The Application Of Sum Frequency Generation Spectroscopy And Surface Enhanced Raman Spectroscopy In The Characterization Of Surface And Interfacial Structures

Molecular level understanding of physical and chemical process at surfaces and interfaces is of great importance, aiding in penetrating into the nature of surface and interfacial properties. However, up to now, it’s still challenge to probe interfacial structure and alignment in situ. Recently, Sum Frequency Generation (SFG) Vibrational Spectroscopy has been developed into a powerful technique to study the molecular structures of the surfaces and interfaces, with inherent interfacial selectivity and submolecular sensitivity. Meanwhile Surface Enhanced Raman Spectroscopy (SERS) reemerged as one of the most promising metal interfacial assay techniques with almost millions of signal enhancement in the nearest decades. Excellent researches have been carried out using these two powerful techniques. In particular, combination of the two techniques will lead to a systematic methodology to investigate more complicated system, providing a deep insight into the nature of surface and interfacial phenomenon.Ligand exchange reaction is a common approach to fabricate functionalized nanopartciles. With the combination of SERS and SFG, we explored the ligand exchange mechanism on planar gold surface and gold nanoparticle system. As we all know, disulfides generally possessed large steric structures. When the long alkyl chain thiols served as the protecting ligands, the steric hindrance of the disulfide molecules played a vital role in the ligand exchange reaction on the planar metal surfaces and this effect can be counteracted on curved gold nanoparticle surface. Meanwhile the increasing substrate defects and more gauche conformation of protecting ligands on curved surface would facilitate the ligand exchange reaction. On the other hand, the electronic effect of para substitute of the ligand phenol ring emerged when the steric effect no more dominated the ligand exchange reaction. Generally, the stronger the electron dominating ability, the stronger the ligand exchange reactivity, and the stronger the ligand affinity.As a powerful combined methodology of SFG and SERS, we believe that it will not only be important for providing fundamental understanding, but will also greatly facilitate the development of functional materials for many important applications.However, in some cases, it’s still challenge to probe molecule details even combining these two techniques (e.g., in the case of disorder molecules or molecules with inherent symmetry on planar metal surface). In this concern, a simple and efficient approach, silver mirror reaction method, was employed to fabricate appropriate silver particles overlayer. With the incorporation of silver particles, a sandwiched "gold substrate-ligands-silver particle overlayer" structure was fabricated, leading to tremendous SERS enhancement. Further experiments verified that this sandwiched structure could provide much stronger SERS enhancement than previously nitric acid etched silver foil sample, and better spectra details. Solid experiment results confirmed that this methodology could be applied to study various molecules on metal surface. Besides, we found that for some structure sandwiched with special ligand, charge transfer effect could be observed. The most important is that, with the combination of SFG and SERS, more precise interfacial information thus can be deduced.In our previous work of polymer/metal interface, we have successfully applied SFG to examine buried polymer/metal interface using a poly(methyl methacrylate)(PMMA) thin film deposited on a silver surface. However, the collected spectra were contributed from PMMAr/air interface, PMMA/Ag interface and metal nonresonant background. Heavy work need to be done to fit the spectra. In this study, we developed our sample geometry to a sandwiched silica/polymer/metal system. It was found that the observed SFG signal intensity does not depend on the sandwiched poly(methyl acrylate)(PMA) film thickness. Detailed data analysis confirmed that the spectra collected in this geometry were directly contributed from PMA/Ag interface. Further studies indicated that the ester methyl groups at the PMA/Ag interface tilt away from the silver interface, which was quite similar to previous PMMA/Ag system. We successfully applied this methodology to silica/PBMA/silver system, and obtained thickness independent spectra. Solid data confirmed that this methodology could be generally applied to various polymer/metal systems.SFG has been successfully applied to study the surface and interfacial structures of industrial materials. The spectra collected from phenolic resin surface indicated that substantial surface restructuring could be induced by the adsorbed water molecules after exposure to humid air. The surface phenyl groups were reoriented closer to a perpendicular position to the surface after exposure to humid air from a more parallel position in air. Further study indicated that the surface restructuring of BPP in humid air is reversible. Besides, we also applied SFG to investigate the interfacial structure of model epoxy in contact with silica substrate. Spectra analysis revealed that Octadecyltrimethoxysilane (OTMS) in the epoxy system preferred to aggregate at the silica/epoxy interface. Combined with lap shearing adhesion test, we believed that hydrophobic assembly of OTMS alkyl chain would play a vital role in depressing the adhesion performance of epoxy at silica interface. Further study about3-Aminopropyltrimethoxysilane (ATMS) added in the epoxy system also confirmed our conclusion.SFG has been widely used in planar solid supported lipid bilayers system. Conventionally used inorganic substrate, such as CaF2prsim, even well performed, could not resemble a real cell membrane environment. In order to mimic more real cell environment, a soft hydrophilic polymer cushion was introduced between the hard inorganic substrate and lipid bilayer. SFG was used to examine and compare lipid bilayers assembled on the CaF2prism and on modified poly(L lactic acid)(PLLA) cushion. Data analysis revealed that the lipid bilayers deposited on both substrates exhibited similar structures. Further study indicated that molecular interactions between an antimicrobial peptide Cecropin P1(CP1) and the lipid bilayers assembled on both substrates were quite similar. This research demonstrated that the hydrophilic PLLA cushion can serve as an excellent substrate to support lipid bilayers. We believe that it can be an important cell membrane model for future studies on transmembrane proteins.