Formation of Biomimetic Membrane Rafts on Bare and Modified Gold

Abstract

Lipid domains are essential functional entities of biological membranes, therefore lipid bilayers containing liquid disordered/liquid ordered (lipid rafts) phase coexistence are improved biomimetic systems. Moreover, supported lipid bilayers (SLB) of such ternary lipid mixtures formed on gold are promising interfaces for biosensing purposes. However, gold’s hydrophobic nature has been an obstacle for direct deposition, and most studies rely on previous modification of its surface. In addition, the formation and properties of lipid domains in these systems only raised attention very recently. In this work, ternary lipid mixtures (e.g. DOPC/DPPC/Cholesterol (2:2:1 molar ratio)) were deposited on gold, either bare or modified with hydrophilic self-assembled monolayers (SAMs) of 11-mercaptoundecanoic-acid (MUA) and cystein, which display distinct packing density and organization. Atomic force microscopy was used to study the topography of the lipid films at the nanoscale, allowing the first reported observation of well-organized raft-containing SLB on unmodified gold. This was achieved under optimized conditions, as changing slightly buffer composition may lead e.g. to the formation of tubular structures. Coverage and continuity of SLB were addressed by ellipsometry and cyclic voltammetry, taking advantage of gold optical/electrical properties. The cyclic voltammograms of ferricyanide redox process suggest the presence of continuous bilayers with small pores. The bilayer thicknesses, 4-5nm, estimated by ellipsometry were further confirmed by force spectroscopy. The raft-containing bilayer is stable over a wide range of potential sweep, enabling the development of new lipid raft-based biosensing interfaces. Planar and continuous SLB could be also prepared onto gold modified electrodes. In the case of a short cystein SAM, pronounced decrease of the redox process intensity was observed after lipid deposition revealing a high coverage of the modified gold surface.Acknowledgements: SFRH/BD/64442/2009, PTDC/QUI/66612/2006, PEst-OE/QUI/UI0612/2011 and Ciencia2007 (FCT Portugal).