Electrochemical cell design for the impedance studies of chlorine evolution at DSA(®) anodes.

A new electrochemical cell design suitable for the electrochemical impedance spectroscopy (EIS) studies of chlorine evolution on Dimensionally Stable Anodes (DSA(®)) has been developed. Despite being considered a powerful tool, EIS has rarely been used to study the kinetics of chlorine evolution at DSA anodes. Cell designs in the open literature are unsuitable for the EIS analysis at high DSA anode current densities for chlorine evolution because they allow gas accumulation at the electrode surface. Using the new cell, the impedance spectra of the DSA anode during chlorine evolution at high sodium chloride concentration (5 mol dm(-3) NaCl) and high current densities (up to 140 mA cm(-2)) were recorded. Additionally, polarization curves and voltammograms were obtained showing little or no noise. EIS and polarization curves evidence the role of the adsorption step in the chlorine evolution reaction, compatible with the Volmer-Heyrovsky and Volmer-Tafel mechanisms.

Electrochemical sensor for simultaneous determination of herbicide MCPA and its metabolite 4-chloro-2-methylphenol. Application to photodegradation environmental monitoring.

The development and application of a polyaniline/carbon nanotube (CNT) cyclodextrin matrix (PANI-ß-CD/MWCNT)-based electrochemical sensor for the quantitative determination of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) and its main transformation product 4-chloro-2-methylphenol in natural waters are described. A simple cyclic voltammetry-based electrochemical methodology, in phosphate buffer solution at pH 6.0, was used to develop a method to determine both MCPA and 4-chloro-2-methylphenol, without any previous extraction or derivatization steps. A linear concentration range (10 to 50 µmol L(-1)) and detection limits of 1.1 and 1.9 µmol L(-1), respectively, were achieved using optimized cyclic voltammetric parameters. The proposed method was successfully applied to the determination of MCPA and 4-chloro-2-methylphenol in natural water samples with satisfactory recoveries (94 to 107%) and in good agreement with the results obtained by an established high-performance liquid chromatography technique, no significant differences being found between the methods. Interferences from ionic species and other herbicides used for broad-leaf weed control were shown to be small. The newly developed methodology was also successfully applied to MCPA photodegradation environmental studies.

Enhanced host-guest electrochemical recognition of herbicide MCPA using a ß-cyclodextrin carbon nanotube sensor.

An electrochemical sensor for the determination of the chlorophenoxy herbicide MCPA has been developed, based on a combination of multi-walled carbon nanotubes with incorporated ß-cyclodextrin and a polyaniline film modified glassy carbon electrode. The proposed molecular host-guest recognition based sensor has a high electrochemical sensitivity for the determination of MCPA. The electrochemical behaviour of MCPA at the chemically modified electrode was investigated in detail by cyclic voltammetry. The results indicate that the ß-CD/MWCNT modified glassy carbon electrode exhibits efficient electrocatalytic oxidation of MCPA with high sensitivity, stability and lifetime. The analytical characteristics of this film were used for the quantitative determination of MCPA in natural waters. Cyclic voltammetry in phosphate buffer solution at pH 6.0, allowed the development of a method to determine MCPA, without any previous steps of extraction, clean-up, or derivatization, in the range of 10-100 µmol L(-1), with a detection limit of 0.99 µmol L(-1) in water. The results were statistically compared with those obtained through an established high-performance liquid chromatography technique, no significant differences having been found between the two methods.

Mechanism of formation and construction of self-assembled myoglobin/hyaluronic acid multilayer films: an electrochemical QCM, impedance, and AFM study.

Self-assembled multilayer films of hyaluronic acid (HA) and the protein myoglobin (Mb) were built up layer by layer on Au covered quartz crystal microbalance (AuQCM) electrode substrates. Film formation and growth were monitored by an electrochemical quartz crystal microbalance (EQCM), and the step-by-step construction was investigated through quantification of the mass variation corresponding to adsorption of each monolayer together with cyclic voltammetry. A decrease of friction at the liquid/electrode interface was observed, indicating that the electrode surface becomes less rough after deposition of several monolayers. The properties of the {HA/Mb}(n) films were evaluated by electrochemical impedance spectroscopy (EIS). Modeling of the impedance spectra shows smoothing of the modified electrode surface with reorganization of the film structure after few monolayers, and the contribution of each layer to the electron transfer process was analyzed. The smoothing of the surfaces and the structural differences between successive bilayers was confirmed by morphological observations by using atomic force microscopy.

Carbon film electrodes for oxidase-based enzyme sensors in food analysis.

Carbon film resistor electrodes have been evaluated as transducers for the development of multiple oxidase-based enzyme electrode biosensors. The resistor electrodes were first modified with Prussian Blue (PB) and then covered by a layer of covalently immobilized enzyme. Electrochemical impedance spectroscopy was used to characterize the interfacial behaviour of the Prussian Blue modified and enzyme electrodes; the spectra demonstrated that the access of the substrates is essentially unaltered by application of the enzyme layer. These enzyme electrodes were used to detect the substrate of the oxidase (glucose, ethanol, lactate, glutamate) via reduction of hydrogen peroxide at +50mV versus Ag/AgCl in the low micromolar range. Response times were 1-2min. Finally, the glucose, ethanol and lactate electrochemical biosensors were used to analyse complex food matrices-must, wine and yoghurt. Data obtained by the single standard addition method were compared with a spectrophotometric reference method and showed good correlation, indicating that the electrodes are suitable for food analysis.

Copper-modified gold electrode specific for monosaccharide detection Use in amperometric determination of phenylmercury based on invertase enzyme inhibition.

The electrochemical oxidation of mono- and disaccharides at various copper-modified electrodes is reported: glassy carbon modified at open circuit or by electrochemical deposition of copper, gold modified by electrochemical deposition, and at bulk copper electrodes. A comparative study of these four electrodes was made by linear sweep voltammetry and amperometry. The maximum oxidation peak separation between disaccharides and monosaccharides is about 200mV. After optimization, amperometric determination of monosaccharides was done at +0.30 versus Ag/AgCl in 0.15M NaOH at the copper-modified gold electrode. Using the developed method, the enzymatic activities of invertase and beta-galactosidase were determined through their reaction with sucrose and lactose, respectively. Validation was carried out by a spectrophotometric method based on 3,5-dinitrosalicylic acid, and it was shown that the proposed electrochemical method is more sensitive. The analytical utility of the copper-modified gold electrode was tested for the determination of organic mercury. Addition of phenylmercury standards to the invertase solution caused a decrease in the enzyme activity, and allowed the determination of phenylmercury in pharmaceutical samples. The concentration has been determined in the 10-55ngml(-1) range.

An impedance study of the adsorption of nucleic acid bases at glassy carbon electrodes.

Electrochemical impedance has been used to study the adsorption at glassy carbon electrodes of guanine, its corresponding nucleoside, guanosine, and adenine. Impedance studies at different concentrations and applied potentials show clearly that all three bases are adsorbed on the electrode, blocking the surface. Irradiating the electrode with low-frequency (20 kHz) ultrasound whilst recording the impedance spectra increased transport of molecules to the electrode surface with cavitation cleaning the surface and removing strongly adsorbed molecules of bases. In this way, sonoelectrochemical experiments enabled the electrode processes to be studied in the absence of adsorption.