Direct electrochemical conversion of bilirubin oxidase at carbon nanotube-modified glassy carbon electrodes

We report on direct electron transfer reactions of bilirubin oxidase at multi-walled carbon nanotube (MWCNT) modified glassy carbon electrodes (GCE). The bioelectrocatalytic oxygen reduction was recorded using linear sweep voltammetry (LSV) with BOD in solution, adsorbed and covalently linked to the nanotubes. The MWCNT modification of GC electrodes strongly enhances the oxygen reduction compared to the signals at unmodified GCE. Under anaerobic conditions with a high protein concentration in solution a pair of redox peaks with a formal potential of 450 ± 15 mV vs Ag/AgCl, 1 M KCl (pH 7.4) was found with cyclic voltammetry. The redox conversion is indicated to be surface-controlled and pH-dependent (54.5 mV/pH). The quasi-reversible redox reaction might be attributed to the trinuclear T2/T3 cluster of BOD.     

Carbon-nanotube-modified glassy carbon electrodes for amplified label-free electrochemical detection of DNA hybridization

The preparation and attractive performance of carbon-nanotube modified glassy-carbon (CNT/GC) electrodes for improved detection of purines, nucleic acids, and DNA hybridization are described. The surface-confined multiwall carbon-nanotube (MWCNT) facilitates the adsorptive accumulation of the guanine nucleobase and greatly enhances its oxidation signal. The advantages of CNT/GC electrodes are illustrated from comparison to the common unmodified glassy carbon, carbon paste and graphite pencil electrodes. The dramatic amplification of the guanine signal has been combined with a label-free electrical detection of DNA hybridization. Factors influencing the enhancement of the guanine signal are assessed and optimized. The performance characteristics of the amplified label-free electrochemical detection of DNA hybridization are reported in connection to measurements of nucleic-acid segments related to the breast-cancer BRCA1 gene.     

Direct electrochemistry of catalase on glassy carbon electrodes.

Catalase was investigated as a possible catalyst of the electrochemical reduction of oxygen on glassy carbon electrodes. The presence of catalase dissolved in solution only provoked a moderate current increase, which was fully explained by the catalase-catalysed disproportionation of hydrogen peroxide (Scheme I). When catalase was adsorbed from dimethylsulfoxide on the surface of electrodes that did not undergo any electrochemical pre-treatment (EP), catalase efficiently catalysed oxygen reduction via direct electron transfer from the electrode (Scheme II). The results are discussed with respect to the electrode surface properties and the enzyme structure.     

Activation of glassy carbon electrodes by alternating current electrochemical treatment

Glassy carbon electrodes subjected to “severe” electrochemical treatment is saturated chloride solutions exhibit enhanced electrochemical reversibility compared to untreated electrodes. Apparently faster rates of electron-transfer result in lowering of the overpotential by 110–300 mV for eight electroactive species tested. Systematic alterations in the response in terms of peak potential, sensitivity, background current, and stability are reported. Scanning electron micrographs show the introduction of pits of random size distribution. From the practical point of view, the most significant improvement is observed in differential pulse measurements, that are highly sensitive to small changes in the rate of electron transfer. In amperometric detection for liquid chromatography, the pretreated electrodes are shown to facilitate the quantitation of different eluting species. It appears that the surface and bulk properties of the glassy carbon have profound effects on the observed behavior.     

Electrochemical behavior of glassy carbon electrodes modified by electrochemical oxidation

Glassy carbon electrodes were modified electrochemically by pretreatment in sulfate, phosphate or carbonate solutions by means of cycling the potential well into the positive limit of the solvent. Electrodes treated in this manner were then used to incorporate and concentrate a variety of redox species that were either cations or aromatic containing compounds, including Ru(bpy)²⁺₃, Ru(NH₃)³⁺₆, Cu(NH₃)²⁺₄, ferrocene, methylviologen, 1,4-benzoquinone, anthraquinone-2-sulfonate, riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Surface-equivalent concentrations ranged from 5 × 10^?9 to 1 × 10^?7 mol cm^?2 for electrodes pretreated for 10 min in sulfuric acid. An E_1/2 vs. pH study of 1,4-benzoquinone, riboflavin, FMN and FAD in modified electrodes shows that the pK_a values shift toward higher pH (nearly 2 pH units). Results concerning the incorporation of redox compounds detected only by mediation with other electroactive complexes and the study of the modified electrodes in electrocatalysis are also discussed.     

