Preparation and analytical testing of mediator containing photolithographically patterned enzyme membrane electrodes

The application of mediators for measurements with amperometric enzyme sensors have been investigated to improve the behavior of sensors with respect to interfering substances or for working under anaerobic conditions. The aim of this investigation is to develop photolithographically patterned enzyme membranes containing mediators, which facilitate the inexpensive technological preparation of patterned sensors. Thin layer platinum electrodes were coated with the enzyme membranes and cross-linked by means of UV light. Measurements were made in a wall-jet configuration using flow injection techniques with or without oxygen in the solutions. Optimum properties can be obtained with glucose oxidase containing membranes using tetrathiafulvalenes. The interfering substances ascorbic acid, uric acid and acetaminophenol showed no influence on the glucose measurements in the range of physiological concentrations. The membrane served as a diffusion barrier; a decrease in the applied potential to 300 mV vs. SCE also improved the ratio of the glucose response to the interference response.     

Fission barriers of preactinide nuclei from electrofission experiments

Absolute electrofission cross sections for ^182,184,186W, ^natPt, and ²⁰⁹Bi have been measured using solid-state track detectors and parallel-plate avalanche counters for the detection of the fission fragments. The measured cross sections — together with others known from literature — have been analyzed within an extended statistical model to deduce fission barriers and compared with theoretical calculations. From the derived barrier height for platinum, it is concluded that the pairing strength G most likely does not change with deformation (G = const). All experimental fission barriers agree very well with theoretical predictions for G = const.     

Thermoviscoplastic modelling of asymmetric effects for polymers at large strains

Glassy polymers such as polycarbonate exhibit different behaviours in different loading scenarios, such as tension and compression. To this end a flow rule is postulated within a thermodynamic consistent framework in a mixed variant formulation and decomposed into a sum of weighted stress mode related quantities. The different stress modes are chosen such that they are accessible to individual examination in the laboratory, where tension and compression are typical examples. The characterisation of the stress modes is obtained in the octahedral plane of the deviatoric stress space in terms of the Lode angle, such that stress mode dependent scalar weighting functions can be constructed. Furthermore the numerical implementation of the constitutive equations into a finite element program is briefly described. In a numerical example, the model is used to simulate the laser transmission welding process.     

Properties of galactocerebroside layers transferred to glassy carbon electrodes: effect of an applied electric field

Galactocerebroside films deposited onto glassy carbon electrodes have been previously studied through the electrochemical response of a redox couple present in solution. Those experiments indicated that the film is inhomogeneous and that there are lipid-free places. In this work, we present experimental results indicating that those bare regions are formed when the electrode is introduced in an aqueous solution, and that the size and/or amount of uncovered domains increase when negative potentials are applied to the film. The experimental techniques employed for these findings are epifluorescence microscopy and ellipsometry.     

Optimization of poly(dimethylsiloxane) hollow prisms for optical sensing

A new generation of simple, robust and compact microfluidic systems with optical readout is presented. The devices consist of hollow prisms fabricated by soft-lithography, together with microlenses and self-aligned channels for fibre optic positioning, conferring the system with a high degree of monolithic integration. Its working principle is based on the absorption of the working wavelength (lambda = 460 nm) by the different substances that can fill the hollow prisms. By modifying the volumes and geometries, optimization of the presented systems has been achieved. Results show how the limit of detection (LOD) for fluorescein and methylorange diluted in phosphate buffer can be significantly lowered, by increasing the size of the prism or increasing the total deviation angle. For our investigations we used concentrations for which the Beer-Lambert law is fulfilled and the measurements showed a LOD in the microM range for both species. Finally, since the change in the fractions of the methylorange as a function of the pH causes a variation of the total absorption of the solution, the hollow prisms have also been used for pH measurements.     

Crystallite size in highly drawn polyethylene

The size and distortion of crystallites in samples of linear polyethylene were determined before and after plastic deformation. A slowly cooled sample, a quenched sample, and highly drawn films (draw ratio 16) were investigated by different methods. Wide-angle x-ray patterns were analyzed to study the average size of the crystalline mosaic blocks and their distortion. In addition, the longitudinal crystal thickness (in the chain direction) was evaluated by two other approaches, determination of the long period, and the melting temperature of irradiated samples. The results show clearly that the size of the crystalline mosaic blocks changes substantially with drawing of polyethylene. Not only is the lateral crystal thickness affected, but the longitudinal crystal dimensions also change during the drawing process. By the three independent methods we find that the longitudinal crystal thickness after drawing is independent of the value for the undrawn samples, as was reported earlier by Peterlin. The change in crystallite size after drawing is accompanied by a large decrease in crystal volume to about 10% of the value for the undrawn sample. The degree of distortion in the crystals seems not to be affected by the deformation process. These experimental data can be considered evidence for high chain mobility and for the possibility of rearrangement of chain molecules during the process of plastic deformation.     

Measurement of the fine structure splitting of the 32D multiplet of Li by stepwise excitation with cw dye lasers

The fine structure splitting of the 3²D multiplet of Li has been investigated using stepwise dye laser excitation of a strongly collimated atomic beam via the 2²P_{$\text{3}\text{2}$} level. The splitting is determined by a beat frequency measurement of two single mode dye lasers exciting the two 3²D levels. The result is Δν(3²D)=(1074±3) MHz.