Impractical and Theoretical Considerations

The success of the enzyme electrode depends, in part, on the immobilization of the enzyme layer. The objective is to provide intimate contact between the enzyme and the sensing surface while maintaining (and even improving) the enzyme stability. Several physical and chemical schemes can thus be used to immobilize the enzyme onto the electrode. The simplest approach is to entrap a solution of the 

FIGURE 6-1 Enzyme electrode based on a biocatalytic layer immobilized on an electrode transducer. 

FIGURE 6-2 Steps in the preparation of an ainperometric enzyme electrode with simple enzyme immohilization hy trapping between an inner cellulose acetate and outer collagen membrane, cast on the electrode body. (Reproduced with permission from reference 1.) 

The immobilization procedure may alter the behavior of the enzyme (compared to its behavior in homogeneous solution). For example, the apparent parameters of an enzyme-catalyzed reaction (optimum temperature or pH, maximum velocity, etc.) may all be changed when an enzyme is immobilized. Improved stability may also accrue from the minimization of enzyme unfolding associated with the immobilization step. Overall, careful engineering of the enzyme microenvironment (on the surface) can be used to greatly enhance the sensor performance. More information on enzyme immobilization schemes can be found in several reviews (7,8). 

The response characteristics of enzyme electrodes depend on many variables, and an understanding of the theoretical basis of their function would help to improve their performance. Enzymatic reactions involving a single substrate can be formulated in a general way as 

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Impractical and Theoretical Considerations The operation of an enzyme electrode is illustrated in Figure 6-1. The immobilized enzyme layer is chosen to catalyze a reaction, which generates or consumes a detectable species ... 

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