Quasi-reference electrodes in confined electrochemical cells can result in in situ production of metallic nanoparticles

Abstract

Nanoscale working electrodes and miniaturized electroanalytical devices are valuable platforms to probe molecular phenomena and perform chemical analyses. However, the inherent close distance of metallic electrodes integrated into a small volume of electrolyte can complicate classical electroanalytical techniques. In this study, we use a scanning nanopipette contact probe as a model miniaturized electrochemical cell to demonstrate measurable side effects of the reaction occurring at a quasi-reference electrode. We provide evidence for in situ generation of nanoparticles in the absence of any electroactive species and we critically analyze the origin, nucleation, dissolution and dynamic behavior of these nanoparticles as they appear at the working electrode. It is crucial to recognize the implications of using quasi-reference electrodes in confined electrochemical cells, in order to accurately interpret the results of nanoscale electrochemical experiments.

Stability and Placement of Ag/AgCl Quasi-Reference Counter Electrodes in Confined Electrochemical Cells

“…1 mV h -1 over a 6.5 hour time period. Contrary to a previous report 25 , WE fouling by soluble redox by-products (e.g., Ag + )…”

“…-0.23 V vs. standard hydrogen electrode, SHE, sufficient to electro-deposit any Ag + by reduction to Ag at the diffusion-limited rate) 31 , before cycling to +0.4 V at 0.1 V s -1 to electro-oxidatively strip Ag + . For comparison, Perera and Rosenstein 25 continually cycled the potential between ±0.4 V vs. Ag/AgCl QRCE at 0.1 V s -1 . The hopping distance (i.e., separation between each pixel) was set to 1 µm to ensure each CV was obtained on a 'fresh' GC surface (i.e., each measurement was independent of the previous).…”

“…Ag/AgCl quasi-reference counter electrode placement. As alluded to above, QRCE placement within confined electrochemical cells is an important consideration due to the possibility of Ag/AgCl contamination at the WE 24,25. Ag + and Clare both electroactive species and are initially present in the vicinity of the QRCE due to the limited solubility of AgCl, as shown above in Eq.…”

“…A simple 'back-of-the-envelope' calculation of diffusion time31 , assuming a distance of 3 cm and a diffusion coefficient of approximately 1.65 × 10 -5 cm 2 s -1 , 40 indicates that Ag/AgCl contamination should only be a problem on the tens of hours timescale. Yet, the experimental set up used herein is analogous to that employed by Perera and Rosenstein25 , who claimed to have observed Ag/AgCl contamination (i.e., in situ metallic nanoparticle formation) after only 40 minutes of operation. CVs obtained at various times under both ambient and O2-free (Ar atmosphere)…”

A Redox Conjugated Polymer-Based All-Solid-State Reference Electrode

“…However, the actual potential of QRE vs. a true reference electrode must be calibrated under the same conditions. QRE is not suitable in experiments in which changes in the composition of the bulk solution can cause concomitant variations in the potential of QRE, such as bulk electrolysis or miniaturized electrochemical sensors [5,6].…”