effectively diminishes in the mixed solution. These features make the Ag₂S significant for selective and sensitive measurement of DA in the presence of excess AA.     

Ionothermal synthesis of gallophosphate molecular sieves in 1-alkyl-3-methyl imidazolium bromide ionic liquids

Gallophosphate (GaPO₄) molecular sieves were ionothermally prepared in 1-alkyl-3-methyl imidazolium bromide ionic liquids (ILs) with alkyl chain length varying from 2 to 6 carbons for the first time. Three kinds of GaPO₄ materials such as GaPO₄-a, cloverite (denoted as CLO below) and GaPO₄-LTA were obtained, and their structures and morphologies were characterized by powder and single crystal XRD, SEM and multi-nuclear NMR. In the case of 1-ethyl-3-methyl imidazolium bromide ionic liquid, large single crystals of GaPO₄-LTA with well-defined octahedral morphology was obtained in both sealed (ca. 400 μm in size) and open vessels (ca. 300 μm in size). Ionothermal synthesis is an effective method for the preparation of large single crystal of molecular sieves. With the alkyl chain length of the ionic liquids increasing from 2 carbons to 6 carbons, the size of GaPO₄-LTA decreased sharply. The organic template cations in the as-synthesized GaPO₄-LTA crystals may have two different kinds of conformational structures. Fluorine plays a role of co-template along with organic cations of ILs in the synthesis of GaPO₄-LTA and CLO, while only GaPO₄-a was obtained in the absence of fluorine. The reaction temperature can also affect the structure of the final structure. In the reaction medium of 1-ethyl-3-methyl imidazolium bromide IL, GaPO₄-LTA was obtained at 180 °C, while CLO was obtained at 150 °C.     

Corrosion inhibition of mild steel by alkylimidazolium ionic liquids in hydrochloric acid

The acid corrosion inhibition process of mild steel in 1 M HCl by 1-butyl-3-methylimidazolium chlorides (BMIC) and 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO₄) has been investigated using electrochemical impedance, potentiodynamic polarization and weight loss measurements. Potentiodynamic polarization studies indicate the studied inhibitors are mixed type inhibitors. For both inhibitors, the inhibition efficiency increased with increase in the concentration of the inhibitor and the effectiveness of the two inhibitors are in the order [BMIM]HSO₄ > BMIC. The adsorption of the inhibitors on mild steel surface obeyed the Langmuir's adsorption isotherm. The effect of temperature on the corrosion behavior in the presence of 5 × 10⁻³ M of inhibitors was studied in the temperature range of 303-333 K. The associated activation energy of corrosion and other thermodynamic parameters such as enthalpy of activation (ΔH), entropy of activation (ΔS), adsorption equilibrium constant (K_ads) and standard free energy of adsorption (ΔG_ads) were calculated to elaborate the mechanism of corrosion inhibition.     

Polymethylthiophene/Nafion-modified glassy carbon electrode for selective detection of dopamine in the presence of ascorbic acid

The possible use of an electrode modified with electroactive conductive poly(3-methylthiophene) (PMeT)/Nafion as a chemical sensor was investigated for the voltammetric analysis of Dopamine (DA). The electrochemical behavior of dopamine was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. By using a PMeT-modified glassy carbon (GC/PMeT) electrode, DA and Ascorbic Acid (AA) signals could be separated but the AA at high concentrations still caused significant interference by overlapping the DA peak. In comparison to the GC/PMeT electrode, the glassy carbon (GC/Nafion/PMeT) electrode modified with hybrid film Nafion/PMeT was found to permit a superior separation by shifting the oxidation of AA peak toward the less positive potential. The DPV curves for a mixture of DA and AA at an GC/Nafion/PMeT electrode in a 0.1 M H₂SO₄ solution showed peaks of DA and AA, at 0.45 and 0.21 V, respectively, indicating that the difference in the oxidation potential was 240 mV. In the 0.1 M H₂SO₄ solution, the oxidation peak current on the differential pulse voltammograms for the GC/PMeT electrode increased linearly with the concentration of DA in the range 1 × 10⁻⁶ to 1 × 10⁻³ M, and the oxidation peak current on the differential pulse voltammograms for the GC/Nafion/PMeT electrode in the range 5 × 10⁻⁷ to 2 × 10⁻⁴ M. The DA detection sensitivity of the GC/Nafion/PMeT electrode (26.7 μA μM⁻¹ cm⁻²) was 22 times higher than that of the GC/PMeT electrode (1.21 μA μM⁻¹ cm⁻²).     

