Electrospun nanofibers as solid phase extraction sorbents and support for alkylphenols colorimetric probes
- Authors: Tancu, Yolanda
- Date: 2014
- Subjects: Nanofibers , Electrospinning , Extraction (Chemistry) , Sorbents , Phenols , Colorimetry , Transmission electron microscopy , High resolution spectroscopy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4486 , http://hdl.handle.net/10962/d1012997
- Description: The thesis reports on fabricating alternative solid phase extraction (SPE) sorbents and colorimetric probes based on electrospun nanofibers for alkylphenols (APs). Hydroxyl methylated styrene [poly(co-styrene-CH₃OH)] and 3-oxobutanoate styrene [poly(co-styrene-OCOCH₃COCH₃)] copolymers were synthesized and fabricated into sorbent materials by electro-spinning/spraying. The fabricated morphologies consisting of bead free fibers, beaded fibers and particles were evaluated as SPE sorbents using batch experiments. Electropun fibers proved to be better sorbents as they exhibited extraction efficiency that exceeded 95% compared to 60% for beaded fibers and 40% for particles. In view to reduce sample and solvent volumes, smooth fibers were packed into pipette tips as SPE devices that yielded quantitative recoveries of APs from spiked wastewater samples. Recoveries ranged from 70% to 125% with LOD of 0.008, 0.01 and 0.1 μg mL⁻¹ for 4-tert octylphenol (4-t-OP), 4-octylphenol (4-OP) and 4-nonylphenol (4-NP) respectively, when using high performance liquid chromatography-fluorescence detector (HPLC-FLD). Furthermore, amino functionalised polydiacetylene polymers (PDAs), citrate capped gold (AuNPs) and silver nanoparticles (AgNPs) were evaluated as colorimetric probes for visual detection of APs. In colloidal studies, AuNPs probe showed a colour change from wine red to green upon introduction of analyte. UV-vis spectroscopy revealed the shifting of the surface plasmon resonance (SPR) peak from 525 nm to 729 nm induced by aggregation of AuNPs. For AgNPs probe, a colour change was observed from yellowish green to brown. Transmission electron microscopy (TEM) studies showed growth of AgNPs. A presumed oxidation of the analyte, forming an absorbing compound at 279 nm in both AgNPs and PDAs probes was also observed. For PDAs probe the colour change was from purple to pink. Concentrations as low as 30 μg mL⁻¹ were detectable in all colloidal based probes. Further colorimetric investigations were conducted with electrospun AuNPs-nylon 6 fiber mat. A colour change from purplish red to navy blue at concentrations of 1000 μg mL⁻¹ was observed. Electrospun AgNPs –nylon 6 fiber mat did not show a distinct colour change. High resolution scanning electron microscopy (HRSEM) revealed the analyte inducing the assembly of AuNPs and AgNPs as they covered the surface of the nanofiber mat. Electrospun nanofibers are a platform for analysis and thus tuning their chemistry will lead to sensitive and selective methods
- Full Text:
- Date Issued: 2014
Characterization and application of phthalocyanine-gold nanoparticle conjugates
- Authors: Tombe, Sekai Lana
- Date: 2013
- Subjects: Phthalocyanines , Gold , Zinc , Nanoparticles , Bioconjugates , Photochemistry , Photocatalysis , Electrospinning , Polymers , Pollutants , Phenols , Azo dyes
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4293 , http://hdl.handle.net/10962/d1004517 , Phthalocyanines , Gold , Zinc , Nanoparticles , Bioconjugates , Photochemistry , Photocatalysis , Electrospinning , Polymers , Pollutants , Phenols , Azo dyes
