Asymmetric alloy formation at the Fe-on-Ti and Ti-on-Fe interfaces.

The Fe-on-Ti and Ti-on-Fe interfaces were studied experimentally by Mössbauer spectroscopy (MS), transmission electron microscopy (TEM) and x-ray reflectometry (XRR) on Ti/Fe/Ti trilayers grown on Si(1 1 1) substrates by vacuum evaporation. The nanoscale structure and composition were explored in cross sections using TEM, the layer structure and the interface widths by specular x-ray reflectometry. MS was applied to identify the interface alloy phases and to determine the pure and alloyed Fe layer fractions. The experimental results were compared with molecular dynamics (MD) simulations of layer growth on Fe or Ti underlayers of different orientations. The concentration distributions provided by MD simulations show an asymmetry at the interfaces in the layer growth direction. The transition is atomically sharp at the Ti-on-Fe interface for the (0 0 1) and (1 1 0) crystallographic orientations of the Fe underlayer, while it spreads over a few atomic layers for Fe(1 1 1) underlayer and for all studied Ti underlayer orientations at the Fe-on-Ti interface. MS and XRR data on Ti/Fe/Ti trilayers confirm the asymmetry between the bottom and top Fe interface, but the inferred interface widths considerable exceed those deduced from the MD simulations.

Ion-erosion induced surface nanoporosity and nanotopography on Si.

The low-energy ion-bombardment induced surface nanotopography and the nanopatterning of Si has been simulated by atomistic simulations using an approach based on molecular dynamics (MD). In order to speed up simulations a reasonable cutoff in simulation time and increased cooling rates for keeping in hand the system temperature have been used. We get an unexpectedly rich variety of disordered nanopatterns formed by the self-organization of the crater rims and adatoms islands generated by the individual ion impacts. Our results reveal that the low-energy (0.5 keV impact energy) ion-sputtered Si surface is not smooth at the sub-20 nm length scale and deep nanoholes rule the landscape. Moreover substantial nanoporosity is found beneath the surface with the size range of a few nanometer. Scanning tunneling microscopy (STM) images are also shown obtained for low-fluence ion-sputtering of Si at 2 keV impact energy at 30° angle of incidence. STM images reveal similar features obtained by computer simulations: nanoholes can be seen with a few nanometer diameter. The overall topography landscape as well as the rms surface roughness also show similar features for the images obtained by STM or MD at 2 keV impact energy. The applied approach could make it possible the simulation of nanotopographic images at the molecular dynamics level of theory and could help resolve scanning probe microscopy images in the sub-20 nm length scale regime.

The molecular dynamics simulation of ion-induced ripple growth.

The wavelength-dependence of ion-sputtering induced growth of repetitive nanostructures, such as ripples has been studied by molecular dynamics (MD) simulations in Si. The early stage of the ion erosion driven development of ripples has been simulated on prepatterned Si stripes with a wavy surface. The time evolution of the height function and amplitude of the sinusoidal surface profile has been followed by simulated ion-sputtering. According to Bradley-Harper (BH) theory, we expect correlation between the wavelength of ripples and the stability of them. However, we find that in the small ripple wavelength (lambda) regime BH theory fails to reproduce the results obtained by molecular dynamics. We find that at short wavelengths (lambda<35 nm) the adatom yield drops hence no surface diffusion takes place which is sufficient for ripple growth. The MD simulations predict that the growth of ripples with lambda>35 nm is stabilized in accordance with the available experimental results. According to the simulations, few hundreds of ion impacts in lambda long and few nanometers wide Si ripples are sufficient for reaching saturation in surface growth for for lambda>35 nm ripples. In another words, ripples in the long wavelength limit seems to be stable against ion-sputtering. A qualitative comparison of our simulation results with recent experimental data on nanopatterning under irradiation is attempted.

A combination of assays reveals biomass differences in biofilms formed by Escherichia coli mutants.

AIMS: The aim of this study was to develop an assay system that can quantify the amount of biomass in biofilms formed by different isogenic mutants of an Escherichia coli K-12 strain. METHODS AND RESULTS: The reported assay, which is based on the BacTiter-Glo assay from Promega, uses bioluminescence to detect the intracellular concentration of ATP, which correlates with viable bacterial cell numbers. The quantitative data obtained with this ATP assay were compared to those obtained with the conventional crystal violet assay. As a qualitative control, scanning electron microscopy was performed. CONCLUSIONS: The ATP assay, the crystal violet assay and scanning electron microscopy yielded similar results for six of the eight strains tested. For the remaining two strains, the images from the scanning electron microscopy confirmed the results from the ATP assay. SIGNIFICANCE AND IMPACT OF THE STUDY: The ATP assay, in combination with other quantitative and qualitative assays, will allow us to perform genetic studies on the regulatory network that underlies the early steps in E. coli biofilm formation.

Anisotropy driven ultrafast nanocluster burrowing.

We explore the occurrence of low-energy and low-temperature transient cluster burrowing leading to intact cluster inclusions. In particular, the anomalously fast (ballistic) Pt nanocluster implantation into Al and Ti substrates has been found by molecular dynamics simulations using a tight-binding many-body potential with a 1-10 eVatom low impact energy. A similar behavior has also been found for many other cluster/substrate couples such as CuAl, NiTi, and CoTi. At this few eV/atom low impact energy regime, instead of the expected stopping at the surface we find the propagation of the cluster through a thin Al slab as thick as approximately 50 A with a nearly constant speed ( proportional, variant1 eVatom). Hence the cluster moves far beyond the range of the impact energy, which suggests that the mechanism of cluster burrowing cannot be explained simply by collisional cascade effects. In the couples with reversed succession (e.g., TiPt, AlPt) no burrowing has been found; the clusters remain on the surface (the asymmetry of burrowing). We argue that cluster penetration occurs at a few eV/atom impact energy when the cluster/substrate interaction is size mismatched and mass anisotropic atomically.

