Detection of Chloride Using Microelectrodes in Closed Bipolar Electrode Scheme

Abstract

Introduction: We present an electrochemical sensor to monitor Cl- concentration using a chloride specific silver electrode connected to electrodes of different surface areas and materials in a bipolar arrangement (figure 1). Using a bipolar arrangement may pose benefits for an ion selective electrode compared to traditional methods. Two key benefits are 1) the partition of the measurement versus reference compartments, and 2) the ability to apply various overpotentials across the system. These two attributes help eliminate the effect of some interference and may allow us to tune the linear range of the sensor.The sensor function relies on balancing the association of Cl- and Ag+ with the conversion of Prussian blue to Prussian white. The entire system is held at a defined potential using a potentiostat across working electrode 1 and reference electrode 1 (below). As Cl-are added to the sample compartment, Cl-will associate with Ag+ to form Ag/AgCl altering the potential across the bipolar electrode. To ensure the conservation of charge the ratio of Prussian blue to Prussian white will also shift. This change in ratio is measured using open circuit potential against a second working electrode. The open circuit potential response for each of these electrodes is then modeled using the Nernst equation, and mixed potential theory1–3. Methods: Deposition of Prussian blue was carried out using 1mL volume of 2.5mM FeCl3, and 2.5mM ferricyanide, in a supporting electrolyte of 0.1M hydrochloric acid and 0.1M KCl solution. An overpotential of 0.4V versus Ag/AgCl was applied to the working electrode for 3 cycles of 4 minutes while monitoring the current. After deposition, the electrode was vigorously rinsed in MQ water and activated using cyclic voltammetry. This procedure was carried out on both the gold and platinum electrodes.For proof of concept, the chloride sensitive electrode was fabricated by taking a single junction Ag/AgCl electrode and removing the silver wire. This wire was then anodized using a positive potential of 0.4V versus Ag/AgCl was held against a platinum counter electrode in 1M KCl solution. The electrode was stored in 1M KCl until further use.The various potentials applied across the entire cell were chosen by running cyclic voltammetry in 10mM potassium phosphate buffer with pH 7. The oxidation and reduction peaks were then selected, as well as the formal potential, which we calculated by taking the average of the 2 peaks. Results and Discussion: The bipolar reference electrode was tested against various interfering substances such as glucose, Na+, K+, and pH based on the in vivo environment (interstitial fluid ranges). The linear range, and limit of detection for Cl- was found across each electrode type and size. We also investigated the effect of applying different potentials across the cell and compare the limit of detection, and linear range. Based on this we found the reduction potential resulted in best limit of detection. Finally, the impact of the measurement compartment ion concentration was tested by changing the concentration of Cl- in the measurement compartment.