- Description: This work presents the syntheses, photophysical and photochemical characterization of arylthio zinc phthalocyanines and their gold nanoparticle conjugates. Spectroscopic and microscopic studies confirmed the formation of the phthalocyanine-gold nanoparticle conjugates which exhibited enhanced photophysicochemical properties in comparison to the phthalocyanines. The studies showed that the presence of gold nanoparticles significantly lowered fluorescence quantum yields and lifetimes. However, this interaction did not restrict the formation of excited singlet and triplet states and hence the formation of singlet oxygen required for photocatalysis. The conjugates showed significantly higher singlet oxygen quantum yields and therefore enhanced photocatalytic activity compared to the phthalocyanines. The zinc phthalocyanines and their gold nanoparticle conjugates were successfully incorporated into electrospun polymer fibers. Spectral characteristics of the functionalized electrospun fibers indicated that the phthalocyanines and phthalocyanine-gold nanoparticle conjugates were bound and their integrity was maintained within the polymeric fiber matrices. The photophysical and photochemical properties of the complexes were equally maintained within the electrospun fibers. The functionalized fibers were applied for the photoconversion of 4-chlorophenol and Orange G as model organic pollutants. , Microsoft� Word 2010 , Adobe Acrobat 9.53 Paper Capture Plug-in
- Full Text:
- Date Issued: 2013
Molecular and biochemical analysis of the diet of the black rhinoceros
- Authors: Kgopa, Ananias Hodi
- Date: 2009 , 2013-07-15
- Subjects: Black rhinoceros -- South Africa -- Eastern Cape , Browse (Animal food) -- South Africa -- Eastern Cape -- Analysis , Black rhinoceros -- Manure -- Analysis , Phenols , Antioxidants , Plant ecology -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4064 , http://hdl.handle.net/10962/d1004721 , Black rhinoceros -- South Africa -- Eastern Cape , Browse (Animal food) -- South Africa -- Eastern Cape -- Analysis , Black rhinoceros -- Manure -- Analysis , Phenols , Antioxidants , Plant ecology -- South Africa -- Eastern Cape
- Description: The black rhinoceros, Diceros bicornis, is listed as critically endangered. The black rhinoceros population in the Great Fish River Reserve (GFRR) has increased steadily to a current estimate of one hundred animals since the re-introduction of four animals in 1986. In an effort to contribute to the effective conservation and management of this species, dietary composition was studied in the medium Portulcaria thicket vegetation of the GFRR. This study used a molecular approach to determine the diet of the black rhinoceros of the GFRR by sequencing the ribulose bisphosphate carboxylase large subunit (rbcL) gene in plants and dung. Twenty-three plant species were collected from the reserve, and 802 bp of the rbcL gene were sequenced. These plant sequences were used as a reference database for the identification of plant sequences generated from black rhinoceros dung. Initial studies investigated the amplification, cloning and sequencing of DNA extracted from the dung samples which indicated the viability of the molecular approach. Thereafter, dung generated rbcL DNA was analyzed by GS FLX sequencing. Of the plant sequences identified by comparison to the GenBank database, Carissa bispinosa was the most prevalent. The study further characterized the antioxidant activities and phenolic content of plants eaten by the black rhinoceros using four different assays. Phyllanthus verrucosus, Putterlickia pyracantha, Maytenus capitata, Euclea undulata and Ozoroa mucrunata consistently had high antioxidant activities when assayed against 2,2-azinobis (3-ethyl benzothiazolium-6-sulfonic acid) (ABTSʹ⁺), 2,2-diphenyl-1-picrylhydrazyl (DPPHʹ), and ferric reducing antioxidant potentials (FRAP) and phenolic content when evaluated using the Folin-Ciocalteu assay. The majority of plants investigated showed low antioxidant potentials and low phenolic content. The extent to which antioxidants influenced the browse selection by the black rhinoceros remains inconclusive.
