On the application of supported bilayers as receptive layers for biosensors with electrical detection

The application of impedance analysis to biosensor devices based on supported lipid/protein membranes is explored. Lipid membranes with a high degree of surface coverage (≈99.5%) have been prepared by two methods. Firstly, very stable high-resistance bilayers are formed by deposition of a covalently bound monolayer of mercaptoalkyls on gold-covered substrates and subsequent transfer of a lipid monolayer via monolayer transfer. Secondly, the preparation of planar membranes by fusion of vesicles onto charged surfaces is demonstrated. A negatively charged monolayer of a carboxy mercaptan is first deposited onto gold onto which vesicles of positively charged dioctadecyldimethylammonium bromide (DODAB) fuse spontaneously while vesicles from negatively charged dimyristoylphosphatidylglycerol (DMPG) fuse after addition of ≈1 mM of calcium ions. Combination of impedance analysis and surface plasmon spectroscopy yields a powerful tool for the detection of ligand binding to supported receptor/lipid membranes. It also allows one to discriminate between specific and nonspecific binding. It is demonstrated that careful analysis of the impedance data yields information about the electric properties of the membrane and of defects.