The surface modification of solution-gated organic field-effect transistors is investigated. The introduction of different surface groups leads to a control of the pH sensitivity, determined by the pKa value of the added surface moiety. Together with the successful demonstration of enzyme modification of the surface, this work reveals the large potential of organic SGFETs for biosensor applications.

We report on the electrolytic gating of α-sexithiophene thin film transistors, in which the organic semiconductor is in direct contact with an electrolyte. Due to the large capacitance of the electrical double layer at the electrolyte/semiconductor interface, modulation of the channel conductivity via an electrical field effect is achieved at low voltages. The transistors are stable for several hours and are sensitive to variations in the pH resulting from a pH-dependent surface charge, which modulates the threshold voltage. The response to different ion concentrations is described by the influence of the ions on the mobility and an electrostatic screening effect.

We investigate the electronic properties of solution-gated carbon nanotube (CNT) thin-film transistors, where the active layer consists of a randomly distributed single-walled CNT network of >90% semiconducting nanotubes, deposited from an aqueous solution by spin-coating. The devices are characterized in different electrolytic solutions, where a reference electrode immersed in the liquid is used to apply the gate potential. We observe a gate-potential shift in the transfer characteristic when the pH and/or ionic strength of the electrolytic solution is changed with a pH sensitivity of ≈19 mV/pH. This sensitivity is attributed to a surface charging effect at the CNT/electrolyte interface.