Electronic Theses and Dissertations (PhDs)
Permanent URI for this collectionhttps://hdl.handle.net/10539/38002
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Item Antimicrobial activity of silver and copper nanoparticles incorporated with biodegradable polymeric scaffolds for wound healing application(University of the Witwatersrand, Johannesburg, 2022) Shumbula, Ndivhuwo Prince; Moloto, NosiphoThere has been a significant increase in the prevalence of chronic diseases such as diabetes and cancer which cause wounds like diabetic foot ulcers and pressure ulcers. This usually causes infection(s), leading to the amputation of limbs or even death. Chronic wounds do not follow the typical stages of wound healing and thus either take a very long time to heal or do not heal at all. Therefore, the need for developing advanced and novel wound treatment products that offer infection control and sharp debridement is vital. To combat this issue, this project focussed on fabricating bioactive nanomaterials to design an advanced wound dressing in the form of 3D scaffolds. The scaffolds were made of nanocomposites containing biocompatible and biodegradable polymers decorated with antimicrobial active Cu and Ag NPs. Prior to designing the scaffolds, pristine Ag and Cu NPs were fabricated and their antimicrobial activity and cytotoxicity were tested. The antimicrobial activity of these NPs was tested against E. coli and S. aureus bacterial strains, while their cytotoxicity was evaluated using baby hamster kidney fibroblasts (BHK-21) cells. Ag NPs with various sizes and spherical shapes were synthesized using dopamine (DA) as a reducing agent. Ag NPs with the smallest diameter exhibited the most antimicrobial activity compared to those with big diameters. The cytotoxicity of Ag NPs was very low, with cell viability of over 70% between 2 μg/mL and 32 μg/mL. At concentrations above 32 μg/mL, a decrease in cell viability was observed, indicating increasing toxicity, especially for small NPs. Cu-based NPs were also synthesized using hydrazine and DA as reducing and capping agents, respectively. Pure metallic Cu, Cu 2 O and the mixture of Cu and Cu 2 O (Cu/Cu2 O) NPs were separately prepared. Cu and Cu2 O NPs exhibited promising antimicrobial activity against E. coli and S. aureus. An improved antimicrobial activity was observed for Cu/Cu2 O NPs, compared to the Cu and Cu2O NPs, and this was attributed to the synergistic effect. The cytotoxicity of Cu2 O NPs was higher compared to that of Cu and Cu/Cu 2 O NPs. The antimicrobial activity and cytotoxicity of both Ag and Cu NPs incorporated with polydopamine (PDA) were also studied. Cu and Ag NPs were either embedded within the matrix of PDA (Cu-PDA or Ag-PDA) or incorporated on the surface of PDA (Cu@PDA or Ag@PDA). Cu@PDA and Ag@PDA showed stronger antimicrobial activity than Cu-PDA and Ag-PDA due to the NPs exposure to the bacteria. These nanocomposites showed very iii low cytotoxicity with cell viability of over 80 % at concentrations as high as 250 μg/mL. However, a drastic decrease in cell viability was observed when the concentrations of Cu@PDA and Ag@PDA were increased above 250 μg/mL. Taking advantage of the strong biological activity exhibited by Cu and Ag NPs, bimetallic nanoalloys of these two metals were prepared. Nanoalloys with a fibrous shape were obtained as bimetallic nanofibres (BNF). The mole ratio of Cu:Ag was found to have an effect the antimicrobial activity of the BNFs. The BNFs prepared with a mole ratio of 1:2 (BNF-2) exhibited a strong antimicrobial activity compared to 1:1 (BNF-1) and 2:1 (BNF-3). Furthermore, the BNFs showed minimal cytotoxicity towards the BHK-21 cells at low concentrations (7.8 μg/mL), with the cell viability ranging between 75-90 %. Compared to BNF-1 and BNF-2, BNF-3 was found to have a negative impact on cell viability when the concentrations were increasing. The novel and porous 3D scaffolds were prepared by coating chitosan/gelatine (CS/Gel) scaffolds with PDA using the freeze-drying method. This was followed by decorating the surface of the scaffolds with either Ag or Cu NPs, in situ. The presence of the catechol functional group inherent from PDA facilitated the reduction of Ag ions to form pure zerovalent Ag NPs on the surface of the scaffolds. However, to reduce Cu ions to form Cu NPs, an access reducing agent (hydrazine) was added. The scaffolds showed reasonably high fluid uptake (FU) with time-dependent biodegradability, a property that would be advantageous in controlling wound exudates. These scaffolds also showed excellent antimicrobial activity against E. coli and S. aureus with low cytotoxicity towards human fibroblast cells. Taken together, the designed scaffolds could act as a barrier from the external environment, help prevent bacterial infections and further accelerate skin cells regeneration. The most important feature of these scaffolds is their potential ability to penetrate deep wounds and provide a conducive environment for skin cells to regenerate without any difficulties from bacterial colonization.Item