ISFET principle

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. 

In principle the ISFET is derived from a MOSFET, where the metal is replaced by the couple solution-reference electrode and where a CIM (Chemically Interactive Material) is deposited on the S1O2, the gate oxide. 

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],... 

Examples of the use of FIA with ISE detection involve the determination of nitrate and total nitrogen in environmental samples [48, 49, 125, 166], potassium, sodium [125], calcium [51] and urea [124] in serum or major nutrients in fertilizers [73]. An interesting combination of an ISFET sensor with the FIA principle [52] is shown in fig. 5.17. This is a simultaneous determination of potassium, calcium and pH in serum during dialysis on an artificial kidney. 

The ISFET-based integrated coulometric sensor-actuator system was introduced in 1985 [154] in order to facilitate in situ calibration of ISFETs. The essential components of a prototype sensor based on this operational principle are shown in Fig. 4.20.B. The system was built by integrating a large noble-metal actuator electrode and a counter-electrode in a piece of silicon. A window in the actuator electrode was etched to receive the gate of the ISFET, which functioned as a pH indicator. The flow-through cell was constructed by sealing a silicon cover with an etched cavity of the chip. The system operation resembles that of a conventional coulometric titration system very closely. The sample was first injected into the cavity and the... 

Such an ISFET was originally used for detecting pH changes but by casting with ion selective membranes a lot of different ion-selective sensors can be obtained in principle [38,39]. Even a multi-parameter electrolyte sensitive chip for clinical applications was invented [39]. 

Ion-sensitive field effect transistor (ISFET) — In a semiconductor device based on the principle of the field effect transistor (FET) the current between two - semiconductor electrodes (designated source and drain) is controlled by a third electrode, the gate. In an ISFET this gate is modified on its surface in a way which makes the surface ion-responsive (-selective and -sensitive). Changes in the concentration of the species in the solution in contact with the gate surface thus control the current between source and drain. In order to establish proper working conditions a reference electrode (e.g., a -+ REFET) is needed. See also - CHEM-FET. 

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 ion sensitive field-effect transistor (ISFET) is a special member of the family of potentiometric chemical sensors [6,7. Like the other members of this family, it transduces information from the chemical into the electrical domain. Unlike the common potentiometric sensors, however, the principle of operation of the ISFET cannot be listed on the usual table of operation principles of potentiometric sensors. These principles, e.g., the determination of the redox potential at an inert electrode, or of the electrode potential of an electrode immersed in a solution of its own ions (electrode of the first kind), all have in common that a galvanic contact exists between the electrode and the solution, allowing a faradaic current to flow, even when this is only a very small measuring current. 

The working principle of an ISFET is essentially different, which is evident from the name of this transducer information is transferred via an electric field. As is known, the source of any static electric field is charge. The nature of this charge in our case is concealed in the first two letters of the acronym ISFET ions form the source of the charge, of which the resulting electric field controls the electronic behavior of the transistor. It is important to observe that in this case no galvanic contact exists between the solution and the conducting part of the sensor, so there is no faradaic current. 

The ISFET is today a well-known transducing element for the development of chemical sensors and biosensors. The transducing principle of an ISFET is based on the dependence of the drain current of the transistor on the surface charge... 

The simulation results presented show clearly the dependence of the ISFET response on the concentration of protein immobilized in a membrane on top of the device. Since the number and type of proton dissociating groups determine the protein buffer capacity (dc rM/ dp ), and since the ISFET response directly reflects the change in pH as a function of time, in principle the shape of an ISFET response also contains information on the type of protein that is immobilized. 

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. 

For a detailed explanation of the theory of ISFETs. see J. Janata. Principles of Chemical Sensors, pp. 125-141. New York Plenum. 1989. 

Field-effect Transistors Enzyme FETs and immuno FETs (IMFETs) are based on principles similar to those valid in potentiometric membrane biosensors. The enzyme is immobihzed on top of the ion-selective membrane on the gate of the FET. For construction of ENFETs, usually double-gate FETs are used employing one gate as a reference system, covered only with a layer of the immobilization matrix, and allowing for the real-time compensation of pH modulations, temperature, and drift. Mostly, pH-sensitive FETs (ISFET)... 

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