Monitoring acidic and basic volatile concentration using a pH-electrode based wireless passive sensor

Abstract

We present a near-field coupled passive sensor capable of measuring acidic and basic volatile concentration in its surrounding environment. The sensor employs an LC resonant circuit consisting of a spiral inductor connected in parallel with a voltage dependent capacitor (varactor). The volatile detector comprises a hydrogel coated pH-sensitive electrode pair connected in parallel with the varactor. The hydrogel is used as an absorptive medium for the volatiles and acts to contain the electrolyte. On absorption of the acidic or basic volatiles the hydrogel pH changes, which in turn changes the voltage across the pH-sensitive electrode pair shifting the resonant frequency of the sensor. An interrogator coil is inductively coupled to the sensor inductor and remotely tracks the resonant frequency of the sensor. The ability to sense and quantify different concentration levels of ammonia and acetic acid vapor is demonstrated. The sensor has a linear response to the logarithm of the volatile concentration and a detection limit of 1.5ppm and 2ppm for ammonia, and acetic acid vapor, respectively. Effects of varying relative humidity, temperature and hydrogel coating thickness on the sensor's performance are presented. Exhibiting a response time of less than 20min and irreversible operation in a closed environment, the sensor has potential in packaged food condition monitoring.