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    Determination of neonicotinoid insecticides in water, soil and sediment samples: acute and chronic risk assessment.
    (2022) Ngomane, Nkosinathi Chris.; Mahlambi, Precious Nokwethemba.
    Neonicotinoids are a type of insecticides pesticides widely used worldwide as a result of their low vertebrates toxicity, relative environmental stabilities, good bioavailability and high level of selectiveness. These insecticides are commonly employed in agricultural activities, in grass management and horticulture as well as in households to control domestic pet flea. Due to neonicotinoids intensive usage, they are continuously introduced to the water bodies where they can adversely affect the aquatic life and accumulate in sediments. Moreover, they can end up in drinking and unintentionally consumed by human beings resulting to health effects. With this regard, this work reports for the first time on the occurrence of neonicotinoids in sediment, soil tap, sludge, wastewater and river water samples from the province of KwaZulu-Natal. Also, the ecological risk of neonicotinoids in water sources was also assessed for the first time in the samples from this province.The liquid chromatography coupled with a photo-diode array detector (LC-PDA) method was modified and applied for the simultaneous detection of neonicotinoids (clothianidin, thiamethoxam and imidacloprid). Ultrasonic extraction (UE), soxhlet extraction (SE) and solid-phase extraction (SPE) methods were developed and applied for the extraction of nitro-guanidine neonicotinoids in water, soil and sediment samples. The SPE, SE, and UE parameters that influence the recoveries of the analytes were first optimized before application to real samples for the analytes recovery improvement. The SPE was used for the extraction of neonicotinoids in sludge and water samples, while SE and UE were both used to extract soil and sediment samples. The extraction conditions optimized for SPE were conditioning solvent and sample volume. While for the UE were extraction time, extraction solvent, and the solvent volume. And for SE method, extraction solvent and the extraction solvent volume were optimized. The LC-PDA method used for detection was also first optimized to improve peak separation, retention times, detection limits and quantification limits. The optimized parameters for the LC-PDA method were the mobile phase, flow rate, and the PDA detection wavelength. Optimum water recoveries of the neonicotinoids ranged from 79 to 112%. The detection and quantification limits of the analytes in water samples were 0.013 - 0.031 μg/L and 0.041 - 0.099 μg/L, respectively. The obtained analytes concentration ranged from 0.061 - 0.10 μg/L, 0.077- 3.76 μg/L and 0.99 - 15 μg/L in tap, river and wastewater, respectively. Analyte recoveries ranged from 85 - 102% in soil and 92 - 103% in sediment for the ultrasonic extraction method. The neonicotinoid recoveries ranged from 83 to 109% in soil and between 84 to 94% in sediment samples for the Soxhlet extraction method. The method’s detection limits and quantification limits in solid samples ranged from 40 - 80 μg/kg and 140 - 270 μg/kg, respectively. The relative standard deviation was less than 4%. The concentration determined in real environmental samples were 47 to 410 μg/kg in soil and 25 to 410 in sediment. The toxicity studies showed that clothianadin pose a high risk towards daphnia species in the river. Imidacloprid, clothianidin and thiamethoxam posed medium risk against algae, daphnia and fish species in the effluent receiving water bodies. These results imply the necessity to continuously monitor these neonicotinoids in the water sources. In South Africa there is limited data concerning the environmental occurrence of neonicotinoids, therefore this work will contribute towards the information available for the analysis of neonicotinoids. This will assist the policy makers to establish the MRL values that are precise for the African continent.
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    Syntheses of (pyridyl) pyrazine carboxamide palladium(II) complexes, their DNA/BSA interactions, and cytotoxicity studies.
    (2023) Mvelase, Sabathile Thandeka.; Ojwach, Stephen Otieno.
    Reaction of pyrazine-2,3-dicarboxylic acid with a respective amine in the presence of triphenyl phosphite afforded the corresponding carboxamide ligands: [N2, N3-bis(pyridin 2-yl)pyrazine2,3-dicarboxamide] (L1), [N2, N3-bis(6-methylpyridin-2-yl)pyrazine-2,3-dicarboxamide] (L2), [N2,N3-bis(4-methylpyridin-2-yl)pyrazine-2,3-dicarboxamide] (L3), and [N2, N3bis(quinoline-8-yl)pyrazine-2,3-dicarboxamide] (L4). Treatments of the corresponding (pyridyl)pyrazine carboxamide ligands with [PdCl2(NCCH3)2 afforded new mononuclear and dinuclear palladium(II) complexes with a general formula, [Pd2(L1)2Cl2] (Pd1), [Pd2(L2)2Cl2] (Pd2), [Pd2(L3)2Cl2] (Pd3) and [Pd(L4)Cl] (Pd4). The identities and coordination nature of the palladium(II) complexes were established through a combination of characterization techniques such as NMR, FT-IR spectroscopy, mass spectrometry, elemental analysis, and single X-ray crystallography. The molecular structures of the dinuclear Pd1 and Pd3 complexes reveal that the two (pyridyl) pyrazine carboxamide ligands coordinate with the palladium atom via one arm, while the other arm remains non-coordinating. The ligands are bridged by two palladium atoms to form dinuclear palladium(II) complexes. While one ligand coordinates to the palladium in a bidentate fashion via the nitrogen atoms of the pyrazine and amide groups, the other ligand unit coordinates to the palladium through the pyridine nitrogen atom to give two ligand units and two palladium atoms in the complex coordination sphere. On the other hand, ligand L4 binds to palladium atom in a tridentate fashion via the pyrazine, amide, and pyridine nitrogen atoms to give complex Pd4 as a mononuclear species. The interaction of the palladium complexes (Pd1-Pd4) with calf thymus DNA (CT-DNA) was monitored using UV-Vis, and fluorescence spectroscopy. Absorption spectroscopic studies revealed that complexes Pd1-Pd4 interact with CT-DNA via intercalative mode, and the computed intrinsic binding constant (Kb) values range from (4.28-13.12) x 106 M-1. In addition, Ksv values of (1.82-28.41) x 106 M-1 and KF values of (1.01-53.44) x 104 M-1 determined in competitive binding studies confirmed the intercalative binding mode. The interaction of the complexes with CT-DNA decrease in the order of Pd3 > Pd2 > Pd1 > Pd4. Furthermore, bovine serum albumin (BSA) interaction was evaluated using fluorescence studies and the results revealed the existence of a static quenching mechanism with bimolecular constant, kq range of (0.66-13.99) x 1014 M-1 s-1. The Ksv values of (1.48-29.67) x 106 M-1 and KF values of (0.10-16.10) x 105 M-1 confirmed the interaction between BSA and palladium complexes. The interaction follows this order Pd2 > Pd1 > Pd3 > Pd4, which is inconsistent with the CT-DNA trend. In general, ligands bearing electron-donating methyl groups (L2 and L3) contributed to higher binding constants in their respective complexes Pd2 and Pd3 compared to the unsubstituted complex Pd1. In addition, the mononuclear complex Pd4 showed the weakest interactions with both the DNA and BSA, pointing to some beneficial effects of increased metal atoms in the complexes. The cytotoxic effect of the ligands L1-L3 and complexes Pd1-Pd4 were examined against a human breast cancer cell line (MCF-7) using 3-(4,5-Dimethyl-2thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. In general, the ligands displayed poor cytotoxicity L1 (IC50 > 400 μM), L2 (IC50 = 182.4 μM), L3 (IC50 = 80.2 μM), when compared to their respective palladium(II) complexes Pd2 (IC50 = 154.9 μM), Pd3 (IC50 = 230.1 μM). Complexes Pd1 (IC50 = 11.4 μM), and Pd4 (IC50 = 61.5 μM) displayed high and moderate cytotoxic activity which was attributed to the planarity of the complexes.
