The EMOSFET as an oxygen sensor: constant current potentiometry

In a previous paper, a novel type of potentiometric dissolved oxygen sensor was introduced. The transduction principle of the sensor is based on the modulation of the work function of an iridium oxide film by the ratio of IrIII/IrIV oxide in the film. This ratio depends on the oxygen concentration in the solution, so that the work function of the iridium oxide is a measure of the amount of dissolved oxygen. The work function changes are determined by using the iridium oxide film as the gate contact of a MOSFET. Because the threshold voltage of a MOSFET depends on the work function of the gate contact, it can be used as a sensor signal. In the present paper, a reducing current of constant magnitude is in addition externally applied to the iridium oxide film, so that an overpotential is generated. The influence of this overpotential on the sensor signal is studied and it is shown that the sensitivity towards oxygen increases compared to the equilibrium potential. Measurement results are shown and compared to the theory. The applied current leads to sensitivities to oxygen and pH that correspond well to the values that may be expected from the literature