As reduções eletroquímicas dos complexos de ferro(II) FeL32+, com ligantes diimínicos alifáticos, L=CH3-N=C(R)-C-(R\')=N-CH3, onde R,R\' = H,H; H,CH3; CH3,CH3; e ligantes diimínicos mistos, L = C5H4N-C(R\')=N-(R\"), onde R\',R\"= H,CH3; CH3,CH3 foram estudadas através de polarografia e voltametria cíclica em acetonitrila em perclorato de tetraetilamônio 0,2M a 25,0ºC. Utilizam-se eletrodo plano de platina.ou eletrodo gotejante de mercúrio como eletrodos de trabalho para a voltametria cíclica e polarografia, respectivamente. Os eletrodos auxiliar e de referência são fio de platina e Ag/AgCl , respectivamente. Os polarogramas obtidos para esses complexos no intervalo de potenciais de 0,0 a -2,4 V vs Ag/AgCl mostram duas a quatro ondas de redução. As duas primeiras etapas são controladas por difusão e os processos de eletrodo podem ser descritos como monoeletrônicos e reversíveis, com a estabilização dos baixos estados de oxidação Fe(I) e F:(0) em acetonitrila. Para o derivado R,R\' = H,CH3, observam-se três ondas reversíveis e monoeletrônicas indicando a estabilização do complexo com ferro no estado de oxidação formal (-I).Comportamento semelhante foi encontrado para complexos de ferro(II) com 2,2\'-dipiridina e 1,10 fenantrolina (. Electrochim. Acta. 13. 335 (1968) ). A estabilização dos baixos estados de oxidação deve-se ao caráter aceptor de elétrons dosoligantes diimínicos, como indicado pelo espectro de transferência de carga e,depende da presença do grupo cromofórico. Verifica-se ainda que quanto maior o valor de 10 Dq, maior a retrodoação e, maior a estabilização dos baixos estados de oxidação. Os voltamogramas cíclicos apresentam dois a três picos de redução no intervalo de potenciais de 0,0 a - 2,2V vs Ag/AgCl. A primeira etapa de redução é bem caracterizada como processo monoe1etrônico e reversível Na redução dos derivados alifáticos R,R\' = H,H; CH3.CH3; há um grande aumento da corrente de pico e os potenciais são deslocados cerca de 0,18V para regiões mais negativas. Isso é interpretado em termos de adsorção do reagente na superfície do eletrodo de platina. É interessante notar que apenas os complexos. que apresentam substituintes simétricos adsorvem na superfície do eletrodo. / The electrochemical reduction of the iron(II) complexes, FeL32+ with aliphatic diimine ligands, CH3-N=C(R)-C(R\')=N-CH3, where R,R\'= H,H; H,CH3; CH3,CH3, and mixed diimine ligands. L = C5H4N-C(R\')=N(R\"), where R\',R\" = H, CH3; CH3,CH3, was studied by means of polarography, and cyclic voltammetry in acetonitrile containing 0,2M tetraethylammonium perchlorate at 25,0ºC. A platinum disk or a dropping mercury electrode were used as working e1ectrodes for the cyclic voltammetric and polarographic experiments, respectively. A platinum wire and Ag/AgCl were employed as auxiliar and reference electrodes, respectively. The polarograms obtained for these complexes in the 0.0 to -2,4 V vs Ag/AgCl potential range exhibit two to four reduction waves. The first two reduction waves were shown to correspond to reversible one electron reductions yielding stable complexes of iron in the formal oxidation states (I) and (O). For the derivative R\',R\" = H,CH3, three reversible one electron waves were found, indicating the stability of the complex with iron in the formal oxidation state (-I). A similar be havior has been found for the 2,2\'-dipyridine and 1,10-phenan -throline complexes of iron(II) (Electrochim. Acta,.13, 335(1968)). The stabilization of the low valence states is due to the strong acceptor properties of the diimine ligands. This acceptor character is reflected in the appearence of a characteristic intense inverse charge transfer band in the visible region. in the presence of the diimine chromophore. Increased stabilization of the low oxidation states is correlated with an increase in the magnitude of the ligand-field strength (10 Dq), i.e., increased back-donation. Two or three reduction peaks were observed in the cyclic voltammograms in the region of 0.0 to -2.2 V vs Ag/AgCl. The first reduction of the aliphatic derivatives R,R =\' H,H ; CH3, CH3, there is a large increase in peak currents and a shift of 0.18 V to more negative potentials. This is interpretable in terms of the platinum electrode, It is interesting to note that only the complexes which have symmetrical ligands exhibit adsorption at the electrode surface.