Synthesis and evaluation of flexible pyrimethamine analogues as antifolates against drug-resistant malaria(University of the Witwatersrand, Johannesburg, 2025-03) Maree, Matthew; Ngwira, Kennedy; Rousseau, AmandaIn many parts of the world today, malaria still represents a major health crisis, with millions of cases and hundreds of thousands of deaths reported each year. The major malaria causing parasite is Plasmodium falciparum, which accounts for the majority of cases and deaths worldwide. This parasite has shown a remarkable ability to rapidly mutate and develop resistance against initially effective drug molecules. The continued threat of malaria today motivates the development of new molecules which can remain active despite point mutations that arise in the active site of the targeted enzymes. Previous work done by A. Rousseau and colleagues showed that modifying existing class II antifolates, which target the bifunctional parasite enzyme dihydrofolate reductase – thymidylate synthase (DHFR-TS), by adding a flexible four-atom linker allowed for high levels of antiplasmodial activity to be maintained against drug-resistant forms of the parasite. In the present work, molecular modelling techniques were used to optimize the structural elements of flexible pyrimethamine analogues for binding ability against mutant enzymes. The effects of various functional groups around a conserved structure were evaluated, and a library of compounds was selected for synthesis. Numerous flexible pyrimethamine analogues were successfully synthesized making use of partially established experimental procedures, and were submitted for various antiplasmodial assessments. The synthesized compounds showed excellent activity in single enzyme assays, inhibiting drug-resistant PfDHFR enzymes at low nanomolar concentrations. Whole cell assays were also conducted, where a significant decrease in activity was observed, with the most active compounds inhibiting the parasite cells at low micromolar concentrations. These results suggested that while the compounds were effective binders of the target enzyme, they had some pharmacokinetic limitations which prevented them from effectively exhibiting their mode of action inside the cell. A second-generation of analogues was then envisaged, taking inspiration from existing antifolate compounds which are known to have favourable pharmacokinetic properties. Methods for the synthesis of the devised second-generation analogues were developed as part of this work, and the binding ability of the compounds was validated with further molecular modelling studies. We are currently awaiting the results for the biological assessments of the second-generation analogues.Item Synthesis and characterization of marula nut derived carbon and modified manganese fluorophosphates for electrochemical energy storage applications(University of the Witwatersrand, Johannesburg, 2023-11) Shaku, Bokome; Maubane-Nkadimeng, Manoko S.; Coville, Neil J.; Ozoemena, Kenneth IkechukwuIn this thesis, six (6) different electrode materials for supercapacitor applications were created and studied. Electrode materials are one of the most important components in supercapacitors. The increase in energy density requires require electrode materials that have high surface area, conductivity, redox properties. Hence, in this study, the electrochemical characteristics of nitrogen doped activated carbons (N-ACs) material made from marula nuts as a carbon source and melamine as a nitrogen source were studied in both symmetric and asymmetric capacitors. Again, potassium and lithium intercalation in manganese fluorophosphate (MFP) were studied in asymmetric capacitor. Ceria oxide (CeO2) was used to form a composite with MFP and the electrode material was studied in asymmetric capacitor. All of the synthetic materials' morphology, crystallinity, defects, elemental composition, and structural porosity were examined. Additionally, electrochemical analysis of all materials in different electrolytes were ran to obtain current response, charge and discharge time and charge transfer resistance using two (2) and three (3) electrode systems respectively. Firstly, marula nutshell waste, was used as a source of carbon to form activated carbons (ACs) doped with nitrogen. The high surface area and nitrogen present on the material resulted in energy density that is high (17.2 Wh/kg). Secondly, a novel chemical intercalation synthesis method based on potassium hydroxide-induced hydrothermal process was used to create alkali ion-containing triplite MFP. Research findings in the study indicate that d-spacing increase when intercalating potassium into the triplite structure. An asymmetric supercapacitor was fabricated using potassium intercalated material (K+/MFP) as a cathode and N-ACs as anode in alkaline and neutral electrolytes. Higher energy density (26.9 Wh/kg) was observed in 1 M KNO3 than 6 M KOH with 25.2 Wh/kg. Thirdly, assembly of asymmetric supercapacitor of N-ACs and Li+/MFP was done. The electrode material exhibited energy of 18.8 