Enantiospecific adsorption of alanine on the chiral Cu{531} surface

We have studied enantiospecific differences in the adsorption of (S)- and (R)-alanine on Cu{531}R using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. At saturation coverage, alanine adsorbs as alaninate forming a p(1 4) superstructure. LEED shows a significantly higher degree of long-range order for the S than for the R enantiomer. Also carbon K-edge NEXAFS spectra show differences between (S)- and (R)-alanine in the variations of the ð resonance when the linear polarization vector is rotated within the surface plane. This indicates differences in the local adsorption geometries of the molecules, most likely caused by the interaction between the methyl group and the metal surface and/or intermolecular hydrogen bonds. Comparison with model calculations and additional information from LEED and photoelectron spectroscopy suggest that both enantiomers of alaninate adsorb in two different orientations associated with triangular adsorption sites on {110} and {311} microfacets of the Cu{531} surface. The experimental data are ambiguous as to the exact difference between the local geometries of the two enantiomers. In one of two models that fit the data equally well, significantly more (R)-alaninate molecules are adsorbed on {110} sites than on {311} sites whereas for (S)-alaninate the numbers are equal. The enantiospecific differences found in these experiments are much more pronounced than those reported from other ultrahigh vacuum techniques applied to similar systems.

Water adsorption on O-covered Ru{0001}: coverage-dependent change from dissociation to molecular adsorption

When water is coadsorbed with oxygen at coverages above 0.25ML an intact water species is observed in high resolution X-ray photoelectron spectroscopy up to 220 K, which is significantly more stable than intact water on the clean surface. The presence of this species causes a shift in the O 1s binding energy of the pre-adsorbed oxygen, which indicates the formation of hydrogen bonds between the two adsorbates. Low coverages of oxygen induce partial dissociation and recombinative desorption in the same temperature range, which illustrates that desorption temperatures alone cannot be used to determine whether water is molecularly intact or not.

Effect of oxygen adsorption on the chiral pt{531} surface

The adsorption of oxygen on the chiral Pt{531} surface was studied by high-resolution X-ray photoelectron spectroscopy (HRXPS) and low energy electron diffraction (LEED). After the surface is annealed in oxygen (3 x 10(-7) mbar), three O 1s peaks are observed in XPS. One peak, at 529.5 eV, is assigned to chemisorbed oxygen; it disappears after annealing in vacuo to temperatures above 900 K. The other two peaks at 530.8 and 532.3 eV are stable up to at least 1250 K. They are associated with oxide clusters on the surface. These clusters readily react with coadsorbed carbon monoxide at temperatures between 315 and 620 K.

The surface geometry of carbonmonoxide and hydrogen co-adsorbed on Ni 111

A combination of photoelectron spectroscopy, temperature programmed desorption and low energy electron diffraction structure determinations have been applied to study the p(2 x 2) structures of pure hydrogen and co-adsorbed hydrogen and CO on Ni {111}. In agreement with earlier work atomic hydrogen is found to adsorb on fcc and hcp sites in the pure layer with H-Ni bond lengths of 1.74Angstrom. The substrate interlayer distances, d(12) = 2.05Angstrom and d(23) = 2.06Angstrom, are expanded with respect to clean Ni {111} with buckling of 0.04Angstrom in the first layer. In the co-adsorbed phase Co occupies hcp sites and only the hydrogen atoms on fcc sites remain on the surface. d(12) is even further expanded to 2.08Angstrom with buckling in the first and second layer of 0.06 and 0.02Angstrom, respectively. The C-O, C-Ni, and H-Ni bond lengths are within the range of values also found for the pure adsorbates.

Experimental evidence for a partially dissociated water bilayer on Ru{0001}

Core-level photoelectron spectra, in excellent agreement with ab initio calculations, confirm that the stable wetting layer of water on Ru{0001} contains O-H and H2O in roughly 3:5 proportion, for OHx coverages between 0.25 and 0.7 ML, and T<170 K. Proton disorder explains why the wetting structure looks to low energy electron diffraction (LEED) to be an ordered p(root3xroot3)R30degrees adlayer, even though approximate to3/8 of its molecules are dissociated. Complete dissociation to atomic oxygen starts near 190 K. Low photon flux in the synchrotron experiments ensured that the diagnosis of the nature of the wetting structure quantified by LEED is free of beam-induced damage.

