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

814 Fiber-Optic Chemical Sensors

FIGURE 11 A fiber-optic pH sensor employing reversible protonation of dye (Dy ).
−

long-path or multipass cell and a well-designed optoelec- and as a result, the absorption, reflection, or fluorescence
tronic system are required reliably to detect low levels of properties of the indicator change. A schematic of a typ-
gases. Although IR fibers can be used, they are fragile and ical pH sensor is shown in Fig. 11. Some common dyes
expensive compared to conventional silica fibers. Direct used for pH sensors are listed in Table I.
absorption methods are mostly used to monitor low con- The fiber-optic, absorbance-based pH sensor monitors
centrations of gases such as nitrogen dioxide, methane, the absorbance change of the immobilized dye as a func-
and hydrogen. tion of the pH change of the analytical medium. The Beer–
Lambert law can be applied to an absorbance based pH
sensor. The concentration of the dye [Dy] is related to the
absorbance according to the following equation:
B. Extrinsic Sensing Mechanism—Indicator
Chemistry A = log(I 0 /I) = ε [Dy]l, (5)
Extrinsic fiber-optic chemical sensors are constructed by
where I 0 and I are the intensity of transmitted light in
immobilizing indicator chemistries on the fiber tip or on
the absence and presence of dye, respectively, l is the
the annulus of the fiber. In this section, the mechanism of
effective path length, and ε is the molar absorption co-
the chemical and biological reagents employed for sensing
efficient. Usually the concentration of the base form of a
are described as well as methods for immobilizing sensing
weak acid indicator is measured, which is a measure of
materials.
the pH-dependent degree of ionization of the indicator.
The dissociation of a dye and the acidity constant K a are
expressed in the following equations, respectively:
1. Chemical Sensing Reagent—Optrodes
+
−
a. pH sensing. Among all types of sensors, the pH Dy − H = Dy + H , (6)
sensor is the most widely developed and has received the −
[Dy ][H ]
+
most attention because of the importance of pH measure- K a = . (7)
[Dy − H]
ments in scientific research as well as various physiolog-
ical, environmental, and industrial processes. Fiber-optic Combining the equilibrium expression for the acid dis-
pH sensors offer several advantages compared to stan-
sociation of a dye with the expressions for pH and pK a
dard glass electrodes for pH measurement, including their gives
immunity from electromagnetic interference, their small
size, their ability to perform remote sensing, and their ap- Measured absorbance (A)
plication to both in vivo and in situ measurements. Optical Absorbance of the total dye in base form
sensors also possess some disadvantages, such as photo-
1
bleachingofthedye,leachingofdyefromtheimmobilized = (pK a −pH) (8)
10 + 1
surface, and interference by ambient light. The pH mea-
surement is based on pH-dependent changes of the optical This relationship results in a sigmoidal plot of ab-
properties of an indicator dye immobilized on the fiber- sorbance versus pH centered on the pK a value. In com-
optic surface. The indicator reversibly reacts with protons, parison to fluorescence, the absorbance method is simple