Self-assembled monolayers on Au microelectrodes

Long chain alkanethiols self-assembled monolayers (SAMs) formed on Au microelectrodes showed higher sensitivity towards defects than the same monolayers on macroelectrodes. The analysis of cyclic voltammetry and electrochemical impedance spectroscopy (EIS) experiments performed on covered microelectrodes were consistent with the formation of pinholes of about 10 nm in diameter. Moreover, the EIS data exhibited a specific behavior that was interpreted invoking the short circuiting of the pinholes impedance by the surrounding surface of the microelectrode in the high frequency domain, whereas in the low frequencies, the surface covered by the SAM was assume to act as an insulator.     

Erratum to: Mathematical working space and paradigms as an analysis tool for the teaching and learning of analysis

ZDM – Mathematics Education -     

Impedance Response of a Thin Film on an Electrode: Deciphering the Influence of the Double Layer Capacitance

The different contributions of the interfacial capacitance are identified in the case of passive materials or thin protective coatings deposited on the electrode surface. The method is based on a graphical analysis of the EIS results to determine both the passive-film capacitance in the high-frequency domain and the double-layer capacitance in the low-frequency domain. The proposed analysis is shown to be independent of the physicochemical origins of the frequency dispersion of the interfacial capacitances which results, from an analysis point of view of the experimental results, in the use of a constant-phase element However, for a correct evaluation of the thin-film properties such as its thickness, the high-frequency data must be corrected for the double-layer contribution. In particular, it is shown that if the double-layer capacitance gives a frequency-dispersed response, it is necessary to correct the high-frequency part for the double-layer constant-phase elements. This is first demonstrated on synthetic data and then used for the determination of the thickness of thin oxide film formed on Al in neutral pH solution.     

Monitoring and Quantifying the Passive Transport of Molecules Through Patch–Clamp Suspended Real and Model Cell Membranes

Transport of active molecules across biological membranes is a central issue for the success of many pharmaceutical strategies. Herein, we combine the patch–clamp principle with amperometric detection for monitoring fluxes of redox‐tagged molecular species across a suspended membrane patched from a macrophage. Solvent‐ and protein‐free lipid bilayers (DPhPC, DOPC, DOPG) patched from single‐wall GUV have been thoroughly investigated and the corresponding fluxes measurements quantified. The quality of the patches and their proper sealing were successfully characterized by electrochemical impedance spectroscopy. This procedure appears versatile and perfectly adequate to allow the investigation of transport and quantification of the transport properties through direct measurement of the coefficients of partition and diffusion of the compound in the membrane, thus offering insight on such important biological and pharmacological issues.     

MO study of the benzylperoxyl radical, an intermediate in atmospheric ozone formation

Unrestricted Hartree-Fock calculations have been carried out to study the benzylperoxyl radical, an intermediate atmospheric contaminant. For this purpose, quantum mechanical MP2//UHF ab initio calculations were performed by using 6-31G^** and 6-31 + G^** basis sets. Because no previous experimental nor theoretical data were available for benzylperoxyl, the results were compared with those of the smaller unsaturated peroxyl radicals, such as allylperoxyl. It is found that the calculated absolute minimal energy corresponds to a trans conformation in which the O---O bond is sticking away from the benzene ring. Benzylperoxyl is found also to exhibit an unusually O---O small bond order, which reflects the weakness of the bond, i.e. its ability to react with NO forming NO₂ and subsequently ozone. A peculiar MO diagram is obtained, with the unpaired MO deeply located in the “doubly” occupied MO space. This effect is also observed in allylperoxyl. Structural parameters, charges, spin densities and dipole moments are also reported.     

Characterization of viscous fingering in a radial Hele-Shaw cell by image analysis

Methods based on image analysis and mathematical morphology are proposed to study fingering patterns obtained in a radial Hele-Shaw cell. They have been used to study miscible displacement patterns obtained under various conditions of initial viscosity, viscosity ratio and injection rate. Their application domain can be extended to other type of fingering patterns as well as diffusion-limited aggregates. Received: 28 October 1997/Accepted: 16 April 1998     

Teachers’ conceptions of tangent line

In order to study the conceptions, and their evolutions, of the tangent line to a curve an updating workshop which took place in México was designed for upper secondary school teachers. This workshop was planned using the methodology of cooperative learning, scientific debate and auto reflection (ACODESA) and the conception-knowing-concept model (cK¢) developed mainly by Balacheff. In order to initiate the conceptions reorganization, an initial activity was made in the graphic frame; it seems optimal to start the debates without leading participants to failure. The mathematical core of the workshop was formed by an algebraic method to find tangents to algebraic curves which is close to Descartes’ method. The ACODESA methodology allowed some intense debates mainly concerning the local character of a tangent, generating the teachers’ cognitive unbalance which is a starting point for the refinement or transformation of their conceptions. On the other hand, the cK¢ model allows to understand the conceptions of participants and to analyze the evolution of their knowings.     

Progress in Scanning Electrochemical Microscopy by Coupling with Electrochemical Impedance and Quartz Crystal Microbalance

Scanning electrochemical microscopy (SECM) is a powerful technique for performing quantitative measurements at a local scale. This paper covers the development of combinations of SECM with electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM). Basic aspects are described and potential applications reported by several research groups are covered. The unique advantages of the coupled techniques—with additional information being obtained from each coupling—are also discussed.     

On the stored and dissipated energies in heterogeneous rate-independent systems: theory and simple examples

The aim of the present work is to determine the amount of dissipated and stored energies in structures containing frictional cracks and elasto-plastic zones. The proposed theory combines micromechanical and thermodynamic tools to calculate both energies. Using simple examples, it is shown that the Taylor–Quinney coefficient is not a constant, and can be much less than the values usually considered (i.e. close to unity).