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Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes.
Analytical Chemistry ( IF 6.7 ) Pub Date : 2020-01-08 , DOI: 10.1021/acs.analchem.9b04952 Mariana C O Monteiro 1 , Leon Jacobse 2 , Thomas Touzalin 1 , Marc T M Koper 1
Analytical Chemistry ( IF 6.7 ) Pub Date : 2020-01-08 , DOI: 10.1021/acs.analchem.9b04952 Mariana C O Monteiro 1 , Leon Jacobse 2 , Thomas Touzalin 1 , Marc T M Koper 1
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Probing pH gradients during electrochemical reactions is important to better understand reaction mechanisms and to separate the influence of pH and pH gradients from intrinsic electrolyte effects. Here, we develop a pH sensor to measure pH changes in the diffusion layer during hydrogen evolution. The probe was synthesized by functionalizing a gold ultramicroelectrode with a self-assembled monolayer of 4-nitrothiophenol (4-NTP) and further converting it to form a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. The pH sensing is realized by recording the tip cyclic voltammetry and monitoring the Nernstian shift of the midpeak potential. We employ a capacitive approach technique in our home-built Scanning Electrochemical Microscope (SECM) setup in which an AC potential is applied to the sample and the capacitive current generated at the tip is recorded as a function of distance. This method allows for an approach of the tip to the electrode that is electrolyte-free and consequently also mediator-free. Hydrogen evolution on gold in a neutral electrolyte was studied as a model system. The pH was measured with the probe at a constant distance from the electrode (ca. 75 μm), while the electrode potential was varied in time. In the nonbuffered electrolyte used (0.1 M Li2SO4), even at relatively low current densities, a pH difference of three units is measured between the location of the probe and the bulk electrolyte. The time scale of the diffusion layer transient is captured, due to the high time resolution that can be achieved with this probe. The sensor has high sensitivity, measuring differences of more than 8 pH units with a resolution better than 0.1 pH unit.
中文翻译：
无介体SECM，用于使用功能化金超微电极探测扩散层的pH。
在电化学反应过程中探查pH梯度对于更好地理解反应机理，以及将pH和pH梯度的影响与固有电解质效应区分开是很重要的。在这里，我们开发了一种pH传感器，用于测量析氢过程中扩散层中的pH变化。通过用自组装的4-硝基硫代苯酚（4-NTP）单层功能化金超微电极，然后将其进一步转化为羟基氨基硫代苯酚（4-HATP）/ 4-亚硝基硫代苯酚（4-NSTP）氧化还原对，来合成探针。通过记录尖端循环伏安法并监测中峰电势的能斯特位移来实现pH感测。我们在自制的扫描电化学显微镜（SECM）设置中采用了电容方法，该方法将交流电施加到样品上，并记录尖端产生的电容电流作为距离的函数。该方法允许末端接近电极，该末端不含电解质，因此也不含介体。作为模型系统，研究了中性电解液中金在氢气上的析氢行为。用探针在距电极一定距离（约75μm）的条件下测量pH值，同时电极电位随时间变化。在所用的非缓冲电解质（0.1 M Li2SO4）中，即使在相对较低的电流密度下，在探头位置和大块电解质之间也可以测量到三个单位的pH差。捕获扩散层瞬态的时间尺度，由于此探头可以实现高时间分辨率。该传感器具有很高的灵敏度，可测量8个以上pH单位之间的差异，分辨率优于0.1个pH单位。
更新日期：2020-01-09
中文翻译：
无介体SECM，用于使用功能化金超微电极探测扩散层的pH。
在电化学反应过程中探查pH梯度对于更好地理解反应机理，以及将pH和pH梯度的影响与固有电解质效应区分开是很重要的。在这里，我们开发了一种pH传感器，用于测量析氢过程中扩散层中的pH变化。通过用自组装的4-硝基硫代苯酚（4-NTP）单层功能化金超微电极，然后将其进一步转化为羟基氨基硫代苯酚（4-HATP）/ 4-亚硝基硫代苯酚（4-NSTP）氧化还原对，来合成探针。通过记录尖端循环伏安法并监测中峰电势的能斯特位移来实现pH感测。我们在自制的扫描电化学显微镜（SECM）设置中采用了电容方法，该方法将交流电施加到样品上，并记录尖端产生的电容电流作为距离的函数。该方法允许末端接近电极，该末端不含电解质，因此也不含介体。作为模型系统，研究了中性电解液中金在氢气上的析氢行为。用探针在距电极一定距离（约75μm）的条件下测量pH值，同时电极电位随时间变化。在所用的非缓冲电解质（0.1 M Li2SO4）中，即使在相对较低的电流密度下，在探头位置和大块电解质之间也可以测量到三个单位的pH差。捕获扩散层瞬态的时间尺度，由于此探头可以实现高时间分辨率。该传感器具有很高的灵敏度，可测量8个以上pH单位之间的差异，分辨率优于0.1个pH单位。
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