No abstract

This paper describes a study of the electrical properties of supported lipid bilayer membranes on semiconductor and gold surfaces. The study is aimed to foster the understanding of supported membrane systems and to allow the rational design of biosensor assays for ion channel analysis. Impedance spectroscopy was applied for the electrical characterization of the supported membrane systems. A novel equivalent circuit model is introduced for the data evaluation, which accounts for the deviation of the impedance response of supported membranes from that of an ideal RC element. As a result of the improved accordance of model and data, the resistance and the capacity of supported membranes can be determined more accurately and independently from each other. Experimental results describe the phenomenology of the electrical properties of supported bilayers regarding variations in preparation, composition, and environmental conditions. We discuss the findings in terms of membrane−substrate interactions and models of membrane permeability. The important role of the electrostatics between the lipid bilayer and the solid substrate for the formation of an electrically dense supported membrane is identified. Bilayer permeability models explain the correlation between the structure of the lipid bilayer and its insulating properties. These models are also in accordance with the observed dependence of the electrical resistance of the lipid bilayer on the temperature and the ion concentration of the electrolyte.

We report the design of polymer/lipid composite films on polarizable indium−tin−oxide (ITO) electrodes and the characterization of their electrical properties by impedance spectroscopy. The polymer films separating the solid surfaces and the lipid membranes were composed of “hairy-rod” macromolecules and provided a fluid and homogeneous ultrathin “cushion” for the deposition of lipid membranes. Two types of polymer cushions were designed:  First, electrically insulating multilayers of cellulose derivatives with substituted alkyl chains forming a fluid hydrophobic “brush” at the surface. Second, conducting cellulose multilayers exposing hydrophilic surfaces for the deposition of self-healing lipid bilayers. The electrical properties, the defect densities, and the homogeneities of the stratified films were studied by impedance spectroscopy in the frequency range between 90 mHz and 50 kHz. Impedance spectra were analyzed in terms of equivalent circuits with resistances, capacitances, and so-called “constant phase elements”. Resistances of up to 440 kΩ cm2 were achieved for the polymer-supported lipid bilayers, which corresponds to about 10% of the value for black lipid membranes containing hexane. The gramicidin channels reconstituted into the supported lipid bilayers exhibited the expected cation selectivity.

We present an improved theory of image formation by reflection interference contrast microscopy (RICM) for structural studies of stratified films on planar substrates and propose a new theoretical approach to analyzing the surface profile of nonplanar films. We demonstrate the validity of the new approach by analyzing the fringe patterns of RICM images from wedge-shaped liquid films and spherical probes. By simulation of various scenarios, we study the effect of finite-aperture illumination and the shape of the nonplanar interface on the interference fringe pattern of RICM images. We show how the reconstruction of the microscopic topography of the sample from the fringe spacing is corrected by angular and curvature correction terms. We discuss the variation of the mean intensity of the fringe patterns and the decay in the fringe amplitude with increasing fringe order that is caused by nonplanar interfaces of different slope.

Self-assembled monolayers of octadecylthiol (ODT) were reconstituted on freshly etched gallium arsenide (n-GaAs) for the electrochemical stabilization against decomposition of the surfaces (passivation) in aqueous buffers. The surface composition was evaluated by x-ray photoelectron spectroscopy to optimize the surface treatment before ODT deposition. Electrochemical properties of the monolayers were monitored by cyclic voltammetry and impedance spectroscopy. The impedance spectrum of the photoetched n-GaAs after the deposition of the ODT monolayer was stable in an aqueous electrolyte at pH=7.5 for more than 24 h within the sensitivity of our experimental technique. The effective passivation of GaAs surfaces is an essential step towards biosensor applications.