Reference-electrode free pH sensing using impedance spectroscopy.

We present a reference-electrode free sensor able to measure both pH and conductivity based on impedance spectroscopy. The electrode is made of a layer of indium-tin-oxide (ITO). The impedance of this electrode at low frequencies depends on its double layer capacity, which varies with pH due to modification of oxide groups at the ITO surface. At high frequencies, the impedance is determined by the resistance in the system, which corresponds to the inverse conductivity of the solution. Because no reference electrode is needed for this technique, miniaturization of the pH sensor is simple. We demonstrate a proof-of-principle experiment of the sensor for human plasma pH measurements.