In this report we describe the fabrication and characterization of a phospholipid/alkanethiol hybrid bilayer membrane in air. The bilayer is formed by the interaction of phospholipid with the hydrophobic surface of a self-assembled alkanethiol monolayer on gold. We have characterized the resulting hybrid bilayer membrane in air using atomic force microscopy, spectroscopic ellipsometry, and reflection-absorption infrared spectroscopy. These analyses indicate that the phospholipid added is one monolayer thick, is continuous, and exhibits molecular order which is similar to that observed for phospholipid/phospholipid model membranes. The hybrid bilayer prepared in air has also been re-introduced to water and characterized using neutron reflectivity and impedance spectroscopy. Impedance data indicate that when moved from air to water, hybrid bilayers exhibit a dielectric constant and thickness that is essentially equivalent to hybrid bilayers prepared in situ by adding phospholipid vesicles to alkanethiol monolayers in water. Neutron scattering from these samples was collected out to a wave vector transfer of 0.25 A(-1), and provided a sensitivity to changes in total layer thickness on the order of 1-2 A. The data confirm that the acyl chain region of the phospholipid layer is consistent with that observed for phospholipid-phospholipid bilayers, but suggest greater hydration of the phospholipid headgroups of HBMs than has been reported in studies of lipid multilayers.

Self-assembled monolayers (SAMs) of methyl 1-thiahexa(ethylene oxide), HS(CH2CH2O)6CH3, on gold with different molecular conformations were evaluated for their ability to inhibit protein adsorption. Assembled from 95% ethanol, reflection-absorption infrared spectroscopy (RAIRS) data show that the HS(CH2CH2O)6CH3 SAMs adopt the well-ordered 7/2 helical conformation of the folded-chain crystal polymorph of poly(ethylene oxide). Electrochemical impedance spectroscopy (EIS) data indicate that these ordered helical SAMs behave as near ideal capacitors in an electrochemical environment. Assembled from other solvents, the SAMs of HS(CH2CH2O)6CH3 were more variable and less ordered. For samples assembled from 100% ethanol, the RAIRS data were virtually identical to those obtained from 95% ethanol indicating the predominant 7/2 helical structure but the EIS spectra show consistently higher specific capacitance values indicating a slight increase in disorder or number of defect sites. In contrast to the ethanolic solvents, assembled from tetrahydrofuran, the HS(CH2CH2O)6CH3 SAMs are significantly less ordered with RAIRS data clearly showing mixtures of the 7/2 helical and nonhelical conformations and EIS data showing much higher specific capacitance values. Exposure of these ordered and disordered HS(CH2-CH2O)6CH3 SAMs to bovine serum albumin and lysozyme solutions showed that the less ordered SAMs exhibit better inhibition of protein adsorption.

Cells receive signals from the extracellular matrix through receptor-dependent interactions, but they are also influenced by the mechanical properties of the matrix. Although bulk properties of substrates have been shown to affect cell behavior, we show here that nanoscale properties of collagen fibrils also play a significant role in determining cell phenotype. Type I collagen fibrils assembled into thin films provide excellent viewing of cells interacting with individual fibrils. Cells can be observed to extensively manipulate the fibrils, and this behavior seems to result in an incompletely spread stellate morphology and a nonproliferative phenotype that is typical of these cells in collagen gels. We show here that thin films of collagen fibrils can be dehydrated, and when seeded on these dehydrated fibrils, smooth muscle cells spread and proliferate extensively. The dehydrated collagen fibrils appear to be similar to the fully hydrated collagen fibrils in topology and in presentation of beta(1) integrin ligation sites, but they are mechanically stiffer. This decrease in compliance of dehydrated fibrils is seen by a failure of cell movement of dehydrated fibrils compared to their ability to rearrange fully hydrated fibrils and from direct measurements by nanoindentation and quantitative atomic force measurements. We suggest that increase in the nanoscale rigidity of collagen fibrils can cause these cells to assume a proliferative phenotype.

Hybrid bilayer membranes (HBMs), consisting of a lipid monolayer covering a self-assembled alkanethiol monolayer on a metal surface, are useful models for studying the structure and function of cell membranes. Surface-enhanced Raman spectroscopy (SERS) and reflection absorption infrared spectroscopy (RAIRS) are used to study HBMs with various alkanethiol and lipid components. Together, these two techniques clearly indicate that the lipid forms a well-ordered, non-interdigitated layer on the alkanethiol with the head groups of the lipids oriented away from the metal surface. Both techniques reveal that the formation of an HBM produces small changes in the alkanethiols, which are similar to those caused by a reduction in temperature, indicative of an increase in order. The small magnitude of the perturbations in the alkanethiol monolayer upon addition of the lipid layer will simplify the further study of HBMs.

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Self-assembled monolayers (SAMs) of a series of novel linear thiols containing a 1-thiahexa(ethylene oxide) (THEO) moiety, i.e., HS(CH2CH2O)6R, where R = C10H21, C10D21, C18H37, and C18D37, were prepared on polycrystalline gold (Au) and characterized by reflection absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry, surface plasmon resonance (SPR), and contact angle measurements. The SAMs are hydrophobic with contact angles, θ a (H2O), of 108°−109° for all surfaces. For all compounds, our data provide evidence for well-ordered monolayers in which the THEO moiety is arranged in a 7/2 helix oriented normal to the substrate. The alkyl group conformation and tilt are similar to the corresponding n-alkanethiol SAMs.

The structure and ordering conditions of the self-assembled monolayers (SAMs) of HS(EO)xR, where R ) CH3, EO ) CH2CH2O, and x ) 3-6, on polycrystalline gold (Au) were determined by reflectionabsorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and electrochemical impedance spectroscopy (EIS). For x ) 5 and 6, RAIRS and SE data show that the 1-thiaoligo(ethylene oxide) [TOEO] segments adopt the highly ordered helical structure of the folded-chain crystal polymorph of crystalline poly(ethylene oxide) and are oriented normal to the substrate. RAIRS and SE data obtained as a function of immersion time show that x ) 6 attains the highly ordered, all helical conformation faster than x ) 5. For x ) 3 and 4, RAIRS and SE data indicate largely disordered structures, with some all-trans conformation but little evidence of the helical conformation. EIS measurements, in an aqueous environment, indicate a significant presence of water in all SAMs with x < 6 while x ) 6 remains essentially free of water and/or ions for extensive periods (2 h). These data suggest that a minimum of five ethylene oxide units are necessary for TOEO SAMs to adopt the helical conformation and that SAMs with x < 6 are penetrated by water or ionic species in an aqueous environment, regardless of the order of the TOEO segment in air.

Recent improvements in neutron reflectometry methodology have afforded enhanced sensitivity for the study of biomimetic membranes. The technique has been used to probe the interactions of the peptide toxin, melittin, with supported bilayers of phospholipid and octadecanethiol or thiahexa(ethylene oxide) alkane on gold. Improvements in instrumentation and experimental design permit neutron reflectivity measurements out to a wavevector transfer of 0.7 Å -1 and down to reflectivities approaching 10 -8 , allowing unprecedented resolution of structural details in the bilayer. The data indicate that melittin strongly perturbs the phospholipid headgroup region and also affects the alkane chain region of the bilayer. There is no evidence for hydration of the ethylene oxide spacer region between the gold and alkane regions of the thiahexa(ethylene oxide) alkane/phospholipid bilayer, but a distinct shift of up to 3 Å in the apparent location of the interface between the alkane and phospholipid regions is observed. This work shows that the neutron reflectometry technique is now sensitive to small changes in the reflected intensities, and these small changes can result in significant contributions to the resultant scattering length density profiles.