IEEE SENSORS JOURNAL, VOL. 11, NO. 11, NOVEMBER 2011 2813 Electrode Potential-Based Coupled Coil Sensor for Remote pH Monitoring Sharmistha Bhadra, Greg E. Bridges, Senior Member, IEEE, Douglas J. Thomson, Senior Member, IEEE, and Michael S. Freund (Invited Paper) Abstract—We present a coupled coil pH sensor for high-resolu- tion remote pH monitoring. The sensor is based on a passive LC coil resonator whose resonant frequency is monitored remotely by measuring the change in impedance of an interrogator coil cou- pled to the sensor coil. The sensor resonator consists of an induc- tive coil connected in parallel with a voltage dependent capacitor and a pH combination electrode. When the pH of the contact solu- tion changes, the electrode potential changes the capacitance, and therefore the resonant frequency of the sensor. A linear response with a 0.1 pH resolution is achieved over a 2–12 pH dynamic range at room temperature. The response time of the sensor is demon- strated to be less than 30 s and is limited by the response time of the pH combination electrode. Effects of varying separation dis- tance and temperature change on the sensor’s performance are shown. The described sensor technology has potential application for remote pH monitoring in numerous fields such as biomedical sensing, environmental monitoring, industrial and chemical pro- cessing, and structural health monitoring. Index Terms—Combination electrode, coupled coil, inductive coupling, passive, pH, prototype, resonant frequency, wireless. I. INTRODUCTION T HE development and applications of chemical sensors are growing rapidly. pH is one of the fundamental parameters desired in a chemical sensor. It is important to monitor and con- trol pH in numerous fields such as structural health monitoring, environmental monitoring, industrial and chemical processing, and biomedical sensing [1], [2]. pH sensors have received major attention in biomedical sensing since maintaining proper pH in the blood, digestive tract, tissues and fluids is essential to sup- port optimal health. Numerous efforts have been directed to- wards the development of pH sensors to facilitate continuous monitoring of blood pH for critically ill patients [3]–[5]. Tissue Manuscript received February 01, 2011; revised September 23, 2011; ac- cepted September 26, 2011. Date of publication October 03, 2011; date of cur- rent version October 21, 2011. This research was funded by the Natural Sciences and Engineering Research Council of Canada. This is an expanded paper from the IEEE SENSORS 2010 Conference and was published in its proceedings. The associate editor coordinating the review of this paper and approving it for publication was Dr. Thomas Kenny. S. Bhadra, G. E. Bridges, and D. J. Thomson are with the Department of Electrical and Computer Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6 (e-mail: umbhadra@cc.umanitoba.ca; bridges@ee.umani- toba.ca; thomson@ee.umanitoba.ca). M. S. Freund is with the Department of Chemistry, University of Manitoba, Winnipeg, MB, Canada R3T 5V6 (e-mail: michael_freund@umanitoba.ca). Digital Object Identifier 10.1109/JSEN.2011.2170563 pH can serve as an indicator of anaerobic metabolism and tissue pH sensors have been applied to asses low blood flow states [6]. An important medical application of pH sensors has been found for diagnosis of Gastroesophageal reflux disease, which refers to symptoms or tissue damage caused by the reflux of stomach contents into the esophagus and pharynx [7]–[9]. pH sensors have also been used for monitoring wound healing processes as the pH level of a wound liquid can be related to the healing process [10]. pH sensors have been found very useful in envi- ronmental monitoring applications. pH sensors have been used for monitoring pH of the soil, or more precisely pH of the soil solution, since it is a factor that affects a plant’s absorption of different essential nutrients [Nitrogen (N), Potassium (K), and Phosphorus (P)] for growth and fighting disease [11]. Moni- toring the quality of drinking water is another useful applica- tion of a pH sensor. Water with low pH values ( 6.5) can con- tain elevated levels of toxic metals. This can cause corrosion to metal piping and pose potential health risks. Water with high pH values ( 8.5) typically does not pose a health risk but causes problems like an alkali taste and formation of insoluble precipi- tates on clothing [12]. pH sensors for high acidity and alkalinity are useful in industries such as manufacturing plants, photo- graphic developers, and waste treatment facilities [2]. They have been applied to monitor pH change during beer fermentation [13] and to monitor localized corrosion [14]. In structural health monitoring, the value of pH is a crucial factor for assessing the deterioration of a reinforced concrete structure. When a struc- ture is first built, a passive layer is formed on the surface of re- inforcing steel which protects it from corrosion. In healthy con- crete the pH is around 12.6 and a pH value above 9.5 is required to maintain this passive layer. Carbon dioxide gas in the atmos- phere can be dissolved by the concrete pore solution and react with some calcium compounds to form carbonates. This lowers the pH of the concrete which results in depassivation of the rein- forcement steel and initiation of corrosion. In situ measurement of pH at the reinforcing steel/concrete interface has been used for monitoring the corrosion process [15]. The pH combination electrode is among the most common types of pH sensor, consisting of a sensing electrode and a ref- erence electrode. The sensing electrode provides a potential pro- portional to the pH value of the sample and the reference elec- trode ideally provides a stable and consistent potential indepen- dent of the sample. The combination electrode provides the po- tential difference between the reference and the sensing elec- trode, which is proportional to the pH of the sample. The pH 1530-437X/$26.00 © 2011 IEEE