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Biomedical subjects

Christof Wöll

Publications and source records attributed to Christof Wöll.

18 recordsLinked to original sources
PubMed2006

Organic molecular beam deposition of oligophenyls on Au111: A study by X-ray absorption spectroscopy.

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy has been applied to reveal the molecular arrangement of ultrathin oligophenyl films [p-quaterphenyl (4P) and p-hexaphenyl (6P)] on Au(111). In the half-monolayer films the molecules lie flat on the surface but still have a considerable inter-ring twist of 30 degrees -40 degrees , similar to the gas-phase conformation. In the saturated monolayer film the second half of the molecules is side-tilted by an angle of less than 66 degrees with respect to the surface. This arrangement is already similar to that in bulk net planes of thicker films parallel to the surface, that is, the 4P(211) and 6P(21-3) planes, respectively.

Benzene Derivatives↗
PubMed2006

Competition as a design concept: polymorphism in self-assembled monolayers of biphenyl-based thiols.

Self-assembled monolayers (SAMs) of two omega-(4'-methylbiphenyl-4-yl)alkanethiols (CH(3)(C(6)H(4))(2)(CH(2))(n)SH, BPn, n = 4, 6) on Au(111) substrates, prepared from solution at room temperature and subsequently annealed at temperatures up to 493 K under a nitrogen atmosphere, were studied using scanning tunneling microscopy (STM), high-resolution X-ray photoelectron spectroscopy (HRXPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In striking contrast to BPn SAMs with n = odd, for which only one phase is observed, the even-numbered BPn SAMs exhibit polymorphism. Irreversible phase transitions occur which involve three phases differing substantially in density and stability. Upon annealing, BP4 and BP6 transform into a beta-phase, which is characterized by an exceptionally high structural quality with virtually defect-free domains exceeding 500 nm in diameter. Exchange experiments, monitored by contact angle measurement, reveal that the beta-phase exhibits a dramatically improved stability. The fundamental differences in the phase behavior of even- and odd-numbered BPn SAMs are discussed in terms of two design strategies based on cooperative and competitive effects.

Journal Article↗
PubMed2006

Asymmetry induction by cooperative intermolecular hydrogen bonds in surface-anchored layers of achiral molecules.

The mesoscale induction of two-dimensional supramolecular chirality (formation of 2D organic domains with a single handedness) was achieved by self-assembly of 1,2,4-benzenetricarboxylic (trimellitic) acid on a Cu(100) surface at elevated temperatures. The combination of spectroscopic [X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS)], real-space-probe [scanning tunneling microscopy (STM)], and computational [density functional theory (DFT)] methods allows a comprehensive characterization of the obtained organic adlayers, where details of molecular adsorption geometry, intermolecular coupling, and surface chemical bonding are elucidated. The trimellitic acid species, comprising three functional carboxylic groups, form distinct stable mirror-symmetric hydrogen-bonded domains. The chiral ordering is associated with conformational restriction in the domains: molecules anchor to the substrate with an ortho carboxylate group, providing two para carboxylic acid moieties for collective lateral interweaving through H bonding, which induces a specific tilt of the molecular plane. The ease of molecular symmetry switching in domain formation makes homochiral-signature propagation solely limited by the terrace width. The molecular layer modifies the morphology of the underlying copper substrate and induces mum-sized strictly homochiral terraces.

Journal Article↗
PubMed2006

A comprehensive study of self-assembled monolayers of anthracenethiol on gold: solvent effects, structure, and stability.

The formation and molecular structure of self-assembled monolayers (SAMs) of anthracene-2-thiol (AnT) on Au(111) have been characterized by reflection adsorption infrared spectroscopy, thermal desorption spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption spectroscopy, scanning tunneling microscopy, and low energy electron diffraction. It is demonstrated that highly ordered monolayer films are formed upon immersion, but their quality depends critically on the choice of solvents and rinsing conditions. The saturated monolayer is characterized by a closed packed arrangement of upright standing molecules forming a (2 x 4)rect unit cell. At about 450 K a partial desorption takes place and the remaining molecules form a dilute (4 x 2)-phase with an almost planar adsorption geometry, while further heating above 520 K causes a thermally induced fragmentation. According to their different densities both phases reveal very diverse chemical reactivities. Whereas the saturated monolayer is stable and inert under ambient conditions, the dilute phase does not warrant any protection of the sulfur headgroups which oxidize rapidly in air.

Journal Article↗
PubMed2006

Adsorption of atomic hydrogen on ZnO(1010): STM study.

The adsorption of atomic hydrogen on a single crystal ZnO(1010) surface has been studied by scanning tunneling microscopy (STM) under ultrahigh vacuum conditions at room temperature and at elevated temperatures. High resolution STM images indicate that a well-ordered (1x1) H adlayer is formed on the ZnO(1010) surface. The STM data strongly indicate that the hydrogen adsorbs on top of the oxygen atoms forming hydroxyl species. Scanning tunneling spectroscopy (STS) studies reveal a H atom induced metallization at room temperature. In contrast to the clean surface for the hydrogen-covered surface distinct defects structures consisting of missing O and Zn atoms could be identified.

