Detection of buried reference structures by use of atomic force acoustic microscopy

The miniaturization of micro- and nanoelectronic components requires new methods for the inspection of buried inner structures at the nanoscale. We used the atomic force acoustic microscopy technique (AFAM) to image subsurface defects. This technique combines high lateral resolution with the capability to determine local elastic properties of materials near the surface. As the structures buried near the surface change the effective tip-sample contact stiffness it is possible to detect them. For the verification of the detection capabilities of AFAM we fabricated well-defined buried void structures with different geometries and dimensions. Large, thin, plate like structures of silicon nitride with a local filling were our first test samples. Then, sets of nine small, square, thin plates with thicknesses increasing stepwise from 30 to 270 nm were etched in a thinned silicon wafer. The last two samples contained wedge structures of widths varying between 1.6 and 10 μm. Our results showed that it was possible to detect buried void structures at depths between 180 and 900 nm. We also observed that the depths at which the buried defects can be detected by the use of the AFAM method depend on the defect dimensions and geometry, and on the mismatch in the elastic properties of the sample and the defects. The experimental results obtained for the groups of small, thin plates were verified by quantitative analysis via finite element method (FEM) simulations.     

Reactivity of NO/NO2–NH3 SCR system for diesel exhaust aftertreatment: Identification of the reaction network as a function of temperature and NO2 feed content

We present a systematic study of the NH₃-SCR reactivity over a commercial V₂O₅–WO₃/TiO₂ catalyst in a wide range of temperatures and NO/NO₂ feed ratios, which cover (and exceed) those of interest for industrial applications to the aftertreatment of exhaust gases from diesel vehicles. The experiments confirm that the best deNO_x efficiency is achieved with a 1/1 NO/NO₂ feed ratio. The main reactions prevailing at the different operating conditions have been identified, and an overall reaction scheme is herein proposed.

Particular attention has been paid to the role of ammonium nitrate, which forms rapidly at low temperatures and with excess NO₂, determining a lower N₂ selectivity of the deNO_x process. Data are presented which show that the chemistry of the NO/NO₂–NH₃ reacting system can be fully interpreted according to a mechanism which involves: (i) dimerization/disproportion of NO₂ and reaction with NH₃ and water to give ammonium nitrite and ammonium nitrate; (ii) reduction of ammonium nitrate by NO to ammonium nitrite; (iii) decomposition of ammonium nitrite to nitrogen. Such a scheme explains the peculiar deNO_x reactivity at low temperature in the presence of NO₂, the optimal stoichiometry (NO/NO₂ = 1/1), and the observed selectivities to all the major N-containing products (N₂, NH₄NO₃, HNO₃, N₂O). It also provides the basis for the development of a mechanistic kinetic model of the NO/NO₂–NH₃ SCR reacting system.     

Metadynamics as a tool for exploring free energy landscapes of chemical reactions

The metadynamics or hills method is a relatively new molecular dynamics technique aimed to enhance the sampling of separated regions in phase space and map out the underlying free energy landscape as a function of a small number of order parameters or collective variables. The high efficiency allows for the application of metadynamics in combination with first principles dynamics methods, in particular with Car-Parrinello molecular dynamics, to study processes in which changes in the electronic structure play a dominant role, such as chemical reactions. The option to choose several independent collective variables is important to tackle complex and concerted transformations that lack an obvious a priori choice for a single reaction coordinate. In this Account, we discuss the role of metadynamics in the search of transition states, local minima, reaction paths, free energy profiles, and reaction coordinates among a growing list of alternative methods.     

Yeast prion-protein, sup35, fibril formation proceeds by addition and substraction of oligomers

In analogy to human prions, a domain of the translation-termination protein in Saccharomyces cerevisiae, Sup35, can switch its conformation from a soluble functional state, [psi-], to a conformation, [PSI+], that facilitates aggregation and impairs its native function. Overexpression of the molecular chaperone Hsp104 abolishes the [PSI+] phenotype and restores the normal function of Sup35. We have recently shown that Hsp104 interacts preferably with low oligomeric species of a Sup35 derived peptide, Sup35[5-26]; however, due to possible exchange between different oligomeric states, it was not possible to obtain information on the distribution and stability of the oligomeric state. We show here, that low-molecular-weight oligomers (Sup35[5-26])n (n approximately = 4-6) are indeed important for the fibril formation and disassembly process. We find that Hsp104 is able to disaggregate Sup35[5-26] fibrils by substraction of hexameric to decameric Sup35[5-26] oligomers. This disaggregation effect does not require assistance from other chaperones and is independent of ATP at high Hsp104 concentrations. Furthermore, we demonstrate that critical oligomers have a preference for alpha-helical conformations. The conformational reorganization into beta-sheet structures seems to occur only upon incorporation of these oligomers into fibrillar structures. The results are demonstrated by using an equilibrium dialysis experiment that employed different molecular-weight cut-off membranes. A combination of thioflavin-T (ThT) fluorescence and UV measurements allowed the quantification of fibril formation and the amount of peptide diffusing out of the dialysis bag. CD and NMR spectroscopy data were combined to obtain structural information.     

A reconsideration of the relationship between the crystallite size La of carbons determined by X-ray diffraction and Raman spectroscopy

Non-graphitic carbon materials produced by pyrolyzing wood at temperatures from 400 to 2400 °C and various types of commercial carbon fibers were examined by X-ray diffraction and Raman spectroscopy. The specimens cover a wide range of crystallite sizes L_a, in particular also very small sizes below 2 nm. The X-ray data were evaluated using the Scherrer equation and by an advanced approach using full curve fitting. The ratio of the D/G band intensities was determined from the Raman data by different evaluation techniques. A critical assessment of the classical linear relationship between 1/L_a and the D/G ratio shows that the relationship breaks down for crystallite sizes below 2 nm in accordance with recent theoretical predictions. The results are compared with data from the literature, showing that there are additional discrepancies between the data from various carbon types at large L_a due to different methods of data evaluation.     

