Integrated Electronics in Chemical Analysis
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Integration of chemical recognition elements with solid state electronic devices has been subject of intense interest for over three decades, with some notable achievements achieved in electroanalytical chemistry. Initially, the “chemistry” has been added to more or less conventional silicon electronics with advantages in miniaturization, noise reduction and promise of multivariate analysis. That was the era of chemically sensitive field-effect transistors (CHEMFET), i.e. ion-sensitive field-effect transistors and enzymatic field-effect transistors. In the second phase, it has been recognized that modulation of electronic properties of organic semiconductors leads to creation of solid state work function sensors for gases, again based on the traditional silicon platform. Development of organic electronics took place almost in parallel. In that case silicon, as the functional material, has been replaced with organic semiconductors. The motivation for this development has been the promise of flexible and inexpensive electronics. What has not been recognized is that the physics of operation of so-called organic field-effect transistors (OFET) is fundamentally different from the physics of their silicon-based counterparts. In the last decade the chemically responsive OFETs have been added to the toolbox of electroanalytical chemistry. All chemically sensitive silicon based field-effect transistors are high input impedance potentiometric sensors. In such case the transistor current passes only through silicon, which is protected from the environment by nearly ideal passivation with silicon dioxide/silicon nitride. The corollary of this fact is that WF of silicon does not change and the WF-FET sensors do not require separate reference electrode. On the other hand in OFETs the transistor current passes through the organic semiconductor, which is subject to modulation by the operating environment. The chemical response to gases and vapors then originates at multiple points in the device. The contacts, the bulk of the organic semiconductor and all the interfaces can be involved making the interpretation of the response very difficult. Because of this fact OFETs are chemiresistors and can be classified as conductimetric chemical sensors.
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