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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2022)

In this thesis, two second-order vibrational nonlinear optical techniques – spatially resolved vibrational sum-frequency generation (VSFG) spectroscopy and line-scanning vibrational sum-frequency microscopy are developed and applied to organometallic and biologically relevant materials respectively to elucidate host-guest interactions as well as chemical specific geometric information.Spatially resolved VSFG spectroscopy is used to study water-host interactions in a single crystal of the metal-organic framework (MOF) zeolitic imidazolate framework 90 (ZIF-90). ZIF-90 follows a

layer/cluster pore filling mechanism in which water first interacts with hydrophilic metal centers of the MOF. These initial water bindings ultimately serve as binding sites for additional water molecules to adsorb into the MOF pore. To design higher performance MOF candidates in the future however, open questions such as whether a single pore is filled entirely before additional pores are filled or if all pores are filled simultaneously need to be addressed. By comparing experimental spatially resolved VSFG spectroscopy and Fourier transform infrared (FTIR) spectroscopy results with that of simulation, it was determined that ZIF-90 pores are filled individually prior to its adsorption isotherm point of inflection highlighting the balance between enthalpic and entropic characteristics in adsorption processes.Further utilizing spatially resolved chemical specific techniques, VSFG microscopy was developed into a laser scanning platform with a broadband detection scheme. Coupling the broadband detection scheme with laser beam steering methodologies enabled the rapid collection of all eight polarization dependent measurements with negligible mechanical and electronic drift. This experimental technique was used to image a hierarchically organized mesoscopic molecular self-assembly (MSA) composed of β-cyclodextrin (β-CD) and sodium dodecyl sulfate (SDS) called SDS@2β-CD. With all eight polarization combinations collected for individual sheets a neural network machine learning solver was implemented to resolve chemical specific geometric information of individual sheets of the MSA. The broad range of application and multimodal detection scheme for the line scanning VSFG hyperspectral microscope are then developed further and explored in depth through the analysis of additional biologically relevant hierarchical organizations: collagen, diphenylalanine aggregates (FF) and SDS@2β-CD. Here we highlight additional chemical specific information that is not revealed when using chemically ignorant optical techniques, such as SHG or traditional bright field microscopy. Our previous studies using the line scanning VSFG microscope focused solely on the 3.5μm region of the electromagnetic spectrum so, we further developed the instrument to accept the full output of our optical parametric amplifier (OPA) extending our coverage from 3μm-10μm. This development enables the sub-diffraction limited IR analysis of common biological spectral signatures such as the amide regions, which are important in determining protein secondary structure.

    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2022)
    In this work, we determined the tilt angles of molecular units in hierarchical self-assembled materials on a single-sheet level, which were not available previously. This was achieved by developing a fast line-scanning vibrational sum frequency generation (VSFG) hyperspectral imaging technique in combination with neural network analysis. Rapid VSFG imaging enabled polarization resolved images on a single sheet level to be measured quickly, circumventing technical challenges due to long-term optical instability. The polarization resolved hyperspectral images were then used to extract the supramolecular tilt angle of a self-assembly through a set of spectra-tilt angle relationships which were solved through neural network analysis. This unique combination of both novel techniques offers a new pathway to resolve molecular level structural information on self-assembled materials. Understanding these properties can further drive self-assembly design from a bottom-up approach for applications in biomimetic and drug delivery research.
      Cover page: Imaging Orientation of a Single Molecular Hierarchical Self-Assembled Sheet: The Combined Power of a Vibrational Sum Frequency Generation Microscopy and Neural Network
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2020)
      Understanding hydrogen-bond interactions in self-assembled lattice materials is crucial for preparing such materials, but the role of hydrogen bonds (H bonds) remains unclear. To gain insight into H-bond interactions at the materials' intrinsic spatial scale, we investigated ultrafast H-bond dynamics between water and biomimetic self-assembled lattice materials (composed of sodium dodecyl sulfate and β-cyclodextrin) in a spatially resolved manner. To accomplish this, we developed an infrared pump, vibrational sum-frequency generation (VSFG) probe hyperspectral microscope. With this hyperspectral imaging method, we were able to observe that the primary and secondary OH groups of β-cyclodextrin exhibit markedly different dynamics, suggesting distinct H-bond environments, despite being separated by only a few angstroms. We also observed another ultrafast dynamic reflecting a weakening and restoring of H bonds between bound water and the secondary OH of β-cyclodextrin, which exhibited spatial uniformity within self-assembled domains, but heterogeneity between domains. The restoration dynamics further suggest heterogeneous hydration among the self-assembly domains. The ultrafast nature and meso- and microscopic ordering of H-bond dynamics could contribute to the flexibility and crystallinity of the material--two critically important factors for crystalline lattice self-assemblies--shedding light on engineering intermolecular interactions for self-assembled lattice materials.
        Cover page: Spatially dependent H-bond dynamics at interfaces of water/biomimetic self-assembled lattice materials
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