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

804 Fiber-Optic Chemical Sensors

biological molecule) that is immobilized to the optical of optical fibers for sensing applications was first sug-
fiber surface and can change its physicochemical gested in the mid-1960s. Since then, various fiber-optic
properties upon interaction with an analyte. chemical sensor types have been developed for detecting
Total internal reflection Reflection of light at the inter- numerous analytes. In the last decade, due to the rapidly
face between two materials of different refractive in- growing use of fiber optics for telecommunication appli-
dex. In optical fibers, the core refractive index is higher cations, new fiber-optic technologies have been developed
than the clad refractive index. When light is introduced resultinginhigh-qualityandinexpensiveopticalfibersthat
into the core, it is reflected at the core–clad interface. can be used for chemical sensing applications. Optical
fibers are remarkably strong, flexible, and durable. These
features and their nonelectrical nature make them highly
A CHEMICAL SENSOR is an analytical device that suitable for different industrial and environmental applica-
can measure the concentration of a specific chemical or tions where safe sensing in hazardous and harsh environ-
a group of chemicals in a sample of interest. The basic ments is needed (e.g., monitoring of chemicals in nuclear
structure of a chemical sensor includes (a) a sensing ma- plants or toxic gases in petrochemical plants). Fiber-optic
terial that selectively interacts with the analyte and (b) chemical sensors are also widely used in the clinical field
a transducer (e.g., electrochemical, optical, thermal, or since their small dimensions allow them to be used for
mass) that can transform this interaction into a measurable in vivo sensing thereby eliminating the need to procure
signal. This signal should be proportional to the magnitude samples. Furthermore, as described later in Section IV,
of the changes in the physicochemical properties associ- fiber-optic chemical sensors can be incorporated into op-
ated with the interaction between the sensing material and tical fiber bundles used for in vivo imaging (endoscopes)
the analyte. Ideally, chemical sensors should operate in a to provide both analytical information and imaging capa-
continuous and reversible manner. bilities. One significant advantage of employing optical
Fiber-optic chemical sensors are analytical devices in- fibers is that multiple optical signals can be transmitted
corporating optical fibers as part as their optical transduc- and measured simultaneously, thereby offering multiplex-
ing system. Optical fibers are small and flexible “wires” ing capabilities.
made out of glass or plastic that can transmit light sig- Optical fibers and the instrumentation used in fiber-
nals, with minimal loss, for long distances. The signals optic chemical sensors are described in Section II. We
generated by the sensing materials, which are usually im- describe how different optical phenomena, generated by
mobilized to the fiber surface, are transmitted through the different sensing mechanisms, can be applied to opti-
optical fibers and can be measured by using different opti- cal fibers to measure analytical signals. These sensing
cal methods such as absorption, fluorescence, and Raman mechanisms are described in Section III. Section IV re-
spectroscopy. Fiber-optic chemical sensors can be used for views several fiber-optic chemical sensor analytical appli-
remote analytical measurements in applications including cations in the clinical, industrial, and environmental fields
clinical, environmental, and industrial process monitor- and Section V reviews recent developments in fiber-optic
ing.Severalfiber-opticchemicalsensorsarecommercially chemical sensors.
available and it is expected that recent developments in
optical technologies and the research efforts to use these
technologies for fiber-optic sensor development will lead
to a number of commercially available fiber-optic chemi- II. FUNDAMENTAL PRINCIPLES OF
cal sensors for various applications. FIBER-OPTIC CHEMICAL SENSORS

The basic design of a fiber-optic chemical sensor system
I. INTRODUCTION is shown in Fig. 1. The fiber-optic chemical sensor’s main
components are (a) a light source, (b) optical fibers to
Fiber-optic chemical sensors are composed of a sensing both transmit the light and act as the substrate for (c) the
material and a transducer. The transducer converts the sensing material, and (d) a detector to measure the output
recognition and sensing events obtained by the sensing light signal. Usually computers or microprocessors con-
materials into a response such as an optical signal. Opti- trol the fiber-optic chemical sensor instrumentation and
cal measurements can provide rapid, sensitive, and nonde- are employed to analyze the output signals.
structive analysis of many important compounds. In fiber- In this section, optical fibers and their basic char-
optic chemical sensors, optical fibers are used to transmit acteristics are described. Fiber-optic chemical sensor
the optical signal to the measurement device, enabling a instrumentation and the optical phenomena employed are
remote detection of the analyte in the sample. The use also described.