Applications of microelectrodes in capillary electrophoresis electrochemical detection

The applications of the microelectrodes for capillary electrophoresis/electrochemical detection are reviewed. The attractive advantages of the microelectrode provide a wide scope for the developments in capillary electrophoresis with electrochemical detection.

Voltammetric study of vitamin K-3 at interdigitated array microelectrodes

Cyclic voltammetry of Vitamin K-3 (V-K3) was measured with Pt disk electrode, platinum interdigitated array (Pt-IDA) and Au-IDA microelectrodes in single and dual modes. The effects of pH, scan rate and collector potential on the current of generator and collector were studied. The collection efficiency of V-K3 at IDA electrodes was measured. The linear ranges for current response as a function of V-K3 concentration were found to he 10 mu M-1 mM (i(g)) and 1 mu M - 1 mM (i(c)) for the generator and collector of the Pt-IDA electrode, respectively. The effects of waiting time, potential difference and pulse electrolysis time in differential pulse voltammetry (DPV) on the peak current of V-K3 were studied to get the optimal condition at 0.1 M Na2HPO4, pH 11.50 and 11.0 for Au-IDA and Pt-IDA, respectively.

In situ UV-vis and CD thin-layer spectroelectrochemistry study of the electrochemical behavior of L-noradrenaline in alkaline solution

Electrochemical redox behavior of noradrenaline in alkaline solution on a glassy carbon electrode has been investigated by in situ UV-vis and CD spectroelectrochemistry by using a long optical path thin-layer cell. The experimental data were processed by using a double logarithmic method of analysis together with nonlinear regression which confirmed that the first step in both the oxidation of noradrenaline and reduction of noradrenochrome is a two-electron irreversible process governed by an EE mechanism. The kinetic parameters of the electrode reactions, i.e., charge transfer coefficient and the number of electrons transferred, alpha(1)n(1) = 0.11 and alpha(2)n(2) = 0.23, formal potentials modified with kinetics, E-1(0') = 0.65 (+/- 0.01) V and E-2(0') = 0.72V and standard rate cnstants, k(1)(0) = 7.0(+/-0.5)x10(-5) cm s(-1), for the first and second steps in the oxidation process of noradrenaline, and similarly, alpha(1)n(1) = 0.33, alpha(2)n(2) = 0.58, E-1(0') = 0.37(+/-0.01) V, E-0' = -0.25 (+/-0.01) V and k(1)(0) approximate to k(2)(0) = 1.06 (+/-0.05)x10(-4) cm s(-1) for the first and second steps in the reduction process of noradrenochrome were also determined.

Preparation of a composite film electrode containing 12-tungstosilicic acid and its electrocatalytic reduction for nitrite

A novel organic-inorganic composite film was formed by attaching Keegin-type heteropolyanion, SiW12O404- (devoted briefly as SiW12), on a glassy carbon electrode derivatized by 4-aminophenyl group. The composite film has an ionic bonding character between SiW12 and the surface amino group, which greatly improves the Blm stability and exhibits a more reversible electrochemical behavior. The modified electrode offers an excellent and stable electrocatalytic response for the reduction of nitrite. Possible mechanism was provided for the reaction of nitrite with SiW12O404-/aminophenyl composite film.

Molecular characterization of beef liver catalase by scanning tunneling microscopy

Beef liver catalase molecules can stick tenaciously to the highly oriented pyrolytic graphite (HOPG) surface which has been activated by electrochemical anodization. The immobilized sample is stable enough for high resolution scanning tunneling microscope (STM) imaging. When the anodized conditions are controlled properly, the HOPG surface will be covered with a very thin oxide layer which can bind the protein molecules. Individual molecules of native beef liver catalase are directly observed in detail by STM, which shows an oval-shape structure with a waist. The dimensions of one catalase molecule in this study are estimated as 9.0 x 6.0x 2.0 nm(3), which are in good agreement with the known data obtained from X-ray analysis, except the height can not be exactly determined from STM. Electrochemical results confirm that the freshly adsorbed catalase molecules maintain their native structures with biological activities. However, the partly unfolding structure of catalase molecules is observed after the sample is stored for 15 days, this may be caused by the long-term interaction between catalase molecules and the anodized HOPG surface.

