Sophie Griveau

Electro-oxidation of 2-mercaptoethanol on adsorbed monomeric and electropolymerized cobalt tetra-aminophthalocyanine films. Effect of film thickness

Abstract We have investigated the electrocatalytic activity of cobalt tetra-aminophthalocyanine (CoTAPc) for the one-electron oxidation of 2-mercaptoethanol (2-ME) using adsorbed monomeric CoTAPc and electropolymerized poly-CoTAPc films of different thickness on vitreous carbon electrode. Our results show that the activity of poly-CoTAPc increases slightly with the thickness of the film, while the activity of the adsorbed monomeric CoTAPc is still greatest. The trend in activities, analysed by cyclic voltammetry, is the same for the electro-oxidation of 2-ME and for the electroreduction of the corresponding disulphide. Tafel plots obtained from rotating disk measurement for adsorbed CoTAPc and poly-CoTAPc film modified electrodes show slopes close to RT/F (ca. 70 mV) at low polarisation potentials which increase gradually with potential to values close to 2RT/F (120 mV). This suggests that at low polarisation the rate-determining step is a chemical step (formation of a metal complex–thiol adduct) preceded by a fast electron-transfer step involving the CoTAPc. At higher polarisations the one electron oxidation of the adduct to give the thiyl radical becomes rate controlling. Even though monomeric CoTAPc adsorbed on glassy carbon presents higher activity for the electro-oxidation of 2-ME, the high stability of the electropolymerized poly-CoTAPc makes them more attractive for applications in the activation and/or the amperometric detection of thiols.

Enhanced electrochemical sensing of thiols based on cobalt phthalocyanine immobilized on nitrogen-doped graphene.

A hybrid nanocomposite based on cobalt phthalocyanine (CoPc) immobilized on nitrogen-doped graphene (N-G) (N-G/CoPc) has been developed to modify glassy carbon electrode (GCE) for the sensitive detection of thiols. The nanocomposites were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). Cyclic voltammetric studies showed that cobalt phthalocyanine and nitrogen doped graphene have a synergic effect and significantly enhance the electrocatalytic activity of the modified electrode towards thiols oxidation compared with electrodes modified with solely CoPc or N-G. The electrochemical oxidation responses were studied and the reaction mechanisms were discussed. The sensors exhibited a wide linear response range from 1μΜ to 16mM and a low detection limit of 1μΜ for the determination of l-cysteine, reduced l-glutathione and 2-mercaptoethanesulfonic acid in alkaline aqueous solution. The proposed N-G/CoPc hybrids contribute to the construction of rapid, convenient and low-cost electrochemical sensors for sensitive detection of thiols.

Real-time electrochemical detection of extracellular nitric oxide in tobacco cells exposed to cryptogein, an elicitor of defence responses

It was previously reported that cryptogein, an elicitor of defence responses, induces an intracellular production of nitric oxide (NO) in tobacco. Here, the possibility was explored that cryptogein might also trigger an increase of NO extracellular content through two distinct approaches, an indirect method using the NO probe 4,5-diaminofluorescein (DAF-2) and an electrochemical method involving a chemically modified microelectrode probing free NO in biological media. While the chemical nature of DAF-2-reactive compound(s) is still uncertain, the electrochemical modified microelectrodes provide real-time evidence that cryptogein induces an increase of extracellular NO. Direct measurement of free extracellular NO might offer important new insights into its role in plants challenged by biotic stresses.

Electrocatalysis of 2-mercaptoethanesulfonic acid oxidation on cobalt phthalocyanine modified electrodes. Effect of surface concentration of the catalyst

The catalytic oxidation of 2-mercaptoethanesulfonic acid (2-MESA) was studied on ordinary pyrolytic graphite (OPG) and vitreous carbon electrodes modified with different surface concentrations of the adsorbed cobalt phthalocyanine (CoPc) and with electropolymerized cobalt tetra-aminophthalocyanine (CoTAPc). The electrocatalytic oxidation of 2-MESA at the examined cobalt phthalocyanine modified electrodes is sensitive to the surface concentration of the catalyst, suggesting that the oxidation of 2-MESA is of first-order on the surface concentration of CoPc, for high coverage. They also tend to indicate that it must be the presence of electronic factors in the monolayer levels of the adsorbed cobalt phthalocyanines which are completely different from those present in the tri-dimensional arrays of the active part of the thicker adsorbed or electropolymerized complex deposits.

Expanded Pyridiniums: Bis-cyclization of Branched Pyridiniums into Their Fused Polycyclic and Positively Charged Derivatives—Assessing the Impact of Pericondensation on Structural, Electrochemical, Electronic, and Photophysical Features