Selective synthesis of triacetin from glycerol catalyzed by HZSM-5/MCM-41 micro/mesoporous molecular sieve

HZSM-5/MCM-41 molecular sieve(H-ZM) catalysts with well-defined micro/mesoporous structures were synthesized and showed high performance for selective synthesis of triacetin via the esterification reaction of glycerol with acetic acid. The conversion of glycerol was demonstrated to be 100% and the triacetin selectivity was over 91%, which can be attributed to the synergistic effect regarding suitable acidic property, excellent diffusion efficiency and good stability derived from the combined advantages of microporous molecular sieve HZSM-5 and mesoporous molecular sieve MCM-41.     

Development of a biomimetic chitosan film-coated gold electrode for determination of dopamine in the presence of ascorbic acid and uric acid

A gold electrode surface was modified using a dinuclear copper complex [Cu^II₂ (Ldtb)(μ-OCH₃)](BPh₄) and then coated with a chitosan film. This biomimetic polymer film-coated electrode was employed to eliminate the interference from ascorbic acid and uric acid in the sensitive and selective determination of dopamine. The optimized conditions obtained for the biomimetic electrode were 0.1 M phosphate buffer solution (pH 8.0), complex concentration of 2.0 × 10⁻⁴ M, 0.1% of chitosan and 0.25% of glyoxal. Under the optimum conditions, the calibration curve was linear in the concentration range of 4.99 × 10⁻⁷ to 1.92 × 10⁻⁵ M, and detection and quantification limits were 3.57 × 10⁻⁷ M and 1.07 × 10⁻⁶ M, respectively. The recovery study gave values of 95.2-102.6%. The lifetime of this biomimetic sensor showed apparent loss of activity after 70 determinations. The results obtained with the modified electrode for dopamine quantification in the injection solution matrix were in good agreement with those of the pharmacopoeia method.     

Enhanced electrochemical response in oxidative differential pulse voltammetry of dopamine in the presence of ascorbic acid at carboxyl-terminated boron-doped diamond electrodes

The differential pulse voltammetric (DPV) peak for dopamine (DA) oxidation was found to be highly amplified by the addition of ascorbic acid (AA) when carboxyl-terminated boron-doped diamond (BDD) electrodes were used as the working electrode. The DP voltammogram for a solution containing DA and AA obtained using a 4-pentenoic acid-modified BDD (4PA-BDD) electrode showed well-separated oxidation peaks for DA and AA at 0.4 and 0.6 V vs. Ag/AgCl, respectively. In addition, as the DA concentration increased at constant AA concentration, a simultaneous increase in the DA peak current density and decrease in the AA peak current density were observed. The slope of the calibration curve for the [DA] range of 1-10 μM in the presence of AA (500 μM) was seven times larger than that obtained in the absence of AA. Such an enhancement was found to be more efficient at 4PA-BDD than at oxidized-BDD, partly due to simple electrostatic effects and partly due to suppression of the polymerization of DA oxidation products by the terminal carboxyl groups. The enhanced detection method using a carboxyl-terminated BDD electrode should be an effective analytical tool, especially for determining low concentrations of DA in the presence of high concentrations of AA.     

Accelerating rate calorimetry studies of the reactions between ionic liquids and charged lithium ion battery electrode materials

Using accelerating rate calorimetry (ARC), the reactivity between six ionic liquids (with and without added LiPF₆) and charged electrode materials is compared to the reactivity of standard carbonate-based solvents and electrolytes with the same electrode materials. The charged electrode materials used were Li₁Si, Li₇Ti₄O₁₂ and Li_0.45CoO₂. The experiments showed that not all ionic liquids are safer than conventional electrolytes/solvents. Of the six ionic liquids tested, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) shows the worst safety properties, and is much worse than conventional electrolyte. 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI) and 1-propyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (Py13-FSI) show similar reactivity to carbonate-based electrolyte. The three ionic liquids 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide (BMMI-TFSI), 1-butyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide (Pp14-TFSI) and N-trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imide (TMBA-TFSI) show similar reactivity and are much safer than the conventional carbonate-based electrolyte. A comparison of the reactivity of ionic liquids with common anions and cations shows that ionic liquids with TFSI⁻ are safer than those with FSI⁻, and liquids with EMI⁺ are worse than those with BMMI⁺, Py13⁺, Pp14⁺ and TMBA⁺.     

