Flexible Organic Thin-Film Transistors for pH Monitoring

A novel freestanding flexible device based on an organic field effect transistor (OFET), able to detect pH changes in chemical solutions thanks to a functionalized floating-gate, was realized and successfully tested. The device is assembled on a flexible film (Mylar), which acts at the same time as gate insulator and as mechanical support for the whole structure. On one side of the foil a control gate and drain/source contacts are photolithographically patterned, and a pentacene active layer deposited; on the opposite side a gold floating gate is defined. The sensor performs the detection of the chemical species placed over the probe area by detecting the associated electric charge: the structure, basically, works as a floating-gate transistor whose threshold voltage is modulated by the surface charge due to the solution under investigation. By properly functionalizing the floating gate surface, sensitivity to different species and the detection of different reactions can be achieved, with the same sensor. In this work we present its application as ion-sensitive device. pH sensitivity is achieved by functionalizing the sensing surface with thio-aminic groups as such groups protonate proportionally to the concentration of H₃O⁺ ions in the solution. Such a structure does not require a counter-electrode as the OFET is biased through a control gate. Moreover, the working mechanism is independent of the choice of semiconductor, gate or dielectric material, since the OFET is insulated from the solution. The application as DNA sensor is currently under investigation as well.