Self-organized transient facilitated atomic transport in Pt/Al(111).

During the course of atomic transport in a host material, impurity atoms need to surmount an energy barrier driven by thermodynamic bias or at ultralow temperatures by quantum tunneling. In the present article, we demonstrate using atomistic simulations that at ultralow temperature, transient interlayer atomic transport is also possible without tunneling when the Pt/Al(111) impurity/host system self-organizes itself spontaneously into an intermixed configuration. No such extremely fast athermal concerted process has been reported before at ultralow temperatures. The outlined novel transient atomic exchange mechanism could be of general validity. We find that the source of ultralow temperature heavy particle barrier crossing is intrinsic and no external bias is necessary for atomic intermixing and surface alloying in Pt/Al, although the dynamic barrier height is a few eV. The mechanism is driven by the local thermalization of the Al(111) surface in a self-organized manner arranged spontaneously by the system without any external stimulus. The core of the short lived thermalized region reaches the local temperature of approximately 1000 K (including a few tens of Al atoms), while the average temperature of the simulation cell is approximately 3 K. The transient facilitated intermixing process also takes place with repulsive impurity-host interaction potential leading to negative atomic mobility; hence, the atomic injection is largely independent of the strength of the impurity-surface interaction. We predict that similar exotic behavior is possible in other materials as well.

Comparative pharmacodynamic-pharmacokinetic correlation of oral sustained-release theophylline formulation in adult asthmatics.

A sustained-release formulation of theophylline with an innovative release mechanism was evaluated in adult asthmatics. The pharmacodynamics and pharmacokinetic behavior of this formulation was compared with a market formulation (Theobid). The formulations, each containing 200 mg of anhydrous theophylline, were evaluated in six male subjects, 40-55 years of age, 151-169 cm in height, 41-60 kg in weight, who were nonsmokers with moderate chronic obstructive pulmonary disease (COPD); the study was a randomized, single-dose, open, complete crossover study with an interval of 1 week. Written consent was obtained from the patients prior to the trial. Plasma samples were obtained at 0, 1, 2, 4, 6, 8, 10, and 12 hr postadministration. Pulmonary functions were simultaneously recorded using an Erich Jaeger spirometer. Plasma theophylline assays were performed using high-performance thin-layer chromatography (HPTLC). Individual bioavailability parameters were obtained using the S-Inv computer program. Pharmacodynamic-pharmacokinetic correlation was studied using the Excel 95 version 7.0 Regression Statistics program. The test formulation (innovator) was found to be comparable with the marketed product with respect to tmax, t1/2 and Kel (p < .05). A significant difference in the means of Cmax and AUC0-12 between the innovator and the market formulation indicated a superior extent of absorption from the innovator formulation. A good pharmacodynamic-pharmacokinetic correlation was observed when plasma theophylline concentration was compared with forced expiratory volume.

High-performance thin-layer chromatographic determination of theophylline in plasma.

A high-performance thin-layer chromatographic method for quantification of theophylline from plasma is described. The calibration curves of theophylline in methanol and in plasma were linear in the range 20-100 ng. The correlation coefficients were 0.9971+/-0.0011 and 0.9955+/-0.0003 for standard curves in methanol and in plasma, respectively. The limit of quantitation of theophylline in human plasma (assay sensitivity) was 20 ng and no interference from endogenous compounds was observed. The recovery of theophylline from human plasma using the described assay procedure was 89%. The mean relative standard deviations for intra- and inter-day analyses were 1.67% and 2.34% for 50 ng and 2.25% and 3.14% for 75 ng theophylline concentration, respectively. The method was utilized to monitor plasma concentration of theophylline post-administration of sustained release tablets in human patient volunteers.

The substrate recognition domain in the Na+/dicarboxylate and Na+/sulfate cotransporters is located in the carboxy-terminal portion of the protein.

The Na+/dicarboxylate cotransporter, NaDC-1, and the Na+/sulfate cotransporter, NaSi-1, share 43% sequence identity, but they exhibit no overlap in substrate specificity. A functional chimera, SiDC-4, was prepared from NaDC-1 and NaSi-1 by homologous recombination and expressed in Xenopus oocytes. SiDC-4 contains putative transmembrane domains 1-4 of NaSi-1 (amino acids 1-139) and putative transmembrane domains 5-11 of NaDC-1 (amino acids 141-593). SiDC-4 retains the substrate specificity of NaDC-1, which suggests that the substrate recognition domain is found in the carboxy-terminal portion of the protein, past amino acid 141. However, residues that affect substrate affinity and inhibition by furosemide and flufenamate are found in the amino terminal third of the protein. The cation binding properties of SiDC-4, including a stimulation of transport by lithium, differed from both parental transporters, suggesting that cation binding is determined by interactions between the amino- and carboxy-terminal portions of the protein. We conclude that the substrate recognition site of NaDC-1 and NaSi-1 is found in the carboxy-terminal portion of the protein, past amino acid 141, but residues in the amino terminus can affect substrate affinity, inhibitor sensitivity, and cation selectivity.