abstract: Oxidoreductases catalyze transformations important in both bioenergetics and microbial technologies. Nonetheless, questions remain about how to tune them to modulate properties such as preference for catalysis in the oxidative or reductive direction, the potential range of activity, or coupling of multiple reactions. Using protein film electrochemistry, the features that control these properties are defined by comparing the activities of five [FeFe]-hydrogenases and two thiosulfate reductases. Although [FeFe]-hydrogenases are largely described as hydrogen evolution catalysts, the catalytic bias of [FeFe]-hydrogenases, i.e. the ratio of maximal reductive to oxidative activities, spans more than six orders of magnitude. At one extreme, two [FeFe]-hdyrogenases, Clostridium pasteuriaunum HydAII and Clostridium symbiosum HydY, are far more active for hydrogen oxidation than hydrogen evolution. On the other extreme, Clostridium pasteurianum HydAI and Clostridium acetobutylicum HydA1 have a neutral bias, in which both proton reduction and hydrogen oxidation are efficient. By investigating a collection of site-directed mutants, it is shown that the catalytic bias of [FeFe]-hydrogenases is not trivially correlated with the identities of residues in the primary or secondary coordination sphere. On the other hand, the catalytic bias of Clostridium acetobutylicum HydAI can be modulated via mutation of an amino acid residue coordinating the terminal [FeS] cluster. Simulations suggest that this change in catalytic bias may be linked to the reduction potential of the cluster. Two of the enzymes examined in this work, Clostridium pasteurianum HydAIII and Clostridium symbiosum HydY, display novel catalytic properties. HydY is exclusively a hydrogen oxidizing catalyst, and it couples this activity to peroxide reduction activity at a rubrerythrin center in the same enzyme. On the other hand, CpIII operates only in a narrow potential window, inactivating at oxidizing potentials. This suggests it plays a novel physiological role that has not yet been identified. Finally, the electrocatalytic properties of Pyrobaculum aerophilum thiosulfate reductase with either Mo or W in the active site are compared. In both cases, the onset of catalysis corresponds to reduction of the active site. Overall, the Mo enzyme is more active, and reduces thiosulfate with less overpotential. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2020

Battery research has been going full steam and with that the search for alternative anodes. Among many proposed electrode materials, little attention has been given to phosphorus. Phosphorus boasts the third highest gravimetric charge capacity and the highest volumetric charge capacity of all elements. Because of that, it would be an attractive battery anode material were it not for its poor cyclability with significant capacity loss immediately after the first cycle. This is known to be the consequence of considerable volume changes of phosphorus during charge/discharge cycles. In this work, we propose circumventing this issue by mixing amorphous red phosphorus with carbon nanotubes. By employing a non-destructive sublimation-deposition method, we have synthesized composites where the synergetic effect between phosphorus and carbon nanotubes allow for an improvement in the electrochemical performance of battery anodes. In fact, it has been shown that carbon nanotubes can act as an effective buffer to phosphorus volumetric expansions and contractions during charging and discharging of the half-cells [1]. By modifying the synthesis parameters, we have also been able to change the degree of crystallinity of the phosphorus matrix in the composites. In fact, the less common phase of red phosphorus, named fibrous phosphorus, was obtained, and that explains some of the varying electrochemical performances observed in the composites. Overall, it is found that a higher surface area of amorphous phosphorus allows for a better anode material when using single-walled carbon nanotubes as fillers.

The stability constants for the ligand 1-hydroxyl-3-aminopropilydene diphosphonic acid (APD) or Pamidronate with metal ions CdII, PbII and ZnII were established in this work by sampled direct current polarography (DCTAST). Due to precipitation of the metal-ligand complexes in the pH range about 4.0 to 5.0 at typical glass electrode potentiometric conditions, these systems could not be studied by glass electrode potentiometry (GEP). The concept of Virtual Potentiometry (VP) was used in the modelling of the metal-ligand system and refinement of stability constants to evaluate further the metal-ligand models derived from DCTAST. Virtual potentiometry uses virtual potentials to refine polarographic data by employing dedicated potentiometric software, ESTA. The structure of the metal complexes determined in this work is also proposed and compared to the reported crystal structures of the metal complexes of the ligand APD. The Linear Free Energy Relationship, LFER (log KML′ vs. log KM(OH) ) for the ligand APD is derived here for the first time using the log KML′ values from literature as well as the values for CdII, PbII and ZnII determined in this work. The log KML′ values of 153SmIII–APD and 166HoIII–APD, which cannot be determined by these two techniques (GEP and DCTAST), were predicted in this work using the LFER methodology. / Dissertation (MSc (Chemistry))--University of Pretoria, 2006. / Chemistry / MSc / Unrestricted

This thesis presents findings of experiments conducted to determine the most feasible method to electroplate metal.