Low-temperature partial dissociation of water on Cu(110)

The low-temperature reactivity of water (D2O) adsorbed on clean and oxygen pre-covered Cu(1 1 0) was studied using high resolution X-ray photoelectron spectroscopy (HRXPS) and low energy electron diffraction (LEED). On the clean surface partial dissociation to hydroxyl was observed already at 95 K. Upon annealing to 220 K hydrogen bonded water-hydroxyl chains are formed. Upon further annealing water desorbs leaving behind a layer of hydroxyl, most of which desorbs recombinatively eventually. With pre-adsorbed oxygen water reacts to hydroxyl lifting the added-row reconstruction even below 225 K. Upon annealing this adsorbate layer passes through essentially the same stages as without pre-adsorbed oxygen.

The adsorption geometry and chemical state of lysine on Cu{110}

Chemisorbed layers of lysine adsorbed on Cu{110} have been studied using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. XPS indicates that the majority (70%) of the molecules in the saturated layer at room temperature (coverage 0.27 ML) are in their zwitterionic state with no preferential molecular orientation. After annealing to 420 K a less densely packed layer is formed (0.14 ML), which shows a strong angular dependence in the characteristic π-resonance of oxygen K edge NEXAFS and no indication of zwitterions in XPS. These experimental results are best compatible with molecules bound to the substrate through the oxygen atoms of the (deprotonated) carboxylate group and the two amino groups involving Cu atoms in three different close packed rows. This μ4 bonding arrangement with an additional bond through the !-amino group is different from geometries previously suggested for lysine on Cu{110}.

A kinetics and mechanistic study of the atmospherically relevant reaction between molecular chlorine and dimethyl sulfide (DMS)

A gas-phase kinetics study of the atmospherically important reaction between Cl2 and dimethyl sulfide (DMS)Cl2 + CH3SCH3 → productshas been made using a flow-tube interfaced to a photoelectron spectrometer. The rate constant for this reaction has been measured at 1.6 and 3.0 torr at T = (294 ± 2) K as (3.4 ± 0.7) × 10−14 cm3 molecule−1 s−1. Reaction (1) has been found to proceed via an intermediate, (CH3)2SCl2, to give CH3SCH2Cl and HCl as the products. The mechanism of this reaction and the structure of the intermediate were investigated using electronic structure calculations. A comparison of the mechanisms of the reactions between Cl atoms and DMS, and Cl2 and DMS has been made and the relevance of the results to atmospheric chemistry is discussed.

A study of the atmospherically relevant reaction between molecular chlorine and dimethylsulfide (DMS): establishing the reaction intermediate and measurement of absolute photoionization cross-sections

In a study using UV photoelectron spectroscopy (PES) of the atmospherically relevant reaction CH3SCH3 + Cl2 → CH3SCH2Cl + HCl bands associated with a reaction intermediate have been observed. These have been assigned to ionization of the covalently bound molecule (CH3)2SCl2 on the basis of the intensity of the observed bands as a function of reaction time, molecular orbital calculations of vertical ionization energies and evidence from infrared spectroscopy. A method has also been developed, with the flow-tube/PE spectrometer combination used, to measure photoionization cross-sections of the reagents and products at the photon energy utilized and this has allowed the photoionization cross-section of the intermediate to be estimated. This work augments an earlier study in which the rate constant of the reaction between CH3SCH3 (DMS) and Cl2 has been measured at room temperature.

Influence of fermentation conditions on the surface properties and adhesion of Lactobacillus rhamnosus GG

Background: The surface properties of probiotic bacteria influence to a large extent their interactions within the gut ecosystem. There is limited amount of information on the effect of the production process on the surface properties of probiotic lactobacilli in relation to the mechanisms of their adhesion to the gastrointestinal mucosa. The aim of this work was to investigate the effect of the fermentation pH and temperature on the surface properties and adhesion ability to Caco-2 cells of the probiotic strain Lactobacillus rhamnosus GG. Results: The cells were grown at pH 5, 5.5, 6 (temperature 37 °C) and at pH 6.5 (temperature 25 °C, 30 °C and 37 °C), and their surfaces analysed by X-ray photoelectron spectrometry (XPS), Fourier transform infrared spectroscopy (FT-IR) and gel-based proteomics. The results indicated that for all the fermentation conditions, with the exception of pH 5, a higher nitrogen to carbon ratio and a lower phosphate content was observed at the surface of the bacteria, which resulted in a lower surface hydrophobicity and reduced adhesion levels to Caco-2 cells as compared to the control fermentation (pH 6.5, 37 oC). A number of adhesive proteins, which have been suggested in previous published works to take part in the adhesion of bacteria to the human gastrointestinal tract, were identified by proteomic analysis, with no significant differences between samples however. Conclusions: The temperature and the pH of the fermentation influenced the surface composition, hydrophobicity and the levels of adhesion of L. rhamnosus GG to Caco-2 cells. It was deduced from the data that a protein rich surface reduced the adhesion ability of the cells.

