M.Tech. (Chemistry) / Please refer to full text to view abstract

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Philosophiae Doctor - PhD / Polypyrrole composites of polyaromatic hydrocarbon sulphonic acids (β–naphthalene sulphonic acid (NSA) and 1, 2-napthaquinone-4-sulphonic acid (NQS)), as well as those of transition metal oxides (tungsten (VI) oxide (WO3) and zirconium (IV) oxide (ZrO2)), were prepared and characterised for use as electrocatalytic sensors. The polymerization of pyrrole in β–naphthalene sulphonic acid (NSA) gave rise to nanotubules, nanomicelles or nanosheets polypyrrole (PPy) morphologies depending on the amount of NSA in the polymer and the polymerisation temperature. Scanning electron microscopy (SEM) measurements showed that the diameters of the nanostructured polypyrrole-β-naphthalene sulphonic acid (PPyNSA) composites were 150-3000 nm for the tubules, 100-150 nm for the micelles and 20 nm for the sheets. A red shift in the UV-Vis absorption spectra of PPy was observed for PPyNSA which is indicative of the involvement of bulky β-naphthalene sulphonate ion in the polymerization process. The UV-Vis also showed the existence of polaron and bipolaron in the polymer which may be responsible for the improved solubility of PPyNSA compared to PPy. All the characteristic IR bands of polypyrrole were observed in the FTIR spectra of PPyNSA, with slight variation in the absolute values. However, the absence of N–H stretching at 3400 cm-1 and 1450 cm-1 usually associated with neutral polypyrrole confirms that the polymer is not in the aromatic state but in the excited polaron and bipolaron defect state. Electrochemical analysis of PPyNSA reveals two redox couples: a/a′ - partly oxidized polypyrrole-naphthalene sulphonate radical cation/neutral polypyrrole naphthalene sulphonate; b/b′ - fully oxidized naphthalene sulphonate radical cation/partly reduced polypyrrole-naphthalene sulphonate radical anion. The corresponding formal potentials measured at 5 mV/s, Eº'(5 mV/s), are 181 mV and 291 mV, respectively. Analysis of the amperometric response of GCE/PPyNSA film to phenol gave sensitivities of 3.1 mA/mole dm-3 with a linear correlation coefficient of 0.982 for phenol concentrations of 19.8 μM to 139.5 μM. The apparent Michaelis-Menten constant (Km′) was estimated as 160 μM. Novel polypyrrole thin film microelectrodes prepared from an aqueous solution of the sodium salt of 1, 2- apthaquinone-4-sulphonic acid and pyrrole in hydrochloric acid as the supporting electrolyte was characterized electrochemically for the first time and found to exhibit good electronic and spectroscopic properties. The modified PPyNQS consisted of nano micelles with diameters of 50–100 nm. It also exhibited more pronounced voltammetric redox responses, improved solubility and stronger UV-Vis absorptions at wavelengths for polarons (380 nm), bipolarons (750 nm) and overlapped bi-polarons (820 nm) compared to conventional PPy. Voltammetric investigations showed that the polymer exhibited quasi-reversible kinetics in a potential window of - 400 mV to +700 mV, with a formal potential of 322 mV vs. Ag/AgCl. The diffusion coefficient was calculated to be 1.02 x 10-6 cm2/s for a thin film with a surface concentration of 1.83 x 10-7 mol/cm2 and a standard rate constant of 2.20 x 10-3 cm/s at 5 mV/s. Substractively normalised in situ Fourier transform infrared spectroscopy (SNIFTIR) confirmed the incorporation of the surfactant into the polypyrrole film, and for the first time structural changes within the polymer were observed and used to explain the electrochemistry of the polymer. Electrochemical impedance spectroscopy (EIS) results validated the quasi-reversible kinetics observed in the voltammetric experiment. The changes in electrical properties of the polymer during electrochemical p-doping and n-doping were quantified by equivalent electrical circuit fitting. Impedimetric nanosensor systems for the determination of two anthropogenic organic pollutants, namely benzidine and naphthalene, were constructed with smart Pt/PPyNQS nanomaterials. Analysis of sensor systems containing tungsten oxide or zirconium oxide-modified polypyrrole showed that nanohybrids of the polypyrrole were generated by the in-situ polymerisation of pyrrole in acidic solutions. Results from morphological and spectroscopic investigation confirmed the pattern of metal distribution within the nanohybrid polymers matrix. However, this class of polymers were devoid of charge carriers characteristics required for electrocatalytic sensor applications. The thesis provided justification for the preparation of nanostructured conducting polypyrrole for use as anodes for the determination of phenol, benzidine and naphthalene. / South Africa

