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Browsing by Author "Levinger, Nancy E., advisor"

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    Femtosecond to nanosecond transient absorption studies of aqueous solvation and deprotonation dynamics in confinement
    (Colorado State University. Libraries, 2011) Cole, Richard Leo, author; Levinger, Nancy E., advisor; Bernstein, E. R. (Elliot R.), committee member; Ladanyi, Branka M., committee member; Van Orden, Alan K., committee member; Bartels, Randy, committee member
    We explore the use of logarithmic based optical delay in time-resolved data collection. We show that logarithmic spacing of data points provides an economical way to collect data over many decades of time which speeds data collection. We present a simple algorithm to generate time delay points for application in time-resolved data collection. We test the use of logarithmic vs. linear data collection over six orders of magnitude by measuring broadband femtosecond transient absorption (BFTA) spectra of HPTS in pH-7 water from femtoseconds to nanoseconds. Statistical analysis of logarithmic and linear data collection show that linear data collection shows a clear advantage by requiring a fewer number of time-delay points to achieve a given precision in subsequent data analysis. We investigate solvation dynamics (SD) via coumarin 343 (C343) in Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate, AOT) reverse micelles with varying water content through broadband femtosecond transient absorption experiments. These studies build upon our previous studies of SD in the AOT reverse micelles through time-resolved fluorescence Stokes shift (TRFSS) experiments (J. Phys. Chem. B, 1998, 102, 2705) which limited data collection to approximately 100 ps. We extend the experimental time window to 2 nanoseconds and recover the entire solvation response. These results combined with steady-state spectra and reorientation dynamics indicate that C343 exists in two distinct populations within the reverse micelles which correlate with interfacial and core water. Our results suggest that translational motion of C343 may contribute to the total observed solvation response. We study excited state proton transfer (ESPT) of HPTS (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt) in cationic (cetyltrimethylammonium bromide, CTAB), anionic (AOT), and nonionic (polyoxyethylene (5) isooctylphenyl ether, IGE) reverse micelles by BFTA. For larger AOT RM, ESPT dynamics are found to be approximately equal to the dynamics found in bulk water. As the size of the AOT RM approaches the size of the probe molecule, ESPT becomes increasingly quenched. For all sizes of CTAB RM, HPTS ESPT is found to be 10-20 times slower than HPTS ESPT in bulk water. This result combined with reorientation measurements suggest that HPTS resides at the interfacial region in CTAB RM and thus remains immobilized. In IGE RM, ESPT is 4-10 times slower than bulk water behavior which we contribute to immobilization of HPTS in the micelle interface. HPTS reorientational motion is hindered with respect to bulk HPTS motion. We measure the kinetic isotope effect (KIE) on HPTS ESPT dynamics and results suggest that the solvent plays a significant role in the observed dynamics only in the largest IGE reverse micelles. Steady-state absorption measurements show that HPTS exists in a unique environment within IGE RM which contrasts with HPTS in other nonionic reverse micelle systems.
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    ItemOpen Access
    Mesoscopic revelations: studying the shape of AOT reverse micelles
    (Colorado State University. Libraries, 2024) Gale, Christopher D., author; Levinger, Nancy E., advisor; Krummel, Amber, committee member; Prieto, Amy, committee member; Buchanan, Kristen, committee member
    Aerosol-OT (AOT) reverse micelles are a quintessential model system for studying nanoconfinement, creating consistent reverse micelles with a repeatable and very small size (~1-10 nm) using just 3 components. These reverse micelles have been used for studying the behavior of water and solutes in nanoconfinement, modeling the behavior of key solutes and proteins in a system more analogous to in vivo work, synthesizing nanoparticles, and even as a vehicle for suspending proteins in a low-viscosity solvent for high quality NMR experiments. Despite their usefulness, AOT reverse micelle's shape is poorly understood, but important to understanding behavior within a reverse micelle. Interfacial properties have been found to be key to many aspects of behavior within AOT reverse micelles and distance from the interface as well as the actual amount of interface present are highly dependent on shape. Therefore, the study of shape is key to a better understanding of AOT reverse micelles and behavior in nanonconfinement. In this work, I develop a series of metrics for shape--- coordinate-pair eccentricity (CPE), convexity, and the curvature distribution--- and apply them to several simulations of AOT reverse micelles. The simulations were designed to test the impact of the force field on the shape and behavior of the reverse micelles, including the first parameterization of AOT into the OPLS force field. The system was extensively checked to ensure equilibration was achieved and the system was not biased by the starting configuration. To aid in the shape analysis, I have developed a model and a formal proof to predict how the CPE changes for an arbitrary shape as it grows to model the shape behavior of general