Enzyme ISFET-based

Miyahara et al. (1985) developed an integrated enzyme FET based on a silicon-on-sapphire (SOS) sensor for simultaneous determination of glucose and urea. Three ISFETs and two metal insulator semiconductor FETs (MISFETs) were integrated on a surface area of 2.5 mm x 2.5 mm (Fig. 54). One of the ISFETs served as reference sensor in order to compensate the signals caused by pH changes of the solution the two others were covered by GOD and urease, respectively. The MISFETs can be used as pH electrodes. For enzyme immobilization the chip was covered with a laminated photosensitive layer of 75 pm thickness and,... 

Ion-sensitive field-effect transistors (ISFETs) were also used to fabricate poten-tiometric biosensors for OP compounds. Nyamsi-Hendji et al. have constructed a differential ISFET-based system. Acetyl or butyryl cholinesterase with BSA was fixed from solution to the surface of one pH-FET by glutaraldehyde treatment. Inhibition by solutions of diisopropyl fluorophosphate (10 -10 M) was almost complete in about 30 min. However, reactivation of the inhibited enzyme with 1 mM 2-PAM was insufficient (30-40 %), making the system of limited usefulness for repeated analyses. Stripping off the used enzyme membrane and recoating the FET is a time-consuming process. 

Dumschat C, Miiller H, Stein K, Schwedt G (1991) Pesticide-sensitive ISFET based on enzyme inhibition. Anal Chim Acta 252 7-9... 

Integrated multi-biosensors based on ISFET can be produced by using a photosensitive polymer which incorporates the enzyme and is spinned onto the gate [54]. The reported measuring range for glucose is 5 mM which is rather weak, and some cross sensitivities due to problems regarded with buffer type and capacity were also reported. 

Enzyme sensors can measure analytes that are the substrates of enzymatic reactions. Thermometric sensors can measure the heat produced by the enzyme reaction [31], while optical or electrochemical transducers measure a product produced or cofactor consumed in the reaction. For example, several urea sensors are based on the hydrolysis of urea by urease producing ammonia, which can be detected by an ammonium ion-selective ISE or ISFET [48] or a conductometric device [49]. Amperometric enzyme sensors are based on the measurement of an electroactive product or cofactor [50] an example is the glucose oxidase-based sensor for glucose, the most commercially successful biosensor. Enzymes are incorporated in amperometric sensors in functionalised monolayers [51], entrapped in polymers [52], carbon pastes [53] or zeolites [54]. Other catalytic biological systems such as micro-organisms, abzymes, organelles and tissue slices have also been combined with electrochemical transducers. 

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). 

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)...

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... 

Potentiometric biosensors based in both ISE and ISFET for water analysis have been widely developed in the last few years, with recent research leading to nanomaterial-based devices. New nanoparticle (NP)-based signal amplification and coding strategies for bioaffinity assays are in use, along with molecular carbon-nanotube (CNT) wires for achieving efficient electrical communication with redox-enzyme and nanowire-based label-free DNA sensors. ... 

These devices are based on the measurement of either electrochemical potential or faradaic current associated with redox reactions at an electrode. They are particularly suitable for enzyme-substrate receptor systems by virtue of the ionic products often produced in such reactions. The sensing membranes of the ion-selective electrodes previously described have been combined with semiconductor devices for miniaturization, low-impedance output, signal amplification, and capability of on-chip processing. The ion-sensitive field effect transistor (ISFET) is based on replacement of the conventional transistor gate with the ion-... 

Each type of transducer has its own advantage, e.g., small size and easy way of mass production for ISFETs, reference electrode free-action of the LAPSs, and general availability and simplicity in the case of pH electrodes. The analytical performance of biosensors based on pH detection depends, in general, much more on the origin of the enzyme, the way of its immobilization, and the measurement methodology rather than the type of transducer. 

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