Neutron reflectivity studies of single lipid bilayers supported on planar substrates

Neutron reflectivity was used to probe the structure of single phosphatidylcholine (PC) lipid bilayers adsorbed onto a planar silicon surface in an aqueous environment. Fluctuations in the neutron scattering length density profiles perpendicular to the silicon/water interface were determined for different lipids as a function of the hydrocarbon chain length. The lipids were studied in both the gel and liquid crystalline phases by monitoring changes in the specularly-reflected neutron intensity as a function of temperature. Contrast variation of the neutron scattering length density was applied to both the lipid and the solvent. Scattering length density profiles were determined using both model-independent and model-dependent fitting methods. During the reflectivity measurements, a novel experimental set-up was implemented to decrease the incoherent background scattering due to the solvent. Thus, the reflectivity was measured to Q approximately 0.3 A-1, covering up to seven orders of magnitude in reflected intensity, for PC bilayers in D2O and silicon-matched (38% D2O/62% H2O) water. The kinetics of lipid adsorption at the silicon/water interface were also explored by observing changes in the reflectivity at low Q values under silicon-matched water conditions.