The aim of this thesis was the development of a voltammetric method for the determination of 7-dehydrocholesterol as a biomarker of congenital disease Smith-Lemli-Opitz syndrome. The concentration of 7-dehydrocholesterol in plasma will increase hundred-fold to hundreds to thousands µmol L-1 in patients suffering from Smith-Lemli-Opitz syndrome. The determination of 7-dehydrocholesterol was carried out in artifical serum in this work, which was prepared to mimic the conditions of postnatal diagnosis of Smith-Lemli-Opitz syndrome. 7-dehydrocholestrol provides a well developed voltammetric signal at approximately +0.8 V vs. Ag/AgNO3 in acetonitrile. Differential pulse voltammetry with optimized parameters was used for the determination of 7-dehydrocholesterol. The presence of protein in artificial serum (human serum albumin) resulted in an unacceptably high detection limit of the method (178 µmol L-1 in artificial serum). Therefore, human serum albumin was eliminated from the artificial serum samples by precipitating it with acetonitrile and subsequently centrifuging the resulting suspension. At the same time, the ratio of aqueous and organic components in the studied medium was also adjusted with acetonitrile to achieve an optimal voltammetric response of 7-dehydrocholestrol. With regard to the...

This thesis is devoted to the study of electrochemical oxidation of meta-substituted phenolic substances on a boron-doped diamond electrode. In the first phase, it was studied the effect of pH on their electrochemical oxidation by the methods of classical "direct current" voltammetry (DCV) and differential pulse voltammetry (DPV). The study was performed on three types of boron doped diamond (BDD) surfaces: polished, anodic and cathodically activated. In the second phase, attention was paid to the Hammett correlation to study the effect of the substituent on the potential of electrochemical oxidation of selected phenolic compounds. Among the substituents studied were (-H, -CH3, -OCH3, -(CH2)2COOH, -COOH a -NO2). In general, the oxidation potentials were lower in the basic medium than in the acidic medium on all studied surfaces. An exception was the studied phenolic acids, which could not be detected in an alkaline medium on the O-terminated BDD, probably due to electrostatic repulsion between the partially negative electrode surface and the acid dianion. The highest correlation coefficient for the dependence composed of all studied phenols was obtained for the polished and H-terminated surface at pH 11, ie the dissociated form of phenolic substances. The high electron density leads to easy...

This master's thesis contains a study of electrochemical processes of selected bile acids possessing 7 hydroxyl group (cholic, chenodeoxycholic and −muricholic). The measurements were performed on boron−doped diamond electrode in the non-aqueous medium of acetonitrile and perchloric acid (water content 0.55 %) by cyclic voltammetry. It is known that the electrochemical activity of 7 bile acids is increased by a dehydration reaction between perchloric acid and the 7 bile acid. The subject of the study was the stability of the voltammetric response of chemically activated bile acids in the region of negative potentials. It was found that the presence of oxygen in the measured solution is an important factor for obtaining the cathodic signal of 7 bile acids. It probably performs a regenerative function; the product of the electrochemical reduction is re-oxidized in its presence, which leads to an increase in the voltammetric response. At the same time, it is important that the direction of the scan in cyclic voltammetry first proceeds to positive values. A potential of +2.0 V (vs. Ag/AgNO3 in acetonitrile) must be reached for the HO● radicals to be formed. It is these radicals that presumably lead to the formation of the product(s) of bile acids electrochemical oxidation that can be subsequently...

Cholesterol is an irreplaceable sterol found in animal cells, lathosterol is one of its precursors. The first aim of this Thesis is to develop a method for determining cholesterol after liquid- liquid extraction from milk matrices by differential pulse voltammetry on a boron doped diamond electrode in the presence of perchloric acid in acetonitrile where cholesterol provides an irreversible anodic response at the potential between +1300 and +1600 mV depending on the water content. The second aim is a study of the voltammetric behavior of lathosterol on boron doped diamond and glassy carbon electrodes using cyclic voltammetry and differential pulse voltammetry mainly in perchloric acid but also sodium perchlorate where lathosterol provides an irreversible anodic response at the potential of about +1650 mV on the boron doped diamond electrode and at the potential of +1350 mV on a glassy carbon electrode. The effect of other acids - sulfuric, nitric and phosphoric - on the response of lathosterol was also investigated. The influence of the water content in the measured solution and the influence of the polarization rate on the response of lathosterol were studied. Furthermore, the calibration dependence of lathosterol was measured using differential pulse voltammetry after optimization of its...

No description available.

