Phase separation in mixed self-assembled monolayers and its effect on biomimetic membranes

Self-assembled monolayers (SAMs) comprised of a mixture of thiol-lipid and spacer-thiol have been used to tether bilayer lipid membranes. However, the properties of these mixed SAMs and their effect on the tethered bilayer lipid membranes (tBLMs) are not fully understood. In this paper, these SAMs have been characterised by electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy and cyclic voltammetry. A complex pattern of mixing and phase separation is observed that is dependent on the type of spacer-thiol used. The influence of phase separation, lipid composition and surface flatness on the formation of the tBLMs have also been studied. Large differences between EggPC and lipid extracts of Escherichi coli were detected, while the surface flatness of the gold has a relatively minor influence. tBLM formation is also relatively insensitive to the type of spacer-thiol used, while the quality of the bilayer is strongly influenced by the relative amount of thiol-lipid on the surface. Impedance spectroscopy of the tBLMs shows varying degrees of frequency dispersion and these results are discussed with respect to the quality of the tBLM.