Novel Ion-sensitive Field-effect Transistor with Ion Sensing Polymerized Silsesquioxane Membrane on Detachable Metal Disk

We developed a new type of an ion-sensitive field-effect transistor (ISFET) with an ion sensing polymerized silsesquioxane (polySQ) membrane carrying a quaternary ammonium salt (QAS) as an ionophore. A noticeable feature of this ISFET is that the metal disc part is designed to be detachable from an FET body. Owing to this structure, the full polymerization of SQ at relatively high temperature became possible; leading to preparation of a durable ion sensing polySQ membrane on a metal disk connected to the FET gate terminal. The ISFET with the polySQ membrane carrying QAS (polySQ-QAS) (optimal content of QAS, 40 wt% in the matrix), prepared at 150°C for 3 h, showed not only the theoretical potential response to the nitrate ion concentration in the range of 1.0 × 10−6 to 0.10 mol dm−3 (M) with the Nernst slop of −61.5 mV·decade−1, but also the rapid response (3 s for 90% of response). Furthermore, the ion sensing property is favorably sustained even after being soaked in methanol for 24 h.