Grafting of phosphonate groups on the silica surface for the elaboration of ion-sensitive field-effect transistors

Ion-sensitive field-effect transistors (ISFETs) sensitive to Ca2+ ions could be elaborated by means of a new grafting process of the phosphonate group at the surface of the silica gate of FETs. A grafting process involving only one chemical reaction step at the surface afforded a significant improvement of the ISFET properties. The sensitivity of the ISFET towards Ca2+ ions at pH 10 was quasi-linear in the concentration range from 10(-1) to 10(-3) M, and the slope was 10 mV pCa(-1). The site-binding model works well in predicting the experimental data, giving the complexation constant of 10(2.7) and a low value of the grafting density. The origin of the poor response of ISFETs sensitized by means of a multistep grafting process was investigated on silica powders of high specific area: the cleavage of the organic grafts at the Si-O-Si bonds occurring at each step could be disclosed by means of elemental analyses, infrared, and cross-polarization and magic angle spinning nuclear magnetic resonance of the grafts. (C) 2000 Elsevier Science B.V. All rights reserved.