Mechanism of formation of vesicle fused phospholipid monolayers on alkanethiol self-assembled monolayer supports

Abstract

We investigated the process of phospholipid vesicle fusion to a hydrophobic surface using electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM). The kinetics of the fusion of dimyristoyl phosphatidylcholine (DMPC) vesicles to an octadecanethiol (ODT) self-assembled monolayer (SAM) was followed with EIS and found to be slower than previously reported observations by surface plasmon resonance. AFM images of the DMPC layer taken at various stages of completion show they are uniform with topography similar to that of the bare ODT SAM. The AFM was used in contact mode to scrape away a portion of a completed DMPC monolayer and the hole was stable for several hours. Surprisingly, the hole remained stable even after the temperature of the system was raised to 38 °C, well above the gel transition temperature for DMPC of 23 °C. We were unable to scrape holes in the partially completed monolayers. We conclude that in the early stages of monolayer growth the DMPC molecules spread across the ODT surface after vesicle fusion rather than remaining in dense islands. However, when the DMPC monolayer is complete this mobility is lost and the DMPC will not spread into a vacant area.