Chantal Degrand

Sonoelectrochemistry: Effects of ultrasound on voltammetric measurements at a solid electrode

Utilization of ultrasound in the field of electrochemistry is rather old. As early as the 1930’s, work on the depolarizing effect of ultrasound was published [1,2]. Increase of mass transport rate by sonication was described first by Bard in 1965 [3]. Sonication was used to increase the electroanalytical current [4,5], to improve quality in electroplating [6,7] and for electrode pretreatments [83 (for a recent review on sonoelectrochemistry see ref. 9). One of the most promising fields of sonochemistry lies in electrosynthesis. Cavitation at and/or near the electrode continuously cleans and activates its surface and accelerates the transport of material to and from it. Several articles have been published dealing with ultrasound assisted electropolymerization (see for example refs 10 and 11) and, in our group, with ultrasound induced electrochemical synthesis of seleno and telluro derivatives [12-181. Ultrasound was used for instance to promote the electrochemical reduction of insoluble Se and Te powder [12,13]. Mason et al. [19,9] and Toshiro et al. [20] have studied the influence of ultrasound on the electrochemical synthesis of alkanes by the Kolbe reaction. The former authors have shown that application of ultrasound favors a two-electron pathway as compared with a single-electron transfer process. The presence of ultrasound thus changes the product yields significantly. Until recently ultrasound has been utilized mainly as a tool to accelerate heterogeneous processes and/or mass transport to the electrode. The mechanisms by

Sonoelectrochemistry: transient cavitation in acetonitrile in the neighbourhood of a polarized electrode

The voltammograms recorded with ultrasound at a stationary electrode present a sigmoidal shape and their limiting current is the sum of steady-state and transient components. A semi-quantitative description of the integrated transient component is proposed in the case of the one-electron reduction of methyl viologen in acetonitrile at a platinum disc electrode of small radius (250 μm) under the action of ultrasound (acoustic intensity = 1.4 W cm−2, i.e. acoustic pressure = 1.7 atm). With the help of the basic theoretical and experimental data provided by Neppiras, Plesset and Chapman, it can be estimated that transient cavitation involves gas- or vapour-filled bubbles of similar radius RM (160–200 μm) collapsing at the surface of the electrode. Jets of liquid of almost cylindrical shape develop, which repeatedly strike the electrode surface with a constant velocity (228 m s−1) and submicrosecond duration. Hence 4.2 × 10−9 C are expected to be consumed in the first reduction step of methyl viologen (0.23 mM) when the largest collapsing bubbles (RM = 200 μm) are involved. The experimental result (4.2 × 10−9 C) is consistent with the above description. The voltammetric results show that the increase of the electrolytical current due to sonication is mainly given by the transient component. The consequences of the formation of violent jets striking the electrode surface are discussed.

Voltammetric properties of electrodes modified by poly-[N-(9,10-anthraquinone-2-carbonyl)ethylenimine] and used in aqueous solution

Abstract Three differently loaded anthraquinone polymers have been prepared by condensation of 2-anthraquinonecarbonylchloride and poly(ethylenimine). They have been adsorbed on vitreous carbon or mercury electrodes by dipping the electrode in a solution of pyridine or methylene chloride containing 0.01–0.05% polymer. The influence of adsorption parameters and of electrochemical variables on the voltammetric behavior of the polymer-coated electrodes in aqueous buffer solutions is described. Changing the dip time in pyridine from 30 s to 5 min and changing the polymer concentration have very little effect on the apparent coverage (=1.3-1.8×10−9 mol cm−2) and peak shape. In contrast, an increase of the loading (the number of monomer units loaded with quinone) or the use of methylene chloride as a dip-coating solvent instead of pyridine affected the shape of the cathodic and anodic peaks which broaden and tend to separate. The peak shape is characterized by a tailing which tends to disappear at slow scan rate. The modifications of the peak shape and position when the scan rate is changed, have been shown to fit qualitatively with a proposed polylayer model in which electron (proton) transfer to the sublayer nearest the underlying conductor is slow. It has been observed that the weak acid dissociation constants for the dihydroanthraquinone and hydroanthraquinone anion are both different for the corresponding monomer in solution (pKA=8.65 and 11.6) and the adsorbed polymer (pKA=9 and 13).

Ferrocenylethyl Phosphate: An Improved Substrate for the Detection of Alkaline Phosphatase by Cathodic Stripping Ion-Exchange Voltammetry. Application to the Electrochemical Enzyme Affinity Assay of Avidin

The ferrocenylethyl phosphate disodium salt was synthesized and used as a new substrate for alkaline phosphatase (AP). The enzyme-generated ferroceneethanol was selectively and sensitively detected at a Nafion film-coated electrode by anodic preconcentration of the ferricinium salt, followed by cyclic voltammetry. The accumulated ferricinium units could be expelled from the polymer film in their neutral form by cathodic stripping, and so the Nafion-modified electrode could be reused for more than 10 measurements with a standard deviation less than 3%. Values of 0.75 mM for the Michaelis constant and 1.42 μmol s(-)(1) (mg of protein)(-)(1) for the maximal velocity were found. The regenerable Nafion-coated electrode was employed for the indirect detection of AP down to 2 × 10(-)(12) M and for the noncompetitive heterogeneous enzyme assay of avidin, whose detection limit was 5 × 10(-)(12) M.

