Potential Control for Bipolar Electrochemical Systems By Closed Circuits Including Ion-Selective Electrodes
Authors: Deng Yi , Ab Mutalib Nurul Asyikeen , Hsueh An-Ju , Xiong Jian , Suzuki Hiroaki , Tsujimura Seiya 
Published: December 2024

Introduction

Bipolar electrode (BPE) is an independent conductor immersed in the electrolyte and without directly connected to the external electronic circuit. Compared with the traditional three-electrode system, a BPE has unique advantages including a simple electrode structure, flexible sensing design, as well as the wireless driving (1). Therefore, in promoting multiplexed electrochemical detection, bipolar electrochemistery provides an effective solution for integrated design. However, given the absence external electronic circuit connection, precise control of the interfacial potential for a single BPE has been an issue in realizing reliable quantitative sensing (2). To solve this problem, we reported a new strategy for highly sensitive potential control of bipolar systems. An ion-selective electrode (ISE) which maintain a stable potential in ISE/solution interface with different ionic strengths coupled with BPE. An electrochemical closed circuit forms by Ag/AgCl electrode bridged BPE and ISE cells. The potential at the BPE/solution interface was controlled based on the potential of ISE and the position of the closed circuit. This technique enables single BPE on a highly array device to achieve sensitive potential control, which promotes the identification of informative biomarkers in large scale and real-time clinical studies.

Experiment method

Fig. 1 (A) shows the schematic of the closed bipolar system used for illustrating the potential control. The BPEs, ISE, and driving electrodes (Pt) are consolidated on a glass substrate. BPE (Ⅱ) branched out an arm electrode in the middle and modified with Na+ ion-selective membrane (ISM) at the terminal. BPE (Ⅰ) was employed as a control group without potential control unit. Electrochemiluminescence (ECL) was introduced as a powerful biosensing technique for BPE. Adopting the concept of closed BPE, signaling reaction (chemiluminescence) at anode as a reporter for sample reduction at cathode of BPE that can be triggered by electrical potential. These two reactions took place in separate chambers that were only connected through a BPE. In the anodic chamber (red in Fig. 1A), 5 mM tripropylamine (TPA) and 1 mM [Ru(bpy)3]2+ in 100 mM phosphate buffer (PBS) (pH 6.9) was used for ECL reaction. The cathodic chamber was filled with 100 mM PBS (pH 6.9) containing 1 mM KCl (green in Fig. 1A). The ISE chamber (blue in Fig. 1A) was filled with NaCl solution of predetermined concentrations. Given the isolated chambers of BPE, two cases are explored, the ISE chamber and anodic chamber or cathodic chamber are bridged with a silver electrode with AgCl growing at both ends. When a constant voltage is applied into BPE chambers, [Ru(bpy)3]2+/TPA are oxidized on the anodes accompanied by photon emitting. These ECL signals are observed by CCD camera correspond to the reduction of O2 at the cathode due to the current flow through BPE. The different of ECL intensity reveals the situation of potential different at the BPE/solution interface.

Results and discussion

The closed bipolar system was first characterized with O2 reduction in the cathodic chamber, the curve for ECL intensity with driving voltage moved significantly depending on the location of the Ag/AgCl (Fig. 1 (B)). The peak of ECL was observed under larger driving voltage when the Ag/AgCl coupled the cathodic and control chamber. The effect of using the ISE was also examined by changing the concentration of NaCl in the control chamber. ECL intensity of BPE (Ⅱ) was decreased with the concentration of NaCl when Ag/AgCl connected anodic chamber (Fig. 1 (C)), in converse, ECL increased with NaCl when Ag/AgCl connected cathodic chamber. (Fig. 1 (D)). The results can be explained considering that the interfacial potential differences at both the anodic and cathodic poles shift in a more positive direction under the rate-limiting condition for the anodic pole.
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Figure 1