Supported phospholipid membranes: Comparison among different deposition methods for a phospholipid monolayer

Supported lipid membranes consisting of self-assembled alkanethiol and lipid monolayers on gold substrates could be produced by three different deposition methods: the Langmuir-Blodgett (L-B) technique, the painted method, and the paint-freeze method, By using cyclic voltammetry, chronoamperometry/chronocoulometry and a.c. impedance measurements, we demonstrated that lipid membranes prepared by these three deposition methods had obvious differences in specific capacitance, resistance and thickness. The specific capacitance of lipid membranes prepared by depositing an L-B monolayer on the alkanethiol alkylated surfaces was 0.53 mu Fcm(-2), 0.44 mu Fcm(-2) by the painted method and 0.68 mu Fcm(-2) by the paint-freeze method. The specific conductivity of lipid membranes prepared by the L-B method was over three times lower than that of the painted lipid membranes, while that of the paint-freeze method was the lowest. The difference among the three types of lipid membranes was ascribed to the influence of the organic solvent in lipid films and the changes in density of the films. The lipid membranes prepared by the usual painted method contained a trace amount of the organic solvent. The organic solvent existing in the hydrocarbon core of the membrane reduced the density of the membrane and increased the thickness of the membrane. The membrane prepared by depositing an L-B monolayer containing no solvent had higher density and the lowest fluidity, and the thickness of the membrane was smaller. The lipid membrane prepared by the paint-freeze method changed its structure sharply at the lower temperature. The organic solvent was frozen out of the membrane while the density of the membrane increased greatly. All these caused the membrane to exist in a ''tilted'' state and the thickness of this membrane was the smallest. The lipid membrane produced by the paint-freeze method was a membrane not containing organic solvent. This method was easier in manipulation and had better reproducibility than that of the usual painting method and the method of forming free-standing lipid film. The solvent-free membrane had a long lifetime and a higher mechanical stability. This model membrane would be useful in many areas of scientific research.