Effect of Electrolyte Ions on the Formation, Electroactivity, and Rectification Properties of Films Obtained by Electrografting

Abstract

The oxidation of 4‐(4‐nitrophenyl)butyrate ions in acetonitrile was used to modify glassy carbon surfaces with films bearing 3‐(4‐nitrophenyl)propyl groups. The main features of the voltammetric reduction of the grafted nitrophenyl groups were studied in two different supporting electrolytes; n‐Bu4NPF6 and Me4NPF6. The best response was obtained with the less bulky electrolyte, which was explained by the major capability of these ions to diffuse inside the channels of the grafted film, as well as the better stabilization of the radical anions generated during the reduction process. It was also shown that the films electrografted in presence of the bulky n‐Bu4NPF6 are thicker but less compact than those prepared in Me4NPF6. The amount of nitrophenyl groups in the grafted film that can be reduced depends on the size of the electrolyte ions. The permeation of the electrolyte ions inside the film channels determines also the voltammetric behavior of ferrocene as redox probe. A current rectification phenomenon was observed in acetonitrile solutions of bulky and hydrophobic electrolytes like n‐Bu4NPF6 and n‐Hx4NPF6, whose marginal inclusion inside the film channels promotes both the entrance of ferrocene and the expelling of ferrocenium ions from these ones. This phenomenon was emulated through a CEC mechanism in solution, where the diffusion inside the channels was described as single chemical equilibria.

Electrografting of Carbon Surfaces with Aliphatic Chains and its Effect on the Rectification of Ferrocene as Redox Probe in Solution

“…Overall, the supporting electrolyte ions, the redox probe and/or the presence of inert additives influence the porous structure of the electrografted film. In a similar context, some evidence about the role of additives and the size of the supporting electrolyte ions on the film permeability, during the electrografting process itself, have been previously reported [31,41] …”

“…Contrasting with these features, the voltammogram of ferrocene on the electrode modified by mediated oxidation of octanoate shows a well‐defined oxidation peak, but without its corresponding reduction peak. This behavior corresponds to a rectification phenomenon, which can also be explained by changes in the mass transport properties inside the film channels (see next section) [30,31] . In this case, mass transport of ferrocene is favored by a higher size of these channels, which are formed during electrografting because of the major number of conformational freedom degrees of the heptyl chains in the polymeric film structure.…”

“…In agreement with this last property, the size of the redox probe as well as the size of the supporting electrolyte ions are regarded as determining factors of a rectification phenomenon observed during cyclic voltammograms of the redox probe [30] . This rectification was first associated with the behavior of redox probes on grafted electrodes bearing electrochemically active groups, however, it occurs also for films without redox groups, which means that the phenomenon could also be related to the hydrophilic or hydrophobic properties of the grafted film as well as the capability of the redox probes and the supporting electrolyte ions to permeate the channels in the grafted film [30,31] . Therefore, in this work, the surface of glassy carbon electrodes was modified with non‐electrochemically active films that bear saturated aliphatic chains of different length.…”

Mechanistic Aspects on the Electrografting of Carbon Surfaces by Oxidation of Carboxylates Bearing Unsaturated Groups

“…Under these conditions, it was impossible to reproduce the behaviour with variations of the transfer coefficient, which indicates that this asymmetry keeps some analogy with that expected for redox probes on partially blocked electrodes, where the peak to peak separation increases by the presence of thin and porous films [16] . Due to this porosity, in some particular cases, this asymmetry could reach a condition where the cathodic current approach a steady state, which is typical of rectification phenomena [13c,16a,17] …”

Nanofabrication Techniques in Large-Area Molecular Electronic Devices

“…Concerning the direct grafting, Guselnikova et al [324] have reported the surface functionalization of a gold substrate by UV-light grafting of the 3,5-bis (trifluoromethyl)phenyl)iodonium salt. Ramírez-Chan et al [325] recently described the fabrication of electrografted films by oxidation of a nitrophenylbutyrate derivative (NO 2 Ph(CH 2 ) 3 COO − ) exploring the influence of different supporting electrolyte ions on the film formation. Madsen et al [326] used a two-phase bipolar grafting system to simultaneously functionalize gold bipolar electrodes with diazonium salts and primary amines or thiophenes.…”