Search

We used two-photon excitation fluorescence correlation spectroscopy (FCS) and neutron reflectometry to study in situ the effect of salt concentration on the degree of protein binding to polyelectrolyte brushes. The binding of bovine serum albumin (BSA) to poly(acrylic acid) (PAA) brushes was characterized at neutral pH values where both the protein and the brushes carry a negative charge. Spherical PAA brush particles were used in the FCS experiments, whereas a planar PAA brush served as protein substrate in the neutron reflectometry experiments. It has been found that BSA binds strongly to both the spherical and the planar PAA brushes under electrostatic repulsion at low ionic strength. The BSA volume fraction profile, as determined from the neutron reflectivities, indicates a deep penetration of the BSA molecules into the PAA brush. However, the analysis of the FCS data reveals that the protein affinity of the spherical PAA brush particles decreases drastically when increasing the concentration of sodium chloride to a few 100 mM. This observation is in line with the measured neutron reflectivities of the planar PAA brush. The reflectivity curve obtained in the absence of protein is virtually overlapping with that measured when the PAA brush is in contact with a BSA solution but containing 500 mM sodium chloride which suggests protein resistance of the planar PAA brush at this elevated salt concentration. The results of this study provide evidence for a new kind of protein-resistant interfaces. Whereas protein binding to the PAA brush is likely to be dominated by the release of counterions, this driving force vanishes as the ionic strength of the solution is raised and protein molecules are repelled from the interface by steric interactions. In a general view, the "switching" of the protein affinity of a PAA brush by varying the ionic strength of the protein solution over a relatively small range may appear to be useful for biotechnological applications.

The influence of the free surface on the structure of the hexagonal lyotropic mesophase (H1) of the binary surfactant system pentaoxyethylene-n-dodecylether + water may be analyzed using grazing incidence X-ray diffraction. The reflected diffraction patterns consist of hexagonally arranged discrete spots showing that the orientation of lattice planes is parallel to the surface. Experimental evidence shows the coexistence of two lattices with different d-spacings. Diffraction data indicates the existence of a surface layer of hexagonally packed micelles.

The design and structural characterization of modulated heterostructure films based on conjugated polymers by means of x-ray and neutron reflectivity were investigated. Positively charged prepolymer of D-PPV, polyanion SPS and polycation PAH were used as polyelectrolytes to fabricate the self-assembled heterostructures. Results revealed quasi-Bragg reflections and higher harmonics in both neutron and x-ray reflectivity of the self-assembled films, implying that ordered superlattice structures were perpendicular to the plane of the films.

We have investigated the correlation between magnetic and structural properties in exchange coupled polycrystalline CoO/Fe thin films. It has been found that an increase in interface roughness increases the exchange bias field as well as the coercivity. The magnetization reversal mechanism is also influenced by the interfacial morphology. Smooth interfaces are characterized by an asymmetric hysteresis loop, which is associated with domain wall motion for the first magnetization reversal after field cooling and spin rotation in all subsequent reversals. This asymmetry diminishes as the interface roughness increases, i.e., all magnetization reversals are dominated by spin rotation. Moreover, we have observed that the blocking temperature decreases with increasing interface roughness. We also report on a logarithmic time dependence of the magnetization which is different for both branches of the hysteresis loop of smooth CoO/Fe bilayers. [ABSTRACT FROM AUTHOR]

The crystalline structure of two-dimensional aggregates of different carbocyanine molecules was investigated in detail by means of electron diffraction. The aggregates are formed by electrostatic adsorption of anionic dye molecules from an aqueous solution at a positively charged ammonium lipid monolayer. From the diffraction pattern we deduce a herringbonelike arrangement of the dye molecules as described by a centered, rectangular unit cell containing two identical molecules. The aggregate structures of the systems investigated here differ only by the mutual orientation of the molecules and the axis ratios of the unit cell. For further optical studies we determine the angle α between the main axes of the differently oriented molecules with very high precision. We calculate electron diffraction intensities of several structures which are simulated on the computer and compare them to experimental data. We observe that the dye aggregates are aligned to one of the crystal axes of the lipid lattice. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Operando neutron reflectometry measurements were carried out to study the insertion of lithium into amorphous silicon film electrodes during cyclic voltammetry (CV) experiments at a scan rate of 0.01 mV s−1. The experiments allow mapping of regions where significant amounts of Li are incorporated/released from the electrode and correlation of the results to modifications of characteristic peaks in the CV curve. High volume changes up to 390% accompanied by corresponding modifications of the neutron scattering length density (which is a measure of the average Li fraction present in the electrode) are observed during electrochemical cycling for potentials below 0.3 V (lithiation) and above 0.2 V (delithiation), leading to a hysteretic behaviour. This is attributed to result from mechanical stress as suggested in the literature. Formation and modification of a surface layer associated with the solid electrolyte interphase (SEI) were observed during cycling. Within the first lithiation cycle the SEI grows to 120 Å for potentials below 0.5 V. Afterwards a reversible and stable modification of the SEI between 70 Å (delithiated state) and 120 Å (lithiated state) takes place. [ABSTRACT FROM AUTHOR]