A novel potentiodynamic method of electrochemical growth for layer-by-layer film formation based on electrostatic interaction

A novel method of electrochemical growth was developed for layer-by-layer film formation and proven more advantageous than the commonly used immersion growth in obtaining more uniform multilayer assemblies, as well as being able to proceed in salt-containing solutions without competitive adsorption from the salt ions. (C) 1999 Elsevier Science S.A. All rights reserved.

End-column electrochemical detection for aromatic amines with high performance capillary electrophoresis

Electrochemical detection of five species of aromatic amines at a carbon fiber microdisk electrode after separation by capillary electrophoresis is described. Under the optimum conditions, the detection limit for 3,4-dihydroxybenzylamine, N,N-dimethylaniline, p-phenylenediamine, p-aminophenol and aniline sulfate was 0.9, 0.03, 0.075, 1.2 and 0.15 mu M (S/N = 3), respectively. The linear response range was 5-1000, 0.1-500, 0.5-500, 5-500 and 1-200 mu M, respectively The effect of the electrode position and buffer pH on the detection was also studied. This method is very simple, sensitive and stable for the detection of these compounds.

A mechanistic study of electrochemical oxidation processes of (OEP)Mg(II) in the presence of anions

In the presence of F-, OH-, Cl-, electrochemical redox of (OEP)Mg(II) [2,3, 7,8,12,13,17,18 octaethyl-21H, 23H-porphine magnesium (II)] are investigated in DCE/0.1 mol/L solution by cyclic voltammetry and spectroelectrochemistry, In the presence of anions, anions are axially coordinated to (OEP)Mg(II) generate (OEP)Mg(II)Y, the E-1/2 of (OEP)MS(II)Y oxidation are negatively shifted. A chemical reaction following the second oxidation step is observed, the E-1/2 of the reaction product is obtained. Mechanism of (OEP)Mg(II) in the halogen and OH- anions titration process has been proposed.

A double logarithmic method for studying the adsorption effects in an irreversible electrochemical process

The adsorption of an electroinactive product greatly influences an irreversible electrochemical reaction in three ways, including self-block, self-inhibition, and self-acceleration, and changes not only the heterogeneous electron-transfer rate constant but also the modified formal potential and electron-transfer coefficient of the electrochemical reaction. In order to study these adsorption effects, a double logarithmic method was suggested to be used in processing the potential-controlled thin layer spectroelectrochemical data. The result shows three types of double logarithmic plots for three kinds of adsorption effects. These double logarithmic plots can be a diagnostic criterion of the adsorption effects and enable us to determine some thermodynamic and kinetic parameters. The combination of nonlinear regression with double logarithmic method is a convenient way to examine the suggested mechanism and to extract more information from the limited experimental data. Some examples are given to test the theoretical results. (C) 1999 The Electrochemical Society. S0013-4651(98)05-012-5. All rights reserved.

A double logarithmic method in thin-layer spectroelectrochemistry for distinction among mechanisms of an irreversible electrochemical process

A simple double logarithmic method in potential-controlled thin-layer spectroelectrochemistry for an irreversible electrochemical process has been studied by numerical analysis and examined by experimental examples. This simple algorithm has a novel function offering some important information about the mechanism of a complex electrochemical process directly from a limited amount of potential-spectrum data, and can be used to distinguish different reaction mechanisms such as E, EC, EE, as well as to determine the electron-transfer coefficient, a, and the kinetically modified E-0'. Combination of the double logarithmic method with nonlinear regression provides a powerful tool to examine the proposed mechanism and also to estimate other thermodynamic and kinetic parameters. (C) 1999 The Electrochemical Society. S0013-4651(98)06-090-X. All rights reserved.

Photoinduced electrochemical reduction of nitrite at an electrochemically roughened silver surface

Photoelectrochemical reduction of nitrite and nitrate was studied on the surface of an electrochemically roughened silver electrode. The dependence of the photocurrent on photon energy, applied potential, and concentration of nitrite was determined. It was concluded that the photoelectrochemical reduction proceeds via a photoemission process followed by the capture of hydrated electrons by electron accepters. The excitation of plasmon resonances in nanosize metal structures produced during the roughening procedure resulted in the enhancement of the photoemission process. Ammonia was detected as one of the final products in this reaction. Mechanisms for the photoelectrochemical reduction of nitrite and nitrate are proposed.

