Nitrate-sensitive field-effect transistor with silica gate insulator modified by chemical grafting

Abstract

Nitrate-sensitive ion-sensitive field effect transistors (ISFETs) were prepared by chemical grafting of the silica gate insulator surface. A differential measurement mode between a reference ISFET with an ion-insensitive surface and an ISFET was used. The insensitive layer was prepared by grafting 3-chloropropyldimethylchlorosilane on to silicon dioxide and the sensitive layer by functionalizing the grafted chlorosilane with trimethylamine. This sensor showed a linear response of about 13 mV (pNO 3) −1 from 10 −3 to 10 −1 M. The pH response was 5 mV (pH) −1 from pH 2 to 7. The response for other anions (Cl −, H 2PO − 4, SO 2− 4) was tested; the selectivity ratios compared with nitrate ions were 0.6, 0.4 and 0.2, respectively. The response for nitrate ion was evaluated according to the extended site binding theory applied to the chemically modified silica/electrolyte interface. This allowed the grafting density to be estimated, and it was found to be 0.25%; the complexation constant was found to be 10 +2.3.