The characterization, properties and applications of the novel nanostructured microgel (nanoparticle network and microgel crystal) composed of poly-N-isopropylacrylanmide-co-allylamine (PNIPAM-co-allylamine) and PNIPAM-co-acrylic acid(AA) have been investigated. For the novel nanostructured hydrogels with the two levels of structure: the primary network inside each individual particle and the secondary network of the crosslinked nanoparticles, the new shear modulus, drug release law from hydrogel with heterogeneous structure have been studied. The successful method for calculating the volume fraction related the phase transition of colloid have been obtained. The kinetics of crystallization in an aqueous dispersion of PNIPAM particles has been explored using UV-visible transmission spectroscopy. This dissertation also includes the initial research on the melting behavior of colloidal crystals composed of PNIPAM microgels. Many new findings in this study area have never been reported before. The theoretical model for the columnar crystal growth from the top to bottom of PNIPAM microgel has been built, which explains the growth mechanism of the novel columnar hydrogel colloidal crystals. Since the unique structure of the novel nanostructured hydrogels, their properties are different with the conventional hydrogels and the hard-sphere-like system. The studies and results in this dissertation have the important significant for theoretical study and valuable application of these novel nanostructured hydrogels.

Nesta tese foi explorado o controle de arquitetura molecular pelo uso das técnicas de Langmuir-Blodgett (LB) e automontagem (LBL) para produzir filmes nanoestruturados de azopolímeros e azocorantes de baixa massa molecular. A primeira contribuição foi a síntese química de azopolímeros, nos quais os grupos azo são ligados covalentemente a uma cadeia polimérica. Para produção de filmes LBL, que requer compostos solúveis em água, fez-se a sulfonação do PAZO (poli(p-azofenileno)) resultando no PAZOS (poli(p-azofenileno sulfonado)). Este azopolímero conjugado apresentou luminescência em solução, confirmando cálculos teóricos, mas não em filmes LBL com poli(alilamina) (PAH). Os filmes LBL são condutores elétricos quando dopados com iodo e têm propriedades de birrefringência opticamente induzida. A fotoindução da birrefringência com laser linearmente polarizado foi muito lenta, com tempos consideravelmente maiores que em outros filmes LBL, devido não só às interações eletrostáticas, mas também à rigidez da cadeia conjugada. Outro polímero sintetizado foi o DR19CL-IPDI, que é solúvel em clorofórmio e foi utilizado para fabricação de filmes de Langmuir e LB. Os filmes LB só puderam ser produzidos utilizando a estratégia de co-espalhamento com estearato de cádmio (CdSt). Os experimentos de birrefringência opticamente induzida indicaram que os filmes LB de DR19CL-IPDI/CdSt têm maior birrefringência que os outros azopolímeros derivados de poliuretanos, pois o seu máximo de absorbância coincide com o comprimento de onda do laser de escrita. Por outro lado, a birrefringência residual, após desligamento do laser, foi menor que em sistemas similares porque a relaxação do azocromóforo é facilitada devido a este polímero possuir menor temperatura de transição vítrea. Ainda com filmes de Langmuir, utilizamos a espectroscopia de UV-VI in situ para analisar os filmes de HPDR13 (poli 4\'-2-(metacriloiloxi)etiletilamino -2-cloro- 4-nitroazobenzeno)). Em acordo com as isotermas de pressão de superfície, os resultados indicaram que com maiores temperaturas de subfase, as cadeias de HPDR 13 ganham flexibilidade e são mais bem empacotadas, ocupando menor área por molécula. A importância do controle da arquitetura molecular foi demonstrada no estudo da birrefringência fotoinduzida e formação de grades de relevo de superfície (SRG) nos filmes LBL de azocorantes com quitosana e em filmes do azopolímero PS119 com dendrímeros DAB (Polipropilenimina). Os filmes LBL de quitosana com o corante SunsetYellow (SY) possuem birrefringência espontânea devida a uma organização molecular proveniente da técnica de automontagem. Para os filmes Ponceau S (PS), a birrefringência pode ser fotoinduzida e a dinâmica depende do pH utilizado na fabricação do filme, devido a alterações nas interações intermoleculares. A estrutura interna do filme LBL de dendrímero DAB teve grande efeito na adsorção de PS119, que aumentou com a geração, de 1 para 5. A maior adsorção do dendrímero G5 foi atribuída ao maior número de sítios ionizados para interagir com os azocromóforos do PS119. Entretanto, a