The majority of power generated worldwide is from combustion of fossil fuels. The sustainability and environmental impacts of this non renewable process are severe. Alternative fuels and power generation systems are needed, however, to cope with increasing energy demands. Ammonia shows promise for use in power generation, however it is costly to produce and very few methods of using it as a fuel are developed. To address the need for alternative methods of ammonia synthesis, this research designed and tested a bioelectrochemical device that generates NH3 through electrode induced enzyme catalysis. The ammonia generating device consists of an electrode modified with a polymer that contains whole cell Anabaena variabilis, a photosynthetic cyanobacterium. A. variabilis contains nitrogenase and nitrate/nitrite reductase, catalysts for the production of ammonia. In this system, the electrode supplies driving force and generates a reductive microenvironment near cells to facilitate enzymatic production of NH3 at ambient temperatures and pressures. Farm animal wastes contain significant amounts of NO2- and NO3-, which can leech into groundwater sources and contaminate them. The system described here recycles NO2- and NO3- to NH4sup+ by the nitrate/nitrite reductase enzyme. Unlike nitrogen fixation by the nitrogenase enzyme whose substrate is atmospheric N2, the substrates for nitrate/nitrite reductase are NO2- and NO3-. The ammonia produced by this system shows great potential as a crop fertilizer. While the substrates and enzymatic basis for ammonia production by nitrogenase and nitrate/nitrite reductase are very different, there is utility in the comparison of commercially produced ammonia by the Haber Bosch synthesis and by the bioelectrocatalytic device described here. In one day, the Haber Bosch process produces 1800 tons of NH3 at an energetic cost of $500/ton. Per ton of ammonia, the Haber Bosch process consumes 28 GJ of energy. The bioelectrocatalytic device produces 1 ton of NH3 for $10/ton, consuming only 0.04 GJ energy, which can be obtained by sunlight via installation of a photovoltaic device. Thus, the system presented here demonstrates ammonia production with significant impact to the economy. NH3 production by the bioelectrocatalytic is dependent upon A. var. cell density and electrode polarization. The faradaic current response from cyclic voltammetry is linearly related to cell density and ammonia production. Without electrode polarization, immobilized A. var. do not produce ammonia above the basal level of 2.8 ± 0.4 ΜM. Ten minutes after cycled potential is applied across the electrode, average ammonia output increases to 22 ± 8 ΜM depending on the mediator and substrate chemicals present. Ammonia is produced by this system at 25 °℃ and 1 atm. The electrochemical basis for enhanced NH3 by immobilized cyanobacteria is complex with multiple levels of feedback.

