1,720,987 research outputs found
An investigation into the influence of substituents and extended π-conjugation on the substitution reactions of bifunctional platinum (II) complexes.
Full text linkMaster of Science in Chemistry. University of KwaZulu-Natal, Pietermaritzburg 2015.Abstract available in PDF file
Determination of polycyclic aromatic hydrocarbons in the water, soils and surface sediments of the Msunduzi River, KwaZulu-Natal, South Africa.
Full text linkMaster of Science in Chemistry. University of KwaZulu-Natal, Pietermaritzburg 2016.Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants which are known carcinogens. Their presence in the environment has been linked to cancer, neurological and reproductive illnesses to name but a few. Hence it is important to monitor the levels of these PAHs in order to identify areas of high pollution and possible toxic exposure to aquatic and human life. The concentration of seven polycyclic aromatic hydrocarbons (namely naphthalene, acenaphthylene, fluorene, phenanthrene, anthracene, pyrene and chrysene) were determined in 28 surface water, 8 wastewater, 26 soil and 26 surface sediment samples from the Msunduzi River. Water samples were extracted using a liquid-liquid extraction technique into dichloromethane and dehydrated with sodium sulfate anhydrous. The soils and surface sediments were extracted with a mixture of dichloromethane and n-hexane (1:1 v/v) using the soxhlet extraction technique. The crude extracts were purified by silica gel packed column chromatography. The concentrations of PAHs in the extracts were analysed by GC-MS. The instrument was calibrated using internal standardization (deuterated PAH) and PAH standards. Percentage recoveries of 7 PAHs in the spiked and equilibrated samples varied from 79.16±0.01 to 101.28±0.02 and 80.30±0.02 to 105.56±0.01 for solid and water samples, respectively. The grand average in the summed concentrations of concentrations of the 7 PAHs in the water for all seasons decreased in the order: Σ[7-PAH] spring > Σ[7-PAH] summer > Σ[7-PAH] autumn > Σ[7-PAH] winter while in the surface sediments was in the order: Σ[7-PAH] spring > Σ[7-PAH] autumn > Σ[7-PAH] summer > Σ[7-PAH] winter and in the soils was in the order: Σ[7-PAH] spring > Σ[7-PAH] autumn > Σ[7-PAH] winter > Σ[7-PAH] summer. The concentration of PAHs was found to be comparatively higher in the soils and surface sediments than in the water samples
Controlling the reactivity of mononuclear palladiun (II) complexes. Substitution kinetics and mechanisms.
Full text linkDoctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.Abstract available in PDF
Role of Substituents on the reactivity of mononuclear cis-platinum(II) complexes with 2 (phenylthiomethyl)pyridyl/quinolyl non-leaving ligands.
Full text linkMaster of Science in Chemistry. University of KwaZulu-Natal, Durban 2017.Abstract available in PDF file
The rates and mechanisms of substitution from Ru(II) complexes with different non-leaving ligand environments.
No full textDoctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.Results from multiple studies have confirmed that the nature of ligands on the metal centre determines the properties of an anti-cancer agent in a biological environment. Ligands affect solubility, substitution reactivity, stability of compound and product after substitution and the type of interaction between the agent and DNA among others properties. Due to competition for sulphur biomolecules by anti-cancer agents in the cells, substitution reactions of potential anti-cancer metal complexes with biologically relevant ligands sheds light on the possible interaction modes of the Ru(II) complexes and stability of the resulting products. This helps in the design, synthesis and administration of new pharmacological agents and in the concept of chemo-protection. On this basis, the study of rate of substitutions from the Ru(II) complexes by thiourea nucleophiles under pseudo-first order conditions was undertaken. The reactions were studied as a function of concentration and temperature using standard Stopped Flow technique for ultrafast reactions or UV-Visible Spectrophotometer.
The first series of the complexes investigated the role of arene ligands on the rate of substitutions in (aqua)(η6-arene)Ru(II) complexes. The rate of substitution for the tri-aqua Ru(II) complexes was controlled by the π-acceptor ability of the arene ligands. For the complexes bearing 2,2’-bipyridyl co-ligand, the leaving aqua ligands are located trans to the arene ligands. For these complexes, the reactivity increase in accordance to the number and type of alkyl substituents on the η6-arene ligands which donate inductively into the -molecular orbitals, causing increased trans labialisation of the coordinated aquo co-ligand. Compared to the reactivity of tri-aquo complexes, the auxiliary bipyridyl ligands lower the rate of substitution for the later complex by a factor of about 100, due to its steric hindrance at the Ru(II) metal centre. The significantly negative activation entropies and positive activation enthalpies suggest that the activation process is dominated by bond making. In the second study, the role of arene and phosphino ligands on the rate of chloride substitution from Ru (II) complexes containing arene and phosphino co-ligands was investigated. It was observed that the coordinated arene ligand donates electrons towards the Ru(II) metal centre and its -electron cloud presents an electrostatic repulsive effect onto and around the Ru centre as measured by the projected cone angle. The bidentate bis(diphenylphosphino)-methane ligand hinders the approach of nucleophiles during the substitution process. When the bis(diphenylphosphino)methane chelate is expanded through the introduction of a methylene carbon within the bridge, the steric hindrance to the approach of nucleophiles is reduced and
ii
the ligand assumes a trough like conformation which traps the nucleophile within the coordination sphere. This enhances the reactivity by a factor of 103.
