Magister Scientiae - MSc / The main objective of this work was to develop simple and sensitive electrochemical sensors for the detection of bisphenol A. To investigate the electrochemical behavior of BPA on a bare glassy carbon electrode. To apply the developed biosensor for the determination BPA by differential pulse voltammetry, electrochemical impedance spectrometry, square wave voltammetry and steady-state amperometry. To characterize the synthesized PDMA-PSS by cyclic voltammetry (CV), UV-Vis spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM). / South Africa

Magister Scientiae - MSc / In electrochemical oxidation treatment of wastewater, the electrode material is an important parameter in optimizing oxidative electrochemical processes, since the mechanism and products of several anodic reactions are known to depend on the anode material. The electrochemical oxidation of benzaldehyde, nitrobenzene and m-cresol on bare boron-doped diamond (BDD) electrode was investigated. Cytochrome c was then electrochemically immobilsed onto the functionalized BDD electrode by cyclic voltammetry. Oxidation and reduction reaction mechanism of each flavonoid was studied. There was one oxidation and reduction peaks for quercitin and catechin respectively, and two oxidation and two reduction peaks for rutin. The cytochrome c modified BDD electrode showed good sensitivity for all three flavonoids and low detection limits i.e. 0.42 to 11.24 M as evaluated at oxidation and reduction peaks, respectively. / South Africa

Magister Scientiae - MSc / Coeliac disease (CD) is a gluten intolerance condition that results in the flattening of the villi, which line the bowel. It is the most common cause of malabsorption of food nutrients. This inability to absorb sufficient levels of nutrients causes many of the common symptoms experienced by CD patients. Some of the symptoms, which lead to an increase in mortality rate, include chronic diarrhea, fatigue, iron-deficient anemia and osteoporosis. People with CD have higher than normal levels of certain antibodies in their blood. Thus, the concentration of anti-transglutaminase antibody (anti-tTG) in human sera is an important analytical marker for the diagnosis of CD. An immunosensor is a type of biosensor that has an antigen or antibody fragment as its biological recognition component. The specificity of the molecular recognition of antigens by antibodies to form a stable complex is the basis of immunosensor technology. In this work, overoxidized polypyrrole (OvoxPpy) was electrosynthesized as a noval sensor platform on a glassy carbon electrode (GCE). The OvoxPpy was then doped with gold-nanoparticles (GNP) by electrodeposition using cyclic voltammetry to form GNP|OvoxPpy||GCE electrode system. Morphology and size of the GNP|OvoxPpy||GCE nanocomposite were determined using scanning electron microscopy. The electrochemical immunosensor for anti-tTG antibodies was prepared by immobilizing transglutaminase antigen (tTG-antigen) onto the GNP|OvoxPpy||GCE by drop coating and allowed to incubate for 2 hrs. The electrochemical characterization of the nanocomposite platform and immunosensor were studied by voltammetry and electrochemical impedance spectroscopy (EIS). Charge transfer resistance, Rct (obtained from EIS data fitting) of [Fe (CN)6]3-/4- redox probe was used as the analytical parameter for measuring the interfacial kinetics which occurred as a result of the bio-recognition event (affinitive binding) between the tTG-antigen and anti-tTG antibody. Rct was found to increase with increased concentration of the antibody as a result of the reluctance to the flow of redox probe charge across the interface. Antibody concentration as low as in 1:4000 dilutions was detected. / South Africa

Magister Scientiae - MSc / In this research project, nanostructured composites based on Tin dioxide (SnO2) and Titanium dioxide (TiO2) with poly-4-styrene sulfonic acid (PSSA) doped polyaniline (PANI) conducting polymer has been investigated based on their structural, electrical and electrochemical properties. The synthesis of conducting polymers and their metal oxide or composites have been carried out chemically or electrochemically according to methods modified from the literature. Layer-by-layer construction of nano-Metal Oxide/PSSA doped polyaniline composites were successfully constructed by electroanalytical methods on the surface of a glassy carbon working electrode (GCE). / South Africa

