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

826 Fiber-Optic Chemical Sensors

interest in chemical sensors. Effective process control is technologies have been developed. New fiber-optic chem-
a basic requirement for the optimal utilization of chemi- ical sensor systems are based on integrating technologies
cal and biological materials. Conventional analytical tech- from several different fields and disciplines including op-
niques for measuring industrial or biological materials tics, chemistry, biology, and mechanical, electrical, and
such as GC, HPLC, and flow injection analysis have sev- computer engineering. Recent advances in these fields
eral drawbacks such as price, large instrument size, inter- have supported the development of improved fiber-optic
ference by medium components, and drift. chemical sensors with capabilities that are superior to
Intrinsic fiber-optic chemical sensors are widely used current analytical methods. Technologies that influence
in industry and bioprocess control. Two basic approaches fiber-optic chemical sensor development include (a) new
are used in remote applications. In the first approach, the data acquisition and data analysis software, (b) improved
spectroscopic parameters are chosen such that the ana- technologies for the production and design of new sens-
lyte gives a unique signal compared with all other compo- ing materials through biological (e.g., recombinant DNA
nents in the sample. Examples of such approaches are as technologies) and chemical (e.g., molecular imprinting)
follows: (a) Fiber-optic remote fluorescence spectroscopy approaches, and (c) development of new materials. In this
is used to measure reduced nicotinamide adenine dinu- section novel fiber-optic chemical sensors systems based
cleotide (NADH) in bioreactors. The idea is to correlate on these new and advanced technologies are described.
the viability of a population of cells within the reactor
to the total amount of NADH present by monitoring the
A. Multianalyte Sensing
magnitude of NADH fluorescence. (b) Fiber-optic trans-
mission spectroscopy is used to measure the copper sul- A key strength of optical fibers is the high information ca-
fate concentration in an electroplating bath. (c) Fiber-optic pacity they carry (high bandwidth). The high bandwidth
Raman spectroscopy is used to monitor the temperature capabilities of optical fibers are extensively employed
and extent of curing in an industrial epoxy curing reac- for telecommunication applications and more recently for
tion (described in Section III.A.1). Raman spectroscopy fiber-optic chemical sensors. Many different wavelengths
is also used in the nuclear industry for detecting water in can propagate through the fiber simultaneously allowing
a sodium nitrate slurry and also has potential applications the transmission of multiple sensing signals arising from
in the power industry such as on-line monitoring of boiler multiple analytes.
water chemistry, on-line monitoring of corrosion and de- Multianalyte sensing is important for clinical, biolog-
posits, and in situ inspection of steam generators during ical, environmental, and industrial analysis in which si-
outages.Thesecondapproachisbasedoncorrelatingspec- multaneous detection of more than one analyte is re-
tral characteristics found over a range of wavelengths with quired. For example, measurement of pH, O 2 ,CO 2 , anti-
the parameter of interest. Typically, an entire spectrum, bodies, DNA sequences, antibiotics, viruses, and bacteria
such as an absorption or fluorescence spectrum, is col- in single blood samples can provide physicians with rapid
lected and information is extracted from each spectrum and comprehensive information about a patient’s medical
by a suitable data-processing algorithm. This technique is condition.
used in the agriculture industry to determine parameters Several approaches have been described for multian-
such as protein, water, and carbohydrate levels in grains. alyte fiber-optic chemical sensor construction. One ap-
In general, all the fiber-optic pH sensors as well as CO 2 proach involves the use of direct spectroscopy where
and NH 3 sensors can be used for monitoring and for indus- different wavelengths are transmitted through the opti-
trial process control. In particular, fiber-optic pH sensors cal fiber to the sample and the returning light signals
for measuring acidity and alkalinity are employed in in- are analyzed by advanced data analysis procedures (see
dustries such as manufacturing, photographic developers, also Section IV.C). Another approach, briefly described
and waste treatment. A fiber-optic CO 2 sensor described in Section IV.A, involves assembling several fibers each
above based on the pH-sensing mechanism has been used with a different immobilized indicator dye into one fiber
for on-line fermentation monitoring. It was also demon- bundle. Alternatively, imaging fibers can be used by im-
strated that imaging fiber-optic pH sensors could be used mobilizing discrete sensing regions, each containing a dif-
to monitor pH changes continuously during beer fermen- ferent indicator, at a precise location on the fiber’s distal
tation and to monitor localized corrosion. end (Fig. 20). A CCD detector is used to spatially resolve
the signal obtained from each sensing element. In this way,
as shown in Fig. 20, multiple analytes (O 2 , pH, and CO 2 )
V. RECENT DEVELOPMENTS are monitored using a single imaging optical fiber. Fur-
thermore, the use of an imaging fiber allows combined
Over the past several years, fiber-optic chemical sensors imaging and chemical sensing as will be described later
have received increasing attention because promising new in this section.