Morphology and mechanical properties of polypropylene/polyamide 6 nanocomposites prepared by a two‐step melt‐compounding process

Polypropylene/polyamide 6 blends and their nanocomposites with layered silicates or talc were prepared in a melt‐compounding process to explore their mechanical performance. The thermomechanical behavior, crystallization effects, rheology, and morphology of these materials were studied with a wide range of experimental techniques. In all cases, the inorganic filler was enriched in the polyamide phase and resulted in a phase coarsening of the polypropylene/polyamide nanocomposite in comparison with the nonfilled polypropylene/polyamide blend. The mechanical properties of these nanoblends were consequently only slightly better than those of the pure polymers with respect to the modulus, whereas the impact level was below that of the pure polymers, reflecting the heterogeneity of the nanoblend. Polymer‐specific organic modification of the nanoclays did not result in a better phase distribution, which would be required for better overall performance. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 283–291, 2006     

Diacetyldiamidoximeester of pentamidine, a prodrug for treatment of protozoal diseases: synthesis, in vitro and in vivo biotransformation

Pentamidine is an effective antimicrobial agent. To increase its poor oral bioavailability due to the strong basic amidine functionality, the less basic O-acetylamidoxime prodrug, the diacetyldiamidoximeester, was used, which has greatly improved lipophilicity. The objectives of this investigation were the synthesis of all potential metabolites of the double prodrug, the conformational analysis of its structure, and to study the in vitro and in vivo biotransformation by ester cleavage and N-reduction to pentamidine via four intermediate metabolites. The biotransformation of diacetyldiamidoximeester to pentamidine involving the reduction of the amidoxime function and the ester cleavage could be demonstrated. The kinetic parameters were determined. Amidoximes were efficiently metabolized by several enzyme systems located in microsomes and mitochondria of different organs including the final formation of the active metabolite pentamidine. The formation of pentamidine after oral administration of the diacetyldiamidoximeester to rats could be demonstrated as well.     

Optimized synthesis of AMPA receptor antagonist ZK 187638 and neurobehavioral activity in a mouse model of neuronal ceroid lipofuscinosis

Previous structure-activity relationship studies in the search for a potent, noncompetitive alpha-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor antagonist led to 2,3-dimethyl-6-phenyl-12H-[1,3]dioxolo[4,5-h]imidazo[1,2-c][2,3]benzodiazepine (ZK 187638). However, the first synthesis had some drawbacks regarding reagents, processes, and overall yield, which furthermore decreased when the synthesis was scaled up. Therefore, we now report a new synthetic route for this compound which requires fewer steps and is suited for large-scale production. This compound significantly relieved the symptoms of neuromuscular deficit in mnd mice, a model of neuronal ceroid lipofuscinosis with motor neuron dysfunction. After oral administration, the concentrations of the compound in the brain and spinal cord were about threefold higher than those in the plasma. In summary, this novel AMPA antagonist is accessible through an optimized synthetic route, has good neurobehavioral activity, oral bioavailability, and favorable brain penetration. This opens new possibilities for the treatment of devastating neurological diseases that are mediated by the AMPA receptor.     

Long-Term Effects on Food Choice of Land Snail Arianta arbustorum Mediated by Petasin and Furanopetasin, Two Sesquiterpenes from Petasites hybridus

Sesquiterpenes (STs) from the Senecioneae have been found to be potent snail repellents. We investigated the range of activity of the STs petasin, isopetasin, furanopetasin, kablicin, and cacalol, which were isolated from Petasites hybridus, P. kablikianus, and Adenostyles alpina. We found the petasin content of leaves of P. hybridus to lie within the range of deterrence of the isolated compound. Furthermore, leaf extracts containing petasin proved to be deterrent, and leaf discs with low petasin content were preferred over discs with higher petasin content. The cacalol-containing fraction of a leaf extract of A. alpina was not deterrent to the snails. When the snails had experience with the relevant ST one week before a choice test, their sensitivity towards petasine and furanopetasine increased whereas for the other ST it remained at the same level. We speculate that this sensitivity increase could be the result of a rapid long-term associative learning process, but there is also the possibility that these STs are directly interfering with the feeding motor program of the snails, thereby eliciting a direct neurophysiological sensitization reaction which prevents them from further feeding.     

Micropatterned silicone elastomer substrates for high resolution analysis of cellular force patterns

Cellular forces are closely related to many physiological processes, including cell migration, growth, division, and differentiation. Here, we describe newly developed techniques to measure these forces with high spatial resolution. Our approach is based on ultrasoft silicone elastomer films with a regular microstructure molded into the surface. Mechanical forces applied by living cells to such films result in elastomer deformation which can be quantified by video microscopy and digital image processing. From this deformation field forces can be calculated. Here we give detailed accounts of the following issues: (1) the preparation of silicon wafers as molds for the microstructures, (2) the fabrication of microstructured elastomer substrates, (3) the in-depth characterization of the mechanical properties of these elastomers, (4) the image processing algorithms for the extraction of cellular deformation fields, and (5) the generalized first moment tensor as a robust mathematical tool to characterize whole cell activity. We present prototype experiments on living myocytes as well as on cardiac fibroblasts and discuss the characteristics and performance of our force measurement technique.