In situ characterization of phospholipid coated electrodes

Four in situ techniques, quartz crystal microbalance, electrochemical impedance spectroscopy, ellipsometry and atomic force microscopy, have been used to study the deposition of dioleoylphosphatidylglycerol (DOPG) films from vesicle solution onto 1-decanethiol coated gold surfaces. The experiments show that, although no one technique can give an unequivocal picture of the deposition process and structure of the final film, by combining the results from the four different techniques a consistent picture emerges. Initially, over the first 200 s a monolayer of DOPG is formed on the 1-decanethiol coated surface. Then at longer times and in the presence of 0.1 M NaCl and 0.1 M TRIS buffer at pH 8 a further bilayer of DOPG is deposited on top of this initially formed monolayer. Evidence for this is provided by the quartz crystal microbalance measurements and from atomic force microscopy. The results from electrochemical impedance measurements and ellipsometry are consistent with these findings but these two methods are less able to distinguish between the formation of the monolayer and subsequent deposition of the bilayer on top of the monolayer. For DOPG films, where the head group is negatively charged at pH 8, the ionic strength of the solution has an important effect in controlling the deposition of the bilayer on top of the initially formed monolayer.