Stéphane Trevin

Production of hydroxyl radicals by electrochemically assisted Fenton's reagent : Application to the mineralization of an organic micropollutant, pentachlorophenol

Abstract Hydroxyl radicals are very powerful oxidizing agents. They are involved in hydroxylation reactions, in biological and atmospheric phenomena. A recent application of these radicals is their use in decontamination of water polluted by toxic organic substances like pesticides. Chemically, these radicals are produced by the use of a mixture of (H 2 O 2 +Fe 2+ ), the so called Fentons reagent. In this work Fentons reagent is generated by electrochemistry in a catalytic way. The reaction of the hydroxyl radicals with pentachlorophenol (PCP) was studied. These radicals generated in situ in aqueous solution react with PCP and thus lead to its degradation. The evolution of the composition of the solution was followed by chromatographic analysis, COD analysis and the measurement of the total organic carbon (TOC) of the studied aqueous solution. Tetrachloro- o -benzoquinone and tetrachloro- p -benzoquinone (TCBQ) are the only aromatic intermediates identified. They result from the oxidation of the corresponding tetrachlorohydroquinones (TCHQ). Just like PCP, the TCBQs are degraded and disappear in their turn. The mineralization of the initial toxic substrate is confirmed on the one hand, by the regular decrease in quantity of the total organic carbon of the solution (TOC analysis) and on the other hand, by the quantitative release of chloride ions according to the electrical charge passed during electrolysis. The degradation of PCP appears relatively slow compared to that of the other organic pollutants studied by the electro-Fenton process and also to the degradation of other less substituted chlorophenols. The kinetic rate of the appearance of chloride ions is slower than that in the degradation of PCP. This phenomenon highlights the formation of chlorated aliphatic intermediates.

Elaboration and use of nickel planar macrocyclic complex-based sensors for the direct electrochemical measurement of nitric oxide in biological media.

We describe here the electrochemical detection of nitric oxide, NO, in biological systems by using chemically modified ultramicro carbon electrodes. In the first part of the paper, the different steps involved in the electrochemical preparation and characterization of the nickel-based sensor are described. This is illustrated by the use of nickel(II) tetrasulfonated phthalocyanine complex. The second part of the paper describes two examples of the direct electrochemical measurement of NO production in human blood platelets and endothelial cells from umbilical cord vein.

The use of gold electrodes in the electrochemical detection of nitric oxide in aqueous solution

It was recently discovered that vascular endothelial cells could synthesize the free-radical gas nitric oxide (NO) [1,2] and since that discovery, it has been possible to demonstrate the considerable physiological importance of NO as for example in the control of human blood flow and pressure. Several works are now dealing with the understanding of the mechanism by which NO is synthesized [3]. As a matter of fact, it has appeared that measuring NO in biological models is very difficult because of its low stability and high fugacity. In a recent review Archer [4] has summarized the different NO measurement strategies reported in the literature. The most widely used techniques involve the ex-situ detection of NO by (i) electron paramagnetic resonance, (ii) spectrophotometry and (iii) chemiluminescence. New amperometric microelectrode probes are now developed to detect NO and the use of electrochemistry, as a potential way to do so is very promising. Indeed, ultramicroelectrode design and elaboration are now reaching very high levels of sophistication [5] contributing very actively to the promotion of the use of the electrochemical techniques for in vivo NO detection in intact tissues and from single cells. There are two amperometric methods reported in the literature for measuring NO in vitro and from tissues and single cells. The first method is based on the direct oxidation of NO on platinum electrode. Shibuki [6] reported the detection of NO by using a miniature Clark-type electrode. The electrode was designed by introducing a platinum wire within a pipette with a fire-polished 150-250 pm tip (with a thin chloroprene rubber seal) filled with an aqueous solution of 30 mM NaCl and 0.3 mM HCI. The platinum indicator electrode was placed as close as possible to