Philosophiae Doctor - PhD / In the presented work, immunosensors for detection of Aflatoxin B1 based on different immobilization platforms were studied. Synthesis of an electroactive hydrogel was also carried out. Aflatoxins are a group of mycotoxins that have deleterious effects on humans and are produced during fungal infection of plants or plant products. Electrochemical immunosensor for the determination of Aflatoxin B1 (AFB1) was developed with anti-aflatoxin B1 antibody immobilized on Pt electrodes modified with polyaniline (PANi) and polystyrene sulphonic acid (PSSA). Impedimetric analysis shows that the electron transfer resistances of Pt/PANi-PSSA electrode, Pt/PANi-PSSA/AFB1-Ab immunosensor and Pt/PANi-PSSA/AFB1-Ab incubated in BSA were 0.458, 720 and 1066 kΩ, respectively. These results indicate that electrochemical impedance spectroscopy (EIS) is a suitable method for monitoring the change in electron-transfer resistance associated with the immobilization of the antibody. Modelling of EIS data gave equivalent circuits which showed that the electron transfer resistance increased from 0.458 kΩ for Pt/PANi-PSSA electrode to 1066 kΩ for Pt/PANi-PSSA/AFB1-Ab immunosensor, indicating that immobilization of the antibody and incubation in BSA introduced an electron transfer barrier. The AFB1 immunosensor had a detection limit of 0.1 mg/L and a sensitivity of 869.6 kΩL/mg. / South Africa

Philosophiae Doctor - PhD / One of the most powerful aspects of conducting polymers is their ability to be nanostructured through innovative, synthetically manipulated, transformations, such as to tailor-make the polymers for specialized applications. In the exponentially increasing wide field of nanotechnology, some special attention is being paid to innovative hybrid dendrimer-core based polymeric smart materials. Star copolymers are a class of branched macromolecules having a central core with multiple linear polymer chains extending from the core. This intrinsic structural feature yields a unique 3D structure with extended conjugated linear polymer chains, resulting in star copolymers, which have higher ionic conductivities than their corresponding non-star conducting polymer counterparts. In this study an in-depth investigation was carried out into the preparation and characterization of specialized electronic smart materials. In particular, the preparation and characterization of novel conducting dendrimeric star copolymers which have a central poly(propylene imine) (PPI) dendrimer core with conducting polypyrrole (PPy) chains extending from the core was carried out. This involved, first, the preparation of a series of dendrimeric polypyrrole poly(propylene imine) star copolymers (PPI-co-PPy), using generations 1 to 4 (G1 to G4) PPI dendrimer precursors. The experimental approach involved the use of both chemical and electrochemical synthesis methods. The basic procedure involved a condensation reaction between the primary amine of a diamino functional PPI dendrimer surface and 2-pyrrole aldehyde, to afford the pyrrole functionalized PPI dendrimer (PPI-2Py). Polymerization of the intrinsically contained monomeric Py units situated within the dendrimer backbone was achieved via two distinctly different routes: the first involved chemical polymerization and the second was based on potentiodynamic oxidative electrochemical polymerization. The star copolymers were then characterized using various sophisticated analytical techniques, in-situ and ex-situ. Proton nuclear magnetic resonance spectroscopy (1HNMR) and Fourier transform infrared spectroscopy (FTIR) were used to determine the structures. Scanning electron microscopy (SEM) was used to determine the morphology. Themogravimetric analysis (TGA) was used to study the thermal stability of the prepared materials. X-ray diffraction analysis (XRD) was used to study the structural make-up of phases, crystallinity and amorphous content. Hall effect measurements were carried out to determine