Wh/kg in 1 M LiFSTI with capacitance retention of 69.5% after 15 000 cycles. N-ACs and MFP-CeO2 was assembled in 1 M K2SO4. The material gave energy of 23.8 Wh/kg with specific power of 525.6 Wh/kg. The MFP-CeO2//N ACs had a good cycling stability of 69.8% after 15 000 cycles.Item Source Apportionment, transport and fate of pollutants in the paper recycling chain - An analytical exploration of the South African context(University of the Witwatersrand, Johannesburg, 2024-08) Mofokeng, Nondumiso Nomonde Radebe; Chimuka, Luke; Madikizela, Lawrence M.This thesis presents the analytical exploration of pollutants in the South African paper recycling chain. Recycled paper is a valuable commodity that forms an intrinsic part of actualising a circular economy. In South Africa, the recycled paper value chain includes largely unsorted waste paper recovered from household waste, businesses, retail, schools, industry as well as paper collected from public areas, solid waste disposal sites and landfills by informal waste pickers. Extensive research has been undertaken in Europe, America, and Asia on the implications of using recycled paper; specifically, in packaging intended for food, toys, and electric equipment. In South Africa, however, research on pollutants in recycled paper does not exist. As the global impetus towards sustainability increases, it is imperative to determine the type of pollutants that may arise from the recycled paper value chain as these may negatively affect humans and the surrounding biota. This study extended beyond typical food packaging migration studies and investigated the holistic extraction of both food and non-food paper-based materials. The stages of paper recycling that were investigated were pre-consumer, retail and post-consumer. In Paper 1, liquid and gas chromatography, both in tandem with mass-spectrometry, were used for the chemical characterisation of various paper recycling grades prepared by accelerated solvent extraction and ultrasonic assisted extraction, respectively. The findings indicated that polymer-associated chemicals such as plasticizers, antioxidants, flame retardants, and polymer degradants have infiltrated the paper recycling chain. Multi-residue quantification of 11 pollutants in recycling paper grades in South Africa using GC-MS after accelerated solvent extraction was the main focus of Paper 2. The target analytes included BBP, DEP, DBP, DEHP, DIBP, DIDP, DIPN, NBBS, BHT, AO168, and AO168O. A plot of the VIPscores showed that DEHP and DBP were the most prominent pollutants, whilst AO168 and BHT were the least significant pollutants in the examined samples. Paper 3 explored the previously unexplored contamination of paper by pharmaceuticals, emerging contaminants that have been historically detected in South African environmental matrices. The analyses were achieved by ultrasonic-assisted extraction of paper samples before targeted and suspect screening by UHPLC-Q Orbitrap. This study was able to uncover the presence of dexamethasone, ketoprofen, and 17β-estradiol in paper-based samples. In addition, suspect screening was able to tentatively identify additional pharmaceuticals through spectral database matches. Papers 4 and 5 studied PFAS prevalence in recycling paper grades used for the manufacture of recycled paperboard. In Paper 4, targeted analysis of per- and polyfluoralkyl carboxylic and sulphonic acids was undertaken. The possible propagation and amplification of PFAS was explored by comparing the concentrations detected in pre-consumer samples to those detected in retail and post-consumer samples. Suspect screening in Paper 5 tentatively identified additional PFAS present in the samples through the development of a study-specific suspect screening protocol reported with its associated confidence level. The samples were prepared using two extraction techniques; accelerated solvent extraction with solid-phase extraction and ultrasonic-assisted extraction. The different functional groups identified indicated the importance of both selective and non-selective extraction in understanding PFAS occurrence. It further demonstrated an urgent global need to understand the different PFAS that can occur in the paper recycling chain. In these Papers, manufacturing additives and retail activities were identified as possible exposure sources. Post-consumer usage, collection, sorting, and comingling of various waste materials were also identified factors that influence the prevalence of pollutants. The presence of certain pollutants in pre-consumer showed that certain compounds may potentially remain within the paper recycling chain, propagate and accumulate along with those from the retail and postconsumer stages. The lower abundance in comparison to retail and post-consumer samples indicated that other pollutants may likely be removed during the reprocessing of recycled fibre. This study thus showed that it is imperative that South Africa develop and implement legislation and guidelines that address the use, risk, and standardised waste management strategies