Reference-electrode free pH sensing using impedance spectroscopy

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