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Open Access Publications from the University of California

Scholarly Works (5 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 (2023)
    Vibrational sum-frequency generation (VSFG), a second-order nonlinear optical signal, has traditionally been used to study molecules at interfaces as a spectroscopy technique with a spatial resolution of ~100 µm. However, the spectroscopy is not sensitive to the heterogeneity of a sample. To study mesoscopically heterogeneous samples, we, along with others, pushed the resolution limit of VSFG spectroscopy down to ~1 µm level and constructed the VSFG microscope. This imaging technique not only can resolve sample morphologies through imaging, but also record a broadband VSFG spectrum at every pixel of the images. Being a second-order nonlinear optical technique, its selection rule enables the visualization of non-centrosymmetric or chiral self-assembled structures commonly found in biology, materials science, and bioengineering, among others. In this article, the audience will be guided through an inverted transmission design that allows for imaging unfixed samples. This work also showcases that VSFG microscopy can resolve chemical-specific geometric information of individual self-assembled sheets by combining it with a neural network function solver. Lastly, the images obtained under brightfield, SHG, and VSFG configurations of various samples briefly discuss the unique information revealed by VSFG imaging.
      Cover page: Multimodal Nonlinear Hyperspectral Chemical Imaging Using Line-Scanning Vibrational Sum-Frequency Generation Microscopy.
      • 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
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2024)
          Photoluminescence (PL) imaging has broad applications in visualizing biological activities, detecting chemical species, and characterizing materials. However, the chemical information encoded in the PL images is often limited by the overlapping emission spectra of chromophores. Here, we report a PL microscopy based on the nonlinear interactions between mid-infrared and visible excitations on matters, which we termed MultiDimensional Widefield Infrared-encoded Spontaneous Emission (MD-WISE) microscopy. MD-WISE microscopy can distinguish chromophores that possess nearly identical emission spectra via conditions in a multidimensional space formed by three independent variables: the temporal delay between the infrared and the visible pulses (t), the wavelength of visible pulses (λvis), and the frequencies of the infrared pulses (ωIR). This method is enabled by two mechanisms: (1) modulating the optical absorption cross sections of molecular dyes by exciting specific vibrational functional groups and (2) reducing the PL quantum yield of semiconductor nanocrystals, which was achieved through strong field ionization of excitons. Importantly, MD-WISE microscopy operates under widefield imaging conditions with a field of view of tens of microns, other than the confocal configuration adopted by most nonlinear optical microscopies, which require focusing the optical beams tightly. By demonstrating the capacity of registering multidimensional information into PL images, MD-WISE microscopy has the potential of expanding the number of species and processes that can be simultaneously tracked in high-speed widefield imaging applications.
            Cover page: Multidimensional Widefield Infrared-Encoded Spontaneous Emission Microscopy: Distinguishing Chromophores by Ultrashort Infrared Pulses.
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