The rate of chloride substitution from 2,4,6-tris-(2-pyridyl)-1,3,5-triazineRu(PPh3)(Cl) and analogous complexes was done in the third study. The study showed that higher π-acceptor ability of cis ligands increase the electrophilicity of the metal centre resulting in enhanced reactivity. Electron donating substituents on the ligands at the cis position lower the π-acceptor ability of the ligand hence lower electrophilicity of the Ru(II) metal centre leading to slower rate of substitution. The investigation on the effect of 2-(2-Pyridyl)azole-based ancillary ligands (L) on the chloride substitution from [RuII(tpy)(L)(Cl)]+ in the fourth study revealed that strong electronegative atoms (O or S) in the auxiliary ligands enhance their π-back-donation capacity thereby increasing the electrophilicity of the metal centre and hence the reactivity. On the other hand, the –NH group donate electron density to the metal centre by outer sphere proton donation causing trans-effect thereby which increases the reactivity more than in the former case. The fifth study sought to understanding the effect of substituents on rate of chloride substitution from Ru(II)tpy complexes. Ru(II)tpy complexes with the tpy having electron donating substituents trans to the labile ligand were dominated by trans-effect while those where the tpy bears electron accepting substituents had enhanced π-back-bonding controlling the reactivity. The rate of substitution from the Ru(II) complexes was more strongly affected by electron donating substituents. Electron donating ligands at the cis position slow down the rate of substitution from the Ru(II) metal centre.
Data from DFT calculations performed using Gaussian09 suite of programmes was used to support the observed rates of substitution from the Ru(II) complexes. Large negative values of entropies of activation and positive enthalpies of activation indicate associative mode of activation. On the other hand small positive values of entropies of activation indicate dissociatively activate interchange mode of substitutions. The studies were explored on model Ru(II) complexes with bio-relevant thiourea nucleophiles to predict possible interaction with biomolecules which has become part of the methods used in the endeavour to search for alternative anti-cancer agents with improved efficacy and higher spectrum of activity
Construction of functional and robust cobalt phthalocyanines modified electrodes for the electrocatalytic detection of metal-based and pharmaceutically derived pollutants.
No full textMasters Degree. University of KwaZulu-Natal, Pietermaritzburg.Water pollution has become a detrimental global concern in a world that continues to grow through industrialisation, population, and demand in sales from agricultural and pharmaceutical industries. It is therefore imperative for innovative methods of continuous water monitoring to be implemented to avoid the harsh effects that pollution poses to human, animal and environmental preservation. Advances from traditional analytical methods have been made to combat associated drawbacks such as tedious sample preparation, high maintenance costs, and lack of mobility. Electrochemical sensors can be used for the analysis of a vast range of water pollutants while offering on-site, simple analysis and inexpensive fabrication. Metallophthalocyanines have been utilised extensively as electrode modifiers due to their excellent redox properties and stability which can be fine-tuned by alteration of the metal centre and substituents. In addition, thes3e alterations improve selectivity, solubility and immobilisation onto electrode substrates. This research is aimed at the application of gold electrodes modified with CoPc-cou nanoconjugates and CoPc-cou electrospun nanofibers (ENFs) for the electrocatalytic detection of pollutants, paraquat and mercury, in real water samples. Experimental chapter one explores the optimization and application of a gold-modified electrode, CoPc-cou-f-MWCNTs/3-HT|Au, for the electrocatalytic detection of a water pollutant, paraquat (PQ). It was fabricated via a sequential modification procedure entailing the formation of self-assembled monolayers (SAMs) of a nanocomposite comprising of a coumarin tetra-substituted cobalt phthalocyanine (CoPc-cou) and carboxylic acid functionalized multiwalled carbon nanotubes (f-MWCNTs). This was followed by the in-situ immobilization of poly(3-hexylthiophene) ([3-HT]n) through electropolymerisation to render the chemically modified electrode (CME). Subsequently, the CME illustrated enhanced sensitivity towards PQ compared to the bare or CoPc-cou-f-MWCNTs modified electrodes. The CoPc-cou-f-MWCNTs/3-HT|Au electrode displayed a linear PQ detection range of 0.193 – 1000 μM with a limit of detection (LOD) and limit of quantification (LOQ) of 0.193 μM and 0.584 μM, respectively. Comparison between calibration curves for the modified electrode and HPLC-MS illustrates that the former method has a lower but comparable calibration sensitivity for PQ. In addition, this CME could electrocatalytically distinguish PQ within a real water sample collected from the Durban lagoon. Furthermore, the direct recovery of PQ in the lagoon water by the modified Au electrode was found to be 86%, which is lower than the calculated value of 97% obtained by HPLC-MS after rigorous solid-phase microextraction of the analyte. However, the lower percentage recovery could be rationalized by the interference studies. In experimental chapter two fabricated electrospun nanofibers containing CoPc-cou, polyaniline (PANI) and poly-vinyl alcohol (PVA) were used to modify a gold substrate which was subsequently immobilised using a 5% Nafion solution affording the CoPc-cou-ENFs-Nf|Au modified electrode. Comparison of the chemically modified electrode with the bare and other modified electrodes under optimised conditions displayed superior detection of mercury (Hg(II)) attaining a linear range of 10 – 3000 μM and an LOD and LOQ of 0.14 μM and 0.46 μM, respectively. This can be attributed to the affinity between Hg(II) and the mercaptocoumarin substituent (Hg-S) as well as the higher surface area occupied by the ENFs resulting in an increased number of active sites. Furthermore, the chemically modified electrode exhibit selectivity and sensitivity in an interference sample containing multiple heavy metals (Pb2+, Cd2+ and Hg2+). A good percentage recovery of 96% was attained when the CoPc-cou-ENFs-Nf|Au electrode was applied to a real water sample which was comparable to a percentage recovery of 98% which was attained using the ICP-OES to analyse the same water samples
The synthesis, characterization and electrocatalytic behaviour of novel cobalt (II) and iron (II) phthalocyanines bearing benzopyrone, benzoxazole, tetrahydropyran and furan moieties.
