Potentiometric Adsorption Isotherm Analysis of Protein Sensing Compering Two EGOT Architectures

Electrolyte Gated Organic Transistors (EGOTs) are rapidly emerging as one of the architectures of choice for label-free biosensing, characterized by their high sensitivity, low-voltage operation, low-cost fabrication, flexibility, and biocompatibility. The main feature of EGOTs is that they operate in an aqueous environment, using an electrolyte containing the analyte as dielectric between the gate electrode and the semiconductive organic channel. We fabricated two EGOT immunosensors for the detection of pro-inflammatory cytokine Interleukin-6 (IL-6) based on Organic Electrochemical Transistor (OECT) and Electrolyte Gated Organic Field Effect Transistors (EGOFET), using PEDOT:PSS and TIPS-Pentacene as channel material, respectively. Both architectures were operated as potentiometric biosensors, in which the gate electrode was functionalized with the biorecognition unit (anti-IL6 antibody). In order to quantify the biosensor response, we constructed a dose curve, which was fitted to adsorption isotherms, under the hypothesis of dynamic equilibrium between the biorecognition unit and the target analyte. Our study shows that Frumkin isotherm describes better the dose curve obtained by either EGOFET or OECT, compared to the most typically used Langmuir and Hill isotherms. Frumkin isotherm provides a physical view of the competing phenomena at the gate/electrolyte interface, characterized by the electrostatic repulsions between the antibody/antigen couples at the surface, which increase with the surface coverage at the functionalized gate. Moreover, application of Frumkin model allows estimation of binding constants: the corresponding analysis of free energy unifies the results obtained by both EGOT architectures, suggesting that the response is due to the binding events at the gate/electrolyte interface, and independent of the transduction mechanism. Therefore, EGOT can be used not only as biosensors for analytical purposes but also as a tool for investigating fundamental aspects of biorecognition phenomena. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 813863.