A wireless volatile organic absorption pH sensor using a mixed metal oxide electrode

We present a wireless pH sensor which utilizes water as an absorptive medium for gaseous organic acids. The potential of a pH-sensitive commercial mixed metal oxide (MMO) electrode with respect to a pH-insensitive Ag/AgCl electrode influences LC sensor resonant frequency via a varactor diode. The sensitivity of MMO electrode potential to electrolyte pH is verified using a series of pH buffer solutions. The absorption of gaseous acetic acid into deionized water and its influence on electrode potential is also demonstrated. Finally, change in electrode potential is shown to predictably influence LC sensor resonant frequency when the electrodes are embedded in a hydrogel. This sensor has potential application in food spoilage detection where direct electrode contact with food is undesirable.