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    Vinyl-addition polymerization of norbornene catalyzed by (pyridyl)imine Ni(II), Pd(II), Co(II), and Fe(II) complexes.
    (2023) van der Westhuizen, Barend Danielle.; Ojwach, Stephen.
    The thesis reports the syntheses, and structural characterization of (pyridyl)imine transition metal complexes and their applications as pre-catalysts in the vinyl-addition polymerization of norbornene. The bidentate ligand (E)-N-(1 phenylethyl)-1-(pyridin-2-yl)methanimine (L1) was synthesized by reactions of 2-pyridine carboxaldehyde with (R)-(+)-α-methyl benzylamine in the presence of p-TsOH and obtained in moderate yields of 64%. On the other hand, the tridentate ligand (E)-1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanimine (L2) was obtained by reacting 2-pyridine carboxaldehyde with 2-picolylamine in the presence of p-TsOH in high yields of 90%. Reactions of bidentate ligand L1 and tridentate ligand L2 with NiCl2, [Pd(COD)Cl2], FeCl2, and CoCl2 metal salts gave the corresponding complexes [Ni(L1)2Cl2] (Ni1), [Ni(L2)Cl2] (Ni2), [Pd(L1)Cl2] (Pd1), [Co(L1)3][2PF6] (Co1), [Co(L2)Cl2] (Co2), [Fe(L1)3][2PF6] (Fe1), [Fe(L2)Cl2] (Fe2) in low to moderate yields (18% - 60%). The identities of the isolated complexes were confirmed by characterization with FT-IR spectroscopy, mass spectrometry, elemental analysis, single crystal X-ray crystallography for Co1, and nuclear magnetic resonance where applicable. The solid state structure of complex Co1 was established as tris-chelated, containing three ligands to give an octahedral coordination environment. The metal complexes were evaluated as pre-catalysts in the vinyl-addition polymerization of norbornene to produce poly(2,3-bicyclo[2.2.1]heptene) using modified methyl aluminoxane (MMAO) as the co-catalyst. Complex Ni1 was the most active with catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ followed by complex Pd1 which showed catalytic activity of 17.6 g×10³(PNBE).mol(M)-¹.h-¹ whereas complex Co1 showed 0.7 g×10³(PNBE).mol(M)-¹.h-¹ and complex Fe1 showed catalytic activity of 0.3 g×10³(PNBE).mol(M)-¹.h-¹ concluding that the choice of metal center is of absolute importance to achieve high catalytic activity. The number of electron donor atoms in the ligand structure influenced catalytic activity as bidentately chelated complex Ni1 showed catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ whereas the tridentately chelated Ni2 showed catalytic activity of 81.9 g×10³(PNBE).mol(M)-¹.h-¹. The influence of reaction parameters were investigated using Ni1 and Ni2 as pre-catalysts and it was concluded that monomer/metal ratios, co-catalyst/metal ratios, reaction temperatures, reaction times, and solvent choice influenced catalytic activity. A higher monomer/metal ratio of 1250 resulted in catalytic activity of 17.5 g×10³(PNBE).mol(M)-¹.h-¹ compared to the value of 6.8 g×10³(PNBE).mol(M)-¹.h-¹ obtained from a lower monomer/metal ratio of 625. An optimum co-catalyst/metal ratio of 1500 was established and recorded catalytic activity of 33.7 g×10³(PNBE).mol(M)-¹.h-¹. Polymerization reactions at room temperature gave higher monomer conversions of 70% as opposed to lower conversions of 17% obtained at 50 °C. The choice of solvent influenced catalytic activities of the complexes, with the more polar o-chlorobenzene solvent giving the highest monomer conversion of 70% in comparison to 31% obtained in toluene solvent. Polymers formed from all complexes were of the vinyl type with possible ring-opening metathesis polymeric inserts present in the polymer backbone. Thermal gravimetric analysis and differential scanning calorimetry of the formed polymers demonstrated that polymers formed from complex Co2 and complex Fe2 displayed degradation temperatures of 492 °C and 478 °C respectively opposed to polymers formed from complex Ni2 and complex Pd2 which gave values of 478 °C and 462 °C.
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    Synthesis of ketones through aerobic transition metal-catalyzed cross-coupling of thioesters.
    (2021) Mthombothi, Zabenguni Zakithi.; Sithebe, Siphamandla.
    Asymmetric diaryl ketones are an important class of compounds in organic chemistry due to their presence in natural products, synthesis, cosmetics as well as in biological active compounds. The aim of this project was to expand the scope of thiophilic metal carboxylates catalysts that can be applied to the aerobic Liebeskind-Srogl cross coupling reaction between various thioesters and phenylboronic acids. Thioesters bearing electron neutral, withdrawing and donating groups were successfully synthesized in yields ranging from 35 to 54%Xcv. The electron neutral thioester was used in the optimization of the aerobic Liebeskind-Srogl reaction and CuMeSal proved to be the most effective catalyst in this protocol. CuMeSal was applied in the aerobic synthesis of the asymmetric diaryl ketones bearing a wide range of functional groups including CF3, SMe, OMe and F yielding up to 65% in 24hrs. Aerobic Liebeskind-Srogl cross-coupling reaction was applied in the successful synthesis of chalcones, through the coupling of commercially available phenylboronic acids with previously synthesised thioesters catalysed by CuMeSal in DMF for 24hrs,furnishing desired products in poor to excellent yields 26-89%.
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    Synthesis, characterization and substitution reaction studies of pyridyl N,N’-bidentate palladium(II) complexes. A kinetic and mechanistic study.
    (2023) Mjwara, Pinky Ncomela.; Sithebe, Siphamandla.; Papo, Tshephiso Rose.
    The influence of structural as well as electronic properties of bidentate N,N chelates with different substituents on the mononuclear Pd(II) complexes were investigated. The complexes were synthesized and characterized by various spectroscopic methods such as 1H & 13C NMR, FT-IR, LC-MS, CHN and single x-ray crystallography. For the first set of complexes (Chapter 3), we studied the unexplored kinetics and mechanistic behaviour of N,N’-pyridyl Pd(II) complexes, viz. dichloro-(N-((pyridin-2-yl)methyl)aniline)palladium(II) (PdL1), dichloro-(4-fluoro-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL2), dichloro-(4-bromo-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL3), dichloro-(4-methoxy-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL4) and dichloro-(4-ethyl-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL5). The substitution behaviour of coordinated chloride atoms by three bio-relevant thiourea nucleophiles, viz. thiourea (TU), N,N’-dimethylthiourea (DMTU) and N,N,N’,N’-tetramethylthiourea (TMTU), of different steric demands was studied in a 0.1 M solution of ultra-pure water under pseudo-first order conditions. The reactions were studied as a function of concentration and temperature using standard Stopped-Flow and UV-Vis spectrophotometric technique. The substitution of the chloride atoms from the Pd metal by thiourea nucleophiles was a two-step reaction where the chloride trans to the pyridine ligand was substituted first, since the pyridine has a stronger trans effect compared to the amine group. The reactivity of mononuclear Pd(II) complexes containing bidentate N,N’-donor ligands with different substituents depends on the electronic effects of the complexes. The reactivity of the complexes increased with the presence of electron withdrawing substituents and decreased when an electron donating group was attached on the para position of the aniline moiety. The electron withdrawing groups influence the pull of electrons from the electron deficient amine that is coordinated to the metal center which results in the loss of electron density from the ligand moiety and increases the electrophilicity of the metal center and thus the substitution reaction. The reactivity of the nucleophiles depends on steric effects, with the bulky TMTU being the least reactive. The negative entropies and second order kinetics for all the substitution reactions support an associative mode of substitution mechanism. DFT calculations were performed to account for the observed reactivity of all the complexes studied. For the second set of novel Pd(II) complexes (Chapter 4), viz. bis[N-(4-bromophenyl)pyridine-2-carboxamidato] Palladium (Pd1) and Palladium(II) [N-(4-bromophenyl)-2-pyridinecarboxamide), pyridine chloride (Pd2), crystals were obtained and the structures were studied. Pd1 crystallizes in the monoclinic crystal system and in the P21/c space group, and Pd2 crystallizes in the orthorhombic system, with the space group Pbca.