- Full Text:
- Date Issued: 2009
Determination of the botanical composition of black rhinoceros (Diceros bicornis) dung using the rbcL gene as a molecular marker, and analysis of antioxidant and phenolic content of its browse
- Authors: Bulani, Siyavuya Ishmael
- Date: 2007 , 2013-06-25
- Subjects: Black rhinoceros -- Food , Black rhinoceros -- South Africa -- Eastern Cape , Browse (Animal food) -- Analysis -- South Africa -- Eastern Cape , Plant ecology -- South Africa -- Eastern Cape , Genetic markers , Black rhinoceros -- Manure -- Analysis , Phenols , Antioxidants
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4070 , http://hdl.handle.net/10962/d1006468 , Black rhinoceros -- Food , Black rhinoceros -- South Africa -- Eastern Cape , Browse (Animal food) -- Analysis -- South Africa -- Eastern Cape , Plant ecology -- South Africa -- Eastern Cape , Genetic markers , Black rhinoceros -- Manure -- Analysis , Phenols , Antioxidants
- Description: The black rhinoceros remains one of the world's extremely endangered species despite a variety of policies to protect it. The black rhinoceros population at the Great Fish River Reserve (GFRR) in the Eastern Cape in South Africa has increased steadily since their re-introduction in 1986. This megaherbivore is a browser, with a diet obtained largely from the short and medium succulent thicket of the GFRR. Knowledge of the preferential diet of the black rhinoceros on the reserve is an important factor for the effective management of the land and the herbivores that compete for its resources. The dietary preferences of the black rhinoceros at the reserve have been established using backtracking methods. In this study the rbcL gene was used to establish an rbcL gene database of the plants from the GFRR and determine the botanical composition of the black rhinoceros dung from the GFRR. Due to the limited number of rbcL gene plant sequences from the GFRR deposited in the GenBank database, 18 plant species from the GFRR were sequenced. Sequence analyses between the partial rbcL gene sequences generated were able to distinguish between plants down to species level. Plant species from the family Euphorbiaceae and Fabaceae showed sequence variation at intra-specific level compared to those of Tiliaceae which were more conserved. The generated rbcL gene sequences from seasonal dung samples were compared to the rbcL gene sequenced from 18 plant species obtained from the GFRR and those from the GenBank database. A wide range of plant species were identified from the dung samples. There were no major differences in botanical composition between the dung samples, except that Grewia spp. were found to dominate in almost all seasons. The results obtained on the free radical scavenging activity of the extracts against 2,2-Diphenyl-l-picrylhydrazyl (DPPH) increased in the order of methanol > ethyl acetate > chloroform. The DPPH free radical scavenging activity of the methanol plant extracts increased in the order Brachylaena elliptica > Plumbago auriculata > Grewia robusta > Azima tetracantha. Methanol extracts on the TLC plate sprayed with Fe³⁺-2,4,6-Tri-2-pyridyl-s-triazine (TPTZ) showed that the compounds present in the extracts react differently to ferric ion, with most compounds unable to reduce ferric ion. Furthermore the methanol extracts were able to exhibit reduction potentials vs. Ag/AgCl at low concentrations. The compounds in the extracts were shown to be phenolic acids and flavonoid glycosides.
