The Effect of Glassy Carbon Surface Oxides in Non-Aqueous Voltammetry: The Case of Quinones in Acetonitrile

Abstract

Glassy carbon (GC) electrodes are well-known to contain oxygenated functional groups such as phenols, carbonyls, and carboxylic acids on their surface. The effects of these groups on voltammetry in aqueous solution are well-studied, but there has been little discussion of their possible effects in nonaqueous solution. In this study, we show that the acidic functional groups, particularly phenols, are likely causes of anomalous features often seen in the voltammetry of quinones in nonaqueous solution. These features, a too small second cyclic voltammetric wave and extra current between the two waves that sometimes appears to be a small, broad third voltammetric wave, have previously been attributed to different types of dimerization. In this work, concentration-dependent voltammetry in acetonitrile rules out dimerization with a series of alkyl-benzoquinones because the anomalous features get larger as the concentration decreases. At low concentrations, solution bimolecular reactions will be relatively less important than reactions with surface groups. Addition of substoichiometric amounts of naphthol at higher quinone concentrations produces almost identical behavior as seen at low quinone concentrations with no added naphthol. This implicates hydrogen bonding and proton transfer from the surface phenolic groups as the cause of the anomalous features in quinone voltammetry at GC electrodes. This conclusion is supported by the perturbation of surface oxide coverage on GC electrodes through different electrode pretreatments.

Effect of Anodic Pretreatment on the Performance of Glassy Carbon Electrode in Acetonitrile and Electrooxidation of Para-substituted Phenols in Acetonitrile on Platinum and Glassy Carbon Electrode

“…The results suggest that during the preanodisation a highly resistive layer forms which is the product of acetonitrile electrooxidation. The surface of glassy carbon is usually covered by phenolic and oxo functional groups and they can bond substrate and product molecules through hydrogen bounds in acetonitrile leading to diminished currents after the first scan which was verified in the investigation of quinones [18]. At higher potentials acetonitrile can be oxidized also on its methyl group and the formed radicals can couple with the surface phenolic hydroxyl groups making less accessible the electrode surface for the substrate molecules.…”

Experimental and Theoretical Reduction Potentials of Some Biologically Active ortho-Carbonyl para-Quinones

“…Several electrochemical studies have been carried out regarding the reduction of quinones, mainly to gain insights about the underlying mechanisms and the effects provoked by changing conditions such as electrolytic medium and electrode surface, among others [ 22 ]. A study on the effects of hydrogen bonds in different Q/HQ systems in non-aqueous media showed that the presence of oxygenated functional groups on the surface of the electrode could affect the interaction between dissolved compounds and the surface, which had a significant impact on the electron transfer process [ 23 ]. The electrochemistry of quinones in the presence of added hydroquinone as a hydrogen bonding proton donor, the effective pH at the electrode surface [ 24 ] as well as the electrochemical effect of CO 2 in dimethylformamide solution, were all studied [ 25 ].…”

Activation of Electron‐Deficient Quinones through Hydrogen‐Bond‐Donor‐Coupled Electron Transfer

“…Am echanism involving as ingle binding step (K 1Q )w ith subsequent ET (E 2 )w as found to best describe the experimental data. [29] As noted above,t hese simulations revealed that bis(amidinium) salt 4 binds QC À as a1 :1 complex, in contrast with HBDs 1-3,w hich form 2:1c omplexes with QC À .B ecause of this change in stoichiometry,t he efficacyo ft he different HBDs in promoting ET to Q was gauged by comparing the value of K Q C À for 4 to that of K 1Q C À K 2Q C À for 1-3.T his analysis reveals that 4 is exceptionally effective at promoting ET and six orders of magnitude more potent than 2 at binding QC À .…”