Page 186 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN009J-69 July 19, 2001 22:50

686 Microanalytical Assays

we limit discussion to three classes of biomolecules: en-
zymes, binding proteins (there are a number of binding
substances that can be utilized in this fashion), and anti-
bodies. Antibodies are large proteins (MW about 150,000)
naturally found in blood, which are produced by the
immune system for protection of an individual against
foreign substances. Fortunately with new techniques in
molecular biology, antibodies can be made to virtually
any organic substance with a molecular weight greater
than several hundred.
Enzymes are the class of bioactive agents that are
the most readily adapted to biosensors because many
enzymes are commercially available in large quantities
and have been characterized in a great deal of detail.
Antibodies would appear to be the most versatile type
of biomolecule that can be employed in sensors, since
they can be developed for virtually any analyte, but they
are costly substances at this time. The most difficult
kind biological substance to use would be tissues them-
selves because methods of sustaining stability are not well
developed.
Enzymes are the choice for biosensors directed toward
analytes that fall into the category of biochemicals that
FIGURE 7 Components of the Clark glucose biosensor. This first
are metabolic intermediates. Because the metabolic en-
portable biosensor is based on the measurement of the consump-
tion of molecular oxygen due to the oxidation of glucose to gluconic zymes (e.g., in glycolysis or protein synthesis) already
acid as catalyzed by the enzyme glucose oxidase. Both glucose exist in nature, and thus are a fruitful choice of materials
and oxygen diffuse into the sensor through a dialysis membrane. that adapted for use in biosensors. Also, since there is vast
The first compartment encountered contains the enzymes that technology associated with enzymes, usually one can find
cause the oxidation of glucose with a simultaneous reduction in
the ambient oxygen concentration. The second compartment con- a commercial source for an enzyme for nearly any organic
tains a polarographic electrolytic oxygen measuring sensor that material. On the other hand, enzymes vary widely in their
results in a current. Due to the consumption of oxygen in the first degree of specificity as compared with the relatively high
chamber, the current produced is inversely related to the glucose
in the external fluid.

different immobilized enzyme membranes, YSI has pro-
duced a general sensor configuration to measure many dif-
ferent analytes. Some of the analytes that can be measured
utilizing this enzyme biosensor are choline, D-glucose,
dextrose, ethanol, galactose, hydrogen peroxide, lactose,
L-glutamate, L-glutamine, L-lactic acid, methanol, and su-
crose. This illustrates the range of selectivity that is ob-
tainable using enzymes as the recognition element.

III. BIOLOGICAL RECOGNITION ELEMENTS
FIGURE 8 YSI Corporation manufactures a complete line of
Figure 1 lists many other biological systems that could be enzyme-based biosensors that follow from Clark’s invention. By
utilized as the recognition element in biosensors. Gener- providing kits containing different enzymes immobilized on a
membrane substrate, this generic approach allows a single de-
ally speaking, one can select systems starting at the very
tector device (a polarographic electrode) to be used for a number
simple molecular level and proceed through to much more of analytes. In this system the electrode measures the hydrogen
complex systems, such as whole cells or even tissues. Here peroxide generated by the enzyme reaction.