Page 110 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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

Fiber-Optic Chemical Sensors 823

can be easily extracted from water, wastewater, or clinical Fiber-optic chemical sensors were originally used to
samples and the presence of pathogenic microorganisms measure oxygen saturation in the blood. The oxygen car-
can be determined by the sensor. In general, such DNA- rier in the blood is hemoglobin (Hb) and the blood satura-
sensing schemes are rarely conducted using a single probe tion is the ratio (as a percentage) between the oxygen that
sequence but are usually performed with an array of hun- is actually carried by Hb and the Hb’s maximum carrying
dreds to thousands of probes. capacity. The measurement is based on the large differ-
ence in the light absorption of Hb and OxyHb (oxyhe-
moglobin, i.e., oxygen bound to Hb) at λ = 660 nm. Typ-
IV. APPLICATIONS OF FIBER-OPTIC ically the light is delivered through two bifurcated fibers at
CHEMICAL SENSORS two different wavelengths, 660 nm (where Hb absorption
is higher than that of OxyHb) and 805 nm (where there is
a small difference in absorption between Hb and OxyHb);
A. Clinical Applications
the absorption is measured and the blood saturation is
Clinical analytical devices are important for various di- calculated.
agnostics applications since the use of these devices can Fiber-optic chemical sensors have been developed for
directly lead to more efficient and effective medical treat- measuring all three blood gas parameters pH, PO 2 , and
ment. Most diagnostic tests are performed in a centralized PCO 2 . These parameters are used to determine the effi-
laboratory. Samples must be collected with the attendant cacy of gas exchange within tissue and are crucial in sur-
transport, storage, and chain-of-custody issues. The re- gical procedures such as heart bypass surgery and critical
mote location of the laboratory delays the medical diag- care monitoring for patients with compromised respira-
nosis. For these reasons, analytical devices such as sensors tory conditions. The sensing mechanism is based on dyes
that can be located near the patient’s bed or even inside the that change their fluorescence intensities as a function of
patient’s body are of great utility. In recent years, fiber- analyte concentration (see Section III.B.1). Several sen-
optic chemical sensors have been developed for both in sor configurations have been developed for intravascular
vivo diagnostics (monitoring physical and chemical pa- measurement including one that incorporates the three dif-
rameters inside the body) and for clinical sample analy- ferent sensors into a single device. At present these sensors
sis at the patient bedside (i.e., blood or urine tests). The have not been implemented because of blood compatibil-
principal advantage of fiber-optic chemical sensors over ity problems in which a thrombus (clot) forms around the
electrochemical technologies is that no electrical current sensor tip and affects the measurement accuracy.
is involved in the measurement. Thus, the measurement Several pH in vivo fiber-optic chemical sensors for other
inside the body is much safer, and analysis at the patient’s clinical applications have been developed. These include
bedside does not affect other electrical medical devices. pH monitoring in the stomach and the detection and ex-
amination of malignant tumors. Both sensors are based on
pH-sensitive fluorescent dyes. In gastric sensors, the dyes
1. Fiber-Optic Chemical Sensors
are immobilized on the fiber tip and the emitted light inten-
for In Vivo Analysis
sities are correlated to pH changes. Since a wide pH range
In vivo analytical devices ideally should be capable of (1–8) is monitored, there is a need to use several differ-
monitoring several different physiological parameters si- ent dyes (each for a different pH range). pH sensors have
multaneously without interfering with an ongoing medical also been used for locating suspicious tissue and deter-
procedure, such as surgery. The devices should be biocom- mining if malignancy is present. This oncological sensor
patible, simple to implement and operate, and highly reli- is based on the observation that malignant tumors can in-
able and safe. Fiber-optic chemical sensors can meet most duce a decrease in their microenvironment pH. Thus, by
of these requirements since the optical fibers are small inserting the optical fiber into the tissue and then injecting
(fewhundredmicrometersindiameter),flexible,nontoxic, a nontoxic pH indicator (fluorescein based), it is possible
and chemically inert. Optical fibers have already proven to identify and map malignant tumor locations.
to be valuable for in vivo clinical applications such as An interesting fiber-optic chemical sensor device for
endoscopic procedures and laser power transmission for in vivo gastric diagnostics is the Bilitec 2000 (Medtronic
surgical applications. Synectics AB). This device is used to measure the biliru-
Typicalfiber-opticchemicalsensorsforinvivomonitor- bin concentration in the stomach and the esophagus. The
ing are constructed from optical fibers that are connected bilirubin concentrations are related to bile-containing re-
to an external compact unit containing the light source and flux in these organs and can reveal several pathological
the detector, as shown in Fig. 18. The optical fiber’s distal conditions such as gastric ulcers and gastric cancer. The
end is inserted into the patient through a catheter. device, shown in Fig. 18, consists of two light-emitting