The Ion-Selective Field Effect Transistor ISFET

Two problems have accompanied the development of the ISFET from the very beginning. The first one is the problem of direct contact for making 

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The ion-selective field-effect transistor (ISFET) represents a remarkable new construction principle [7, 63], Inverse potentiometry with ion-selective electrodes is the electrolysis at the interface between two immiscible electrolyte solutions (ITIES) [28, 55],... 

The recently developed field-effect transistors (FETs)41 have also been used as biosensors. The ion-selective field-effect transistor (ISFET) uses ion-selective membranes, identical to those used in ion-selective electrodes, over the gate. 

Bergveld introduced the ion selective field effect transistor (ISFET) [105[. 

Joining of an ion-selective electrode and a field-effect transistor (FET) in a single device has made it possible to create a new type of ion concentration detector called the ion-selective field-effect transistor (ISFET.) High sensitivity and selectivity, as well as the very small size of this device, have resulted in its constantly growing applications in recent years in industry, medicine, biology, scientific research, etc. 

For pesticide analysis, the potential of enzyme biosensors has been tested. In this field, biosensors based on the inhibition of acetylcholinesterases, acylcholinesterases, or butylrylchol-inesterases by organophosphorus compounds are widely used. Their specific activity can be monitored by electrochemical methods such as the ion-selective electrode and the ion-selective field effect transistor (ISFET). 

The ion-selective field-effect transistor (ISFET) is derived from the well-established MOSFET device familiar in silicon integrated circuits. Field effect devices have three major advantages to offer (see Chapter 10 of this volume). 

These problems might be resolved by the ion-selective field-effect transistor (ISFET). In an IS-FET the ion-selective membrane is placed directly on the gate insulator of the field-effect transistor alternatively, the gate insulator itself, acting as a pH-selective membrane, may be exposed to the analyte solution (Fig. 28). If one compares an ISFET with the conventional ISE measurement system, the gate metal, connecting leads, and internal reference system have all been eliminated. An ISFET is a small, physically robust, fast potentio-metric sensor, and it can be produced by microelectronic methods, with the future prospect of low-cost bulk production. More than one sensor can be placed within an area of a few square millimeters. The first ISFETs were described independently by Bergveld [142] and Matsuo, Esashi,... 

The working principle of LAPS resembles that of an ion-selective field effect transistor (ISFET). In both cases the ion concentration affects the surface potential and therefore the properties of the depletion layer. Many of the technologies developed for ISFETs, such as forming of ion-selective layers on the insulator surface, have been applied to LAPS without significant modification. 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

Ion-Selective Field Effect Transistors [22b,c,d] An ion-selective field effect transistor (ISFET) is a hybrid of an ion-selective electrode and a metal-oxide semiconductor field effect transistor (MOSFET), the metal gate of the MOSFET being replaced by or contacted with a thin film of a solid or liquid ion-sensitive material. The ISFET and a reference electrode are immersed in the solution containing ion i, to which the ISFET is sensitive, and electrically connected as in Fig. 5.37. A potential which varies with the activity of ion i, o(i), as in Eq. (5.38), is developed at the ion-sensitive film ... 

Successful operation of potentiometric chemosensors opened up the possibility for the fabrication of chemical field-effect transistors (chemFETs) and ion-selective field-effect transistors (ISFETs). A sensing element in these devices, i.e. the MIP film loaded with the molecular, neutral or ionic, respectively, imprinted substance is used to modify surface of the transistor gate area. Apparently, any change in the potential of the film due to its interactions with the analyte alters the current flowing between the source and drain. 

The first representative of a potentiometric sensor was the pH-glass electrode invented in 1906 [35]. Decades of development resulted in the invention of many more ion-selective electrodes including more recently those based on neutral carrier membranes [36] and of the microelectronic fabricated ion selective field effect transistor (ISFET) [37]. 