Ce travail est une contribution à la conception d'un système portatif et intégré de type µTAS (micro-Total Analysis System) pour le traitement (préconcentration et simplification) de solution aqueuses contenant des métaux lourds et leur analyse en ligne. Pour la préconcentration amont, l'étude a porté sur la réalisation d'un micro-électrodialyseur manipulant des µL de solutions. L'optimisation des paramètres opératoires a permis d'augmenter d'un facteur 100 environ les concentrations d'analytes. Pour l'étage de détection aval, un système sur puce de type polarographique a été étudié. Le traditionnel mercure est remplacé par du diamant polycristallin dopé au bore (DDB) préparé par CVD micro-ondes. La quantification des métaux lourds par redissolution anodique sur ce matériau a été validée ainsi que les techniques de gravure compatibles avec la réalisation de microréseaux d'électrodes. / This work is a contribution to the conception of an integrated μTAS (micro-Total Analysis System) for treatment (preconcentration and simplification) of aqueous solutions containing heavy metals and their on-line analysis. For upstream preconcentration, a micro-electrodialyser handling µL of solutions was developped. The optimization of operating parameters permitted to reach an analyte pre-concentration factor of about 100. For the downstream detection step, an on-chip polarographic system, where mercury was replaced by boron doped diamond (BDD), was studied. BDD films were prepared by assited microwave plasma CVD. Quantification of heavy metals by anodic stripping on BDD was validated as well as the etching technique compatible with the realization of microarray electrodes.

L'usage intensif de pesticides dans l'agriculture engendre une contamination sans précédent des eaux de surface et des nappes phréatiques. Les traitements classiques appliqués aux eaux usées contenant des produits organiques polluants sont basés sur la biodégradation ou sur des méthodes physiques de transfert de masse (décantation, filtration, adsorption des polluants sur du charbon actif) ou des procédés chimique tels que l'oxydation à l'ozone ou au chlore. Cependant, ces procédés demeurent inefficaces dans le cas de traitement des eaux contaminées par les polluants organiques persistants (POPs). Les procédés d'oxydation avancés sont mis en œuvre pour dégrader ou même minéraliser ces polluants. Ce travail a donc pour objectif de déterminer un protocole expérimental pour détruire efficacement les pesticides, produits chimiques largement utilisés dans l'agriculture au Maroc, tels que le monochloro-benzène, le 1,2-dichlorobenzène, l'acide 2,4,5-trichlorophénoxy-acétique, et l'imazalil en utilisant les procédés d'oxydation avancée électro-Fenton et oxydation anodique avec différentes anodes telles que BDD, Pt et DSA. Ainsi, nous avons trouvé que le taux de minéralisation par procédé électro-Fenton avec l'anode BDD était de 92%, 95%, 92% et 97%pour le monochloro-benzène, le 1,2-dichlorobenzène, l'acide 2,4,5-trichlorophénoxy-acétique, et l'imazalil, respectivement, après 4 h de traitement. Ces résultats confirment l'efficacité des procédés d'oxydation avancés électrochimiques dans le traitement des eaux polluées par des pesticides / The intensive use of pesticides in agriculture generates, nowadays, an unprecedented contamination of surface water and groundwater. Conventional treatments applied to waste water containing organic pollutants are based on biological treatments méthods or physical mass transfer methods (decantation, filtration, adsorption of the pollutants on activated carbon), chemical oxidation with ozone, chlorine, etc. However, these methods remain ineffective for the treatment of water polluted by persistent organic pollutants (POPs). Advanced oxidation processes are implemented to degrade and mineralize these pollutants. This PhD thesis work aims to establish an experimental protocol to degrade and mineralize pesticides, chemicals widely used in agriculture in Morocco, such as monochlorobenzene, 1,2-dichlorobenzene, 2,4,5-trichlorophenoxy-acetic acid , and imazalil using the electro-Fenton and anodic oxidation processes with DD Pt or DSA anodes. Thus, the rate of mineralization was 92%, 95%, 92% and 97% for the monochloro-benzene, the 1,2-dichlorobenzene, the 2,4,5-trichlorophenoxyacetic acid , and the imazalil, respectively, after 4 h treatment by electro-Fenton process. These results confirm the effectiveness of electrochemical advanced oxidation processes for treating water polluted by pesticides