the electrical conductivity of the chemically prepared star copolymers. The PPI-co-PPy exhibited improved thermal stability compared to PPI-2Py, as confirmed by TGA. SEM results showed that the surface morphology of the functionalized dendrimer and star copolymer differed. The surface morphology of the chemically prepared star copolymers resembled that of a flaky, waxy material, compared to the ordered morphology of the electrochemically grown star copolymers, which resembled that of whelk-like helixes. In the case the electrochemically grown star copolymers, SEM images recorded at higher magnifications showed that the whelk-like helixes of the star copolymers were hollow tubes with openings at their tapered ends, and had an average base diameter of 2.0 mu;m. X-ray diffraction analysis of the first generation star copolymer G1PPI-co-PPy revealed a broadly amorphous structure associated with PPy, and crystalline peaks for PPI. Cyclic voltammetry (CV), square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) techniques were used to study and model the electrochemical reactivity of the star copolymer materials. Electrochemical impedance spectroscopy data showed that the G1PPI-co-PPy exhibited slightly higher ionic conductivity than pristine PPy in lithium perchlorate. The second generation star copolymer G2PPI-co-PPy electrochemically deposited on a platinum (Pt) electrode had a lower electrochemical charge transfer resistance compared to electrodeposited polypyrrole (PPy) on a Pt electrode, and bare Pt. The decrease in charge transfer resistance was attributed to an increase in the conjugation length of the polymer as a result of the linking of the highly conjugated PPy to the PPI dendrimer. Bode impedimetric analysis indicated that G2PPI-co-PPI was a semiconductor, with a maximum phase angle shift of 45.3° at 100 MHz. The star copolymer exhibited a 2- electron electrochemistry and a surface coverage of 99%. Results of Hall effect measurements showed that the star copolymer is a semiconducting material, having a conductivity of 0.7 S cm-1, in comparison to the 1.5 S cm-1 of PPy. To the best of my knowledge, these new star copolymers have not been reported in the open literature. Their properties make them potentially applicable for use in biosensors. / South Africa

Philosophiae Doctor - PhD / In the presented work, immunosensors for detection of Aflatoxin B1 based on different immobilization platforms were studied. Synthesis of an electroactive hydrogel was also carried out. Aflatoxins are a group of mycotoxins that have deleterious effects on humans and are produced during fungal infection of plants or plant products. Electrochemical immunosensor for the determination of Aflatoxin B1 (AFB1) was developed with anti-aflatoxin B1 antibody immobilized on Pt electrodes modified with polyaniline (PANi) and polystyrene sulphonic acid (PSSA). Impedimetric analysis shows that the electron transfer resistances of Pt/PANi-PSSA electrode, Pt/PANi-PSSA/AFB1-Ab immunosensor and Pt/PANi- PSSA/AFB1-Ab incubated in BSA were 0.458, 720 and 1066 kΩ, respectively. These results indicate that electrochemical impedance spectroscopy (EIS) is a suitable method for monitoring the change in electron-transfer resistance associated with the immobilization of the antibody. Modelling of EIS data gave equivalent circuits which showed that the electron transfer resistance increased from 0.458 kΩ for Pt/PANi-PSSA electrode to 1066 kΩ for Pt/PANi- PSSA/AFB1-Ab immunosensor, indicating that immobilization of the antibody and incubation in BSA introduced an electron transfer barrier. The AFB1 immunosensor had a detection limit of 0.1 mg/L and a sensitivity of 869.6 k ΩL/mg. In the second platform an immunosensor based on gold nanoparticles (AuNP) and polythionine-modified glassy carbon electrode (GCE) for the determination of aflatoxin B1 (AFB1) was developed. Aflatoxin B1-BSA conjugate was immobilised on the modified GCE. Horseradish peroxidase (HRP) or Bovine serum albumin (BSA) were used to block sites against non-specific binding of the AFB1- conjugate with other compounds such as the salts used in preparing the buffer when the antibody interacts with the AFB1 conjugate and free AFB1. Competition