Electrochemical and spectrophotometric study of the behavior of electropolymerized nickel porphyrin films in the determination of nitric oxide in solution

We describe in this paper an electrochemical and spectrophotometric study of the behavior of an electropolymerized nickel porphyrin film as a sensor for the determination of nitric oxide (NO) in aqueous solution. Our results show that the anodic oxidation of NO at the modified electrode may not be the result of a catalytic effect induced by the porphyrinic complex. However, the current (measured by differential pulse amperometry) and calculated NO concentration showed a linear relationship in the range 15 nM-6 muM in aerobic phosphate buffer solution (pH 7.4). These results provide a fruitful example of calibration of such electrochemical sensors for the selective detection of NO with a calculated detection limit, at a signal-to-noise ratio of three, equal to 1.5 nM.

Design and characterization of chemically modified electrodes with iron(III) porphyrinic-based polymers: study of their reactivity toward nitrites and nitric oxide in aqueous solution

Abstract This study gives new examples of iron porphyrin film electrodes prepared either by electrochemical polymerization or by incorporation in pre-electropolymerized pyrrole derivatives. It shows also the different kinds of interactions between nitric oxide, nitrites and the supported iron porphyrins in acidic and neutral aqueous solutions. It gives clear indications, by cyclic voltammetry and UV-visible spectrophotometry of the formation of the suggested iron-nitrosyl intermediate, [Fe(III)(NO)]+, in supported films.

Electropolymerized nickel macrocyclic complex-based films: design and electrocatalytic application

The voltammetric behaviour of electropolymerized nickel macrocyclic complexes in alkaline aqueous solution is described. The nickel-based modified electrodes were prepared by oxidative polymerization of nickel porphyrin, nickel salen and nickel cyclam complexes by repeated potential scans in 0.1 m NaOH solution. All the electroformed films exhibited the same behaviour which is similar to that shown by nickel hydroxide-modified electrodes. In addition, the nickel complex-based polymers act as efficient materials for the electrocatalytic oxidation of methanol, ethanol and hydrazine.

Direct electrochemical characterization of superoxide anion production and its reactivity toward nitric oxide in solution

Abstract We report in this study, and for the first time the direct and simultaneous electrochemical measurements of both nitric oxide and superoxide anion evolution and reactivity in solution. For this purpose, we have combined the use of two electrodes: a previously developed NO-sensor for the amperometric determination of NO and a carbon microelectrode for the amperometric detection of superoxide anion. The latter electrode displayed a potential use for the selective detection of O 2 − (generated by the xanthine/xanthine oxidase reaction) with an acceptable selectivity against uric acid. Our electrochemical experiments were validated by the Cytochrome c method and by the use of a different procedure for the generation of the superoxide anions. Our results show that the use of the NO sensor provides the selective detection of NO without any interface of the superoxide enzymatic generator.

Practical aspects and methodological approaches to achieve electrochemical detection of submicromolar NO in biological systems

Amperometry is one of the electrochemical techniques which may be used for real-time detection of nitric oxide, NO, in biological systems. We address here possible hazards that may be associated with some of the various electrochemical sensors and techniques reported. Specifically, it is not only important to consider the design of the microsensor for high performance NO detection, but one must also consider the parameters associated with the amperometric technique that is utilized in making the measurements. Thus, the conception of any efficient sensor should judiciously combine the optimum organized layer electrode design with the most appropriate amperometric technique, such as differential normal pulse amperometry and/or pulsed chronoamperometry.

Direct electrochemical measurement of nitric oxide production by cytochrome P450-catalyzed oxidation of N,N'-substituted hydroxyguanidines

The production of nitric oxide by oxidative cleavage of N- hydroxyguanidines catalyzed by rat liver microsomes was measured directly, and for the first time by differential-pulse amperometry at a Nafion–porphyrin coated micro-sensor. The measured concentration of the nitric oxide produced (0.18–0.25 µM) was lower than the total concentration of the stored nitrites. The difference in the obtained data is discussed.