- Full Text:
- Date Issued: 2007
The oxidative coupling of phenols using stoichiometric metal oxidants
- Authors: Hoffmann, Eric
- Date: 2005
- Subjects: Phenols , Oxidation , Stoichiometry
- Language: English
- Type: Thesis , Doctoral , DTech
- Identifier: vital:10977 , http://hdl.handle.net/10948/180 , Phenols , Oxidation , Stoichiometry
- Description: The oxidative coupling of 2,6-di-t-butylphenol under mild reaction conditions is well documented and the subject of many patents. However, the coupling of other monoand di- substituted phenols is not as well documented and thus there is scope for further investigation for providing a convenient, environmentally friendly and economically viable method for the oxidative coupling of these phenols. In this study, the oxidative coupling of a variety of alkylated phenolic substrates, 2-tbutylphenol, 2,6-di-t-butylphenol, 2,4 -di-t-butylphenol and ,4-dimethylphenol, using a range of different oxidizing agents, were investigated by means of experimental and/or theoretical means. The dibutylated aromatics provided the highest selectivities to their respective coupled products, with results obtained with the dimethyl analogue being only satisfactory, and that for 2 -t-butylphenol being totally inefficient. PM3 Molecular orbital (MO) calculations were used to predict the possible modes of coupling for the substrates 2,6 -di-t-butylphenol and 2,4-di-t-butylphenol, and these results were then compared with those obtained experimentally in the laboratory. Preliminarily, the coupling of unsubstituted phenolics was also assessed by means of MO calculations. Much emphasis was placed on Ce(IV) as the oxidant, and the reaction conditions under which it was used and the results that were obtained have not been reported before and are therefore novel. The oxidation of 2,4-di-t-butylphenol using Ce(IV) in the presence of methanesulphonic acid was optimized to afford high yields and selectivities to the desired ortho C-ortho C coupled product under mild reaction conditions. Various reaction parameters were also investigated in this case, such as varying the MeSO3H concentration, the solvent, the reaction temperature, the reaction time, the substrate loading, the rate of oxidant addition and the substrate to oxidant ratio. Ce(IV) also gave a high selectivity to the para C-para C coupled product when IX using 2,6-di-t-butylphenol as the substrate. However, it was not as effective with 2,4- dimethylphenol, and even less so with 2-t-butylphenol. The oxidation reactions of 2-t-butylphenol and 2,4-dimethylphenol with various coupling agents were also investigated with the intention of obtaining high selectivities to the respective desired coupled products. In these studies, 2-t-butylphenol afforded a large number of products, irrespective of the oxidant used. The dimethyl analogue was more selective, but results were not optimal. It was clear that the number of substituents on the phenol ring, their nature and their position with regards to the hydroxyl moiety was of great importance and made a significant impact on the preferred coupling mode of the substrate. It was observed that steric effects also played a major role in the outcome of these reactions: 2,6-di-t-butylphenol never afforded any C-O coupled products whereas 2-t-butylphenol, 2,4-di-t-butylphenol and 2,4-dimethylphenol all appeared to undergo some C-O coupling. Finally, reaction mechanisms were provided for both the K3Fe(CN)6 and Ce(IV) work, these reacting in basic and acidic media, respectively. It was proposed that both of these mechanisms operate through the initial formation of the phenoxyl radical.
- Full Text:
- Date Issued: 2005
Investigation of the bioconversion of constituents of olive effluents for the production of valuable chemical compounds
- Authors: Notshe, Thandiwe Loretta
- Date: 2002
- Subjects: Phenols , Sewage -- Purification , Effluent quality
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4078 , http://hdl.handle.net/10962/d1007446 , Phenols , Sewage -- Purification , Effluent quality
- Description: Olive mill wastewater is produced in large quantities during the production of olive oil and olive production effluents are produced during the processing of olives. This project was planned to find a use for constituents found in olive production wastewater. The task was carried out by first characterizing the olive effluents, then screening microorganisms for growth in the effluents and reduction of the pollutant properties of the effluents. An investigation into the biotransformation of aromatic compounds present in the effluents into useful chemicals, was carried out. The olive production effluents were collected from different stages in the process for treating olive wastewater, viz, a fermentation tank (FB), the surface of a digester (LV) and an evaporation pond (SO). The three effluents were