S. Sotiropoulos

Study on the electrochemical detection of the macrolide antibiotics clarithromycin and roxithromycin in reversed-phase high-performance liquid chromatography

The optimal conditions of the amperometric detection of the macrolide antibiotics clarithromycin and roxithromycin were found by cyclic voltammetric studies and HPLC-electrochemical detection responses obtained in different temperatures (25.5-60 degrees C) and different but almost isoelutropic binary, ternary and quaternary mixtures of aqueous buffer (pH 7), methanol, acetonitrile and isopropanol. These conditions were also proved to be applicable for the quantitative detection of clarithromycin in human plasma using roxithromycin as an internal standard and vice versa. It was demonstrated that increased attention has to be paid to eluent composition and column temperature to ensure sensitive and reproducible electrochemical responses as well as regularly shaped peaks for both macrolides tested.

Thermodynamic methods for analysis of adsorption data of organic compounds at electrified interfaces

Abstract The thermodynamic methods used for analysis of adsorption data at charged interfaces are reviewed critically. It is shown that the conventional methods depend the validity of their results upon the choice of the molecular model of the adsorbed layer. For this reason, contradictory conclusions may be drawn. A new thermodynamic approach which is free from the above shortcoming is described. In particular, if is shown that all the adsorption parameters, including that of size ratio, can be obtained on the basis of purely thermodynamic arguments without any reference to molecular models. Thus, the popular Frumkin and Bennes isotherms are proved to be valueless. It is also shown that useful information about the adsorption process and the interfacial interactions can be gained only when the adsorption parameters are interpreted by means of up-to-date molecular models for charged interfaces. The field effect and the physical content of the various adsorption parameters are examined. Finally, an analysis of the accuracy of the experimental data needed for the complete application of the new thennodynamic approach is also presented and discussed.

Preparation and characterization of microporous Ni coatings as hydrogen evolving cathodes

A number of industrial electrochemical processes employ high surface area Ni electrodes. These include the use of Ni anodes in alkaline or molten carbonate fuel cells [1‐3], NiO(OH) cathodes in nickel‐cadmium and nickel‐hydrogen batteries [4] and hydrogen evolving Ni cathodes in alkaline water electrolysis [5] as well as in electrochemical hydrogenation of organics [6]. There are various types of high surface area Ni electrodes and an even larger variety of preparation methods. Sintered microporous Ni coatings are usually manufactured according to the ceramic foil-casting technology, by mixing of a micrometer size nickel powder with an organic binder, which is subsequently thermally decomposed with further sintering at elevated temperatures in a hydrogen atmosphere [3]. The methods for the production of nanoporous Raney‐Ni coatings include cold rolling, plasma spraying, annealing, sherardizing and cathodic codeposition of the Raney‐nickel precursor alloys (Ni/Al or Ni/Zn) on a nickel support [7]. PolyHIPE Polymer (PHP) [8, 9] is a microporous material produced through the formation of a high internal phase water-in-oil emulsion, in which the volume of the aqueous dispersed phase is greater than about 75%, and the subsequent polymerization (at 60 C) of the oil phase which contains the monomer (styrene and occasionally other monomers too) and the cross-linker (divinylbenzene). This results in the production of a porous polymer matrix due to the evaporation of the water droplets, which were present in the precursor emulsion. The structure of PHP is characterized by the presence of numerous cells (of 1‐100 lm diameter) interconnected by smaller pores (of 0:1‐10 lm diameter).We have recently reported the incorporation of Ni into the PHP matrix by electroplating through its pores and onto a thin Au layer electrode pasted on one side of a polymer sample [10] or onto a Ni mesh in a Ni/ PHP/Ni composite cell [11]. Thermal decomposition of the polymer resulted in a granular Ni structure of BET surface areas in the range of 1‐50 m 2 g ˇ1 depending on the method and plating current density. The main advantages of this process for producing nickel coatings of morphology comparable to that of sintered Ni are the inexpensive raw materials for the polymer matrix production and the relatively low temperature processing. Furthermore, the high surface area coating can be deposited on a variety of substrate electrode materials (e.g., stainless steel, reticulated vitreous carbon) and on substrates of diAerent geometries. The work presented here introduces a further modification of the technique, whereby only the Ni cathode wire is immersed into the precursor emulsion and finally entrapped into a well-defined PHP coating after polymerization. A preliminary characterization of the porous Ni coating, produced after electroplating and polymer decomposition, with respect to hydrogen evolution from alkaline solutions is also presented.