reaction was allowed to take place between the free AFB1 and AFB1-conjugate for the binding sites of the anti-aflatoxin B1 antibody. Cyclic voltammetry (CV) was employed to characterize the electrochemical properties of the modified process. The peak separation of the immunosensor (ΔEp) was 62 mV indicating a quasi reversible process. Differential pulse voltammetry (DPV) was used to monitor the analytical signal. The response decreased with an increase in AFB1 concentration in the range of 0.6-2.4 ng/mL with a limit of detection of 0.07 and 0.16 ng/mL for HRP and BSA blocked immunosensors respectively. Significantly the low detection limit of 0.07 ng/mL is within the limits set by worl health organization (WHO) for AFB1 and its derivatives which is 2 ng/mL The proposed method eliminates the use of secondary antibody enzymatic labels. Synthesis and characterization of (p-(HEMA)-polyaniline hydrogels were investigated. The hydrogels were synthesized using: 2-Hydroxyeththyl methacrylate (HEMA), N-Tris (hydroxymethyl) methyl] acrylamide, 3- Sulfopropyl methacrylate potassium salt, Tetraethylene glycol diacrylate, Poly-(2- hydroxyethyl methacrylate), 2, 2-Dimethoxy-2-phenylacetophenone and aniline by UV irradiation. Two sets of the hydrogels were prepared using water / 1, 3, 3, 3-(tetramethyl butyl phenyl polyethylene glycol [Triton X-100] and water / ethylene glycol as the solvent. Scanning electron microscopy (SEM) revealed a more uniform pore size when Triton X 100 (TX-100 HG) was used as compared to ethylene glycol (EG-HG). Thermogravimetric analysis (TGA) showed that both hydrogels were stable up to 270 oC. Fourier transform-Infra red (FTIR) spectrum confirmed the incorporation of polyaniline (PANi) and HEMA in the composite. Electrochemical properties of the hydrogels evaluated using Cyclic Voltammetry and Electrochemical Impedance Spectroscopy (EIS) demonstrated the electroactivity and conductivity.

Philosophiae Doctor - PhD / A highly dispersive gold nanoparticle-dotted 4-nitrophenylazo functionalised graphene nanocomposite (AuNp/G/PhNO2) was successfully synthesised and applied in enhancing sensing platform signals. Three label-free electrochemical immunosensors for the detection of deoxynivalenol mycotoxin (DON) based on the systematic modification of glassy carbon electrodes (GCE) with AuNp/G/PhNO2 was effectively achieved. General electrochemical impedance method was employed for the sensitive and selective detection of DON in standard solutions and reference material samples. A significant increase in charge transfer resistance (Rct) of the sensing interface was observed due to the formation of insulating immune-complexes by the binding of deoxynivalenol antibody (DONab) and deoxynivalenol antigen (DONag). Further attachments of DONab and DONag resulted in increases in the obtained Rct values, and the increases were linearly proportional to the concentration of DONag. The three immunosensors denoted as GCE/PDMA/AuNp/G/PhNH2/DONab, GCE/Nafion/[Ru(bpy)3]2+/AuNp/G/PhNH2/DONab and GCE/Nafion/[Ru(bpy)3]2+/G/PhNH2/DONab have detection range of 6 – 30 ng/mL for DONag in standard samples. Their sensitivity and detection limits were 43.45 ΩL/ng and 1.1 pg/L; 32.14ΩL/ng and 0.3 pg/L; 9.412 ΩL/ng and 1.1 pg/L respectively. This result was better than those reported in the literature and compares reasonably with Enzyme Linked Immunosorbent Assay (ELISA) results. The present sensing methodology represents an attractive alternative to the existing methods for the detection of deoxynivalenol mycotoxin and other big biomolecules of interest due to its simplicity, stability, sensitivity, reproducibility, selectivity, and inexpensive instrumentation. And they could be used to develop high-performance, ultra-sensitive electrochemiluminescence, voltammetric or amperometric sensors as well.