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Effects of annealing on the structural and optical properties of nanostructured TiO2

  • Authors: Talla, Assane , Urgessa, Zelalem
  • Date: 2023-12
  • Subjects: Titanium dioxide , Nanostructured materials , Nanotubes
  • Language: English
  • Type: Doctorial theses , text
  • Identifier: http://hdl.handle.net/10948/62655 , vital:72907
  • Description: In this thesis, the structural, morphological and optical properties of titanium dioxide (TiO2) are investigated. Titanium dioxide (TiO2) nanotubes are prepared by anodic oxidation of titanium foil. The as-anodised samples are thermally annealed at various temperatures in nitrogen, air, oxygen and vacuum. The purpose is to study how the annealing conditions affect the properties of the nanostructures, including the anatase to rutile phase transformation. In all annealing atmospheres, except in vacuum, the dominant phase is found to be anatase when annealing is performed up to 600 oC. Above 700 oC the rutile phase becomes dominant. The anatase phase is stable above 600 oC in vacuum and does not evolve significantly up to 900 oC. The morphologies of the tubes tend to deteriorate with increased annealing temperature, in nitrogen, air and oxygen atmospheres, due to sintering effects. However, the integrity of the nanotubes is maintained up to 900 oC in vacuum. The photoluminescence (PL) spectra suggest mainly the presence of oxygen vacancies and self-trapped excitons, with respective emission bands around 2.5 eV and 2.3 eV. The results show that both the annealing temperature and atmosphere strongly influence the crystalline and optical properties of the TiO2 nanotubes. In addition, the phase transformation from anatase to rutile for samples annealed in an oxygen-rich environment is investigated in detail. Complementary structural information obtained from transmission electron microscopy and Raman analysis for oxygen-annealed samples reveals that the nucleation of the rutile phase starts from the titanium substrate and then propagates along the tubes. The results provide suitable annealing conditions to control the phase content and morphology of anodic TiO2 nanotubes. The PL characteristics of bulk crystalline anatase TiO2, namely virgin and hydrogen-annealed at 600 oC for 1 h, are studied. The low temperature PL spectra at 5.5 K shows near band edge (NBE) emissions with two dominant lines ascribed to shallow donor bound exciton and possibly free to bound recombination. The two main transitions are assisted by optical phononmodes. Temperature-dependent PL measurements performed on these anatase crystals reveal that the donor bound exciton is stable below 90 K. Hydrogen trapped in oxygen vacancies is proposed to be the shallow donor. In addition, two activation processes are involved for the thermal quenching of donor bound excitons. The total activation energy is found to correlate well with the localisation energy of the bound exciton. Site-selective PL spectra obtained from anodic TiO2 tubes reveals that the luminescence of the nanostructures depends on the morphology. The result shows unusual near-band edge emission (NBE) for these structures, which is rarely observed in indirect band gap TiO2. , Thesis (PhD) -- Faculty of Science, School of Biomolecular and Chemical Sciences, 2023
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  • Date Issued: 2023-12

Dual and targeted photodynamic therapy ablation of bacterial and cancer cells using phthalocyanines and porphyrins in the presence of carbon-based nanomaterials

  • Authors: Openda, Yolande Ikala
  • Date: 2022-10-14
  • Subjects: Phthalocyanines , Porphyrins , Active oxygen , Biofilms , Breast Cancer Treatment , Nanostructured materials , Combination therapy , Photochemotherapy
  • Language: English
  • Type: Academic theses , Doctoral theses , text
  • Identifier: http://hdl.handle.net/10962/365945 , vital:65804 , DOI https://doi.org/10.21504/10962//365946
  • Description: Phthalocyanines (Pcs) and porphyrins bearing substituents that possess antibacterial/anticancer properties are used as photosensitizers (PS) for the first time in the work. For targeting specificity and improved photoactivity, the PSs were afterward functionalized with carbon nanomaterials such as graphene quantum dots (GQDs) and detonation nanodiamonds (DNDs) via covalent conjugation (amide or ester bonds) or by non-covalent conjugation (π-π stacking and electrostatic interactions). Furthermore, the PSs-DNDs nanoconjugates were conjugated to either chitosan-capped silver nanoparticles (CSAg) via amide bonds or to the bare silver nanoparticles (Ag NPs) using the silver- nitrogen affinity. The as-synthesized nanoconjugates were also fully characterized by spectroscopic and microscopic methods together with thermal analysis. The potential photocytotoxicity of the complexes alone and their nanoconjugates against S. aureus and/or E. coli planktonic and biofilm cultures has been evaluated in vitro. Compared to the non- quaternized PSs, the cationic analogs exhibited a higher photodynamic inactivation against the planktonic cells with log10 reduction values above 9 in the viable count using a concentration of ca. 1.25 μM following 30 min exposure to light (Light dose: 943 J/cm2 for Pcs and 250 mW/cm2 for porphyrins). Whereas, at a concentration of ca. 100 μM the cationic PSs showed complete eradication of biofilms upon 30 min exposure to light. As a result of conjugation to carbon-based nanomaterials and silver nanoparticles, the compounds proved to be more effective as they exhibited stronger antibacterial and anti-biofilm activities on the multi-drug resistant bacteria strains due to synergetic effect, compared to PSs alone. This suggests that the newly prepared nanohybrids (PS concentration ca. 100 μM) could be used as potential antimicrobial agents in the treatment of biofilm-related infections. The target nanoconjugates showed all the advantages of two different groups existing on a single entity. In light of the potential advantages of combined chemotherapy and photodynamic antimicrobial chemotherapy (PACT), this work reports for the first time the use of PACT-ciprofloxacin (CIP) dual therapy using selected indium quaternized PSs which showed higher photoactivity with complete eradication of both Gram-positive and Gram-negative bacteria biofilms at concentrations of 8 μM of PS versus 2 μg/mL of the antibiotic following 15 min irradiation time (light dose: 471 J/cm2 for Pcs and fluence: 250 mW/cm2 for porphyrins) on S. aureus. Whereas the total killing of E. coli was obtained when combining 8 or 16 μM of PS combined with 4 μg/mL of CIP. The combined treatment resulted in the complete eradication of the matured biofilms with the highest log10 reduction values of 7.05 and 7.20 on S. aureus and E. coli, respectively. Used as a model, positively charged dimethylamino-chalcone Pcs also exhibited interesting photodynamic therapy (PDT) activity against MCF-7 cancer cells giving IC50 values of 17.9 and 7.4 μM, respectively following 15 min irradiation. Additionally, the TD-B3LYP/LanL2DZ calculations were run on the dimethylaminophenyl- porphyrins to compare the singlet excitation energies of quaternized and non-quaternized porphyrins in vacuo. the study shows excellent agreement between time-dependent density- functional theory (TD-DFT) exciting energies and the experimental S1>S0 excitation energies. The small deviation observed between the calculated and experimental spectra arises from the solvent effect. The excitation energies observed in these UV-Vis spectra mostly originated from electron promotion between the highest occupied molecular orbital (HOMO) for the less intense band and the HOMO-1 for the most intense band of the ground states to the lower unoccupied molecular orbital (LUMO) of the excited states. , Thesis (PhD) -- Faculty of Science, Chemistry, 2022
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  • Date Issued: 2022-10-14

