Biomembrane interfaces, expecially cell membrance interfaces, involve many biological processes including transportation, signal transduction, absorption and desorption of protein etc. So it is very critical to reveal the biological events occurring at cell membrane interfaces. Given the complexity of real cell membranes, lipid monolayers and lipid bilayers acting as the model cell membrances were widely studied. In the past decades, many experimental and computational techniques, e.g., X-ray and neutron diffraction, Fourier transform infrared spectroscopy, and molecular dynamics simulation, have been extensively employed to investigate the model cell membrances. However, these traditional techniques are not the ideal tools to characterize surfaces and interfaces due to the lack of the sensitivity and the selectivity of surfaces and interfaces. Fortunately, Sum frequency generation (SFG) vibrational spectroscopy with inherent ultrahigh selectivity and submolecular sensitivity of surfaces and interfaces has been rapidly developed into one of the most powerful techniques for surface and interfacial characterization. SFG has the ability to get the molecular structure, orientation, and orientation distribution at surfaces and interfaces in real time and in situ, these advantages were hardly obtained by other characterization techniques.
In this thesis, model cell membranes and polymer thin films were investigated using the traditional and the phase-sensitive SFG. Firstly, two significant and fundamental issues were addressed. One is concerning the change of molecular orientation and SFG signal intensity when a lipid monoayer on prism in air contacts with a lipid monolayer on water, founding that the SFG signal enhancement is mainly caused by the increscent interfical Fresnel coefficient rather than the orientation change of the lipids. The other one is about the SFG spectral features of a lipid monolayer on silica window (external reflection) and a lipid monolayer on right-angle silica prism (total internal reflection). The results showed that SFG ssp spectra were the same while SFG ppp spectra were different for these two geometries, which was attrituted to the interaction between effective second-order nonlinear susceptibilities and interfical Fresnel coefficients. The settlements of these two issues are extremely important for deep understanding of the SFG spectral features and lipid molecular structures when various contacting media and experimental geometries were employed in SFG study. Based on above studies, quantitative and qualitative analyses of the interaction between memantine (a drug for treating Alzheimer’s disease) and model cell membranes (PC bilayers and PG bilayers) were carefully performed at the molecular level, indicating memantine cannot disrupt the PC bilayer, but can dramatically interact with the distal leaflet of the PG bilayer because of the electrostatic attraction, which makes methyl groups of PG lipids in the distal leaflet tend to lie down at the interface, the average orientation angle was calculated to be around 82° with respect to the surface normal. This research is benifical to better understand the interaction mechanism between them, pharmacokinetics, and will potentially promote the development of new drugs for treating the neurodegenerative diseases.
Given the surface and interfacial selectivity and sensitivity of SFG and its wide application, SFG was applied to probe surfaces and interfaces of polymer thin films. The Tgs of polyacrylates thin films supported on the metal substrate and sandwiched between metal and silica substrates were investigated. It turned out that the Tgs decrease and Tgs increase with decreasing film thickness for the supported and the sandiwiched films, respectively. This experiment demonstrated that, without free surface, the interaction between silica and polymer can significantly affect the Tgs of polymer thin films. A novel approach to detect the Tg of polymer thin films based on the metallic surface plasmon polariton and SFG was developed in this study. To continually promote the development of SFG for the accurate detection, the nonlinear phase-sensitive femtosecond SFG was successfully built up. And then using the phase-sensitive femtosecond SFG, the dewetting behaviors of polystyrene thin films serving as the model were studied, showing that main chains and end groups of polystyrene are inclined to be orderly oriented towards the air whereas side phenyl groups are prone to lie flat at the surface after dewetting.
In summary, the state-of-the-art SFG was used to quantitatively and qualitatively study the model cell membranes and polymer thin films at the molecular level, which helps to understand/build up the correlation between the microscopic molecular structure and the macroscopic performance with respect to interfaces of biomembranes and polymer thin films, and thus promoting the development and application of SFG in soft mater field.