Page 187 - 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

Microanalytical Assays 687

TABLE II Enzymes That Have Been Used in Commercial sands; thus the number analytes that might be measured
Biosensors by biosensors based on bioreceptors is somewhat limited.
K M (mol 1 −1 ) Substrate On the other hand, this class of cell regulators that inter-
act with bioreceptors may be very important biologically,
Alcohol EC 1.1.3.13 1 × 10 −2 Ethanol e.g., hormones.
dehydrogenase
Catalase EC 1.11.1.6 1 Sucrose, glucose
Choline EC 1.1.99.1 7 × 10 −3 Phospholipids IV. IMMOBILIZATION
dehydrogenase
Lactate EC 1.1.2.3 2 × 10 −3 Lactate Technologies for immobilizing proteins on surfaces that
dehydrogenase
have been developed over the last 25 years are particu-
Galactose oxidase EC 1.1.3.9 2 × 10 −1 Galactose
larly critical to the continued development of biosensors.
Glycerol kinase EC 2.7.1.30 1 × 10 −6 Glycerol
There a dozens of techniques for activating surfaces so as
Glucose oxidase EC 1.1.3.4 1 × 10 −2 Glucose
to be able to covalently bond proteins while maintaining
Lactate EC 1.1.1.27 2 × 10 −2 Lactate their biological activity. These methods have been used
dehydrogenase
Lactate oxidase EC 1.1.3.2 2 × 10 −2 Lactate routinely for manufacturing adsorbents for affinity chro-
Mutarotase EC 5.1.3.3 3 × 10 −2 Glucose matography, dip-stick type analytical methods, e.g., im-
munoassays, and immobilized enzymes for commercial
Sulfite oxidase EC 1.8.3.1 3 × 10 −5 Sulfite
catalysts.
Urate oxidase EC 1.7.3.3 2 × 10 −5 Uric acid
Techniques that are used to immobilize biological com-
ponents for use in sensors include entrapment and en-
capsulation covalent binding crosslinking and absorption
degree of specificity of antibodies. Some of the common these various methods have been developed for proteins
enzymes that have been used in sensor applications are and there is a wide variety of chemical techniques to carry
shown in Table II. out each of these methods.
Another class of biomolecules that can be used as recog-
nition elements in sensors are bioreceptors. These are V. DETECTOR ELEMENTS
proteineous species that normally are found in the mem-
branes of biological cells. Many of these proteins have The appropriate detector element depends on the type re-
been identified that have a role as signaling agents to pro- action that is being affected by the enzyme. For example,
vide information transfer between the two sides of a cell for an enzyme that involves an oxidation-reduction mech-
membrane—in other words, these molecules are the prim- anism, such as glucose oxidase, an electrochemical trans-
itive manifestation of senses (taste, touch, smell, vision) ducer would be a natural choice. If a substrate is utilized
for single cells. Membrane bioreceptors have been iden- that produces a color, then one can use optical techniques
tified with a responsiveness to many different substances to measure the extent of reaction, and if the enzymatic
that either excite cells or to regulate the cell’s behavior. reaction results in a change in pH, then one can use a pH
Some examples are given below. electrode to determine the extent of the reaction.
In addition to electrochemical and optical detectors,
Receptor Candidate analyte there have been a very wide variety of other kinds of de-
tector systems that have been employed in experimental
Nicotinic Organic phosphates biosensors as noted in Fig. 1, including surface wave de-
Adrenergic Propanolol tectors, which are essentially mass detectors, conductivity,
Serotonergic Imipramine
and thermal transducers. However, because of the need
GABA Barbiturates
for reliability, flexibility, and sensitivity, electrochemi-
Sodium channel DDT
cal transducers are usually the system of choice because
Estrogen Tamoxifern
of the extensive research and manufacturing experience
Auxin 2,4-D
available.
E colicins Aminobenzypurines
The basic behavior of the electrochemical detector is
shown in Fig. 9. Here the current response as a function
However, the number of bioreceptors that have been iden- of voltage applied is displayed. The rather nonlinear re-
tified, numbering into the hundreds, is much more limited sponse is typical of electrochemical systems. Through-
than the diversity of antibodies that number into the thou- out the most of the range of applied voltages, positive or