Glucose oxidase electrodes polymer films

Liu et al., 1998 [106] Milk Glucose oxidase (GOD)/ cross-linked with fi-cyclodextrin polymer. Coated with Nafion film Glassy carbon electrode/ +0.35 V vs. Ag/AgCl Ferrocene (also included in the polymer)... 

In this paper, we have evaluated three different protocols for the preparation of AQ-enzyme film using choline and glucose oxidases and mixture of AQ 29D and AQ 55D (1 1). This AQ mixture is recommended by Eastman Kodak to increase the adherency of the film to a surface such as a platinum electrode (17). The values 29 and 55 represent the glass transition temperature of each polymer. Also, the main structural difference between the two polymers is that, in the case of AQ 55D, an aliphatic glycol moiety replaces the cycloaliphatic glycol moiety found in the AQ29 (17,18). 

Film Fabrication. The platinum electrode (0.28 cm area) was fabricated and cleaned as previously described (19). Thin films of AQ-enzyme were prepared by dissolving an amount of the enzyme, as indicated below, in 10 il of 1.5% AQ polymers solution at room temperature. Two aliquots of 5 il were deposited atop the platinum electrode and the first aliquot was allowed to dry before the second addition. This procedure corresponds to the first protocol. In addition, for the second protocol, 10 il of the 0.5% Nafion solution was casted atop the dried AQ-enzyme film and the methanol was allowed to evaporate at room temperature. The third protocol consisted in the deposition of 10 il of a 1% of AQ solution containing the enzyme, atop the platinum electrode followed by heating in an oven at 50°C during 30 min. In each case, 2 U of glucose oxidase were used. 

Figure 9. (A) The preparation of a nonordered polymeric layer of glucose oxidase electrically wired by ferrocene groups incorporated in the polymer film. (B) Cyclic voltammograms of the GOx/ferrocene-modified electrode in the absence (a) and presence (b) of glucose, 30 mM. Performed under argon, in phosphate buffer, pH 7 potential scan rate, 10 mV s. Inset calibration curve for the amperometric response to glucose at 0.35 V vs. SCE measured under N2(a) and air (b). Adapted from Ref. [90] with permission.

Depositing polymer membranes from solution is probably the oldest and simplest method for polymer deposition. Typically a solution of the polymer is deposited onto the transducer surface and allowed to dry. Although this technique is simple to practice, it is a fine art to perfect. Polymer concentration, solvent composition, amount of solution deposited, and solvent evaporation rate are all crucial parameters to control in order to obtain films that adhere well to the substrate, are uniform in thickness, and are free from defects. Gregg and Heller have solution cast redox-functional hydrogels containing glucose oxidase on an electrode to fabricate a glucose sensor [19]. 

The electron transfer rate can be improved by using an electrically conductive polymer like polypyrrole [16] as an interface between the electrode and the non-conductive polymer matrix bearing the glucose oxidase and the mediator. Even better results can be achieved in a copolymerization of enzyme and mediator into a polypyrrole film on top of a platinum electrode [17, 18]. Here, the mediator ferrocene is copolymerized with the pyrrole. 

An enzyme sensor for glucose fabricated by the electrochemical polymerization method is described. Electropolymerized films of 2,2 -bithiophene is used to immobilized glucose oxidase at platinum electrodes. The responses of the enzyme-polymer electrodes toward glucose are analyzed under various conditions. The relation between performance of the polymer-modified electrode and properties of the polymer formed is also discussed. 

The electropolymerlzatloa was done with the enzyme already Immobilized on the electrode surface, resulting in a complete glucose biosensor. The limiting factor is now the electropolymerized film and the length of time that common interferents can be screened from the electrode surface. The cross-linked glucose oxidase enzyme is stable for months cross-linked on RVC, with or without the polymer film. Although the polymer film does provide additional enzyme stability. 

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