Theory and experiment for the substrate generation tip collection mode of the scanning electrochemical microscope: Application as an approach for measuring the diffusion coefficient ratio of a redox couple
Theory and experiment for the substrate generation tip collection mode of the scanning electrochemical microscope: Application as an approach for measuring the diffusion coefficient ratio of a redox couple
复制标题
DOI：
10.1021/ac970681p
复制
发表时间：
1998-01-15
影响因子：
7.4
通讯作者：
Unwin, PR
中科院分区：
文献类型：
--
作者：
Martin, RD;Unwin, PR
建立了扫描电化学显微镜（SECM）的基底产生/尖端收集（SG/TC）模式（采用安培电极）的数值模型，并进行了实验验证。在相关条件下，该模式涉及溶液中的物质R在宏观基底（产生）电极上进行扩散限制的电解以产生O。当O扩散开时，一部分物质被位于基底上方的尖端（收集）超微电极（UME）拦截，在那里发生逆反应，导致R形成并扩散反馈至基底。结果表明，随时间变化的尖端电流响应对于O/R电对的扩散系数之比很敏感。此外，当在这些条件下确定的长时间（稳态）尖端电流与在远离基底的尖端处对R电解的扩散限制电流的测量值相结合时，可以高精度地确定电对的还原态和氧化态的扩散系数差异。在铂产生电极和收集电极上对乙腈溶液中的二茂铁（Fc）/Fc⁺电对进行的测量表明，可以很容易地从瞬态和稳态尖端响应中获得扩散系数之比。通过对乙腈中对苯醌（BQ）单电子还原形成自由基阴离子BQ⁻以及水溶液中亚铁氰化物/铁氰化物电对的稳态测量，进一步证明了该技术的简便性。对于这些体系以及Fc/Fc⁺电对，结果表明，通过在特定的尖端/基底距离下测量SG/TC模式和反馈模式中的稳态尖端电流，可以在不知道两个电极的尺寸以及它们之间的间距的情况下确定介体电对的扩散系数之比。
A numerical model for the substrate generation/tip collection (SG/TC) mode of the scanning electrochemical microscope (SECM), with amperometric electrodes, is developed and examined experimentally. Under the conditions of interest, this mode involves the diffusion-limited electrolysis of a solution species, R, to produce O, at a macroscopic substrate (generator) electrode. As O diffuses away, a fraction of the species is intercepted by a tip (collector) ultramicroelectrode (UME), positioned above the substrate, where the reverse reaction occurs, leading to the formation and diffusional feedback of R to the substrate. The time-dependent tip current response is shown to be sensitive to the ratio of diffusion coefficients of the O/R couple. Moreover, when the long-time (steady-state) tip current, determined under these conditions, is combined with measurements of the diffusion-limited current for the electrolysis of R at a tip positioned far from the substrate, differences in the diffusion coefficients of the reduced and oxidized forms of the couple can be determined with high precision. Measurements on the ferrocene (Fc)/Fc(+) couple in acetonitrile solution at Pt generator and collector electrodes demonstrate that the diffusion coefficient ratio can readily be obtained from both the transient and steady-state tip responses. The simplicity of the technique is further demonstrated through steady-state measurements on the one-electron reduction of p-benzoquinone (BQ) to form the radical anion, BQ(.-), in acetonitrile and the ferrocyanide/ferricyanide couple in aqueous solution. For these systems, and the Fc/Fc(+) couple, it is shown that, by making measurements of the steady-state tip current in both the SG/TC and feedback modes at a particular tip/substrate distance, the ratio of diffusion coefficients of the mediator couple may be determined without any knowledge of the sizes of, or the separation between, the two electrodes.