The electrochemical oxidation of catechol and dopamine on platinum in 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 2mim][NTf 2]) and 1-Butyl-3-methylimidazolium tetrafluoroborate ([C 4mim][BF 4]): Adsorption effects in ionic liquid voltammetry

Abstract

The electrochemical oxidation of catechol and dopamine has been studied at a platinum micro-electrode (10 μ m diameter) in two room temperature ionic liquids (RTILs): 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 2mim][NTf 2]) and 1-Butyl-3-methylimidazolium tetrafluoroborate ([C 4mim][BF 4]). For catechol in [C 2mim][NTf 2], an electrochemically quasi-reversible oxidation peak was observed at 1.1 V vs. Pt with a back peak at 0.4 V vs. Pt. This is assigned to the two-electron oxidation of catechol to doubly protonated o-benzoquinone. Double-step chronoamperometry gave a diffusion coefficient for the catechol and the oxidised species which is 3.8 × 10 - 11 m 2 s - 1 for both. For catechol in [C 4mim][BF 4], a two-electron oxidation wave was observed at 1.0 V vs. Pt with no back peak. Another peak at less positive potential was also observed at 0.6 V vs. Pt in [C 4mim][BF 4] but not in [C 2mim][NTf 2] which is assigned to the adsorption of electrochemically formed neutral o-benzoquinone on the platinum electrode. The oxidised protonated o-benzoquinone is suggested to be deprotonated by the [ BF 4 ] - anion, but not by the [ NTf 2 ] - anion: hence adsorption of the neutral species at the platinum electrode, not the charged species. For dopamine in both RTILs, two chemically irreversible oxidation peaks were observed at 0.75 V and 1.1 V vs. Pt, and assigned to the oxidation of dopamine to the corresponding semi-quinone and the quinone. Potential-step chronoamperometry was carried out on the oxidation waves of dopamine in [C 2mim][NTf 2] and the diffusion coefficient of species in solution was calculated to be 6.85 × 10 - 12 m 2 s - 1 and confirmed that the waves corresponded to one and two electron processes. A third wave was observed at 1.8 V vs. Pt which is attributed to the oxidation of the amine group to a radical cation with likely subsequent follow up chemistry. In [C 4mim][BF 4] a peak at less positive potential was observed for dopamine, similar to catechol which is assigned to the adsorption of the neutral quinone species on the platinum electrode formed by the reaction of the removal of protons from the oxidised dopamine with the [ BF 4 ] - anion.