characterized by investigating their phenolic composition. Protocatechuic acid, vanillic acid, syringic acid, hydroxyphenyl acetic acid, coumaric acid and ferulic acid were identified in an olive effluent, FB, using thin layer chromatography (TLC) and High perfomance liquid chromatography (HPLC). Hydroxyphenyl acetic acid constitutes almost 60% of the organics in olive effluent FB. Five bacteria, namely RU-LV1; RU-FBI and RU-FB2; RU-SOI and RU-S02, were isolated from the olive effluents LV, FB and SO respectively. These isolates were found to be halotolerant and were able to grow over a broad temperature and pH range, with the maximum temperature and pH for growth being 28°C and pH 7 respectively. A range of microorganisms were evaluated for their ability to grow and reduce the total phenolic content of the olive effluents. Among these Neurospora crassa showed the highest potential for the biological reduction of total phenolics in olive effluents. Approximately 70% of the total phenolic content was removed by N. crassa. Trametes verscilor, Pseudomonas putida strains, RU-KMI and RU-KM3s, and the bacteria isolated from olive effluents could also degrade the total phenolic content of olive effluents, but to a lesser extent. The ability of the five bacterial isolates to grow and degrade aromatic compounds was assessed by growing them in medium with standard aromatic compounds. RU-L V1 degraded 96%, 100%, 73% and 100% of caffeic acid, protocatechuic acid, p-coumaric acid and vanillic acid respectively. The other isolates degraded caffeic acid and protocatechuic acid, but their ability to degraded p-coumaric acid and vanillic acid was found to be lesser than the ability of RU-LV1 to degrade the same aromatic compounds. Whole cells of RU-LV1 degraded vanillic acid but no metabolic products were observed on HPLC analysis. Resting cells, French pressed extract, cell free extracts and cell debris from RU-LV1 cells induced with vanillic acid degraded vanillic acid, ferulic acid and vanillin at rates higher than those obtained from non-induced cultures. No products were observed during the degradation of vanillic acid. Ferulic acid was converted into vanillic acid by French pressed extract, cell free extract and cell debris of RU-LV1. The maximum yield of vanillic acid as a product (0 .23 mM, 50 %yield) was obtained when cell free extracts of RU-LVI, grown in glucose and induced by vanillic acid, were used for the degradation of 0.4 mM ferulic acid. Vanillin was rapidly converted into vanillic acid by resting cells, cell free extracts and French pressed extract of RU-LVI. Using molecular techniques, the similarity ranking of the RU-LVI 16S rRNA gene and its clone showed a high similarity to Corynebacterium glutamicum and Corynebacterium acedopltilum. The rapid degradation of vanillin to vanillic acid suggests that extracts from RU-LV1 degrade ferulic acid into vanillin which is immediately oxidized to vanillic acid. Vanillic acid is also considered as a high value chemical. This project has a potential of producing useful chemicals from cheap substrates that can be found in olive effluents. , KMBT_363
- Full Text:
- Date Issued: 2002
The development and evaluation of procedures for the synthesis of phenolic ethers by Baeyer-Villiger oxidation
- Authors: Hoffmann, Petra
- Date: 2001
- Subjects: Ethers , Phenols
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10955 , http://hdl.handle.net/10948/51 , Ethers , Phenols
- Description: A novel preparative method has been discovered in our laboratories which produces hydroxy-substituted phenolic ethers in a simple one-pot process. Initially it was found that 4-hydroxyacetophenone, when oxidized in the presence of ammonium peroxydisulfate and concentrated sulfuric acid in methanol, resulted in 4- methoxyphenol as product in high yield. Being novel, no information about the method was available in the literature, and hence it was our task to study the reaction more extensively. The scope of the reaction was investigated in that different reagents and substrates, as well as alternative oxidants, catalysts and alcohols (or reagents which may be envisaged to behave similarly to alcohols) were used. The studies showed that 4- hydroxy-substituted benzaldehyde and acetophenone, vanillin, fuchsones and stilbenequinones as substrates all successfully afforded the phenolic ether. Other oxidants such as hydrogen peroxide, sodium perborate etc. also afforded the phenolic ether but were less effective. The use of other acids than sulfuric acid resulted in comparable or lower yields of the desired phenolic ether. When the use of alcohols or similar reagents was varied, it was found that methanol, ethanol, benzyl alcohol, allyl alcohol, but-3-en-1-ol, hex-5-en-1-ol and undec-10-en-1-ol all afforded the desired ethers in significant yields. In an investigation to determine how this method may be useful