Surface-tethered biomimetic bilayer membranes (tethered bilayer lipid membranes (tBLMs)) were formed on gold surfaces from phospholipids and a synthetic 1-thiahexa(ethylene oxide) lipid, WC14. They were characterized using electrochemical impedance spectroscopy, neutron reflection (NR), and Fourier-transform infrared reflection-absorption spectroscopy (FT-IRRAS) to obtain functional and structural information. The authors found that electrically insulating membranes (conductance and capacitance as low as 1 microS cm(-2) and 0.6 microF cm(-2), respectively) with high surface coverage (>95% completion of the outer leaflet) can be formed from a range of lipids in a simple two-step process that consists of the formation of a self-assembled monolayer (SAM) and bilayer completion by "rapid solvent exchange." NR provided a molecularly resolved characterization of the interface architecture and, in particular, the constitution of the space between the tBLM and the solid support. In tBLMs based on SAMs of pure WC14, the hexa(ethylene oxide) tether region had low hydration even though FT-IRRAS showed that this region is structurally disordered. However, on mixed SAMs made from the coadsorption of WC14 with a short-chain "backfiller," beta-mercaptoethanol, the submembrane spaces between the tBLM and the substrates contained up to 60% exchangeable solvent by volume, as judged from NR and contrast variation of the solvent. Complete and stable "sparsely tethered" BLMs (stBLMs) can be readily prepared from SAMs chemisorbed from solutions with low WC14 proportions. Phospholipids with unsaturated or saturated, straight or branched chains all formed qualitatively similar stBLMs.

Self-assembled monolayers are formed by exposing freshly cleaved mica to a solution of octadecylphosphonic acid in tetrahydrofuran. Atomic force microscope images of samples immersed in solution for varying exposure times show that prior to forming a complete monolayer the molecules aggregate into dense islands (1.8 ± 0.2 nm high) on the surface. The islands have a compact, rounded morphology. The cosine of the contact angle between hexadecane and the partial monolayers has an approximately linear dependence on coverage. However, the cosine of the contact angle of water decreases linearly to about 50% coverage (the percolation threshold of the phosphonate islands) and then appears to saturate.

Thin films of the extracellular matrix protein, collagen, were prepared by adsorbing native or heatdenatured type I collagen onto hexadecanethiol self-assembled monolayers. The resulting films were characterized by atomic force microscopy, ellipsometry, and light microscopy. Denatured collagen formed a topographically smooth ∼3.6 nm thick film, consistent with an adsorbed protein monolayer. In contrast, the native collagen thin film consisted of supramolecular collagen fibrils. The density of the large fibrils could be varied by changing the native collagen concentration in the solution from which the films were prepared. The biomimetic nature of the thin collagen films was partially assessed by examining their effects on vascular smooth muscle cells. Automated quantitative analysis indicated that the morphology of smooth muscle cells on the thin films was dependent on whether the collagen was heat-denatured or was in its native fibrillar form. The area of cells on denatured collagen films was significantly larger than that of cells on thin films of native fibrillar collagen. This response closely mimicked the response of these cells to thick collagen gels. Examination of the relationship between collagen fibril density and cell area indicated that large fibrils play a role in determining how cells respond to collagen. Cells assumed a larger morphology on native collagen films with a lower density of large fibrils. In this study, it is clear that cell morphology on these films is determined by micron-scale interactions between cells and the matrix molecules and is not dependent on the bulk materials properties of collagen gels.

Abstract:The interaction of small phospholipid vesicles with well-characterized surfaces has been studied to assess the effect of the surface free energy of the underlying monolayer on the formation of phospholipid/ alkanethiol hybrid bilayer membranes (HBMs). The surface free energy was changed in a systematic manner using single-component alkanethiol monolayers and monolayers of binary mixtures of thiols. The binary surfaces were prepared on gold by self-assembly from binary solutions of the thiols HS-(CH2)n-X (n ) 11, X ) CH3 or OH) in THF. Surface plasmon resonance (SPR), electrical capacitance, and atomic force microscopy (AFM) measurements were used to characterize the interaction of palmitoyl,oleoyl-phosphatidylcholine (POPC) vesicles with the surfaces. For all surfaces examined, it appears that the polar part of surface energy influences the nature of the POPC assembly that associates with the surface. Comparison of optical, capacitance, and AFM data suggests that vesicles can remain intact or partially intact even at surfaces with a contact angle with water of close to 100°. In addition, comparison of the alkanethiols of different chain lengths and the fluorinated compound HS-(CH 2)2-(CF2)8-CF3 that characterize with a low value of the polar part of the surface energy suggests that the quality of the underlying monolayer in terms of number of defects has a significant influence on the packing density of the resulting HBM layer.

