Department of Chemistry, University of Pittsburgh , 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States.
Anal Chem. 2016 Aug 16;88(16):8323-31. doi: 10.1021/acs.analchem.6b02273. Epub 2016 Aug 1.
Formation of a nanometer-wide gap between tip and substrate electrodes by scanning electrochemical microscopy (SECM) enables voltammetric measurement of ultrafast electron-transfer kinetics. Herein, we demonstrate the advantage of SECM-based nanogap voltammetry to assess the cleanness of the substrate surface in solution by confirming that airborne contamination of highly oriented pyrolytic graphite (HOPG) causes the nonideal asymmetry of paired nanogap voltammograms of (ferrocenylmethyl)trimethylammonium (Fc(+)). We hypothesize that the amperometric response of a 1 μm-diameter Pt tip is less enhanced in the feedback mode, where more hydrophilic Fc(2+) is generated from Fc(+) at the tip and reduced voltammetrically at the HOPG surface covered with airborne hydrophobic contaminants. The tip current is more enhanced in the substrate generation/tip collection mode, where less charged Fc(+) is oxidized at the contaminated HOPG surface. In fact, symmetric pairs of nanogap voltammograms are obtained with the cleaner HOPG surface that is exfoliated in humidified air and covered with a nanometer-thick water adlayer to suppress airborne contamination. This result disproves a misconception that the asymmetry of paired nanogap voltammograms is due to electron exchange mediated by Fc(2+) adsorbed on the glass sheath of the tip. Moreover, weak Fc(+) adsorption on the HOPG surface causes only the small hysteresis of each voltammogram upon forward and reverse sweeps of the HOPG potential. Significantly, no Fc(2+) adsorption on the HOPG surface ensures that the simple outer-sphere pathway mediates ultrafast electron transfer of the Fc(2+/+) couple with standard rate constants of ≥12 cm/s as estimated from symmetric pairs of reversible nanogap voltammograms.
通过扫描电化学显微镜（SECM）在尖端和基底电极之间形成纳米级间隙，可实现超快电子转移动力学的伏安测量。在此，我们通过证实空气中的高定向热解石墨（HOPG）污染会导致（二茂铁甲基）三甲基铵（Fc(+））的成对纳米间隙伏安曲线出现不理想的非对称性，证明了基于 SECM 的纳米间隙伏安法在溶液中评估基底表面清洁度的优势。我们假设，在反馈模式下，1 μm 直径的 Pt 尖端的电流响应增强较小，因为在尖端处从 Fc(+)生成更亲水的 Fc(2+)，并且在覆盖有空气中疏水性污染物的 HOPG 表面上还原伏安法还原。在基底生成/尖端收集模式下，在污染的 HOPG 表面上氧化的电荷量较小的 Fc(+)会使尖端电流增强。实际上，与在加湿空气中剥离并用纳米级厚的水吸附层覆盖以抑制空气中污染的更清洁的 HOPG 表面获得了对称的纳米间隙伏安对。该结果否定了一种误解，即成对纳米间隙伏安曲线的不对称性是由于吸附在尖端玻璃护套上的 Fc(2+)介导的电子交换所致。此外，Fc(+)在 HOPG 表面上的弱吸附仅导致 HOPG 电势正向和反向扫掠时每个伏安图的小滞后。重要的是，HOPG 表面上没有 Fc(2+)吸附可确保简单的外层途径介导 Fc(2+/+)对的超快电子转移，其标准速率常数≥12 cm/s，这是从对称的可逆纳米间隙伏安对估计得出的。