Understanding the structure and kinetics of electrochemical oxidation of platinum single-crystal surfaces at the atomic level is important to understanding and improving the electrocatalysis of platinum in fuel cells. This thesis concerns the analysis of data from combined electrochemical and surface X-ray diffraction (SXRD) experiments on Pt(111) and Pt(100). In the early stages of oxidation, Pt atoms are extracted from their metal lattice sites and become part of a metal oxide. Their locations have been measured by collaborators using SXRD. The corresponding charges measured electrochemically are determined by integration of cyclic voltammetry, potential step and potential sweep-hold experiments, and used to propose reactions that are occurring in the oxidation. The measured charge consists of the wanted charge that passes in the electrochemical reactions and a capacitive charge associated with charging the electrical double-layer. Different ways of subtracting the double-layer charge were investigated. Reactions were proposed for optimally corrected charges, and for the worst case where no correction is made, in order to determine the reliability of the proposed reactions. From cyclic voltammetry experiments on Pt(111) surfaces, during oxidation 0.5 ML adsorbed oxygen and a few extracted PtO (less than 0.1 ML) are formed. Without baseline correction the data are consistent with 0.5 ML Oads and a few extracted PtO2. After fast scans, the Pt(111) surface is restructured as shown by the X-ray signal, but the oxide peaks in CV are nearly unchanged, which suggests that the electron transfer and Pt extraction do not need to be tightly coupled. From CV experiments on Pt(100) surfaces, during oxidation many more extracted Pt are formed than on Pt(111). These form long chains with PtO2 units (0.25 to 0.39 ML) together with Oads and a total coverage of 0.5 ML. If CV isn't baseline corrected, the data is not consistent with long chains with PtO2 units, but short Pt3O8 chains with equal numbers of independent PtO2 groups. From potential step experiments on Pt(100), there is an expectation from the literature that the charge will grow linearly with log t. However, this relationship was only found during the first second of each step, and then is unchanged after ~1 s. The slopes of the logarithmic plots are linearly related to potential. From sweep hold experiments on Pt(100), both charges and coverage of extracted Pt atoms are not very linear vs log t, and their slopes are also not linearly related to potential but change sharply during the potential range of Pt extraction. / Graduate

This thesis aimed to study some of the general principles underlying electrodeposition together with experimental facts regarding the effects of changing constituents of the plating solutions, variations in hydrogen-ion concentrations, and variation in current density used in the electrodeposition of lead.

Early detection is crucial in delivering timely treatment and improving patient outcomes. Point-of-care (POC) biosensors play an essential role in early detection, allowing for rapid and accurate diagnosis of diseases at the patient’s bedside without the need for expensive equipment or specialized personnel. By performing the analysis on-site, POC diagnostics can offer continuous monitoring and real-time data acquisition of a patient's health status. Thus, there is strong incentive in creating POC biosensors to provide healthcare professionals with greater access to diagnostic information, ultimately improving outcomes and reducing healthcare costs. Herein, the development of a POC lab-in-a-tube biosensor that utilizes simple and scalable fabrication techniques is presented. Electrodes are patterned on low-cost plastic substrates, which can be subsequently rolled and heat-shrunk into miniaturized tubing for flow-through analysis of liquid samples. Heat-shrinking of the device results in 3-dimensional, hierarchically wrinkled electrodes with morphological feature that span several orders of magnitude in size. These wrinkled electrodes demonstrate dramatically increased surface area in a given footprint compared to traditional planar electrodes. Incorporation of modified gold and silver wires allows for sensitive and stable electrochemical detection, enabling fast and quantitative results. These devices are capable of millilitre-per-minute flow rates to allow for rapid sample processing and for increased mass-transport to the electrode surface. The ability to capture analytes was characterized with nucleic acid sequences using pump-driven and blood-collection tube induced flow for rapid and accurate detection. Overall, this work demonstrates the successful development of an electrochemical platform integrated into a plastic tubing capable of rapid detection of flowing analytes. With its ease-of-use and compatibility with a wide range of flow rates, the device has the potential to be incorporated with existing medical tubing and procedures to achieve POC diagnostics. / Thesis / Master of Applied Science (MASc) / Early detection is critical for timely treatment and better patient outcomes. Point-of-care (POC) biosensors allow for disease diagnosis and real-time monitoring of a patient's health status directly at the patient's bedside without the need for expensive equipment or specialized personnel. A lab-in-a-tube biosensor was developed using sensing surfaces on low-cost plastic that is rolled and heat-shrunk into miniaturized tubing for analysis of liquid samples. The wrinkled sensing surface that results from heat-shrinking dramatically increase surface area and interaction with the sample, enabling sensitive detection that is fast and quantitative. These devices are capable of capturing samples at high flow rates, allowing for rapid analysis of large samples. Overall, this work demonstrates the successful creation of a biosensor platform that could be incorporated with existing medical tubing for POC diagnostics.

The importance of addressing environmental issues such as lowering the emission of greenhouse gases, preventing eutrophication of aquatic life due to high concetrations of nitrogen oxyanions, and producing sustainable energy is at an all-time high. With a concentration on employing earth abundant metals, a Mn(I) complex was synthesized with an integrated photocatalyst in an attempt at developing a new electrophotocatalytic system. Additionally, a Ni(II) pincer complex was synthesized and structurally and computationally characterized as well as observed to perform electrocatalytic hydrogen generation in water. Lastly, with inspiration from the Ni(II) pincer complex, two macrocyclic complexes, one with a Ni(II) center and the other with a Co(II) center, were structurally characterized and observed to electrocatalytically reduce aqueous nitrite at neutral pH. DFT calculations were also performed to elucidate the nitrite reduction reaction mechanism.

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No description available.