Morphology, Structure and Photoelectrocatalytic Activity of TiO2 / WO3 Coatings Obtained by Pulsed Electrodeposition onto Stainless Steel

A simple two-step pulsed electrodeposition/ electrosynthesis technique is employed for the preparation of a bicomponent photocatalyst, TiO2/WO3, onto metal substrates. TiO2 can be activated under UV light illumination and is well known for its water detoxification capabilities. The coupling between this wide band-gap semiconductor with a suitable narrow band-gap one, WO3, is used for effective separation of the photogenerated charge carriers. Besides the reduced surface recombinetion due to directional charge transfer, the combination with the visible light (Vis)-activated WO3 entails an extended photoactivity towards Vis wavelengths. In addition, the photocatalytic decomposition of organic water pollutants at TiO2/WO3 layers supported on conductive substrates can be further enhanced by applying a positive bias in an appropriate electrochemical cell. Drawing the electrons away from the surface through the external circuit reduces surface recombination rates of photogenerated electron-hole pairs. Recently, bilayer TiO2/WO3 photocatalysts were prepared onto stainless steel substrates by continuous cathodic electrodeposition of WO3 followed by TiO2 electrosynthesis [1, 2]. Their photoelectrocatalytic efficiency is very promising and superior to both their single-component counterparts. Also, there have been indications of the considerable impact of composition, morphology and structure on photoelectrocatalytic activity [3]. This implicates the necessity for appropriate monitoring and design of these factors. By applying a consecutive pulsed electrodeposition/electrosynthesis method for WO3 and TiO2, a favorable modification of the electronic properties at the TiO2-WO3 junction and an increased catalyst surface area has been sought. The morphology, structure and related composition distribution of the pulsed-deposited films onto metal substrates have been characterized by high resolution Field Emission SEM (FE SEM) (Fig. 1), SEMEDS and Raman spectroscopy. The photocurrents at photoanodes with various loadings, structure and morphology have been evaluated in the presence and absence of the model pollutants Na-oxalate and 4chlorophenol under UV and Vis light illumination. Similar to the case of continuous electrodeposition [3], a trend was observed of the impact and the need for optimization of the TiO2/WO3 loading ratio, surface morphology, structure and composition distribution to design high-performance photocatalysts. The performance for bulk photo-decomposition of 4-chlorophenol has been evaluated at photoanodes WO3 and TiO2/WO3 prepared by continuous electrodeposition and compared with that at pulsed-plated TiO2/WO3. Long-term photoelectrolysis at constant potential was applied, using spectrophotometry to monitor the variation of the pollutant concentration.

Pt-Ni carbon-supported catalysts for methanol oxidation prepared by Ni electroless deposition and its galvanic replacement by Pt

Pt–Ni particles supported on Vulcan XC72R carbon powder have been prepared by a combination of crystalline Ni electroless deposition and its subsequent partial galvanic replacement by Pt upon treatment of the Ni/C precursor by a solution of chloroplatinate ions. The Pt-to-Ni atomic ratio of the prepared catalyst has been confirmed by EDS analysis to be ca. 1.5:1. No shift of Pt XPS peaks has been observed, indicating no significant modification of its electronic properties, whereas the small shift of the corresponding X-ray diffraction (XRD) peaks indicates the formation of a Pt-rich alloy. No Ni XRD peaks have been observed in the XRD pattern, suggesting the existence of very small pockets of Ni in the core of the particles. The surface electrochemistry of electrodes prepared from the catalyst material suggests the existence of a Pt shell. A moderate increase in intrinsic catalytic activity towards methanol oxidation in acid has been observed with respect to a commercial Pt catalyst, but significant mass specific activity has been recorded as a result of Pt preferential confinement to the outer layers of the catalyst nanoparticles.