The electrocatalytic response of metallophthalocyanines when clicked to electrodes and to nanomaterials

  • Authors: Mpeta, Lekhetho Simon
  • Date: 2021
  • Subjects: Phthalocyanines , Nanostructured materials , Electrocatalysis , Nanoparticles , Environmental chemistry , Electrodes , Organic wastes -- Purification
  • Language: English
  • Type: text , Thesis , Doctoral , PhD
  • Identifier: http://hdl.handle.net/10962/172191 , vital:42174 , 10.21504/10962/172191
  • Description: Conjugates of nanomaterials and metallophthalocyanines (MPcs) have been prepared and their electrocatalytic activity studied. The prepared nanomaterials are zinc oxide and silver nanoparticles, reduced graphene oxide nanosheets and semiconductor quantum dots. The MPcs used in this work are cobalt (II) (1a), manganese(III) (1b) and iron (II) (1c) 2,9(10),16(17),23(24)- tetrakis 4-((4-ethynylbenzyl) oxy) phthalocyaninato, 2,9(10),16(17),23(24)- tetrakis(5-pentyn-oxy) cobalt (II) phthalocyaninato (2), 9(10),16(17),23(24)- tris-[4-tert-butylphenoxy)-2- (4-ethylbezyl-oxy) cobalt (II) phthalocyaninato (3), 9(10),16(17),23(24)- tris-[4-tertbutylphenoxy)-2-(pent-4yn-yloxy)] cobalt (II) phthalocyaninato (4), cobalt (II) (5a) and manganese (III) (5b) 2,9(10),16(17),23(24)- tetrakis [4-(4-(5-chloro-1H-benzo [d]imidazol-2-yl)phenoxy] phthalocyaninato and 9(10),16(17),23(24)- tris tert butyl phenoxy- 2- [4-(4-(5-chloro-1H-benzo[d]imidazole-2-yl)phenoxy] cobalt (II) phthalocyaninato (6). Some of these MPcs (1a, 3 and 4) were directly clicked on azide grafted electrode, while some (1b, 1c, 2, 5a and 5b) were clicked to azide functionalised nanomaterials and then drop-dried on the electrodes. One phthalocyanine (5b) was drop-dried on the electrode then silver nanoparticles were electrodeposited on it taking advantage of metal-N bond. Scanning electrochemical microscopy, voltammetry, chronoamperometry, electrochemical impedance spectroscopy are among electrochemical methods used to characterise modified electrodes. Transmission electron microscopy, X-ray photoelectron spectroscopy, Xray diffractometry, Raman spectroscopy and infrared spectroscopy were employed to study surface functionalities, morphology and topography of the nanomaterials and complexes. Electrocatalytic activity of the developed materials were studied towards oxidation of 2-mercaptoethanol, hydrazine and hydrogen peroxide while the reduction study was based on oxygen and hydrogen peroxide. In general, the conjugates displayed superior catalytic activity when compared to individual materials. Complex 2 alone and when conjugated to zinc oxide nanoparticles were studied for their nonlinear optical behaviour. And the same materials were explored for their hydrazine detection capability. The aim of this study was to develop sensitive, selective and affordable sensors for selected organic waste pollutants. Conjugates were found to achieve the aim of the study compared to when individual materials were employed.
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  • Date Issued: 2021

Evaluation of surface functionalized electrospun polyacrylonitrile nanofibers for heavy metal ions removal from synthetic wastewater

  • Authors: Maqinana, Siphosethu Sesethu
  • Date: 2020-12
  • Subjects: Nanofibers , Nanostructured materials
  • Language: English
  • Type: Master's theses , text
  • Identifier: http://hdl.handle.net/10353/20871 , vital:46680
  • Description: Heavy metal ions are elements that are discharged into water streams from municipal or industrial waste. In abundance, they can be threating to the environment and human health. Amongst other several convention methods, adsorption has been proven to be the most effective method in the removal of heavy metal ions from wastewater. The aim of this research was to fabricate electrospun polyacrylonitrile (PAN) nanofibers and modify their surface to improve the adsorption efficiency for Chromium and Cadmium metal ions from synthetic wastewater. Electrospun nanofibers PAN nanofibers were fabricated via electrospinning process though careful monitoring of its parameters and modified via two-step process: hydrolysis with sodium hydroxide (NaOH), hydrochloric acid (HCl) and ethylenediamine (EDA). The morphologies, functional groups, thermal stability, chemical composition and crystallinities or amorphous structures of the nanofibers were characterised by Scanning electron microscopy (SEM), Fourier-Transform Infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA), Energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD), respectively. The experiments were conducted in batch tests to analyse the effect to pH, contact time and initial concentration of the solution. Ultraviolet-Visible spectroscopy (UV-Vis) was used to analyse the concentration of metal ions. The adsorption equilibrium was reached after 120 min with a maximum adsorption capacity of 301.1 mg/g for Cd(II) ions and 195.02 mg/g for Cr(VI) ions. The adsorption capacity increased with increasing pH and initial concentration of the solution. The adsorption capacity of Cd(II) and Cr(VI) ions was higher at pH 8 and 10, respectively. The equilibrium data was best described using Freundlich isotherm with a maximum adsorption capacity for Cd(II) ions than Cr(VI) ions. Pseudo-second order kinetic model best fitted both heavy metal ions with R2 value of 0.99 for Cr(VI) ions and 0.67 for Cd(II) ions. Heavy metal ions were desorbed from the nanofibers after one regeneration cycle with an equilibrium concentration of 4.83 mg/L for Cr(VI) ions and 43.06 mg/L for Cd(II) ions. , Thesis (MSc) (Chemistry) -- University of Fort Hare, 2021
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  • Date Issued: 2020-12