Investigation of oxygen- and hydrogen peroxide-reduction on platinum particles dispersed on poly(o-phenylenediamine) film modified glassy carbon electrodes

Composite membrane modified electrodes were prepared by electrochemical deposition of platinum particles in a poly(o-phenylenediamine) (PPD) him coated on glassy carbon (GC) electrodes. The modified electrodes showed high catalytic activity towards the reduction of oxygen and hydrogen peroxide. A four-electron transfer process predominated the reduction process. The pH dependence and the stability of the electrodes were also studied.

The electrochemical behavior of polypyrrole from self-assembly polymerization

The electrochemical behavior and the charge transport of polypyrrole film prepared by self-assembly polymerization have been investigated. Ir is found that the microstructure of the film influences the electrochemical behavior of polypyrrole, and that the p-toluenesulfate (Tos(-)) ion plays avery important role in this system.

Electrochemical quartz crystal microbalance study of the electrochemical behavior of riboflavin at gold electrodes

The electrochemical and adsorption behaviors of riboflavin (RF) at gold electrodes has ken studied by using an electrochemical quartz crystal microbalance (EQCM). Useful information is obtained not only about electrochemical behavior but also about mass changes on the electrode surface. The electrochemical properties and frequency shifts were investigated in RF solutions at different pH values, concentrations and scan rates. Reversible voltammograms were observed for pH less than or equal to 9.71. There was no electrochemical reaction for pH > 9.71. The maximum current response was obtained at about pH 8. The current response was proportional to the square root of scan rates when the concentration of RF was lower than 1.0 x 10(-4) mol L-1 (pH 6.92). On the contrary, at concentrations higher than 1.0 x 10(-4) mol L-1 (pH 6.92), it was proportional to the scan rates.

Electrocatalytic oxidation and flow amperometric detection of hydrazine at an electropolymerized 4-vinylpyridine/palladium film electrode

A poly(4-vinyl)pyridine (PVP)/Pd film electrode was constructed for the electrocatalytic detection of hydrazine. The preparation of the PVP/GC electrode was performed by electropolymerization of the monomer 4-vinylpyridine onto the surface of a glassy carbon electrode. Subsequently, palladium is electrodeposited onto the polymer modified electrode surface. The ion-exchange function of PVP polymer is helpful to this process in view of the tetrachlorapalladate anion. Compared with the Pd/GC electrode, the modified electrode displays a better mechanical stability in a flowing stream. The PVP/Pd film electrode exhibits higher sensitivity when detecting hydrazine with a detection limit of 0.026 ng (S/N=3).

Spontaneous assembly of the inclusion complexes of viologen-attached alkanethiols and alpha-, beta-cyclodestrin on gold electrode

Electroactive self-assembled monolayers (SAMs) with well-defined electrochemical responses were prepared by spontaneous assembly of the inclusion complexes (CD/C8VComegaSH) of viologen-attached alkanethiols (C8VComegaSH) and alpha- and beta-cyclodextrin (CD). They were characterized by X-ray photoelectron spectroscopy and cyclic voltammetry. The results demonstrate that the chemisorption process of CD/C8VComegaSH on gold substrate occurs through S-Au bonds, and that the redox sites in SAMs of CD/C8VComegaSH are in a much more uniform environment than those in SAMs of C8VComegaSH.

Electrochemical study of interactions between DNA and viologen-thiol self-assembled monolayers

We investigated the binding characteristics of double-stranded DNA to self-assembled monolayers (SAMs) containing viologen groups formed on the surface of gold electrodes via Au-S bonds. The positive charged and hydrophobic surfaces of the viologen SAMs modified gold electrodes are suitable to bind strongly dth DNA, whose interactions to solution DNA and adsorbed DNA both lead to positive shifts (22.5 mV and 65 mV, respectively) in the first redox potential ci viologen centers, indicating that the main interaction is from a hydrophobic interaction. Meanwhile, the binding of DNA strongly affects the kinetics of electron transfer of the viologen group so that the separation of anodic and cathodic peak potentials becomes larger and the heterogeneous electron transfer constant becomes smaller.