birrefringência fotoinduzida foi maior para os filmes LBL com dendrímeros de menor geração, pois os filmes de geração maior apresentam maior interpenetração entre as camadas. Esta restringe a mobilidade dos cromóforos, gerando menor birrefringência. Esta explicação foi corroborada pela observação de maiores amplitudes das SRG, 31 nm e 5nm, para filmes de 35 bicamadas de PS119/DAB G1 e PS119/DAB G5, respectivamente. Estas grades foram formadas por transporte de massa causado por efeitos fotônicos, sendo observadas só para luz com polarização p e não s / In this thesis we exploit the control of molecular architecture provided by the Langmuir-Blodgett (LB) and layer-by-layer (LBL) techniques to produce nanostructured films from azobenzene-containing materials, azopolymers and low molecular weight azodyes. The first contribution was associated with the chemical synthesis of azopolymers, in which azochromophores were attached to polymer chains. For LBL film fabrication, which requires water-soluble materials, the sulfonation of the polymer PAZO (poly(p-azophenylene)) led to PAZOS (sulphonated poly(p-azophenylene)) This conjugated azopolymer was luminescent in solution, confirming theoretical predictions, but not in LBL films when alternated with poly(allylamine hydrochloride) (PAH). The latter LBL films displayed electric conductivity when doped with iodine and were also amenable to photoinduced birefringence. The writing of information with a linearly polarized laser was very slow, with writing times considerably longer than for other LBL films due not only to the electrostatic interactions in the film but also to the rigidity of the conjugated chain. The other polymer synthesized and characterized here was DR19CL-IPDI, which is soluble in organic solvents and was used to fabricate Langmuir and LB films. The LB films could only be produced by employing the strategy of co-spreading with an amphiphile, in this case DR19CL-IPDI mixed with cadmium stearate (CdSt). Experiments with photoinduced birefringence in the LB films of DR19CL-IPDI/CdSt indicated higher birefringence than in other polyurethane-based azopolymers because the laser wavelength almost coincides with the wavelength for maximum absorbance. On the other hand, the residual birefringence - after the writing laser was switched off - was smaller than in those similar systems because azochromophore relaxation is facilitated in DR19CL-IPDI due to its lower glass transition temperature. Still with regard to Langmuir films, we have introduced the in situ UV-VIS. spectroscopy technique to analyze films of HPDR13 (poly 4 ’-2-(methacryloyloxy)ethylethylamino -2-chloro-4-nitroazobenzene). Consistent with the pressure-area isotherms, the results from this spectroscopy indicated that at higher subphase temperatures the HPDR13 chains gain flexibility and the molecules can be packed in a more condensed manner, leading to a smaller occupied area per molecule. The importance of molecular architecture control was demonstrated in the study of photoinduced birefringence and formation of surface-relief gratings in LBL films consisting of azodyes alternated with chitosan, and the azopolymer PS119 alternated with DAB (polypropylenimine) dendrimers. In the chitosan LBL films, those containing the azodye Sunset Yellow (SY) exhibited spontaneous birefringence owing to the anisotropy in molecular organization imparted by the LBL technique. For Ponceau S (PS) films, birefringence could be photoinduced with the dynamics of writing depending on the solution pH employed for film fabrication, owing to changes in the intermolecular interactions. The internal structure of LBL films from DAB dendrimers had a large effect on the adsorption of PS119, which increased with the generation, from 1 to 5. The more effective adsorption for G5 dendrimer was due to a larger number of ionized sites for interacting with the azochromophores of PS119. In contrast, the photoinduced birefringence was higher for LBL films of the low generation G1 dendrimer, which was explained by the stronger interpenetration between adjacent layers in the higher generation dendrimers. In LBL films from PS119/DAB G5, this interpenetration restricts the chromophore mobility, leading to a smaller birefringence. Consistent with this explanation, higher amplitudes were obtained for 35-bilayer films of PS119/DAB G1 (31 nm) in comparison with films from PS119/DAB G5 (5 nm). These gratings were formed with mass transport arising from a light-driven mechanism, as photoinscription was only successful with p-polarized light and not with s-polarized light

