Possibilities and limitations of direct detection of protein changes by means of an immunological field-effect transistor

An outline of the requirements for the construction of an immunological field-effect transistor (ImmunoFET) which should operate on the direct potentiometric sensing of protein charges is given. Selectivity of the ImmunoFET can be obtained by immobilizing antibodies on the gate area of the ISFET, enhancing the surface affinity to the corresponding antigens over other molecules in the solution. A theoretical approach is given based on the Donnan equilibrium description, which provides an insight into the potential and ion distribution in the protein layer on the ImmunoFET. It is shown that the Donnan potential and the internal pH shift, induced by the protein charges, compensate each other to a great extent. If the ISFET shows Nernstian behaviour, it is concluded that a direct detection of protein charge is impossible. In order to construct an ImmunoFET, a reference FET (REFET) or ISFET with low sensitivity would satisfy the detection of the partially compensated Donnan potential in the presence of an adsorbed protein layer. However, the application of such an ImmunoFET is limited to samples with low ionic strength.