New insight into 4-nitrobenzene diazonium reduction process: Evidence for a grafting step distinct from NO2 electrochemical reactivity

Electrochemical and spectroscopic investigations were performed in order to clarify the mechanism of 4-nitrobenzene diazonium reduction on glassy carbon in protic medium. The number and nature of the electron transfer processes were found to be strongly correlated to the electrode surface state. On polished electrode two different reduction peaks were observed. Selective electrolyses realized at the corresponding potential definitely proved that the grafting process actually occurs at a potential distinct from NO2 electroreduction, this latter inducing the presence of the quasi-reversible NO/NHOH redox couple at the electrode surface. These results were confirmed by XPS analyses. Furthermore, voltammetric experiments using Fe(CN)63- showed that the electrochemical properties of the modified electrode strongly depend on the potential applied for grafting, which modulates the nitro group oxidation state. All the results suggested that the electrode functionalization was more efficient when grafting and NO2 reduction were performed separately

Evidence of an EC' mechanism occurring during the simultaneous assay of ascorbic and uric acids on poly(3,4-ethylenedioxythiophene) modified gold microsensor.

A voltammetric microsensor has been developed for the simultaneous assay of ascorbic (AA) and uric (UA) acids in aq. soln. The electrode surface has been modified by means of electropolymd. conductive poly(3,4-ethylenedioxythiophene) (PEDOT) org. films. The anodic peak potential sepn. between both acids was more than 300 mV. The sensitivity of the microsensor for UA was found to be dependent on the presence of AA in the mixt. By using square wave voltammetry (SWV), it increased from 77.5 mA mM-1 cm-2 without AA to 86.2 mA mM-1 cm-2 with AA 1 mM. An EC' catalytic mechanism was highlighted, inducing the regeneration of reduced UA by AA at the vicinity of the electrode surface

Hg(II) trace electrochemical detection on gold electrode: Evidence for chloride adsorption as the responsible for the broad baseline

Investigations were performed in order to clarify the origin of the broad baseline observed during Hg(II) trace electrochemical detection on gold electrode in the presence of Cl- anions. The influence of Cl- concentration on the shape of the voltammograms was studied in the presence and in the absence of Bovine Serum Albumin (BSA) in order to bring out adsorption/desorption processes. On the basis of these experiments, and contrary to what has been proposed by several authors in the literature, it was proved that the broad baseline does not result from calomel (Hg2Cl2) formation but is rather related to an interaction between Cl- and polycrystalline Au electrode surface. The evolution of the shape of the baseline was also studied in the presence of other halide anions, namely F-, Br-, and I-. The latter two were found to induce a broad baseline similar to that recorded in the presence of Cl-. Finally, it was shown that BSA addition is not suitable for Hg(II) detection since it prevents Hg(0) deposition onto the electrode surface

Influence of the gold nanoparticles electrodeposition method on Hg(II) trace electrochemical detection

Gold nanoparticles (AuNPs) were deposited on Glassy Carbon (GC) substrate by using three electrochemical techniques: Cyclic Voltammetry (CV), Chronoamperometry (CA) and Potentiostatic Double-Pulse (PDP). For each electrodeposition method, the resulting AuNPs-modified electrodes were characterized by CV in H2SO4 and Field Emission Gun Scanning Electron Microscopy (FEG-SEM). CA was found to be the best electrodeposition mode for controlling the morphology and the density of AuNPs. The modified electrodes were used for low Hg(II) concentration detection using Square Wave Anodic Stripping Voltammetry (SWASV). AuNPs obtained by CA afforded the best amperometric response while involving the lowest amount of charge during the electrodeposition step (QAu(III)). This analytical response is correlated to both the smallest particle size (ca. 17 nm in diameter) and the highest particle density (332 particles μm−2), thus displaying high electrode effective surface area. In these optimal conditions, using a Hg(II) preconcentration time of 300 s, the nanosensor array exhibited a linearity range from 0.80 to 9.9 nM with a sensitivity of 1.16 μA nM−1. A detection limit of 0.40 nM (s/n = 3) was reached

Complexes bi- et polymétalliques à base de palladium : synthèse, électrosynthèse et applications en catalyse