Interfacial micellization of cetyl-dimethyl-benzylammonium chloride and Tween-80R at the Hg/electrolyte solution interphase

Abstract The adsorption of a cationic micelle—forming surfactant, cetyl—dimethyl-benzylammonium chloride, and of a neutral surfactant, Tween 80 R , from aqueous electrolyte solutions on a polarized mercury electrode was studied by means of differential capacitance measurements in order to examine the influence of the structure and charge of the adsorbate on interfacial micellization. For both surfactants, deformed capacitance peaks were recorded, accompanied by capacitance plateaux. These are sufficient criteria for the occurrence of a two- or three-dimensional aggregation process of the adsorbate molecules within the electrical double layer. The adsorption behaviour of cetyl-dimethyl-benzylammonium cations shows striking similarities with that of sodium dodecylsulphate. Their aggregates on the electrode surface collapse to a compact layer at potentials negative of the ecm and a polylayer is formed under favourable conditions at far negative or positive polarizations respectively. Since these adsorbates have quite different structures, these similarities are attributed to their ionic character. In contrast, the surface aggregates of Tween 80 R do not collapse to a compact layer but are stable over an extended potential range, forming two micellar plateaux. Finally, general conclusions of our theoretical and experimental studies on micellization at charged interphases are also presented.

Preparation of microporous nickel electrodeposits using a polymer matrix

Nickel-mesh electrodes were embedded into polyHIPE (a generic hollow fiber polymer) by immersion in the precursor emulsion and subsequent entrapment into the solid microporous matrix produced by polymerization and drying. Immersing this Ni/polyHIPE/Ni composite into a nickel electroplating bath and passing direct current through the two electrodes resulted in growing Ni electrodeposits on the cathode and through the polymer cells and pores. When the polymer matrix was subsequently burned off, a granular microporous Ni coating was produced on the cathode. Variation of the electroplating time and current density showed that the structure of the Ni coating is determined by the local distortions of the electric field inside the tortuous microporous body of the insulating polymer.

Gas Phase Photoelectrochemistry in a Polymer Electrolyte Cell with a Titanium Dioxide/Carbon/Nafion Photoanode

Mixtures of Ti0 2 and carbon powders are tested as photoanodes in mini-photoelectrochemical cells based on Nafion electrolyte-impregnated microporous membranes. The composite TiO 2 + C working electrode is formed onto one face of the membrane (where a Ag/AgCl reference electrode is also adhered), whereas a stainless steel mesh counter electrode is attached to its opposite face. Photovoltammetry and constant potential photoamperometry carried out under UV-A illumination in the presence of water and methanol vapors result in well-defined photoelectrochemistry, characteristic of water and methanol photo-oxidation, and point to the possibility of scaling up similar devices to be used in electrically enhanced photocatalytic air treatment.

Adsorption of sodium dodecylsulphate on mercury as an example of micellization within a multilayer interphase

Abstract The adsorption of sodium dodecylsulphate from aqueous electrolytic solutions on a polarized mercury electrode was studied by means of differential capacitance measurements over a wide range of concentration and potential, and in the presence of different supporting electrolytes with different ionic strengths. An interpretation of the differential capacitance vs. applied potential curves is given, based on theoretical treatments developed previously. It is shown that at concentrations below the cmc, two-dimensional aggregates are formed on the electrode surface within a polarization region which is bounded by two capacitance peaks at extreme positive and negative polarizations. This film is not particularly stable and is transformed into a compact layer at polarizations close to the potential of maximum adsorption, resulting in a capacitance pit. With increasing bulk concentration the aggregation process extends across the interphase and at least two layers of aggregates—micelles are formed at concentrations around and above the cmc. This three-dimensional aggregation is characterized by the appearance of deformed and/or split capacitance peaks which determine the polarization region where this phenomenon occurs.

Direct RP‐HPLC determination of underivatized amino acids with online dual UV absorbance, fluorescence, and multiple electrochemical detection

The combined use of a dual-UV detector, a fluorimetric one and of a multiple electrochemical (EC) detector equipped with a dual electrode, consisting of a conventional size 3 mm diameter glassy carbon electrode (GCE) and of a pair of 30 mum thick carbon microfibers, is proposed for the determination of 15 amino acids, two dipeptides and creatinine. This online coupling of the above detection modes could partially replace amino acid analysis by derivatization methods, since it solves problems concerning the direct detection of selected underivatized amino acids. Additionally, it was proved that the use of multiple-detection allows positive peak identification in a single chromatographic run, yields more information for free amino acids and solves in some cases the problem of chromatographic resolution. In order to optimize the detection conditions of the underivatized amino acids and related compounds by different detectors, their detection characteristics were determined by adequate preliminary experiments. The electro-oxidation characteristics of the underivatized compounds of interest were determined by hydrodynamic voltammetry using a flow cell with a macrodisc GCE and by ex-situ voltammetry using both a GCE of conventional size and a carbon fiber disk microelectrode. Important practical advantages of microfiber and microdisk electrodes with respect to macroelectrodes were demonstrated.