Simultaneous voltammetric determination of ascorbic acid, dopamine and uric acid by methylene blue adsorbed on a phosphorylated zirconia-silica composite electrode

This paper describes the preparation, characterization and application of a composite electrode based on methylene blue adsorption to phosphorylated zirconia-silica mixed oxide particles prepared by a sol-gel process. This electrode electrocatalytically oxidizes ascorbic acid (AA), dopamine (DA) and uric acid (UA), allowing their simultaneous voltammetric detection. Well-defined and -separated oxidation peaks were observed by differential pulse voltammetry in a 0.35 mol l⁻¹ Tris-HCl buffer solution (pH 7.4) containing 0.5 mol l⁻¹ KCl. The anodic peak currents observed at −74, 94 and 181 mV increased with increasing concentrations of AA, DA and UA, respectively. Linear calibration plots were obtained over the range of 100-1600 μmol l⁻¹ for ascorbic acid, 6-100 μmol l⁻¹ for dopamine and 22-350 μmol l⁻¹ for uric acid with detection limits of 8.3 ± 0.1, 1.7 ± 0.1 and 3.7 ± 0.2 μmol l⁻¹, respectively. DA and UA concentrations could also be determined under conditions of excess AA (1 mmol l⁻¹).     

Towards understanding the structure and capacitance of electrical double layer in ionic liquids

In order to understand basic principles of the double layer formation in room temperature ionic liquids, we have performed Molecular Dynamic simulations for a simplified system: dense assembly of charged Lennard-Jones spheres between charged walls. For simplicity, in this first investigation we have considered the cations and anions of the same size. We have calculated the corresponding values of the double layer capacitance as a function of the electrode potential and compared the results with existing theories. We have found that the capacitance curve does not follow the U-shape of the Gouy-Chapman theory, but has a bell-shape in agreement with the mean-field theory that takes into account the effect of limited maximum packing of ions. The wings of capacitance decrease inversely proportional to the square root of the electrode potential, as prescribed by the mean-field theory and the charge conservation law at large electrode polarizations. We have found, however, that the mean-field theory does not quantitatively reproduce the simulation results at small electrode potentials, having detected their remarkable overscreening effects (ionic correlations). The plots for the distributions of ions near the electrode at different electrode charges show that for the considered system, unlike it is often assumed, the double layer is not one layer thick. The overscreening effects, dominating near the potential of zero charge (p.z.c.), are suppressed by the high electrode polarizations, following the onset of the so-called ‘lattice saturation effect’. The maximum of the capacitance coincides with the p.z.c., but it is true only for this ‘symmetric’ system. If sizes of cations and anions are different the maximum will be shifted away from the p.z.c., and generally the shape of the capacitance curve could be more complicated.     

Solubility of neutral and charged polymers in ionic liquids studied by laser light scattering

The solubility and chain conformation of different types of homopolymers in low viscosity ionic liquids (ILs), 1-allyl-3-methylimidazolium chloride ([AMIM][Cl]) at 50 °C and 1-butyl-3-methylimidazolium formate ([BMIM][COOH]) at 25 °C, were studied by laser light scattering (LLS). For neutral polymers, such as polyvinyl alcohol and polysulfonamide, aggregation occurred in all the cases except for polyvinyl alcohol in [BMIM][COOH]. For negative polyelectrolytes, such as DNA and polystyrene sulfonate, single chain conformation was observed. However, the hydrodynamic radius of both polymers was much smaller than that in good solvents, suggesting that the chains were condensed. Cellulose was soluble in [AMIM][Cl], and non-diffusive mode was observed by dynamic light scattering. Zeta potential analysis indicated that cellulose exhibited the feature of polyelectrolyte. The solubility of homopolymers could be qualitatively explained by treating polymer/IL as a ternary system: polymer, cation, and anion. It was the mutual interactions determined the solubility and conformation of polymers in ILs.     