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Técnicas de limpeza química ou mecânica são comumente empregadas para evitar a formação de biofilmes e problemas associados com a colonização de superfícies por micro-organismos. Neste trabalho, amostras de aço inox 304L foram submetidas a ensaios acelerados de crescimento de biofilme, o qual foi posteriormente removido por tratamentos de limpeza e desinfecção (choque térmico com água a 70C por 1h, limpeza com ácido fosfórico 20 % v/v por 30min e desinfecção com peróxido de hidrogênio 0,17 % v/v por 1h). Com o objetivo de verificar a influência destes tratamentos na remoção do biofilme, este foi caracterizado (por quantificação microbiana e análises de espectroscopia de impedância eletroquímica - EIE), antes e após a aplicação dos tratamentos. Dois casos foram estudados. No primeiro caso, simularam-se condições presentes em tubulações de ambientes hospitalares e sistemas de distribuição de água quente. O micro-organismo utilizado foi a bactéria Serratia marcences. O processo de formação de biofilme e posterior limpeza e desinfecção foi realizado de modo contínuo durante 15 semanas. Os resultados de quantificação microbiana e de EIE mostraram que desde a primeira semana de exposição e ao longo dos ensaios, formou-se um biofilme aderente à superfície do aço e que o emprego dos tratamentos de limpeza e desinfecção não foi eficaz na remoção do biofilme. Em um segundo caso, simularam-se condições presentes em tubulações da indústria de água mineral, empregando-se a bactéria Pseudomonas aeruginosa. Neste caso, as técnicas de limpeza e desinfecção foram aplicadas individualmente e em conjunto. A aplicação de choque térmico, assim como da limpeza ácida, sobre um biofilme com 72 h de formação foi capaz de eliminar as bactérias viáveis, presentes na superfície do aço. Entretanto, a desinfecção com peróxido de hidrogênio não foi capaz de eliminá-las. Nas duas condições, porém, as análises de EIE do sistema aço/biofilme mostraram que o mesmo não foi completamente removido da superfície do metal. Correlacionando os dois casos, pode-se inferir que a superfície do aço inox 304L é rapidamente colonizada pelas duas espécies microbianas e que as técnicas de limpeza e desinfecção são capazes de reduzir e até eliminar as células viáveis, embora não removam completamente o biofilme da superfície. Por outro lado, é importante ressaltar que fatores como a arquitetura, espessura e porosidade do biofilme, e propriedades intrínsecas da superfície, são fatores que afetam os valores de impedância medidos, sendo necessário considerá-los na análise, tanto da formação do biofilme, quanto dos efeitos dos procedimentos de limpeza e desinfecção / Techniques for chemical or mechanical cleaning are usually employed to prevent the formation of biofilm and the problems associated with the colonization of surfaces by micro-organisms. In this work, samples of 304 L stainless steel were submitted to experiments of accelerated growth of biofilm, which was subsequently removed by cleaning and disinfection treatment (heat shock with water at 70 C for 1h, cleaning with phosphoric acid 20% v/v for 30 min and disinfection with hydrogen peroxide 0,17% v/v for 1h). In order to verify the influence of these treatments on biofilm removal, analyses of microbial quantification and electrochemical impedance spectroscopy (EIS), before and after the treatments were employed. Two cases were studied. In the first case, it was simulated the conditions found in piping for hospital environment and systems for the distribution of hot water. The micro-organism used was the bacteria Serratia marcences. The process of biofilm formation and subsequent cleaning and disinfection was performed continuously for 15 weeks. The results of measurement of microbial and EIS showed that since the first week of exposure and during the testing, an adherent biofilm was formed on the steel surface and that the use of cleaning and disinfecting treatments were not effective in removing biofilm. In a second case, it was simulated the conditions present in the pipes of the mineral water industry, using the bacterium Pseudomonas aeruginosa. In this case, the cleaning and disinfection techniques were applied individually and as a whole. The application of heat shock, as well as acid cleaning, to the biofilm formated after 72 h was able to eliminate the viable bacteria present in the steel surface. However, disinfection with hydrogen peroxide was not able to eliminate them. In both conditions, however, the EIS analysis of the steel/biofilm system showed that it was not satisfactorily