Preparation and characterization of sodium alginate-based dissolvable bandages as potential wound dressings

  • Authors: Ndlovu, Sindi Prescila
  • Date: 2020
  • Subjects: Nanostructured materials
  • Language: English
  • Type: Thesis , Masters , MSc (Chemistry)
  • Identifier: http://hdl.handle.net/10353/18800 , vital:42734
  • Description: Burn wounds are among the leading causes of mortality and morbidity globally. Burn wounds are painful, cause social isolation and causes post-traumatic stress disorder (PTSD) due to prolonged hospital treatment. The challenges encountered in the treatment of burn wounds are infections resulting from the use of wound dressings that do not protect the wounds from microbial invasion. Sodium alginate-based dissolvable bandages were prepared and encapsulated with various antibiotics (ampicillin and ciprofloxacin) and nanoparticles (i.e. zinc oxide and silver). The dissolvable bandages were characterized by FTIR, SEM/EDX, TEM, XRD and TGA. Furthermore, the porosity, water uptake, biodegradation, antibacterial studies, and water vapour transmittance properties of the bandages were also studied. The prepared sodium alginate-based dissolvable bandage incorporated with varied amount of drugs (such as ampicillin, ciprofloxacin, zinc oxide and silver nanoparticle) exhibited good porosity, high water uptake, excellent WVTR with sustained drug release profiles. The loading of ZnO Nps, CLP/ZnO Nps and Ag Nps/CLP into the bandages improved the antibacterial activity of the bandages against both gram-negative and gram-positive strains of bacteria. XRD confirmed the amorphous nature of the dissolvable bandage and the absence of free drugs. FTIR revealed the successful encapsulation of the antibiotics and nanoparticle into the dissolvable bandages. The dissolvable bandages exhibited high water uptake ranging from 870-4468% with good porosity suggesting that they can absorb large amount of wound exudates. The drug release for all the bandages obeyed the Korsemeyer Peppas drug release model with n values in the range 0.1-1.0. The results obtained indicate that the bandages are potential wound dressings for burns and for patients with sensitive skins.
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  • Date Issued: 2020

Integration of nanostructured metal sulfides into titanium (iv) oxide for high performance dye sensitized solar cell

  • Authors: Agoro, Adedoyin Mojeed
  • Date: 2020-01
  • Subjects: Nanostructured materials , Sulfur compounds
  • Language: English
  • Type: Doctoral theses , text
  • Identifier: http://hdl.handle.net/10353/21523 , vital:48857
  • Description: The work in this thesis describes synthesis, characterization and integration of nanostructured metal sulfides into titanium (IV) oxide for high performance dye sensitized solar cell. The synthesised single source precursors were evaluated using TGA, FTIR, , UV-Vis, PL,1H and 13CNMR, SEM, EDS, HRTEM, XRD, AFM, Raman, I-V, CV, Bode plot and EIS. TGA revealed the thermal degradation and quantified the mass percentage of metal sulphides as 33percent, 45percent and 27percent for CuS, PbS and SnS nanoparticles, respectively. FTIR analysis showed bands at 1580-1450 cm-1, and 1060-940 cm-1 tentatively assigned to C-N and C-S stretching frequencies correspondingly. The (M-S) stretching was observed at 577-406 cm-1 for the complexes using bis(N-di-isopropyl-N-octyldithiocarbamato) as primary ligands. IR spectra of bis(N-1,4-phenyl-N-(4-morpholinedithiocarbamato) as secondary ligands suggested the presence of (C-N) vibrations at 1508-1513 cm-1 for the complexes and 1507-1584 cm-1 for the ligands. The bands in the region of 973-1030 cm-1 were assigned to the complexes and 974- 983 cm-1 were attributed to the ligands, whereas vibration frequencies at 416-625 cm-1 were Nelson's Choice were included in these preliminary variety trials (PVTs) as checks. Twenty-five hybrids expressing high values for the Smith – Hazel selection index were identified across sites over years. Among those hybrids were two checks, namely Q16 (PAN5Q649R) and Q33 (Phb31MO7BR). The top five high – yielding hybrids selected based on the selection index were considered to be the most productive, stable and adaptable based on the GGE biplot and AMMI stability values. None of these single cross hybrids over yielded the best hybrid check (Q16) in the current study. A high potential environment, Centane, was the ideal environments for evaluating genotypes in the present study. The studies showed inbred lines L22, L23, L26, L28, L25, L29 and L31 to be low N stress tolerant at 0 kg N ha-1 based on the low N stress tolerance indices under glasshouse and the field conditions; they were also among the top ten grain-yielders under field conditions at 0 kg N ha-1. Also, in the NUE study, they were found to be among the top ten most N-efficient inbred lines under low N soils, under 30 kg N ha-1, and were among the top twenty-four inbred lines with high NUE values across the N levels in the study. Inbred lines L29 and L22 also produced testcross hybrids that were among the top twenty based on GY under 0 kg N ha-1. The testcross hybrids produced from these inbred lines were also among the top twenty-five with outstanding SCA effects for GY. These inbred lines were also parental lines of some of the top twenty- five best hybrids selected based on the Smith – Hazel selection index in the PVT study. Inbred lines L22, L23, L26, L28, L25, L29 and L31 can therefore further be evaluated and used as sources of N-tolerance genes in QPM breeding programs. tentatively assigned to M-S bond in the complexes. UV-Vis spectroscopy analysis for the complexes and from primary and secondary ligands are between within the range of 338 – 340 nm. PL studies of the primary ligands with metal complexes indicated emissions at 464 nm, 462 nm, 462 nm for Pb(II) and Sn(II), and Cu(II). The presence of secondary ligands was revealed by the absorption peaks at 455, 456 and 457 nm, exhibiting both the signals and chemical constituents of the respective ligands and their corresponding complexes. The electrochemistry in chapters 3 and 4 reveals that phase angle in the Bode plots changes with frequencies for Sn(II) complexes at 58, 57 and 8 respectively. Bode plots showed remarkable electronics of Cu(II) and Pb(II) complex interfaces. The CV curves exhibit two kinds of redox peaks indicating reduction at the negative potentials and oxidation at the positive potentials. The EIS, electrodes produced Rct for Pb(II), Cu(II) and Sn(II) electrodes in chapter 3. Moreover The EIS revealed that Sn(II) sensitizer displaced a stronger chemical capacitance and improved efficiency which could lead to better electron lifetime yield. The obtained CV exhibited anodic and cathodic peaks for Pb(II), connoting a reduction in Pb2+ and oxidation in Pb2-. Bode plot phase angle displayed Sn(II) and Pb(II) with maxima phase, indicating the presence of time constants of the electrode processes in Bis(N-1,4-Phenyl-N-Morhpo-dithiocarbamato bis(Mo/1,4-PHDTC) complexes. From the results in chapter 4, the XRD patterns exhibited good crystalline nature of CuS as a result of sharp and strong diffraction peaks obtained. There are eight peaks of 2θ angle between 26° and 79° relating to orthorhombic structure of SnS. While PbS has a cubic structure with polycrystalline nature, PbS/HDA and PbS both correspond to their crystalline planes of (200), (111), (220), (311), (222), (400), (331), (420) and (422) affirming to PbS QDs structure. SnS/HDA and SnS photosensitizers displayed eleven peaks between the values of 27.02° to 66.05° for SnS/HDA and 26.03° to 66.04° for SnS, in confirming the orthorhombic structure. SEM analysis revealed hexagonal structure for CuS, while PbS and SnS exhibited mesoporous nanostructures with spherical nanoparticles. HRTEM images indicated spherical nano-particles with particle sizes in the range of 3.14 - 44.39 nm. The outcome of HRTEM analysis revealed crystallite sizes varying as 10.90 – 11.57 nm, 3.14 – 5.95 nm and 14.96 – 44.39 nm for the CuS, PbS and SnS, originated from the primary ligands. HRTEM images originating from the Bis(N-1,4-Phenyl-N-Morhpo-dithiocarbamato bis(Mo/1,4-PHDTC) complexes indicated spherical nano-particles with particle sizes of 3.14 - 44.39 nm. Raman images in chapter 5 revealed the presence of Raman active modes of E2g 66 cm-1 and 304, cm-1 and E1g 627 cm-1 and 706 cm-1 for CuS, the active modes of TO 65 cm-1 and 2LO 626 cm-1 for PbS, active modes of B1g 69 cm-1 for SnS and 266 cm-1 for Ag. From the AFM results in chapter 5, CuS photosensitizer displayed a smooth surface and particle estimated to be 0.50 μm and height profile of 12percent. The PbS photosensitizer exhibited particle size of 1.82 μm and 0.654 μm for PbS/HDA, depicting a regular crystal growth rate. The size distribution of SnS nanoparticle at 357 nm connotes smooth surface and good compactness on the substrate. However, SnS/HDA at 122 nm displayed shape and size of non-symmetrical particles. AFM analysis in chapter 8 revealed good size roughness for CuS film. PbS exhibited particle size of 365 nm and size height of 18percent as the smoothest film, while PbS/HDA revealed 1.22 um size with 9percent size height. The evaluated particle sizes varied as 0.11- 1.18 um for SnS/HDA and 0.054 – 0.54 um for SnS films grown at 360 ℃ with size height of 16.8 and 8.4percent. The I-V efficiency obtained indicated that the CuS exhibited a much better efficiency in the QDSCs with higher Voc and the highest η being 2.85percent compared to CuS/HDA and Cu(II). High JSC of 11 mA/cm has been observed in the PbS/HDA QDSSCs, compared to the PbS and Pb(II) cells. The SnS/HDA exhibited a better performance compared to SnS and Sn(II) sensitizers due to the presence of HDA capping agent. , Thesis (MSc) -- Faculty of Science and Agriculture, 2020
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  • Date Issued: 2020-01

