Tungsten trioxide (WO3) as an actuator electrode material for ISFET-based coulometric sensor-actuator systems

Acid or base concentrations can be determined by performing an acid-base titration with OH− or H+ ions, coulometrically generated by the electrolysis of water at a noble metal actuator electrode. This can be done very rapidly if the actuator electrode is in close proximity to an ISFET which is used as the indicator electrode to detect the equivalence point in the titration curve. In order to restrict the effect of interfering redox reactions at the actuator electrode during coulometric generation, electroactive actuator materials have been studied which can exchange H+ ions at a lower electrode potential than the potential of anodic water electrolysis. In this paper, electrochemically grown tungsten trioxide (WO3) is proposed as an actuator electrode material. At a WO3 electrode, H+ ions can be generated by a redox reaction at approximately 0.1 V versus SCE in a mildly alkaline solution (0.5–7 mM KOH) (anodic water electrolysis at a Pt electrode occurs at 1.5 V versus SCE). The observed thermodynamic and kinetic behaviour of the redox reaction is in good agreement with the theoretical predictions. Disadvantages of WO3 are its slow dissolution in aqueous solutions and the restriction that a titration at a WO3 electrode can only be performed in alkaline solutions.