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    Synthesis and the application of molecularly imprinted polymers as solid-phase extraction and dispersive solid-phase extraction sorbents in the extraction of antiretroviral drugs in water.
    (2022) Xolo, Thabiso.; Mahlambi, Precious Nokwethemba.
    A multi-template molecularly imprinted polymer was synthesized, characterized, and applied to real water samples for the specific extraction of antiretroviral drugs (ARVDs), abacavir, nevirapine, and efavirenz. A MIP was synthesized by bulk polymerization at 60 ºC using abacavir, nevirapine and efavirenz (templates), 2-vinylpyridine (functional monomer), 1,1’- azobis-(cyclohexanecarbonitrile) (initiator), ethylene glycol dimethacrylate (cross-linker) and toluene: acetonitrile (9:1, v/v) (porogenic solvent mixture) for 16 hours. The temperature was then increased to 80 ℃ for 24 hours to ensure complete polymerization. The synthesized MIP was washed with acetic acid: acetonitrile (1:9, v/v) via soxhlet extraction until all three ARVDs were undetected in the washing solvent using high performance liquid chromatography coupled with a photo-diode array detector. A non-imprinted polymer (NIP) was synthesised using the same reagents and quantities except for the templates. Both MIP and NIP were characterized using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscope (SEM), N2 physisorption analysis, and elemental analysis. The FTIR results showed that the polymers were similar in structure and BET showed that they were mesoporous. The SEM showed that the MIP surface was rougher when compared to the NIP and characterization with TGA showed that they were both thermally stable. The synthesized MIP was used to study its adsorption kinetics and isotherms. Kinetics modelling revealed that the Pseudo-second-rate order kinetics was the best fitting model with correlation coefficient of 1 compared to Pseudo-first-rate order kinetics which had a correlation efficient of 0.81-0.983 for all target analytes. The best fitting adsorption isotherm was the Freundlich model with a correlation range of 0.9451-0.986 for compared to the Langmuir model which had correlation efficient of 0.6692-0.93390.0198-0.6782 for all target analytes. The traditional solid phase extraction and the MIP-based solid phase extraction methods were applied in distilled water samples spiked with 1 mg.L-1 of ARVDs and recoveries obtained were ranging from 91.68-94.59% and 97.20 to 99.68%, respectively. The MIP-based dispersive solid phase extraction method was successfully optimized with recoveries ranging from 100.28% to 102.60% for all three analytes. Selectivity studies were conducted using both the NIP and MIP with lamivudine and diclofenac as competitors. The recoveries obtained for the MIP ranged between 92% to 98% for the target analytes while they were 63% to 79% for competitors. These results showed good selectivity and strong affinity of the polymer towards the target analytes than the competitors. This is justified by the presence of imprinted recognition sites that have the same functional groups, size, and shape as the target analytes/templates hence recoveries were low for competitors. The MIP was more selective towards analytes of interest compared to the NIP (recoveries ranged from 87.9% to 91%) for the analytes of interest which indicates successful imprinting on the MIP. Reusability studies showed that the MIP can be reused for up to 8 cycles with recoveries above 92% for all target analytes. The developed, adopted, and validated methods were then applied to wastewater, tap water and river water samples from around KwaZulu-Natal. The concentrations obtained for abacavir, nevirapine and efavirenz were 10.65-295.90 μg.L-1 in wastewater, 1.95-13.15 μg.L-1 in river water, and 2.17-6.27 μg.L-1 in tap water. Efavirenz was the most dominant and consistently detected ARVD in all samples. The MIP-DSPE was the most sensitive and selective extraction technique compared to SPE and MIP-SPE. Umhlathuzana and Amanzimtoti were the most ARVD’s polluted wastewater treatment plants, whilst Northern wastewater water works was the least polluted. Camps Drift was the most ARVD’s polluted sampling point in Msunduzi river. Napierville and Scottsville showed the most contaminated tap water samples from suburbs around Pietermaritzburg.
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    One-pot, multicomponent oxidative synthesis of 2,4,5-trisubstituted imidazoles from internal alkenes using an I2/DMSO system.
    (2023) Majola, Nonhlelo.; Jeena, Vineet.
    Imidazoles are vital heterocyclic compounds usually incorporated in natural products such as biotin, vitamin B12, histamine, and histidine. 2,4,5-trisubstituted imidazoles, in particular, possess versatile biological and pharmaceutical activities such as antidiabetic, antimalarial, and analgesic properties. A traditional procedure for the synthesis of these elegant compounds involves the cyclocondensation reaction between a 1,2-diketone, an aldehyde, and ammonia in the presence of an acid or metal catalyst. However, this methodology suffers from various shortcomings such as the use of acid or metal catalysts, tedious work-up procedures, use of toxic reagents, and substrate scope limitations. Hence, the development of new methods to synthesize 2,4,5-trisubstituted imidazoles is of vital importance. This study describes the preparation of 2,4,5-trisubstituted imidazoles from alkenes using an environmentally benign iodine/DMSO system. This novel methodology was applied to a broad substrate scope such as substituted benzaldehydes, heterocyclic aldehydes, bulkier aldehydes, and substituted stilbenes, and afforded the target compounds in moderate to high yields under mild reaction conditions. Preliminary mechanistic studies revealed that 1,2-diketone is a key intermediate and that the mechanism is not radical-mediated. It also revealed that the oxygen source is DMSO and that the coupling step is catalyzed by iodine coordination and hydrogen bonding from the solvent. Based on the results obtained from the preliminary mechanistic investigations, a reasonable mechanism is proposed.
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    Synthesis of non-natural amino acids as covalent inhibitors for protein-protein interactions.
    (2023) Dladla, Siphamandla Austen.; Sithebe, Siphamandla.; Veale, Clinton Gareth Lancaster.
