Analyse des propriétés de sorption de la silice vis-à-vis des bases et des espèces cationiques par les méthodes diélectriques et électrochimiques

Non disponible / Not availableL'étude du comportement des silices synthétiques vis-à-vis des bases et des ionscuivriques a été réalisée dans le but de mieux connaître les interactions ayant lieu à l'interface solide-solution. Deux types de silices ont été considérées, un gel de silice à grande aire spécifique et une silice dite de Stôber qui se caractérise par une microporosité. Deux méthodes ont été principalement mises en oeuvre pour permettre l'analyse des phénomènes dans des conditions in situ et non destructives : la diélectricité et l'électrochimie aux électrodes à pâte de carbone modifiées par la silice. Il a été montré pour la première fois que la mesure des pertes diélectriques sur la silice décantée dans une solution basique rend directement compte de la quantité de groupements silanol ionisés. De même, il a été démontré que l'évolution du taux d'ionisation est peu liée à la force de la base mais est fortement dépendante du degré de dissociation des groupements silanol conformément au modèle de Sonnefeld. La spectroscopie Raman confirme la présence de groupements silanol de réactivité différente. Les billes de Stüber se comportent tout à fait différemment. En effet, en faisant réagir des bases moléculaires de différentes tailles, la propriété de tamis moléculaire a été mise en évidence pour la première fois. L'ouverture des pores de ce matériau a été estimée à 0,3 nm. Ses groupements silanol, majoritairement situés à l'intérieur de sa structure, sont chimiquement réactifs et se comportent comme des groupements de surface lorsque le réactif est suffisamment petit que pour entrer librement dans la structure. L'application de la diélectricité pour suivre la fixation du cuivre(ll) en milieu ammoniacal démontre sans ambiguïté le caractère covalent de la liaison SiO-Cu. L'insertion de la silice dans une électrode à pâte de carbone a permis d'étudier les processus d'adsorption-désorption du cuivre sur ce matériau en milieu ammoniacal. Elle démontre une sélectivité très marquée de la silice pour les ions cuivriques. Cette propriété a d'ailleurs été exploitée pour la mise au point d'un capteur ampérométrique du cuivre(ll) sur base du schéma «accumulation en milieu ammoniacal - détection voltampérométrique en milieu acide ». La limite de détection obtenue pour un temps d'accumulation de 2 minutes a été estimée à 10-8 mol l-1

Mesoporous silica thin films for molecular sieving and electrode surface protection against biofouling

International audienceElectrode fouling is a major challenge for the long term use of sensors in real samples as it leads to the decay of the electroanalytical signal and is often caused by the formation of an inhibiting layer formed by biomolecules. We demonstrate here that ordered and vertically aligned mesoporous silica generated at the surface of an indium tin oxide electrode by electrochemically assisted self-assembly act as a molecular sieve and a protective layer for the electrode surface. They indeed prevent the adsorption of size excluded large undesired molecules (e.g. haemoglobin) while allowing the detection of small redox active molecules likely to reach the electrode surface through the film (e.g. propranolol) with almost no loss of sensitivity. At a bare electrode, the oxidation of propranolol is completely inhibited in the presence of 5 mu M haemoglobin. At a modified electrode, the sensitivity for propranolol in the absence of haemoglobin is (72.8 +/- 2.9) mA mol(-1) (R-2= 0.992, N= 7) and it remains similar in the presence of 5 mu M haemoglobin with a value of (67.4 +/- 7.2) mA mol(-1) (R-2= 0.992, N= 7)

Thickness control in electrogenerated mesoporous silica films by wet etching and electrochemical monitoring of the process

Vertically aligned mesoporous silica films can be generated by electrochemically assisted self-assembly (EASA) but the accurate control of their thickness is essentially restricted to the 100 nm range. Here we have developed a wet etching approach using dilute ammonium fluoride to gradually decrease the thickness of ferrocene-functionalized films down to 20 nm by increasing the etching time. The effectiveness of the process was followed by monitoring the decrease in the voltammetric response of ferrocene moieties that are progressively removed from the electrode surface upon silica etching. Film thickness variations have also been confirmed by profilometry and were consistent with the electrochemical measurements. Electron microscopy analyses indicated a uniform thickness decrease but also some loss in the integrity of the mesostructure after prolonged etching. Keywords: Oriented mesoporous silica membrane, Ferrocene functionalized thin film, Wet etching, Film thickness control, Electroactive organic-inorganic hybri

Carbon paste electrode modified with 5-Br-PADAP as a new electrochemical sensor for the detection of inorganic mercury(II)

International audienc

Interaction of ammonium with birnessite: Evidence of a chemical and structural transformation in alkaline aqueous medium

International audienc

Immobilization of membrane-bounded (S)-mandelate dehydrogenase in sol-gel matrix for electroenzymatic synthesis