for industrial applications, it was observed that compounds such as 3,5-di-tert-butyl-4-hydroxysubstituted alkenyloxy phenols, having potential application as antioxidant precursors in the polymer industry, may be prepared. Results showed that the reaction became less efficient when the alkenyl chain length was increased. The use of different acids and oxidants, the change of the oxidant:substrate ratio and the addition of a potential lanthanide catalyst did not improve the yield of the desired ether. A mechanism for the reaction was proposed which involved an initial Baeyer-Villiger oxygen insertion step to afford an ester which was hydrolysed in situ to hydroquinone. The remaining oxidant was then involved in oxidizing some of the soformed hydroquinone to benzoquinone. It was then proposed that these two compounds, i.e., hydroquinone and benzoquinone, interacted with each other in solution, forming a charge-transfer complex. According to our proposal, the carbonyl group of the benzoquinone, when complexed with hydroquinone in solution, was then attacked by nucleophilic reagents such as alcohols. Etherification hence resulted as a consequence of this attack. A statistical experimental evaluation and optimization study was then performed which led to a system that produced the phenolic ether, 4-methoxyphenol, in an 86% yield. An analysis of the effects of the reaction parameters on the reaction showed that the reaction temperature and the oxidant:substrate ratio played a major role in deciding the effectiveness of the reaction, while the amount of substrate and acidic catalyst had a much smaller influence on the ether yields.
- Full Text:
- Date Issued: 2001
Capillary membrane-immobilised polyphenol oxidase and the bioremediation of industrial phenolic effluent
- Authors: Edwards, Wade
- Date: 1999
- Subjects: Membranes (Technology) , Effluent quality , Pollutants , Phenols , Water -- Purification
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4095 , http://hdl.handle.net/10962/d1008458
- Description: Waste-generating industrialisation is intrinsically associated with population and economic proliferation. This places considerable emphasis on South Africa's water shortage due to the integral relationship between population growth rate and infrastructure development. Of the various types of industry-generated effluents, those containing organic pollutants such as phenols are generally difficult to remediate. Much work has been reported in the literature on the use of enzymes for the removal of phenols from these waste-streams but little application of this bioremediation approach has reached practical fruition. This study focuses on integrating and synergistically combining the advantages of enzyme-mediated dephenolisation of synthetic and industrial effluent with that of membrane teclmology. The ability of the enzyme polyphenol oxidase to convert phenol and a number of its derivatives to chemically reactive o-quinones has been reported extensively in the literature. These o-quinones can then physically be removed from solution using various precipitation or adsorption techniques. The enzyme is, however, plagued by a product-induced phenomenon known as suicide inactivation, which renders it inactive and thus limits its application as a bioremediation tool. Integrating membrane technology with the enzyme's catalytic ability by immobilising polyphenol oxidase onto polysulphone and poly(ether sulphone) capillary membranes enabled the physical removal of these inhibitory products from the micro-environment of the immobilised enzyme which therefore increased the phenol conversion capability of the immobilised biocatalyst. Under non-immobilised conditions it was found that when exposed to a mixture of various phenols the substrate preference of the enzyme is a function of the R-group. Under immobilised conditions, however, the substrate preference of the enzyme becomes a function of certain transport constraints imposed by the capillary membrane itself. Furthermore, by integrating a quinone-removal process in the enzyme-immobilised bioreactor configuration, a 21-fold increase in the amount of substrate converted per Unit enzyme was observed when compared to the conversion capacity of the inunobilised enzyme without the product removal step. Comparisons were also made using different membrane bioreactor configurations (orientating the capillaries transverse as opposed to parallel to the module axis) and different immobilisation matrices (poly(ether sulphone) and polysulphone capillary membranes). Conversion efficiencies as high as 77% were maintained for several hours using the combination of transverse-flow modules and novel polysulphone capillary membranes. It was therefore concluded that immobilisation of polyphenol oxidase on capillary membranes does indeed show considerable potential for future development.
