A. Szucs

Fullerene film electrodes in aqueous solutions Part 1. Preparation and electrochemical characterization

Solution cast films of C60 on gold and glassy carbon substrates were studied in aqueous solutions as possible electrode materials. These films could be reduced in 1 M NaOH solutions to became conductive, possibly due to the formation of NaxC60. The reduced films were quite stable even on exposure to air, and their conductivity decreased only after a long time or when they were oxidized electrochemically. Both the reduction and the oxidation were completely irreversible. If oxidation is avoided, i.e. the potential is kept below +600 mV/SHE, these films can be used as electrodes, displaying individual properties as electrocatalysts; some electrochemical reactions were faster and others were slower than on bare gold or glassy carbon electrodes.

Fullerene film electrodes in aqueous solutions part 2. Effects of doping cations

Abstract Solution cast C 60 films were studied in aqueous solutions containing a variety of doping cations. Li + , Na + , K + , Rb + , Cs + , Ca 2+ , and Ba 2+ were used as dopants and it was established that the reduction of the C 60 films shifted in parallel with the hydration energy of these cations. The reduction peak potentials also changed with the concentration of the cations, and this was assumed to be the consequence of the formation of reduced films which acted as cation exchange membranes. The cation exchange character was also detected after the reduction, but only with smaller ions (Li + , Na + ), suggesting that, unlike the larger ions, they remain mobile in the films. The reduced C 60 films had quite different electrochemical activity and stability depending on the dopants. The most active and stable were those formed with K + and Rb + dopants. Some of the films (those doped with Li + , Na + and Ba 2+ ) could be oxidized electrochemically, while the others were probably oxidized only on their surfaces. In these cases an increased compactness and the formation of some protecting layers were assumed.

Electrochemical reactions of cytochrome c on electrodes modified by fullerene films

Abstract C60 fullerene film modified electrodes were investigated for the electrochemical reactions of cytochrome c. It was established, that depending on the oxidation state of the fullerene films, the electrochemistry of the protein varied. The best, stablest redox reaction could be measured on partially reduced film. The response of cytochrome c was quite independent of the underlying substrate (gold or glassy carbon), indicating that the fullerene film acted as a promoter. Molecular sieve character and the possible effect of negative charges or polar groups on the surface of the films were suggested to be the reason of enhanced stability of the electrochemical response compared to bare electrodes.

Stable and reversible electrochemistry of cytochrome c on bare electrodes Part 1. Effect of ionic strength

Abstract Stable and reversible electrochemical responses of cytochrome c can be measured on bare unmodified gold and glassy carbon electrodes. It has been found that in phosphate buffers the electrochemical behaviour varies from completely irreversible to reversible on both electrodes depending on the concentration of the supporting electrolyte. The optimum phosphate concentration was found to be between 20 and 30 mM. It was also observed that the electrochemical response improved with the lifetime of the protein samples and could be destroyed by mechanical aggregation of cytochrome c molecules. The importance of oligomeric forms of cytochrome c on the electrode surfaces was emphasized; they determine the overall electrochemical behaviour.

Ellipsometry of cytochrome c on gold surfaces: effect of 4,4′-dipyridyl disulfide

Abstract Ellipsometry has been used to study the mode of adsorption of horse heart cytochrome c on gold in the presence and absence of the promoter 4,4′-dipyridyl disulfide. In the absence of 4,4′-dipyridyl disulfide, cytochrome c unfolds on the surface and forms an irreversibly adsorbed layer that completely covers the electrode. Adsorbed cytochrome c can mediate the reduction of cytochrome c in solution via electron transfer through the unfolded protein layer; the oxidation of cytochrome c in solution was not observed. On gold modified with 4,4′-dipyridyl disulfide, cytochrome c adsorbs irreversibly but retains its native configuration. Ellipsometry and charge transfer measurements show that the layer of adsorbed cytochrome c completely covers the gold surface in the presence of the promoter. Both the reduction and oxidation of cytochrome c in solution occurs through the adsorbed protein layer on the modified surface. These results show that 4,4′-dipyridyl disulfide plays a key role in preventing the unfolding of adsorbed cytochrome c at the electrode solution interface.

Fullerene film electrodes in aqueous solutions

Abstract Solution-cast C60 films on gold and glassy carbon substrates can be reduced electrochemically in aqueous solutions producing an electron-conducting cation exchanger material. These films remain active even on exposure to air; their activity decreases measurably only after several hours due to some partial oxidation by oxygen. However, they can be converted irreversibly to an insulator form by electrochemical oxidation starting from about 600 mV versus SHE. Below this potential limit the reduced C60 films can be used as electrode materials, and their specific behaviour is demonstrated with some test compounds.

Electrochemistry on partially reduced fullerene films

Abstract Electrochemical studies on solution cast fullerene (C60, C70) films were carried out in aqueous solutions. It was observed that a special pretreatment of films in quinhydrone solution could provide suitable conditions to investigate the electrochemical behavior of fullerene films exclusively. It was found that fullerene films can conduct electrons to the solution phase in their partially reduced forms. They behave as diodes with pronounced current rectifying character. Results on these partially reduced layers also support conclusions on the bulk reduction of the fullerene films in aqueous media.

Stable and reversible electrochemistry of cytochrome c on bare electrodes Part II: Effects of experimental conditions

Abstract The electrochemical behaviour of cytochrome c was studied on gold and glassy carbon electrodes as a function of pH, the type of anions present in the electroyltes and temperature. It was established that the stability and the reversibility of electron transfer processes are determined by the adsorption of the protein on the electrode surface, which can be greatly influenced by factors having an effect on the double layer around the molecules. Stable electrochemistry can be observed if the conditions are suitable for the native, monomeric form of cytochrome c to be preserved on the surfaces.

Electrochemical reactions of glucose oxidase at graphite electrodes

Abstract Differential pulse voltammetry has been used to show that a number of direct electron transfer reactions can occur between glucose oxidase and graphite electrodes. Two of these reactions are due to strongly adsorbed species, indicating that some of the enzymes have undergone extensive and slow unfolding during the interaction with the electrode, leading to the adsorption of the FAD directly onto the graphite surface. The electron transfer reactions due to these strongly adsorbed molecules were not affected by the addition of the enzyme substrate. In contrast, reduction peaks can be observed that are due to molecules either from the solution or from weakly adsorbed molecules. At low concentrations of enzyme in solution (

Separation of faradaic and capacitive current regions in the redox transformation of poly(3-methylthiophene) with the exclusion of overoxidation processes

Abstract The electrostability of the poly(3-methylthiophene) film prepared in nitrobenzene provided the opportunity to study its redox transformations in an extended anodic potential region with the exclusion of the overoxidation. From spectroelectrochemical results a capacitive region could be separated in the highly oxidized state of the layer which exhibited a large capacitance. This pattern—assumed to be related to charge separation/redistribution within the film at high potential field connected to solvent movements—is promising from the point of view of symmetric p-type/n-type superconductors.