Biochemically Modified Devices

The Ir/Pd-MOS condensator described in Section 3.1.21.1 (Winquist et al., 1985) has also been combined with urease to form a urea probe. Urease was entrapped between a dialysis membrane and an NH3-per-meable foil, the latter being separated from the MOS structure by an air gap of 0.1 mm width (Fig. 72). The measuring range was 0.01-5 mmolA, the response time being 3 min. The sensor had an operational lifetime of only 4 days. 

Immobilization of urease to a silanized Si3N4 gate by glutaraldehyde 

Enzyme-FET systems for glucose and urea, in which the pH within the enzyme layer is kept constant by electrolytic production of H+ or OH ions, have been described by Van der Schoot and Bergveld (1987/B8). By the application of direct feedback, this pH-static sensor becomes independent of the buffer capacity and pH of the sample solution the response is linear, and the dynamic range can be expanded. 

Urease has also been adsorbed to the surface of a Langmuir-Blodgett film and the film transferred to the gate of an ISFET (Karube 1986 Moriizumi and Onoue, 1986). 

Another urea microbiosensor, based on an iridium oxide electrode, 

---

The results obtained so far with biosensors based on biochemically modified small scale electronic devices demonstrate the possibility of producing multianalyte sensors by Si-chip technology. Although these... 

This review gives a brief summary of the "types of chemically modified electrodes, their fabrication, and some examples of their uses. One especially promising area of application is that of selective chemical analysis. In general, the approach used is to attach to the electrode surface electrochemically reactive molecules which have electrocatalytic activity toward specific substrates or analytes. In addition, the incorporation of biochemical systems should greatly extend the usefulness of these devices for analytical purposes. 

Due to high biocompability and large surface are of cobalt oxide nanoparticles it can be used for immobilization of other biomolecules. Flavin adenine FAD is a flavoprotein coenzyme that plays an important biological role in many oxidoreductase processes and biochemical reactions. The immobilized FAD onto different electrode surfaces provides a basis for fabrication of sensors, biosensors, enzymatic reactors and biomedical devices. The electrocatalytic oxidation of NADH on the surface of graphite electrode modified with immobilization of FAD was investigated [276], Recently we used cyclic voltammetry as simple technique for cobalt-oxide nanoparticles formation and immobilization flavin adenine dinucleotide (FAD) [277], Repeated cyclic voltammograms of GC/ CoOx nanoparticles modified electrode in buffer solution containing FAD is shown in Fig.37A. 

Wittman-Liebold, B., Graffunder, H., and Kohls, H., A device coupled to a modified sequenator for the automated conversion of anilinothiazolinones into PTH amino acids, A aZ. Biochem., 75, 621-633, 1976. 

It will be apparent that the analytical use of bioelectrochemical methods depends on many factors including the use of the novel design of electrodes and the employment of the direct electrochemistry of enzymes, whether modified or not. Probably the defect of all the present methods is the modest sensitivity of bioelectrochemical methods. This is obviously important if these techniques are going to give rise to devices capable of sensing components of the immune system, to say nothing of DNA and RNA analyses. Obviously it is possible to provide considerable amplification from the electronic apparatus associated with the biochemical materials employed but the latter will have to be made more sensitive, perhaps, for example, by a series of coupled enzymatic reactions, before the full advantages of these techniques can be exploited. 

Electrochemical biosensor A self-contained integrated device that is capable of providing specific quantitative or semiquantitative analjrtical information using a biological recognition element (biochemical receptor), which is retained in direct spatial contact with an electrochemical transduction element [6]. A biosensor with an electrochemical transducer may represent a chemically modified electrode. 

For electrospray ionization, initially, 100 lm i.d. stainless-steel capillaries, i.e. hypodermic needles, were used for sample introduction. With such a device, the flow-rate is limited to 10 pL min i, which is too high for many biochemical applications and too low for effective LC-MS coupling. Therefore, the initial system was modified. 

---