Page 189 - Analytical Electrochemistry 2d Ed - Jospeh Wang

174 ELECTROCHEMICAL SENSORS

FIGURE 6-2 Steps in the preparation of an amperometric enzyme electrode with simple
enzyme immobilization by 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 immobiliza-
tion 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 formu-
lated in a general way as

k 1 k 2
E ‡ S „ ES ! E ‡ P 6-2†
k 1

In this mechanism, the substrate S combines with the enzyme E to form an
intermediate complex ES, which subsequently breaks down into products P and