Selective determination of dopamine in the presence of ascorbic acid using ferrocenyl-tethered PAMAM dendrimers modified glassy carbon electrode

Determination of dopamine (DA) in the absence and presence of ascorbic acid (AA) by ferrocenyl-tethered PAMAM G3 dendrimers (Fc-D) modified glassy carbon electrode (GCE) was reported. The modified electrode was characterized with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Factors influencing the detection processes were optimized and kinetic parameters were calculated. The sensor exhibited excellent catalytic activities for the oxidation–reduction reactions of DA and eliminated the interference of AA. Under optimal condition, the linear range of 1 × 10⁻⁵–1.5 × 10⁻³ mol L⁻¹ and the detection limit of 4.7 × 10⁻⁶ mol L⁻¹ was obtained. This study provides a new idea for the determination of DA in the presence of AA.     

Temperature dependence of kinetics and diffusion coefficients for ferrocene/ferricenium in ammonium-imide ionic liquids

The electrochemical behavior of ferrocene (Fc) and ferricenium (Fc⁺) was investigated by cyclic voltammetry with convolutional and semi-differential electroanalyses in the temperature range 298-373 K in the hydrophobic room temperature ionic liquid systems consisting of bis(trifluoromethanesulfone)imide anion with quaternary ammonium cation. The experimental results indicated that the redox reaction of Fc/Fc⁺ was reversible in this ionic liquid and the charge-transfer rate constant (k⁰) has been obtained. Mass transport towards the electrode is a simple diffusion process and the diffusion coefficient (D) for Fc/Fc⁺ has been also calculated. These results indicated that the k⁰ and D increased with increasing temperature in ionic liquids. The validity of the Arrhenius law was verified by investigating the temperature dependences of k⁰ and D.     

A novel and efficient catalytic epoxidation of monoterpenes by homogeneous and heterogeneous methyltrioxorhenium in ionic liquids

A convenient and efficient synthesis of monoterpene epoxides by application of methyltrioxorhenium and heterogeneous poly(4-vinylpyridine)/methyltrioxorhenium and microencapsulated polystyrene/methyltrioxorhenium catalytic systems in ionic liquids is described here. It was found that even highly sensitive terpenic epoxides were obtained in excellent yields. Environmentally friendly, easily available and low cost (UHP) urea hydrogen peroxide adduct was used as the primary oxidant. Catalysts were stable systems for at least four recycling oxidations. Experimental results showed that the reactions performed in ionic liquids were more selective and efficient than the ones performed in molecular solvents.     

Electrochemical oxidation of dopamine in the presence of sulfhydryl compounds: Application to the square-wave voltammetric detection of penicillamine and cysteine

Electro-oxidation of dopamine at a glassy carbon electrode was investigated in the presence of some biologically important thiols (R-SH), e.g. cysteine and penicillamine. Results of cyclic voltammetric studies together with the spectrophotometric foundations via Ellman's test during the controlled-potential coulometry show a nucleophilic addition/reduction of thiol to the electrochemically generated dopaminoquinone by a 1 + 4 Michael addition. The resulting ring substituted substrate (as RS-form) is more easily oxidized leading to an increase in the anodic current of dopamine, which is proportional to the concentration of thiol. The square-wave voltammetry (SWV) were applied as a very sensitive voltammetric detection method for the detection of cysteine and penicillamine. A linear response from 0.10 to 2.5 μM with a detection limit of 0.08 μM is resulted by the voltammetric determinations for penicillamine. The effect of other biological thiols, such as N-acetyl cysteine, glutathione, methionine, captopril and mercaptoglycine were also assessed and found to present no appreciable change in the voltammetric response of dopamine. The proposed method can be considered as a relatively selective method for the voltammetric detection of penicillamine and cysteine. The practical utility of the method was investigated in the determination of thiol species in pharmaceutical preparations.     