removed from the metal surface. Comparing the two cases, it can be inferred that the surface of 304L stainless steel is rapidly colonized by the two microbial species studied and that the cleaning and disinfection techniques are able to reduce and even eliminate viable cells, although they could not completely remove the biofilm from the surface. On the other hand, it is important to note that factors such as properties of the electrode surface (roughness, porosity and adsorption), the electrode potential, and the architecture, thickness and porosity of the biofilm, are factors that affect the measured impedance values, it is necessary to consider them in the analysis of biofilm formation and effects of cleaning and disinfection

CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / A extração de petróleo em águas profundas impõe diversos desafios às empresas de óleo e gás e, dentre eles, destaca-se o desgaste sofrido na parte interna dos dutos de transporte devido à ação da água produzida (elevado teor salino), gases e particulados, principalmente a areia produzida. O objetivo deste trabalho foi estudar a erosão-corrosão de um aço com baixo teor de carbono (VMEC 134AP) em um flowloop sob condições dinâmicas: fluido em escoamento com 3,5 % de NaCl, areia em concentração de 40 g cm-3 e adição de substância inibidora de corrosão, com foco principal em observar a possível interação entre o agente erosivo e o inibidor. Para isso, realizaram-se ensaios de erosão-corrosão em um circuito fechado de tubulações com quatro polegadas de diâmetro interno (101,2 mm) e foram aplicadas as técnicas de medição da taxa de desgaste: cupons de perda de massa, resistência à polarização linear e espectroscopia de impedância eletroquímica. Foram monitoradas também a concentração de inibidor no fluido e as modificações na superfície dos corpos de prova por microscopia eletrônica de varredura. Ao fim deste trabalho, concluiu-se que o aço apresenta uma taxa de corrosão de 3,5 mm ano-1 sem a adição de inibidor e 0,4 mm ano-1 com a adição de inibidor. Não foi observada interação direta entre a areia e o inibidor de corrosão que pudesse comprometer a eficiência do mesmo. A técnica de espectroscopia eletroquímica foi empregada com sucesso em condições dinâmicas por meio de uma célula projetado especificamente para tubulações. Os resultados mostraram que a célula e a técnica foram capazes de identificar o efeito sinergético da erosão-corrosão na parte inferior da tubulação e também a atuação do inibidor de corrosão. / Deep water oil production imposes several challenges for oil and gas companies, and among them, the wear suffered in the interior of the transportation pipelines due to the action of produced water (high salt content), gases and particulates, especially produced sand. The objective of this work was to study the erosion-corrosion of a low carbon steel in a flowloop under dynamic conditions: fluid with 3.5% NaCl, sand at a concentration of 40 g cm-3 and addition of a corrosion inhibitory substance, with a main focus on observing the possible interaction between the erosive agent and the inhibitor. For this purpose, we performed erosion-corrosion tests in a closed circuit of pipes of four inches of internal diameter and we applied the corrosion rate measurement techniques of mass loss coupons, linear polarization resistance and electrochemical impedance spectroscopy. We also monitored the concentration of inhibitor in the fluid and the modifications on the surface of the specimens by scanning electron microscopy. At the end of this work, we concluded that the steel presents a corrosion rate of 3.5 mm year-1 without the addition of inhibitor and 0.4 mm year-1 with the addition of inhibitor. We did not observe any direct interaction between the sand and corrosion inhibitor that could compromise the efficiency of the substance. We also concluded that electrochemical techniques can be successfully employed in dynamic conditions by means of a cell specifically designed for pipelines. The results showed that the cell and the technique were able to identify the synergistic effect of erosion-corrosion in the bottom of the pipe and also the action of the corrosion inhibitor. / Tese (Doutorado)