This Thesis deals with the construction of an electrochemical cell with two types of integrated composite electrodes based on carbon films. Using these electrodes, the voltammetric behavior of environmental pollutant triclosan (5-chlor-2-(2,4- dichlorophenoxy)phenol) was studied. For the construction of composite electrodes, graphite and glassy carbon were used as conductive microparticles. Several types of polymers served as the nonconductive binder. For the measurements, combinations of graphite with polystyrene and graphite with polycarbonate were selected as optimum materials. From the dependence of peak heights on solution composition, a mixture of a buffer of pH 7 and methanol (1:9, V/V) was selected as the optimum medium. In this medium, concentration dependences were measured; calculated triclosan detection limit were 0,49 µmol dm-3 for carbon polystyrene composite electrode and 0,25 µmol dm-3 for carbon polycarbonate composite electrode, respectively. The possibility of further increasing the sensitivity of the determination by the accumulation step was studied. Accumulation of triclosan on carbon polystyrene composite electrode was observed, but the increase in response was connected with the loss of repeatability. Accumulation of triclosan on carbon polycarbonate composite electrode was...

Rare earth and actinide metals, critical to security, medicine, and the economy, have been processed through methods such as solvent extraction and electrorefining. To minimize radiological waste and improve yield, a 'chloride volatility' process--also known as the chlorination and volatilization process (CVP)--has been proposed and demonstrated for processing rare earths. The process takes advantage of the low vapor pressure of rare earth chlorides (<700 >°C), CaCl2 was added to LaCl3 to lower the melting temperature. LaCl3 electrochemical behavior has not previously been studied in CaCl2. Cyclic voltammetry (CV) and square wave voltammetry (SWV) were applied to LaCl3 salts in CaCl2-LiCl and CaCl2 to study the metal ion behavior. Various electrode materials were compared at low CV scan rates (s-1) to determine compatibility with chlorine gas evolution. Experiments of eutectic LaCl3-CaCl2 were performed and analyzed to determine the efficacy of chlorine gas generation via electrolysis for the CVP. Through galvanostatic electrolysis, oxidation of chloride ions and subsequent chlorination of rare earth was demonstrated, with cerium chosen as the representative rare earth metal. Through a quadrupole mass spectrometer plumbed in line with the electrolytic cell, the quality of the generated gas was analyzed.

Electrochemical biosensors are renowned for their ability to detect a wide range of analytes in biological fluids for clinical diagnosis. The implementation of biomarkers in electrochemical biosensors for clinical diagnosis is essential for the specific and accurate diagnosis of the disease with high sensitivity and selectivity. Therefore, this thesis evaluates the challenges pertaining to the stability, reproducibility, and obtaining a low limit of detection for the internal/external biomarkers associated with two distinct electrochemical biosensors. The first study tackles the challenge of detecting low analyte concentrations in a label-free biosensor. It introduces an innovative label-free electrochemical biosensing method for the detection of glycosylated hemoglobin (HbA1c) and C-reactive protein (CRP) to predict Coronary Heart Disease (CHD) progression using tailored redox probes, proposing a dual biomarker biosensing platform for future research. Calibration curves reveal an LOD of 5 mg/mL in PBS (8) FeCN (II) and 6 mg/mL in SB for a linear range of 0 – 30 mg/mL of HbA1c. Similarly, an LOD of 0.007 mg/mL and 0.008 mg/mL in PBS (7.4) PcA-NO2 and SB, respectively, is reported for a linear range of 0 – 0.05 mg/mL of CRP. The second study focuses on stabilizing a biomolecule-free sensor for the ultra-low detection of Δ9-tetrahydrocannabinol (THC) in roadside testing. Pre-depositing THC, an external biomarker for drug-impaired driving, onto the biosensor's working electrode enhances its interaction with analytes. However, THC's oxidative nature compromises sensor stability during manufacturing. Consequently, optimal electrode storage conditions were explored, indicating frozen storage as ideal for up to six months, effectively preventing THC oxidation at -18°C, while degradation occurs at 4°C. Modified electrodes stored under optimal conditions exhibit improved calibration curves when exposed to various THC samples. / Thesis / Master of Applied Science (MASc) / An electrochemical biosensor is a sensing device with the ability to detect biological species via the transduction of a specific biological event into electrochemical signals. These sensors are extremely useful for the detection of analytes in biological fluids for clinical diagnostics, to determine the presence or absence of diseases. This manuscript addresses the challenges associated with the stability, reproducibility, and the low limits of detection associated with screen-printed carbon electrodes used in electrochemical biosensing. Subsequently, due to the strong correlation between glycated hemoglobin (HbA1c) and C-reactive protein (CRP) to connote the risk of contracting coronary heart disease (CHD), the manuscript presents a novel label-free electrochemical biosensing method for the detection of HbA1c and CRP with low detection limits. Secondly, the manuscript identifies ambient storage conditions for the long-term stability of a biomolecule-free sensing device for the roadside detection of ultra-low concentrations of Δ9-tetrahydrocannabinol (THC).

