Abstract
Anodic aluminum oxide (AAO) membranes with aligned, cylindrical, nonintersecting pores were selectively functionalized in order to create dual-functionality substrates with different pore-rim and pore-interior surface functionalities, using silane chemistry. We used a two-step process involving an evaporated thin gold film to protect the underlying surface functionality of the pore rims. Subsequent treatment with oxygen plasma of the modified AAO membrane removed the unprotected organic functional groups, i.e., the pore-interior surface. After gold removal, the substrate became optically transparent, and displayed two distinct surface functionalities, one at the pore-rim surface and another at the pore-interior surface. We achieved a selective hydrophobic functionalization with dodecyl-trichlorosilane of either the pore rims or the pore interiors. The deposition of planar lipid membranes on the functionalized areas by addition of small unilamellar vesicles occurred in a predetermined fashion. Small unilamellar vesicles only ruptured upon contact with the hydrophobic substrate regions forming solid supported hybrid bilayers. In addition, pore-rim functionalization with dodecyl-trichlorosilane allowed the formation of pore-spanning hybrid lipid membranes as a result of giant unilamellar vesicle rupture. Confocal laser scanning microscopy was employed to identify the selective spatial localization of the adsorbed fluorescently labeled lipids. The corresponding increase in the AAO refractive index due to lipid adsorption on the hydrophobic regions was monitored by optical waveguide spectroscopy. This simple orthogonal functionalization route is a promising method to control the three-dimensional surface functionality of nanoporous films.
Highlights
Anodic aluminum oxide (AAO) is one of the most promising ordered nanoporous materials
The AAO membranes were covered with a thin metal coupling layer (2 nm of Cr and 25 nm of Au) on the aluminum oxide barrier side and mounted on glass supports using an optical adhesive[43] (Figure 1B) to allow the characterization of their refractive index using optical waveguide spectroscopy (OWS)
We have developed a strategy based on silane chemistry to generate orthogonally functionalized nanoporous anodic aluminum oxide (AAO) substrates, where the pore-rim surface chemistry is distinctly different from the pore-interior surface
Summary
Anodic aluminum oxide (AAO) is one of the most promising ordered nanoporous materials. A porous membrane with hydrophilic pore rims and hydrophobic pore interiors was achieved using plasma-polymerization of a fluorocarbon layer.[41] Recently, the wetting properties of AAO were tuned by the selective deposition, by electrospray technique, of a thin polymer film at the pore rims.[42] In this contribution, we present a simple technique, where AAO silanization is performed after the anodization and pore-widening processes, i.e., on the desired final substrate, which ensures that the surface functionality is created just before use, ensuring optimal chemical integrity, degree of functionalization and spatial localization of the surface chemistry. Depending on the size of the phospholipid vesicles, we were able to produce hybrid solid-supported lipid membranes on the pore-interior AAO surface using small unilamellar vesicles (SUVs), or hybrid porespanning membranes on functionalized AAO with hydrophobic pore-rim surfaces using giant unilamellar vesicles (GUVs)
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