The aim of this Thesis is development of electroanalytical methods for determination of cholesterol and 7-dehydrocholesterol on boron doped diamond electrodes based on electrochemical oxidation of products of acid-induced dehydration reaction. Two approches have been applied, relying on their detection in 0,1 mol.l-1 perchloric acid in acetonitril containing variable water content 0,43 % - 40 %. Cyclic voltammetry revealed substantial difference in oxidation peak pontecials for 7-dehydrocholesterol (+850 mV) and cholesterol (+1500 mV) in 0,1 mol.l-1 perchloric acid in acetonitrile containing 5 % of water. The limit of detection using differential pulse voltammetry were 6,19 µmol.l-1 a 19,2 µmol.l-1 for a mixture of 7-dehydrocholesterol and cholesterol, respectively. For flow injection analysis with amperometric detection (FIA-AD) the water content in run buffer, detection potencial and the distance between the capillary outlet and electrode surface in wall-jet arrangement of amerometric flow-through detection cell were optimized. Achieved detection limits are in range of 0,61 - 3,43 µmol.l-1 depending on the detection potencial and water concent in the run buffer. The developed FIA-AD method was used for determination of a model mixture solution of 7- dehydrocholesterol and cholesterol. Key words...

Este trabalho mostra estudos das propriedades eletroquímicas do eletrodo de diamante dopado com boro (DDB) e descreve a utilização de eletrodos de DDB, modificados direta ou indiretamente pelo método Sol-gel, para a oxidação de metanol e de etanol em meio ácido. Do estudo das propriedades superficiais do diamante concluiu-se que a polarização catódica é bastante apropriada para um bom desempenho do material é que não introduz diferenças estruturais detectáveis no filme de DDB, indicando que a melhora na resposta eletroquímica é devida somente a mudanças superficiais. Entretanto, polarizações catódicas consecutivas e extensivas por longos períodos de tempo produzem uma erosão da superfície do eletrodo. Adicionalmente, estudos realizados usando voltametria cíclica e espectroscopia fotoeletrônica de raios X, mostraram que a superfície de eletrodos de DDB pré-tratados catodicamente tem um comportamento eletroquímico dinâmico, apresentando uma perda da reversibilidade para o sistema Fe(CN)63-/4- em função do tempo de exposição ao ar. Isto é devido, possivelmente, à perda de hidrogênio superficial decorrente da oxidação da superfície do eletrodo pelo oxigênio do ar. Este comportamento dinâmico da superfície do DDB é inversamente dependente ao teor de boro do eletrodo. Foi observado também que a quantidade de sítios ativos disponíveis na superfície do eletrodo é diretamente proporcional à dopagem do mesmo e deve ser conseqüência da quantidade de boro superficial. Desses estudos concluiu-se que todos os eletrodos de DDB com distintas dopagens usados nesta tese (300, 800, 2000 e 8000 ppm de B) apresentam uma superfície eletroquímica heterogênea (sítios ricos em boro que apresentariam uma alta condutividade e sítios de diamante com terminação hidrogênio com menor condutividade), quando polarizados catodicamente e parcialmente bloqueada (diamante com terminações oxigênio) quando polarizados anodicamente, formando arranjos de microeletrodos (provavelmente sítios ricos em boro). As superfícies do diamante foram modificadas diretamente com Pt, Pt-RuO2 e Pt-RuO2-RhO2 pelo método Sol-gel. Estes depósitos de catalisadores apresentavam tamanhos nanométricos e mostraram um bom contato elétrico com a superfície do diamante e elevada pureza, assim como, uma composição atômica bem controlada e uma distribuição homogênea na superfície do DDB. Os estudos da oxidação eletroquímica de metanol e de etanol sugerem fortemente que o envenenamento da superfície do eletrodo é grandemente inibido sobre o catalisador ternário Pt-RuO2-RhO2/DDB quando comparado com os eletrodos Pt-RuO2/DDB e Pt/DDB. Adicionalmente, esse eletrodo mostra uma excelente atividade catalítica para a oxidação de etanol, provavelmente devido à habilidade que o ródio tem para quebrar a ligação C-C somada ao mecanismo bifuncional que acontece no catalisador contendo Pt e Ru. Em seguida, realizaram-se modificações indiretas do DDB pela síntese de compósitos de pó de carbono modificados com metais e óxidos metálicos e posterior fixação usando uma solução de Nafion. A oxidação dos álcoois foi estudada por voltametria cíclica e curvas de polarização em estado estacionário (diagramas de Tafel). Os sistemas investigados incluíram misturas metálicas binárias, ternárias e quaternárias contendo Pt, Ru, Ir, Rh, PbOx, TaOx e MoOx num total de 16 sistemas diferentes. Assim, os melhores catalisadores para a oxidação de metanol foram: Pt-Ru-TaOx-PbOx > Pt-Ru-TaOx-MoOx ≈ Pt-Ru-MoOx-Ir enquanto que, para a oxidação de etanol foram Pt-Ru-TaOx-PbOx e Pt-Ru-PbOx-Rh. Consequentemente, catalisadores do tipo