OECT sensors

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. 

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...

The preceding OECT sensors were based on addition of the enzyme in solution, and it was shown that addition of free-floating enzyme can result in quantitative measurement of metabolites down to the micromolar range. Other OTFT-based sensors for glucose have immobilized the enzyme within a... 

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...

OECTs for humidity sensing based on the conducting polymer PEDOTPSS and the proton-conducting film (Nation) were presented by Nilsson et alr The entire sensor was fabricated with solution-processable organic materials and was, therefore, compatible with printing technology (see also Chapter 8.07). 

Low-cost, single-use sensors were demonstrated by the authors, who printed the devices on plastic foil and fine paper substrates. Owing to the OECT mode of operation, the operating voltages were of the order of the reduction and oxidation potentials of the conducting polymer (PEDOT PSS), that is, typically of the order of IV. The source-drain current was considered as the sensing parameter and showed a near-exponential relationship to air humidity. It was argued that mobile ions in the electrolyte (which depend on humidity) modulate the electronic conductivity within the polymer channel. ... 

The first demonstration of the OECT for glucose sensing was by Zhu et al. using a commercially available PEDOTiPSS formulation. This work was subsequently refined and the technique was used to measure glucose in the micromolar range, appropriate for detection of glucose in human saliva. In this sensor,... 

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