Ionsensitive Field-Effect-Structures with Langmuir-Blodgett Membranes

Abstract

The electrolyte/insulator/silicon-structure is well suited to investigate and to optimize membranes for an application in ion-sensitive-field-effect-transistors (ISFETs). Examples for new membrane materials for chemical sensors are offered by special Langmuir-Blodgett (LB) films, which show very good properties with respect to long-term stability and sensitivity. An indispensable condition for the use of the insulator as a carrier for different sorts of chemically sensitive groupsis its protection against water. Membranes made of a synthetic rodlike Phthalocyaninato-Polysiloxane (PcPS)-Polymeror a special Polyglutamate (PG) fulfill this demand. They both exhibit lifetimes of more than 6 months in permanent contact with electrolyte. While the PcPS-membranes show a pH-sensitivity depending ontheir thickness, the PG membranes show a much reduced pH-sensitivity. The latter can thus be used for a reference-element. There are two waysto obtain ion-sensitive LB-materials: Covalent fixing of ionophores like crown ethers or physical mixing of special sensitive groups to the polymers. In this paper an Ag*-sensitive membrane with fixed olefin-groups and a K*- sensitive membrane with a mixed film of PG and Valinomycin are presented.