Ion-selective field-effect transistors (ISFETs) are ion sensors that combine the electric properties of gate-insulator field-effect transistors and the electrochemical properties of ion-selective electrodes (ISEs). ISFETs have attracted much attention for clinical and biomedical fields because they could contain miniaturized multiple sensors and could be routinely used for continuous in vivo monitoring of biological fluid electrolytes (e.g., Na+, K+, Ca +, Cl", etc.) during surgical procedures or at the bedside of the patients in clinical cate unit (2). 

The integration of chemically sensitive membranes with solid-state electronics has led to the evolution of miniaturized, mass-produced potentiometric probes known as ion-selective field effect transistors (ISFETs). The development of ISFETs is considered as a logical extension of coated-wire electrodes (described in Section 5.2.4). The construction of ISFETs is based on the tech-... 

Lilienfeld, Julius Edgar — (Apr. 18, 1881, Lemberg, Austro-Hungarian Empire, now Lviv, Ukraine - Aug. 28, 1963, Charlotte Amalie, the Virgin Islands, USA) Lilienfeld proposed the basic principle behind the MOS field-effect transistor in 1925 [i]. This was the background of all field-effect transistors used now, including - ion-selective field-effect transistors (ISFETs) used in numerous electrochemical sensors. 

The acid-base behavior of proteins can reveal some important properties with respect to both their composition (selectivity) and their concentration (sensitivity). The most direct way to exploit these acid-base properties is to make use of acid-base titration, Titrant should be added somehow and the resulting change in pH should be measured. Since the ion-sensitive field-effect transistor (ISFET) is suitable for fast (and local) pH detection, an ISFET can be used for protein titration if the protein to be detected can be immobilized in a membrane, deposited on top of the device. Advantages are the small amount of protein necessary for the characterization owing to the small membrane volume, and the relatively short time needed to perform a full titration. 

A new development in the field of potentiometric enzyme sensors came in the 1980s from the work of Caras and Janata (72). They describe a penicillin-responsive device which consists of a pH-sensitive, ion-selective field effect transistor (ISFET) and an enzyme-immobilized ISFET (ENFET). Determining urea with ISFETs covered with immobilized urease is also possible (73). Current research is focused on the construction and characterization of ENFETs (27,73). Although ISFETs have several interesting features, the need to compensate for variations in the pH and buffering capacity of the sample is a serious hurdle for the rapid development of ENFETs. For detailed information on the principles and applications of ENFETs, the reader is referred to several recent reviews (27, 74) and Chapter 8. 

Semiconductor electrodes ion-selective field effect transistors (ISFETs) are semiconductor devices used to measure ionic species in solution. They are sometimes called chemical field-effect transistors or ChemFETs. The transistor is coated with silicon nitride, which is in contact with the test solution via an analyte-sensing membrane and also connected to a reference electrode. A variation in the concentration of the analyte ions changes proportionally with the voltage of the ISFET. ISFETs are rugged, have a faster response time than membrane electrodes and can be stored dry. 

Ion-Selective Field Effect Transistors. Ion-selective field effect transistors (ISFETs) are semiconductor devices related to the solid-state detectors used in spectroscopy (discussed in Chapter 5). In this case, the surface of the transistor is covered with silicon nitride, which adsorbs H ions from the sample solution. The degree of adsorption is a function of the pH of the sample solution and the adsorption of H" " ions results in a change in the conductivity of the ISEET channel. The cell requires an external reference electrode. ISEET pH sensors can be made extremely small (about 2 mm ) and are extremely rugged, unlike the fragile glass bulb pH electrode. They have rapid response times and can operate in corrosive samples, slurries, and even wet solids such as food products. The sensor can be scrubbed clean with a toothbrush, stored in a dry condition, and does not require hydrating... 

Over the past few decades, effort has been devoted to exploit semiconductor field-effect transistors (FETs) in chemical and biological sensors due to the potential of these devices to meet some of the requirements discussed above. Most of this work concerned the development of the ion-sensitive field-effect transistor (ISFET) for the detection of specific ions and anal3d es using appropriate ion-selective or enzymatic membranes. One of the advantages of the ISFET is that it operates in equilibrium conditions. Due to the presence of the insulating layer on top of the semiconductor, no current flows across the biological layer. 

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