Elena Volanschi

Evidence for OH radical production during electrocatalysis of oxygen reduction on Pt surfaces: consequences and application.

Multielectronic O(2) reduction reactions (ORR) at Pt surface (and at Au surface for comparison purpose) were examined both in water and in organic solvents using a strategy based on radical footprinting and scanning electrochemical microscopy (SECM). Experiments reveal a considerable and undocumented production of OH radicals when O(2) is reduced at a Pt electrode. These observations imply that the generally admitted description of ORR as simple competitive pathways between 2-electron (O(2) to H(2)O(2)) and 4-electron (O(2) to H(2)O) reductions is often inadequate and demonstrate the occurrence of another 3-electron pathway (O(2) to OH radical). This behavior is especially observable at neutral and basic pHs in water and in organic solvents like dimethylformamide or dichloromethane. In view of the high reactivity of OH radical versus organic or living materials, this observation could have important consequences in several practical situations (fuel cells, sensors, etc.) as far as O(2) reduction is concerned. This also appears as a simple way to locally produce highly reactive species as exemplified in the present work by the micropatterning of organic surfaces.

Spectral Studies on the Molecular Interaction of Anticancer Drug Mitoxantrone with CTAB Micelles

The interaction of anticancer drug mitoxantrone with cationic surfactant cetyltrimethylammonium bromide (CTAB) has been investigated by absorption spectroscopy as a function of surfactant concentration ranging from the premicellar to postmicellar region at pH 7.4 and 10. Interaction of mitoxantrone with CTAB micelles induces a bathochromic shift of both absorption maxima and spectral data showed that the micellization reduces the dimerization process and mitoxantrone is bound into micelles in the monomeric form. Binding constant and partition coefficient were estimated using the red shifts of the absorption maxima in the presence of surfactant. From the resulting binding constants for mitoxantrone-surfactant interactions, it was concluded that the hydrophobic interactions have a great effect on the binding of mitoxantrone to CTAB micelles. Also, by comparing the partition coefficients obtained using pseudo-phase model, the hydrophobic interactions have a major role in the distribution of mitoxantrone between micelle-water phases. Gibbs free energy of binding and distribution of mitoxantrone between the bulk aqueous medium and surfactant micelles were calculated.

Spectroscopic investigations of the molecular interaction of anticancer drug mitoxantrone with non-ionic surfactant micelles.

Objectives  The aim of this study was to investigate the interaction of the anticancer drug mitoxantrone with non‐ionic micelles, as simple model systems of biological membranes.

Scanning Electrochemical Microscopy Studies of Glutathione-Modified Surfaces. An Erasable and Sensitive-to-Reactive Oxygen Species Surface

A surface sensitive to reactive oxygen species (ROS) was prepared by reduction of a diazonium salt on glassy carbon electrode followed by the chemical coupling of glutathione (GSH) playing the role of an antioxidant species. The presence of active GSH was characterized through spectroscopic studies and electrochemical analysis after labeling of the -SH group with ferrocene moieties. The specific reactivity of GSH vs ROS was evaluated with scanning electrochemical microscopy (SECM) using the reduction of O(2) to superoxide, O(2)(•-), near the GSH-modified surface. Approach curves show a considerable decrease of the blocking properties of the layer due to reaction of the immobilized GSH with O(2)(•-) and the passage of GSH to the glutathione disulfide (GSSG). The initial surface could be regenerated several times with no significant variations of its antioxidant capacity by simply using the biological system glutathione reductase (GR)/NADPH that reduces GSSG back to GSH. SECM imaging shows also the possibility of writing local and erasable micropatterns on the GSH surface by production of O(2)(•-) at the tip probe electrode.

Cyclic voltammetry, ESR and optical study of the electrochemical reduction of dibenz[b,e]-thiepinon-5,5-dioxides

Abstract Cyclic voltammograms, ESR and optical spectra were recorded during the electrochemical reduction of dibenz[b,e]-thiepin-11-one-5,5-dioxide (I) and dibenz[b,e]-thiepin-11-thione-5,5-dioxide (II). The experimental results provide evidence for the cleavage of the CH 2 SO 2 bond in the median ring, leading to a sulphonic derivative, and not a transposition leading to a stable six-membered central ring, as in the case of dibenz[b,e]-thiepinone derivatives. Molecular orbital calculations in the frame of LCAO—HMO and McLachlan methods account well for the spin distribution and different cleavage pathways for both compounds investigated.