Electrochemistry of vanadium hexacyanoferrate film

Two stable redox couples, accompanying clear color switches between yellow green and blue, can be observed when the VHCF-coated film platinum electrodes are cyclic potential scanned in 3.6 M H2SO4 and 0.2 M K2SO4 electrolyte solution. Electrochemical results and in situ Fourier transfer infrared (FT-IR) spectroscopy demonstrate that the redox reaction of the electroactive iron sites is related to the first redox couple (E-1/2 = 0.81 V) while the second redox couple (E-1/2 = 1.01 V) is due to the redox reactions of the electroactive vanadyl ions. Under the proper conditions, such as in high acidic solutions or thin films (deposition time is less than 2 min) and so on, the third redox couple (E-1/2 = 0.89-0.94 V) can be observed on the cyclic voltammograms, which originates from the redox reactions of the interstitial vanadyl ions. This electrochemical reaction mechanism is investigated by in situ probe beam deflection technique, exchange of K+ ions accompanies with redox reaction of the iron sites, but for redox reaction of the vanadyl ions, both H+ ions, K+ ions and water molecules are involved.

Study on diffusion of electroactive species in polyelectrolyte polyethylene glycol lithium perchlorate by chronoamperometry at a microdisk electrode

The mass transport dynamics of Ferrocene in polyelectrolyte polyethylene glycol lithium perchlorate (PEG . LiClO4) was studied by using chronoamperometry at a microdisk electrode. Chronoamperometry is a powerful method for the study of mass transport in polyelectrolyte, it has many advantages over the conventional methods at a microelectrode and the steady-state method at an ultramicroelectrode. By using this method the apparent diffusion coefficient D-app and concentration C-a of the electroactive species, can be estimated from a single experiment without previous knowledge of either one. We have estimated D-app and C-a of ferrocene in PEG . LiClO4 polyelectrolyte from 25 degrees C to 75 degrees C. The dependence on the concentration of electroactive species was observed. The diffusion coefficients decrease with increasing ferrocene concentration and decreasing temperature. The mass transport mechanism is explained, by using a free volume model.

Conversion from electronic conducting phase to redox conducting phase of polypyrrole

Square-wave voltommetry is used to study the oxidation of polypyrrole doped with dodecylsulfate. The net current curve in this experiment shows why the oxidation current does not display the capacitive-like shape common in cyclic voltammetry. In cyclic voltammetry, the redox behavior of polypyrrole is attributed to the size of dodecylsulfate, irreversible incorporation and the complete consumption of dodecylsulfate. After the polypyrrole film was scanned in aqueous NaCl solution, square wave voltammetric measurements show different results, indicating the change of the polymer nature with regard to the charge transport. This is explained by anion replacement, exclusion and the change of the charge transport mechanism.

Electrochemical studies of lipophilic ion transport through BLM. The influence of sterols on its transport

Cyclic voltammetry was employed to study the influence of sterols on the lipophilic ion transport through the BLM. The mole fraction of the sterols (cholesterol, oxidized cholesterol). as referred to total lipid, was varied in a range of 0-0.8. Data demonstrate that a thin-layer model is suitable to this BLM system. By this model, the number of charges transported per lipophilic ion, the concentration of the ion in the membrane bulk phase and the aqueous/membrane phase partition coefficient could be calculated. These parameters proved that sterols had an obvious influence on the lipophilic ion transport. Cholesterol had a stronger influence on the ion transport than oxidized cholesterol. Its incorporation into egg lecithin membranes increased the partition coefficient beta of the ion up to more than 3-fold. Yet, oxidized cholesterol incorporated into egg lecithin membranes only increased the beta up to less than 2-fold, and the beta had no great variation at different oxidized cholesterol mole fractions. The higher beta obtained was partly due to the trace amount of solvent existing in the core of the lipid bilayers. At the different sterol mole fractions, combining the change of beta with the change of peak current, we also concluded that sterols had somewhat inhibiting effect on the ion transport at the higher sterols mole fraction (>0.4). These results are explained in terms of the possible change of dipole potential of the membrane produced by sterols and the decrease of the membrane fluidity caused by the condensation effect of sterols and the thinning effect caused by sterols. The substituting group (in the oxidized cholesterol) had some inhibiting effects on the ion transport at higher mole fractions (oxidized cholesterol mole fraction >0.4).