published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The overall goal of this research is to characterize the potential-dependent structure and composition of the alcohol-Ag electrochemical interface. The approach involves the use of a variety of electrochemical and spectroscopic tools to arrive at a consistent model for a series of straight-chain alcohols (methanol, ethanol, 1-propanol, and 1-pentanol) at Ag electrodes. There are essentially four areas of investigation presented in this dissertation. The first portion of this work has been directed at the development of charge coupled device detection in Raman spectroscopy so that many of the interface investigations could be performed. The advantages and limitations of these detectors in Raman spectroscopy are addressed. The second area involves the characterization of in-situ alcohol-Ag electrochemical interfaces using Raman spectroscopy and associated surface selection rules for the evaluation of solvent orientation and bonding. Since the series of alcohols offers a systematic variation in solvent properties, these studies provide substantial insight regarding some of the chemical interactions which can dictate orientation. The development of emersed electrode technologies is also presented as a means to improve selectivity for surface molecular species over bulk molecular species. The utility of this approach is demonstrated for a variety of straight-chain alcohols at both rough and smooth Ag electrodes. Conditions for emersing the molecular interface, intact, under potential control are presented. Finally, double layer capacitance measurements are performed to offer additional insight regarding alcohol solvent structure and interfacial composition as a function of electrode potential. In addition, capacitance-potential plots are used along with the Hurwitz-Parsons analysis to determine absolute surface coverage of Br⁻ as a function of electrode potential. These results are correlated with the Raman spectroscopic results to obtain a consistent model for the structure and composition of the alcohol-Ag electrochemical interface.

Electrokinetics based microfluidic systems are potentially promising for lab-on-a-chip applications due to their effectiveness in manipulating nanoscale and biological objects, label-free operation, simple fabrication processes, small voltage requirements, and most importantly simple system integration strategy. Among various electrokinetics techniques, AC electrothermal flow (ACEF) is the most promising technique in microfluidic manipulation toward biomedical applications due to its effectiveness in high conductivity biological and physiological fluids. As relatively little is known about the ACEF induced fluid motion at highly conductive samples, the characteristics of electrothermal manipulation of fluid samples with different conductivities were investigated systematically. For low conductivity sample (below 1 S/m), the characteristics of the electrothermal fluid motion was in quantitative agreement with the theory. For high conductivity samples (greater than 1 S/m), the fluid motion appeared to deviate from the model as a result of electrochemical reactions and the temperature effect. Here, a universal electrode approach which directly implements ACEF-induced sample preparation on a SAM based electrochemical sensor for point-of-care diagnostics of urinary tract infections has also been demonstrated. Using uropathogenic E. coli clinical isolates as model systems, we demonstrate that "on-chip" ACEF-induced sample preparation can improve the sensor performance without complicated system integration strategy and presents a pathway for implementing truly lab on a chip, instead of chip in a lab. Finally an integrated chip approach has been proposed for transforming electrochemical sensing system from laboratory research into point-of-care diagnostics with multiple microelectrodes.

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The purpose of this study is evaluating a new water treatment technology; electrochemical technique and see if the technique is an alternative for municipal wastewater treatment. The aim of the project was also to test the electroflotation by purification of phosphorus from synthetic wastewater and separation of green algae from the culture medium so the technology can be evaluated. To answer the purpose, two different experiments were made at laboratory. In the first experiment, wastewater was purified from phosphorus with electroflotation. In the second experiment, green algae were separated with the same technology, but with different machine. The results of the study show that the electrochemical technology is a good technique to separate green algae from the culture medium. In the tests the removal efficiency was over 90 %. Results from the phosphorus purification did not go so well, because sources of error in the system. To summarize the results of the project the electrochemical technology is a good alternative to other wastewater treatments because it’s more environmental friendly and easy to operate.