    There is still a need to develop new cancer therapies for troubling cancers. Hence, a resurging interest in compounds that engage their target through covalent interactions. Lysine’s amine can be engaged covalently with a weak electrophile (SO2F) extending the potential of covalent inhibitors. Herein, we were prompted to investigate the synthesis of non-natural amino acids, modified to include weakly electrophilic warheads, which could potentially target specific lysine residues. Three new non-natural amino acids were successfully synthesized, methyl (S)-2-((tert-butoxycarbonyl)amino)-3-(4-((fluorosulfonyl)oxy)phenyl)propanoate, 3.5, methyl (S)-2-((tert-butoxycarbonyl)amino)-2-(4-((fluorosulfonyl)oxy)phenyl)acetate, 3.9, and methyl (S)-2-((tert-butoxycarbonyl)phenyl)propanoate, 3.35, in 85%, 89%, and 63.7% yield, respectively. Our study explored the synthetic pathway of a three-step procedure toward the target compounds, with the initial esterification of the carboxylic acid group, followed by the N-Boc protection of the amine group. Finally, the key sulfonation of the N-Boc protected amino methyl ester, where for 3.5 and 3.9, was performed through ex-situ generation of sulfuryl fluoride, which was installed following the substitution of the hydrogen on the hydroxyl group by SO2F. For 3.35, it was achieved through a palladium-catalyzed system and an in-situ fluorine introduction, where para iodine was substituted by the SO2 generated from DABSO. Under physiological conditions, compound 3.5 was assessed for possible interaction through its electrophilic warhead, with nucleophilic N-Boc-lysine side chain. The LCMS and NMR buffered assays were conducted, and in both these studies, the characteristics of a possible binding happening can be observed, hence an adduct N2-(tert-butoxycarbonyl)-N6-((4-((S)-2-((tert-butoxycarbonyl)amino)-3-methoxy-3-oxopropyl)phenoxy)sulfonyl)-L-lysine 3.5a formation.
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    Oxidative dehydrogenation of n-octane using morphologically different VMgO catalysts.
    (2017) Magwenyane, Ayanda Mandlenkosi.; Friedrich, Holger.
    ABSTRACT Gas phase paraffin oxidative dehydrogenation (ODH) has been explored extensively using medium chain paraffins such as n-hexane and n-octane as feedstock over various catalytic systems. Among these systems, the activity and stability of magnesium vanadates have been studied during the ODH of these paraffins. In most cases, it was found that 15 wt% vanadium was the optimum loading on the support, i.e. magnesium oxide. In addition, it was found that the morphology of the magnesium oxide influenced the performance of the catalyst. However, this has not been explored in much detail. In this work, magnesium oxide was synthesized using several methods to produce surfaces that differed morphologically, such as materials with a cubic morphology, denoted MgO-A, spherical morphology (MgO-B) and MgO with a nano-sheet type morphology (MgO-C). These supports were then loaded with 15 wt% vanadium by wet impregnation method using different synthesis methods. They were characterized using X-ray diffraction (XRD), Brunauer-Emmet-Teller technique, inductively-coupled plasma-optical emission spectroscopy, Raman spectroscopy, scanning and transmission electronic microscopy, temperature programed reduction (TPR) and temperature programed desorption (TPD). Electron micrographs generated for all the catalysts confirmed their respective morphologies. The XRD and Raman results showed the formation of magnesium orthovanadate and pyrovanadate phases for the cubic MgO. Only the orthovanadate phase was present for the other VMgO catalysts. This was confirmed by TPR which showed two reduction peaks for VMgO-A and one reduction peak for the other catalysts. The catalysts were tested for the ODH of n-octane at 450 °C using air as an oxidant. At an iso-conversion of 14 ± 1 %, VMgO-B (water), whose support had a spherical morphology was more selective towards ODH products, whereas VMgO-A (oxalic acid) where a cubic morphology was observed for the support, carbon oxides (COx) were the dominant products and this correlated well with the ammonium-TPD results which showed that catalysts with weak and medium acidic sites promoted the formation of ODH products, whereas catalysts with strong acidic site were more selective towards COx, in agreement with published results. Scanning electron microscopy images of the used catalyst showed agglomeration of particles while XRD showed a peak for the reduced phase of magnesium orthovanadate. This study has shown that catalysts with the same formulation but with different morphologies synthesised by methods modified to suit its physical and chemical properties, influenced the variance in the catalysis.
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    Plant-mediated synthesis of zinc oxide nanoparticles using extracts and isolated compounds from senecio serratuloides and their biological activity.
    (2018) Joyisa, Nonhlanhla.; Moodley, Roshila.; Pruessner, Karin.
    Senecio serratuloides DC of the Asteraceae family is a medicinal plant used in South African traditional medicine for the treatment of skin diseases, sexually transmitted infections (STIs) and wounds. Despite the ethnomedicinal significance of the plant, a phytochemical investigation to determine the active components for future pharmacological developments has not been conducted. Nanotechnology is a promising field in the development of biocompatible metal nanoparticles from bio-resources. Zinc oxide nanoparticles (ZnONPs) are of great interest due to their wide range of applications in the field of biomedical sciences. Research on the exploitation of plant materials for the green synthesis of nanoparticles is increasing rapidly. The aim of this study was therefore to extract, isolate and identify secondary metabolites from S.serratuloides, to use the extracts and isolates as reducing agents in the synthesis of zinc oxidenanoparticles, and to compare the antioxidant, antibacterial and anti-quorum sensing activities of extracts, isolated compounds, freestanding and plant-derived zinc oxide nanoparticles. The phytochemical investigation yielded one sesquiterpene (farnesylamine), five tripernoids (sitosterol, α- and β-amyrin, stigmasterol and taraxerone) and two jacaranones (jacaranone and a mixture of jacaranone and methyl-2-(1-hydroxy-4-oxocyclohexyl)acetate). The synthesised zinc oxide nanoparticles were characterised using spectroscopic and microscopic techniques. Spherical zinc oxide nanoparticles were successfully synthesised but exhibited a wide size range. ZnONPs synthesised using jacaranones showed good antioxidant activity whilst ZnONPs synthesised using extracts of S. serratuloides showed moderate antioxidant activity. The extracts, phytocompounds and nanoparticles were tested for antibacterial activity against three Gram-positive bacteria (Pseudomonas aeruginosa, Staphylococcus aureus and Enterococcus faecalic) and two Gram-negative bacteria (Escherichia coli and Chromobacterium violaceum). The extracts demonstrated promising antibacterial activityagainst Chromobacterium violaceum. Amongst the isolated phytocompounds, jacaranones showed promising antibacterial activity against two Gram-positive bacteria, Staphylococcus aureus and Enterococcus faecalic, with good antibacterial activity against Chromobacterium violaceum. Nanoparticles did not possess antibacterial activity. The isolated jacaranone, extracts and nanoparticles were further evaluated for quorum sensing inhibitory activity using a qualitative agar-overlay assay. The extracts showed promising anti-quorum sensing activity whilst the jacaranone showed good anti-quorum sensing activity. Nanoparticles did not show anti-quorum sensing activity. This can be attributed to freestanding nanoparticles not possessing activity against the bacterial strains tested and in plant-derived nanoparticles, the amount of the active compound capping particles could be too little to impart activity.
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    Influence of different fuels on the properties of solution-combustion synthesized palladium/ceria catalysts for low-temperature methane combustion.
    (2018) Mkhwanazi, Thabiso Perfect Oscar.; Friedrich, Holger Bernhard.; Singh, Sooboo.; Mahomed, Abdul Samad.