that ensure a safe circular economy in the paper recycling chain.Item Mineral beneficiation from seawater: development and optimization of selective extraction techniques for essential minerals from seawater(University of the Witwatersrand, Johannesburg, 2024-09) Ntombela, Silindile Cynthia; Tutu, Hlanganani; Richards, Heidi; Chimuka, LukeThe growing demand for essential minerals such as lithium and magnesium has underscored the need for sustainable extraction methods. Lithium plays a significant role in various industries since it is a promising metal for energy storage in electric vehicles as well as in electric devices. Magnesium is commercially used in the automotive industry. The governments of developed countries such as those in Europe, have imposed strict laws when it comes to vehicle emissions and have made the use of electric vehicles an alternative for more environmentally sustainable transportation. Traditional mining such as minerals in rock ore demands large amounts of water and energy, which is known to pose substantial environmental and health risks to the miners. Therefore, seawater mining has been reported as one of the strategies to mitigate the depletion of high-grade ores while offering reduced waste generation. This research contributes to finding technologies that align with the blue economy and addresses the environmental challenges of traditional mining. This study focuses on the synthesis and optimization of polymer inclusion membranes (PIMs) to selectively extract essential minerals from seawater. However, the challenge is that lithium is present in extremely low levels approximately 0,17 mg/L in seawater. The approach was to synthesize PIM that will selectively extract the targeted analytes, leaving the non-targets behind when applied to real seawater samples. The research was conducted in three phases. The first phase involved synthesizing PIMs and optimizing parameters such as membrane composition, stripping solution concentration, the effect of the pH and extraction time. The selectivity of the synthesized PIMs was tested in ultrapure water spiked with 15 mg/L of mineral salts such as magnesium carbonate, calcium carbonate, sodium carbonate and potassium carbonate, yielding a selectivity order of Mg2+ > Ca2+ > Na+ > K+ > Li+. In real seawater samples, the selectivity was Mg2+ > Ca2+ >Na+ > K+ and lithium was not detected. The density functional theory (DFT) studies were also conducted to investigate the binding ability of the carriers towards the targeted metal ions. The obtained selectivity was Mg2+ > Ca2+ > Li+ Na+ > K+. The selectivity of the metal ions obtained from the experiments slightly differs from DFT. However, the computational study contributes to finding suitable technologies that will take advantage of the blue economy. The method was optimised successfully and further applied to real seawater samples. The second part of the study involved the synthesis of a PIM with different membrane compositions. The optimized PIMs demonstrated excellent selectivity for lithium which varied with the concentration of the HCl receiver solution. The selectivity obtained for the PIM that was in a 1:1 ratio utilising 0,05 M HCl of the receiver solution was Li+ > Na+ > Ca2+ > Mg2+. As the receiver solution was increased to 0,1 M HCl and 1 M HCl, the selectivity shifted to Li+ > Ca2+ > Na+ > K+ > Mg2+ and Li+ > Ca2+ > Na+ > K+, respectively. The selectivity obtained for the 2:1 ratio was Li+ > Ca2+ > Na+ > Mg2+. When the concentration of the receiver solution was increased to 0,1 M HCl and 1 M HCl, the selectivity was Li+ > Ca2+ > Na+ > K+ for both concentrations. Furthermore, the optimal parameters were further tested on the real seawater. The selectivity obtained was the same for the PIM at a 1:1 ratio for 0,05 M HCl, 0,1 M HCl and 1 M HCl receiver solution which was Ca2+ > Na+ > K+. When the second PIM in a 2:1 ratio was applied in seawater, the selectivity obtained for 0,05 M HCl of the receiver solution was Ca2+ > Na+ > K+ > Mg2+ whereas for 0,1 M HCl the selectivity was Ca2+ > Na+ > K+ and for 1 M HCl the selectivity was Na+ > K+. The third part of this study was a continuation of the first part. The optimised PIM was further employed for the extraction of minerals in seawater using a semi upscaled approach. A much bigger flat sheet membrane of approximately 270 mm (width) and 370 mm (length) was synthesized based on the optimised membrane composition. The other optimised parameters such as the concentration of the receiver solution and the effect of extraction time were tested in 0,05 M HCl and 0,1 M HCl receiver solutions for 39 days, respectively. The volume of the receiver solution was also investigated between 1 L and 2 L. The application was done in real seawater and the selectivity obtained for both volumes of the receiver solutions at 0,05 M HCl was found to be the same: Na+ > Mg2+ > Ca2+ > K+. However, when the concentration of the receiver solution was increased to 0,1 M HCl the selectivity changed to Mg2+ > Ca2+ > K+ for both 1 L and 2 L respectively. The