Organic Electrochemical Transistor to measure electrochemical potentials

Organic Electrochemical Transistors (OECTs) have been proposed as low cost chemical sensors for the detection of several analytes thanks to their remarkable features such as signal amplification, the use of an easy and cheap readout electronics, low supply voltage (usually < 1 V), low power operation (< 100 μW), bio-compatibility, and, moreover, they can be easily miniaturized and adapted to non-flat and/or flexible devices. An OECT is composed by a stripe of conductive polymer that works as a channel and by another electrode, usually a metal, that works as a gate. When the device is dipped in an electrolyte solution, the current flowing in the channel can be modulated through the gate voltage because it promotes electrochemical reactions that change the charge carrier concentration in the polymer and, consequently, its conductivity. Chemical sensors based on OECTs are usually obtained by exploiting as gate electrode a chemically modified electrode that acts as amperometric transducer. In such a way, the rate of electrochemical processes and thus of the gate action depends on the analyte concentration. In our previous work, we have demonstrated that the OECT transduction is generated by a variation of electrochemical potential of the channel, because the electrical conductivity of a conductive polymer is a function of its oxidation level. Consequently, OECTs could be also used as transducers for potentiometric sensing. This contribution describes a strategy to exploit OECTs in the measurement of potentiometric signals with the final aim of obtaining sensors that cannot be fabricated with consolidated technology. In order to thoroughly investigate the potentiality of these devices, we fabricate transistors with different architectures wherein the channel is made of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and the gate is an Ag/AgCl electrode. Since the Cl- concentration in the electrolyte rules the gate potential measured with respect to a reference electrode, it also controls the electrochemical potential of the channel and thus its conductivity. The current that flows in the channel linearly depends on the logarithm of Cl- concentration as expected from Nernst equation. Since the Ag/AgCl electrode can force the channel potential without the action of an external potential, the transistor can also operate when the gate and the source are short circuited in order to produce a two terminal device with the features of a transistor. The proposed approach is general and we have already fabricated similar devices for the detection of Br-, I- and S2-, based on AgBr, AgI and Ag2S gate electrodes, respectively. Moreover, we exploited such evidences to design new composite materials for the production of two terminal devices that maintain the amplification of a transistor. These evidences could pave the way to the production of both new materials and new sensors based on potentiometric transduction.