Philosophiae Doctor - PhD / Nitric oxide (NO) plays a key role as biological messenger in the biological system, however detection and quantification thereof has always posed significant problems. NOx is a principal constituent of air pollutants. There are seven oxides of nitrogen of which N2O, NO and NO2 are most important. NO is a free radical and reacts extremely fast with oxygen, peroxides and superoxides. It’s these reactions which are responsible for NO’s fleeting existence. The specific detection and quantification of NO still remains challenging. Most techniques rely on the measurements of secondary nitrite and nitrate species. Electrochemical techniques using ultra micro-electrode systems presented the possibility of direct detection of NO, offering a range of favourable characteristics; good selectivity towards NO, good sensitivity, fast response, long-term stability and ease of handling. Electrochemical detection of NO relies on the modification of electrode surfaces and exploiting the redox properties of NO. NO can either be oxidized or reduced electrochemically depending on the nature of the solution. Under cathodic current NO is reduced to nitrosyl, a highly unstable derivative of NO. These nitrosyls are subject to a serious of chemical reactions to eventually form nitrous oxide. Due to the interferences presented by the electrochemical reduction of NO, the electro oxidation of NO is therefore the methodology of choice for NO detection. The electrochemical oxidation of NO occurs at positive potentials around 800 mV vs. Ag/AgCl. However this potential range is not only favourable to NO oxidation but can lead to the oxidation of several other biological species. These interfering species are biologically present at concentrations higher than NO therefore selectivity is of the highest order when designing these electrode systems. Some nitric oxide sensors are limited in their sensitivity, stability and reproducibility. Direct electron transfer between redox proteins and conductive membrane layers has been scrutinized for years in an attempt to reproduce the mechanistic charge transfer processes for sensor application. However, literature reports have presented many arguments on the complexities associated with depositing these enzymes on electrode surfaces for the purpose of reproducing direct electron transfer at metalloprotein centres. The study sets out to design a material which could mimic the electrochemistry of denatured cytochrome-c. To achieve this it was imperative to design a polymer which could reproduce the electrochemistry of the ligands coordinated to the metal centre of the metalloprotein. A novel Schiff base was synthesized by cross-linking naphthalene to pyrrole to produce the monomer, N,N-bis((1H-pyrrol-2-yl)methylene)naphthalene-2,3-diamine). The monomer was electrodeposited on a screen print carbon electrode (SPCE) vs. Ag/AgCl and served as a supporting layer for denatured cytochrome-c. Cytochrome-c is classified as a metalloprotein. These metalloproteins possess metal centres which when denatured unfolds and allows access to the metal centre. Cytochrome-c was subjected to thermal denaturation which opened up the iron centre. The denatured metalloprotein was cross-linked to the ligand to reconstruct the heme centre environment. This was believed to facilitate the electrochemical activity of the system and allow for electrochemical analysis of these metalloproteins for sensor application. The redox behaviour of the sensors were modelled in phosphate buffer solution (PBS) with cyclic voltammetry. Electrochemical analysis reported the sensors to possess reversible electrochemistry with diffusion control characteristics. The sensor recorded a redox system in the negative potentials range. Following the establishment of the electrochemical profile of the sensor an attempt was made to produce a synthetic analogue of denatured cytochrome-c. Iron (II) was chelated to the monomer N,N-bis((1H-pyrrol-2-yl)methylene)naphthalene-2,3-diamine) to form an iron ligand complex. The complex was subjected to a series of characterization techniques which confirmed coordination to the metal centre. The iron ligand complex was electrodeposited on a SPCE over the potential window of -1 V and 1 V to model the electrochemical behaviour of the sensor. The material was found to be electroactive. Subsequent electrochemical analysis revealed the system to have electrochemical properties, analogous to that of the denatured cytochrome-c system. The sensor was applied in NO and NO2 studies and displayed an affinity towards NO. Based on extrapolated values it was postulated that the lower limit range for NO detection was in the range of 30 to 40 nM. The potentials recorded were lower than the reported oxidation potentials for nitric oxide. The sensor displayed stability and selectivity towards nitric oxide within a complex matrix. The complex matrix employed in this study was synthetic urine that was synthesised in the lab. The sensor displayed the capacity for linear range of NO detection with very low error margins. / 2021-09-01

In this thesis, cyclic biamperometry (CB) as a new method in electrochemistry, has been introduced and investigated. The hallmark of this method is the absence of a reference electrode which potentially allows simplification and miniaturization of the measurement apparatus. Similarities and differences of this method and cyclic voltammetry (CV) have been studied and it was shown that under conditions of using standard electrodes, CB has a better sensitivity and a lower detection limit than CV. A new equivalent circuit model for the cell has been proposed and parameters affecting the sensitivity of CB, such as keeping the concentration of one redox species in excess and having a larger W2 electrode, have been described. The redox cycling effect in biamperometric systems has been investigated and it is shown that improvements of at least two orders of magnitude in sensitivity can be achieved by using interdigitated electrodes (IDEs). In addition, an example for applications of this method, including biamperometric dead-stop titration of 1-naphthol with ferricyanide, has been presented and possible fields in which CB can be incorporated (e.g. monitoring the activity of alkaline phosphatase) have been illustrated. Finally, a few suggestions for future studies and further improvements have been outlined.

