Scanning electrochemical microscopy: theory and experiment for the positive feedback mode with unequal diffusion coefficients of the redox mediator couple
Scanning electrochemical microscopy: theory and experiment for the positive feedback mode with unequal diffusion coefficients of the redox mediator couple
复制标题
DOI：
10.1016/s0022-0728(97)00377-x
复制
发表时间：
1997-12-10
影响因子：
4.5
通讯作者：
Unwin, PR
中科院分区：
文献类型：
--
作者：
Martin, RD;Unwin, PR
扫描电化学显微镜（SECM）计时电流正反馈模式的理论被扩展到包括氧化还原媒介物对的氧化态和还原态具有任意扩散系数的情况。在典型的正反馈条件下，溶液最初包含一种氧化还原活性物质R以及过量的支持电解质。靠近感兴趣界面放置的尖端超微电极（UME）的电位被调节到一个值，在此值下R被电解以扩散控制的速率产生物质O。O从尖端向界面扩散，在界面处发生逆氧化还原反应，导致R产生并扩散反馈到尖端电极进行电解。当在扩散控制的计时电流条件下进行正反馈测量时，发现在特定的尖端到界面距离下，归一化电流 - 时间行为的形式对O/R对的扩散系数之比敏感。当达到稳态时，归一化电流变得与扩散系数比无关，而仅取决于尖端到界面的距离。在靠近铂基底电极的铂尖端超微电极上对乙腈溶液中二茂铁（Fc）的计时电流氧化进行的实验测量为理论预测提供了支持，并表明扩散系数比可以很容易地从SECM计时电流法中确定。尽管Fc和Fc(+)通常被假定具有相同的扩散系数，但在常规超微电极上的稳态和计时电流测量得出Fc的扩散系数值为2.0×10⁻⁵ cm² s⁻¹，而SECM计时电流法表明Fc(+)的扩散系数为1.6×10⁻⁵ cm² s⁻¹。（C）1997年爱思唯尔科学出版社
Theory for the chronoamperometric positive feedback mode of the scanning electrochemical microscope (SECM) is extended to include the situation where the oxidised and reduced forms of the redox mediator couple have arbitrary diffusion coefficients. Under typical positive feedback conditions, the solution initially contains a redox-active species, R, along with excess supporting electrolyte. The potential of the tip ultramicroelectrode (UME), positioned close to an interface of interest, is adjusted to a value where R is electrolysed to produce species O at a diffusion-controlled rate. O diffuses away from the tip towards the interface, where the reverse redox reaction occurs leading to the production, and diffusional feedback of R for electrolysis at the tip electrode. When positive feedback measurements are carried out under diffusion-controlled chronoamperometric conditions, the form of the normalised current-time behaviour, at a particular tip to interface distance, is found to be sensitive to the ratio of the diffusion coefficients of the O/R couple. As a steady-state is established, the normalised current becomes independent of the diffusion coefficient ratio and depends only on the tip to interface distance. Experimental measurements on the chronoamperometric oxidation of ferrocene (Fc) in acetonitrile solution at a Pt tip UME positioned close to a Pt substrate electrode, provide support for the theoretical predictions and demonstrate that the diffusion coefficient ratio can readily be determined from SECM chronoamperometry. Although Fc and Fc(+) are often assumed to have the same diffusion coefficients, steady-state and chronoamperometric measurements at a conventional UME yield a value of 2.0 X 10(-5) cm(2) s(-1) for the diffusion coefficient of Fc, while SECM chronoamperometry indicates that the diffusion coefficient of Fc(+) is 1.6 X 10(-5) cm(2) s(-1). (C) 1997 Elsevier Science S.A.