Cyclic voltammetry and spectral study of the electrochemical reduction of some dibenz[b,e]-thiepinone derivatives

Abstract Cyclic voltammetry, ESR and optical spectroscopy experiments performed on some derivatives in the dibenz [b,e]-thiepinone series are presented and discussed. On the basis of these results, a mechanism for the electrochemical reduction of these compounds is suggested which accounts for the cleavage of the seven-membered ring and the appearance of anthrone and/or anthraquinone as transient or final products.

Spectral and interferometrical study of the interaction of haemin with glutathione

The interaction of haemin with reduced glutathione (GSH) was investigated in vitro and the association constant was determined by both spectrophotometric and interferometric methods. In order to elucidate the nature of this interaction, the reaction of haemin with a typical reductive agent (sodium dithionite) and a typical chelating agent for hem-iron (histidine) was also studied. Haemin-induced haemolysis in the presence and absence of GSH was monitored spectrophotometrically. The results outline the role of GSH at physiological concentration in preventing erythrocyte damage by haemin.

Potential-Induced Conformational Changes in an α-CN-terthiophene Thiolate Film on GaAs(110)

Second harmonic generation (SHG) investigations on alpha-CN-terthiophene-thiolate-covered GaAs(110) electrodes in 1 N H2SO4 solution revealed significant changes in the rotational anisotropy of the SH response. The enhancement of the 1- and the 3-fold contributions around -250 mV suggests changes in the symmetry properties of the delocalized electron system due to an alteration of the adsorption geometry induced by the applied potentials. The analysis of the EIS data showed that in the potential region where the SH signal exhibits the more important changes the Mott-Schottky plot undergoes a pronounced shift to more negative potentials as a result of the charging of the surface states grouped about 1.06 eV below the conduction band edge. Semiempirical MO calculations suggest that the most energetically favorable interaction implies electron transfer from the semiconductor conduction band to the lowest unoccupied molecular orbital of the organic molecule with epsilon(LUMO)=-1.707 eV. Such a chemisorption bond bringing the organic molecule to a quasi-planar position is well supported by the major changes in the XPS spectra of the electrochemically biased samples with respect to the as-prepared ones. Two distinct N 1s species instead of one and a shift of 1.6 eV to higher BE of the terthiophene S 2s core level are strong evidence for a potential-induced change in the adsorption geometry. Taking into account that the acceptor-like surface state group located close to the semiconductor valence edge (EC,S=-1.06 eV) may correspond to the LUMO level (shifted downward by the adsorption process), we assume that the organic molecule, initially adsorbed by the thiolate end, undergoes a conformational change from a tilted to an almost flat position when the applied potential brings the semiconductor Fermi level into its neighborhood. This assumption is in a very good agreement with the potential-induced variation in the thiol thickness estimated from the thiol capacitance.

Electrochemical behaviour and redox reactivity of some 4-R-1,2,4-triazolin-3,5-diones

Abstract Cyclic voltammetry with stationary and rotating disk electrodes results in two 4-R-1,2,4-triazolin-3,5-diones (RTAD) and the corresponding urazoles in dimethylsulphoxide and acetonitrile are reported and discussed. The results show a first reversible reduction step, corresponding to fairly stable anion-radicals, characterised by well resolved EPR spectra. From the electrochemical and spectral data, a reduction mechanism for 4-R-triazolindiones is proposed, leading finally to the corresponding urazoles. Anodic oxidation of 4-methyl urazole gives the corresponding dione, accounting for the reversibility of the reduction process and furnishing further experimental support for the proposed mechanism. Possible intermediates in these redox processes indicated by the electrochemical data are the anion-radical ( A − ), the dianion ( A −2 ), the urazolyl radical ( AH ) and the urazolide anion ( AH − ). MO calculations at the semiempirical AM1 level of the electronic structure of these species account reasonably for the differences in the redox reactivity between the compounds investigated.

The cathodic behaviour of poly(alkyl) and (aryl) sulfones revisited. Mechanistic and theoretical approaches

A series of aromatic sulfones activated by 2, 3 or 4 sulfonyl groups were reduced electrochemically in aprotic media. The electrochemical behaviour is strongly directed by the nature and the number of the substituents bound to the aromatic ring. From the analyses of cyclic voltammetry experiments, the kinetics and thermodynamic parameters were evaluated for each system and a theoretical study at the B3LYP/6-31G* level was performed simultaneously to understand the reactivity of the radical anions stemmed from these aromatic sulfones.