Enzyme field-effect transistors ENFETs

Fig. 16 Enzyme field-effect transistor (ENFET) based on LbL assembly of enzymes with Mn02 nanoparticle and polyelectrolyte on the surface of ion-sensitive field-effect transistor (ISFET)...

MIS field-effect transistors (MISFETs) and enzyme field-effect transistors (ENFETs) based on conducting polymers have also been fabricated. A schematic diagram of such a device is shown in Figure 13.12. Janata and co-workers have investigated the electrical properties... 

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

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. 

Enzyme-immobilized FETs (ENFETs), 22 269. See also Enzyme ion-selective field-effect transistor (ENFET) Field effect transistors (FETs)... 

ENZYME ION-SELECTIVE FIELD-EFFECT TRANSISTOR (ENFET)... 

Furthermore, the gate area of pH-sensitive ISFET s (ion-sensitive field-effect transistor) has been covered with pH changing enzymes, such as urease and )8-lactamase [12]. Furthermore, ATPase, glucose oxidase, and trypsin were used for ATP, glucose, and peptide ENFET s (enzyme-field-effect transistor). 

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. 

ENFET Enzyme-immobilized field effect transistor... 

Under these conditions a dynamic range between 1 pph and 1.3 ppm can be obtained [322]. Hammerle et al. have described an electrochemical ceU containing the enzyme electrode that can directly operate in the gas phase with a limit of detection of 0.3 ppm and no loss of response during 7 h of operation [323]. Formaldehyde dehydrogenase has also been incorporated into a field-effect transistor for the measurement of catalytically-produced protons and the ENFET incorporated to a sampling system that strips the aldehyde into an aqueous phase [324]. 

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)... 

Another approach to eliminate the inner filling solution of conventional ISEs was introduced also in the 1970s and is based on the use of field effect transistors (FETs). These devices are referred to as ISFETs, that is, ion-sensitive FETs, and belong together with enzyme FETs (EnFETs) and gas sensitive FETs to the larger category of ChemFETs (chemically sensitive FETs). In the case of the ISFET, the ISE membrane is applied to the Si3N4... 

The sensitive component of an ISFET operates along the same principles as a potentiometric electrode. In both systems, there is no consumption of reaction product, and the interaction of ions with the sensitive component leads to a membrane potential. This potential is measured directly in the potentiometric electrode and indirectly in the field-effect transistor. The measurement of the activity of the ions is generally made at a constant current the measured output potential is thus a function of the ionic activity of the solution. Once the ISFET and the potentiometric electrode are covered with enzymatic membranes to give an ENFET and an enzyme electrode, respectively, the two biosensors differ only in their method of transduction. 

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