    Catalytic combustion of methane is a leading technology in energy production, emission prevention and gas clean-up. Its main advantage over traditional flame combustion is to carry out complete oxidation of fuel at low temperatures. Noble metals supported over high surface area supports are known to combust methane at low-temperatures. However, noble metals supported on oxide supports has been observed to result in high methane combustion activity at low temperatures. In recent studies on methane combustion, it was observed that the use of ceria as a support can significantly improve the catalyst activity. PdO supported on ceria is known to be a very active catalyst for methane combustion. However, this catalyst still suffers from poor activity at low temperature (below 673 K) and deactivation at high temperature (above 973 K) owing to the formation of metallic Pd from PdO particles. In this study, a comparison between solution combustion synthesis (SCS) and conventional incipient wet-impregnation catalysts was made and discussed for low temperature methane combustion. The PdO/CeO2 catalysts was prepared by the solution combustion synthesis method (SCS) with different fuels including oxalylhydrazide (ODH), citric acid monohydrate, urea, β-alanine, and tartaric acid were subsequently evaluated for low-temperature methane combustion. Each fuel is known to affects the physical and chemical properties of the catalyst which further influences the catalytic performances. To the best of our knowledge, the effect of fuels on the properties of SCS synthesised PdO/CeO2 catalysts, for low temperature methane combustion has not been reported. To evaluate these effects, several fuels such as oxalylhydrazide (ODH), citric acid monohydrate, urea, β-alanine, and tartaric acid were used. Furthermore, all prepared catalysts were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX), laser Raman spectroscopy (LRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2-physisorption analysis and X-ray photoelectron spectroscopy (XPS). Furthermore, all prepared catalysts were tested for methane combustion activity at 4 vol% methane in air and GHSV of 15 000 h-1. The N2-Physisorption analysis revealed that SCS ceria had a high surface area and a smaller crystallite size when compared to the commercial ceria. From Raman spectroscopy, the SCS ceria was established to contain more defects and thus, contained a higher amount of lattice oxygen incomparison to the commercial ceria. The SCS ceria provided almost 2.5-fold higher methane conversion than the commercial ceria at 600 ̊C. When the 2 wt.% PdO/CeO2 catalysts were compared, it was revealed that the SCS synthesized catalysts had a higher surface area and more oxygen vacancies and thus, higher catalytic activity in comparison to the catalysts prepared using wet impregnation. The study of the effect of fuels revealed, that different fuels result in catalysts with different physical and chemical properties. The surface areas of the prepared catalysts were observed to decrease according to the following trend, Pd0.03Ce0.97O2-δ-urea > Pd0.03Ce0.97O2-δ-tartaric acid > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-ODH. Raman spectroscopy, revealed that the amount of oxygen vacancies decreased in the order of Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-ODH > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-tartaric acid > Pd0.03Ce0.97O2-δ-δurea. The H2-TPR and XPS studies, revealed that the citric acid and ODH synthesized catalysts contained supported PdO in comparison to the other SCS synthesized catalysts. The T50 was observed to decrease in the following order Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-ODH > Pd0.03Ce0.97O2-δ-urea > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-tartaric acid. However, the T100 was observed to decrease in following order Pd0.03Ce0.97O2-δ-citric acid > Pd0.03Ce0.97O2-δ-urea > Pd0.03Ce0.97O2-δ-ODH > Pd0.03Ce0.97O2-δ-alanine > Pd0.03Ce0.97O2-δ-tartaric acid.
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    Density functional theory studies of the non-catalytic and catalytic oxidative dehydrogenation reaction of n hexane to 1 and 2 hexene.
    (2017) Damoyi, Nkululeko Emmanuel.; Friedrich, Holger Bernhard.; Kruger, Hendrik Gerhardus.
    One of the important areas in the present petrochemical industry is the catalytic production of olefins that have wide applications, including as starting materials for many valuable products, like polymers. Although the choice and advancement in the development of appropriate catalysts for such processes is a challenging effort, catalytic oxidative dehydrogenation (ODH) methods potentially provide for energy-favourable transformation of widely available hydrocarbon feedstocks into a variety of products, including olefins. The aim of this study was to use Density Functional Theory (DFT) methods to model the non-catalytic and catalytic (VMgO) ODH conversion of n-hexane hydrocarbon to 1- and 2-hexene, which are products that were characterised and obtained in low yields (< 20%) in our laboratory experiments. All the reaction pathways were modelled under experimental conditions of 573, 673 and 773 K and the relative total energies (ΔE#, ΔE, ΔG# and ΔG) were determined and in order to elucidate the non-catalytic and catalytic radical mechanisms for the reaction. In Chapter 2 of the thesis, the kinetically and thermodynamically most favourable non-catalytic reaction pathways were determined, with the rate-determining step (RDS) proposed as the interaction of the n-hexane and the O2 molecules through β-H abstraction (ΔE# = +42.4 kcal/mol) at 573 K. The most stable intermediates were found to be the alkoxy (C6H13O) and hydroxyl (OH) radicals. The propagation steps that lead to 1- and 2-hexene were proposed as likely to involve the two intermediate radicals, although the C6H13O radical may also contribute to side reactions that produce undesired oxygenates. Chapters 3 and 4 discuss the catalytic radical mechanisms of the interaction of n-hexane with H3VO4 and H4V2O7 model catalysts, respectively. Both the models comprise the vanadyl O atoms that are associated with the RDSs, through β-H abstraction at the kinetically and thermodynamically favourable temperature of 573 K. The calculated relative energies were ΔE# = +27.4 and +32.7 kcal/mol, for H3VO4 and H4V2O7, respectively. From the calculated value of ΔE# = +43.9 kcal/mol, the bridging O atom in H4V2O7 is not likely to activate n-hexane molecules. The produced C6H13 radical intermediate may either generate the desired olefin through the second H-abstraction by another vanadyl O in close proximity to it, or it may chemisorb to any of the surface O atoms, thereby enabling side-reaction channels for producing undesired products, such as the oxygenates. For both the catalyst models, the calculations show that the chemisorption pathways are kinetically and thermodynamically more favourable by ~10 kcal/mol than the H-abstraction pathways. This may be the reason for low yields (< 20%) that were obtained in our laboratory experiments for this catalytic system. The H-abstraction pathways that may lead to olefins are also likely to lead to the accumulation of OH groups on the catalyst surface. Low energy barriers for H-transfer and migration between two adjacent OH groups were calculated, with related intermediates stabilizing as a result of the formation of H2O. Barrier-less energies were also calculated for the reoxidation of the reduced V(III) by the O- species, to produce V(V) in both H3VO4 and H4V2O7. Finally, Chapter 5 discusses conclusions and the most likely mechanism for the combined non-catalytic and catalytic systems is proposed.
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    Studies on the short-term planetary wave activity in the MLT region over Southern hemisphere using super DARN HF radar.
    (2016) Ngwane, Ntlakanipho.; Sivakumar, Venkataraman.; Mthembu, Sibusiso Hector.
