RESUMO
A nanogap cell involves two working electrodes separated by a nanometer-wide solution to enable unprecedented electrochemical measurements. The powerful nanogap measurements, however, can be seriously interfered with by resistive coupling between the two electrodes to yield erroneous current responses. Herein, we employ the nanogap cell based on double carbon-fiber microelectrodes to suppress resistive coupling for the assessment of intrinsic current responses. Specifically, we modify a commercial bipotentiostat to compensate the Ohmic potential drop shared by the two electrodes through the common current pathway with a fixed resistance in the solution. Resistive coupling through both non-Faradaic and Faradaic processes is suppressed to eliminate erroneous current responses. Our approach is applied to investigate the mechanism of dopamine oxidation at carbon-fiber microelectrodes as important electrochemical sensors for the crucial neurotransmitter. Resistive coupling is suppressed to manifest the intrinsic current responses based on the oxidation of both adsorbed and non-adsorbed forms of dopamine to the respective forms of dopamine-o-quinone. The simultaneous dual oxidation pathways are observed for the first time and can be mediated through either non-concerted or concerted mechanisms of adsorption-coupled electron transfer. The two mechanisms are not discriminated for the two-electron oxidation of dopamine because it can not be determined whether the intermediate, dopamine semi-quinone, is adsorbed on the electrode surface. Significantly, our approach will be useful to manifest intrinsic current responses without resistive coupling for nanogaps and microgaps, which are too narrow to eliminate the common solution resistance by optimizing the position of a reference electrode.
RESUMO
Here, we demonstrate for the first time that the mechanism of adsorption-coupled electron-transfer (ACET) reactions can be identified experimentally. The electron transfer (ET) and specific adsorption of redox-active molecules are coupled in many electrode reactions with practical importance and fundamental interest. ACET reactions are often represented by a concerted mechanism. In reductive adsorption, an oxidant is simultaneously reduced and adsorbed as a reductant on the electrode surface through the ACET step. Alternatively, the non-concerted mechanism mediates outer-sphere reduction and adsorption separately when the reductant adsorption is reversible. In electrocatalysis, reversibly adsorbed reductants are ubiquitous and crucial intermediates. Moreover, electrocatalysis is complicated by the mixed mechanism based on simultaneous ACET and outer-sphere ET steps. In this work, we reveal the non-concerted mechanism for ferrocene derivatives adsorbed at highly oriented pyrolytic graphite as simple models. We enable the transient voltammetric mode of nanoscale scanning electrochemical microscopy (SECM) to kinetically control the adsorption step, which is required for the discrimination of non-concerted, concerted, and mixed mechanisms. Experimental voltammograms are compared with each mechanism by employing finite element simulation. The non-concerted mechanism is supported to indicate that the ACET step is intrinsically slower than its outer-sphere counterpart by at least four orders of magnitude. This finding implies that an ACET step is facilitated thermodynamically but may not be necessarily accelerated or catalyzed by the adsorption of the reductant. SECM-based transient voltammetry will become a powerful tool to resolve and understand electrocatalytic ACET reactions at the elementary level.
