Novel membrane material for ion-selective field-effect transistors with extended lifetime and improved selectivity

New plasticizer-free polysiloxane membranes modified by trifluoropropyl groups were developed and investigated concerning the detection of different ions, e.g. K+, Na+, Mg2+ and NO3-. Ion-selective membranes prepared with a trifluoropropyl-functionalized polymer were deposited on ion-selective field-effect transistors (ISFETs) by curing at room temperature. For cation-selective FETs high ion selectivity was found (e.g. logK(K+-S) less than or equal to -4.1 for S=Na+, Li+, Mg2+, Ca2+) when using potassium or sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as ionic site instead of potassium tetrakis(4-chloro-phenyl)borate. A very good discrimination of interferences from chloride and sulfate for nitrate-selective FETs (log K-NO3--SO42- = -4.1; logK(NO3--Cl-) = -2.7) was achieved. With this new membrane material, it was also possible to obtain ISFETs with sensitivity towards silver, lead and cadmium ions. The adhesion properties of the new polysiloxane membranes functionalized with trifluoropropyl groups were excellent and the ISFETs showed a lifetime of more than 3 months. The adhesion was improved by using the same polymer as encapsulation material. The membrane resistances in comparison to poly(vinylchloride) (PVC) membranes were investigated.