An inductively coupled passive tag for remote basic volatile sensing

We present a passive tag capable of detecting basic volatiles in the surrounding environment. The tag is based on a voltage dependant frequency shift approach using a LC resonator comprised of a spiral inductor in parallel with a varactor. The volatile detector, comprised of a pair of pH-sensitive electrodes coated with a very thin layer of hydrogel, is connected in parallel with the varactor. The hydrogel is utilized as an absorptive medium for the basic volatiles and acts to contain the electrolyte. Due to the absorption of the basic volatile, the hydrogel pH changes which in turn changes the voltage across the pH-sensitive electrode pair shifting the resonant frequency of the tag. An interrogator coil is inductively coupled to the tag inductor to remotely track the resonant frequency of tag at distances ranging from 5 to 11 cm. Tests with ammonia show that the tag has a detection limit of 1.5 ppm. With less than a 20 min response time, the tag has potential for application in food freshness monitoring where detecting basic volatiles is important.