Comparison of growth stress measurements with modelling in thin iron oxide films

High temperature oxidation of metals leads to residual stresses both in the metal and in the growing oxide. In this work, the evolution of this residual stresses is theoretically predicted in the growing oxide layers. The origin of these stresses is based on a microstructural model. Using experimental results providing from the oxidation kinetics, and an analysis proposed to describe the growth strain occurring in the thin layers, a set of equations is established allowing determining the stresses evolution with oxidation time. Then, the model is compared with experimental results obtained on both α-Fe and phosphated α-Fe, oxidised at different temperatures. Numerical data are extracted from experiments either with an asymptotic formulation or with an inverse method. These two methods give good agreement with experiments and allow extracting the model parameters.     

On-line cysteine modification for protein analysis: new probes for electrochemical tagging nanospray mass spectrometry

A series of electrogenerated selective electrophiles based on substituted benzoquinones has been characterized as tags for l-cysteine and cysteine residues in proteins. The electrophiles are generated electrochemically from the corresponding hydroquinones. It is shown from mass spectrometry analysis that the electrogenerated benzoquinone can tag the biomolecules. The rate constants pertaining to the addition of l-cysteine onto the electrogenerated benzoquinones have been determined using electrochemical techniques. The substitution patterns have been unraveled leading to the assessment of site-specific rate constants. It is shown that the rate constants are primarily dependent on the electronic nature of the substituents as expressed by the Hammett substitution constant. The apparent tagging yields observed for l-cysteine in nanospray mass spectrometry experiments do not correspond to the yields expected from the electrochemical study, as the ionisation efficiencies are highly dependent on the tag. Finally, the on-line tagging has been tested using β-lactoglobulin A and myoglobin. Based on these results, it is concluded that the tagging reaction is selective towards cysteine when it takes place in the nanospray interface. The results show that the methodology presented can be used for a rapid characterization and identification of reactive sites in biomolecules.     

Photoinduced electron transfer at liquid|liquid interfaces: dynamics of the heterogeneous photoreduction of quinones by self-assembled porphyrin ion pairs

The initial stages of the heterogeneous photoreduction of quinone species by self-assembled porphyrin ion pairs at the water|1,2-dichloroethane (DCE) interface have been studied by ultrafast time-resolved spectroscopy and dynamic photoelectrochemical measurements. Photoexcitation of the water-soluble ion pair formed by zinc meso-tetrakis(p-sulfonatophenyl)porphyrin (ZnTPPS(4)(-)) and zinc meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP(4+)) leads to a charge-separated state of the form ZnTPPS(3)(-)-ZnTMPyP(3+) within 40 ps. This charge-separated state is involved in the heterogeneous electron injection to acceptors in the organic phase in the microsecond time scale. The heterogeneous electron transfer manifests itself as photocurrent responses under potentiostatic conditions. In the case of electron acceptors such as 1,4-benzoquinone (BQ), 2,6-dichloro-1,4-benzoquinone (DCBQ), and tetrachloro-1,4-benzoquinone (TCBQ), the photocurrent responses exhibit a strong decay due to back electron transfer to the oxidized porphyrin ion pair. Interfacial protonation of the radical semiquinone also contributes to the photocurrent relaxation in the millisecond time scale. The photocurrent responses are modeled by a series of linear elementary steps, allowing estimations of the flux of heterogeneous electron injection to the acceptor species. The rate of electron transfer was studied as a function of the thermodynamic driving force, confirming that the activation energy is controlled by the solvent reorganization energy. This analysis also suggests that the effective redox potential of BQ at the liquid|liquid boundary is shifted by 0.6 V toward positive potentials with respect to the value in bulk DCE. The change of the redox potential of BQ is associated with the formation of hydrogen bonds at the liquid|liquid boundary. The relevance of this approach toward modeling the initial processes in natural photosynthetic reaction centers is briefly discussed.     

Supercapacitive admittance tomoscopy

A sensor for measuring adsorption on a substrate has been designed including a contactless detection scheme, called supercapacitive admittance tomoscopy (SCAT). The sensor comprises a thin dielectric layer with two parallel band electrodes on the one side and a chemically modified surface on the other onto which the adsorption of molecules occurs. Upon application of a high frequency ac voltage between the two electrodes, a capacitive coupling is established across the dielectric layer, and the admittance measured depends on the surface state of the chemically modified interface. On the basis of this principle, a flow sensor has been developed to measure sensorgrams to follow the dynamics of the adsorption and has been tested for the adsorption of IgG on the modified surface.     