Electrochemical detection of herbicides and plant growth regulators at membrane glassy carbon electrodes

Membrane glassy carbon electrodes modified with cation-exchangers (Eastman AQ-29D, laponite, or polystyrene sulfonate) entrapped between the membrane and the carbon surface were used to study the electrochemical behavior of herbicides and plant growth regulators from the quaternary ammonium family. Cations are shown to incorporate in the entrapped solutions containing negatively charged cation-exchangers, because of favorable electrostatic interactions. In the case of nonelectroactive cations, collection of mepiquat, chlormequat and difenzoquat from the bulk solution is assessed from the competition in ion-exchange equilibrium between one of the above-mentioned cations and the electroactive diquat cation. An attractive property results from the use of spinach ferredoxin as cation-exchanger inside the membrane electrode. Promotion of ferredoxin is shown to be evidenced as a result of the collection of nonelectroactive cations from the bulk solution. Prospects of such modified membrane electrodes in the environmental field are discussed.     

Amperometric detection of cationic neurotransmitters at nafion-coated glassy carbon electrodes in flow streams

Substantial improvements in amperometric monitoring of flowing streams are obtained by using Nafion-coated working electrodes. The charged coating tends to exclude anionic and neutral interferences, thus adding a new dimension of selectivity to electrochemical detection for flow-injection and liquid-chromatographic systems. A highly selective response is observed for cationic neurotransmitters in the presence of otherwise interfering substances (e.g., ascorbic acid, uric acid, bilirubin or chlorpromazine). The permselectivity and transport characteristics are evaluated with respect to solution pH, solvent, flow rate, film thickness, and other variables. The reduced flow-rate dependence results in low noise levels and detection limits of 0.04 and 0.10 ng of norepinephrine and epinephrine, respectively. A bilayer electrode coating, with a cellulose acetate film over the Nafion layer, offers a bifunctional (selectivity, protection) capability. Applicability to urine samples is demonstrated.     

Enhanced electrochemical detection of ketorolac tromethamine at polypyrrole modified glassy carbon electrode.

A glassy carbon electrode modified with a coating of polypyrrole (Ppy) exhibited an attractive performance for the detection and determination of a non-steroidal and non-narcotic analgesic compound, ketorolac tromethamine (KT). Cyclic voltammetry, differential pulse and square wave voltammetry were used in a combined way to identify the electrochemical characteristics and to optimize the conditions for detection. For calibrating and estimating KT, square-wave voltammetry was mainly used. The drug shows a well-defined peak at -1.40 V vs. Ag/AgCl in the acetate buffer (pH 5.5). The existence of Ppy on the surface of the electrode gives higher electrochemical active sites at the electrode for the detection of KT and preconcentrate KT by adsorption. The square-wave stripping voltammetric response depends on the excitation signal and the accumulation time. The calibration curve is linear in the range 1 x 10(-11) to 1 x 10(-7) M with a detection limit of 1.0 x 10(-12) M. Applicability to serum samples was also demonstrated. A detection limit of 1.0 ng ml for serum was observed. Square-wave voltammetry shows superior performance over UV spectroscopy and other techniques.     

Permselectivity of neurotransmitters at overoxidized polypyrrole-film-coated glassy carbon electrodes

The permselectivity of neurotransmitters such as dopamine, epinephrine, and norepinephrine at overoxidized polypyrrole (OPPY)-film-coated glassy carbon electrodes has been investigated. The chemically-modified electrodes exhibit attractive permselectivity and antifouling properties of rejecting anionic species, e.g. ascorbate, etc. Compared with the response of neurotransmitters at modified electrodes overoxidized in phosphate buffer solution (pH 7.4), higher sensitivity and reversibility response can be obtained at modified electrodes overoxidized in sodium hydroxide solution. The effect of film thickness on the permselective response was tested. Rotating disk electrode experiments were used to determine the apparent diffusion coefficients of several electroactive solutes in the OPPY films. The influence of the hydrophobicity of the organic ions on the permeability within the polymer films was discussed. Dopamine and epinephrine were determined at the 1 x 10(-6)-1 x 10(-4) M level by means of voltammetry after an exposure period of 2 min in 0.1 M phosphate buffer (pH 7.4) with detection limits of 8 x 10(-7) M and 6 x 10(-7) M respectively.     

纳米金修饰玻碳电极对儿茶酚的催化氧化

研究了电沉积纳米金修饰玻碳电极对儿茶酚的催化氧化,发现该修饰电极使儿茶酚的氧化电位大大降低,峰电流显著增大并且线性范围较宽;讨论了酸度、沉积量、扫速等条件对儿茶酚在纳米金修饰玻碳电极上催化氧化的影响。该电极制备简单,重现性好,用于儿茶酚的定量分析,发现儿茶酚氧化峰电流与浓度在2.0×10-6~1.2×10-2mol/L范围内呈良好的线性关系,相关系数为0.9965,检出限(3σ)为3.2×10-7mol/L;用于样品检测,效果较好。     

Electrochemical behaviour of platinum at polymer-modified glassy carbon electrodes

In this paper, the preparations and voltammetric characteristics of chitosan-modified glassy carbon (Ct-MGC) and platinum electrodes are studied. Ct-MGC can be used for pre-concentration and quantification of trace amounts of platinum in solution. At low pH medium, the complex of Pt with protonated group-NH³⁺ in the chitosan molecule has been confirmed by FT-IR spectra studies.     

