Glucose sensor electrodes

Entrapment of biochemically reactive molecules into conductive polymer substrates is being used to develop electrochemical biosensors (212). This has proven especially useful for the incorporation of enzymes that retain their specific chemical reactivity. Electropolymerization of pyrrole in an aqueous solution containing glucose oxidase (GO) leads to a polypyrrole in which the GO enzyme is co-deposited with the polymer. These polymer-entrapped GO electrodes have been used as glucose sensors. A direct relationship is seen between the electrode response and the glucose concentration in the solution which was analyzed with a typical measurement taking between 20 to 40 s. 

In the area of consumer products, amperometric glucose sensors hold high potential. Industrially, process monitors for the manufacture of consumer chemicals are under development. However, replacement of defective reference electrodes, which in a laboratory environment may be trivial, may be prohibitively difficult m vivo or in an industrial process environment. 

FIGURE 11.30 Zn Nanowire glucose sensor (a) glucose oxidase impregnated Zn nanowire electrode (b) amperimetric response of different glucose concentration. 

In recent years, there are more applications based on the layer-by-layer fabrication techniques for CNT-modified electrodes. This technique clearly provides thinner and more isolated CNTs compared with other methods such as CNT-composite and CNT coated electrodes in which CNTs are in the form of big bundles. This method should help biomolecules such as enzymes and DNA to interact more effectively with CNTs than other methods, and sensors based on this technique are expected to be more sensitive. Important biosensors such as glucose sensors have been developed using this technique, and further development of other sensors based on the layer-by-layer technique is expected. 

The first enzyme biosensor was a glucose sensor reported by Clark in 1962 [194], This biosensor measured the product of glucose oxidation by GOD using an electrode which was a remarkable achievement even though the enzyme was not immobilized on the electrode. Updark and Hicks have developed an improved enzyme sensor using enzyme immobilization [194], The sensor combined the membrane-immobilized GOD with an oxygen electrode, and oxygen measurements were carried out before and after the enzyme reaction. Their report showed the importance of biomaterial immobilization to enhance the stability of a biosensor. 

The counter electrode is the current carrying electrode and it must be inert and larger in dimension. Platinum wire or foil is the most common counter electrode. For work with micro- or ultramicroelectrode where the maximum current demand is of the order of few microamperes, the counter electrode is not necessary. At very low current, a two-electrode system with the reference electrode can function as the current-carrying electrode with very little change in the composition of the reference electrode. Many commercial glucose sensors and on-chip microcells have such electrode configuration. 

Zhao et al. prepared magnetite (FesO nanoparticles modified with electroactive Prussian Blue [44]. These modified NPs were drop-cast onto glassy-carbon electrodes. They observed the redox processes commonly observed for PB (similar to that seen in Figure 4.8), and also demonstrated that the Prussian White material produced by PB reduction at 0.2 V served as an electrocatalyst for Fi202 reduction. They also prepared LbL films in which PB NPs and glucose oxidase were alternated between PD DA layers [99]. These were demonstrated to act as electrocatalysts for Fi202 reduction. Based on the ability to sense the product of the enzymatic reaction, these structures were shown to act as glucose sensors. 

Enzyme electrodes with amperometric indication have certain advantages over potentiometric sensors, chiefly because the product of the enzymic reaction is consumed at the electrode and thus the response time is decreased. For this reason, the potentiometric glucose enzyme electrode, based on reaction (8.1) followed by the reaction of HjO, with iodide ions sensed by an iodide ISE [39], has not found practical use. 

Figure 3.12 — Interfacing of a fermenter to an FI system. The fermenter medium is continuously recycled by a pump to the filter unit, from which the filtrate is guided to a small reservoir (500 /xL). The sample solution is aspirated through a dialyser, the acceptor stream of which is fed to the injector of the FIA system. The analyte content is assayed amperometrically by using the glucose sensor incorporating the enzyme-containing chemically modified electrode. (Reproduced from [86] with permission of Elsevier Science Publishers).

The disposable micro-glucose sensor consisted of thin-film electrodes positioned on a glass substrate and a small sample chamber (the iimer volume of which weis only 20 nL) was brought into contact with a silicon chip. Measmements were possible with as little as 1 pL of sample. The sensor sUncture is depicted in Fig. 3.18.E. The 10 x 20 mm silicon chip had a V-shaped groove that was 100-pm in wide, 70-pm deep and 5-mm long, in addition to two square sample inlets and five contact holes to connect lead wires to electrodes, all of which were formed by anisotropically etching the silicon. Four working electrodes that were 200 pm in width, and one counter-electrode that was 1.5-mm wide, were formed on a Pyrex substrate. The silicon chip and the Pyrex substrate were thermally bonded. 

Figure 7. Amperometric response of glucose sensors prepared using LB film-coated electrodes. ( ) multilayer GOx (o) monolayer GOx sensors.

Glucose sensors based on this electrochemistry are now commercially available. Furthermore, it seems likely that this concept will soon be expanded to other types of enzyme-based sensors. Hence, sensor development is proving to be one of the great success stories of the chemically modified electrode research area. 

Fig. 7.13 Dual glucose sensor based on two oxygen electrodes, one (a) covered with active glucose oxidase and other (b) with deactivated glucose oxidase...

Fig. 7.14 Two-dimensional, cylindrical glucose sensor. Glucose diffuses to the sensor only in direction z whereas oxygen can reach the electrode by diffusion through the walls of the cylinder of length L...

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