A Platinum Reference Electrode for Ion-Sensitive Field-Effect Transistor

The reference electrode (RE), such as the widely used Ag/AgCl redox-couple electrode, is both hard to maintain and difficult to integrate as a constituent of an on-chip sensing system. Inert metals, such as Pt and Au, have been investigated as a pseudo-RE in miniaturized sensing systems. However, they generally suffer from instabilities arising from variations of ion compositions in the electrolyte. This paper presents a novel microfluidic-controlled Pt-KCl RE (PtRE) system that sets a stable potential independent of ion activities and activity changes. Assisted by extensive simulations to guide the system design, we show experimentally that the microfluidic-controlled PtRE system performs the functions of an excellent RE for ion-sensitive field-effect transistor (ISFET). The PtRE system is then implemented as an RE for an ISFET in pH sensing. Miniaturization of the PtRE system to millimeter-dimensions is shown viable by simulation, which may open the possibility for its on-chip integration.