Nonlinear optical responses of targeted phthalocyanines when conjugated with nanomaterials or fabricated into polymer thin films

  • Authors: Nwaji, Njemuwa Njoku
  • Date: 2019
  • Subjects: Electrochemistry , Phthalocyanines , Nanoparticles , Bioconjugates , Thin films , Polymers , Nonlinear optics , Nonlinear optical spectroscopy , Nanostructured materials , Raman effect
  • Language: English
  • Type: text , Thesis , Doctoral , PhD
  • Identifier: http://hdl.handle.net/10962/71625 , vital:29926
  • Description: A number of zinc, gallium and indium metallophthalocyanines (MPcs) with diverse substituents have been synthesized and characterized using various characterization tools such as proton nuclear magnetic resonance (1HNMR), matrix assisted laser desorption time of flight (MALDI-TOF) mass spectrometry, Fourier-transformed infra-red (FT-IR), Ultraviolet-visible (Uv-vis) spectrophotometry, magnetic circular dichroism and CHNS elemental analysis. The time dependent density functional theory was employed to probe the origin of spectroscopic information in these complexes. Complexes with gallium and indium as central metal showed higher triplet quantum yield compared to the zinc derivatives. Some of the MPcs were covalently linked to nanomaterials such as CdTe, CdTeSe, CdTeSe/ZnO, graphene quantum dots (GQDs) as well as metallic gold (AuNPs) and silver (AgNPs) nanoparticles. Others were either surface assembled onto AuNPs and AgNPs or embedded into polystyrene as polymer source. The phthalocyanine-nanomaterial composites (Pc-NMCs) were characterized with FT-IR, UV-visible spectrophotometry, transmission electron microscopy (TEM), dynamic light scattering (DLS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD). The thickness of the thin films was determined by utilization of the knife edge attachment of the A Bruker D8 Discover X-ray diffraction. The optical limiting properties (using the open-aperture Z-scan technique) of the MPcs and the Pc-NMCs were investigated. The investigated MPcs complexes generally showed good optical limiting properties. The nonlinear optical response of the MPcs were improved in the presence of nanomaterials such as the semiconductor quantum dots (SQDs), graphene quantum dots (GQDs) as well as metallic AuNPs and AgNPs with MPc-QDs showing the best optical limiting behavior. The optical limiting properties of the MPcs were greatly enhanced in the presence of polymer thin films.
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  • Date Issued: 2019

The synthesis of functionalized carbon nanomaterial from waste tyre sourced carbon for chemosensing during purification processes

