Following the formation of supported lipid bilayers on mica: A study combining AFM, QCM-D, and ellipsometry

Supported lipid bilayers (SLBs) are popular models of cell membranes with potential biotechnological applications and an understanding of the mechanisms of SLB formation is now emerging. Here we characterize, by combining atomic force microscopy, quartz crystal microbalance with dissipation monitoring, and ellipsometry, the formation of SLBs on mica from sonicated unilamellar vesicles using mixtures of zwitterionic, negatively and positively charged lipids. The results are compared with those we reported previously on silica. As on silica, electrostatic interactions were found to determine the pathway of lipid deposition. However, fundamental differences in the stability of surface-bound vesicles and the mobility of SLB patches were observed, and point out the determining role of the solid support in the SLB-formation process. The presence of calcium was found to have a much more pronounced influence on the lipid deposition process on mica than on silica. Our results indicate a specific calcium-mediated interaction between dioleoylphosphatidylserine molecules and mica. In addition, we show that the use of PLL-g-PEG modified tips considerably improves the AFM imaging of surface-bound vesicles and bilayer patches and evaluate the effects of the AFM tip on the apparent size and shape of these soft structures.