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Dissertation submitted in partial compliance with the requirements of the Masters Degree in Technology: Chemistry, in the Faculty of Applied Sciences at the Durban University of Technology, 2008. / In this work, a holistic approach to validate analytical methods was assessed by virtue of Monte Carlo simulations. This approach involves a statement of the methodsâ s scope (i.e. analytes, matrices and concentration levels) and requisites (internal or external); selection of the methodâ s (fit-for-purpose) features; pre-validation and validation of the intermediate accuracy and its assessment by means of Monte Carlo simulations. Validation of the other methodâ s features and a validity statement in terms of a â fit-for-purposeâ decision making, harmonized validation-control-uncertainty statistics and short-term routine work with the aim of proposing virtually â ready-to-useâ methods. The protocol could be transferred to other methods. The main aim is to harmonize the work to be done by research teams and routine laboratories assuming that different aims, strategies and practical viewpoints exist. As a result, the recommended protocol should be seen as a starting point. It is necessary to propose definitive (harmonized) protocols that must be established by international normalisation/accreditation entities. The Quality Assurance (Method verification and Internal Quality Control, IQC) limits, as well as sample uncertainty were estimated consistently with the validated accuracy statistics i.e. E U (E) and RSDi + U (RSDi). Two case studies were used to assess Monte Carlo simulation as a tool for method validation in analytical laboratories, the first involves an indirect polarographic method for determining nitrate in waste water and the second involves a direct determination of heavy metals in sea water by differential pulse anodic stripping voltammetry, as an example of the application of the protocol. In this sense the uncertainty obtained could be used for decision making purposes as it is very tempting to use uncertainty as a commercial argument and in this work it has been shown that the smaller the uncertainty, the better the measurement of the instrument or the laboratoryâ s reputation.

In this thesis, mathematical modelling is used to theoretically investigate the electrochemical behaviour of surfaces which can be broadly classified as being ‘electrochemically heterogeneous’. Simulated voltammetry is used in the exploration of a number of specific systems as listed below. The cyclic voltammetry of electrodes composed of two different electroactive materials that differ in terms of their electrochemical rate constants towards any given redox couple. The effect of the distribution of the two materials was investigated and the occurrence of split peak cyclic voltammetry where two peaks are observed in the forward sweep, was studied. The technique was specifically applied to the modelling of highly-ordered pyrolytic graphite (HOPG). The steady-state voltammetry of a conducting spherical particle resting on an insulating supporting surface. An algebraic expression that completely describes the voltammetric waveform in the limit of irreversible kinetics was developed. The cyclic voltammetry of the EC′ (catalytic) mechanism at a regularly distributed array of hemispherical particles on an insulating supporting surface. Particular attention was paid to the ‘split-wave’ phenomenon, where two peaks are observed in the forward scan of a cyclic voltammogram and the conditions under which these peaks are resolvable were elucidated. The linear sweep voltammetry of micro- and nano-particle modified electrodes and other electrodes of partially covered and non-planar geometry. It was demonstrated that the apparent electrochemical rate constant of the reaction and thus the peak position of the voltammetry is dependent only on the relative electroactive surface area of the particles on the surface and not upon their shape or distribution. This has wide reaching implications as it can be used to explain some instances of a purported nano-catalytic effect without appeal to altered properties at the nanoscale. The linear sweep voltammetry of the interior of a partially electroactive cylindrical pore. Four limiting cases were observed and fully characterised. The linear sweep voltammetry of porous surfaces. It was established that if the pores are less than a certain threshold depth, then a porous surface will also display an apparent catalytic effect that is dependent on the relative electroactive surface area (including the area in the interior of the pores).