concentration of sodium at 0,1 M HCl in the receiver solution was not clear, thus it was eliminated from the results. Despite some deviations in selectivity compared to smaller-scale experiments, the study demonstrated the feasibility of using PIMs for mineral extraction from seawater on a larger scale. Future work will focus on understanding the reasons for selectivity deviations in upscaled applications and further refining the PIM method to achieve consistent results. This research contributes to developing sustainable technologies for extracting valuable minerals from seawater aligning with the blue economy and addressing the environmental challenges of traditional mining.Item The study of quaternary semiconducting chalcogenide nanomaterials for application as counter electrodes(University of the Witwatersrand, Johannesburg, 2021) Ngubeni, Grace Nomthandazo; Moloto, NosiphoHerein, the colloidal synthesis of Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and the first time colloidal synthesis of Li2ZnSnS4 (LZTS) and Na2ZnSnS4 (NZTS) nanoparticles respectively were investigated. All the nanoparticles were applied as counter electrodes in dye-sensitized solar cells (DSSCs). For the CZTS and CZTSe nanoparticles in particular, the effect of three substrates, namely, vitreous carbon (VC), indium tin oxide (ITO) and fluorine doped tin oxide (FTO) on the electrocatalytic properties including the overall performance of the solar cells were investigated. The CZTS and CZTSe were successfully synthesized and characterized with X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FT-IR), nuclear magnetic resonance (NMR), and ultra-violet visible (UV-vis) spectroscopy and transmission electron microscope (TEM) for the morphologies. CZTSe on glassy carbon (VC) had better electrocatalytic activity as compared to CZTS, however, the DSSCs from VC were poor due to the reduced transmittance of the substrate. On ITO and FTO, CZTS performed the best. Electrochemically, CZTS had the lowest series resistance and charge transfer resistance however had the largest exchange current density and limiting diffusion current thereby making it the best electrocatalyst. The DSSC using CZTS–ITO gave the best performance with the power conversion efficiency (PCE) of 3.62%. Conversely, for the LZTS and NZTS nanoparticles the effect of the lithium and sodium precursors and the ratio of the constituents e.g. Li:Zn:Sn:S and Na:Zn:Sn:S on the properties of the LZTS and NZTS nanoparticles, respectively were investigated. In addition, the effect of the substrates, that is, ITO vs FTO on the electrocatalytic properties as well as overall performance of the DSSCs were studied. Using the Li2S precursor, the XPS, 7Li MAS NMR and Raman spectroscopy confirmed the presence of lithium and the formation of LZTS. Since the Li2S source and the 2:1:0.25:2 ratio, resulted in the purest particles, these were therefore used as CEs in DSSCs for the first time. LZTS nanoparticles on ITO gave the best performance with 2.26% PCE. Similarly, the study of NZTS indicated that the NaCl regardless of the ratios used resulted in the formation of impurities as observed from XRD patterns. The presence of sodium and the complete formation of NZTS nanoparticles through 23Na MAS NMR, XPS and Raman v spectroscopy were investigated. The results indicated that the Na2S-based nanoparticles in the 2:1:0.5:4 ratio, yielded the purest NZTS nanoparticles. As such, these results indicated a PCE of 3.93%. The study illustrates that alkali metals (Li+ and Na+) can be used as the monovalent cation in quaternary nanoparticles, instead of the commonly used Cu+ transition metal with promising results as counter electrode materials in DSSCs. Notably, the NZTS nanoparticles illustrated a higher PCE to CZTS while LZTS had the lowest PCE. It must be however noted, that these results require further optimization to explore the full potential of these alkali metal-based quaternary nanoparticles.Item Thermo – photocatalytic production of hydrogen from water with methanol/formaldehyde as sacrificial agent under mild conditions(University of the Witwatersrand, Johannesburg, 2024-10) Nyamai, Nancy Akoth; Lemmerer, AndreasFossil fuels are the main source of energy for modern society to this decade. The awareness of scarcity of fossil fuels and the environmental degradation accompanying their use has motivated researchers to conduct extensive research in other forms of energy for mitigation. Hydrogen has the potential to replace the traditional fuels for transportation application and other uses. Comparatively, hydrogen has higher energy (143 MJ/kg) than the conventional fossil fuels (natural gas: 53.6 MJ/kg, petrol: 46.4 MJ/kg, diesel: 45.4 MJ/kg) moreover, its environmental friendly as it produces water as the only by-product of its combustion. In spite of its superiority, public acceptance of hydrogen based technologies are still undermined. Following the main finding of water-based photo-electrolysis in 1972, scientists, engineers and environmental activists have intensified research on