Optically induced gate voltage spectroscopy in a GaAs/AlGaAs heterostructure

We report on measurements of optically induced gate voltage spectroscopy in a GaAs/AlGaAs heterostructure with a high mobility 2-dimensional electron gas (2DEG) in a thin (55 nm) GaAs layer. The optically induced gate voltage between the front gate and the 2DEG is sensitive to excess electron concentrations below 10⁷ cm⁻². In the gate voltage spectrum we observe a peak below the bandgap energy of GaAs, which is not observed in the photocurrent, luminescence or excitation spectra. Due to the extremely high sensitivity of this technique we attribute this below bandgap signal to very weak absorption lines below the GaAs bandgap energy by impurity bands or defect absorption. The fall-off of the below bandgap signal varies as exp (hω/E₀), where E₀ is an indicative for the quality of the heterostructure.     

Chemical and physical analysis of cotton fabrics plasma-treated with a low pressure DC glow discharge

This paper focuses on the modification of cotton fabrics using a low pressure DC glow discharge obtained in air. The influence of different operating parameters such as treatment time, discharge power and operating pressure on the chemical and physical properties of the cotton fabrics is studied in detail. Surface analysis and characterization of the plasma-treated cotton fabrics is performed using vertical wicking experiments, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and weight loss measurements. The cotton fabrics show a significant increase in wicking behaviour; an effect which increases with increasing treatment time, increasing discharge power and increasing pressure. Results also show that low pressure DC glow treatment leads to surface erosion of the cellulose fibres, accompanied by an incorporation of oxygen-containing groups (C–O, C=O, O–C–O and O–C=O) on the cotton fibres. The DC glow treatment has thus the potential to influence not only the chemical but also the physical properties of cotton fabrics and this without the use of water or chemicals.     

Bei der Bestimmung von Invertzucker

Ohne Zusammenfassung     

Surface modification of polypropylene with an atmospheric pressure plasma jet sustained in argon and an argon/water vapour mixture

In this paper, an atmospheric pressure plasma jet sustained in pure argon and an argon/water vapour mixture has been used to modify the surface of polypropylene (PP) films. The gas temperature of the plasma jet was found to be 625 K in an active zone between the electrodes and was found to increase in the afterglow. Based on these results, the PP films are placed as close as possible to the edge of the capillary in order to avoid thermal damage to the polymer. XPS results on the untreated and modified PP samples revealed incorporation of a significant amount of oxygen on the polymer surface, however, this oxygen inclusion is more pronounced for the argon/water vapour jet due to the higher radicals density in the jet afterglow. One can therefore conclude that adding water vapour to an argon plasma jet can be a convenient way to increase the efficiency of plasma surface modification.     

Nachweis fremder Farbstoffe in der Milch

Ohne Zusammenfassung     

Extending the biocatalytic scope of regiocomplementary flavin-dependent halogenase enzymes

Flavin-dependent halogenases are potentially valuable biocatalysts for the regioselective halogenation of aromatic compounds. These enzymes, utilising benign inorganic halides, offer potential advantages over traditional non-enzymatic halogenation chemistry that often lacks regiocontrol and requires deleterious reagents. Here we extend the biocatalytic repertoire of the tryptophan halogenases, demonstrating how these enzymes can halogenate a range of alternative aryl substrates. Using structure guided mutagenesis we also show that it is possible to alter the regioselectivity as well as increase the activity of the halogenases with non-native substrates including anthranilic acid; an important intermediate in the synthesis and biosynthesis of pharmaceuticals and other valuable products.     

Increasing the Hydrophobicity of a PP Film Using a Helium/CF<Subscript>4</Subscript> DBD Treatment at Atmospheric Pressure

Plasma surface modification is widely used to tailor the surface properties of polymeric materials. Most treatments are performed using low pressure plasma systems, but recently, atmospheric dielectric barrier discharges (DBDs) have appeared as interesting alternatives. Therefore, in this paper, an atmospheric He + CF₄ DBD is used to increase the hydrophobicity of a polypropylene (PP) film. The surface characterization of the PP film is performed using contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results show that the hydrophobic properties of the polymer films are greatly enhanced after plasma treatment as evidenced by an increased contact angle. The incorporation of fluorine on the surface is significant (45 at%), demonstrating the ability of the used DBD set-up to generate fluorine-containing functional groups on the PP surface.     

Mutagenesis of morphinone reductase induces multiple reactive configurations and identifies potential ambiguity in kinetic analysis of enzyme tunneling mechanisms

We have identified multiple reactive configurations (MRCs) of an enzyme-coenzyme complex that have measurably different kinetic properties. In the complex formed between morphinone reductase (MR) and the NADH analogue 1,4,5,6-tetrahydro-NADH (NADH4) the nicotinamide moiety is restrained close to the FMN isoalloxazine ring by hydrogen bonds from Asn-189 and His-186 as determined from the X-ray crystal structure. Molecular dynamic simulations indicate that removal of one of these hydrogen bonds in the N189A MR mutant allows the nicotinamide moiety to occupy a region of configurational space not accessible in wild-type enzyme. Using stopped-flow spectroscopy, we show that reduction of the FMN cofactor by NADH in N189A MR is multiphasic, identifying at least four different reactive configurations of the MR-NADH complex. This contrasts with wild-type MR in which hydride transfer occurs by environmentally coupled tunneling in a single kinetic phase [Pudney et al. J. Am. Chem. Soc. 2006, 128, 14053-14058]. Values for primary and alpha-secondary kinetic isotope effects, and their temperature dependence, for three of the kinetic phases in the N189A MR are consistent with hydride transfer by tunneling. Our analysis enables derivation of mechanistic information concerning different reactive configurations of the same enzyme-coenzyme complex using ensemble stopped-flow methods. Implications for the interpretation from kinetic data of tunneling mechanisms in enzymes are discussed.     

Evidence from adiabatic scanning calorimetry for the Halperin-Lubensky-Ma effect at the N-SmA phase transitions in mixtures of 7OCB+heptane with an injected SmA phase

The high-resolution adiabatic scanning calorimetric technique has been used to investigate the nematic-smectic A transition (N-SmA in binary mixtures of the non-smectogenic liquid crystal heptyloxycyanobiphenyl (7OCB) and heptane, exhibiting a so-called injected smectic A phase. With the exception of a mixture with the lowest heptane mole fraction for which only an upper limit of 0.2 ± 0.2 J kg(-1) for a possible latent heat could be obtained, for all other mixtures finite latent heats were obtained. The mole fraction dependence of the latent heat could be well fitted with a crossover function consistent with a mean-field free energy expression with a non-zero cubic term arising from the Halperin-Lubensky-Ma (HLM) coupling between the SmA order parameter and the orientational director fluctuations. The mole fraction dependence of the heat capacity effective critical exponents is similar to that observed in mixtures of the two liquid crystals octyloxycyanobiphenyl (8OCB) and nonylcyanobiphenyl (9OCB). The thermal behavior observed along the N-SmA phase transition line yields further strong evidence for the HLM coupling effect.