Page 192 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

P1: FMX Final Pages
Encyclopedia of Physical Science and Technology EN009J-69 July 19, 2001 22:50

692 Microanalytical Assays

attaches to the surface can be as low as 10 −12 g. Surface
acoustic wave detectors can be used as well.

VIII. SENSOR DYNAMICS

The response time of sensors to changes in concentration
of an analyte depends on the response rate of the various
components of a sensor. Usually, diffusional processes are
the limiting factor; these include diffusion of the analyte to
the surface of the sensor (dependent on external mixing),
diffusion through membranes, and diffusion through the
various regions of the sensor structure. Since diffusion lag
times increase with the square of distance, it is imperative
to maintain the active layers of the sensor to dimensions
on the order of tenths of a millimeter or less. Enzyme re-
actions can usually be made nonlimiting if necessary by
increasing the amount of enzyme in the system. However,
the rate of dissociation of antibody–analyte complexes de-
creases directly with increasing binding affinity. For high
levels of sensitivity, high binding constants are needed.
For example, to measure an analyte at a concentration of
FIGURE 15 The principle of biosensors based on surface plas-
nanomoles, a binding constant of the order of nanomoles
mon resonance. The angle of total internal reflection is a function
of the refractive index change at the interface between the metal- is required. The dissociation rate for such antibodies is on
ized surface and the sample medium. The lower figure shows that the order of tens of minutes.
a large change the angle of maximum reflection occurs between Some of the considerations that are important in deter-
air and water. And in water a minor but measurable effect is seen mining the structure of sensors for use in analytical assays
when an antibody is adsorbed on the surface.
include the following:
in that the selectivity could be low if materials not re-
lated to the analyte can indiscriminately absorb to the Sensitivity: Sensitivity relates to the lowest concentration
surface. that the system can reliably detect. Table III shows that
Another principal that has been evaluated for the con- the concentration levels must be able to measure varies
struction of optically based sensors is the use of chemi- quite greatly among different biochemicals that are im-
luminescence. In these cases an enzyme system specific portant in the body. The biochemicals in blood can have
for the analyte are coupled to reactions that produce light average concentration ranges from milligrams per cu-
through chemiluminescence. In principle, systems of this bic centimeter to nanograms per cubic centimeter. The
type could be very sensitive, first of all due to the amplifi- wide range of concentration of different biochemicals
cationfactorofenzymereactions,andsecondarilybecause that are present in blood presents a technical challenge
fluorescence measurements are among the most sensitive to the measurement of several analytes simultaneously
of optical techniques. in the same detection system.
Dynamic range: In addition to sensitivity of an assay sys-
tem, the range of sensitivity of the device is another
VII. OTHER DETECTORS important consideration. The dynamic range is usu-
ally defined as the ratio of highest concentration to the
One can also measure a temperature produced by the lowest concentration that a particular technique can re-
chemical reaction if the systems is quite well insolated liably measure. For many analytical methods based on
and temperature differences are small as 10 −6 ◦ C can be biosensors, the dynamic range is rarely more than a
detectedwithsensitivebridgetechniquesandthiscanmea- factor of about 10. Thus for any particular application,
sure materials as low as 10 −5 molar. the system needs to be engineered to achieve dynamic
Another type of detection device is based on oscillating range that covers the normal variation in concentration
quartz crystals. These types of devices are very similar for that analyte. For example, for the measurement of
to the systems that are used inside of electronic watches. blood glucose the normal level is about 100 mg/dl and
The limit of detection with these systems where there is the dynamic range should be between 50 and 200 mg/dl
a frequency change based on the amount of material that and somewhat higher for diabetics. For systems that use