    The atmospheric vertical pressure profile, chemical composition and temperature distribution, together, define a set of conditions governing the manifestation of atmospheric dynamics. Observations and extensive research shows that lower atmospheric layers play host to the formation of forced atmospheric waves such as atmospheric planetary waves and solar tides. These waves serve as transportation modes for energy budgets and ascend to upper atmospheric layers where they induce significant meteorological processes. Planetary waves, tides and gravity waves often dissipate energy in the mesosphere lower-thermosphere (MLT) region. This dissertation presents a study of the planetary wave variability in the MLT region using the South African National Antarctic Expedition High Frequency (SANAE HF) radar data, a component of the Super Dual Auroral Radar Network (SuperDARN). The focus is on short term planetary waves with periods ranging from 2 to 6 days. This planetary wave variability in the MLT is investigated during the occurrence of minor sudden stratospheric warming (SSW) events. The study also investigates the assertion that there is a non-linear interaction between planetary waves and atmospheric tides. The mesospheric wind data considered stretched from year 1998 to year 2008. The criterion towards a conclusive investigation of short term planetary waves included determining years within the said interval (1998 to 2008) with minor SSW events. The 11 year long temperature data from the NCEP/NCAR reanalysis project was used for this. The study managed to show the previously stated finding that mesospheric wind reversal occurs a week prior the stratospheric wind reversal linked with the warming. Years 2002, 2003 and 2007 were shown to host minor SSW events. In year 2002, minor SSW events occurred in days 235, 243 and day 255. In year 2003, the minor SSW event occurred in day 280. The minor SSW for year 2007 occurred in day 263. This meant that the planetary wave variability and the non-linear interaction between planetary waves and tides is investigated in the said years only. The short time Fourier transform technique (STFT) was used to reveal the tidal wave behaviour in the MLT region. It was observed that the semi-diurnal tide (SDT) is the most active tide at high latitudes. The wavelet transform was used to show the planetary wave variability in the MLT region. Along the zonal component, the activity in year 2002 was shown to be the most robust compared to the activity in years 2003 and 2007. In the meridional component, the planetary wave behaviour in year 2007 was the most active compared to years 2002 and 2003.The wavelet transform was simultaneously used to implement the first phase towards asserting the non-linear interaction between planetary waves and atmospheric tides. This phase is termed SDT modulation. In year 2002, 2 day and 3 day p-waves possibly modulated an SDT along the zonal component while 3 day and 6 day p-waves possibly modulated an SDT in the meridional component. In 2003, both along the zonal and the meridionalcomponents, the 4 day p-wave possibly modulated an SDT. In year 2007, a 6 day and a 4 day p-wave possibly modulated an SDT along the zonal and meridional components respectively. The proposition that there is a possible non-linear interaction between p-waves and tides was further reinforced using the bispectrum analysis. This method revealed the interaction predicted by the modulation phase. Peaks signifying p-wave SDT wave interaction were observed. These peaks were all consistent with the modulation that was said to have occurred between p-waves and SDTs. The third step in validating the stated assertion involved looking for secondary waves that may have formed due to a possible primary wave (p-waves and SDTS) interaction. This step was conducted on periods suggested by the SDT modulation. In every p-wave SDT wave suspected interaction as per the SDT modulation, secondary waves were shown to exist.
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    Assessment of the impact of reforestation on soil and river water quality based on organic chemical pollutants.
    (2018) Pillay, Vishalan.; Moodley, Brenda.
    Forests are a natural resource and are influential in most countries as they are a source of food, clothing, and form of shelter for many organisms. These sections of forested land have been sacrificed for the development of urban areas, making way for agriculture, cities and the ever increasing human population. Some of the detrimental effects associated with deforestation are as follows: loss of wildlife and fish habitats, increased nutrient and sediment loads in nearby rivers, and ultimately increases in greenhouse gas emissions. Reforestation refers to the planting of trees so as to replenish an area that was previously a forest but due to anthropogenic effects, such as land deforestation, resulted in its deterioration. This project aimed to assess the impact of the eThekwini Municipality reforestation project on the quality of the soil within the reforestation sites, and water from the nearby rivers (White and Black Mhlasini Rivers) situated at the reforested Buffelsdraai area in KwaZulu-Natal. The levels of organic pollutants were assessed from the analysis of soil, sediment and river water. Selected polyaromatic hydrocarbons (PAHs) and pesticides, which had been previously utilised at this site when it was a sugarcane farm, were analysed. The sixteen PAHs analysed were naphthalene, acenaphthylene, acenaphthene, fluorine, phenanthrene, anthracene, fluoranthene, pyrene, chrysene, benzo[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[i]fluoranthene, benzo[a]pyrene, benzo[e]pyrene, indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene, and benzo[g,h,i]perylene which are on the Environmental Protection Agency (EPA) list of priority pollutants. The pesticides analysed were hexazinone, oxamyl, and acetochlor. The soil and sediment samples were extracted using ultrasonication, and liquid-liquid extraction was utilised for the water samples. Gas chromatography mass spectrometry (GCMS) and liquid chromatography mass spectrometry (LCMS) were used to analyse the PAHs and pesticides, respectively. PAH recoveries on the GCMS ranged between 60-110%, and pesticide recoveries on the LCMS were between 83-113%. The PAH LOD values were between 0.30–0.69 μg g-1 and between 0.17-0.32 μg g-1 for pesticides. PAH LOQ values ranged between 0.99-1.9 μg g-1 and between 0.56-1.33 μg g-1 for pesticides. The total PAH concentrations determined were between 4.258 – 6.426 μg g-1 in the soil samples, 2.210 – 13.900 μg g-1 in sediment, and 6.360 – 85.468 ng L-1 in river water. The total pesticide concentration was between 1.271 – 1.742 μg g-1 in soil, 0.197 – 1.175 μg g-1 in sediment, and 0.792 – 12.950 ng L-1 in river water. A comparison between the soil samples and the control,showed that reforestation is potentially reducing the concentration of organic chemical pollutants. The water and sediment samples also provided potential evidence of the positive impact of reforestation, as it revealed the concentration of pollutants to be lower within the reforestation boundaries and higher outside the reforestation boundary. The most abundant PAH determined in the samples was fluoranthene, which could possibly be due to this hydrocarbon being the most abundant aerosol in the atmosphere. Source apportionment analysis showed that most PAHs originated from pyrolytic sources, which was from burning of sugarcane. The total concentration for specific PAHs was above the threshold value for most sampling sites according to Canadian environmental guidelines. However, reforestation was shown to potentially be reducing these pollutant concentrations. The findings from this study will assist the neighbouring communities and eThekwini in future planning for the extension of existing or development of new reforestation sites.
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    Synthesis, characterisation and antibacterial activity of thiadiazolopyrimidinones.
    (2018) Ntshele, Thandokazi Andiswa.; Singh, Parvesh.; Koorbanally, Neil Anthony.
    Seven thiadiazolopyrimidinone hybrid derivatives were successfully synthesized in a three step reaction that involved synthesis of 5-substituted phenyl-1,3,4-thiadiazole-2-amines, condensation of the amines with DMF-DMA (dimethylformamide-dimethylacetal), and the [4+2] cycloaddition reaction of the imidoformamides and phenoxyketene. The DMF-DMA used in the second step was synthesized using sodium methoxide, DMF and dimethylsulfate (DMS). The synthesized compounds were all novel derivatives. Structural elucidation of all synthesized compounds was carried out using both 1D and 2D NMR spectroscopy together with mass spectrometry. All synthesized compounds were subjected to antibacterial testing against both Gram positive and Gram negative strains and showed excellent activity against Gram negative strains but no activity against Gram positive bacteria. In comparison to the two standards used, Levoflaxocin and Ciproflaxocin, compounds 3a (MBC = 0.19 μM) and 3e (MBC = 0.15-3.92 μM) showed better activity against Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumonia and Salmonella typhimurium while compounds 3b, 3d, 3g and 3h showed better activity than these standards for three of the four strains.
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    Green synthesis of silver, selenium and zinc oxide nanoparticles using extracts and isolated secondary metabolites of chrysanthemoides monilifera and harpephyllum caffrum and their biological activity.
    (2018) Vundla, Ntombizodwa.; Moodley, Roshila.; Pruessner, Karin.