Le travail de recherche présenté dans ce document concerne la synthèse, l’électrosynthèse, l’étude de la réactivité de complexes polymétalliques à base de palladium et les éventuelles propriétés catalytiques de ces complexes. Dans la première partie de ce document, nous rappellerons tout d’abord l’intérêt des phosphines et des diphosphines, et plus particulièrement de la diphosphine bis(diphénylphosphino)méthane (dppm), en tant que ligand, ainsi que des dimères de palladium contenant ce ligand en chimie organométallique. Nous décrirons ensuite les propriétés électrochimiques des complexes face-à-face Pd[indice]2(micro-dppm)[indice]2Me[indice]2Cl[indice]2 et en livre ouvert - ou « A-frame » - [Pd[indice]2 (micro-dppm)[indice]2Me[indice]2]I lorsqu’ils sont seuls en solution, en présence de dppm ou d’iodométhane. La structure cristallographique du dérivé en livre ouvert [Pd[indice]2(micro-dppm)[indice]2Me[indice]2I]I, qui a été obtenue dans le cadre de ce travail, sera présentée et donnera lieu à une comparaison avec d’autres structures reportées dans la littérature. La deuxième partie de ce manuscrit traite de catalyse d'hydrogénation par un cluster de palladium à quatre centres métalliques comportant deux hydrures : Pd[indice]4(micro-dppm)[indice]4H[indice]2[exposant]2+. Le premier chapitre offre un rapide tour d’horizon de la catalyse homogène par des complexes et clusters de palladium, et des outils d’investigation des mécanismes. Dans le deuxième chapitre, nous décrirons la structure cristallographique d’un cluster monohydrure proche de Pd[indice]4(micro-dppm)[indice]4H[indice]2[exposant]2+ qui a été obtenue au cours de cette étude. La caractérisation complète de ce cluster sera également indiquée. Dans le troisième chapitre, nous présenterons les résultats obtenus en catalyse d’hydrogénation avec le cluster Pd[indice]4(micro-dppm)[indice]4H[indice]2[exposant]2+ en tant que pré-catalyseur. Les études en fonction de la température et de la pression seront exposées, de même que le mécanisme de cette catalyse et les études cinétiques nous ayant permis de le démontrer. Enfin, dans la troisième partie de ce mémoire, nous nous intéresserons aux propriétés du complexe hétérobimétallique PdPt(micro-dppm)[indice]2Cl[indice]2. Après avoir brièvement rappelé les travaux reportés dans la littérature concernant ce type de dérivés hétérobimétalliques, nous décrirons la réactivité chimique et électrochimique du complexe PdPt(micro-dppm)[indice]2Cl[indice]2, qui a donné lieu à la formation de nouveaux dérivés palladium-platine originaux. Les structures cristallographiques de deux de ces dérivés seront présentées et discutées, de même que la caractérisation spectroscopique de l’ensemble de ces complexes

Electrodeposited gold nanoparticles on glassy carbon: Correlation between nanoparticles characteristics and oxygen reduction kinetics in neutral media

Gold nanoparticles (AuNPs) were deposited onto glassy carbon (GC) by constant potential electrolysis (CPE) using various sets of potential and duration from -0.3 to 0.7 V and 10 to 1800 s, respectively. The physico-chemical characteristics of the as-obtained deposits were investigated by cyclic voltammetry (CV) in H2SO4, field emission gun scanning electron microscopy (FEG-SEM), and Pb underpotential deposition (UPD). Their performances towards the oxygen reduction reaction (ORR) in a NaCl-NaHCO3 (0.15 M / 0.028 M, pH 7.4) neutral solution were examined and correlated to AuNPs size and density. The best results were obtained using the deposits which exhibited a high density (555 +/- 49 µm-2) of relatively small AuNPs (25 +/- 12 nm). The Koutecky-Levich treatment was systematically applied to all the deposits in order to determine the number of electrons n exchanged for the ORR in the potential range from 0.1 to -1 V. The values of the cathodic transfer coefficients Beta n were also extracted and compared to the values reported for unmodified GC and bulk Au. A map of the Beta n values as a function of AuNPs electrodeposition potential and duration was also provided

Kinetics of dioxygen reduction on gold and glassy carbon electrodes in neutral media

The electrochemical reduction of dioxygen (O₂) has been studied on bulk gold (Au) and glassy carbon (GC) electrodes in aqueous neutral solution close to blood ionic composition. The mechanism was found to involve two successive bielectronic steps with hydrogen peroxide (H₂O₂) as the reaction intermediate whatever the electrode material used. On Au, O₂ and H₂O₂ were reduced at close potentials. The determination of the kinetic parameters of O₂ electroreduction was thus achieved after removing the cathodic current corresponding to H₂O₂ reduction. Cyclic voltammograms exhibited one cathodic peak whose both current density (jp) and potential (Ep) evolution as a function of potential scan rate (r) was in accordance with Randles Sevcik and Nicholson-Shain equations, respectively. Rotating disk electrode (RDE) voltammetry was also performed and the data were analyzed using the Koutecky-Levich relationship. The effective number of electrons (n) was found to be roughly independent of the potential and close to n = 2 when removing H₂O₂ reduction current whereas it gradually increased up to n = 4 while considering the total current. The Tafel slopes allowed the cathodic transfer coefficients (Beta n) to be calculated in several neutral aqueous electrolytes. Values varied from 0.25 to 0.49 and were systematically higher on Au than on GC electrode. Similar results were obtained with Tafel slopes deduced from Butler Volmer exploitation of the current-potential curves

A new sensor with increased lifetime based on a mixed diazonium thick film/gold nanoparticles interface for Hg(II) trace detection

This work describes a novel strategy for surface functionalization, the aim of which is to significantly increase the lifetime of an electrochemical sensor dedicated to Hg(II) trace determination. In order to tailor stable mixed organic/inorganic interfaces, gold nanoparticles were electrodeposited onto a glassy carbon electrode previously functionalized by a thick 4-thiophenol diazonium film, which affords a good anchoring to the nanoparticles. AFM and FEG-SEM were used to characterize the film thickness and the nanoparticles average size and density, respectively. By using square wave anodic stripping voltammetry, the sensor exhibited a linear response between 1 and 10 nM Hg(II) and a normalized sensitivity 0.03 µA nM-1 min-1. Compared to previous works, the storage lifetime of the interface was at least three times longer, being more than three weeks

FIREBALL: Detector, data acquisition and reduction

The Faint Intergalactic Redshifted Emission Balloon (FIREBALL) had its first scientific flight in June 2009. The instrument combines microchannel plate detector technology with fiber-fed integral field spectroscopy on an unstable stratospheric balloon gondola platform. This unique combination poses a series of calibration and data reduction challenges that must be addressed and resolved to allow for accurate data analysis. We discuss our approach and some of the methods we are employing to accomplish this task