  • Authors: Du Plessis, Jacolien
  • Date: 2018
  • Subjects: Nanostructured materials , Nanotechnology
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: http://hdl.handle.net/10948/29925 , vital:30794
  • Description: The short lifetime of tyres causes great environmental harm due to their non-biodegradable nature resulting in land and water pollution. Exploration of novel uses for recycled tyres is therefore imperative. The use of recycled waste tyres as carbon source for the synthesis of fluorescent chemosensing carbon nanomaterial for the detection of harmful chemicals in water is proposed in this work. Three key problems are addressed in this study: i) re-using of waste tyre products, ii) synthesis of low-cost carbon nanomaterial, iii) detection of harmful compounds within environment systems. The fluorescent triazolyl-coumarin compound, 7-(diethylamino)-3-(4-(1-hydroxyethyl)-1H-1,2,3-triazol-1-yl)-2H-chromen-2-one, was synthesized in multiple steps from 4-diethylamino salicylaldehyde. The last step included the use of Cu(I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition (“click-reaction”) to form a triazole ring. Carbon spheres were synthesized through catalytic chemical vapour deposition (CVD) from waste tyre pyrolysis oil (WTPO), as the carbon source, in the presence of a catalytic amount of ferrocene. The quality and quantity of the carbon spheres had an overall improvement as the synthesis temperatures increased from 800 oC to 950 oC. The diameter of the carbon spheres reduced from 1017 nm to 577 nm as seen in TEM and SEM imaging. TGA and XRD analysis indicated a direct relationship between an increase in synthesis temperatures with an increase in the crystallinity of the carbon spheres. Commercially available naphthenic oil was used as a reference to compare the impact of the starting carbon precursor on the morphology of the carbon nanomaterial under the same reaction conditions. Multi-walled carbon nanotubes were synthesized at the lower temperatures of 800-850 oC. Carbon fibres and carbon spheres were observed at the synthesis temperatures of 900-950 oC. The carbon source played a role in the morphology of the carbon nanomaterial as it could be seen between the complex natured WTPO and the pure naphthenic oil. The purified carbon spheres synthesized at 950 oC from WTPO were functionalized with the triazolyl-coumarin units on the surface through Steglich esterification using DCC/DMAP to yield fluorescent carbon spheres. Complexation studies of the functionalized carbon spheres were done using UV/Vis spectroscopy and fluorescence with a wide range of ions; Ag+, Al3+, Co2+, Cr3+, Cu2+, Fe3+, ii Hg2+, Li+, Ni2+, Pb2+ and Zn2+. The triazolyl-coumarin carbon spheres showed reasonable sensitivity and selectivity towards Fe3+ in water with a quenching effect in fluorescence. Titration studies with Fe3+ indicating an exponential decay of quenching in the fluorescence up to 68% with detection limit at 2.73 x 10-5 M.
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  • Date Issued: 2018

Synthesis and characterization of titanium dioxide nanotubes on fluorine-doped tin oxide (FTO) glass substrate using electro-anodization technique

  • Authors: Zinya, Simcelile https://orcid.org/0000-0001-5864-0957
  • Date: 2017-12
  • Subjects: Titanium dioxide , Nanotubes , Nanostructured materials
  • Language: English
  • Type: Master's theses , text
  • Identifier: http://hdl.handle.net/10353/23979 , vital:62201
  • Description: One-dimensional (1-D) titanium dioxide nanotubes (TNTs) have attracted much attention as a promising alternative electrode material for dye sensitized solar cell (DSSC). As compared to the randomly packed and disordered TiO2 nanoparticles (TNPs) network with numerous particle-particle interfaces, TNTs prove to have fascinating features than make them suitable candidates in DSSCs. Well-structured TNTs arrays are of great potential among the various types of 1D TiO2 nano-materials owing to their superior electron transport properties with limited grain boundaries. Vectorial transport of photon generated electrons along the TNTs has been reported to lead to higher charge mobility which is crucial for improvement of DSSC performances. In this work, highly adhesive titanium films were deposited on functional substrates (FS) using radio frequency (RF) sputtering technique at a sputtering output power of 1kW, operating pressure of 1.5 Pa and at a deposition temperature of 200 °C to obtain a thickness of 10 μm under an inert argon atmosphere. The duration period for sputter coating 10 μm thickness of titanium film layer was 122 minutes with sputter rate for titanium target of about 82 nm per minutes. Subsequently, the RF sputtered titanium films were anodized with 0.5 wt. percent ammonium fluoride + 0.35 wt. percent deionised water and 96 wt. percent glycerol electrolyte solution at room temperature at 60 V for 72 hours. The resulting TNTs on functional substrates (TNTs-FS) were subjected to thermal treatment at 350 °C, 450 °C, 550 °C and 650 °C for 3 hours under oxygen atmosphere. The effect of annealing temperature on the morphological, and structural properties have been scrutinized. The as prepared and thermally treated TNTs-FS were characterized using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and Confocal Raman Spectroscopy (CRS). SEM and HRTEM techniques were employed to confirm the presence of the TNTs-FS and also to study the structural-morphology of the TNTs as the annealing temperature increases. SEM revealed improvement in morphology with increase in sample annealing temperature, even at high temperatures such as 650 °C with no collapsing and sintering of the TNTs-FS occurring. SEM images revealed TNTs-FS with pore diameter sizes in the range between 85-170 nm. This is in compliance with HRTEM analysis, which revealed smooth and straight tube walls and improved surface morphology with increase in annealing temperature. In addition HRTEM images revealed pore diameter of TNTs-FS in the range between 85-165 nm. Furthermore, HRTEM revealed lattice fringes of 0.351, 0.352 and 0.353 nm between the neighbouring lattice fringes. All corresponding to (101) planes of anatase phase TNTs at different annealing temperatures (350-650 °C). The crystallographic structure of TNTs-FS was characterized by XRD measurements after thermal treatment at 350 °C, 450 °C, 550 °C and 650 °C. The XRD pattern revealed peaks in the wide angle range of 2θ (20° < 2θ > 80°) discovered at 29.43°, 45.10°, 56.52°, 63.5°, 64.92° and 74.81° corresponding to the planes (101), (112), (200), (105), (211) and (204) crystalline structures of the anatase TNTs. The intensity of the peaks increased with increasing annealing temperature. The strong sharp peaks indicate the large quantities and higher degrees of crystallinity of anatase phase of the TNTs. CRS Large Area Scan (LAS) and Depth profiling (DP) were employed to evaluate the crystallinity and phase distribution of TNTs-FS thermally treated at different temperatures. CRS LAS in the XY direction of TNTs-FS revealed the presence of differently crystallized anatase phases of TiO2 with Raman vibrational modes of 159.38 cm-1 (Eg), 208.37 cm-1 (Eg), 399.67 cm-1 (B1g), 514.25 cm-1 (A1g) and 641.58 cm-1 (Eg) for the samples annealed at 350 °C. The effect of annealing temperature on TiO2 phase evolution was meticulously evaluated using CRS for TNTs-FS for the samples annealed at 350 °C, 450 °C, 550 °C and 650 °C. The FWHM was estimated from CRS and decreases with increasing annealing temperature resulting in increasing crystallinity. Increase in anatase FWHM and anatase peak intensity implies higher degree of crystallinity and increasing crystallite sizes were also confirmed by XRD. Growing of titanium dioxide on functional substrates one novel contribution towards the fabrication of efficient electrode materials for solar cell development. Our method of characterizing TNTs-FS from a large area scan along the surface of the samples and depth profiling along the TNTs tube walls using confocal Raman spectroscopy prove to be a pivotal step in the development of efficient photoelectrode materials of the solar devices. , Thesis (MSc) -- Faculty of Science and Agriculture, 2017
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  • Date Issued: 2017-12