Possible analytical application of laponite clay modified electrodes

Abstract Laponite clay modified electrodes (LCME) have been used to detect trace amounts of neutral or cationic organometallic substances, including ferrocene (Fc) and cobaltocenium (Cc+), two molecules covalently attached to cobaltocenium, and a molecule labelled by ferrocene (N-amphetaminecarbonylferrocene) as an electroactive organic test species. During an ion-exchange preconcentration step, the cationic species (cobaltocenium derivatives) are collected in the laponite film from their dilute solutions under open-circuit conditions whereas the procationic species (ferrocene derivatives) are collected in their cationic form by applying a positive potential. Quantification of the surface bound cations is then carried out by applying a negative scan using voltammetry or square wave voltammetry. In the case of the two molecules labelled by Cc+, a detection limit of 4 × 10−8 mol l−1 and a linear calibration range from 1 × 10−7 to 2 × 10−5 mol l−1 are obtained reproducibly by using a new LCME for each measurement. Conversely with small redox molecules such as ferrocene and cobaltocenium, the same LCME can be used repeatedly because applying a negative potential leads to the exclusion of the resulting neutral molecule which can be rinsed efficiently from the film.

Ultrasound-assisted anodic oxidation of diuron

Abstract Bulk electrolysis of diuron (3-(3,4-dichlorophenyl)-1,1-dimethyl urea) was carried out at a glassy carbon anode with ultrasound in order to avoid a total blockage of the electrode surface by a passivating film. The major oxidation product P1 isolated in 23% yield resulted from the loss of one electron and one proton with formation of a nitrogen radical and of the corresponding NN dimer. Two more compounds P2 and P3 were isolated in 12 and 10% yields. They are suggested to originate from an intramolecular Fries rearrangement in dimer P1, i.e. the migration of an amide group from a monomeric unit of the dimer to the aromatic ring of the second one, with formation of a dichloro– N , N –dimethyl benzamide derivative. This rearrangement would be related to a strong adsorption of the intermediate nitrogen radical. It would occur at the positively charged electrode surface concomitantly with the N–N bonding formation. Several non-identified minor compounds were generated, which could involve multi-electron oxidation processes, since the overall oxidation process of diuron required more than one electron.

The effects of cross-linking and anodic surface roughening on quinone polymer/carbon electrodes

A polymer in which anthraquinone-2-carbonyl groups were bound to polyethyleneimine was coated onto a glassy carbon electrode. Electrodes of this kind were studied using cyclic voltammetry and pH 7 aqueous solutions. At pH <10 only those quinone units in contact with the carbon surface are electroactive. It was shown that anodic surface roughening increased the limited number of electroactive groups in the polymer film and gave more stable activity and narrower voltammetric peaks. Above pH 10 redox propagation through the layer is more rapid but the anionic product desorbs. This desorption was inhibited by cathodically cross-linking a layer of mixed polymers on a polyethyleneimine backboned polymer containing fluorenone units as well as anthraquinone units.

Redox processes in polymeric anthraquinone layers: The importance of polymer chain motion

Abstract Anthraquinone polymers whose loadings in electroactive sites range from 20 to 75%, were adsorbed on a hanging mercury drop electrode (HMDE) which can be expanded after the adsorption. The voltammetric results obtained in buffered aqueous or aqueous—alcoholic solutions are presented. It is shown that in acid media, the charging of the coating is limited to the layer adjacent to the electrode. In neutral and alkaline solutions, the charging of some of the bulk of the coating takes place. This process is slower when polymers containing a higher loading of anthraquinone units are used. The factors which control these results are discussed.

Redox cationic or procationic labeled drugs detected at a perfluorosulfonated ionomer film-coated electrode

With the aim of developing a new type of immunoassay with electrochemical detection, a Nafion® film-modified electrode was used as an amplifying sensor to detect redox cationic labeled drugs by square-wave voltammetry. A series of amino drugs was labeled with three model redox labels, i.e. cationic cobaltocenium and procationic ferrocene and nitroxide groups. Several parameters were involved in the optimization of the analytical properties of the sensor, i.e. film processing and film thickness, accumulation time, ionic strength and ethanol content. The preconcentration of the labeled drugs was impeded by the competitive accumulation of hydrophobic amines. The accumulation process of the procationic labeled drugs was influenced significantly by the potential applied during accumulation, and their strong non-specific binding with proteins was observed in the presence of serum. Therefore the choice of cobaltocenium as a label was preferred. Optimized conditions were defined, which took into account these various aspects. The avidin-biotin system was proposed as an applicable extension of this approach.

Determination of alkaline phosphatase using a Nafion®-modified electrode

Abstract Alkaline phosphatase (AP) assays were performed at pH 7.5 with [ N -ferrocenoyl], -6-amino-2,4-dimethylphenyl phosphate ( 3 ) as substrate and a Nafion®-modified glassy carbon (GC + Nafion®) electrode as sensor. At this pH value, the substrate was dianionic and therefore was repulsed by the GC + Nafion® electrode, whereas the corresponding alcohol 4 generated enzymatically was entrapped within the Nafion® film as a ferricinium salt by applying a potential of 0.6 V vs. Ag|AgCl during an accumulation step that followed incubation and preceded the electrochemical determination of 4. In a typical experiment, enzymatic hydrolysis of 3 to the corresponding alcohol 4 was carried out for 15 min in Tris buffer solution at pH 10.2. The pH value was then lowered to 7.5 and the solution was transferred to an electrochemical cell. The accumulation occurred for 5 min at a rotating GC + Nafion® electrode, prior to the determination of 4 by square-wave voltammetry at stationary GC + Nafion® electrode. The AP detection limit was 0.02 U l −1 (signal/residual current = 2). The accumulation of [ N -ferrocenoyl]-4-aminophenol ( 2 ) within the Nafion® film was also examined. It was observed that, at low concentrations, the dimethylated alcohol 4 accumulated more readily in the Nafion® film than 2 , which is consistent with the more hydrophobic character of the former alcohol. Therefore the use of [ N -ferrocenoyl]-4-aminophenyl phosphate as substrate for very sensitive AP assays at the Nafion® electrode is not recommended.