Micro- and nano-electrodes have distinct advantages over large electrodes, including their decreased iR drop and enhanced mass transport due to radial diffusion characteristics which leads to the ready establishment of a steady state (or near steady-state) signal without convection. This enhanced mass transport also leads to increased current densities and signal to noise ratios. However, there is a need for fabrication techniques which reproducibly give micro- and nano-electrodes of controlled size and shape. The optimisation of systematic arrays on the nano-scale, open up possibilities for developing highly sensitive electrode devices, for use in physical chemistry and the determination of fast electrode kinetics and rates of reaction, as well as to provide highly sensitive electroanalytical devices, able to detect very low concentrations of substrates. This thesis first presents work involving the fabrication and characterisation on silicon substrates of square platinum microelectrodes. There is already an established theory for the behaviour of microdisc electrodes however, it is easier to make microsquares reproducibly using pixellated photomasks. The voltammetric and ac impedance characteristics of these electrodes in background electrolyte and in the presence of ferri/ferrocyanide redox couple are presented and the response is theoretically analysed. A combination of computer simulation, theory and experimentation show that these electrodes have increased current densities (14% greater) compared with a microdisc of equivalent radius and an alternative theoretical expression is presented to calculate the limiting current of microsquares at all dimensions. This thesis then discusses the development and optimisation of novel nano-band cavity array electrodes (CaviArE), using standard photo-microlithographic techniques. The resulting architecture encloses a Platinum nanoband of 50 nm width within each array element that is positioned half way up the vertical edges of shallow square cavities (depressions), with a total depth of 1050 nm. The width of the square cavity and the separation of the array elements can be controlled and systematically altered, with great accuracy. The CaviArE devices are shown to give quantitative pseudo-steady-state responses characteristic of multiple nanobands, whilst passing overall currents consistent with a macroelectrode. The array has a much enhanced signal-tonoise ratio compared with an equivalent microsquare array, as it has 0.167% of the area and is therefore markedly less affected by non-Faradaic currents, while it passes comparable Faradaic currents. At high sweep rates the response is also virtually unaffected by solution stirring. The impedammetric characteristics presented show different diffusional regimes at high, medium and low frequencies, associated with diffusion within individual square cavities, outside of the cavity and finally across the whole array as the diffusional fields of the neighbouring array elements overlap. Justification and fitting of equivalent circuits to these frequency regions provide details about the charge transfer, capacitance and diffusional processes occurring. The results show that these systems are highly sensitive to surface transfer effects and a rate constant for ferricyanide of 1.99 cm s-1 was observed, suggesting fast kinetic processes can be detected. Together, these characteristics make nanoband electrode arrays, with this architecture, of real interest for sensitive electroanalytical applications, and development of devices for industrial application is currently being undertaken.

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2007. / Electrodialysis is a method of water desalination which involves the separation of TDS through an ion-exchange membrane under a potential gradient. In this study it was attempted to reverse engineer the composite carbon ion-exchange membrane used in a prototype plant and electrochemically evaluate a prototype desalination cell. The influence of applied potential on the capacitance of the various electrode surfaces and possible electrode reactions was investigated. A model was also suggested to describe the conductivity through the membrane. The composition of composite carbon membranes were determined by compositional analysis using various analytical tools. Elemental analysis, done with PIXE and EDS, showed that the membranes contained chloride, fluoride, oxygen, carbon, and possibly hydrogen. With LC-MS and IR it was established that the membranes consisted of two polymers with no carbonyl or aromatic functional groups. After further thermal analysis the following possible compounds remained: hexafluoropropylene tetrafluoroethylene copolymer, polychlorotrifluoroethylene (PCTFE), polyoxyethylene oxide (PEO) and polyethylene glycol (PEG). This assessment is in good agreement with the contents of US patent 4,153, 661, which describes the composite membrane.

Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / Electrochemical behaviour of Ce, Fe, Cr,V and Mn in the presence of DTPA, EDTA, EDDS, NTA ligands were investigated by using cyclic voltammetry, a rotating disc electrode and electrochemical impedance spectroscopy for use in redox flow battery (RFB) systems. RFB is currently used for energy storage, the vanadium, which is used in most of the RFB’s, however suffers from species crossover and sluggish reactions, which limit the lifetime of the battery. These various ligands and metal complexes mentioned above where all examined to identify the suitable and favoured electrolyte that can be used for a RFB system. Kinetic parameters such as potential, limiting current, transfer coefficient, diffusion coefficients, and rate constants were studied. RDE experiments confirmed that the parameters measured by CV are similar under hydrodynamic conditions and can be used to determine the kinetic parameters of the redox couples. The use of DTPA as a ligand for complexation of Ce(IV) gave more favourable results compared to other ligand with various metal complexes used in this study [1-3]. The results of kinetic studies of Ce(IV)–DTPA complex shows promise as an electrolyte for a redox flow battery. The separation of V(IV)/(V), Fe (III)/(IV),Cr(III)/(IV),Mn (III)/(IV) and Ce(III)/(IV) with various ligands (EDTA, EDDS, NTA and DTPA) were also investigated using capillary electrophoresis. To understand the speciation of these metal complexes as used in this study and particularly the vanadium, for the reason that it has a complicated (V) oxidation state. The charge/discharge performance of all electrolytes used in this work was determined and a high voltage achieved when Ce-DTPA was used, and it is compared to that of the vanadium electrolyte currently in use. This was evaluated with systems studied previously. Therefore, Ce-DTPA will be a suitable electrolyte for redox flow battery systems.

The molecular identity of the green emission of polyfluorene is investigated in the view point of the molecular interactions between modeled segments. The semi-empirical quantum methods, ZINDO/S and AM1 (AM1-CIS), are used in combination to provide reasonable explanations for experimental spectroscopic properties of monodisperse fluorene oligomers and fluorene oligomers with a central keto defect in dilute solutions. Applying the same method, the molecular interactions between model segments are found to exist and are significant. However, the spectroscopic property change from the molecular interactions is negligible. In addition, the effects of mechanical stress and multi-defects on fluorene oligomers are investigated. On the other hand, the redox mechanisms proposed for the oxidation of an amphiphilic cyanine (C8S3) J-aggregates immobilized at ITO electrode and the subsequent dehydrogenated dimmer formation during cyclic voltammetry based on analysis of absorption spectra during the process are verified with the combined semi-empirical quantum methods similar to the previous methods. The absorption spectra assigned by experiment for electrochemical species involved in the proposed mechanism show reasonable match to the theoretically estimated absorption energies of the corresponding simplified model systems. In addition, the standard reduction potentials of the fairly large molecules, C8S3 monomer and its dehydrogenated dimer, are pursued with quantum mechanical calculations. The free energy difference between the oxidized and reduced states of the target systems is decomposed to electronic energy, solvation energy and temperature-dependent free energies terms. Based on AM1 ground state geometries and with the corresponding temperature dependent free energies, the electronic energies and the solvation energies are each evaluated by two different methods. The electronic energies are calculated with AM1 method and DFT calculation and, also, the solvation energies are obtained based on the atomic partial charges from AM1 and DFT wavefunctions with continuum dielectric solvent approximation. The four calculation schemes from the combinations of the electronic and solvation energy estimation methods are tested with the redox compounds with various molecular weights and the estimations are compared with the corresponding experimental redox potentials. The relative redox potentials between two different redox systems are found to be reasonably estimated with the four calculation schemes. / text