International audienceMembrane-bounded (S)-mandelate dehydrogenase has been immobilized on the surface of glassy carbon and carbon felt electrodes by encapsulation in a silica film obtained by sol-gel chemistry. Such bioelectrochemical system has been used for the first time for electroenzymatic conversion of (S)-mandelic acid to phenylglyoxylic acid. Apparent Km in this sol-gel matrix was 0.7 mM in the presence of ferrocenedimethanol, a value in the same order of magnitude as reported previously for vesicles in solution with other electron acceptors, i.e., Fe(CN)(6)(3-) or 2,6-dichloroindophenol. The bioelectrode shows very good operational stability for more than 6 days. This stability was definitively improved by comparison to a bioelectrode prepared by simple adsorption of the proteins on the electrode surface (fast activity decrease during the first 15 h of experiment). Optimal electroenzymatic reaction was achieved at pH 9 and 40 degrees C. Apparent Km of the protein activity was 3 times higher in carbon felt electrode than on glassy carbon surface, possibly because of transport limitations in the porous architecture of the carbon felt A good correlation was found between electrochemical data and chromatographic characterization of the reaction products in the bioelectrochemical reactor

Signal amplification by electro-oligomerisation for improved isoproturon detection

A novel concept is introduced for signal amplification in electrochemical sensing: the electro-oligomerisation stripping voltammetry, which has been applied here to the improved detection of the isoproturon herbicide in spring waters as a proof-of-principle. It involves a potentiostatic accumulation step onto a glassy carbon electrode (at +1.5 V vs Ag/AgCl reference electrode for 300 s) leading to the formation of an oligomeric film, which is then detected by cathodic stripping square wave voltammetry (SWV). The presence and composition of the film are confirmed by confocal Raman spectroscopy. Its characterisation by cyclic voltammetry demonstrates the reversible nature of the electrodeposited material, confirming its interest for sensitive detection by SWV. Adding a mesoporous silica membrane with vertically oriented nanochannels further enhances the sensitivity of the sensor, exhibiting a linear response in the 10–100 μM concentration range. This effect was even more interesting for real media analysis thanks to the permselective properties of such nanoporous coating in rejecting interferences and/or surface fouling agents. The method should be applicable to other analytes that are usually not detectable by conventional accumulation/stripping voltammetry

Study of DNA adsorption on polypyrrole: interest of dielectric monitoring

International audienceThe interest of high frequency impedance measurements performed on decanted solids was demonstrated for the in situ investigation of DNA adsorption onto polypyrrole (PPy). The dielectric method was first applied to distinguish between polymers containing different dopant and to monitor their ion-exchange occurring when PPy was contacting a phosphate buffer solution. Results were compared to those obtained by X-ray photoelectron spectroscopy (XPS) and ion-exchange chromatography. Afterwards, the effects of DNA adsorption were investigated in 0.01 M phosphate buffer at pH 7. A significant decrease in the dielectric parameters was observed upon the adsorption, as a result of the coverage of the conductive PPy by an insulating layer of DNA, while maintaining, however, the mobility of doping anions in the bulk of the polymer. Irreversibility of the adsorption process was assessed. Some conformational changes of the adsorbed DNA have been suspected to occur during the deposition of increasing amounts of DNA

Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat

An electrochemical method was developed for rapid and sensitive detection of the herbicide paraquat in aqueous samples using mesoporous silica thin film modified glassy carbon electrodes (GCE). Vertically aligned mesoporous silica thin films were deposited onto GCE by electrochemically assisted self-assembly (EASA). Cyclic voltammetry revealed effective response to the cationic analyte (while rejecting anions) thanks to the charge selectivity exhibited by the negatively charged mesoporous channels. Square wave voltametry (SWV) was then used to detect paraquat via its one electron reduction process. Influence of various experimental parameters (i.e., pH, electrolyte concentration, and nature of electrolyte anions) on sensitivity was investigated and discussed with respect to the mesopore characteristics and accumulation efficiency, pointing out the key role of charge distribution in such confined spaces on these processes. Calibration plots for paraquat concentration ranging from 10 nM to 10 μM were constructed at mesoporous silica modified GCE which were linear with increasing paraquat concentration, showing dramatically enhanced sensitivity (almost 30 times) as compared to nonmodified electrodes. Finally, real samples from Meuse River (France) spiked with paraquat, without any pretreatment (except filtration), were analyzed by SWV, revealing the possible detection of paraquat at very low concentration (10–50 nM). Limit of detection (LOD) calculated from real sample analysis was found to be 12 nM, which is well below the permissible limits of paraquat in drinking water (40–400 nM) in various countries

Electrochemical analysis of a microbial electrochemical snorkel in laboratory and constructed wetlands

International audienceMicrobial electrochemical snorkel (MES) is a short-circuited microbial fuel cell applicable to water treatment that does not produce energy but requires lower cost for its implementation. Few reports have already described its water treatment capabilities but no deeper electrochemical analysis were yet performed. We tested various materials (iron, stainless steel and porous graphite) and configurations of snorkel in order to better understand the rules that will control in a wetland the mixed potential of this self-powered system. We designed a model snorkel that was studied in laboratory and on the field. We confirmed the development of MES by identifying anodic and cathodic parts, by measuring the current between them and by analyzing microbial ecology in laboratory and field experiments. An important application is denitrification of surface water. Here we discuss the influence of nitrate on its electrochemical response and denitrification performances. Introducing nitrate caused the increase of the mixed potential of MES and of current at a potential value relatively more positive than for nitrate-reducing biocathodes described in the literature. The major criteria for promoting application of MES in artificial wetland dedicated to mitigation of non-point source nitrate pollution from agricultural water are considered