General model for multiple surface reactions in ion-sensitive FETs

The use of Field Effect Transistors as electrochemical sensor combined with integrated readouts in CMOS technology offers many potential advantages in terms of sensitivity, accuracy, repeatability, miniaturization, costs, parallelism, digitalization and communication. Modelling of the transduction mechanisms is necessary to enable quantitative understanding and prediction of the sensor signals, the selectivity and cross-sensitivities for optimized design. We propose a generalized model for the ISFET response to an arbitrary number of receptor types binding with different ionic species in the electrolyte suitable to assess cross-sensitivities in ISFET sensors. The model is implemented in the ENBIOS-1D platform [1], which solves the equilibrium Poisson- Boltzmann equation in the electrolyte coupled to the electrostatics in the FET channel. The interface charge generated at bare or functionalized surfaces by an arbitrary number of chemical reactions among surface binding sites and electrolyte ions is described by a linear system of equations for each type of receptor, whose unknowns are the fractions of sites binding with a given ion. Our approach generalizes the well-known Site Binding model of reactions between amphoteric sites at metal-oxide surfaces and H+ ions, which is commonly used to explain pH sensitivity in ISFETs [2]. It also includes the modified site binding model reported in [3], which considers specific absorption of ions (see Fig. 1). Competing reactions of multiple sites (bare surface plus ligand functionalization) can be described as well. In fact, the model matches the results in [4], where sensing of the FimH protein is demonstrated (Fig. 2).