Potential-Controlled Incorporation of Liposomes into a Solid-Supported Lipid Layer

Background Solid-supported lipid bilayers are important for modelling of cell membranes, biosensors and drug screening. Formation of these layers is easily achieved through bursting and spreading of liposomes over a solid surface, but the mechanism of this process is not fully understood. The ability to predict and control formation of these layers is particularly important, as adsorption of liposomes may lead to heterogenous surface coverage. Use of an octadecanol layer to block the electrode surface allows opportunity for controlling adsorption as it reversibly forms defects at well known potentials. Methods Octadecanol bilayers are deposited onto a Au(111) single-crystal electrode. Liposomes (DOPC, 100 nm diameter) are added to the electrolyte, and a four-step potential profile is applied to incorporate the liposomes into the octadecanol layer: Characterization (0 mV/SCE); Poration of octadecanol (-200 to -800 mV/SCE); Liposome incorporation (-200 mV/SCE) and Closure of defects (0 mV/SCE). The layer is monitored by in-situ fluorescence imaging and capacitance measurements. Fluorescence imaging allows visualization of structures at the electrode surface by the reversible quenching of fluorophores in the layer. Near the electrode surface, fluorescence is quenched by resonance transfer into the metal. However, if the fluorophore moves away from the surface (for example during desorption of the layer), fluorescence resumes. These images compliment the electrochemical data, providing spatially-resolved information of the electrode surface where capacitance provides only a surface average. Results Several poration potentials were tested (-200, -400, -600, and -800 mV/SCE). The degree of liposome incorporation can be estimated by comparing the desorption potentials of octadecanol vs. the hybrid layer. Among those tested, -200 and -800 mV had the poorest response (less than 50 mV shift) and -400 mV the best response (approx. 100 mV shift). Using the -400 mV poration potential, poration times were tested. The shortest time tested (1 min) showed the best response (~175 mV shift). In-situ fluorescence measurements confirm that the layer is desorbed at these potentials. The hybrid layers have a higher fluorescence intensity and larger surface features than octadecanol alone. Conclusions Potential-controlled incorporation of liposomes into an octadecanol layer is demonstrated. The degree of incorporation depends on both the magnitude of potential applied and the residence time at that potential. Fluorescence measurements highlight the changes caused by incorporation of liposomes.