Direct electrochemical redox of tyrosinase at silver electrodes

The direct electron transfer reactions between tyrosinase and silver electrode were investigated by using cyclic voltammetry and potential-step chronoamperometry as well as current-step chronopotentiometry techniques. The kinetics of these reactions is quasi-reversible with two electron transfer reactions and 0.030 s(-1) apparent electrode reaction rate constant. The results demonstrate that neither electrode surface modification nor the inclusion of mediators is necessary to study the electron transfer reactions of tyrosinase at silver electrodes. Moreover, both the anodic and the cathodic currents are linear relationship with the tyrosinase concentration in the range of 1 x 10(-9) approximately 5 x 10(-8)moll(-1). It is possible to be used as a method of analyzing tyrosinase concentration.     

Electrochemical parameters of ethamsylate at multi-walled carbon nanotube modified glassy carbon electrodes

In this paper, some electrochemical parameters of ethamsylate at a multi-walled carbon nanotube modified glassy carbon electrode, such as the charge number, exchange current density, standard heterogeneous rate constant and diffusion coefficient, were measured by cyclic voltammetry, chronoamperometry and chronocoulometry. The modified electrode exhibits good promotion of the electrochemical reaction of ethamsylate and increases the standard heterogeneous rate constant of ethamsylate greatly. The differential pulse voltammetry responses of ethamsylate were linearly dependent on its concentrations in a range from 2.0 x 10(-6) to 6.0 x 10(-5) mol L(-1), with a detection limit of 4.0 x 10(-7) mol L(-1).     

Surface preparation of glassy carbon electrodes

Recent work on glassy carbon electrodes for various applications is reviewed. Activation of glassy carbon electrodes by different types of polishing, heat treatment, and electrochemical methods yields enhanced rates of electron transfer. Characterization of different glassy carbon surfaces by x-ray photoelectron spectroscopy shows that polished and electrochemically pretreated surfaces contain more oxygen on the surface than do unactivated surfaces; much of this oxygen is associated with phenolic groups. Causes of activation, characterization of glassy carbon by spectroscopic methods, and the role of surface cleanliness are summarized. For simple electron-transfer reactions, removal of contaminants from the electrode surface is important. For proton-coupled electrode reactions, specific interactions of reactants with catalytic groups created on the surface during polishing tend to play an important role in electrode activation     

Pulsed amperometric detection at glassy carbon electrodes: A new waveform for sensitive and reproducible determination of electroactive compounds

In this work, the application of a new pulsed amperometric detection (PAD) waveform at a glassy carbon electrode, operating in typical chromatographic mobile phases, is proposed for the sensitive and reproducible determination of arylethanolaminic and phenolic moiety based compounds (e.g. beta-agonists and polyphenols). Preliminary experiments by cyclic voltammetry were carried out to investigate the electrochemical behaviour and to select the detection and cleaning electrode potentials. The proposed potential-time profile was designed to prevent the carbon electrode fouling under repeated analyses, thus ensuring a reproducible and sensitive quantitative determination, without the need of any mechanical or chemical electrode cleaning procedure. The waveform electrochemical parameters, including detection and delay times, were optimized in terms of sensitivity, limit of detection and response stability. The optimized waveform allowed the sensitive and stable detection of model compounds, such as clenbuterol and caffeic acid, that showed detection limits of 0.1 μg L⁻¹ and 14 μg L⁻¹, quantification limits of 0.4 μg L⁻¹ and 46 μg L⁻¹, and linearity up to 100 μg L⁻¹ (r = 0.9993) and 10 mg L⁻¹ (r = 0.9998), respectively. Similar results were obtained for other compounds of the same classes, with precision values under repeatability conditions ranging from 3.0 to 5.9%. The proposed method can be then considered as an excellent alternative to the post-column detection of beta-agonists, phenols and polyphenols.     

Catalysis of slow charge transfer reactions at polypyrrole-coated glassy carbon electrodes

Oxidation of ascorbic acid, dihydroxyphenylacetic acid and dopamine are compared at polypyrrole-coated glassy carbon and naked glassy carbon electrodes. These currents are mass-transport limited and not limited by permeation into or through the polypyrrole film. Ascorbic acid oxidation occurs at potentials 300 mV more negative at polypyrrole-coated electrodes and the rising slope of rotated disk voltammograms changes by over 100 mV. A similar enhancement in electrochemical reversibility is observed for dihydroxyphenylacetic acid, whereas dopamine is oxidized at slightly more positive potentials at polypyrrole-coated electrodes. Comparing the electrochemistry of dopamine and dihydroxyphenyl-acetic acid, it appears that the electrochemical reversibility differences for these substances are to some degree result of electrostatic interactions between the anionic solutes, or anionic reaction intermediates, and anionic functional groups on carbon or cationic fixed sites in oxidized polypyrrole.