Dynamic use of the ISFET in the ion-step method
Fairly soon after the introduction of the ISFET, researchers all over the world started
looking for other applications than the determination of pH only. As the potential over
the oxide-solution interface of the ISFET determines its output, scientists tried to
modify this potential by applying charged molecules on the oxide surface. Doing so, it
was thought to be possible to monitor immuno-reactions. By immobilising antibodies
on the ISFET surface a certain surface charge is created that might change noticeably
after a reaction with antigens. This seemed to be the basis of a whole new family of
biosensors. The results of these experiments, however, were very disappointing. Also
in the biosensor-group of the University of Twente the possibilities of such a
immunosensor were explored. It was concluded, however, that from a theoretical
point of view, it is almost impossible to detect a layer of charged biomolecules on an
ISFET in a situation of thermodynamical equilibrium. This resulted in the
introduction of a new measuring concept: the ion-step method.