This thesis discusses diverse electrochemical strategies for the determination of the concentration of the gases hydrogen sulfide, ammonia and halothane. The chemical tagging of sulfide by a variety of structurally diverse substituted benzoquinone species was studied over a wide range of pH (2<pH<10). Each derivative was found to respond to increasing concentration of sulfide (typically over a range 10-200 μM). The electrochemically initiated reaction of N,N-diethyl-pphenylenediamine (DEPD) with sulfide in N,N-dimethylformamide (DMF) was next examined with quantitative detection of sulfide (linear range= 28-3290 μM, LoD= 22 μM) achieved by analysis of the increase in the second oxidation wave. This is consistent with the sulfide attacking the doubly oxidised species in a 1,4-Michael addition. The direct oxidation of sulfide at a nickel hydroxide film on a nickel electrode in alkaline solution has provided the basis for the design of a simple and inexpensive sensor for monitoring H₂S in the range 20-200 μM. More sensitive (LoD= 1 (μM) amperometric detection of sulfide was obtained at modified nickel electrodes in acidic media in which sulfide was stripped from the nickel oxide layer. This approach was exploited further by using nickel modified screen printed carbon (Ni-SPC) electrodes as economical and disposable sensors for sulfide. Next, two different strategies for determining gaseous ammonia in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluromethylsulfonyl)imide, [EMIM][N(Tf)₂], and in DMF are described. The first approach exploits the effect of ammonia as a proton acceptor species on the anodic oxidation of hydroquinone, resulting in a linear detection range from 10 to 95 ppm ammonia (LoD= 4.2 ppm). The second approach is based on the direct oxidation of ammonia in either DMF or [EMIM][N(Tf)₂]. The possibility of photochemically induced electrocatalytic processes within microdroplets containing p-chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone, TCBQ) was examined as a means of detecting the anaesthetic gas halothane.</p> Finally, two of the more promising routes for sulfide detection were studied at elevated temperatures (up to 70 °C) with a view to developing H₂S sensors capable of meeting the demands of oilfield applications.

The separation of oxidation-reduction reactions into individual half-cells with a resulting "mixed potential" is well known as a dissolution mechanism for metals; however, the mechanism by which non-conducting crystals lose ions to the solution has been studied only slightly.