the production of hydrogen via photocatalytic water splitting adopting different modifications of semiconducting materials such as titanium dioxide, TiO2, cadmium sulphide, graphitic carbon nitride and using sunlight to fragment water molecules to produce hydrogen gas. Titanium is one of the most attractive d-block transition metals in use as water splitting photocatalyst. It is preferred because of its low density, high strength, corrosion resistance, availability and cost effectiveness. However, efficient water splitting using visible light and TO2 has been a challenge. TO2 has a setback of wide bandgap, 3.0 – 3.2 eV, which lowers its prospective for visible light absorption. This study outlines the development and the testing of efficient catalytic materials for the thermo-photocatalytic production of hydrogen from water. Methanol-water mixture was also tested under mild conditions. TiO2 composited with carbon nanofibers, CNFs at different loadings (0,10 and 20%) and co-catalysts platinum (Pt), gold (Au), iridium (Ir), zinc (Zn) and copper (Cu) were synthesized and used in the photocatalytic water splitting and appreciable quantities of hydrogen were obtained. Additionally, 10% methanol was also used as sacrificial reagent with water and tested for hydrogen evolution but the results were not significantly different when compared to the results obtained from the use of photocatalysts without methanol addition. The photocatalysts were tested for hydrogen production using water, and the most active photocatalyst, F_CNFs/20% _TiO2/6%Pt exhibited an activity of 0.45 mol g-1h-1. This was followed by F_CNFs/10% _TiO2/5%Au and F_CNFs/20% _TiO2/1%Ir with activities of 0.28 mol g-1h-1, and 0.21 mol g-1h-1 respectively. In another section of this study, the focus was on the fabrication of nanomaterials for advanced photocatalytic water splitting as a result, structures of TiO2/CNFs, Cu/TiO2/CNFs, and Zn/ TiO2/CNFs nanocomposites of mesoporous nature were synthesized using CNFs and surfactant wrapping sol-gel/hydrothermal method. Impregnation technique was used in copper and zinc loading while characterization of the synthesized catalysts was achieved by RD, Raman, FESEM, TEM, UV/DRS, BET, FTIR, XPS, and PL techniques. Water splitting was performed by UV light irradiation and the effect of compositing TiO2/CNFs-Cu/Zn evaluated. 10%_TiO2/CNFs/5%Zn composite was the most efficient photocatalyst for hydrogen production when compared with the rest of the samples. The most efficient photocatalyst, 10%_TiO2/CNFs/5%Zn, produced 0.53 mol (h g cat.)-1 of H2 which was 90-100 folds the amount generated over commercial TiO2 - P25, and pure TiO2 photocatalysts. The improved photo-activity of the TiO2/CNFs, Pt/TiO2/CNFs, Au/TiO2/CNFs, Ir/TiO2/CNFs, Cu/TiO2/CNFs, and Zn/ TiO2/CNFs composites were due to the synergistic effect between Pt, Au, Ir, Cu, Zn, and CNFs, greater BET surface area, suitable band structure and reduced recombination rate of the photo-generated charge carriers. This work gives promising route for fabricating TiO2/CNFs/metal (Pt, Au, Ir, Cu, and Zn) composites for advanced H2 production using UV- radiation.Item Studies on the chemistry and biochemistry of gold(III) carboxamide pincer chelates(University of the Witwatersrand, Johannesburg, 2024-06) Razuwika, Rufaro; Nowakowska, Monika; Mathura, adhnaCancer, a group of diseases characterised by the uncontrollable growth of abnormal or mutated cells within an organ, is a global concern. Metallodrugs have emerged as promising solutions to this pandemic, leading to intense research on different metal complexes. In this study, gold(III) carboxamide pincer complexes were evaluated as potential chemotherapeutic agents. The novel NNN-type carboxamide pincer molecules (ligands) effectively stabilising the gold(III) metal centre. The strong σ-donor properties of both the anionic and pyridine N groups further enhanced this stability. Ligands 1a-1f exhibited atropisomerism, a common feature in drug discovery, and containing special heterocycles such as quinolones, indazole, benzophenone, and phenanthroline, which are particularly relevant in drug development. Atropisomerism, however, was lost upon metalation of the ligands. Three complexes, 2d, 2e, and 2f, were successfully synthesised and isolated. Complex 2d was subjected to biochemical property testing and in vitro analysis due to its superior stability and solubility compared to 2e (poor stability) and 2f (poor solubility in the buffer solution used in the study). Speciation studies, combined with computational studies, suggested that 2d exists as a neutral complex under physiological conditions. This inert complex demonstrated stability against the reducing agent glutathione, indicating resilience to reduction under physiological conditions. DNA spectroscopic titration studies revealed that 2d exhibited intensive interaction with ct-DNA, with binding constants Ka1 = 1.48 x109 M-1 and Ka2 = 6.59 x105 M-1. This interaction resulted in a notable increase in the DNA melting point by 4 °C and an enhancement in viscosity