Biomedical Engineering Reference
In-Depth Information
without the denaturation of the enzyme. Additionally, enzyme immobilization (dis-
cussed in Section 9.5.2) is important in terms of biosensor operational stability
and long-term use. Since this factor is, to some degree, a function of the strategy
used, the choice of immobilization technique is critical. A large number of reports,
as reviewed elsewhere, can be found involving enzymes physically or chemically
(covalently) entrapped on the transducer.
Since neither mediator nor enzyme must be added, this design facilitates re-
peated measurements. Sensor use for multiple analyses minimizes cost pressures on
sensor design. It also follows that such a sensor could allow for continuous analyte
monitoring. The redox enzyme and wire are immobilized by cross-linking to form
three-dimensional redox epoxy hydrogels. A large fraction of enzymes bound in the
3D redox epoxy gel are wired to the electrode. These wires provide a general ap-
proach to third-generation biosensors, sensitive to glucose and other components
such as sarcosine, L-lactate, D-amino acids, L-glycerophosphate, cellobiose, and
choline.
9.4.2 Optical Transducers
Optical transducers form a group that directly display features, making them ad-
vantageous over other systems such as electrochemical, mass-sensitive, thermal,
acoustic, or other transducers. Generally, these methods use a substrate that turns
into a different color or generates a color due to a reaction with the analyte. The
developed color is assessed by a number of techniques such as absorption spectro-
copy (from the UV to the deep infrared in EM spectrum), conventional fluorescence
(and phosphorescence) spectroscopy, bioluminescence, chemiluminescence, inter-
nal reflection spectroscopy (evanescent wave technology), and laser light scattering
methods.
9.4.2.1 Absorption-Based Techniques
The absorption-based techniques involve the well colorimetric test strips and en-
zyme-linked immunosorbent assays (ELISAs) routinely used in antigen-antibody in-
teractions. Colorimetric strips are disposable, single-use cellulose pads impregnated
with sensing elements and reagents. An example is a glucose biosensor for whole-
blood monitoring in diabetes control. The reactions utilized in the production of
electrochemical signals are used. However, produced peroxide in the reaction is
coupled to a chromogen (for example,
o
-toluidine or 3,3',5,5'-tetramethylbenzi-
dine) rather than to an electrode, which is converted to a dye in the presence of
horseradish peroxidase (HRP).
HRP
Chromogen (2H)
+
H O
→+
dye
2H O
22
2
Dyed strips are useful when only positive or negative signals are necessary. The
strips include glucose oxidase, HRP, and a chromogen. A wide variety of test strips
involving various enzymes are commercially available. However, spectrophotome-
ters are used in conjunction with ELISA for the quantification of the concentration
of a specific molecule (e.g., a hormone or drug) in a fluid such as serum or urine.