In this thesis, cyclic biamperometry (CB) as a new method in electrochemistry, has been introduced and investigated. The hallmark of this method is the absence of a reference electrode which potentially allows simplification and miniaturization of the measurement apparatus. Similarities and differences of this method and cyclic voltammetry (CV) have been studied and it was shown that under conditions of using standard electrodes, CB has a better sensitivity and a lower detection limit than CV. A new equivalent circuit model for the cell has been proposed and parameters affecting the sensitivity of CB, such as keeping the concentration of one redox species in excess and having a larger W2 electrode, have been described. The redox cycling effect in biamperometric systems has been investigated and it is shown that improvements of at least two orders of magnitude in sensitivity can be achieved by using interdigitated electrodes (IDEs). In addition, an example for applications of this method, including biamperometric dead-stop titration of 1-naphthol with ferricyanide, has been presented and possible fields in which CB can be incorporated (e.g. monitoring the activity of alkaline phosphatase) have been illustrated. Finally, a few suggestions for future studies and further improvements have been outlined.

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The microstructure and electrochemical behavior of boron doped and undoped ultra thin diamond film electrodes have been studied in this work. The ultra thin diamond films are deposited on porous silicon (PSi) by microwave plasma chemical vapor deposition (MPCVD). In order to enlarge the surface area of diamond electrodes, the deposition of nano structured diamond thin films is performed only in a short time deposition under a negative bias, so that diamond nuclei grew from the tips of PSi nano structures and the thin film surface remained rough and nano fine structured. Diamond thin films were analyzed by Raman spectroscopy and SEM, and then fabricated to the electrode device. From SEM analysis, the morphology of diamond thin films on PSi reveals in the shape of nano rods diamond crystallites. The electro-chemical response was evaluated by performing cyclic voltammetry in the inorganic K4[Fe(CN)6] and a K2HPO4 buffer solution. Boron doped diamond thin film on porous silicon has demonstrated a high redoxidation current of cyclic voltammetry, which may be due to the rough surface providing more electrochemical surface area and more sp2 conducting bonds exposed on the surface.

Unstable Sn(III) intermediates generated in the Sn(IV)/Sn(II) redox reaction in 2 M HBr + 4 M NaBr media were detected by scanning electrochemical microscopy (SECM) and cyclic voltammetry (CV). In CV, the underpotential deposition of Sn(0) and its stripping peaks severely perturbed the analysis of diffusional reactions. In SECM, however, the detection of diffusional Sn(III) bromide species was clearly observed due to the absence of the perturbation from the surface reactions. The ECEC-DISP mechanism in both the reduction and oxidation reactions was proposed via Sn(III) bromide intermediates. CVs at different concentrations of Sn(IV) and at various scan rates were fit by numerical simulations based on the proposed mechanism with good agreement. Enhanced electrochemical reversibility in the Sn(IV)/Sn(II) redox reaction was observed at the elevated temperature of 80 °C. We attributed such observation to changes in the rate of bromide loss from Sn(IV)Br₆²⁻ to Sn(IV)Br₅⁻ based on the CV simulation. In a similar approach, a short-lived intermediate, presumably bromine anion radical Br₂⁻·, was detected in the Br⁻ /Br₃⁻ electro-oxidation reaction in nitrobenzene solution by SECM and CV. The reaction mechanism was proposed based on a detected Br₂⁻· intermediate as follows: (1) the one electron transfer of Br⁻ to Br·, (2) the dimerization of 2Br· to Br₂, (3) the bromide addition reaction of Br₂ to Br₃⁻ , (4) the bromide addition reaction of Br· to Br₂⁻·, and (5) the Br· addition reaction of Br₂⁻· to Br₃⁻. The simulation based on the proposed mechanism fitted well with the experimental SECM and CV results. At last, the applicability of the Sn/Br system as electrolyte for electrochemical energy storage was tested. A redox flow battery was constructed, where the Sn(IV)/Sn(II) reduction was carried out on the negative electrode, while the Br· /Br₂ oxidation was carried out on the positive electrode during charging. Cyclability was tested up to 35 charge/discharge cycles, and 100 % coulombic efficiency was observed in all cycles. However, only 40 % of voltage efficiency was obtained, mainly due to the large irreversibility of the Sn(IV)/Sn(II) redox reaction in the bromide media.

