Ion Sensitive Organic Field Effect Transistors

Abstract

Ion sensitive organic field-effect transistors (ISOFETs) with a metal–oxide–semiconductor field-effect transistor (MOSFET) architecture have been fabricated by using poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) as the semiconductor and dielectric layers, respectively. To avoid any pin-holes in the dielectric layer, the ISOFET was coated by two separate PMMA layers. An Ag/AgCl double-junction reference electrode was used as the gate. The results show that the uncoated ISOFET exhibited transistor behaviour in aqueous solutions. However, these devices possessed a small sensitivity of about 0.5 nA dec-1 to H+, K+ and Na+ ions.
Langmuir-Blodgett membranes were then used to improve the ISOFET response to the target ions in solution. By coating the gate dielectric (PMMA) with an LB membrane of pure arachidic acid (AA), the ISOFETs showed a significantly higher sensitivity to H+ ions of about 3.5 nA pH-1, but no improvement in the pK response (< 0.5 nA dec-1). The compact ionised layers of carboxylic acid head groups were thought to lead to the improvement in the pH sensitivity; however, the layers of long hydrocarbon chains prevented large monovalent ions, such as K+ and Na+, from interacting with the ionised carboxylic acid head groups.
ISOFETs coated with an arachidic acid/valinomycin (AA/val) mixture did not show any selectivity to K+ ions, but exhibited enhanced sensitivities to both K+ and Na+ ions. Instead of trapping K+ ions, the valinomycin molecules in the AA membrane were thought to disrupt the membrane architecture and provide ion-leakage channels. Pure valinomycin-coated ISOFETs also revealed enhancements in both sensitivity and selectivity to K+ ions over Na+. This may be due to the fact that the cavity in the valinomycin molecules can accommodate a K+ ion but not a Na+ ion.
To study facilitated K+ transport across the membrane, LB films of AA/val mixture and pure valinomycin were coated on porous supports. The responses of both uncoated and coated membranes were similar. After deposition, collapse of the LB film into the pores may provide leakage channels. This probably led to the observed gradual decrease of the potentials across the membranes.