Nanocomposites of carbon nanomaterials and metallophthalocyanines : applications towards electrocatalysis

  • Authors: Nyoni, Stephen
  • Date: 2016
  • Subjects: Nanocomposites (Materials) , Nanostructured materials , Electrocatalysis
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:4561 , http://hdl.handle.net/10962/d1020846
  • Description: Nanohybrid materials have been prepared and examined for their electrocatalytic activity. The nanocomposites have been prepared from carbon nanomaterials (multiwalled carbon nanotubes (MWCNTs) and graphene nanosheets), cadmium selenide quantum dots and metallophthalocyanines (MPcs). The MPcs used in this work are cobalt tetraamino-phthalocyanine (CoTAPc) and tetra (4-(4,6-diaminopyrimidin-2-ylthio) phthalocyaninatocobalt (II)) (CoPyPc). Their activity has also been explored in different forms; polymeric MPcs, iodine doped MPcs and covalently linked MPcs. The premixed drop-dry, sequential drop-dry and electropolymerisation electrode modification techniques were used to prepare nanocomposite catalysts on the glassy carbon electrode (GCE) surface. The sequential drop dry technique for MPc and MWCNTs gave better catalytic responses in terms of limit of detection, catalytic and electron transfer rate constants relative to the premixed. MWCNTs and CdSe-QDs have been used as intercalating agents to reduce restacking of graphene nanosheets during nanocomposite preparation. Voltammetry, chronoamperometry, scanning electrochemical microscopy and electrochemical impedance spectroscopy methods are used for electrochemical characterization modified GCE. X-ray photoelectron spectroscopy, X-ray diffractometry, transmission electron microscopy, scanning electron microscopy, infra-red spectroscopy, Raman spectroscopy were used to explore surface functionalities, morphology and topography of the nanocomposites. Electrocatalytic activity and possible applications of the modified electrodes were tested using oxygen reduction reaction, l-cysteine oxidation and paraquat reduction. Activity of nanocomposites was found superior over individual nanomaterials in these applications.
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  • Date Issued: 2016

Application of catalysts and nanomaterials in the design of an electrochemical sensor for ochratoxin A

  • Authors: Flanagan, Shane Patrick
  • Date: 2011 , 2010-12-06
  • Subjects: Ochratoxins , Filamentous fungi , Electrochemical sensors , Nanostructured materials , Catalysts , Food contamination
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:4121 , http://hdl.handle.net/10962/d1013328
  • Description: Ochratoxin A is the most potent chlorinated derivative of the ochratoxin group, consisting of a 5'-chlorinated dihydroisocoumarin moiety linked by an amide bond to l-phenylalanine. Produced as a secondary fungal metabolite by several species of Aspergillus and Penicillium, ochratoxin A has been shown to readily contaminate a large variety of commodities including cereals, groundnuts, dried fruit, spices and coffee. This has led to widespread contamination of ochratoxin in wine, beer, milk and meat products. As ochratoxin A is a potent nephrotoxin exhibiting teratogenic and carcinogenic properties, the development of a rapid screening platform for the cost effective control of ochratoxin A content in foodstuffs is therefore required. The evaluation of metallophthalocyanine and carbon nanotube electrode modification toward the development of a nanostructured biosensor capable of enhancing the electrochemical detection of ochratoxin A in complex media is presented. Cyclic voltammetry at a glassy carbon electrode allowed for the optimization of detection parameters including pH and type of supporting electrolyte. Britton-Robinson buffer was found to be the most suitable supporting electrolyte in terms of sensitivity and reproducibility obtaining a LOD of 0.28 μM as determined by differential pulse voltammetry. Subsequent analysis determined the dependence of OTA oxidation on pH in acidic media which proceeds with the transfer of two electrons to form a quinone/hydroquinone couple shown to adsorb to the electrode surface. Passivation of the electrode through adsorption of oxidation products was shown to severely limit the detection of OTA upon successive detection cycles. Comparison of various metallophthalocyanine modifiers showed an increase in sensitivity toward the detection of OTA at phthalocyanine complexes with metal based redox processes. However with the exception of NiPc and CoTCPc complexes, phthalocyanine modification was limited by the increase in deviation of current response and extent of fouling. NiPc modification showed an increase in sensitivity by two fold with fouling characteristics comparable to an unmodified electrode while low improvements in fouling was observed at CoTCPc modified electrodes with sensitivity in detection comparable to an unmodified electrode.Modification of the electrode with multi- and single walled carbon nanotubes produced a significant increase in sensitivity toward the detection of ochratoxin A. The electrocatalytic activity of nanotube modifiers was attributed to the increase in surface area and to the addition of oxygenated functional groups upon acid treatment as confirmed by Raman spectroscopy. Acid functionalization of the carbon nanotubes for a period of two hours produced the greatest increase in sensitivity obtaining a respective LOD of 0.09 μM and 0.03 μM for analysis of ochratoxin A at multi- and single walled carbon nanotube modified electrodes. Centrifugal purification of carbon nanotubes was deemed necessary to improve the electrocatalytic activity of the nanotube modifiers through the removal of carbonaceous impurities as visualized by atomic force microscopy. Furthermore, a crude lipase preparation, lipase A, was investigated as a potential biological recognition element for selective detection of ochratoxin A in complex media. Lipase A enabled the hydrolysis of ochratoxin A to the electroactive species ochratoxin α as confirmed by thin layer chromatography and voltammetric analysis. Additional isolation of a pure hydrolase from the lipase A preparation is required prior to utilization within a nanostructured biosensor platform capable of detecting ochratoxin A in complex media.
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  • Date Issued: 2011

