Physical insights from the Frumkin isotherm applied to electrolyte gated organic transistors as protein biosensors

Label free biosensors based on electrolyte gated organic transistors (EGOTs) are ultra-sensitive and versatile sensing devices. The dose curve represents the change of the sensor signal as a function of the concentration of the target analyte, and, under the hypothesis of dynamic equilibrium between the surface-bound probe and its target partner, can be fitted to adsorption isotherms. In this work, we show that the data obtained from both the OECT and EGOFET Interleukin-6 (IL-6) biosensors are best fitted by the Frumkin isotherm compared to the widely adopted Langmuir and Hill isotherms. Comparable values of the equilibrium association constant K a and the Frumkin interaction parameter g ′ are obtained with both OECT and EGOFET sharing the same functionalization of the gate electrode. Our study unambiguously shows that the biosensor response is, to a large extent, due to the specific binding at the gate/electrolyte interface, and that is viable to investigate the thermodynamics of biorecognition. Moreover, the electrostatic repulsions between adsorbed probe-target pairs are shown to decrease the effective equilibrium association constant as coverage increases, thus causing a loss of sensitivity for concentrations above the threshold limit 1/| g ′|. Frumkin isotherm is used to fit data obtained from OECT- and EGOFET-based Intrelukin-6 biosensors and compared to the Langmuir and Hill ones. The model allows extraction of the equilibrium constant K a and the Frumkin interaction parameter g′ .