Elucidation of the Mechanism of Redox Grafting of Diazotated Anthraquinone

Abstract

Redox grafting of aryldiazonium salts containing redox units may be used to form exceptionally thick covalently attached conducting films, even in the micrometers range, in a controlled manner on glassy carbon and gold substrates. With the objective to investigate the mechanism of this process in detail, 1-anthraquinone (AQ) redox units were immobilized on these substrates by electroreduction of 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate. Electrochemical quartz crystal microbalance was employed to follow the grafting process during a cyclic voltammetric sweep by recording the frequency change. The redox grafting is shown to have two mass gain regions/phases: an irreversible one due to the addition of AQ units to the substrate/film and a reversible one due to the association of cations from the supporting electrolyte with the AQ radical anions formed during the sweeping process. Scanning electrochemical microscopy was used to study the relationship between the conductivity of the film and the charging level of the AQ redox units in the grafted film. For that purpose, approach curves were recorded at a platinum ultramicroelectrode for AQ-containing films on gold and glassy carbon surfaces using the ferro/ferricyanide redox system as redox probe. It is concluded that the film growth has its origin in electron transfer processes occurring through the layer mediated by the redox moieties embedded in the organic film.

Electrochemical Behaviour of HOPG and CVD‐Grown Graphene Electrodes Modified with Thick Anthraquinone Films by Diazonium Reduction

“…As seen from Figure 3 A, the broad peak at 0.2 V vs. SCE, assigned to the reduction of AQD, is disappearing while the current density of the redox wave at E = À0.96 V vs. SCE pertaining to the one-electron reduction of GC/AQ to GC/AQC À is increasing during potential cycling. The gradual increase of the latter is due to the continuous attachment of AQ groups, occurring in a mediated reduction of AQD [48][49][50]64]. In general, this electrochemical behaviour is in good accordance with previous results [48].…”

“…Figure 3 B shows for the HOPG/AQ2 case that potential cycling induces a gradual diminish of the diazonium reduction wave along with a concomitant increase of the first redox wave of the AQ moiety at HOPG. This phenomenon is attributed to the continuous attachment of AQ groups by the redox catalysis effect [48][49][50] in accordance with the results obtained for GC (Figure 3 A).…”

“…À1 to obtain GC/AQ [48,50]. As seen from Figure 3 A, the broad peak at 0.2 V vs. SCE, assigned to the reduction of AQD, is disappearing while the current density of the redox wave at E = À0.96 V vs. SCE pertaining to the one-electron reduction of GC/AQ to GC/AQC À is increasing during potential cycling.…”

“…Recently, a new method called redox grafting was proposed for formation of thick covalently attached conducting organic layers on different substrates (GC, gold, and stainless steel) via electrochemical reduction of various aryldiazonium salts (e.g., 9,10-dioxo-9,10-dihydroanthracene-1-diazonium salt) [48][49][50]. A prerequisite for redox grafting is that the aryldiazonium salt used should contain a redox active functionality.…”

“…Furthermore, employment of high sweep rate in cyclic voltammetry is pertinent to avoid clogging of the physisorbed species in the electrolyte channels in the film that would, otherwise, easily prevent the charge propagation in the growing film. Finally, the switching potential should be carefully selected to encompass the reduction of the redox moiety in the growing film during grafting to exploit the repulsion effect to get rid of noncovalently attached material [48][49][50].…”