Nanomaterial modified electrodes : optimization of voltammetric sensors for pharmaceutical and industrial application

  • Authors: Brimecombe, Rory Dennis
  • Date: 2011
  • Subjects: Voltammetry , Electrochemistry , Nanotubes , Nanostructured materials
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:4101 , http://hdl.handle.net/10962/d1009721
  • Description: Nanomaterials, in particular carbon nanotubes have been shown to exhibit favourable properties for the enhancement of electrochemical detection of target analytes in complex matrices. There is however scope for improvement in terms of the optimization thereof in electrochemical sensors surface modification. The aim of this thesis was to examine methods that would result in increased current response, lowered passivation and application of such modified surfaces with application to pharmaceutically and industrially relevant analytes. Current methods for enhancing the performance of carbon nanotubes include acid functionalization which not only increases the hydrophilicity of the nanotubes, and consequently their ability to provide stable (aqueous) suspensions, but also introduces electrochemically active sites. This particular approach is however not normalized in the literature. Over-exposure to acid treatment results in loss of structural integrity of the carbon nanotubes, and as such a fine balance exists between achieving these dual outcomes. Guided by high resolution scanning electron microscopy, atomic force microscopy, voltammetric and impedance studies, this thesis examined the role of the length of time of the acid functionalization process as well as the impact of activation of carbon nanotubes and fullerenes on electrochemical sensor performance. Based on desired charge transfer resistances, rate transfer coefficients and sensitivity towards redox probes the optimal length of acid functionalization for multiwalled carbon nanotubes was 9 hours and 4 hours for single-walled carbon nanotubes. Further improvements in the desired outcomes were achieved through electrochemical activation of the modified electrode surface by cycling in the presence of catechol, in a novel approach. By employing electrochemical impedance spectroscopy it was observed that catechol activation resulted in lowered charge transfer resistance, before and after activation, with functionalized multi-walled carbon nanotubes (9 hours) exhibiting the greatest decrease of 90 % and functionalized single-walled carbon nanotubes (4 hours), a 50 % decrease. Corresponding increases in the heterologous rate transfer coefficient showed a 770 % increase for functionalized multi-walled carbon nanotubes (9 hours), following catechol activation. Comparative observations for fullerenes following partial reduction in potassium hydroxide yielded a 30 % decrease in charge transfer resistance, with an increased heterologous rate transfer coefficient at a fullerene modified surface The performance of the nanomaterial modified electrodes was applied to the detection of wortmannin with applications in bioprocess control and in the pharmaceutical sector as well as to the detection and monitoring of the industrial dye Reactive red. Of particular relevance to these analytes was the assessment of the nanomaterial modified electrodes for enhanced stability, reproducibility, sensitivity and decreased passivation effects. In this study the first known account of wortmannin detection through electrochemical methods is reported. Voltammetric characterization of wortmannin revealed an irreversible cathodic process with a total number of 4 electrons and a diffusion coefficient of 1.19 x 10-7 cm².s⁻¹. At a functionalized multiwalled carbon nanotubes modified glassy carbon electrode a limit of detection of 0.128 nmol.cm⁻³ was obtained, and with limited surface passivation the detection scheme afforded pertinent analyses in biological media representing a substantial improvement over chromatographic detection methods. This study also provided the first account of the voltammetric detection of reactive red, competing favourably with traditional spectroscopic methods for monitoring biodegradation of this compound in real time.
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  • Date Issued: 2011

Nanostructures and metallophthalocyanines : applications in microbial fuel cells

  • Authors: Edwards, Sean
  • Date: 2011
  • Subjects: Microbial fuel cells , Waste products as fuel , Nanostructured materials , Electrochemistry , Nanotubes
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:4107 , http://hdl.handle.net/10962/d1011742 , Microbial fuel cells , Waste products as fuel , Nanostructured materials , Electrochemistry , Nanotubes
  • Description: Microbial fuel cells (MFCs) are a promising form of alternative energy capable of harnessing the potential energy stores in organic waste. The oxygen reduction reaction (ORR) forms an integral role in the generation of electricity in MFCs however it is also a potential obstacle in enhancing the performance of MFCs. Platinum, a commonly used catalyst for the ORR, is expensive and rare. Significant research has been conducted into developing alternative catalysts. Metallophthalocyanines (MPc) have garnered attention for use as catalysts. Iron phthalocyanine (FePc) has been shown to have catalytic activity towards the reduction of oxygen. Coupling of the catalyst to nanostructured carbon materials, such as multi-walled carbon nanotubes, has been observed to have several advantages as nanostructures have a high surface-to-volume ratio. In this study, we have attempted to assess the suitability of FePc, both its bulk and nanostructured form, as an oxygen reduction catalyst and acid functionalized multi-walled carbon nanotubes for use as a catalyst support using electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy. We showed, for the first time, the catalytic nature of nanostructured FePc towards the ORR. Applying the data obtained from the electrochemical analyses, electrodes were modified using FePc and MWCNTs and applied to an Enterobacter cloacae-based MFC. Several operational parameters of the MFC, such as temperature and ionic strength, were optimized during the course of the study. We showed that optimized FePc:MWCNT-modified electrodes compared favourably to platinum-based electrodes in terms of power densities obtained in a microbial fuel cell.
  • Full Text:
  • Date Issued: 2011

Probing the biocompatibility of biomedical interfaces using the Quartz Crystal Microbalance with Dissipation