Electrochemical behavior of a series of undecatungstozincates monosubstituted by first-row transition metals, ZnW11M(H2O)O-39(n-) (M = Cr, Mn, Fe, Co, Ni, Cu or Zn)

The electrochemical behavior of a series of undecatungstozincates monosubstituted by first-row transition metals, ZnW11M(H2O)O-39(n-) (M=Cr, Mn, Fe, Co, Ni, Cu or Zn), was investigated systematically and comparably in aqueous solutions by electrochemical and in situ UV-visible-near-IR spectroelectrochemical methods. These compounds exhibit not only successive reduction processes of the addenda atoms (W) in a negative potential range, but some of them also involve redox reactions originating from the substituted transition metals (M) such as the reduction of Fe-III and Cu-II at less negative potentials and the oxidation of Mn-II at a more positive potential. Some interesting results and phenomena, especially of the transition metals, were found for the first time. Moreover, possible reaction mechanisms are proposed based on the experimental results.

Study of electrochemical behavior of ergosterol by in situ thin layer circular dichroic spectroelectrochemistry

The electroxidation of ergosterol was studied by in situ circular dichroic (CD) spectroelectrochemistry with a long optical path length thin layer cell. It was confirmed that the oxidation of ergosterol in ethanol solution is a two-electron irreversible electrochemical process with strong adsorption of an electroinactive product at the glassy carbon electrode, which blocks the electrochemical reaction. The CD spectroelectrochemical data were treated by the double logarithm method together with nonlinear regression, from which the formal potential, E-0 = 1.00 V, alpha n(alpha) = 0.302, the standard electrochemical rate constant, k(0) = 6.1(+/-0.4) x 10(-4) cm s(-1) and the adsorption constant, beta = 19 +/- 1, were obtained. The number of electrons transferred (n = 1.86) was estimated by cyclic voltammetry.

Water soluble polyaniline and its blend films prepared by aqueous solution casting

Polyaniline (PAn) was doped with phosphonic acid containing hydrophilic tails. The solubility of the doped PAn in water was controlled by changing the length of hydrophilic chain in the dopant. When poly(ethylene glycol) monomethyl ether (PEGME) with molecular weight M-w = 550 was used as the hydrophilic chain of the dopant, the doped PAn was entirely soluble in water. The film cast from aqueous solution showed good electrochemical redox reversibility, Aqueous solution blending of PAn with poly(ethylene glycol) (PEG, M-w = 20 000) and poly(N-vinyl pyrrolidone) (PVP, M-w = 360 000) was achieved. Percolation threshold of the composite film was lower than 3 wt.%. Electrical conductivity of the composite film was in the range of 10(-1)-10(-5) S cm(-1), depending on molecular weight of the acid and the content of PAn in the composite. (C) 1999 Elsevier Science Ltd. All rights reserved.

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.

Electrochemical growth and characterization of polyoxometalate-containing monolayers and multilayers on alkanethiol monolayers self-assembled on gold electrodes

