Organic electrochemical transistors (OECTs), in recent years, have become the devices of choice for fabricating biosensors using semiconducting polymers. Although inorganic materials have long dominated the semiconductor market, organic semiconductors have been found to be much better candidates for interfacing with biological systems due to their high chemical variability, low elastic moduli and ability to perform both electronic and ionic transport. Because ionic species can penetrate the highly porous polymer film leading to large interfacial areas, OECT devices typically exhibit extremely large capacitances and display among the highest transconductance values in published literature. Of specific interest to us here is using OECTs to monitor and quantify ion transport across lipid bilayers and barrier tissues. Such devices would not only be useful as a sensitive diagnostic tool, but also represent a good potential candidate for building lipid-based biosensors.
To this date, the best performing OECTs are fabricated using the highly p-doped, conducting polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Due to its high conductivity and ability to readily uptake water and electrolytes, PEDOT:PSS devices have found their use in several medical applications. Previous studies have shown, for instance, that these devices can detect both the disruption of integral cell layers and lipid membranes as well as specific cation migrations across ion-transporting proteins. Although such experiments have been generally successful and a variety of biological membranes have been characterized, quantification of the impedances across these membranes has not been performed. The exact resistances and capacitances of the cell/lipid layers of interest and how they change upon introduction of toxins, for example, would be of great use for better understanding these physiological processes.
In this letter we present detailed circuit modeling of lipid-functionalized, PEDOT:PSS OECTs. Our results show firstly that both the lipid/cell membranes and the active channel can be simply modeled by two parallel RC circuits. The time and frequency responses of the OECT, which can be experimentally collected using transient current and impedance spectroscopy measurements, yield crucial information about the impedances of both the device and the membranes. By changing the membrane and device areas, it is possible to enhance the device sensitivity to lipid membranes. Finally we show that depending on the specific resistance of the membrane of interest, there is a maximum device size above which detection is not possible. Although we use electrical properties specific to that of spin cast PEDOT:PSS, our modeling results are general to all organic semiconductors and will be of great use to the bioorganic electronics community as we continue to work towards building better biosensors and more effective diagnostic tools.