Directing macromolecular assemblies by tailored surface functionalizations of nanoporous alumina

Abstract

Anodic aluminum oxide (AAO) is a nanoporous material with parallel, hexagonally-ordered, cylindrical pores running straight through the material s thickness. A method to deposit highly ordered AAO thin-films, with adjustable 1-10 mm thicknesses, on various surfaces was developed to allow the in-situ study of its dielectric response by optical waveguide spectroscopy (OWS). The Layer-by-Layer deposition within AAO of different types of macromolecules was studied: dendrimer polyelectrolytes, linear polyelectrolytes and proteins. Working at low ionic strength was found to inhibit macromolecular transport within the cylindrical nanopores due to electrostatic repulsion between macromolecules in solution and those adsorbed atop the AAO. Under optimal charge screening conditions, LbL proceeded in a similar fashion as on a planar surface, until multilayer growth became inhibited due to the confined cylindrical geometry that imposes a physical limit to macromolecular deposition and the pore diameter was reduced to 20-35 nm. The LbL growth was dependent on the physical structure of the LbL film, dictated by the size, shape and nature of the interaction between macromolecules. A method for the orthogonal functionalization of AAO was developed to selectively protect the silanized pore-rim AAO surface with a thin gold layer. Subsequent plasma treatments remove the unprotected silanization, such that AAO with different pore-rim and pore-interior surface chemistries were produced. This method was used to direct the formation of both hybrid and fluid pore-spanning membranes on the AAO surface by giant liposome rupture. These lipid membranes effectively encapsulate the AAO pore-interior liquid environment and act as real physical barriers. We demonstrated that these membranes could exclude proteins from entering the nanopores, or alternatively, that they can prevent encapsulated fluorescent dyes from escaping the nanopores. Finally, the homogeneous modification of AAO with a laterally mobile lipid monolayer in three-dimension s was achieved in order to control the amount of protein deposition within the AAO pores.