O câncer colorretal é uma das malignidades com maior incidência no Brasil e, na maioria dos casos, tem poucos sintomas visíveis até a doença estar em um estágio avançado. Pensando em simplificar o procedimento do exame e fazer um diagnóstico mais rápido para pacientes com câncer colorretal, este estudo teve como objetivo desenvolver um processo de diagnóstico simplificado baseado na caracterização de um biossensor eletroquímico em que fitas simples de miRNAs de interesse foram imobilizadas na superfície de eletrodos de ouro com objetivo de fazer a detecção de sua fita complementar utilizando-se dos métodos de espectroscopia de impedância eletroquímica e microbalança de cristal de quartzo. A sequência que foi imobilizada foi uma sequência de DNA que corresponde ao miRNA-224, cuja elevada expressão foi recentemente correlacionada com a agressividade e a incidência de metástase em pacientes com câncer colorretal. Os dados obtidos através da técnica de EIS foram comparados a dois modelos físicos de circuito equivalente, o circuito Randles clássico e um circuito Randles modificado, onde o capacitor foi substituído por um elemento de fase constante, tentando modelar de maneira mais eficiente a dupla camada que se forma na região de contado do eletrodo com a solução eletrolítica. O modelo modificado apresentou uma janela de funcionamento de 40,11% enquanto o circuito original apresentou uma janela de funcionamento de 23,98%; o que juntamente com o embasamento teórico do modelo de dupla camada fez com que o circuito modificado tenha sido escolhido para modelar os dados dos experimentos feitos na sequência. Tendo em vista que a imobilização das fitas simples de DNA na superfície do eletrodo de ouro é o passo chave para a obtenção de uma resposta satisfatória no biossensor, foram feitos testes alterando a proporção de DNA e espaçadores na fase de funcionalização a fim de determinar qual a proporção que otimiza a resposta do sensor. Comparando os resultados obtidos para as proporções de ssDNA:tiol total na etapa de funcionalização do eletrodo de 1:3, 1:4 e 1:5, a proporção 1:4 que corresponde a 20% de amostra no buffer de imobilização foi a que levou a uma maior variação percentual dos valores de Rct devido às hibridizações. Considerando esses como sendo os parâmetros otimizados para o sistema, foi construída então uma curva de caracterização do sensor, e o limite de detecção encontrado foi de 10pM; um valor considerado alto comparado com outros experimentos semelhantes, entretanto deve-se lembrar que o sistema aqui estudado não fez o uso de elementos amplificadores, como nanopartículas ou intercaladores de DNA. Para fins de comparação entre técnicas, foi conduzida uma análise semelhante utilizando uma microbalança de cristal de quartzo. Inicialmente foram utilizados os parâmetros encontrados como otimizados para a EIS, e posteriormente foram usados parâmetros mais próximos aos otimizados para essa técnica encontrados na literatura. Em ambos os casos foi possível observar variações referentes à hibridização das fitas complementares, entretanto como esperado, a resposta se mostrou mais expressiva para o segundo caso. Essas diferenças de resposta puderam ser relacionadas aos diferentes princípios de funcionamento dos sensores nas duas técnicas, mostrando que para a QCM o fator que influencia na resposta está relacionado à quantidade total de amostra imobilizada na superfície do eletrodo, enquanto que para a técnica de EIS o mais relevante é o quão compacta a camada se mostra. / Colorectal cancer is one of the most prevalent malignancies in Brazil and, in most cases, has few visible symptoms until the disease is at an advanced stage. In order to simplify the examination procedure and to make a faster diagnosis for patients with colorectal cancer, this study aimed to develop a simplified diagnostic process based on the characterization of an electrochemical biosensor in which single strands of miRNAs of interest were immobilized on the surface of gold electrodes with the objective of detecting their complementary strand using the methods of electrochemical impedance spectroscopy and quartz crystal microbalance. The sequence that was immobilized was a DNA sequence corresponding to miRNA-224, whose high expression was recently correlated with the aggressiveness and incidence of metastasis in patients with colorectal cancer. The data obtained through the EIS technique were compared to two equivalent circuit models, the classic Randles circuit and a modified Randles circuit, where the capacitor was replaced by a constant phase element, trying to more efficiently model the double layer that is formed in the interface between the electrode and the electrolytic solution. The modified model presented an operating range of 40.11% while the original circuit presented an operating range of 23.98%; which along with the theoretical understanding of the double-layer model made the modified circuit to be chosen as the model to fit the data of the experiments realized in the sequence. Considering that the immobilization of the simple DNA strands on the surface of the gold electrode is the key step to obtain a satisfactory response in the biosensor, tests were performed by altering the ratio of DNA and spacers in the functionalization phase in order to determine which the ratio that optimizes the sensor response. Comparing the results obtained for the proportions of ssDNA:total thiol in the functionalization stage of the electrode of 1:3, 1:4 and 1:5, the ratio 1:4 corresponding to 20% of sample in the immobilization buffer was the one that led to a greater percentage variation of Rct values due to hybridizations. Considering these as the optimized parameters for the system, a sensor characterization curve was built, and the limit of detection found was 10pM; a value considered high compared to other similar experiments, however it must be remembered that the system studied here did not make use of amplifying elements, such as nanoparticles or DNA intercalators. For purposes of comparing techniques, a similar analysis was conducted using a quartz crystal microbalance. Initially, the parameters found as optimized for the SIS were used, and later parameters that were closer to those optimized for this technique found in the literature were. In both cases it was possible to observe variations related to the hybridization of the complementary tapes, however, as expected, the response was more expressive for the second case. These differences in optimization parameters for the two techniques could be related to the different principles of sensor operation, showing that for the QCM the factor that influences the response is related to the total amount of sample immobilized on the surface of the electrode, while for the EIS technique the most relevant parameter is how compact the layer is.