- Full Text:
- Date Issued: 1999
Homogenous and heterogenous catalytic activity of metallophthalocyanines towards electrochemical detection of organic compounds
- Authors: Mafatle, Tsukutlane J P
- Date: 1998
- Subjects: Electrochemistry , Organic compounds , Phenols
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4316 , http://hdl.handle.net/10962/d1004974 , Electrochemistry , Organic compounds , Phenols
- Description: Cysteine plays an important role in many biological and pharmaceutical systems. Therefore, in view of its importance, it is essential to find means of detecting it at the lowest possible levels. In this regard, electrochemical techniques have been found to be capable of detecting analytes even at micro levels. However, electrochemical determination of cysteine occurs at a very high potentials. These overpotentials makes quantitative analysis or detection of cysteine difficult at most conventional carbon electrodes. On platinum electrode, the oxidation of cysteine has been reported to occur in the potential range 0.7 to 1.45 V (vs NHE). Therefore, the object of this investigative study has been to find an active complex that could replace platinum and other expensive metals as electrodes. Such a complex should also be capable of reducing the potential at which the oxidation of cysteine occurs on carbon electrodes. As a result, this manuscript gives a full report on the investigative study of electrocatalytic activity of molybdenum phthalocyanine complexes towards detection of cysteine. Molybdenum phthalocyanine, OMo(OH)Pc, and its tetrasulfonated derivative, [OMo(OH)TSPc]⁴⁻ were successfully used to reduce the potential needed to initiate the oxidation of cysteine on carbon paste electrodes (CPE). The oxidation of cysteine on CPE modified with [OMo(OH)Pc]⁴⁻ was found to occur at 0.29 V (vs Ag/AgCl), and in the presence of [OMo(OH)TSPc]⁴⁻ species in solution the oxidation occurred at 0.33 V (vs Ag/AgCl). Molybdenum, in the oxidation states of Mo(IV), Mo(V) and Mo(VI), is found in biological systems as an essential trace element, participating in a number of enzymatic reactions, where it is believed to be coordinated to sulphur-containing ligands in many molybdenum enzymes. This therefore explains why molybdenum phthalocyanines were employed in electroanalytical detection of sulphur containing amino acid, cysteine. Electrochemical methods have also been successfully used in detection of environmental pollutants such as phenolic compounds. Phenolic compounds are oxidised at readily accessible potentials. However, like cysteine, there are problems associated with the electrochemical detection of these important environmental pollutants. Their electrooxidation is known to form dimeric and/or polymeric oxidation products which adsorb onto the electrode surface, thus -videactivating it. Therefore, to address this problem, cobalt phthalocyanine (CoPc) and its tetrasulfonated derivative, [CoTSPc]⁴⁻ were employed in electrocatalytic detection of phenolic compounds. These complexes were found to increase the anodic peak currents for the oxidation of o-cresol, m-cresol, p-cresol, phenol, 2-chlorophenol and 4-chlorophenol. In addition, CoPc deposited onto the glassy carbon electrode improved the stability of the electrode, by reducing electrode poisoning caused by the electrooxidation products of the mentioned phenolic compounds. The potential at which the oxidation occurred and the current response of individual phenolic compounds depended on the degree of substitution and the type of substituent on the phenol molecule. In general, the current response was found to be lower for chlorinated phenols compared with the cresols and phenol. To establish the role of the central metal in the catalytic process, comparison of the electrocatalytic activity of some of the first row transition metal phthalocyanines, for the detection of mono-substituted phenolic compounds, showed the following trend: Co⁽¹¹⁾ > Mn⁽¹¹⁾ > Fe⁽¹¹⁾Pc > Ni⁽¹¹⁾Pc > Cu⁽¹¹⁾Pc > H₂Pc > Zn⁽¹¹⁾Pc > Bare GCE. A report is also given on electrocatalysis using [CoTSPc]⁴⁻ electrochemically deposited on the glassy carbon electrode. This was also found to enhance the anodic peak currents for the oxidation of all phenolic compounds. A report on the effects of scan rate, operating potential, analyte concentration and other variables is also given.
- Full Text:
- Date Issued: 1998