A general strategy has been developed for fabrication of ultrathin monolayer and multilayer composite films composed of nearly all kinds of polyoxometalates (POMs), including isopolyanions (IPAs), and heteropolyanions (HPAs). It involves stepwise adsorption between the anionic POMs and a cationic polymer on alkanethiol (cysteamine and 3-mercaptopropionic acid) self-assembled monolayers (SAMs) based on electrostatic interaction. Here a Keggin-type HPA SiMo11VO405- was chosen as a main representative to elucidate, in detail, the fabrication and characterization of the as-prepared composite films. A novel electrochemical growth method we developed for film formation involves cyclic potential sweeps over a suitable potential range in modifier solutions. It was comparatively studied with a commonly used method of immersion growth, i.e., alternately dipping a substrate into modifier solutions. Growth processes and structural characteristics of the composite films are characterized in detail by cyclic voltammetry, UV-vis spectroscopy (UV-vis), X-ray photoelectron spectroscopy (XPS), micro-Fourier transform infrared reflection-absorption spectroscopy (FTIR-RA), and electrochemical quartz crystal microbalance (EQCM). The electrochemical growth is proven to be more advantageous than the immersion growth. The composite films exhibit well-defined surface waves characteristic of the HPAs' redox reactions. In addition, the composite films by the electrochemical growth show a uniform structure and an excellent stability. Ion motions accompanying the redox processes of SiMo11VO405- in multilayer films are examined by in situ time-resolved EQCM and some results are first reported. The strategy used here has been successfully popularized to IPAs as well as other HPAs no matter what structure and composition they have.

Electrochemical studies of cytochrome c disulfide at gold electrodes

The electrochemistry of disulfide in cytochrome c on gold electrodes was reported. The observed electrochemical response was used to explain why the electrochemical reaction of cytochrome c is irreversible at gold electrodes. Disulfide bonds in cytochrome c were strongly adsorbed onto the surface of gold electrodes and caused slow rate of electron transfer of the heme group. It was found that the presence of disulfides in cytochrome c was responsible for the lack of electrochemical response of the heme group on a gold electrode. The mechanisms for this effect were studied using electrochemistry and photoelectron spectroscopy. (C) 1999 Elsevier Science B.V. All rights reserved.

The effect of preparation conditions on the catalyst Nd(P-507)(3)/H2O/Al(i-Bu)(3) for the polymerization of styrene

The catalyst system neodymium phosphonate Nd(P-507)(3)/H2O/Al(i-Bu)(3) for the polymerization of styrene was examined. Effects of the addition order of the catalyst components, catalyst aging time and aging temperature on the catalyst activity and the polymer characteristics were investigated. The catalyst activity for isospecific polymerization of styrene increases with aging time and reaches the maximum with a catalyst aged for 45 min at 70 degrees C. The aging time that the catalyst needs to reach the highest activity for isospecific polymerization decreases with increasing aging temperature. The preformed catalyst and the in situ catalyst were compared with respect to the kinetic behavior of the styrene polymerization and the polymer characteristics.

Catalytic oxidization polymerization of aniline in an H2O2-Fe2+ system

Polyaniline is prepared by chemical polymerization of aniline in an acidic solution using H2O2 as an oxidant and ferrous chloride as a catalyst. A wide variety of synthesis parameters are studied, such as the amount of the catalyst, reaction temperature, reaction time, initial molar ratio of oxidant, monomer and catalyst, and aniline and HCl concentrations. The polymerization of aniline can be initiated by a very small amount of catalyst. The yield and the conductivity of product depend on the initial molar ratio of the oxidant and monomer. The polyaniline with a conductivity of about 10 degrees S/cm and a yield of 60% is prepared under optimum conditions. The process of polymerization was studied by in situ ultraviolet-visible spectroscopy and open-circuit potential technology. Compared to the polymerization process in a (NH4)(2)S2O8 system, the features of the H2O2-Fe2+ system are pointed out, and the chain growth mechanism is proposed. (C) 1999 John Wiley & Sons, Inc.

Preparation of polymer-supported zirconocene catalysts and olefin polymerization

A novel polymer-supported metallocene catalyst with crosslinked poly(styrene-co-acrylamide) (PSAm) as the support has been prepared and characterized. The probability of long sequences of acrylamide (Am) in PSAm is still low even at an Am amount of 32.8 mol %, implying the relatively homogeneous distribution of Am. The infrared spectra of PSAm and the supported catalyst substantiate that an amide group in PSAm coordinates with methylaluminoxane through both oxygen and nitrogen atoms. Ethylene/alpha-octene copolymerization showed that the catalytic activity is not markedly affected by adding alpha-octene. C-13 NMR analysis of the ethylene/alpha-octene copolymer indicated that the composition distribution of the copolymer is uniform. (C) 1999 John Wiley & Sons, Inc.