Electrochemical STM investigations of phospholipid molecules adsorbed on gold (111) surface

This thesis presents an Electrochemical Scanning Tunneling Microscopy (ECSTM) investigation of the structures and mechanisms of formation of phospholipid layers adsorbed on the Au (111) surface. Four lipids, dodecyl-N,N-dimethyl-3-ammonio-1-proponate-sulfonate (DDAPS), di-myristoyl phosphatidyl choline (DMPC), di-palmitoyl phosphatidyl choline (DPPC) and a mixture of di-myristoyl phosphatidyl choline (DMPC) and Cholesterol, have been studied with ECSTM.; At lower solution concentrations, the zwittzerionic surfactant molecule DDAPS, dodecyl-N,N-dimethyl-3-ammonio-1-proponate-sulfonate, first formed a monolayer on the surface with the appearance of molecular stripes, the widths of which were ca. 4.5 nm. When more molecules adsorbed on the gold electrode surface, a thicker organic film was formed, which was imaged by in-situ STM. When further adsorption took place, DDAPS formed a hemimicellar structure on the surface. The structure of the hemimicelles has been visualized by STM for the first time.; DMPC (di-myristoyl phosphatidyl choline) is one of the most important components of the mammalian cell membrane and comprises a polar headgroup and two hydrocarbon tails. In these experiments, we first visualized the DMPC at molecular resolution at +200 mV by spreading DMPC vesicles on a Au (111) surface. The DMPC molecules formed stripes on the surface and the width of the stripes was ca. 4.5 nm. With increasing surface concentrations, the DMPC molecules tilted at certain angles to accommodate more adsorption. The orientation of the DMPC molecular stripes was 120° with respect to the reconstructed surface. When more molecules adsorbed on the surface, a flat molecular layer was formed.; DPPC (di-palmitoyl phosphatidyl choline) was also investigated in these experiments. DPPC contains the same headgroup as DMPC but the hydrocarbon chains are two CH2 units longer which increases the hydrophobicity of the molecules. The interest in DPPC was to investigate the interactions between the charged metal surface and the properties of the molecules. The more hydrophobic DPPC molecule adsorbed on the Au (111) surface much faster than the DMPC in the range from 0 mV to +200 mV, at which potential the Au (111) surface is hydrophobic. The initial adsorption of DPPC was also in the form of molecular stripes of width ca. 4.5 nm. The orientation of the stripes was 120° with respect to the reconstructed Au (111) surface.; The results have greatly enhanced our understanding of the adsorption behavior of phospholipid molecules on Au (111), and of their interactions with one another and with the metal surface. It is envisaged that this knowledge can be used to exploit in the design of molecular nanosystems and biosensors. (Abstract shortened by UMI.)...