    The method of using plant extracts for the synthesis of metal-based nanoparticles is a recently developed technique which is not only cost effective but environmentally friendly. It has very low energy requirements, needs no specialised equipment and reactions are completed within minutes. The resultant materials can be applied to catalysts, medicine, electronics and optics. The biological activity of plant extracts and nanoparticles have each been studied and are well known. In addition to being green, nanoparticles produced by the plant-mediated synthesis route are being utilised in biomedical applications with the added advantage of increased activity arising from the synergistic effects of both the biologically active nanoparticles and plant extracts or phytocompounds. In this project, extracts and phytocompounds from the indigenous, South African, medicinal plant species Chrysanthemoides monilifera and Harpephyllum caffrum were obtained and used to synthesise and stabilise silver, zinc oxide and selenium nanoparticles. The plant extracts and phytocompounds were characterised using spectroscopic techniques. Quercetin was isolated from the extract of C. monilifera and catechin from the extract of H. caffrum. The growth of nanoparticles was investigated at various concentrations of phytocompounds. The synthesised nanoparticles were characterised using spectroscopic and microscopic techniques. Growth seemed to occur by agglomeration and subsequent re-orientation. Changes in concentration had an effect on the yield, shapes and sizes of the synthesised nanoparticles. To investigate the synergistic or antagonistic effect of the capping agents on the biological activity of synthesised nanoparticles, freestanding (uncapped) nanoparticles were synthesised, using sodium borohydride as a reducing agent, for comparison. Results showed biosynthesised nanoparticles to be capped with plant biomolecules. The biosynthesised nanoparticles had a wider size distribution than freestanding nanoparticles. The particles were tested for their antioxidant activity using three different assays (1,1-diphenyl-2-picrylhydrazyl radical (DPPH•) radical scavenging, ferric reducing antioxidant power (FRAP) and H2O2 radical scavenging. The results showed plant biomolecules to enhance the antioxidant activity of the biosynthesised nanoparticles. The anti-microbial activities of the nanoparticles were evaluated with the Kirby-Bauer disc diffusion susceptibility test. Capping of selenium and zinc oxide nanoparticles with plant biomolecules did not have any effect on antibacterial activity. H. caffrum was found to enhance the activity of the silver nanoparticles whilst quercetin promoted its anti-quorum sensing ability.
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    Biosynthesis of silver and selenium nanoparticles using extracts of Commelina Africana and their biological activity.
    (2019) Mhlongo, Silungile Pearl.; Moodley, Roshila.
    Commelina africana is an indigenous medicinal plant that belongs to the Commelinacaea family. This plant is traditionally used for the treatment of venereal diseases and burns, and it is eaten as a vegetable. Despite the plants medicinal and nutritional significance, it has not been phytochemically investigated. Nanoparticles are currently being investigated for their medicinal value. Different synthesis techniques, especially those that are less toxic, inexpensive and more environmentally friendly are being explored. The use of plant material to synthesise nanoparticles is known to be a greener and safer method. These nanoparticles have potential for applications in the biomedical sciences. Currently, silver nanoparticles (AgNPs) are being used for the dressing of wounds and prevention of infections. Selenium nanoparticles (SeNPs) are being used biologically as antioxidants. This study aimed at extracting, isolating and characterising the secondary metabolites from C. africana. The crude aqueous and methanolic extracts from the plant were used to synthesise AgNPs and SeNPs at varying extract volumes. The crude extracts served as reducing agents and reduced Ag+ and Se4+ to their elemental states. The synthesised nanoparticles were tested for their antioxidant activities (using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays) and antibacterial activities relative to the extracts from which they were synthesised and known standards. Four phytosterols (stigmasterol, ergosterol peroxide, β-sitosterol and campestanol) were isolated and identified from the extracts of C. africana. AgNPs and SeNPs were synthesised using the plant material and characterised using spectroscopic and microscopic techniques. AgNPs consisted of spheres and SeNPs showed a mixture of spheres and rods. Both types of nanoparticles had a wide particle size range.For the antioxidant activity, ergosterol peroxide showed good radical scavenging activity by the DPPH test whilst SeNPs synthesised using the methanol extract showed good radical scavenging activity and reducing potential by the DPPH and FRAP tests, respectively. Crude extracts, phytocompounds and nanoparticles were tested against two Gram-positive bacteria (Enterococcus faecalis and Staphylococcus aureus) and four Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae and Chromobacterium violaceum) for their antibacterial activity. Plant synthesised AgNPs showed satisfactory inhibitory activity against the growth of all bacterial strains except E. faecalis. These nanoparticles also exhibited anti-quorum sensing abilities indicating their potential to block cell-to-cell signalling that is used to determine their physiological activities and for co-ordinated gene expression. This study adds to the base of knowledge on the medicinal plant species, C. Africana and highlights the synergistic effects between AgNPs and SeNPs with active plant biomolecules for biological systems. The findings of this study also indicate that medicinal plants can be used as a greener and more environmentally friendly technique for the synthesis of AgNPs and SeNPs.
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    Seasonal and spatial distribution of heavy metals in surface water and sediments of Msunduzi river, KwaZulu-Natal, South Africa.
    (2017) Magwaza, Nontobeko Pamela.; Shozi, Mzamo Lungani.; Ndungu, Patrick Gathura.
    Some physicochemical parameters and heavy metal concentrations of surface water as well as sediments in the Msunduzi River were investigated to provide information on the spatial and seasonal characteristic patterns of surface water quality, concentrations, chemical speciation and environmental risks of heavy metals in surface sediments. The physicochemical properties of surface water were determined following standard analytical procedures as described by the American Public Health Association (APHA). The levels of total metals in water and sediments were quantified by inductively coupled plasma-mass spectrometry (ICP-MS) after digestion with a mixture of acids, while the chemical speciation of heavy metals in the sediment was evaluated following the Community Bureau of Reference (BCR) sequential chemical extraction method. The physicochemical, concentrations and chemical speciation patterns of heavy metals in the investigated samples showed significant (p<0.05) spatial and seasonal variations. The value ranges for the measured physicochemical properties of water were: temperature 12.9 - 28.7 oC, pH 5.08 - 9.02, EC 84.0 - 1165 μS cm-1, TDS 60.9 - 952 mg L-1, DO 3.08 - 6.18 mg L-1, Cl- 8.73 - 102 mg L-1, NO3- nd - 7.96 mg L-1 and SO4- 0.27 - 103 mg L-1. Total metal concentrations in surface water for all seasons ranged from 0.22 - 5.44, 0.00 - 1.35, 0.80 - 50.2, 0.16 - 21.9 and 0.03 - 658 μg L-1 for As, Cd, Cr, Pb and Zn, respectively while those of the sediment varied from 1.5 - 2.79, 42.26 - 246, 8.3 - 63.7 and 36.9 - 402 mg kg-1 for Cd, Cr, Pb and Zn, respectively.The water quality index (WQI) for surface water based on physicochemical and heavy metal characteristics were 50, 85, 134 and 155 for autumn, winter, spring and summer, respectively. The WQI values for this river system fell within the “very poor quality” to “unsuitable for drinking” water categories. The ecological risk index indicated that there is low ecotoxicological risk associated with exposure to heavy metals in these sediments. The speciation patterns indicated that Cd, Pb and Zn were found mostly in the residual and Fe-Mn oxide phases while Cr existed mainly in the residual and oxidisable phases.
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    Production of nanocellulose from renewable resources and conductivity measurements of polypyrrole hybrid nanocomposites.
    (2018) Nirmal, Nadia.; Van Zyl, Werner Ewald.
