OECTs

Figure 2. (a) Schematic cross section of an organic field-effect transistor (OFET). (b) Schematic cross section of an organic electrochemical transistor (OECT). The applied source-drain voltage Vd and gate voltage Vg are also shown. 

In OECT-based sensors, it is the conjugated polymer layer which acts as a transducer upon exposure to analytes. However, it is not essential that the conjugated polymer layer be the one which responds directly to the presence of the analyte. Therefore, OECTs can be broadly divided into four types based on their mechanism of detection of the analyte. These sensor mechanism types are listed in Table 8.1. 

Fig. 8.3. (Left) Solid state OECT humidity sensor printed on polyethylene-coated paper. (Right) The sensor was capable of operating when bent at a radius of curvature of approximately 5 cm. Reprinted from [9] with permission from Elsevier...

Matsue et al. [27] were the first to explore an enzyme-based OECT biosensor. They used Diaphorase as the entrapped enzyme in a polypyrrole transducing layer for the detection of NADH via a redox mediator (the sodium salt of anthraquinone-2-sulfonic acid). The net result was the conversion of polypyrrole from its conducting state to its insulating state in the presence of NADH. The device showed a response time of 15--20 min in the presence of NADH. Later Nishizawa et al. [26] exploited the pH sensitivity of the polypyrrole film for the design and fabrication of OECT sensors for pH and for pencillin. The Penicillinase enzyme was entrapped in a membrane which was coated with a polypyrrole film, in which a decrease in pH was observed in the presence of penicillin due to the hydrolysis of penicillin by Penicillinase. 

Contractor and coworkers were the first to explore the pH sensitivity of polyaniline for the fabrication of an OECT biosensor for the detection... 

In all of the above-mentioned work of Contractor and coworkers, enzyme entrapment was carried out at higher pH after the electropolymerization of aniline at much lower pH. Enzymes could not be added along with the elec-tropolymerizing solution because the enzymes become denaturated at such low pHs. This restricted the amount of enzyme that could be entrapped and hence limited the sensitivity of the OECT devices. In 1999, Tripathy and coworkers [29-31] demonstrated that aniline could be polymerized at pHs as high as 5.5, using a strongly acidic polyelectrolyte like poly(styrene sulfonate). Based on these reports, Contractors and coworkers fabricated OECT... 

There are some interesting advances in the design and fabrication of OECTs. The most promising one is the use of microfluidic-based OECTs reported by Malliaras et al. [33], They have successfully integrated microfluidic channels... 

Fig. 9.1. (Left) the architecture of the PEDOT PSS-based OECT is illustrated viewed from the top and at a cross section, respectively. The area of the PEDOT PSS film, in between the source and drain contact, that is capped by the electrolyte, defines the transistor channel. (Right) the current vs. voltage characteristics of the PEDOT PSS-based OECT. At zero gate voltage, the transistor is in the ON state and at Vg = 0.8 V the channel current is suppressed by more than two orders of magnitude as compared to the ON state current...

The relative lateral dimensions of the different parts of the OECT transistor couple strongly to a number of the crucial performance parameters of the... 

Fig. 9.2. (Left) the impedances of the OECT for the associated electronic (e) and ionic (i) currents. Two current loops exist the gate electrode-electrolyte-channel-source electrode loop and the source-channel-drain. (Right) the steady-state drain current (Ids) vs. voltage (top) and the gate current (Jg) vs. time...

In the table below the typical device parameters are given for a PEDOT PSS-based OECT including an aqueous electrolyte with a concentration of 100 mM. 

Fig. 9.5. (Left) the drain current after 15s of gate biasing (Vg = 1-2 V). Reproduced with permission from [23]. Copyright Wiley-VCH Verlag GmbH Co. KGaA. (Right) the PEDOT PSS/Nafion OECT sensor printed on fine paper. Reproduced with permission from [23], Copyright Wiley-VCH Verlag GmbH Co. KGaA...

In this work, an ionophore-based solvent polymeric membrane is cast on top of the PEDOT PSS channel in order to obtain an ion-selective organic electrochemical transistor (IS-OECT). The polymeric membrane is composed of 2-nitrophenyl octyl ether (o-NPOE, 57.2% ww), poly(vinyl chloride) (PVC, 27.9% ww), V.. V.. V. . V -i( I i i( ycloli( xyl-3-oxapentanedianii(le (ETH 129, 9.6% ww) and potassium tetrakis(4-chlorophenyl)borate (KTpCIPB, 5.4% ww), where ETH 129 is a Ca2+-selective ionophore. The channel region of the transistor that is coated by the Ca2+-selective membrane is immersed in an aqueous solution containing 0.1 M KC1 as a background electrolyte and... 

The organic-based OECT, reported here, can handle remarkably higher power than its OFET relatives. As an example, an Orgacon (particular grade of PEDOT PSS from AGFA)-based transistor with active channel size 0.5 x 0.5 mm2 can handle a switching power of up to 20 mW. Above this power level breakdown occurs, most likely due to thermal stress. The transistor can also handle voltages of up to 70 V as well as currents up to 1 mA. 

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