Page 183 - 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 683

of the detectors that have been used in commercial devices
are electrochemical or optical in nature.
Perhaps the most unique component of a biosensor is
the biological system that is utilized to identify specific
molecules of interest in solutions of complex mixtures.
The biological element of course is primarily responsible
for the selectivity of biosensors. There are many differ-
ent types of biological recognition systems that have been
explored for sensors, ranging from the molecular scale—
e.g., bioreceptors, enzymes, and antibodies—to cellular
structures like mitochondria, and even immobilized whole
cells and tissues. However, to date for practical reasons
most commercially feasible biosensors have primarily uti-
lized enzymes, and to a lesser extent antibodies.
Packaging is to some extent one of the most critical ele-
ments of a biosensor from the point of view of practicality.
There have been hundreds of demonstrations of biosensor
concepts using different types of biological recognition

FIGURE 4 Capillary electrophoresis provides a very efficient and
effective method to identify the components of complex mixtures.
Microfabrication of multiple capillary channels on a chip allows
rapid identification of mixtures because of the small distances
involved, approximately 50 mm. This illustration shows the laser-
excited confocal-fluorescence scanner that is used to determine
the amounts of material passing the detection zone as a function
of time. [From Woolley et al. (1977). Anal. Chem. 69, 2183–2184.
Reprinted with permission of the American Chemical Society,
Washington, DC.]

II. BIOSENSORS

As illustrated in Fig. 1, there are three primary compo-
nents of a biosensor: (1) the detector element, (2) the bi-
ological element, and (3) membranes used to separate the
various structural elements of the sensor. The detector el-
ement performs the task of providing a signal related to
the recognition event—that is, the result of the interaction
of the analyte to be measured with the biological recogni-
tion molecule. The detector translates what is essentially
a chemical interaction to some type of physical signal that
can be manipulated by a variety of a electronic or optical FIGURE 5 This figure illustrates that the entire analysis of mixture
techniques to finally produce an electrical output that is of polynucleotide fragments of from 67 to 622 base pairs can be
related to the amount of the analyte of interest. accomplished in 160 sec. Also, the reproducibility from channel
to channel allows the easy discrimination of minor changes in
The detector element is primarily responsible for the
composition between samples. [From Woolley et al. (1997). Anal.
sensitivity of the device. Some of the very many detectors Chem. 69, 2183–2184. Reprinted with permission of the American
that have been tested are shown in Fig. 1. However, most Chemical Society, Washington, DC.]