Recognizing Molecules and Following Reactions at Interfaces Using the Nonlinear Optical Techniques Sum Frequency Generation and Second Harmonic Generation

• Author / Creator

  Li, Zhiguo

• Silica has been widely used as a solid support to generate functionalized silica in material science, which plays important roles in various areas such as catalysis and disease diagnostics. The applications of functionalized silica involve the special molecular properties of interfaces, which can differ from the properties of molecules in bulk phases. Nonlinear optical techniques, including second harmonic generation (SHG) and sum frequency generation (SFG), are a class of well-defined spectroscopic techniques that are well-suited to probe interfacial molecules and to study interfacial phenomena. Owing to their surface specificity, only the molecules that are ordered at the interface are probed in the measurement while interference from molecules in bulk phases is effectively excluded. Taking planar silica and functionalized planar silica as substrates, this thesis mainly focuses on characterizing the structure and order of molecules at these solid surfaces, and investigating the interaction between immobilized molecules and bulk phase molecules. Specifically, the binding of a model reactant 4-nitroacetophenone with amino and ureido organocatalytic monolayers on silica was investigated at the silica/acetonitrile interface using a combination of SHG and SFG, and the results reveal that the performance of immobilized catalysts is strongly affected by the local environment of surface molecules. Next, the Cu(I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition (CuAAC) was followed at the silica/methanol interface using vibrational SFG; the reaction order with respect to copper catalyst was determined to be 2.1, suggesting that two coppers are involved in the rate-determining step of the interfacial reaction. Using a CuAAC attachment strategy, the thermal evolution of immobilized DNA single strands and duplexes were investigated at the silica/buffer interface using vibrational SFG. Consistent with our SHG work, the melting temperature of the immobilized T15:A15 (thymidine 15-mer: deoxyadenosine 15-mer) duplex at the interface is found to be ~ 12 °C lower than the solution phase melting temperature, indicating immobilization on silica destabilizes the DNA duplex. Finally, as silica is a commonly used stationary phase in chromatography, the organization of acetonitrile molecules at the silica/aqueous interface was studied using vibrational SFG. For a given solution composition, increasing the solution pH is found to effectively decrease the number density of interfacial acetonitrile molecule, which we attribute to the decrease in the number density of surface silanol which can form a hydrogen bond with cyanide within the acetonitrile molecule. Overall, these results demonstrate the study of interfacial phenomena involving self-assembled monolayers on insulated surfaces like silica using the nonlinear optical techniques SHG and SFG. The combination of these techniques represents a useful approach to explore similar systems in their relevant research areas.

• Subjects / Keywords

  • Functionalized Planar Silica
  • Second Harmonic Generation
  • Sum Frequency Generation

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R32V2CJ4H

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.