Biomedical Engineering Reference
In-Depth Information
The number of analytes that can be measured with biosensors can be increased signifi-
cantly by adding biologically specific mediators (reagents that either undergo reactions or
act as catalysts) to the semipermeable membrane structure. Several biosensors that have
been constructed and used mainly for research applications have different enzymes and
bacteria as the primary sensing elements. Although these biosensors have been used suc-
cessfully in vitro to demonstrate unique medical and industrial applications, further techni-
cal improvements are necessary to make these sensors robust and reliable enough to fulfill
the demanding requirements of routine analytical and clinical applications. Examples of
some interesting sensor designs are given in the following sections.
10.5.1 Enzyme-Based Biosensors
Enzymes are the most widely used biological sensing element in the fabrication of vari-
ous biosensors. Enzymes constitute a group of more than 2,000 proteins having so-called
biocatalytic properties. These properties give the enzymes the unique and powerful ability
to accelerate chemical reactions inside biological cells. Most enzymes react only with spe-
cific substrates even though they may be contained in a complicated mixture with other
substances. It is important to keep in mind, however, that soluble enzymes are very sensi-
tive both to temperature and pH variations, and they can be inactivated by many chemical
inhibitors. For practical biosensor applications, these enzymes are normally immobilized by
insolubilizing the free enzymes via entrapment into an inert and stable matrix such as
starch gel, silicon rubber, or polyacrylamide. This process is important to ensure that the
enzyme retains its catalytic properties and can be reusable.
The action of specific enzymes can be utilized to construct a range of different biosensors.
A typical example of an enzyme-based sensor is a glucose sensor that uses the enzyme glu-
cose oxidase. Glucose plays an important role in metabolic processes. In patients suffering
from diabetes mellitus, the pancreas does not produce sufficient amounts of insulin to con-
trol adequately the level of glucose in their blood. Therefore, to manage the disease, these
patients must monitor and regulate their blood glucose level on a regular basis by medica-
tion and insulin injections. Currently available glucose sensors are based on an immobilized
enzyme, such as glucose oxidase, that acts as a catalyst. Glucose is detected by measuring
electrochemically either the amount of gluconic acid or hydrogen peroxide (H
2
O
2
) pro-
duced or by measuring the amount of oxygen consumed, according to the following chem-
ical reaction:
glucose oxidase
Glucose
þ
O
2
þ
H
2
O
!
gluconic acid
þ
H
2
O
2
O
2
sensor and is shown in Figure 10.34. Glucose and
oxygen enter through the outside membrane to allow glucose to interact with the glucose
oxidase enzyme. The remaining oxygen penetrates through the second oxygen-permeable
membrane and is measured by the oxygen electrode.
Biocatalytic enzyme-based sensors generally consist of an electrochemical gas-sensitive
transducer or an ion-selective electrode with an enzyme immobilized in or on a membrane
that serves as the biological mediator. The analyte diffuses from the bulk sample solution
into the biocatalytic layer, where an enzymatic reaction takes place. The electroactive
A glucose sensor is similar to a
p
