Ion Sensitive Field-Effect, Transistors

Fig. 2.16. Ion-sensitive field effect transistor (intersection and symbol), (a) n-p-n transistor, (b) IGFET (MOSFET), (c) ISFET.

T. Uno, T. Ohtake, H. Tabata, and T. Kawai, Direct deoxyribonucleic acid detection using ion-sensitive field-effect transistors. Jpn. J. Appl. Phys. 43, L1584—L1587 (2004). 

M. Zayats, O.A. Raitman, V.I. Chegel, A.B. Kharitonov, and I. Willner, Probing antigen-antibody binding processes by impedance measurements on ion-sensitive field-effect transistor devices and complementary surface plasmon resonance analyses development of cholera toxin sensors. Anal. Chem. 74, 4763-4773 (2002). 

I. Freund, and B. Wolf, Non-invasive measurement of cell membrane associated proton gradients by ion-sensitive field effect transistor arrays for microphysiological and bioelectronical applications. Biosens. Bioelectron. 15, 117-124 (2000). 

Explain the main mechanistic differences between a glass membrane electrode and an ion-sensitive field effect transistor (ISFET). 

The measurement of changes of the surface potential Vo at the interface between an insulator and a solution is made possible by incorporating a thin film of that insulator in an electrolyte/insulator/silicon (EIS) structure. The surface potential of the silicon can be determined either by measuring the capacitance of the structure, or by fabricating a field effect transistor to measure the lateral current flow. In the latter case, the device is called an ion-sensitive field effect transistor (ISFET). Figure 1 shows a schematic representation of an ISFET structure. The first authors to suggest the application of ISFETs or EIS capacitors as a measurement tool to determine the surface potential of insulators were Schenck (15) and Cichos and Geidel (16). 

ION-SELECTIVE ELECTRODES (ISEs) AND ION-SENSITIVE FIELD-EFFECT TRANSISTORS (ISFETs)... 

S.-S. Jan, J.-L. Chiang, Y.-C. Chen, J.-C. Chou, and C.-C Cheng, Characteristics of the Hydrogen Ion-Sensitive Field Effect Transistors with Sol-Gel-Derived Lead Titanate Gate, Anal. Chim. Acta 2002,469, 205. 

M. Lahav, A. B. Kharitonov, O. Katz, T. Kunitakc, and I. Willner, Tailored Chemosensors for Chloroaromatic Acids Using Molecular Imprinted Tt02 Thin Films on Ion-Sensitive Field-Effect Transistor, AnaL Chem. 2001, 73, 720. 

A. B. Kharitonov, A. N. Shipway, and I. Willner, An Au Nanoparticle/ Bisbipyridinium Cyclophane-Functionalized Ion-Sensitive Field-Effect Transistor for the Sensing of Adrenaline, Anal. Chem. 15199, 71, 5441. 

S. P. Pogprelova, M. Zayats, T. Bourenko, A. B. Kharitonov, O. Lioubashevski, E. Katz, and L Willner, Analysis of NAD(P) /NAD(P)H Cofactors by Imprinted Polymer Membranes Associated with Ion-Sensitive Field-Effect Transistor Devices and Au-Quartz Crystals, Anal. Chem. 2003, 75, 509. 

Volume has been the most significant limitation on the size and construction of microreference electrodes, a limitation that complements the small size of the microfabricated ion sensors (Section 6.23.2). There have been many attempts to prepare a liquid junction free microreference electrode that would be comparable in size with the integrated ion sensors, such as ion-sensitive field-effect transistors (Section 6.23.2). These attempts have followed broadly three tines of reasoning scaling down of a macroscopic reference electrode (Comte and Janata, 1978 Smith and Scott, 1986), elimination of the reference solution compartment while preserving the internal element structure (e.g., Ag/AgCl), and utilization of inert materials such as polyfluorinated hydrocarbons and the tike, particularly in the so-called reference FET configuration. 

Ion-selective membranes can be used in two basic configurations. If the solution is placed on either side of the membrane, the arrangement (e.g., Fig. 6.16a) is symmetrical. It is found in conventional ion-selective electrodes in which the internal contact is realized by the solution in which the internal reference electrode is immersed. In the nonsymmetrical arrangement (Fig. 6.16b), one side of the membrane is contacted by the sample (usually aqueous), and the other side is interfaced with some solid material. Examples of this type are coated wire electrodes and Ion-Sensitive Field-Effect Transistors (ISFETs). 

Symmetrical placement of the ion-selective membrane is typical for the conventional ISE. It helped us to define the operating principles of these sensors and most important, to highlight the importance of the interfaces. Although such electrodes are fundamentally sound and proven to be useful in practice, the future belongs to the miniaturized ion sensors. The reason for this is basic there is neither surface area nor size restriction implied in the Nernst or in the Nikolskij-Eisenman equations. Moreover, multivariate analysis (Chapter 10) enhances the information content in chemical sensing. It is predicated by the miniaturization of individual sensors. The miniaturization has led to the development of potentiometric sensors with solid internal contact. They include Coated Wire Electrodes (CWE), hybrid ion sensors, and ion-sensitive field-effect transistors. The internal contact can be a conductor, semiconductor, or even an insulator. The price to be paid for the convenience of these sensors is in the more restrictive design parameters. These must be followed in order to obtain sensors with performance comparable to the conventional symmetrical ion-selective electrodes. 

Field-Effect Transistors (FETs) are part of all modern pH meters. With the introduction of ion-sensitive field-effect transistors, they have both been brought to the attention of chemists. In order to understand the principles of operation of these new electrochemical devices, it is necessary to include the FET in the overall discussion of the electrochemical cell. The outline of the operation of an insulated gate field-effect transistor is given in Appendix C. 

Fig. 6.22 Analog circuits for operation of ion-sensitive field-effect transistor (ISFET) (a) in constant applied voltage mode ((6.62) and (6.63)) and (b) in (source-follower) constant current feedback mode ((6.65) and (6.69))...

Fig. 6.26 Individual and differential response of the NaCI ion-sensitive field-effect transistor (ISFET) sensor (a) response of Na-ISFET and (b) Cl ISFET both measured against regular reference electrode (c) differential current measurement of concentration of NaClin 0.01MMgSO4 solution (adapted from Bezegh et al., 1987)...

Figure 3. Ionic leakage paths in chemfet structures a.Schematic illustration of ionic leakage paths around the chemically sensitive membrane. Leakage through the membrane also occurs but is not illustrated b. Schematic illustration of leakage at the surface of a standard ion sensitive field effect transistor.

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