Inhomogeneous molecular distribution in self-spreading lipid bilayers at the solid/liquid interface

Self-spreading lipid bilayers at the solid/liquid interface can be used as a molecular transport medium in targeting nano-devices such as drug delivery and micro-total analytical systems. To gain physico-chemical insight in the self-spreading lipid bilayer, we have characterized the distribution of dye-labeled lipids in the upper and lower monolayers of a self-spreading lipid bilayer on a hydrophobic substrate by fluorescence quenching experiments using KI as a quencher. TR-DHPE, a molecule with a dye moiety at the head group, was found to be distributed primarily in the upper layer and accumulated especially at the spreading edge because of high steric repulsion. This resulted in an asymmetric distribution of TR-DHPE in the self-spreading bilayer in both the vertical and lateral directions. By contrast, NBD-PC, bearing a dye moiety at the alkyl chain, was distributed almost symmetrically both vertically and laterally. The observed difference is attributed to the difference in interactions between these molecules in the lower layer and the substrate surface. We have also found that the self-spreading velocity was decreased by the addition of KI. Since the spreading dynamics are determined by the interaction energy between the bilayer and solid substrate, a part of the observed velocity decrease could be explained by the change in the lipid density resulting from the adsorption of the I- anion on the lipid head group, thereby reducing the van der Waals interaction energy.