Journal Article↗
PubMed2005

The interaction of C6H6 and C6H12 with noble metal surfaces: electronic level alignment and the origin of the interface dipole.

The electronic interaction of two molecules, the aromatic benzene (C6H6) and the saturated hydrocarbon cyclohexane (C6H12) with a Cu(111) surface, have been determined using precise, ab initio electronic structure calculations. For the interaction of these adsorbates with the substrate, we present a detailed analysis and decomposition of various individual chemical mechanisms that contribute. A novel aspect of this analysis is the use of charge-density difference contour plots to graphically display the chemistry. A wave-function-based approach was used in order to avoid problems when the presently most commonly employed approach, density-functional theory, is applied to weakly chemisorbed molecules, where the interaction is dominated by van der Waals forces. The present information are not only relevant with regard to understanding the chemistry going on when molecules are adsorbed on a Cu surface but also have important consequences with regard to charge injection in molecular electronic devices, e.g., organic field-effect transistors and organic light-emitting diodes.

Journal Article↗
PubMed2005

Selective nucleation and growth of metal-organic open framework thin films on patterned COOH/CF3-terminated self-assembled monolayers on Au(111).

Metal-Organic open Framework coordination polymers (MOFs) are a fascinating class of materials. We demonstrate the selective growth of patterned thin films of MOF-5 and the subsequent loading with a palladium organic compound, which acts as a precursor for the deposition of palladium nanoclusters inside the cavities of the MOF-5 material.

Gold↗
PubMed2005

In deuterated water the unspecific adsorption of proteins is significantly slowed down: results of an SPR study using model organic surfaces.

The control of unspecific adsorption of proteins to natural and technical surfaces plays an important role in biology and also for many applications. Organic model surfaces, e.g., self-assembled monolayers, are often used to identify fundamental surface and/or protein properties that rule protein adsorption. Some techniques involved in biointerface research require the use of heavy water, e.g. neutron scattering techniques. Also in NMR studies D(2)O is the solvent of choice when focusing on biomolecular and hydration dynamics. So far several studies have been concerned with the characterization of the unspecific adsorption of proteins from normal water buffers. In the present work, we report a comparison of the unspecific protein adsorption from normal and heavy water buffers. So far it has been assumed that the surface kinetic of the unspecific adsorption is unaffected by the substitution of water by D(2)O. However, for the four proteins investigated here, this assumption does not hold. The ratio k(H)/k(D) of the adsorption rate constants of the different buffer conditions describe the strength of the isotope effect. We have measured ratios between 1.0 and 2.6, indicating that the adsorption kinetics are strongly affected by a H(2)O-D(2)O substitution.

Adsorption↗
PubMed2005

Stress in self-assembled monolayers: omega-biphenyl alkane thiols on Au(111).

Self-assembled monolayers of omega-(4'-methylbiphenyl-4-yl) alkane thiols CH3(C6H4)2(CH2)(n)SH (BPn, n = 2, 3, and 5) on Au(111) substrates, prepared at room and elevated temperatures, were studied using scanning tunneling microscopy. In contrast to the biphenyl thiol analogues with n = 0 or 1, ordered domains of large size are formed which exhibit small, periodic height variations on a length scale of several nanometers. These are attributed to solitons (or domain walls), resulting from structural mismatch between the molecular adlayer and the gold substrate. The implications of these results for the design of aromatic thiols to cope with stress and yield low-defect density self-assembled monolayers are discussed.

Journal Article↗
PubMed2002

Exchangelike effects for closed-shell adsorbates: interface dipole and work function.

Based on detailed theoretical analyses, we present, for the first time, direct evidence that the significant interface dipole commonly observed for atoms and molecules physisorbed on metal surfaces originates from exchangelike effects. In the case of Xe, previously proposed contributions from chemical interactions do not play a significant role.

Journal Article↗
PubMedDate not supplied

Light-driven water splitting for (bio-)hydrogen production: photosystem 2 as the central part of a bioelectrochemical device.

To establish a semiartificial device for (bio-)hydrogen production utilizing photosynthetic water oxidation, we report on the immobilization of a Photosystem 2 on electrode surfaces. For this purpose, an isolated Photosystem 2 with a genetically introduced His tag from the cyanobacterium Thermosynechococcus elongatus was attached onto gold electrodes modified with thiolates bearing terminal Ni(II)-nitrilotriacetic acid groups. Surface enhanced infrared absorption spectroscopy showed the binding kinetics of Photosystem 2, whereas surface plasmon resonance measurements allowed the amount of protein adsorbed to be quantified. On the basis of these data, the surface coverage was calculated to be 0.29 pmol protein cm(-2), which is in agreement with the formation of a monomolecular film on the electrode surface. Upon illumination, the generation of a photocurrent was observed with current densities of up to 14 microA cm(-2) . This photocurrent is clearly dependent on light quality showing an action spectrum similar to an isolated Photosystem 2. The achieved current densities are equivalent to the highest reported oxygen evolution activities in solution under comparable conditions.

Electrochemistry↗