>Magister Scientiae - MSc / The tuberculosis (TB) disease to this day remains one of the world’s prominent killerdiseases. Pyrazinamide (PZA) is one of the most commonly prescribed anti- tuberculosis (anti-TB) drugs due to its ability to significantly shorten the TB treatment period from the former nine months to the current six months duration. However, excess PZA in the body causes hepatotoxicity and damages the liver. This hepatotoxicity, together with the resistance of the bacteria to treatment drugs, poor medication and inappropriate dosing, greatly contribute to the high incidents of TB deaths and diseases that are due to side effects (such as liver damage). This brings about the calls for alternative methods for ensuring reliable dosing of the drug, which will be specific from person to person due to inter-individual differences in drug metabolism. A novel biosensor system for monitoring the metabolism of PZA was prepared with a Ni-PPI-PPy star copolymer and cytochrome P450 2E1 (CYP2E1) deposited onto a platinum electrode. The nanobiosensor system exhibited enhanced electro-activity that is attributed to the catalytic effect of the incorporated star copolymer. The biosensor had a sensitivity of 0.142 µA.nM-1, and a dynamic linear range (DLR) of 0.01 nM-0.12 nM (1.231 – 7.386 ng/L PZA). The limit of detection of the biosensor was found to be 0.00114 nM (0.14 ng/L) PZA. From the HPLC peakconcentration (Cmax) of PZA determined 2 h after drug intake is 2.79 – 3.22 ng.L-1,which is very detectable with the nanobiosensor as it falls within the dynamic linear range.

Masters of Science / South African dams and reservoirs are increasingly showing the propensity to support sustained populations of Cyanobacteria (blue green algae). These photosynthetic bacteria occur throughout the world and can rapidly form blooms in eutrophic water systems. The occurrence of these photosynthetic bacteria, in our dwindling drinking water source dams, poses a serious, economic, as well as a health, threat to and arid country like South Africa due to is potential to produce of toxic metabolites like Microcystins and Nodularins (MCN). MCN's are cyclic peptides toxins, harmful to humans and animals, and its toxicological mechanism is based on a strong inhibition of protein phosphatises in the liver. This may lead to severe liver damage and increased tumour development. Rural communities consuming untreated water in South Africa are most at risk due the high toxicity of MCN’s at low doses.We endeavour to develop an immunosensor for the detection of Microcystins and nodularins using anti-sheep IgG antibody labelled with horseradish peroxidase (HRP) immobilised on a modified glassy-carbon polymer surface. The immunosensor will be applied to water samples for MCN’s as a group of compounds recognised by the ADDA moiety common to all MCN congeners. The immunosensor will provide immediate confirmation and quantification of MCN’s in situ. A competitive Enzyme Linked Immuno-Sorbant Assay (ELISA) and High Performance liquid Chromatography (HPLC) will be used to validate results of our immunosensor. Elisa's are widely used as a screening test method for MCN's. The antibody-antigen specificity forms the bases for the recognition of target compound (MCN's) by antibodies which bind to a compound which is labelled with a colour indicator, and quantified by spectrophotometry.