Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes

Abstract

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.

Understanding the Voltammetry of Bulk CO Electrooxidation in Neutral Media through Combined SECM Measurements

“…SECM experiments were carried out in our home-built SECM setup, which was previously described in detail. 20 All the equipment and cleaning procedures used here are exactly the same as reported in our previous work. The procedures for preparing the platinum ultramicroelectrode and functionalized gold pH sensor are available in the SI .…”

“…A schematic representation of the experiment is shown in Figure 1 a. The blank and CO stripping cyclic voltammetry (CV) of the Pt-UME can be seen in Figure 1 c. After characterization of the Pt-UME, a capacitive approach 20 , 21 is performed in air in order to determine the tip-to-surface distance. Figure 1 d shows a measured approach curve (data points) together with its fit (line).…”

“…Details on the sensor fabrication and data processing can be found in our previous work. 20 The Au-UME is positioned at the same distance from the surface as the Pt-UME (40 ± 3 μm) and the tip voltammetry is recorded while changing the sample potential. The pH measurements in time can be found in Figure S5 .…”

“…Using a functionalized gold pH sensor, we have used SECM to measure the pH in the diffusion layer under the same reaction conditions and approximately the same tip-to-surface distance. 20 The correlation of these results and additional rotating disc electrode (RDE) measurements provide a clear understanding on the nature of the two different CO oxidation peaks previously observed 13 , 14 , 16 − 18 and how they are influenced by the interfacial pH. Our measurements show the nature of the oxygen donor as a function of pH and how in a narrow pH window the diffusion of these species and not (only) the diffusion of CO itself is what limits the current of the oxidation reaction and gives rise to the specific voltammetry.…”

Electrochemically Generated Interfacial pH Change: Application to Signal‐Triggered Molecule Release

“…On the other hand, scanning electrochemical microscopy (SECM), [32,33] which is based on micro-size electrodes, allows in situ pH measurements at the electrode interface, also providing information on pH gradient measured at different distances from an electrode surface. [34][35][36] A related method based on scanning ion conductance microscopy has been used for high resolution spatial analysis of local pH changes. [37] Another electrochemical method based on the use of a rotating ring-disk electrode (RRDE) with well-characterized hydrodynamics [38] has been successfully applied to the analysis of the interfacial pH change during electrochemical reactions.…”