Ionic detection using differential measurement between an ion-sensitive FET and a reference FET

The possibility of obtaining ion-selective membranes on ISFETs by chemical grafting of the silica insulator has been previously shown. The grafting only slightly modifies the electrical characteristics of the transistor (the dielectric permitivity and thickness being nearly unchanged), which makes it possible to perform differential measurements between an ion-sensitive FET and a reference FET. For the detection of silver ions, the sensitive membrane (dimethylamino)silane onto silicon dioxide; the reference FET is an ISFET fabricated by grafting an aliphatic chain onto silicon dioxide.

The response obtained, i.e., the difference between both source potentials versus a platinum pseudo-reference electrode, can be described with the site binding model.

The differential method allows a miniaturized reference electrode to be used. At the same time, the effects of drift, temperature and residual pH response are reduced.