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

Bipolar electrochemistry is promising in realizing simultaneous multiplexed detection. In open bipolar 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, interfacial potential difference develops between the BPE and the solution (Figure 1A). As a result, oxidation and reduction reactions occur simultaneously on both poles (Figure 1B) [1]. The progress of the reduction reaction on the cathodic pole can be measured by coupling the reaction with a reaction that generates electrochemiluminescence (ECL) [2]. However, a problem with this method is that the interfacial potential difference on the BPE is not precisely controlled. In other words, the potential profile with respect to the electrode potential changes (Figure 1C). 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 at the location without current generation through the interface (Figure 1D).
Arrays of BPEs were formed with gold and Na+ ISM was formed on a part of each BPE. The surface of the BPE was insulated with a polyimide layer except for the anodic and cathodic poles and the active area of the ISM. To form the ISM, a layer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS) was first coated on an exposed area of the BPE. The Na+ ISM was formed on the PEDOT/PSS layer. The solution to form the ISM contained 43.97 wt% polyvinyl chloride (PVC) dissolved in 1 mL tetrahydrofuran (THF). 53.38 wt% of nitrophenyl octyl ether (NPOE), 2.04 wt% of sodium ionophore X, and 0.61 wt% (w/w) of sodium tetrakis(4-fluorophenyl)borate (NaTFPB) were added to the PVC solution and mixed well. After forming the ISM, its peripheries were fixed with a silicone adhesive.
In the experimental setup shown in Figure 1A, platinum wires (diameter: 0.5 mm) were used for the driving electrodes. The distance between the driving electrodes was 30 mm. To cause the ECL reaction, 10 mM phosphate buffer solution (pH 6.9) containing 5.0 mM tripropylamine (TPA) and 1.0 mM [Ru(bpy)3]2+ was used. Oxygen dissolved in the solution was used as an analyte. The experiments were conducted at 25°C.
To control the potential of the BPEs, we previously used Ag/AgCl for the potential control. However, a problem was that current flowed through the non-polarizable metal/solution interface, affecting the ECL intensity. With regard to this, the ISM is superior because current does not flow. The effect of placing the ISM on a part of the BPE was first examined using BPEs of the same length and changing the location of the ISM (Figure 2). The ion selective electrode (ISE) was effective and the ECL intensity changed by changing the location of the ISE. The effect of the ISM was also demonstrated using folded double BPE structures with the ISM formed at either or both ends. Brighter ECL was observed with the folded structure compared with a single BPE. From these experiments, the effect of placing the ISM on BPEs was clearly observed. As an application to biosensing, we also tested the detection of glucose by immobilizing glucose oxidase and osmium polymer on the cathodic pole. The ECL was also observed accompanying the production of hydrogen peroxide by glucose oxidase and following the reduction of hydrogen peroxide.
The effect of placing the ISM was also examined in closed bipolar systems. In the closed bipolar systems, solutions in the cathode and anode side are separated in different compartments. In this study, 15 mm-long electrodes were used and the electrodes extended in the two compartments. The anodic pole was exposed to the solution for ECL, whereas the cathodic pole was immersed in a Na2SO4 solution. A voltage (2.4 V) was applied between the driving electrodes immersed at the end of the solutions. The brightest ECL was observed on the BPE with the ISM nearest to the cathodic pole. The result indicates that the ISM is also effective in controlling the potential for closed bipolar systems.

[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