We have used a precision-calibrated photodiode as the fundamental metrology reference in order to determine the relative throughput of the PanSTARRS telescope and the Gigapixel imager, from 400 nm to 1050 nm. Our technique uses a tunable laser as a source of illumination on a transmissive flat-field screen. We determine the full-aperture system throughput as a function of wavelength, including (in a single integral measurement) the mirror reflectivity, the transmission functions of the filters and the corrector optics, and the detector quantum efficiency, by comparing the light seen by each pixel in the CCD array to that measured by a precision-calibrated silicon photodiode. This method allows us to determine the relative throughput of the entire system as a function of wavelength, for each pixel in the instrument, without observations of celestial standards. We present promising initial results from this characterization of the PanSTARRS system, and we use synthetic photometry to assess the photometric perturbations due to throughput variation across the field of view.

The contact angle of water was measured on surfaces composed of random hydrophilic and hydrophobic patches with typical length scales of 10−100 nm. By quenching self-assembled monolayers at various stages of growth, the fractional surface coverage of the hydrophobic patches was varied in the range 0.04−0.97 as determined by atomic force microscopy. The cosine of the contact angle of water cos θ was systematically lower than the prediction of the mean field Cassie equation cos θC. The deviation from this prediction cos θC − cos θ had an approximately linear dependence on the total contour length between hydrophobic and hydrophilic patches (a measure of the degree of heterogeneity). The contact angle was insensitive to droplet size, suggesting that line tension effects were minimal. Also, contact angles of hexadecane were in good agreement with the Cassie prediction. We propose two possible explanations for the observed behavior. Long-range (approximately 5 nm) hydrophobic interactions may result in a relatively hydrophobic boundary region around each hydrophobic patch which effectively increases the coverage of the hydrophobic phase altering the equilibrium contact angle. Alternatively, an increased density of “pinning” sites may prevent the contact angle from relaxing to the equilibrium value.

The thermodynamic temperature of the point of inflection of the melting transition of Re-C, Pt-C and Co-C eutectics has been determined to be 2747.84 ± 0.35 K, 2011.43 ± 0.18 K and 1597.39 ± 0.13 K, respectively, and the thermodynamic temperature of the freezing transition of Cu has been determined to be 1357.80 ± 0.08 K, where the ± symbol represents 95% coverage. These results are the best consensus estimates obtained from measurements made using various spectroradiometric primary thermometry techniques by nine different national metrology institutes. The good agreement between the institutes suggests that spectroradiometric thermometry techniques are sufficiently mature (at least in those institutes) to allow the direct realization of thermodynamic temperature above 1234 K (rather than the use of a temperature scale) and that metal-carbon eutectics can be used as high-temperature fixed points for thermodynamic temperature dissemination. The results directly support the developing mise en pratique for the definition of the kelvin to include direct measurement of thermodynamic temperature.

We have studied the growth kinetics of self-assembled monolayers of octadecylphosphonic acid on mica by examining films removed from solution before completion. Atomic force microscope (AFM) images of the quenched films showed islands approximately 2 nm high indicating a growth mechanism consisting of nucleation, growth, coalescence, etc. of dense submonolayer islands. This was consistent with previous in situ AFM studies. Infrared spectroscopy data were consistent with well-ordered alkyl chains and indicated only one type of chain conformation, implying that the areas between islands were bare, not covered with loosely packed disordered molecules. The surface coverage of the submonolayer islands was extracted from AFM images as a function of immersion time for solution concentrations ranging from 0.02 to 2 mM. These data were compared to two common models for adsorption kineticsdiffusion-limited kinetics and adsorption-limited (Langmuir) kinetics. The previously reported quasi-Langmuir−Blodgett deposition process occurring during removal was also taken into account in this modeling. The data were completely inconsistent with the adsorption-limited kinetics and were in reasonable agreement with the functional form for diffusion-limited kinetics. However, the diffusion parameters extracted from the latter fits were much smaller than expected for molecular diffusion in solution and did not scale correctly with solution concentration. We conclude that the island growth is limited by a process that obeys diffusion-like kinetics but not by actual solution diffusion.