Nitrophenyl Groups in Diazonium-Generated Multilayered Films: Which are Electrochemically Responsive?

Abstract

Various nitrophenyl-containing organic layers have been electrografted to glassy carbon surfaces using diazonium chemistry to elucidate the extent by which the layer structure influences the solvent (i.e., acetonitrile) accessibility, electroactivity, and chemical reactivity of the films. For most of these films, cyclic voltammetric and impedance spectroscopy measurements show that the electron-transfer process at the electrode is facile and independent of film thickness and structure. This is consistent with the occurrence of self-mediated electron transfers throughout the film with nitrophenyl groups serving as redox stations. Importantly, this behavior is seen only after the first potential sweep, the effect of which is to increase the porosity of the layer by inducing an irreversible desorption of nonchemisorbed material along with a reorganization of the film structure. From a kinetic point of view, the radical anions of surface-attached nitrophenyl groups are reactive toward the residual water present in acetonitrile. Thin layers (thickness of 1 to 2 nm) containing redox-active groups only in the outer part of the layer are protonated two to three times as fast as groups located in a more hydrophobic but still solvent-accessible inner layer. Hence, kinetic measurements can detect small differences in the layer environment. Finally, a deconvolution of the cyclic voltammetric response of an electrode grafted from 4-nitrobenzenediazonium discloses that roughly 25% of the overall signal can be attributed to the presence of 4-azonitrophenyl moieties introduced during the electrografting process.

Kinetic Study of Redox Probes on Glassy Carbon Electrode Functionalized by 4-nitrobenzene Diazonium

“…Only another electrolysis at potential as cathodic as -0.8 V vs. SCE allows a complete reduction of all electroactive groups into NH2. Furthermore, we have also demonstrated that during electrolysis at -0.1 V the whole reaction rate is actually limited by the charge transfer process and that consequently there is no limitation from a lack of solvation, contrary to what was suggested by Ceccato et al [32].…”

Reduction of Nitrophenyl Films in Aqueous Solutions: How Many Electrons?

“…Repeat cyclic voltammograms (CVs) of NP layers in acetonitrile solution typically show a continuous decrease in peak areas, pointing to decomposition of the modifier. It has been pointed out that, on the first CV scan, the reduction peak area usually considerably exceeds that for oxidation, an observation attributed to protonation and further reduction of the NP groups . This must lead to fewer NP radical anions to be re‐oxidized on the return scan and, hence under these conditions, determination of the surface concentrations of NP groups based on the reduction peak of the first scan will give an overestimation, whereas use of the oxidation peak (or peaks in subsequent repeat CVs) an underestimation.…”

“…Although the ratios of the charges of peak 1 to peaks 2, 3, and 4 in basic conditions are at least twice that expected for a total four‐electron reduction of NP to hydroxyaminophenyl groups, the decrease of the ratio with film thickness suggests that, for sufficiently thick films, the reduction may stop at four electrons. However, it is well‐documented that as film thickness increases, the electroactivity of the film can be compromised such that some NP groups remain redox inactive . Hence, estimating surface coverages from CVs under these conditions may give significant underestimations.…”

New insight into 4-nitrobenzene diazonium reduction process: Evidence for a grafting step distinct from NO2 electrochemical reactivity

“…On the other hand, NBD was reported to modify the electrical properties of graphene field-effect transistors [23] or to help at covalent immobilization of organometallic model of enzyme onto carbon surface [24]. All these applications need the electrochemical reactivity of NBD to be clearly defined and this is the reason why it has been extensively investigated in both organic and aqueous media [25][26][27][28][29][30]. It is nowadays well established that the reduction of the nitro moiety in protic medium leads to the formation of hydroxylamine (NHOH) group [31] which belongs to the reversible nitroso/hydroxylamine (NO/NHOH) redox couple.…”