Browsing by Subject "Electron transfer"

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    Electrical current generation by wild type and mutant Shewanella strains
    (2009-12) Baron, Daniel Benjamin
    The genus Shewanella has been reported to have the capacity to couple the transfer of electrons to insoluble metal oxides and solid carbon electrodes with cellular growth. While this process may be useful as an energy generation strategy or biotechnological tool, the electron transfer pathway by which this process occurs is not completely understood, and better techniques for studying the transfer mechanism are needed. This project used single chamber electrochemical cells to show that the current generation capabilities of Shewanella oneidensis are dependent on the ability of cells comprising a biofilm to shuttle soluble electron carriers between an electrode and its outer membrane cytochromes. The extracellular electron transfer capabilities of S. oneidensis mutant strains containing deleted or transposon-interrupted copies of genes known to be involved in extracellular electron transfer were also studied and compared to wild type. Amperometry was utilized to monitor real-time electron flow between attached anaerobic wild type and mutant cells and a poised carbon working electrode. Differential pulse voltammetry and cyclic voltammetry performed on electrode attached S. oneidensis MR-1 wild type cells detected both mediated and direct electron transfer reactions at the electrode surface. Ion exchange HPLC verified the presence of endogenously produced flavin compounds in S. oneidensis liquid cultures and confirmed the most common flavin in S. oneidensis MR-1 electrochemical cell analytes is riboflavin (vitamin B2). It was also discovered that removing the medium surrounding an electrode biofilm caused current production from the electrochemical cell to decrease. Returning the filtered original medium, or adding anaerobic riboflavin resulted in the restoration of current production. The amount of current produced at the carbon working electrode increased with biofilm development and accumulation of soluble electron mediator. A correlation was observed between the concentration of the redox shuttle in potentiostat-controlled electrochemical cells and the maximum sustainable current, as well as maximum electrode biofilm thickness. For example, wild type cultures with twice the natural amount of riboflavin approximately doubled their electrode current production and also attached to the electrode in larger numbers. Electrode phenotypes of Shewanella oneidensis MR-1 mutant strains were also observed and compared to wild type. The deletion of several genes, such as for the outer membrane cytochrome MtrC, the periplasmic cytochrome MtrA, or the membrane beta barrel protein MtrB severely impaired MR-1 cells from attaching to the carbon electrode. As a result, the deletion mutant strains were incapable of producing significant anodic current and were deficient in electrode attached biomass. However, deletion of the outer membrane cytochrome OmcA, or genes related to the formation of mature biofilms resulted in a percentage of the current production being retained. This data supports the theory that MtrC is a key component in the terminal electron transfer step for S. oneidensis MR-1 cells interacting with solid surfaces. Measurements of current production from MR-1 electrode biofilms revealed that the extracellular electron transfer process involves both cell associated enzymes and flavins acting as soluble electron transfer agents. However, these separate pathways most likely utilize many of the same, membrane proteins to accomplish their function. This study indicates it is likely that the deposition of electrons by S. oneidensis MR-1 to a poised electrode can be done both directly by MtrC while stabilized or otherwise assisted by other outer membrane elements such as OmcA or MtrB, as wells as through cycling of redox active shuttles such as flavins between MtrC and the electrode surface. As a result, for electrode attached Shewanella oneidensis MR-1 cells, a complex relationship exists between soluble flavin concentration, biofilm thickness, and electrical current production. These factors pertain greatly to the capabilities and limitations of S. oneidensis, especially while functioning as part of a electrochemical device, and must be taken into account when utilizing this organism for research or other applications.
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    Hot electron dynamics at semiconductor surfaces: implications for quantum dot photovoltaics.
    (2010-07) Tisdale, William A.
