Probing vesicles stability and morphology on the solid-liquid interface

Liposomes are important models for surface modification, biosensors, gene therapy, and drug encapsulation. However, little is known about the structure and stability of adsorbed liposomes on solid surface. In this thesis, our goal is to characterize the morphology and stability of small unilamellar vesicles on mica surface using Atomic Force Microscopy (AFM). Egg yolk phosphatidylcholine (EggPC), cholesterol modified EggPC, and EggPC/Pluronics are used as model liposomes.; The morphology and elastic properties of EggPC, EggPC/Cholesterol, and EggPC/Pluronics liposomes immobilized on mica were probed by AFM imaging and force measurement. Three different morphologies (convex, planar, and concave shape) of the EggPC vesicles on the mica surface were observed and the morphology change of the vesicles was due to lower elastic properties of pure EggPC vesicles. Spherical shape of EggPC/Cholesterol liposomes with certain size range (∼60 nm) on mica was observed. A significant difference in the size distribution of free EggPC/Cholesterol vesicles in solution and supported vesicles was observed suggesting a critical stable size for cholesterol-modified vesicle adsorbed on the surface. Bilayer, and spherical liposomes were observed for EggPC/(PEO)_x-PPO_y-PEO_x) liposomes with varying PEO and PPO chain length (x, y value).; The force curve on the vesicle provides evidence for the structure of vesicles and can be used to pinpoint the elastic compression and stretching, breakthrough and resealing of bilayers in the deformation of a single vesicle. The elastic properties from force plot analysis based on Hertz model showed that the Young's modulus and bending modulus of EggPC/Cholesterol, and EggPC/Pluronic ® liposomes increased by several folds as compared with those of pure EggPC vesicles.; In summary, our results showed that the elastic properties of liposomes can be tailored by biological and polymeric inserts. And this study also demonstrated that AFM not only could provide real-time visualization of individual vesicles and their aggregation behavior but also prove to be a novel technique to probe the micromechanical properties and structure of individual, adsorbed small vesicles.