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Encyclopedia of Physical Science and Technology EN005F-954 June 15, 2001 20:48

824 Fiber-Optic Chemical Sensors

semicontinuously without requiring blood samples to be
taken from the patient.
Fiber-opticbiosensorcanbepotentiallyusedforclinical
sample analysis. As described above, by using enzymes
and antibodies immobilized on the fiber tip or around the
fiber core (evanescent field), different fiber-optic biosen-
sors have been developed. In most cases, the measurement
is based on changes in fluorescence intensity. The target
analytes include glucose, cholesterol, enzymes, antibod-
ies, bacteria, and viruses. Measurements are usually fast
and simple, and in many devices, the probe is disposable;
however, the instability of the biological recognition ma-
FIGURE 18 Bilitec 2000 fiber-optic chemical sensor for in vivo terials reduces the sensor lifetime.
gastric diagnostics. (a) The external unit and the optical fibers.
(b) The optical fiber’s distal end that is inserted into the body. Recently, fiber-optic biosensors for detecting DNA se-
[Reproduced with permission from Medtronic Synectics AB.] quences have been reported. These fiber-optic biosensors
can be used for detecting pathogenic microorganisms and
for identifying defective genes. The sensors are usually
diodes that emit light at λ = 465 nm (bilrubin absorption)
designed in an array format (Fig. 19) allowing the simul-
and 570 nm (reference). The light is transmitted by the
taneous analysis of hundreds or thousands of samples.
fiber bundles to the probe (miniaturized spectrophotomet-
Although just recently developed, these sensors will un-
riccell)atthefiber’sdistalend.Bilirubin’slightabsorption
doubtedly revolutionize the clinical diagnostics field.
is measured by the detector and bilirubin concentrations
are calculated.
Several clinical fiber-optic chemical sensors are com- B. Environmental Fiber-Optic
mercially available and others are in different stages of Chemical Sensors
development. The demand for in vivo fiber-optic chemi-
Both short- and long-term harmful effects of toxic chemi-
cal sensors for monitoring important analytes such as glu-
cal accumulation in the environment have become appar-
cose, potassium, urea, lactate, and some enzymes has led
ent in the last few decades. In order to reduce the envi-
to concerted research efforts in this field.
ronmental damage caused by these pollutants, regulations
were set that either restricted or completely prohibited the
2. Clinical Fiber-Optic Chemical Sensors
release of environmentally hazardous chemicals into the
for In Situ Sample Analysis
environment. The efficient enforcement of these regula-
Fiber-optic chemical sensors can be located at the patient’s tions is highly dependent on sensitive and reliable ana-
bedside or even in the patient’s home for self-use. Such lytical tools for environmental monitoring. These analyt-
sensors are designed as compact, simple–to-use devices. ical tools ideally should be sensitive, simple to construct,
While these sensors employ essentially the same sensing portable, and suitable for continuous field (i.e., in situ)
chemistry as with in vivo sensors, the overall sensor design measurements. Fiber-optic chemical sensor systems can
can be much simpler and a wider range of materials may be meet most of these requirements since they can be minia-
used for sensor fabrication. Fiber-optic chemical sensors turized and integrated into portable analytical devices that
for blood gases (PO 2 , PCO 2 , and pH) are commercially can offer high specificity and sensitivity, fast response,
available. Measurements are based on the use of immobi- mechanical stability, and low cost. Furthermore, by us-
lized fluorescent dyes. In one device, the dyes are incor- ing long optical fibers, the sensors can be used for remote
porated into a disposable apparatus that is inserted into an analytical monitoring.
extracorporeal blood circuit on one side and connected to Most commercially available environmental fiber-optic
a fiber bundle on the other. These sensors are mainly used chemical sensors are for pH and oxygen monitoring in wa-
to monitor blood gases during open heart surgery. ter and wastewater. The monitoring of unusual changes in
Another approach is to measure pH and blood gases us- oxygen and pH can be used as an indirect indication of the
ing a paracorporeal measurement at the patient’s bedside. presence of pollutants. These sensors are based on fluo-
The sensors are three fluorescent-sensing materials re- rescent dyes as the sensing material. The optical fiber and
sponsive to each of the blood gases. The sensors are placed the sensing elements are covered with a metal jacket that
into an external tube connected to an arterial blood line. provides immunity from harsh environmental conditions.
Blood samples are periodically and automatically pumped Environmental sensors capable of detecting specific
into the tube, analyzed by the sensors, and then returned pollutants such as volatile organic compounds and heavy
to the blood line. In this way, the blood can be monitored metals (two of the largest classes of chemical pollutants)