Control of Reactions on Bipolar Electrodes Using Ion-Selective Membranes
Authors: Ab Mutalib Nurul Asyikeen , Tsuyuki Ken , Wu Chia-Chien , Suzuki Hiroaki 
Published: May 2020

Introduction
Bipolar electrochemistry is an attractive technique to realize simultaneous multiplexed detection. In bipolar electrochemical systems, a potential gradient is formed in a solution. When an isolated electrode (bipolar electrode: BPE) that is not in contact with an external device is placed in the potential gradient, potential difference is generated at the interface between the BPE and the solution (Figure 1(A)), which can be a motive force to make anodic and cathodic reactions to proceed on the BPE (Figure 1(B)) [1]. The progress of reduction reactions on the cathodic pole can be measured by coupling the reaction with electrochemiluminescence (ECL) [2]. Although the simple single electrode configuration is attractive in realizing a large array of BPEs, an issue is that the interfacial potential difference on the BPE is not controlled intentionally. In other words, the potential profile in the solution can vary as expressed by 1, 2, and 3, although the potential difference between the two poles is the same for all cases (Figure 1(C)). To solve this problem, we used an ion-selective membrane (ISM) on a part of the BPE. The ISM has an effect to fix the interfacial potential difference there without current generation through the interface (Figure 1(D)).

Method

Arrays of BPEs were formed with platinum (Pt) or gold (Au). An ISM was formed on a part of each BPE. The surface of the BPE was insulated with polyimide layer except for the anodic and cathodic poles and the active area of the ISM. We previously tested the use of thin-film Ag/AgCl instead of the ISM. However, a problem was that current flowed through the interface due to its non-polarized nature, affecting the ECL intensity. With regard to this, current does not flow through the ISM/solution interface. The location of the ISM was changed with BPEs of the same length (Figure 2(A)). Otherwise, BPEs of different lengths were fabricated, and the location of the ISM was kept constant between the ISM and the cathodic pole (Figure 2(B)).
The active areas on the BPEs were cleaned and the cleanliness was checked by taking cyclic voltammograms (CV) in a solution containing 100 mM KCl and 10 mM K3[Fe(CN)6]. To form the ISM, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS) was first coated on an exposed area of the BPE. To form the ISM, 20 µl of Na+ ionophore cocktail containing 89.5% 2-Nitrophenyl octyl ether, 10.0% sodium ionophore II, and 0.5% sodium tetraphenylborate (w/w) (Sigma-Aldrich (Japan)) was dissolved in 79.3% polyvinyl chloride (PVC) and tetrahydrofuran (PVC/THF) solution. The solution was dropped onto the PEDOT/PSS layer to form the ISM and was dried.
Figure 3 shows the setup used for the experiment. Pt wires (diameter: 0.5 mm) were used for the driving electrodes. The distance between the driving electrodes was 30 mm. For ECL, 10 mM PBS (pH 6.9) containing 5.0 mM tripropylamine (TPA) and 1.0 mM [Ru(bpy)3]2+ was used. As an analyte, oxygen dissolved in the solution was used. To change the interfacial potential difference at the ISM, Na+ ions of different concentrations were added to the solution. Before injecting the solution in the reaction chamber, the solution was stirred for at least 30 min to saturate air. Pictures of ECL were taken immediately after the application of the voltage with 60 s exposure. The experiments were conducted at 25°C.

Results and Conclusions

The potential of the ISM was checked using a BPE with an ISM by measuring the potential in NaCl solutions of various concentrations with respect to a commercial liquid junction Ag/AgCl electrode. Figure 4 shows the dependence of the potential of the ISE on the concentration of Na+ ions. The slope was 59.0 mV/decade, which was very close to the value expected from the Nernst equation. The effect of placing the ISM on a part of the BPE was investigated using BPEs of the same length. ECL intensity changed depending on the location of the ISM (Figure 5). Figure 6 shows ECL observed using BPEs of different length maintaining the distance between ISM and the cathodic pole. In both cases, the effect of placing the ISM was clearly observed and the ECL intensity was strongest when the distance between the ISM and the anodic pole is longer. These results indicate that the ECL intensity can actually be controlled by the ISM. We have tested other electrode configurations. The results will also be presented.
The potential control developed in this study is also effective to control the electrode potentials in closed BPE systems in which solutions are separated in different compartments.

References

[1] S. E. Fosdick, K. N. Knust, K. Scida, R. M. Crooks. Bipolar electrochemistry. Angewandte Chemie International Edition, 52 (2013) 10438-10456. doi:10.1002/anie.201300947
[2] K. F. Chow, F. Mavré, R. M. Crooks. Wireless electrochemical DNA microarray sensor. Journal of the American Chemical Society, 130 (2008) 7544-7545. doi:10.121/ja802013q

Figure 1