in a dose-responsive manner. The DNA titrations, melting point, and viscosity studies suggested a dual binding mode of 2d to ct-DNA, involving base binding with a nearly equal preference for A, T, G, and C bases, and groove binding. Complex 2d exhibited a high affinity towards the transport protein HSA (Ka values were 1.57 x104 M-1), suggesting that it can be transported in the body by means of the HSA-mediated pathway, enhancing its efficacy and stability. In comparison to its affinity towards DNA, there is a significant difference allowing for the successful transfer of 2d from HSA to DNA. The poor solubility of complex 2d in aqueous environments may have hindered its cellular uptake, but binding to HSA could mitigate this, ensuring minimal interference with its cytotoxicity towards different cancer cell lines. MTT studies demonstrated that 2d has comparable cytotoxicity towards the breast cancer cell line MCF-7 with an IC50 of 9 µM. The IC50 for HT-29 was, however, too high to measure accurately (>100 µM). In conclusion, complex 2d exhibits promising anticancer properties based on its DNA binding studies and cytotoxicity evaluations. This suggests that this class of compounds can be applied in cancer treatments, with potential modifications to compounds 2e and 2f to improve their solubility and stability.Item Electrocatalytic detection of biomarkers of tuberculosis and cervical cancer(University of the Witwatersrand, Johannesburg, 2024-07) Peteni, Siwaphiwe; Ozoemena, Kenneth IkechukwuThe need for simpler, low cost and efficient diagnostic methods remains a matter of urgency. This has opened numerous streams of research. Electrochemistry is a simple, cost effective and efficient method that has been used for the detection of several diseases such as tuberculosis (TB) and human papilloma virus (HPV). TB has been ranked amongst the most problematic diseases in HIV/AIDS burdened communities, this alone calls for concern. Biomarkers of TB not only indicate mycobacterium infection but can also assist in the early detection of TB which is highly beneficial for the infected person and the health care system. HPV is the causative agent for cervical cancer. Cervical cancer is ranked as the fourth disease that causes mortality amongst women. With that in mind, HPV-16 L1 early detecting means possible early detection of cervical cancer. In this thesis, methyl nicotinate (MN), which is one of TB’s biomarkers was detected in phosphate buffer solution (PBS, pH 6.0) and commercial human serum using cobalt nanoparticles supported on carbon derived from trimesic acid (TMA) (abbreviated as Co-NPs@CTMA) and biphenyldicarboxylic acid (BPDC) abbreviated as Co-NPs@CBPDC) as electrocatalysts. These electrocatalysts were obtained using microwave-assisted metal-organic framework process with TMA and BPDC as ligands. XRD data showed that these electrocatalysts are cobalt nanoparticles with dominant {111} and {200} phase with traces of cobalt oxide (CoO). XPS and Raman data showed that Co-NPs@CBPDC is defect-rich compared to the Co-NPs@CTMA counterpart. BET showed that CoPs@CBPDC has higher surface area and pore size and volume than the Co-NPs@CTMA catalyst. Both electrocatalysts showed reversible cobalt nanoparticle oxidation and reduction reactions, in the absence and in the presence of the MN, thereby allowing for a facile indirect electrochemical detection of this biomarker. The calibration curves showed low limit of detection (LoD) of 0.47 and 0.147 µM for Co-NPs@CTMA and Co-NPs@CBPDC, respectively. The higher performance of the latter is attributed to its enhanced physico-chemical properties compared to the former. Next, HPV-16 L1, which is the conventional high-risk antigen that is present in cervical cancer, was detected using onion-like carbon (OLC) and polyacrylonitrile fibre integrated with OLC (OLC-PAN) as electrode platforms. Two electrode platforms were used; onion-like carbon (OLC) and its polyacrylonitrile (OLC-PAN) composites. Both platforms led to the detection in a wide linear concentration range (1.95 fg/ml to 50 µg/ml), excellent sensitivity (>5.2 µA/log([HPV-16 L1, fg/mL]) and ultra-low detection of ca. 1.0 and 1.4 fg/ml for OLC-PAN and OLC-based immunosensors, respectively. The high specificity of detection was proven by experimenting with an anti-Ovalbumin antibody (anti-Ova) and native Ovalbumin protein (Ova). An immobilized antigenic HPV-16-L1 peptide showed insignificant interaction with anti-OVA in contrast with the excellent interaction with anti-HPV-16 LI antibody. The immunosensors showed satisfactory stability of ~ 3 days of re-usability. The application of the immunosensor as a potential point-of-care diagnostic (PoC) device was investigated with the screen printed carbon electrode which showed the ability to detect ultra-low (~ 0.7 fg/ml) and high (~ 12 µg/ml) concentrations. This study opens the door of opportunity for further investigation with other electrode platforms and realization of PoC diagnostic devicesfor screening and testing of HPV biomarker for cervical cancer.Item Synthesis