Enzyme-based biosensor electrode

In recent years the electrochemistry of the enzyme membrane has been a subject of great interest due to its significance in both theories and practical applications to biosensors (i-5). Since the enzyme electrode was first proposed and prepared by Clark et al. (6) and Updike et al. (7), enzyme-based biosensors have become a widely interested research field. Research efforts have been directed toward improved designs of the electrode and the necessary membrane materials required for the proper operation of sensors. Different methods have been developed for immobilizing the enzyme on the electrode surface, such as covalent and adsorptive couplings (8-12) of the enzymes to the electrode surface, entrapment of the enzymes in the carbon paste mixture (13 etc. The entrapment of the enzyme into a conducting polymer has become an attractive method (14-22) because of the conducting nature of the polymer matrix and of the easy preparation procedure of the enzyme electrode. The entrapment of enzymes in the polypyrrole film provides a simple way of enzyme immobilization for the construction of a biosensor. It is known that the PPy-... 

In common with enzyme based biosensors (equations (7.3) and (7.4)) the reduced mediator can be re-oxidized when it diffuses to an electrode poised at a suitably oxidising potential, e.g.,... 

Table 11.19 Liquid Membrane Electrodes (LME), Gas-Sensing Electrodes (GSME), and Enzyme-Based Biosensors (EBB) [56]...

This enzyme-based biosensor uses glucose oxidase (GO) as a chemical recognition element, and an amperometric graphite foil electrode as the transducer. It differs from the first reported glucose biosensor discussed in the introduction to this chapter in that a mediator, 1,1 -dimethylferricinium, replaces molecular oxygen as the oxidant that regenerates active enzyme. The enzymatic reaction is given in Eq. 7.15, and the electrochemical reaction that provides the measured current is shown in Eq. 7.16. 

Most of transduction elements used in enzyme-based biosensors are electrochemical amperometric or potentiometric. Typically the enzymes used in amperometric biosensors are oxidases. The main advantages of this class of transducer are the low cost a high degree of reproducibility, and the suitability of many of them for incorporation into disposable electrodes. This type of instrumentation is widely available and can be inexpensive and compact this allows this makes it possible to use them for making on-site measurements. Limitations of amperometric measurements include potential interferences to the response from any electroactive compounds that are present in... 

However, these reports of multitudinous enzyme-based biosensors should be viewed with some caution, as it is much easier to demonstrate the possibility of using an enzyme in a laboratory prototype than to convert these observations into a reliable, and reproducible, device that can meet commercial product requirements. This is illustrated by table 7.1, which lists the few enzymes that have been reported to have been used in commercial biosensors only about two dozen enzymes have been used commercially. Most of the enzymes are oxidases partly because of the stability of this class of enzyme, and partly because of ease of linking this type of enzyme with a Clark-type oxygen electrode. 

Electropolymerization of polymers directly onto the surface of an electrode has been used for a number of enzyme-based biosensors. By polymerizing from a solution containing the monomer, as well as the other components of the sensor, enzymes for example, a multifunctional polymer film can be fabricated. As the polymer film grows on the electrode, the enzyme and other components are entrapped in the film [9]. GOD and other enzymes have been incorporated into sensors using electropolymerization. Advantages of electropolymerization are that the film thickness can be easily controlled by the amount of polymerization charge passed, and that the polymer film is deposited only on the sensing electrode. 

In addition to a family of SODs, several other kinds of enzymes and proteins, including tyrosinase [87], galactose oxidase [87], hemin, and cytochrome c (Cyt. c), have been employed to construct enzyme-based biosensors for the O2 determination. Here, we will use Cyt. c as an example to illustrate the analytical mechanism of such a kind of 02 biosensors. For constructing a Cyt. c-based biosensor, Cyt. c is normally inunobilized on the electrode surface and acts as an electron transfer mediator between the electrode and 02. The O2 radical reduces the immobilized Cyt. c (Fe(lll)) to Cyt. c (Fe(II)) and the Cyt. c (Fe(ll)) is reoxidized on the electrode at a potential of... 

Enzymes-based biosensors are well reported in the literature for chemical toxicity screening. The sensor devices produced using enzymes are usually simple and easy to fabricate, inexpensive, and sensitive to low levels of toxicants. Immobilization of enzymes on the electrode surface can include adsorption, covalent attachment, or film deposition using a range of procedures [68-70]. The sensor system relies primarily on two enzyme mechanisms catalytic transformation of a pollutant and detection of pollutants that inhibit or mediate the enzyme s activity. In catalytic enzyme biosensor, the enzyme specific for the substrate of interest (toxin in this case)... 

Application of poIy(AMFc) electrode. The poly(AMFc) modified electrode was used in the construction of a flavin enzyme-based biosensor. Fig. 3B shows the effect of pH on the response of the glucose electrode. This pH profile is similar to that reported in literature with a maximum at pH of around 5.5. A pH of 5.5 was therefore used for characterization of the electrode. 

HPLC with UV-based diode array detection (DAD-UV) or electrochemical detection is normally used to determine ascorbic acid. Many types of electrochemical determinations of ascorbic acid have been proposed. Although the electrochemical determinations using enzyme-based biosensors exhibited high specificity and sensitivity, these methods suffer in the fabrication of the electrodes and in automatic analysis. Recently, chemically modified screen-printed electrodes have been constructed for the determination of ascorbic acid. This is one of the most promising routes for mass production of inexpensive, reproducible, and reliable electrochemical sensors. 

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