Organic field-effect transistor based sensors with sensitive gate dielectrics used for low-concentration ammonia detection

A novel organic field-effect transistor (OFET)-sensor concept is presented based on the application of an ion-conducting organic dielectric material, which is chemically adapted to change its electronic properties upon contact with an analyte, thereby generating an electrically detectable response. By employing pH-sensitive, ring-opening metathesis polymerized materials as gate dielectrics in bottom-contact OFETs with a meander-shaped top-gate structure, the concept was successfully realized and evaluated with ammonia (NH₃) as gaseous analyte, easily providing distinct sensor response at concentration levels as low as 100 ppm. In addition to current–voltage OFET-analysis, optical spectroscopy and capacitance measurements were used to rationalize the underlying sensor mechanism, which is mainly attributed to a deprotonation of the pH-sensitive groups of the active-sensing dielectrics by NH₃ and a resulting generation of mobile ions, leading to an increase of the charge carrier density within the OFET channel. The proposed concept provides several advantages over existing OFET-sensor detection principles, including the separation of the sensing mechanism from the charge-transport functionality of the semiconductor, inherent protection of the latter against air exposure and increased selectivity by the application of specific dielectric materials. It therefore offers a great deal of promise in contributing to the development of cheap, integrated, smart and flexible (bio)sensor systems.