    The extraction of nanocellulose (NC) from i) bacterial cellulose (grown using "Symbiotic 'Colony' of Bacteria and Yeast" or SCOBY), ii) Whatman filter paper, and iii) hardwood pulp was successfully investigated in this study. Acid hydrolysis was applied to these three materials and nano-fibrous whiskers were formed. When the nano-fibres were subjected to dialysis, the nano-fibres exhibited a crystalline structure known as nanocrystalline cellulose (NCC). Harmless disposal of the residual hydrolysis acid byproduct is still an obstacle that hinders large-scale production of NCC and NCC-based nanocomposites. In this work, the hydrolysis products of NC without further separation was studied to produce nanofibrous cellulose (NFC). The NFC structural integrity was compared to conventional NCC. The structural composition was verified using Fourier - transform infrared spectroscopy (FT-IR) and Raman spectroscopy. The nanofibers were examined with Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM-EDS) and Transmission Electron Microscopy (TEM). The dimensions of the nanocellulose were within the nanometre range that has acceptable aspect ratios for ideal stress transfer of the fibre-matrix interaction. For the various nanocellulose samples, i.e. from Bacterial cellulose, Whatman Filter paper and hardwood pulp, it was found that the concentration of the dialysis-free nano-fibrous cellulose has a higher concentration and longer dimensions than that of dialysed nanocrystalline cellulose. The nanocellulose was then utilized as a stabilizer for the synthesis of polypyrrole using a pyrrole monomer and ferric chloride hexahydrate initiator that produced a well dispersed network of polypyrrole@nanocellulose (PPy@NC) hybrid nanocomposite. Various concentrations were produced to find the optimum ratio of polypyrrole to nanocellulose to give excellent stability and dispersity. The ratio of 1:1 polypyrrole to nanocellulose exhibited good suspension of the nanohybrid, forming a well dispersed network. The nanohybrid formation was confirmed using TEM, SEM, FT-IR, Raman spectroscopy and Ultraviolet-visible spectroscopy (UV-Vis). Interestingly, the PPy@NC nanohybrid could be easily isolated from the polymerization products due to the decreased surface charge. The PPy@NC nanohybrid was subsequently suspended in polyvinyl alcohol (PVA), which facilitated the construction of a continuous PPy@NC conductive network in the polymermatrix for both NCC and NFC. The development of a novel resistivity apparatus and tests were conducted for the first time on all samples using an expedient method and the results were similar with that of the commonly used but restrictive four-point probe method. The PPy@NFC/PVA nanocomposite showed significant improvement in electrical conductivity and mechanical properties when compared with neat PPy/PVA composites and exhibited slightly improved performance when compared to the dialysed nanocellulose composite, PPy@NCC/PVA, at the same ratio. This was due to the dialysisfree synthesis process allowing the residual hydrolysis acid to act as a doping agent for the synthesized PPy, endowing PPy@NFC nanohybrid with improved electrical conductivity. Dialysis is a time-consuming step during acid hydrolysis, and this study investigated the both the effect of products formed and their electrical properties with and without the dialysis step. It was found that the nanohybrid synthesized from bacterial nanocellulose in a dialysis-free step, showed a resistivity of 2.341 Ω/m that has an improved resistivity result compared to the nanohybrid prepared from bacterial cellulose that was dialysed which was 2.928 Ω/m; a lower resistance value allows for greater conductance. The nanohybrid made from nanocellulose sourced from Whatman filter paper, that was not subjected to dialysis gave a better resistivity of 2.287 Ω/m as compared to the nanohybrid from the nanocellulose subjected to dialysis, which was 4.954 Ω/m. The dialysis-free hardwood pulp nanohybrid produced a nanohybrid with a resistance of 6.515 Ω/m and the dialysed gave a resistance of 8.402 Ω/m which had the greatest opposition of the flow of current through the material. When the samples were viewed under a microscope before and after being subjected to an applied voltage, the samples retained their integrity and could be re-used several times. This work conclusively demonstrated that by capitalizing on the physiognomies of nanocellulose not subjected to dialysis, its unique traits can be further exploited for useful applications. By avoiding the costly and laborious dialysis step, one could easily utilize the residual acid as a doping agent that contributed to the desired conductance required. The straightforward and sustainability of this dialysis-free and in-situ doping synthesis of the PPy@NFC nanohybrid should facilitate in a significant way the scalable fabrication and application of nanocellulose based conductive nanocomposites with high performance.
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    Synthesis and antibacterial studies of new cadmium,silver and zinc dithiophosphonates.
    (2018) Mkumbuzi, Emmanuel Ranganayi.; Van Zyl, Werner Ewald.
    A variety of dithiophosphonate and bis(dithiophosphonate) compounds were synthesised from the reaction between a dithiophosphetane disulfide dimer, [P(4-C6H4OMe)S(S)]2, commonly referred to as Lawesson’s Reagent (LR), and alcohols. The LR was symmetrically cleaved by nucleophilic attack from primary or secondary alcohols, as well as diols. The alcohols used include isosorbide, 1, 4 cyclohexanol, 2-hexanol, isomannide, 2-butene 1, 4-diol, 2-methyl-cyclohexanol and 2-methyl- 3 hexanol. The reaction of alcohols with LR resulted in the replacement of the P ̶ S bond by a P ̶ O bond due to the oxophilic nature of the phosphorous atom and consequent formation of dithiophosphonic acids (pKa ~ 3-4) which were then readily deprotonated by ammonia gas to form the corresponding ammonium dithiophosphonate and bis(dithiophosphonato) ligand salts of the form NH4[S2PR(OR’)] and (NH4)2[S2PR(OR’O)PS2R] respectively where (R = 4-C6H4OMe, and R’ = a spacer portion from a diol). The ligands were further reacted with aqueous metal nitrate salts of zinc(II), cadmium(II) and silver(I) precipitating copious amounts of dinuclear complexes with yields of at least 70% in most instances. The characterisation of the compounds was achieved using solubility, stability, colour, melting point and several techniques which included FT-IR, mass spectrometry, 1H-NMR, 13C-NMR, 2D-NMR and 31P-NMR and SC-XRD. The 31P-NMR spectra revealed singlet peaks indicative of a single phosphorous centre or equivalent phosphorous centres which were typically in the range ca. 90-110 ppm, values which are consistent with dithiophosphonate chemical shifts. Crystals of Cd(II) [3B] and Zn(II) [3C] complexes were successfully grown and subjected to single-crystal XRD analyses. The structures of the cadmium(II) and zinc(II) complexes were confirmed by SC-XRD as dinuclear complexes with an inversion centre within the molecular structure and comprising three ring structures including an eight-membered ring consisting of the metal, sulfur and phosphorus atoms, The dinuclear core M2S4P2 formed via bridging of the ligands with the two metal centres and with a distorted tetrahedral geometry around the metal atoms. The Zn(II) and Cd(II) dinuclear metalcomplexes had formulae of [Zn2{S2PR(OCH2HC=CHCH2O)PS2R}2] and [Cd2{S2PR(OCH2HC=CHCH2O)PS2R}2], respectively, where (R =4-C6H4OMe). The synthesised dithiophosphonate ligands and complexes were subsequently investigated as antimicrobial agents against Methicillin resistant Staphylococcus Aureus, Staphylococcus Aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia Coli and Salmonella Typhimurium bacterial strains. In general, the metal complexes were found to be more efficacious than the ammonium salts against these virulent bacterial strains. Silver(I) complexes were the most effective, consistent with the general observation that silver is a good antibacterial agent while cadmium generally had poor effectiveness. The most susceptible bacteria was Klebsiella pneumoniae, while the most resistant bacterial strains were Salmonella Typhimurium and Escherichia Coli. Most compounds, especially ammonium salts, were ineffective against these two bacterial strains in particular Salmonella Typhimurium. In general, each of the synthesised compounds had some degree of antibacterial activity against at least two strains.