  • Authors: Cromhout, Mary
  • Date: 2011
  • Subjects: Biomedical materials , Nanostructured materials , Biomedical engineering , Quartz crystal microbalances , Blood proteins , Nanoparticles
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:4102 , http://hdl.handle.net/10962/d1010660
  • Description: The biomedical application of nanotechnology has come into the spotlight, with the promise of ‘personalised’ therapeutics that couple early diagnosis with targeted therapeutic activity. Due to the rapid growth of the biomedical applications of nanoparticles, along with the lack of understanding concerning their interactions with biomolecules, there is a pressing need for the development of standard methods directed at investigating the effect of introducing these unique particles into the human body. The central aim of this research is to establish a platform directed at assessing the biological fate of pioneering therapeutic particulate agents, such as metallophthalocyanines (MPcs) and multi-walled carbon nanotubes (FMWCNTs). In particular, we proposed, that Quartz Crystal Microbalance with Dissipation (QCM-D) technology may be employed to assess the composition of blood protein corona deposited on the therapeutic surface, and subsequently assess the biocompatibility of such particles. The proposed method of protein detection utilises the nanogram sensitivity of QCM-D technology to monitor highly specific antibody-antigen interactions. In particular those interactions which occur when probe antibodies are used to detect adsorbed blood proteins deposited on target particle-modified sensor surfaces. Protein detection analysis was directed toward identification of surface bound human serum albumin, complement factor C3c, and human plasma fibrinogen. Preliminary analysis of generic biomedical surfaces indicated human serum albumin demonstrates a higher binding affinity towards positively charged surfaces (i.e. cysteamine self-assembled monolayer), followed by hydrophobic surfaces. Detection of complement C3c, corresponded with literature, where lower levels were detected on negatively charged surfaces (i.e. mercapto undecanoic acid self-assembled monolayer), and higher levels of more hydrophobic surfaces (i.e. 11-amino undecane thiol self-assembled monolayer). Human plasma fibrinogen was observed to favour hydrophilic over hydrophobic self-assembled monolayer surfaces, which was in accordance with literature. Application of the proposed protein detection method for biocompatibility analysis of target therapeutic molecules, namely metallophthalocyanines and acid functionalised multi-walled carbon nanotubes, demonstrated a dependence on modified-surface film characteristics, such as surface charge and topography with regards to human serum albumin and human plasma fibrinogen analysis representing new insights into their potential biomolecular interactions The highest levels of detected human serum albumin and complement C3c were detected on the GePcSmix-modified surfaces. AlPcSmix-modified surfaces analysis suggested the highest levels of human plasma fibrinogen. Two methods of acid functionalisation were employed, using both nitric and sulphuric acid, and pure nitric acid. A general increase in detected human serum albumin, corresponding with an increase in functionalisation time, was observed. Complement C3c detection suggested an increase in deposited complement C3c, with increasing functionalisation time, when assessing nitric acid functionalised multi-walled carbon nanotubes, and a decrease, with increasing functionalisation time, when assessing nitric and sulphuric acid functionalised multi-walled carbon nanotubes. Analysis of human plasma fibrinogen was inconclusive, as were cytotoxicity experiments utilising MCF-7 cells in the presence of metallophthalocyanine complexes, raising simultaneously important considerations for their application and study. In the first such detailed examination of its kind it was concluded that the proposed method of protein detection, using QCM-D, allows for the rudimentary but rapid means of analysis of select protein corona deposited on particulate biomedical surfaces.
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  • Date Issued: 2011

Polymers, catalysts and nanostructures a hybrid approach to biomolecule detection

  • Authors: Frith, Kelly-Anne
  • Date: 2009
  • Subjects: Polymers , Nanostructured materials , Biomolecules , Tryptophan , Melatonin , Electrodes , Electrochemistry , Tryptophan oxygenase
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:3980 , http://hdl.handle.net/10962/d1004039 , Polymers , Nanostructured materials , Biomolecules , Tryptophan , Melatonin , Electrodes , Electrochemistry , Tryptophan oxygenase
  • Description: The main goals in electroanalytical sensing are towards improved sensitivity and selectivity, or specificity, of an analyte. There are several approaches to achieving these goals with the main approach being modification of an electrode surface with synthetic or natural catalysts (enzymes), polymers and also utilisation of nanostructured materials. At present, there is a strong movement towards hybrid sensing which couple different properties of two or more surface modification approaches. In this thesis, a range of these surface modifications were explored for analysis and detection of two main analytes: the amino acid, tryptophan (Trp); and, the neurotransmitter, dopamine (DA). Specifically, this thesis aimed to utilise these methods to enhance the sensitivity and selectivity for Trp over an interferent, the indoleamine, melatonin (Mel); and, DA over the vitamin, ascorbic acid (AA). For Trp detection, immobilisation of an enzyme, Tryptophanase (Trpase) resulted in poor selectivity for the analyte. However, enhanced sensitivity and selectivity was achieved through pH manipulation of the electrolyte medium at a Nafion®-modified electrode surface for both Trp and Mel. At pH 3.0, the Mel and Trp anodic peak potentials were sufficiently resolved allowing for an LOD of 1.60 and 1.62 nM,respectively, and permitting the accurate analysis of Trp in a dietary supplement containing Mel. Multi-walled carbon nanotubes (MWCNTs) suspended in Nafion® exhibited further increases in the signal responses of these analytes at pH 3.0 and 7.4 with minimal change in the resolution of the anodic peaks. A lower sensitivity was, therefore, observed at the Nafion® and MWCNT modified electrode compared to the Nafion®-modified electrode at pH 3.0 with LODs of 0.59 and 0.80 nM exhibited for Trp and Mel, respectively. Enhanced selectivity for Trp in the presence of Mel can be achieved with MWCNTs in the presence of metallotetrasulphonated phthalocyanines (MTSPcs) particularly at pH 3.0, owing to cation exchange effects. However, the lack of sensitivity towards Trp, and even Mel, at this CoTSPc and MWCNT modified electrode remains a drawback. For DA, detection at the MWCNT and Nafion® surface resulted in improved sensitivity over that of both the bare electrode (613.0 nM) and the Nafion® modified electrode (1045.1 nM) with a calculated LOD of 133.9 nM at this layer. Furthermore, improvements in the selectivity of DA were achieved at the Nafion® and MWCNT modified electrode as exclusion of AA (150 μM) was achieved. At the MWCNT and CoTSPc surface, AA was excluded up to 130 μM with sensitivity for DA extending as low as 14.3 nM, far greater than observed for Trp and Mel. These concentrations are well within physiological concentration ranges and represent the most significant solution yet in terms of AA exclusion and enhanced sensitivity for DA. An examination of the surface layering by impedance spectroscopy and atomic force microscopy indicates that the success of the hybrid sensor utilising CoTSPc and MWCNTs lay in improved dispersion of MWCNTs and improved electron transfer kinetics, facilitated by the net charge of the materials present. This thesis, thus, showed the utility of a judicious selection of synthetic and biological catalysts, polymers and carbon nanomaterials towards a hybrid approach to the electrochemical sensing of Trp, Mel, DA and AA with focus on sensitivity and selectivity of these analytes.
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  • Date Issued: 2009
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