A wireless LC chemical sensor based on a high quality factor EIS capacitor

This paper reports on the concept, design, fabrication and experimental results of a new passive wireless sensor for pH measurements. The sensor is based on a LC resonator formed by an inductor and an electrolyte–insulator–silicon (EIS) capacitor working as a pH-to-capacitance transductor. A reference electrode is used to obtain a stable dc potential at the electrolyte. An additional comb-like electrode located on the surface of the insulator is also provided to obtain a low ac impedance. Using this double-electrode scheme a Q larger than 15 is achieved for a wide range of conductivities. Wireless measurements of the LC sensor yields a sensitivity of 1.0% frequency change per pH unit.