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In this thesis, I firstly report high field measurements of graphene/NbN junctions, in which NbN makes edge contact to graphene. Transport measurements at zero field demonstrate clear features associated with both retro and specular Andreev reflection. By applying perpendicular magnetic field, field dependence of junction transparency at Quantum Hall (QH) / superconductor (SC) interface is calculated and explained by a picture of superposition of electron and hole edge excitation. Zeeman splitting is induced in graphene by applying in plane magnetic field. We observe changes in the Andreev reflection spectrum that are consisting with spin splitting of the graphene band structure. This edge contact technique provides the opportunity to create hybrid SC/graphene or SC/QH system to illustrate new physics such as non-Abelian zero modes of Majorana physics. Secondly, other potential material candidates for SC/graphene junctions are discussed, high field transport measurement of FeSeTe/graphene junction is discussed, Superconductor/quantum spin Hall (QSH) interface and superconductor-graphene-superconductor weak link are also discussed, respectively. At last, via contact, a new contact method for two-dimensional materials, especially air-sensitive materials is discussed, the via contact method provides a new and reliable fabrication technique for two dimensional materials.

Carbon dots (CD) have emerged as the new eye-catching theranostic nanomaterials due to their distinctive features, including tunable emission, facile surface modification, high biocompatibility and low cytotoxicity. These distinguishing features allow full customizations of CD according to the needs of various studies. Of note, they have been widely employed as nano-vehicles with live-tracking systems in many biological applications to deliver medicine with low bioavailability to targeted sites. Candida albicans, a commonly seen commensal fungus accounts for life-threatening infections in humans, is the leading cause of oral candidiasis. Yet, the efficacy of the "gold standard" Amphotericin B (AmB) has been limited due to poor water solubility and dose-dependent cytotoxicity. In addition, the interactions of CD with Candida cells/biofilms and human epithelial tissues have not been fully investigated, and very limited studies have been done on CD-based antifungal drugs delivery for topical administration. Herein, AmB-conjugated guanylated CD (CD-Gu + -AmB) tackling oral fungal infections were synthesized and possessed potent antifungal/anti-biofilm effects against C. albicans. Moreover, CD-Gu + -AmB exhibit low cytotoxicity to primary human oral keratinocytes and can selectively accumulate in the cell nuclei. Above all, the first evidence of studying the penetration and exfoliation profiles of CD in a three- dimensional organotypic human oral epithelial tissue model was provided, and the accumulation of CD-Gu + -AmB in the epithelial tissue can form a 'shielding' layer on oral epithelia against C. albicans. This study demonstrates that CD-Gu + -AmB may serve as a promising antifungal agent for tackling C. albicans and Candida-induced oral candidiasis through fast epithelial penetration, extra-/intra-cellular embedding and gradual exfoliation

In this work, we investigated nano-metal plasmonic coupling between dissimilar metals. We measured the optical transmission of nano-Ag coupled to other nano-metals using glass and Si substrates respectively. The reflected colors shifted from yellow to violet were obtained through the plasmonic coupling with nearest-neighbor nano-metals such as aluminum, magnesium, and ytterbium nano-metals. They were deposited randomly next to the nano-Ag. The metal size is from 8 to 15 nanometers. The results show that the colors changing is essentially due to plasmonic coupling between nano-Ag and another the nano-metals e.g. nano-Al The coupling caused a red shift in plasmonic resonance frequency, thus, changing the reflection color. The resonance shift agrees well with the simulation result using COMSOL. The inter-particle distance and particle size dependency of the optical spectra correspond to surface plasmon resonance extinction peaks for isolated nano-Ag and coupled with those neighboring nano- metals. Due to plasmonic coupling between nanoparticles in small space can create new resonances; red shifts as the interparticle distance reduce. Wavelengths are tuned by the extent of the interparticles interactions which relate to the particles size, interparticles distance and the similarity of nano metals. Using different nano metals to fabricate thin films can change the plasmonic resonance frequency which makes the reflected colours become multihued. When we look into the effect of the nano-particle size, and the distance between nano-particles, we discovered that larger nano-particle size has larger distance between the particles, and since the plasmonic coupling is a function of Inverse Square of the distance between particles. Therefore, smaller nano-particles have the strongest plasmonic coupling. Al produced the smallest nano-particle therefore it has the shortest distance between nano-Al and nano-Ag. Since the size of the particles can be controlled during deposition, the color changing of nano-Ag can be well defined. Thus tunable color changing devices can be fabricated

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