    Finding a viable supply of clean, renewable energy is one of the most daunting challenges facing the world today. Solar cells have had limited impact in meeting this challenge because of their high cost and low power conversion efficiencies. Semiconductor nanocrystals, or quantum dots, are promising materials for use in novel solar cells because they can be processed with potentially inexpensive solution-based techniques and because they are predicted to have novel optoelectronic properties that could enable the realization of ultra-efficient solar power converters. However, there is a lack of fundamental understanding regarding the behavior of highly-excited, or "hot," charge carriers near quantum-dot and semiconductor interfaces, which is of paramount importance to the rational design of high-efficiency devices. The elucidation of these ultrafast hot electron dynamics is the central aim of this Dissertation. I present a theoretical framework for treating the electronic interactions between quantum dots and bulk semiconductor surfaces and propose a novel experimental technique, time-resolved surface second harmonic generation (TR-SHG), for probing these interactions. I then describe a series of experimental investigations into hot electron dynamics in specific quantum-dot/semiconductor systems. A two-photon photoelectron spectroscopy (2PPE) study of the technologically-relevant ZnO(10-10) surface reveals ultrafast (sub-30fs) cooling of hot electrons in the bulk conduction band, which is due to strong electron-phonon coupling in this highly polar material. The presence of a continuum of defect states near the conduction band edge results in Fermi-level pinning and upward (n-type) band-bending at the (10-10) surface and provides an alternate route for electronic relaxation. In monolayer films of colloidal PbSe quantum dots, chemical treatment with either hydrazine or 1,2-ethanedithiol results in strong and tunable electronic coupling between neighboring quantum dots. A TR-SHG study of these electronically-coupled quantum-dot films reveals temperature-activated cooling of hot charge carriers and coherent excitation of a previously-unidentified surface optical phonon. Finally, I report the first experimental observation of ultrafast electron transfer from the higher excited states of a colloidal quantum dot (PbSe) to delocalized conduction band states of a widely-used electron acceptor (TiO2). The electric field resulting from ultrafast (<50fs) separation of charge carriers across the PbSe/TiO2(110) interface excites coherent vibration of the TiO2 surface atoms, whose collective motions can be followed in real time.
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    Redox Potential Controls Electron Transfer Through The Inner Membrane Of Geobacter Sulfurreducens
    (2021-08) Joshi, Komal
    Harnessing energy for growth and survival is universal to all living forms. Bacteria are constantly faced with changing environment forcing them to quickly adapt to the conditions to gain maximum energy available. Acquisition of energy involves transfer of electrons from substrate that gets oxidized to the reduction of electron acceptors. Microorganisms performing extracellular electron transfer have evolved to couple oxidation of electron donors to the reduction of electron acceptors present outside the cell using a chain of redox active proteins. Geobacter sulfurreducens is one such model organism for studying extracellular electron transfer, providing unique opportunities for the development of bioelectronic devices and sensors. Despite the usefulness of G. sulfurreducens extracellular electron transfer ability in biotechnological applications, the complete electron transfer pathway still remains unknown. The factors regulating the electron transfer between different cytochromes, as well as the specific utilization of different cytochromes in energy conservation is one of the lesser studied aspects of G. sulfurreducens physiology. The work presented in this thesis includes discovery and characterization of an inner membrane cytochrome complex, CbcBA essential for respiration of electron acceptors near the thermodynamic limit of acetate respiration (< -0.21 V vs. Standard Hydrogen Electrode (SHE)). A \sigma^{54}–dependent transcription factor, BccR controlling the expression of CbcBA was also characterized. Other inner membrane cytochromes involved in redox dependent electron transfer, ImcH, and CbcL are constitutively expressed. Using genetic and electrochemical approaches, CbcL was found to function as a redox dependent switch showing oxidative inactivation above redox potentials of -0.1 V vs. SHE. Using specific mutants lacking one or more inner membrane cytochromes, cellular yields were measured corroborating earlier reported data that the ImcH-dependent electron transfer pathway supported the highest cellular yield, while the CbcL-dependent pathway supported much lower cell yields. The CbcBA-dependent pathway could not support growth under conditions tested, but was found to be needed for survival under low electron acceptor conditions. Expressing fluorescent proteins in specific inner membrane cytochrome mutants allowed studying metabolic heterogeneity of G. sulfurreducens biofilms visualized using confocal microscopy. At high redox potentials (+0.24 V vs. SHE), G. sulfurreducens utilizes ImcH-dependent pathway in cells closest to the electrode, and CbcL-dependent pathway in cells beyond 10 µm from the electrode surface. At low redox potentials (-0.13 V vs. SHE), only the CbcL-dependent pathway is utilized. The findings reported in this thesis, suggests a route for building biosensors for redox sensing.