Biomedical Engineering Reference
In-Depth Information
Fig. 4.18.
The principle of a biosensor based on electron transfer mediator
electrode, the reducing compounds, such as ascorbic acid and uric acid, in the
blood sample will be oxidized. Hence, these compounds usually influence the
sensor output when hydrogen peroxide is measured. That is why a method
that uses a nonnatural electron acceptor instead of oxygen attracts attention
(Fig. 4.18). This acceptor is called a mediator, since it mediates the electron
transfer reaction between the electrode and the enzyme, and electrochemical
biosensors using mediator are hardly affected by the dissolved oxygen concen-
tration. Hence, a sensor can be constructed to measure highly concentrated
glucose. Ferrocene derivatives [14], benzoquinone [15-17], and a phenazine
derivative [18] have been used for mediators.
Cass et al. [14] have shown that the ferrocene/ferricinium ion couple acts
as an effective mediator between reduced glucose oxidase and a graphite el-
ectrode. The incorporation of 1,1-dimethylferrocene into a graphite electrode
upon which glucose oxidase has been immobilized provides a glucose sensor
that has many of the features required for the analysis of clinical glucose
samples. Its performance depends on a number of features: the rapid rate
of electron transfer between the reduced enzyme and the ferriciniun ion; the
good electrochemical properties of ferrocene; and the low solubility of the
ferrocene, which results in the effective confinement of the mediator to the
electrode surface. The greater solubility of the ferricinium ion allows diffusion
between the immobilized enzyme and the electrode surface.
4.3.2
Biosensors Based on Redox Active Polymers
Although the properties of biosensors using a mediator seem to be excellent,
defects in the biosensors that use a mediator of low molecular weight have
been indicated recently. Mediator molecules are gradually released from the
electrode, even if the insoluble mediator is used. An insoluble mediator such
as ferrocene will be water-soluble in the oxidized form. Hence, the develop-
ment of a novel mediator immobilization method was required. Therefore, a
polymer that can mediate the electron transfer between the electrode and
GOx has been developed. Redox active polymers with an osmium complex
of pyridine and imidazole group have been reported [19-25]. Ferrocene-based
redox active polymers have also been developed [26-33].
Gregg and co-workers have reported biosensor applications of cross-linked
redox gels containing GOx based on polyvinylpyridine (PVP) with comple-