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, in fulfillment of the requirements for the degree of Mastel' of Science in Engineering Johannesburg 1998 / Impala's Minerals Processing Plant in the Rustenburg Area, South Africa, uses flotation to beneficiate precious metal bearing ores from the Bushveld Complex. Pyrrhotite is one of the sulphide minerals that is targeted but it is the least amenable to current flotation conditions having the lowest recovery. Electrochemical techniques (mixed potential measurements, cyclic voltammetry and current transient techniques) were used to study the relevant reactions on the surface of pyrrhotite mineral electrodes. Aspects investigated included the oxidation of the mineral in aqueous alkaline solutions, activation by copper sulphate, kinetics of oxygen reduction and the adsorption of isobutyl xanthate. Mixed potential measurements of mineral electrodes were taken in batch flotation test work. In addition a novel qualitative measure of hydrophobicity was investigated. The oxidised surface of pyrrhotite is likely to be covered with iron hydroxides and a sulphur rich sub-lattice. No direct evidence was found for the activation of pyrrhotite by copper sulphate in alkaline solutions. It was shown however that activation could be achieved in mildly acidic media and that the surface remained activated if subsequently exposed to alkaline conditions. When achieved under acidic conditions activation was observed to enhance the degree of interaction between the mineral and the xanthate collector. Also copper sulphate appeared to aid the formation of a more hydrophobic surface (as indicated by the hydrophobicity tests). Copper activation conducted in acidic media did not significantly enhance the kinetics of oxygen reduction, a reaction seen as crucial to the adsorption of xanthate. No evidence was found for the initial chemisorption of xanthate onto the mineral surface. However evidence was found for the oxidation of xanthate to dixanthogen at sufficiently anodic potentials. It Was concluded that the relatively poor flotation performance of pyrrhotite could be combated by minimising the extent of the oxidation, adding reagents as soon as possible before the mineral becomes extensively oxidised and by removing surface hydroxides through lowering the pH during conditioning. / MT2017

<p>The demand for heat pumping technologies is expected to see tremendous growth over the next century. Traditional vapor compression cycles are approaching practical limits of efficiency and running out of possibilities for environmentally friendly and safe refrigerants. As a result, there is an increasing interest in pursuing non-vapor compression technologies that can achieve higher efficiencies with alternative working fluids. The chemical looping heat pump (CLHP) investigated here utilizes a chemical reaction to alternate a working fluid between more and less volatiles states. This allows the main compression to take place in the liquid phase and enables the utilization of a range of different working fluids that would not be appropriate for vapor compression technology. </p> <p> </p> <p>Thermodynamic models were developed to assess the potential performance of a chemical looping heat pump driven by electrochemical cells. A number of potential working fluids were identified and used to model the system. The thermodynamic models indicated that the chemical looping heat pump has the potential to provide 20% higher COPs than conventional vapor compression systems. </p> <p> </p> <p>An experimental test stand was developed to investigate the efficiency with which the electrochemical reactions could be performed. The working fluids selected were isopropanol and acetone for reasons of performance and availability. The test stand was designed to measure not only the power consumed to perform the conversion reaction but also the concentration of products formed after the reaction. The experimental tests showed that it was possible to perform the reactions at the voltages required for an efficient chemical looping heat pump. However, the tests also showed that the reactions proceed much slower than expected. To increase the rates of the reactions, an optimization effort on the membrane and catalyst selections was performed. </p> <p> </p> <p>Traditional catalyst materials used by solid polymer electrochemical cells, like those used in the testing, perform best in hydrated environments. The fluids isopropanol and acetone tend to displace water in the membranes, reducing the system conductivity. Multiple membrane types were explored for anhydrous operation. Reinforced sPEEK membranes were found to be the most suitable choice for compatibility with the CLHP working fluids. Multiple catalyst mixtures were also tested in the experimental setup. Density functional theory was used to develop a computational framework to develop activity maps which could predict the performance of catalyst materials based on calculated parameters. </p> <p> </p> <p>A detailed model of the CLHP electrochemical cell was developed. Built on open-source tools, the model was designed to determine the charge, mass, and heat transfers within the cell. The conversion of reactants along the channel of the cell as well as overall power consumption are predicted by the model. The model was validated against measurements and used to determine parameters for a CLHP cell that would have improved conversion performance and energy efficiency compared with the tested cell. </p> <p> </p> <p>The cell model was integrated into an overall system model which incorporates the effect of concentration changes throughout the entire cycle. Compared to the early-stage thermodynamic modeling, consideration of incomplete reactions provided more accurate predictions of the potential performance of CLHP systems. Different cell and system architectures were investigated to boost system performance. The model predictions demonstrated that the CLHP has the potential to provide high heat pumping efficiencies, but more work is still needed to improve the energy density of the system. </p>