Thermal and dynamic analysis of the reactions between formic acid and triethylamine

It is possible to study the problems of thermal regulation and energy storage by utilizing chemical reactions. For this purpose, a method of dynamic and global modelling is used to determine the enthalpy of reaction of formic acid and triethylamine.     

Ecdysteroid glycosides: identification, chromatographic properties, and biological significance

Ecdysteroid glycosides are found in both animals and plants. The chromatographic behavior of these molecules is characteristic, as they appear much more polar than their corresponding free aglycones when analyzed by normal-phase high-performance liquid chromatography (HPLC), whereas the presence of glycosidic moieties has a very limited (if any) impact on polarity when using reversed-phase HPLC. Biological activity is greatly reduced because the presence of this bulky substituent probably impairs the interaction with ecdysteroid receptor(s). 2-Deoxy-20-hydroxyecdysone 22-O-beta-D-glucopyranoside, which has been isolated from the dried aerial parts of Silene nutans (Caryophyllaceae), is used as a model compound to describe the rationale of ecdysteroid glycoside purification and identification.     

Pressure pinched injection of nanolitre volumes in planar micro-analytical devices

A new method for injecting and driving fluids by means of a multi-port injection valve and syringe pumps in a micro-channel network is described. A structure composed of two micro-channels arranged as a cross is connected with capillary tubes to an external multi-port injection valve. The fluid flows are driven by pressure and the multi-port valve controls the direction of the flow within the different sections of the structure. The first position of the multi-port valve allows the preparation of the loading of the sample, which is pinched in the cross section of the two micro-channels. The second position allows the precise injection of nL volumes. No dead volume exists between injection and separation modes. The system can be used to prepare a sample plug by pressure in order to perform chromatography with a broad range of buffered or non-buffered solutions. Thanks to the insensitivity to the ionic strength of the sample, this injection method is useful for the injection of complex biological samples in microchip analysis. In order to demonstrate the feasibility of the method, different solutions of ionic or fluorescent molecules were injected and detected in a photoablated planar polymer device.     

Photoinduced electron transfer at liquid|liquid interfaces. Part VII. Correlation between self-organisation and structure of water-soluble photoactive species

The self-organisation of a variety of dyes at the water|1,2-dichloroethane interface was studied by admittance measurements, photocurrent–potential curves and light polarisation anisotropy of the photocurrent. The heterogeneous photo-oxidation of ferrocene was studied at interfaces sensitised by Sn(IV) meso-tetra-(4-carboxyphenyl) porphyrin dichloride (SnTPPC), chlorin e-6, protoporphyrin IX (protoIX) and Fe(III) protoporphyrin IX chloride (Fe-protoIX). Cyclic voltammograms and capacitance voltage curves exhibit different features associated with the self-assembly of the dye species at the liquid|liquid boundary. In the case of SnTPPC, the capacitance curves displayed the characteristic responses commonly associated with the specific adsorption of ionic species. On the other hand, chlorin e-6, protoIX and Fe-protoIX show rather complex behaviour suggesting not only changes in the excess charge but also in the dielectric permittivity of the interface. Differences in the photocurrent efficiency were also observed under the same experimental conditions. The relative magnitude of the photocurrent responses were rationalised in terms of the phenomenological electron transfer rate constant, the photon capture cross-sections and the lifetime of the triplet state as obtained from nanosecond flash photolysis. Finally, the average molecular orientation of the adsorbed photoactive species was estimated from the photocurrent dependence on the angle of light polarisation in total internal reflection. The results show a clear correlation between the orientation of the transition dipole and the distribution of the peripheral carboxyl groups responsible for the hydrophilic nature of the dyes.     

Calorimetric study of the reaction between formic acid and N,N,N′,N′-tetramethylethylenediamine from the aspect of thermal energy storage

The attack by formic acid on the nitrogen electron pairs of N,N,N,N-tetramethylethylenediamine leads to the stoichiometric salt or to many other complexes with different molar fractions. Physical and thermodynamic parameters of two adducts, obtained by calorimetric study, are discussed in terms of their use as potential phase change materials in thermal energy storage because of their large heat of fusion between 288 and 333 K.

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Zusammenfassung Bei der Einwirkung von Ameisensäure auf die Elektronenpaare von N,N,N,N-Tetramethylethylendiamin entsteht ein stöchiometrisches Salz oder mehrere andere Komplexe mit unterschiedlichem Molenbruch. Es werden physikalische und thermodynamische Parameter zweier bei der kalorimetrischen Untersuchung erhaltenen Addukte hinsichtlich ihrer Verwendung als potentielle Phasenwechselsubstanzen bei der thermischen Energiespeicherung diskutiert, da sie im Bereich 288–333 K eine große Schmelz-wärme besitzen.