Preventing carbon contamination of optical devices for X-rays: the effect of oxygen on photon-induced dissociation of CO on platinum

Platinum is one of the most common coatings used to optimize mirror reflectivity in soft X-ray beamlines. Normal operation results in optics contamination by carbon-based molecules present in the residual vacuum of the beamlines. The reflectivity reduction induced by a carbon layer at the mirror surface is a major problem in synchrotron radiation sources. A time-dependent photoelectron spectroscopy study of the chemical reactions which take place at the Pt(111) surface under operating conditions is presented. It is shown that the carbon contamination layer growth can be stopped and reversed by low partial pressures of oxygen for optics operated in intense photon beams at liquidnitrogen temperature. For mirrors operated at room temperature the carbon contamination observed for equivalent partial pressures of CO is reduced and the effects of oxygen are observed on a long time scale.

Chromium nanostructures formed by dewetting of heteroepitaxial films on W(100)

In this paper, we report the surprising formation of square-based facetted islands with linear dimension of the order of 500 nm upon dewetting of a Cr multilayer onW(100).We show that these square islands are composed of inclined facets surrounding a depressed center such that the facet slopes inward with the outer edges of the islands thicker than the centers. The islands’ shapes do not represent traditional equilibrium crystal shapes as expected for a Wulf construction. In situ UV and x-ray photoelectron emission microscopy allied to spatially resolved spectroscopy throws considerable light on the nature of the dewetting and shows that the metal surface between the islands remains covered by a thin pseudomorphic wetting layer of ∼1 ML. Low-energy electron diffraction and scanning tunneling and atomic force microscopies allow quantification of facet slopes, and we identify a predominance of tilted Cr(100) facets ±5◦ off of the substrate normal bound by (210) planes at ∼26◦. The epitaxial Cr islands adopt the bulk Cr lattice constant but are tilted with respect to the surface normal.We suggest that the Cr crystallite tilting creates a vicinal-like interface structure that determines the island morphology

Guided growth of neurons and glia using microfabricated patterns of parylene-C on a SiO2 background

This paper describes a simple technique for the patterning of glia and neurons. The integration of neuronal patterning to Multi-Electrode Arrays (MEAs), planar patch clamp and silicon based ‘lab on a chip’ technologies necessitates the development of a microfabrication-compatible method, which will be reliable and easy to implement. In this study a highly consistent, straightforward and cost effective cell patterning scheme has been developed. It is based on two common ingredients: the polymer parylene-C and horse serum. Parylene-C is deposited and photo-lithographically patterned on silicon oxide (SiO2) surfaces. Subsequently, the patterns are activated via immersion in horse serum. Compared to non-activated controls, cells on the treated samples exhibited a significantly higher conformity to underlying parylene stripes. The immersion time of the patterns was reduced from 24 to 3 h without compromising the technique. X-ray photoelectron spectroscopy (XPS) analysis of parylene and SiO2 surfaces before and after immersion in horse serum and gel based eluant analysis suggests that the quantity and conformation of proteins on the parylene and SiO2 substrates might be responsible for inducing glial and neuronal patterning.

Redox behavior of the model catalyst Pd/CeO2-x/Pt(111)

Photoelectron spectroscopy and scanning tunneling microscopy have been used to investigate how the oxidation state of Ce in CeO2-x(111) ultrathin films is influenced by the presence of Pd nanoparticles. Pd induces an increase in the concentration of Ce3+ cations, which is interpreted as charge transfer from Pd to CeO2-x(111) on the basis of DFT+U calculations. Charge transfer from Pd to Ce4+ is found to be energetically favorable even for individual Pd adatoms. These results have implications for our understanding of the redox behavior of ceria-based model catalyst systems.

Surface chemistry of alanine on Cu{111}: Adsorption geometry and temperature dependence

Adsorption of l-alanine on the Cu{111} single crystal surface was investigated as a model system for interactions between small chiral modifier molecules and close-packed metal surfaces. Synchrotron-based X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy are used to determine the chemical state, bond coordination and out-of-plane orientation of the molecule on the surface. Alanine adsorbs in its anionic form at room temperature, whilst at low temperature the overlayer consists of anionic and zwitterionic molecules. NEXAFS spectra exhibit a strong angular dependence of the π ⁎ resonance associated with the carboxylate group, which allows determining the tilt angle of this group with respect to the surface plane (48° ± 2°) at room temperature. Low-energy electron diffraction (LEED) shows a p(2√13x2√13)R13° superstructure with only one domain, which breaks the mirror symmetry of the substrate and, thus, induces global chirality to the surface. Temperature-programmed XPS (TP-XPS) and temperature-programmed desorption (TPD) experiments indicate that the zwitterionic form converts into the anionic species (alaninate) at 293 K. The latter desorbs/decomposes between 435 K and 445 K.