Controlling the Phase Boundary Potential at the Interface to Ionophore-Doped Sensing Membranes

Traditional electroanalytical sensors with an ionophore-doped polymeric sensing membrane were built with an internal filling solution that separated the sensing membrane from an internal reference electrode. This design suffers from a number of disadvantages, including high membrane resistances and difficulties with sensor miniaturization, and it gave way to so-called solid-contact sensors. Initial efforts to directly interface the polymeric sensing membrane to an electron conductor, such as a metal, suffered from poorly controlled phase boundary potentials. A period followed with numerous efforts to use conducting polymers as transducing layer between the sensing membrane and the electron conductor. However, the reaction of ambient oxygen with conducting polymers leads to slow drifts in the phase boundary potential at the solid contact interface. To this end, nanoporous carbon interfaces with exceptional high surface areas have been explored as an alternative solid contact design and have enabled sensors with some of the most stable long term performance, with potential drifts in the low microvolt per hour range. This presentation will discuss effects of the composition of the sensing membranes on the capacitance of these sensors as well as long-term potential drift upon and after current application. A better understanding of the phenomena thus observed is critical to fabricating ionophore-based solid-contact sensors with the long-term stability needed for wearable and implantable devices as well as calibration-free potentiometry.