Page 192 - Analytical Electrochemistry 2d Ed - Jospeh Wang

6-1 ELECTROCHEMICAL BIOSENSORS 177

can be monitored amperometrically at the platinum surface:

electrode
H O ! O ‡ 2H ‡ 2e 6-5†
‡
2 2 2
The multilayer membrane coverage (Figure 6-4) improves the relative surface
availability of oxygen (by decreasing the ratio of glucose/oxygen ¯ux) and excludes
potential interferences (common at the potentials used for detecting the peroxide
product). The enzymatic reaction can also he followed by monitoring the consump-
tion of the oxygen cofactor.
Further improvements can be achieved by replacing the oxygen with a non-
physiological (synthetic) electron acceptor, which is able to shuttle electrons from
the ¯avin redox center of the enzyme to the surface of the working electrode.
Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer
electrons to conventional electrodes because their redox center is surrounded by a
thick protein layer. This insulating shell introduces a spatial separation of the
electron donor±acceptor pair, and hence an intrinsic barrier to direct electron
transfer, in accordance with the distance dependence of the electron transfer rate
(11):

K ˆ 10 e e 6-6†
13 0:91 d 3† ‰ DG‡l†=4RTlŠ
et
where DG and l correspond to the free and reorganization energies accompanying
the electron transfer, respectively, and d is the actual electron transfer distance.
As a result of using arti®cial (diffusional) electron-carrying mediators, measure-
ments become insensitive to oxygen ¯uctuations and can be carried out at lower
potentials that do not provoke interfering reactions from coexisting electroactive
species (Figure 6-5). Many organic and organometallic redox compounds have been
considered for this role of enzyme mediator. Some common examples are displayed
in Figure 6-6. In particular, ferrocene derivatives (e.g., Figure 6-6a) have been very
successful for shuttling electrons from glucose oxidase to the electrode by the
following scheme:

Glucose ‡ GOx ! gluconic acid ‡ GOx 6-7†
ox† red†
GOx ‡ 2M ! GOx ‡ 2M ‡ 2H ‡ 6-8†
red† ox† ox† red†
2M ! 2M ‡ 2e 6-9†
red† ox†
where M and M are the oxidized and reduced forms of the mediator. This
ox† red†
chemistry has led to the development of a pen-sized meter for personal glucose
monitoring in a single drop of blood. Such rapid ( 30 s) self-testing assays
commonly rely on a chronoapmerometric operation (e.g., Example 6-1). The
single-use disposable strips used with this device are made of poly(vinyl chloride)
and a screen-printed carbon electrode containing a mixture of glucose oxidase and