Operational mechanism
Using the ion-step method it appeared to be possible to detect the presence of a layer
of charged biomolecules (e.g. proteins) on the surface of an ISFET. The essential
difference between this new method and many others is the fact that the ion-step
method is a dynamic measuring principle in contrast to most of the other methods,
being static and relying on thermodynamical equilibrium. In a dynamical method of
measuring the equilibrium is disturbed by a stimulus. The response on this stimulus
contains the required information. After some time the system regains equilibrium
again, mostly being identical to the situation before the stimulus. The stimulus in the
ion-step measuring method consists of a sudden, stepwise change in the ion
concentration of the solution surrounding the ISFET. In practice, this is accomplished
by a flow-system, in which the solution flowing over the ISFET is suddenly changed
with a solution having a higher ion-concentration than the original one. The ion-step
measuring method is schematically depicted in figure 14