and electrochemical properties of high-entropy spinel oxides, cobalt atomic clusters and zinc oxide as electrode materials for rechargeable zinc-air batteries(University of the Witwatersrand, Johannesburg, 2024-07) Gaolatlhe, Lesego; Ozoemena, Kenneth IkechukwuThis thesis investigated cathode and anode electrode materials for application in rechargeable zinc-air battery (RZAB). Two types of cathode materials were strategically studied in RZAB applications: (a) cobalt carbon composites of (i) cobalt atomic clusters (Co AC@CBPDC) and (ii) cobalt nanoparticles (Co NP@CBPDC), and (b) high-entropy spinel oxide (HESOx, containing five transition metals – Cu, Mn, Fe, Ni, and Co). The activities of these materials toward oxygen reduction reaction (ORR and oxygen evolution reaction (OER) were investigated in both half- and full-cell configurations as a proof-of-concept in RZAB cells in alkaline electrolyte. Considering that conventional zinc plate has several short-comings as an anode for RZAB, a new material, polydopamine-derived carbon-coated zinc oxide (ZnO@PDA-DC), was also synthesised and applied in RZAB as a possible alternative anode to the popular zinc plate. First, Co AC@CBPDC and Co NP@CBPDC were prepared using the metal-organic framework (MOF) route through the microwave-assisted solvothermal method and acid treatment. From the XRD results, the spectra showed dominant {111} and {200} phases, characteristic of metallic cobalt with a face-centred cubic (fcc). There were trace amounts of CoO observed indicating the coexistence of Co/CoO. From TEM imaging, Co AC@CBPDC was highly defective with a visible porous carbon structure than its counterpart (Co NP@CBPDC) and showed dispersed atomic clusters. BET data showed that Co AC@CBPDC had a higher surface area (144.8 m2/g) than the Co NP@CBPDC (33.25 m2/g). The improved physicochemical merits of the Co AC@CBPDC allowed for better ORR and OER activities than the Co NP@CBPDC in terms of low halfway potential (E1/2), onset potential (Eonset), overpotential at 10 mA/cm2 (ƞ10), potential gap (∆E) between the overpotential of OER and the halfway potential, and a higher kinetic current density (jk). The enhanced electrochemistry of the Co AC@CBPDC was attributed to the defects created by the acid treatment. As proof of real-life applicability, the Co AC@CBPDC electrocatalyst delivered an excellent air cathode in a parallel plate RZAB cell with notable OCV (1.23 V), peak power density (49.9 mW/cm2), a real energy density (477 mAh/cm2), long-term stability for 210 h, enhanced voltage retention, Coulombic efficiency (ca. 100 %) and DOD (51.3%), comparable to literature. In addition, an all-solid-state RZAB based on the Co AC@CBPDC catalyst gave a higher and constant OCV (1.73 V) at varied bending angles (0 – 180 degrees) and excellent stability. Second, new HESOx materials were prepared via the Pechini method at two different annealing temperatures of 500 and 750 oC (abbreviated herein as HESOx-500 and HESOx-750). P-XRD results showed that these are inverse spinel oxides, with {311} as the dominant phase. HR-TEM images proved that they are single nanocrystalline materials. XRD and BET data showed that the HESOx-500 is smaller in size, more porous, and has a higher surface area than its counterpart (HESOx-750). HESOx-500 showed superior ORR performance with an onset potential of 0.93 V and a E1/2 of 0.88 mV. The OER performance also showed improved ƞ10 compared to IrO2 with an overpotential of 340 mV at a current density of 10 mA/cm2, and a 45 ± 5.0 mV/dec Tafel slope, above the performance of IrO2 (66 ± 6.1 V/dec). The ∆E of HESOx-500 was 0.69 V. The material was further tested as a cathode material in a RZAB cell. The optimised RZAB cell showed remarkable performance with a theoretical potential of 1.67 V and long-term stability of 375 h at 10 mA/cm2. The performance was attributed to the high-entropy compositional design with a high number of surface oxygen vacancies and different metal oxidation states. Finally, having dealt with the issue of bifunctionality in RZAB, a new ZnO@C anode material was also considered. The ZnO@PDA-DC (where PDA-DC means polydopamine-derived carbon) was used due to its ability to form Zn2+ pathways. Electrochemical potentiodynamic polarisation tests were performed to understand and compare the corrosion inhibition effects in an alkaline medium (6 M KOH). The ZnO@PDA-DC showed better corrosion inhibition properties than the zinc plate and other samples: low corrosion current (icorr = 0.107 uA/cm2) and corrosion potential (Ecorr = 1.077 V), and a mixed inhibition effect, indicating reduced hydrogen evolution reaction and zinc dissolution. Due to the excellent corrosion inhibition properties of the ZnO@PDA-DC, it was then evaluated in the RZAB cell. The shallow galvanostatic charge-discharge cycle stability at 2 mA/cm2 was able to maintain 150 h in a RZAB at a voltage gap of 0.76 V to 0.80 V. The results demonstrated that enhanced rechargeability is possible with ZnO@PDA-DC for RZAB.
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