ENFET sensors

Penicillinase is immobilized on a pH ISFET to obtain an ENFET that is sensitive to penicillin [7], The variation in local pH, arising from the hydrolysis of penicillin into penicilloic acid, is measured by the adjacent ISFET. The response time of this ENFET varies from 30 to SO seconds according to the thickness of the enzymatic membrane its sensitivity is related to the buffering capacity of the solution. A differential measurement compensates for variations in pH or temperature, and a concentration range of 10 to 10 M penicillin can be obtained. 

A urea-sensitive ENFET is constructed by immobilizing urease on a pH ISFET. The pH at the enzymatic layer varies in the presence of urea 

The pH-sensitive ISFET can also be used to determine hypoxanthine, an indicator of the freshness of fish [192]. Xanthine oxidase (XO) is immobilized on a polyvinylbutyral membrane over the insulating layer of Si02, and catalyses simultaneously two successive reactions  

The ENFET thus detects hypoxanthine by detecting uric acid. 

Cells from microorganisms can also be immobilized on a pH ISFET. A membrane containing cells from Gluconobacter suboxydans catalyses the conversion of ethanol inK acetic acid, and can be coupled with a pH ISFET for the determination of ethanol [193], 

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An ENFET sensor is constructed by placing an enzymatic membrane over the ion-selective membrane of an ISFET (Figure 4.31). An ENFET is effectively a miniature version of an enzyme electrode. The product of the enzymatic reaction chosen for measurement is detected by the ISFET. 

Comparison between ENFET sensors and potentiometric enzyme sensors... 

A. Poghossian, T. Yoshinobu, A. Simonis, H. Ecken, H. Liith and M.J. Schoning, Penicillin detection by means of field-effect based sensors EnFET, capacitive EIS sensor or LAPS Sens. Actuators B 78, 237-242 (2001). 

Due to the fact that nearly all ENFET approaches lead to insufficient or unsatisfactory sensor performance, no ISFET based biosensor has been fully commercialized for wider applications to date. 

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. 

This, in addition to the less common detection techniques for these oxygenases (e.g., ENFET [180] or calorimetric [181]), the search for new laccases, the degree of maturity that these sensors have generally achieved and, mainly, the search for better or fit-for-purpose performance requires a focus on the following features ... 

Combined (e.g., transducer and analyte) amperometric urea sensor, immunologically sensitized field-effect transistor (IMFET), enzyme field-effect transistor (ENFET)... 

The modification of the ISFET gate by immobilization of various enzymes results in a number of enzyme-FETs (EnFETs) [1 ] Generally hydrolytic enzymes eg urease or penicillinase were applied Principal problems of these sensors rise from the dependence on the buffer capacity and the pH-sensitivity of the enzyme s activity... 

MOS sensors for liquid solutions, known as ISFET, ENFET or IMFET, are largely a close combination of a chemical receptor layer with the MOSFET acting as a voltage amplifier. Such a combination belongs to the field of electrochemical sensors, which are discussed in Chap. 7. 

Potentiometric enzyme electrodes were the first biosensors. In such sensors, an IBS was coated by an enzyme layer acting as a biocatalyst for reaction of a specific substance. The reaction product subsequently was detected by the ISE. This feature had been transferred soon to ISFETs. A special term, ENFET, has even been informally proposed for enzyme-modified ISFETs. Attempts were also made to utilize further biological interactions for recognition of analytes and construction of potentiometric biosensors. Immunologic sensors on the basis of antigen-antibody reaction may be called IMFETs if they are built on top of a MOSFET. Immunologic reactions, however, maybe used much more efficiently in combination with other transducers besides potentiometric... 

En me sensors involving semiconductors are called enzyme field-effect transistors, ENFET, and, as their name implies, exploit the association of an en me with a field-effect transistor (Fl. The transistor has a metal oxide semiconductor field-effect transistor (MOSFET) structure, which is constructed from, for example, a p-type silicon substrate (Figure 4.30). This central channel is defined by placing two n-type semiconducting zones, called the source and the drain, on opposite sides of the substrate. A metallic gate is isolated from the channel by a thin insulating film (Si02), which also covers the upper face of the substrate. 

Figure 4.31 Schematic representation of a semiconductor enzyme sensor (ENFET), in which the enzyme is immobilized on the sensitive component of an ISFET (S - source D - drain Si - silicon substrate SiOt insulator).

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