Application of thionine-nafion supported on multi-walled carbon nanotube for preparation of a modified electrode in simultaneous voltammetric detection of dopamine and ascorbic acid

A modified carbon-paste electrode (CPE) is prepared by incorporating thionine-nafion supported on multi-walled carbon nanotube (MWCNT). The electrochemical behavior of dopamine (DA) and ascorbic acid (AA) on the surface of the modified electrode is investigated by cyclic voltammetry (CV). The results show that thionine effectively immobilized in the matrix of the paste by using an appropriate mixture of nafion/MWCNT under the ultrasonic condition. On the other hand, presence of nafion enhances the stability of the thionine supported by MWCNT in the composite electrode and improves the reproducibility of the surface of the modified electrode under renewing process by polishing. The results of cyclic and differential pulse voltammetric investigations show that the modified electrode possesses an efficient electrocatalytic activity for the electrochemical oxidation of DA and AA and a peak potential separation nearly 379 mV is resulted for two compounds. The prepared modified electrode does not show any considerable response toward the electro-oxidation of sulfhydryl compounds, such as, cysteine, penicillamine and glutathione, revealing a good selectivity for voltammetric response to AA and DA in clinical and pharmaceutical preparations. The effective electrocatalytic property, excellent peak resolution and ability for masking the voltammetric responses of the other biologically reducing agents, make the modified electrode suitable for simultaneous and sensitive voltammetric detection of sub-micromolar amounts of AA and DA.     

Simultaneous determination of ascorbic acid, dopamine and uric acid by use of a MWCNT modified carbon-ceramic electrode and differential pulse voltammetry

Multi walled carbon nanotube modified carbon-ceramic electrode (MWCNT/CCE) was employed for the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The MWCNT/CCE displayed very good electrochemical catalytic activities with respect to CCE. The oxidation over-potentials of AA, DA and UA decreased dramatically, and their oxidation peak currents increased significantly at MWCNT/CCE compared to those obtained at the bare CCE. Differential pulse voltammetry was used for the simultaneous determination of AA, DA and UA in their ternary mixture. The peak separation between AA and DA, and DA and UA was large up to 205 and 160 mV, respectively. The calibration curves for AA, DA and UA were obtained in the range of 15.00-800.00, 0.50-100.00, and 0.55-90.00 μM, respectively. The detection limits (S/N = 3) were 7.71, 0.31, and 0.42 μM for AA, DA and UA, respectively.The present method was applied to the determination of AA, DA and UA in human serum and some commercial pharmaceutical samples, using standard adding method and the results were quite promising.     

无机膜回收分子筛催化剂过程阻力的研究与测定

介绍了无机膜回收分子筛催化剂过程中的膜污染机理及影响因素,阐述了滤饼层的形成是膜污染的主要来源,同时对膜阻力进行了测定。得出实验所用的无机微滤膜在23℃、错流过滤速度2 m/s、操作压力为0.1 MPa的操作条件下过滤分子筛催化剂过程中所产生的总阻力Rt为3.26×1012m-1,污染所产生的阻力Rf为2.733×1012m-1,组件自身的阻力Rm为5.27×1011m-1,Rf约为Rm的5倍,是导致膜通量下降的主要原因。     

Voltammetric determination of ascorbic acid and dopamine in the same sample at the surface of a carbon paste electrode modified with polypyrrole/ferrocyanide films

Functionalized polypyrrole film were prepared by incorporation of [Fe(CN)₆]⁴⁻ as a doping anion, during the electropolymerization of pyrrole onto a carbon paste electrode in an aqueous solution by potentiostatic method. The electrochemical behavior of dopamine (DA) and ascorbic acid (AA) in one solution was studied at the surface of bare and modified carbon paste electrodes using cyclic voltammetry (CV), linear sweep voltammetry (LSV) and differntial pulse voltammetry (DPV) methods. The well separated anodic peaks for oxidation of DA and AA were observed at the surface of the modified carbon paste electrode under optimum condition (pH 6.00), which can be used for determination of these species simultaneously in mixture by LSV and DPV methods. The linear analytical curves were obtained in the ranges of 0.10-1.00 mM and 0.10-0.95 mM for ascorbic acid and 0.10-1.20 mM and 0.20-0.95 mM for dopamine concentrations using LSV and DPV methods, respectively. The detection limits (2σ) were determined as 3.38 × 10⁻⁵ M and 1.34 × 10⁻⁵ M of ascorbic acid and 3.86 × 10⁻⁵ M and 1.51 × 10⁻⁵ M of dopamine by CV and DPV methods.