Lipid bilayers on topochemically structured planar colloidal crystals: a versatile platform for optical recording of membrane-mediated ion transport

Abstract

Phospholipid membranes assembled on a planar colloidal crystal functionalized with a proton sensitive fluorescence probe offers a useful model system for a convenient, on-chip, parallel, and fluorescence based assay of membrane-mediated ion transport. The fusion of small unilamellar vesicles on surface derivatized hydrophilic/hydrophobic planar colloidal crystals results in a single lipid bilayer on the hydrophilic region and a monolayer on the hydrophobic region. We show that this composite structure exhibits a laterally continuous outer leaflet spanning the two mono- and bilayer morphologies, thereby providing an effective seal against ionic permeability. This is in contrast to the structure of lipids assembled on a patterned wettability planar coverslip where there a lipophobic “moat” formed at the junction between the mono- and the bilayer configurations prevents forming well-sealed membrane. By embedding a pH sensitive fluorescent probe, namely, fluorescein, within the colloidal crystal interstices, we find that the colloidal crystal supported membrane configuration provides a practical assay for optical characterization of membrane mediated ionic transport. We demonstrate the ability of our platform for the case of passive transport of protons across a fluid phospholipid bilayer. The membrane permeability coefficients derived, including those associated with the biphasic permeation mechanism, match well with those reported for vesicle based assays and confirm the quantitative optical transduction. We also illustrate the use of this experimental platform for parallel measurements such as may be useful for the characterization of stochastic transport and high throughput measurements.