Formation of Supported Bilayers on Silica Substrates
We have investigated the formation of phospholipid bilayers of the neutral (zwitterionic) lipid dimyristoyl-phosphatidylcholine (DMPC) on various glass surfaces from vesicles in various aqueous solutions and temperatures using a number of complementary techniques: the surface forces apparatus (SFA), quartz crystal microbalance (QCM), fluorescence recovery after photobleaching (FRAP), fluorescence microscopy, and streaming potential (SP) measurements. The process involves five stages: vesicle adhesion to the substrate surfaces via electrostatic and van der Waals forces, steric interactions with neighboring vesicles, rupture, spreading via hydrophobic fusion of bilayer edges, and ejection of excess lipid, trapped water, and ions into the solution. The forces between DMPC bilayers and silica were measured in the SFA in phosphate buffered saline (PBS), and the adhesion energy was found to be 0.5−1 mJ/m2 depending on the method of bilayer preparation. This value is stronger than the expected adhesion predicted by van der Waals interactions. Theoretical analysis of the bilayer−silica interaction shows that the strong attraction is likely due to an attractive electrostatic interaction between the uncharged bilayer and negatively charged silica owing to the surfaces interacting at “constant potential.” However, the bilayer−silica interaction in distilled water was found to be repulsive at all distances, which is attributed to the surfaces interacting at “constant charge.” These results are consistent with QCM measurements that show vesicles readily forming bilayers on silica in high salt but only weakly adhering in low salt conditions. We conclude that the electrostatic interaction is the most important interaction in determining the adhesion between neutral bilayers and charged hydrophilic surfaces. SP and FRAP experiments gave insights into the bilayer formation process as well as information on the surface coverage, lateral diffusion of the lipid molecules, and surface potential of the bilayers during the spreading process.