Electrochemical modification of a carbon electrode using aromatic diazonium salts. 2. Electrochemistry of 4-nitrophenyl modified glassy carbon electrodes in aqueous media

The electrochemical behavior of a 4-nitrophenyl modified glassy carbon electrode has been investigated in the absence and presence of electroactive species such as ferricyanide, ruthenium hexaamine and hydroquinone following its electrochemical reduction in aqueous acidic media. The blocking properties of the grafted film are significantly altered following the reduction of the 4-nitrophenyl film. The reduction yields nitrosophenyl, hydroxyaminophenyl and aminophenyl moieties. A set of redox waves, attributed to the products of the reduction of the 4-nitrophenyl, is observed at about 0.42 V versus Ag/AgCl and is related to the nitrosophenyl/hydroxyaminophenyl interconversion. XPS measurements at the Nls core level have confirmed a loss of nitro groups upon electrochemical reduction of the grafted film. The decrease of the 406 eV peak associated with nitro groups is larger than the increase of the peak at about 400 eV that would correspond to the nitro group reduction products. This can be attributed to the cleavage of a fraction of the substituted phenyl group from the glassy carbon electrode surface. The XPS data also suggest that only a fraction of the grafted 4-nitrophenyl groups are electrochemically reduced under our experimental conditions. Nonetheless, the barrier properties of the resulting layer are maintained as shown by the suppression of the Ru(NH3)(6)(3+) electrochemistry. (C) 1998 Elsevier Science S.A. All rights reserved.