This study evaluates the impact of the extension of the π-conjugated system of pyridiniums on their various properties. The molecular scaffold of aryl-substituted expanded pyridiniums (referred to as branched species) can be photochemically bis-cyclized into the corresponding fused polycyclic derivatives (referred to as pericondensed species). The representative 1,2,4,6-tetraphenylpyridinium (1(H)) and 1,2,3,5,6-pentaphenyl-4-(p-tolyl)pyridinium (2(Me)) tetra- and hexa-branched pyridiniums are herein compared with their corresponding pericondensed derivatives, the fully fused 9-phenylbenzo[1,2]quinolizino[3,4,5,6-def]phenanthridinium (1(H)f) and the hitherto unknown hemifused 9-methyl-1,2,3-triphenylbenzo[h]phenanthro[9,10,1-def]isoquinolinium (2(Me)f). Combined solid-state X-ray crystallography and solution NMR experiments showed that stacking interactions are barely efficient when the pericondensed pyridiniums are not appropriately substituted. The electrochemical study revealed that the first reduction process of all the expanded pyridiniums occurs at around -1 V vs. SCE, which indicates that the lowest unoccupied molecular orbital (LUMO) remains essentially localized on the pyridinium core regardless of pericondensation. In contrast, the electronic and photophysical properties are significantly affected on going from branched to pericondensed pyridiniums. Typically, the number of absorption bands increases with extended activity towards the visible region (down to ca. 450 nm in MeCN), whereas emission quantum yields are increased by three orders of magnitude (at ca. 0.25 on average). A relationship is established between the observed differential impact of the pericondensation and the importance of the localized LUMO on the properties considered: predominant for the first reduction process compared with secondary for the optical and photophysical properties.

Carbon nanotubes and metalloporphyrins and metallophthalocyanines-based materials for electroanalysis

We discuss here the state of the art on hybrid materials made from single (SWCNT) or multi (MWCNT) walled carbon nanotubes and MN4 complexes such as metalloporphyrins and metallophthalocyanines. The hybrid materials have been characterized by several methods such as cyclic voltammetry (CV), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electrochemical microscropy (SECM). The materials are employed for electrocatalysis of reactions such as oxygen and hydrogen peroxide reduction, nitric oxide oxidation, oxidation of thiols and other pollutants.

Electropolymerized cobalt macrocomplex-based films for thiols electro-oxidation: Effect of the film formation conditions and the nature of the macrocyclic ligand

Abstract We report here on the study of the electrocatalytic behavior of adsorbed and thin electropolymerized cobalt porphyrin and phthalocyanine films towards thiols oxidation with a special emphasize on the effect of the anion used for preparing the films and the nature of the macrocyclic ligand on the catalytic activity. Our results show that there are no effects related to the anions used during the preparation of the films on the electrocatalytic activity of the complex coatings, as evaluated by the cyclic voltammetry investigation of various thiols of different sizes. In contrast, the nature of the ligand around the cobalt centre of the macrocyclic complex has a strong effect on the catalytic activity for the oxidation of 2-mercaptoethanol (2-ME). This clearly illustrates the fact that the Co II /Co I formal redox potential of the macrocycle needs to be located in a rather narrow window to obtain maximum activity.

Overview of significant examples of electrochemical sensor arrays designed for detection of nitric oxide and relevant species in a biological environment

Ultramicroelectrode sensor arrays in which each electrode, or groups of electrodes, are individually addressable are of particular interest for detection of several species concomitantly, by using specific sensing chemistry for each analyte, or for mapping of one analyte to achieve spatio–temporal analysis. Microfabrication technology, for example photolitography, is usually used for fabrication of these arrays. The most widespread geometries produced by photolithography are thin-film microdisc electrode arrays with different electrode distributions (square, hexagonal, or random). In this paper we review different electrochemical sensor arrays developed to monitor, in vivo, NO levels produced by cultured cells or sliced tissues. Simultaneous detection of NO and analytes interacting with or released at the same time as NO is also discussed.

Rhenium Complexes Based on 2-Pyridyl-1,2,3-triazole Ligands: A New Class of CO2 Reduction Catalysts

A series of [Re(N^N)(CO)3(X)] (N^N = diimine and X = halide) complexes based on 4-(2-pyridyl)-1,2,3-triazole (pyta) and 1-(2-pyridyl)-1,2,3-triazole (tapy) diimine ligands have been prepared and electrochemically characterized. The first ligand-based reduction process is shown to be highly sensitive to the nature of the isomer as well as to the substituents on the pyridyl ring, with the peak potential changing by up to 700 mV. The abilities of this class of complexes to catalyze the electroreduction and photoreduction of CO2 were assessed for the first time. It is found that only Re pyta complexes that have a first reduction wave with a peak potential at ca. -1.7 V vs SCE are active, producing CO as the major product, together with small amounts of H2 and formic acid. The catalytic wave that is observed in the CVs is enhanced by the addition of water or trifluoroethanol as a proton source. Long-term controlled potential electrolysis experiments gave total Faradaic yield close to 100%. In particular, functionalization of the triazolyl ring with a 2,4,6-tri-tert-butylphenyl group provided the catalyst with a remarkable stability.

CATALYTIC ACTIVITY OF ELECTRODE MATERIALS BASED ON POLYPYRROLE, MULTI-WALL CARBON NANOTUBES AND COBALT PHTHALOCYANINE FOR THE ELECTROOXIDATION OF GLUTATHIONE AND L-CYSTEINE

Electrodes prepared from overoxidized pyrrole, multi-walled carbon nanotubes (MWCNT) and cobalt phthalocyanine (CoPc) exhibit a clear catalytic activity for the electrooxidation of L-cysteine and reduced gluthatione. The electrodes are more stable than the ones modified by simple adsorption of CoPc and/or MW CNT. Therefore, these materials offer a good alternative for the development of electrocatalytic applications and thiols detection in biological interest.