Charge Effects Provide Ångström-Level Control of Lipid Bilayer Morphology on Titanium Dioxide Surfaces
Dennis J. Michalak
Oxidic surfaces in electrolytic solutions and their interaction with molecular organic architectures are important for many applications in science and technology, such as nanofabrication, biosensor design, microfluidics, and biomimetic membrane devices. The versatility of such structures derives from, and is at the same time limited by, the complicated surface chemistries of both the oxides and the organic layer. For phospholipid bilayers, the large range of pH- and ionic strength-dependent surface charge densities adopted by oxidic surfaces leads to a rich landscape of phenomena and provides exquisite control of membrane interactions with the substrate. We exploit a solvent-exchange method to prepare homogeneous, unpinned bilayers of zwitterionic phospholipids on SiO2 as well as on TiO2, where membrane formation by conventional methods often fails, and characterize the initial conditions that lead to bilayer formation. On SiO2, neutron reflectometry shows membranes stably locked into a narrow window of membrane-substrate separations at all pH and ionic strength values while on TiO2 sharp, reversible transitions occur between closely surface-associated and weakly coupled. This is fully consistent with predictions based on a generalized potential derived from Van der Waals attraction, electrostatic interactions, fluctuation repulsion and short-range (protrusion or hydration) forces without any free parameters: the length scale of electrostatic replusion can be tuned with respect to the fixed length scale of attraction on TiO2, while on SiO2 it cannot. The same formalism rationalizes a variety of related observations concerning the unbinding of bilayers from supporting substrates reported over the past two decades.