Pt-Ru-TaOx-M são promissores para futuras aplicações práticas. Finalmente, estudos preliminares realizados usando pó de DDB modificado diretamente com Pt-RuOx pelo método Sol-gel mostraram que este compósito apresenta uma excelente atividade catalítica para oxidação de metanol, que foi maior do que a observada sobre um catalisador comercial considerado como o estado da arte (Pt-Ru/C da E-TEK) nas mesmas condições experimentais. Desta forma, foi mostrada a possibilidade do uso de pó de DDB como suporte para eletrocatalisadores para aplicações em células a combustível que funcionem diretamente com álcoois. / The electrochemical properties of boron-doped diamond (BDD) surfaces as well as studies of the oxidation of methanol and ethanol in acid media on BDD surfaces modified with catalysts by the Sol-gel method are presented here. Studies of the surface properties of BDD electrodes revealed that a cathodic polarization (-3,0 V vs. HESS, 30 min) is necessary for a good performance of the system while no important bulk structural differences are introduced in the film indicating that the enhanced electrochemical responses brought on by the cathodic pre-treatment is only due to superficial changes. Meanwhile, repeated and extensive cathodic polarizations led to severe erosion of the electrode surface. On the other hand, studies performed using cyclic voltammetry and X-ray photoelectron spectroscopy have shown that, after a cathodic pre-treatment, the BDD electrode surface presents a dynamic behaviour that results in a loss of the reversibility towards the Fe(CN)6 4−/3− redox couple as a function of the time exposed to atmospheric conditions. This dynamic behaviour must be associated to a loss of superficial hydrogen due to oxidation by the air and is inversely proportional to the BDD doping level suggesting that the boron content has a stabilizing effect on the H-terminated surface. It was also observed that the amount of electrochemical active sites of the BDD electrode has a direct dependence with the doping level (maybe due to the amount of superficial boron). From those studies, it was concluded that all BDD electrodes used in this work having different doping levels (300, 800, 2000 and 8000 ppm of boron) posses, after cathodic polarization, electrochemical heterogeneous surfaces (i.e. boron-rich sites with high conductivity and H-terminated diamond sites with lower conductivity) while after anodic polarization the surface is partially blocked by O-terminated diamond sites, resulting in a behaviour that resembles microelectrode arrays (probably formed by the few boron-rich sites). The BDD surface was also modified directly with Pt, Pt-RuO2 and Pt-RuO2-RhO2 using the Sol-gel method. These catalysts deposits have nanometric sizes, good electrical contact with the diamond surface and high purity. They also show a well controlled atomic composition and a homogeneous distribution on the BDD surface. Methanol and ethanol electrochemical oxidation studies on those modified electrodes suggested that poisoning of the surface by undesired intermediates (most xvi probably CO) is highly inhibited on the ternary catalyst Pt-RuO2-RhO2/BDD when compared with the Pt-RuO2/BDD and Pt/BDD responses. Additionally, the observed catalytic activity for ethanol oxidation was excellent, probably due to the rhodium ability to break the C-C bond which was added to the bifunctional mechanism operating in Pt-Ru catalysts. In the sequence, indirect modifications of the BDD surface were carried out by the production of carbon powder composites modified with metals and metallic oxides by the Sol-gel method and their subsequent anchoring to the BDD using a Nafionsolution. The alcohols oxidation on these electrodes was studied by cyclic voltammetry and steady-state polarization curves (Tafel plots). The investigated systems included binary, ternary and quaternary catalysts containing Pt, Ru, Ir, Rh, PbOx, TaOx and MoOx in a total of 16 different systems The higher catalytic activity towards methanol oxidation was observed in the sequence: Pt-Ru-TaOx-PbOx > Pt-Ru-TaOx-MoOx _ Pt-Ru-MoOx-Ir while for ethanol oxidation, the catalysts activity sequence was: Pt-Ru-TaOx-PbOx _ Pt-Ru-PbOx-Rh > Pt- Ru-Rh. Consequently, it was concluded that catalysts of the type Pt-Ru-TaOx-M are very promising systems to be used in future practical applications. Finally, preliminary studies carried out using BDD powder directly modified with Pt-RuOx by the Sol-gel method showed that this material has an excellent catalytic activity towards methanol oxidation reaction, being higher than that observed on a state of the art commercial catalyst (Pt-Ru/C from E-TEK) under the same experimental conditions. Thus, the feasibility of the use of BDD powder as catalysts support in direct-alcohol fuel cell systems was clearly established.

<p>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.</p>