Full text linkMaster of Science in Chemistry. University of KwaZulu-Natal, Pietermaritzburg 2015.The fabrication of metallophthalocyanine (MPc)-based modified electrodes has
proven to be effective for the electrocatalysis of various bio-analytes and pollutants.
The selectivity of these chemically modified electrodes can be fine-tuned by the
attachment of biologically relevant substituents to MPcs which has shown to
facilitate the detection of numerous analytes. Hence, this study focuses on the design
of MPcs bearing chromone, coumarin, flavone, benzoxazole, tetrahydropyran and
furan moieties. The formulated MPcs were characterized using UV-Vis and FT-IR
spectroscopy, ESI-TOF mass spectrometry and elemental analysis. The redox
properties of the complexes were investigated via voltammetry and the subsequent
voltammetric assignments were corroborated by UV-Vis spectroelectrochemistry.
Each metal complex displayed four redox processes of which the Pc ring oxidation is
irreversible and the remaining redox couples are quasi-reversible.
Novel cobalt and iron phthalocyanines peripherally tetra-substituted with chromone
(chr) or coumarin (cou) moieties were formulated and characterized in chapter three.
The structural elucidations of the ligands, 4-(chromone-7-oxy)phthalonitrile (1) and
4-(4-(trifluoromethyl)-coumarin-7-oxy)phthalonitrile (2) were complemented by
NMR spectroscopy and single crystal X-ray analysis (for 1). Utilizing the respective
MPcs, modified working electrodes were prepared by electropolymerization and
their electrocatalytic activities towards nitrite oxidation were explored. All the metal
complexes showed an increase in nitrite oxidation currents and a minor decrease in
Sumayya Chohan II
oxidation potentials which is indicative of electrocatalysis. The trend of electrocatalytic
activity was found to be as follows: CoPc-chr (3) > FePc-cou (4) > CoPc-cou
(5).
Chapter four focuses on the synthesis and characterization of cobalt phthalocyanines
(CoPcs) containing flavone (flav) and benzoxazole (bo) moieties. CoPc-flav (3), CoPcbo
(4), multi-walled carbon nanotubes (MWCNTs) and CoPc-MWCNT conjugates
were used to prepare modified glassy carbon electrodes (GCEs) which were tested
for dopamine electrocatalysis. Both CoPc modified electrodes (3-GCE and 4-GCE)
showed higher peak currents, slightly lower peak potentials and improved
reversibility compared to the bare GCE. The respective CoPc-MWCNT conjugates
were found to further enhance dopamine detection. 3-MWCNT-GCE and 4-
MWCNT-GCE showed lower peak to peak separations than the respective CoPc
modified electrodes indicating faster electron transfer kinetics. Chronoamperometry
was employed to determine the catalytic rate constants of each electrode which were
superior to previously reported values. 4-MWCNT-GCE was noted to be the most
effective electron mediator in the electrocatalysis of dopamine. Chapter five reports on the synthesis of tetrahydropyran (thp) and furan (fur)
substituted CoPcs. The electrocatalysis of L-cysteine was tested using CoPc-thp (3),
CoPc-fur (4) and CoPc-cou (5) reported in chapter three. Modified electrodes were
prepared using the drop-dry method. While the bare GCE and 4-GCE showed no
peaks for L-cysteine oxidation in the 0.0 - 0.70 V potential window; the modified electrodes showed a well-defined peak at 0.40 V for 3-GCE and a broad peak at 0.52
V for 5-GCE. Kinetic parameters were determined using chronoamperometry,
rotating disc electrode (RDE) studies and construction of Tafel plots. It was found
that L-cysteine oxidation using 3-GCE proceeded at a faster rate
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Screening of water pollutants using metallophthalocyanine-based chemically modified electrodes.
Full text linkMasters Degree. University of KwaZulu-Natal, Pietermaritzburg.Abstract available in PDF
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