Sum-Frequency Generation Characterization of Lipid Monolayers at the Solid-Air and Solid-Water Interface
Vasso, Victoria Lynn
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Optical techniques have been in use by the scientific community for many decades, helping to elucidate the mechanics of chemical reactions as well as the basic structures of various compounds (biological or otherwise). Non-linear optical (NLO) techniques have become increasingly popular spectroscopic tools, as is the case for sum frequency generation (SFG) and second harmonic generation (SHG). In fact, the first SFG/SHG spectra to be taken and analyzed occurred in the year of 1987, and has since been used more frequently in various experimental studies. Of particular interest are solid-liquid and solid-air interfaces, in this case the interface between 1,2-diheptadecanoyl-sn-glycero-3-phosphate (17:0 PA) monolayers and air and water. These monolayers were created by the Langmuir-Blodgett/Langmuir- Schaefer approach on a calcium fluorite (CaF 2 ) window. Both sides of the window were coated with a monolayer of the lipid, therefore both the front and the back side were analyzed by way of sum-frequency generation spectroscopy at the solid-air interface. For studies of the solid-water interface, only the back side of the window, the one in direct contact with water, was considered. The experimental results show that is likely that the lipid coverage on the window was in the sub-monolayer regime, though the attachment of the lipid to the window is confirmed by the presence of the CH 2 and CH 3 stretching peaks which are well in agreement with previous studies of long chain hydrocarbons at the solid-air and solid-water interfaces. Utilizing the same method as the 17:0 PA, cholesterol monolayers were created on a CaF 2 window and on a glass slide., It was determined that cholesterol monolayers were formed on the substrates due to the clear vibrational signature in the CH stretching regions. Several achiral and chiral-sensitive polarization dependent SFG experiments were performed and will provide interfacial molecular orientation information on the cholesterol interfacial molecules once future molecular and quantum simulations are performed...