Thermodynamic issues associated with combined cyclic voltammetry and wafer curvature measurements in electrolytes

A thermodynamic framework has been provided for the interpretation of combined cyclic voltammetry and surface stress measurements, the latter being obtained from wafer curvature or beam deflection measurements of a solid electrode as a function of applied potential (so-called voltstressograms). Firstly, the derivation of electrocapillarity equations for solid electrodes has been critically reviewed by starting from the Gibbs adsorption equation appropriate for solid–electrolyte interfaces. This allowed us to demonstrate the critical importance of elastic surface strain in the thermodynamic boundary conditions of the partial derivatives intervening in the interpretation of voltstressograms. From these considerations, it was shown for the first time that the electrocapillarity equations for solid electrodes are not appropriate for describing the variation of surface stress with potential obtained from wafer curvature measurements, because such measurements are intrinsically incompatible with the constant strain condition implied in the electrocapillarity equations. An alternative explanation is provided for the experimentally observed proportionality between the current density, measured in cyclic voltammograms, and the first derivative of surface stress with respect to potential, obtained from voltstressograms.     

In situ monitoring of electrostriction in anodic and thermal silicon dioxide thin films

Stresses due to electric fields in thermal and anodic silica thin layers can impact the devices using these films as dielectrics. Accurately quantifying the internal stress as a function of the electric field is thus of technological importance. In this work, electrostrictive stresses are monitored during cyclic polarization of silica thin films on silicon and during the growth of anodic silica. These are obtained by combining curvature and ellipsometry measurements in situ. In silica films grown by thermal oxidation of silicon, the electric field can generate either tensile or compressive stresses depending on its magnitude and on the silica polarization history. The electromechanical coupling in thermal silica is assumed to be controlled by a reversible change of the dipole organization. For anodic silica films, the stress generated by the electric field is tensile and varies linearly with the square of the electric field above 0.26 V²/nm² under both cyclic polarization and oxidation conditions.     

What controls the pore spacing in porous anodic oxides?

In this paper, we use energy-based perturbation criteria to examine whether strain or electrostatic energy acts as a driving force for porosity initiation in anodic oxides. By doing so, we also succeeded to rationalise the dependence of pore spacing on anodising conditions. Our experimental approach consists of measuring in-situ the internal stress in anodic oxide films grown galvanostatically on aluminium in phosphoric acid, and to correlate these data with the measured pore spacing of the obtained porous films. Our results indicate that the possibility of a strain energy-induced surface instability is unlikely, as for this case the constitutive dependence of pore spacing on internal stress was not verified. Instead, the measured pore spacing, electric field and barrier oxide thickness obtained on our anodic alumina films indicate that electrostatic energy is the main driving force for pore initiation, as well as the factor controlling the pore spacing. Corroborative quantitative evidence for this novel electrostatic-based scaling law is provided by data compiled from the literature for a range of other anodic oxide systems, including nanoporous alumina and nanotubular titania films.     

Simultaneous measurement of drug metabolic stability and identification of metabolites using ion-trap mass spectrometry

The use of in vitro drug metabolism data in the understanding of in vivo pharmacokinetic, safety and toxicity data has become a large area of scientific interest. This has stemmed from a trend in the pharmaceutical industry to use in vitro data generated from human tissue as a criterion to select compounds for further investigation. As well as measuring metabolic stability in vitro using human liver microsomal preparations, the identification of possible metabolite(s) formed may play a vital role in Hit-to-Lead and Lead optimisation processes. The data-dependent scan function mode with the ion-trap instrumentation provides the ability to measure the metabolic stability and identification of possible metabolites of a compound. A gradient liquid chromatographic method with a run time of 6 min/injection was developed for this purpose. The approach of simultaneous metabolic stability measurements and rapid identification of metabolites of drugs with high (verapamil), medium (propranolol and cisapride) and low (flunarazine) metabolic stabilities using ion-trap mass spectrometry is described. The metabolites identified after 15 min incubation for verapamil, propranolol and cisapride are in good agreement with those reported as the major metabolites in human in vivo studies.     

Highly Amino Acid Selective Hydrolysis of Myoglobin at Aspartate Residues as Promoted by Zirconium(IV)‐Substituted Polyoxometalates

SDS‐PAGE/Edman degradation and HPLC MS/MS showed that zirconium(IV)‐substituted Lindqvist‐, Keggin‐, and Wells–Dawson‐type polyoxometalates (POMs) selectively hydrolyze the protein myoglobin at Asp? X peptide bonds under mildly acidic and neutral conditions. This transformation is the first example of highly sequence selective protein hydrolysis by POMs, a novel class of protein‐hydrolyzing agents. The selectivity is directed by Asp residues located on the surface of the protein and is further assisted by electrostatic interactions between the negatively charged POMs and positively charged surface patches in the vicinity of the cleavage site.     

The use of liquid chromatography-atmospheric pressure chemical ionization mass spectrometry to explore the in vitro metabolism of cyanoalkyl piperidine derivatives

A LC/MS method using atmospheric pressure chemical ionization, positive ion mode and full scan to measure the in vitro metabolic stability of cyanoalkyl functionalized compounds with the human liver microsomes was employed. Percentage metabolism examined for the five cyanoalkyl piperidines revealed the optimal chain length and positioning of these functions to produce the most metabolically stable compound. The 4-cyanomethyl piperidine derivative was the most stable compound with 15% metabolism after 15 min incubation with human liver microsomes. In general, the major metabolites formed from the cyanoalkyl piperidine derivatives were due to oxidation of the cyanoalkyl chain or the piperidine fragment, resulting in a M+16 ion. However, the 2-cyanomethyl piperidine derivative exhibited an interesting biotransformation pathway with unusual metabolite peaks corresponding to M+5, M-11 and M+21 ions. Data-dependent MS/MS scanning was used to generate daughter ion spectra from the parent compound and its metabolite peaks. Based on the fragmentation analysis, a carboxylic acid, aldehyde and oxidative metabolite of the carboxylic acid structure have been proposed for M+5, M-11 and M+21 ions, respectively.     

An in situ study of the hydriding kinetics of Pd thin films

The hydriding kinetics of Pd thin films has been investigated in detail. The in situ experimental technique used in this work consists of a high resolution curvature measurement setup, which continuously monitors the reflections of multiple laser beams reflecting off a cantilevered sample. After mounting the sample inside a vacuum chamber, a H-containing gas mixture is introduced to instantaneously generate a given hydrogen partial pressure (p(H(2))) inside the chamber. The resulting interaction of hydrogen with the Pd layer then leads to a volume expansion of the thin film system. This induces in turn changes in the sample curvature as a result of internal stresses developing in the Pd film during a hydriding cycle. Based on such in situ curvature data, three different kinetic regimes have been resolved. The first two exhibited a linear increase of the internal stress in the compressive direction with time. A systematic study of the p(H(2))-dependency of the two constant slopes was performed, based on newly derived constitutive kinetic equations. This resulted in the identification of the first linear regime to be limited by absorption and the second one by adsorption. After adsorption equilibrium is reached at the end of the second regime, a third, non-linear kinetic regime, limited by absorption, was found to precede the final hydriding equilibrium. This switch back to absorption-limited kinetics likely occurs due to a coverage dependent change in the adsorption enthalpy of the surface hydrogen. Furthermore, from our in situ experimental data, relevant kinetic and thermodynamic hydriding parameters have been derived. As a result, this study was able to provide a self-consistent quantitative interpretation of the entire Pd room temperature hydriding cycle in the alpha-phase domain.