Organic Thin-film Transistors for pH Detection
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Organic ThinFilm Transistors for pH detection. M. Demelas1,2, A. Caboni1, M. Barbaro1, A. Bonfiglio1,2 1 University of Cagliari Department of Electrical and Electronic Engineering, Italy. 2 S3INFMCNR, Modena, Italy. ABSTRACT A novel, flexible and ductile organic fieldeffect transistor (OFET) able to detect pH changes in chemical solutions has been realized and successfully tested. With respect to other organic pH sensors, based on an ISFETlike structure, in our approach the organic transistor is completely separated from the sensing active area and its gate is left floating. The device is biased with a fourth electrode (controlgate) capacitively coupled to the floatinggate. The floatinggate is functionalized by deposition of a layer of thioamines able to protonize proportionally to the pH value of the solution thus modulating the drain current. The structure does not need an Ag/AgCl counterelectrode since the controlgate is not in contact with the solution. Moreover, the sensing mechanism does not depend on the choice of the dielectric and semiconductor material since the working principle is based on charge separation in the metal induced by the electric field. This structure also simplifies the realization of the fluidics since all the contactable electrodes (drain, source and controlgate) are on the same side of the substrate. A differential measurement approach was adopted in order to get rid of device aging and processrelated fluctuations. With the same structure, other chemical species may be detected provided that a proper functionalization procedure is adopted. INTRODUCTION In recent years interest in organic electronics has grown increasingly because of some specific properties of organic devices. Organic devices may be produced following very simple procedures such as lowtemperature deposition and solution processing [1][3]. Besides the relative ease of processing, organic materials are flexible and tough; these good mechanical properties make organic materials naturally compatible with lowcost substrates such as glass, plastic or metal foils [4]. A lot of research has been done on organic thinfilm transistors (OTFTs) and there are many practical applications: recently, organic thinfilm transistors have been employed to make disposable sensors [5]. Organic electronics are not intended to replace inorganic electronics: low mobility in organic semiconductors, for example, makes the OTFTs not suitable for use in applications requiring high speed switching, or high mobility of charge carriers. In any case, the devices to be interfaced with biological systems require more biocompatibility than high speed switching; therefore, plastic electronics can suit that need [6]. Furthermore, typical intrinsic restrictions like shortterm stability and lifetime, performance degradation and instability are less important when the aim is to obtain singleuse devices. Bartic et al. [7] proposed a first example of an ionsensitive organic field effect transistor (I
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