Contactless monitoring of the diameter-dependent conductivity of GaAs nanowires

Contactless monitoring with photoelectron microspectroscopy of the surface potential along individual nanostructures, created by the X-ray nanoprobe, opens exciting possibilities to examine quantitatively size- and surface-chemistry-effects on the electrical transport of semiconductor nanowires (NWs). Implementing this novel approach-which combines surface chemical microanalysis with conductivity measurements-we explored the dependence of the electrical properties of undoped GaAs NWs on the NW width, temperature and surface chemistry. By following the evolution of the Ga 3d and As 3d core level spectra, we measured the position-dependent surface potential along the GaAs NWs as a function of NW diameter, decreasing from 120 to ?20 nm, and correlated the observed decrease of the conductivity with the monotonic reduction in the NW diameter from 120 to ~20 nm. Exposure of the GaAs NWs to oxygen ambient leads to a decrease in their conductivity by up to a factor of 10, attributed to the significant decrease in the carrier density associated with the formation of an oxide shell. Open image in new window     

Machinability analysis in turning tungsten–copper composite for application in EDM electrodes

Electro-discharge machining (EDM) is widely used in tooling industry, where it is applied on materials, which are too hard to be machined with conventional techniques. The tungsten–copper is broadly used as an EDM electrode for machining of die steel and tungsten carbide workpieces. As, tungsten–copper electrode is more costly than conventional electrodes, there is a need to understand the machinability aspects in turning of this material. Hence, an attempt has been made in this paper to study the effects of cutting conditions on machinability characteristics such as cutting force, feed force, depth force, machining force, power, specific cutting force, arithmetic average surface roughness and maximum peak to valley height during tungsten–copper turning with K10 carbide cutting tool. The response surface methodology (RSM) based second order mathematical models of machinability aspects are developed using the data obtained through full factorial design (FFD). The adequacy of the machinability models is tested through the analysis of variance (ANOVA). The response surface analysis reveals that a combination of higher cutting speed with low-to-medium feed rate is advantageous in reducing the forces, power and surface roughness, which in turn increases the specific cutting force.     

Amylolytic enzymes produced by Bacillus amyloliquefaciens MIR-41 in batch and continuous culture

Bacillus amyloliquefaciens MIR-41 has been isolated from natural sources and characterized using statistical methods. The strain showed three types of glucanase activities: α-amylase, α-glucosidase and pullulanase in defined medium and starch as carbon source. Extracellular activity of α-amylase was approximately three times higher in continuous culture than in batch. Maximal productivity was found at a dilution rate of 0·16 h^?1. Alpha-glucosidase was expressed as a cell-bound enzyme in batch culture, but in continuous culture was mostly extracellular. Pullulanase was cell-bound in all cases. The expression of the enzymes was dependent upon the system and the dilution rate used. No cell lysis was detected in the chemostat as compared with batch cultures.     

Novel Approaches in Selective Tryptophan Isotope Labeling by Using Escherichia coli Overexpression Media

NMR‐based investigations of large protein complexes require optimized isotopic labeling schemes. We report new methods to introduce stable isotopes into tryptophan residues; these are fine‐tuned to the requirements of the particular protein NMR experiment. Selective backbone labeling was performed by using a new α‐ketoacid precursor as an additive in cell‐based overexpression media. Additionally, we developed synthetic routes to certain isotopologues of indole with ¹³C–¹H spin systems surrounded by ¹²C and ²H. The corresponding proteins, overexpressed in the presence of these precursor compounds, can be effectively analyzed for conformational changes in tryptophan residues in response to external stimuli, such as interaction with other proteins or small molecules.     

Thermal stabilization by polyols of β-xylanase from Bacillus amyloliquefaciens

Purified endo-β-1,4-xylanase of Bacillus amyloliquefaciens MIR 32 retained 100% of its activity after 4 days of incubation at 50°C. Sorbitol (400 mg cm⁻³) produced a 63-fold increase in the half-life of the enzyme at 65°C, which was only 29 min at this temperature in the absence of the polyol. This thermal stabilizing activity increased exponentially in respect to sorbitol concentration in the range 250–400 mg cm⁻³ and was dependent on the pH, showing a maximum at pH values between 5·25 and 8·0. The circular dichroism (CD) thermal scanning profile (50°C h⁻¹) at 224 nm showed that changes in the secondary structure of xylanase started at 65°C, while in the presence of sorbitol (400 mg cm⁻³) these modifications started at 80°C. This study indicated that sorbitol might be a valuable stabilizer for the use of β-xylanase from B. amyloliquefaciens at high temperatures. © 1998 SCI