Covalent modification of carbon electrodes has been developed during the last 30 years with the implementation of different reduction or oxidation electrochemical strategies [1]. These methods have been of interest for many applications due to the physical and chemical stability of the grafted films. Among these methods, the direct and mediated oxidation of carboxylates allowed the attachment of organic groups like saturated [2] and unsaturated alkyl chains [3] and substituted benzylic [4] and phenylalkylic groups. When the grafted film bears moieties electrochemically actives, they can be characterised through their response in cyclic voltammetry experiments, however, the main features of the redox active groups are far from the ideal behaviour for adsorbed species [4]. Therefore, in this work is analysed the effect of the cation size of the supporting electrolyte used during the grafting by mediated oxidation of 4-nitrophenylbutyrate on the ideality of the redox process of the nitrobenzene group. It was found that the reversible reduction process of the adsorbed nitrophenyl groups is closer to an ideal behaviour when the size of the supporting electrolyte cation follows the next order: n-Hx4N+ > n-Bu4N+ > Et4N+ > Me4N+. This order reveals different levels of compactness of the grafted film and explain the behaviour of ferrocene as redox probe, whose apparent reversibility decreases in the same order of cation size.

References
[1] D. Bélanger, J. Pinson, Chem. Soc. Rev., 2011, 40, 3995
[2] P.D. Astudillo, A. Galano, F.J. González, J. Electroanal. Chem., 2007, 610, 137
[3] D.M. Hernández-Morales, F.J. González, Electrochem. Commun., 2014, 46, 48
[4] C.P. Andrieux, F. González, J.M. Savéant, 1997, 119, 4292