Pore-suspending lipid bilayers on porous alumina investigated by electrical impedance spectroscopy

Nonordered and ordered porous alumina substrates with pore diameters of 20 and 50 nm, respectively, were utilized to immobilize lipid membranes spanning the pores of the porous material. The substrates were characterized by means of interferometry and electrical impedance spectroscopy. For impedance data reduction, an equivalent circuit representing the electrical behavior of porous alumina was developed on the basis of the parallel layer model. It turned out that the electrical parameters of the as prepared alumina substrates prevent a sensitive monitoring of the formation of immobilized lipid membranes. Thus, we established a technique to modify the substrates with respect to their electrical properties, leading to a significantly increased capacitance of porous alumina, which allowed for a sensitive detection of pore-spanning lipid bilayers by impedance spectroscopy. Two different membrane preparation techniques based on vesicle spreading were investigated. First, negatively charged giant liposomes were spread onto the porous alumina surface under an applied dc voltage of +100 mV. Second, large unilamellar vesicles containing lipids bearing a thiol anchor were used to chemisorb on gold functionalized porous alumina substrates and subsequently rupture to form planar pore-spanning membranes. For both techniques, impedance spectra were obtained, which indicate the formation of lipid bilayers on top of the porous alumina substrates.