core-shell structures. Additionally, I measured the dipole moment of AOT, the rotational anisotropy decay of water, and several radial distribution functions to provide experimental verification where possible and further explore the behavior of the AOT reverse micelle system. Several key findings emerge from this work. Most notably, I find that AOT reverse micelles are significantly aspherical and non-convex over every force field tested, providing robust evidence that AOT reverse micelles are aspherical at any given moment in time. This provides strong evidence in support of the idea that experimental observations of spherical particles are the result of ensemble averaging. I also observe that the shape at the AOT/oil interface is comparatively more spherical with a "Goldilock's" value of convexity, neither too high nor too low, compared to the water/AOT interface. My model predicts that the CPE should fall with the addition of a shell, here provided by the AOT surfactant layer, suggesting this is largely the result of geometry. There is great variation between simulations and metrics in their dynamics, but in general, the shape appears to change on the order of 10 ns. This provides a useful method of deducing which values may or may not be impacted by shape, based on the time scale. For instance, it can reasonably be said that shape likely has no impact on water dynamics based on the roughly four orders of magnitude difference in the time scales of each process, which is supported by my own findings. Across all metrics studied, there are noticeable differences between simulations, but none of the differences are consistent. I believe this observation has important implications for both the behavior and simulation of AOT reverse micelles. First, this implies that the forces and interactions giving rise to different aspects of the reverse micelle are complex and largely independent, and that there is a disconnect between molecular-level measurements like radial distribution functions and and mesoscopic-level measurements like shape. Second, this implies that any simulation parameterized on one measure has no guarantee that it reproduces any other aspect of the reverse micelle accurately.
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    ItemOpen Access
    Oh cryoprotectant, wherefore art thou cryoprotectant? Investigation of the permeation of common cryoprotectants into live rice callus cells by coherent Raman microscopy
    (Colorado State University. Libraries, 2023) Samuels, Fionna M. D., author; Levinger, Nancy E., advisor; Van Orden, Alan, committee member; Crans, Debbie C., committee member; Wilson, Jesse W., committee member
    Conserving a diverse selection of plant species is vital as climate change begins shifting the planet's ecosystems in earnest. Many plants can be preserved through maintaining their seeds in cool, dark chambers, like that of the Svalbard Seed Vault, but others do not reliably reproduce through seeds. This can include wild plants that lack seeds entirely, ferns for example, or agricultural plants where clonal propagation is the only way to conserve desirable traits, grapes, hops and bananas for example. Rather than collecting and saving seeds from these plants, researchers collect tissue samples, cool and preserve them in liquid nitrogen then warm and regrow the plants years later. To maximize post-freezing viability, tissue samples are exposed to mixtures of cryoprotectants. Only a few cryoprotectant formulations have been used almost exclusively since their development in the early 1990s, including Plant Vitrification Solution 2 (PVS2) and Plant Vitrification Solution 3 (PVS3). Unfortunately, these formulations are not universally protective—some plant species respond extremely well to exposure while others are killed. When PVS2 or PVS3 fail to preserve a species, researchers must either empirically develop a new formulation or method of cryopreservation or settle for not conserving the species. A lack of mechanistic understanding of how these formulations work to protect tissue from extreme cold prevents straightforward development of new formulations. Since the development of PVS2 and PVS3, advances in instrumental techniques have opened the door to improved physical characterizations of the components of these mixtures. Vibrational microscopies, like Raman or infrared microscopy, allow the direct visualization of cryoprotectants interacting with living cells. By exciting vibrations unique to the bonds in a molecule, these techniques can effectively image nearly unadulterated molecules. Through deuteration, cryoprotectants can be imaged without disturbing other molecules in the cell. The work presented in this dissertation demonstrates how deuterated dimethyl sulfoxide (d6-DMSO), deuterated ethylene glycol (d6-ethylene glycol) and deuterated glycerol (d8-glcyerol) can be directly imaged inside living rice callus cells. Readers are first introduced to the cell system, rice callus cells, and the analytical technique, coherent anti-Stokes Raman scattering (CARS) microscopy. Then, they will learn about initial static experiments searching for the location of d6-DMSO within the callus cells. The next two chapters explore the real-time permeation of d6-DMSO, d6-ethylene glycol and d8-glcyerol individually and then solvated in PVS2. The final chapter describes future experiments I think should come from this work that will increase fundamental understanding of cryoprotectant-cell interactions and streamline the process of developing new cryoprotectant formulations.