Wireless passive sensor for pH monitoring inside a small bioreactor

A wireless passive pH sensor for continuous remote bioprocess monitoring is presented. The sensor is small enough to fit inside a small bioreactor or test tube. It consists of a planar spiral inductor connected in parallel to a varactor forming a LC resonant circuit. A pH combination electrode made of an iridium/iridium oxide sensing electrode and a silver/silver chloride reference electrode, is connected in parallel to the varactor. As the medium pH changes, the voltage across the electrodes varies, shifting the resonant frequency of the sensor. For in-fluid monitoring the sensor is hermetically sealed to encapsulate, and reduce parasitic capacitive coupling and eddy current loss. The resonant frequency of the sensor is tracked remotely by an interrogator inductor inductively coupled to the sensor. The sterilizable sensor was used for remote pH monitoring of Yarrowia lipolytica fermentation in a shake flask over 67 hours. Experiment shows that the medium pH can be monitored with 2.46 MHz/pH sensitivity and maximum deviation of 0.07 pH from a commercial pH probe measurement over a 6.55.26 pH range.