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

Fiber-Optic Chemical Sensors 817

sity of the immobilized dye. The concentration of CO 2 is
determined by measuring the pH of the buffer solution.
The second type of CO 2 fiber-optic chemical sensor is
constructed by using ion pairs consisting of a pH indicator
anion and an organic quaternary cation. First, a pH indi-
cator dye (DH) and a quaternary ammonium hydroxide
−
+
(Q OH ) are entrapped into a proton-impermeable but
CO 2 -permeable polymer membrane, which is then immo-
bilized onto the fiber’s surface. The mechanism of this
CO 2 sensor is based on the interaction between the dye
+
−
molecules (DH) and the quaternary cations (Q OH )to
+
form hydrated ion pairs (Q D · xH 2 O). The hydrated ion
−
pair is dissolved in the polymer, where it reacts with CO 2
according to the following reaction:
+ − + −
{Q D · xH 2 O}+ CO 2 ⇐⇒ {Q HCO · (x − 1)H 2 O}
3
FIGURE 12 Fiber-optic sensor for carbon dioxide with a buffer + DH. (19)
solution entrapped in a polymer gel at the fiber tip and covered
with a CO 2 -permeable membrane. The indicator dye becomes protonated and changes its
absorption (or emission) maximum. Such sensors can be
used for determining CO 2 in dry gases as well as in aque-
sensors are obtained by entrapping a bicarbonate buffer ous solutions.
along with an absorbance or fluorescence indicator into Fiber-optic chemical sensors for the detecting organic
a polymer gel matrix and subsequently covering the ma- vapors (such as methanol, chloroform, benzene, toluene,
trix by a CO 2 -permeable membrane at the fiber tip. A and nitroaromatics) are constructed by immobilizing a sol-
schematic arrangement of such CO 2 sensors is shown in vatochromic fluorescent dye into various polymers on the
Fig. 12. The sensor measures the pH of the bicarbonate fiber tip. Solvatochromic indicators change their fluores-
buffer solution, which is in equilibrium with CO 2 outside cent properties (such as intensity, emission wavelength
the membrane, i.e., maximum) as the polarity of the medium changes. For
membrane example, the solvatochromic indicator Nile red exhibits a
CO 2 ←→ CO 2 + H 2 O,
large shift in emission wavelength maximum with changes
CO 2 + H 2 O ⇐⇒ H 2 CO 3 , in local polarity. Typically, its absorption and/or emission
(15) spectra shift to higher wavelengths when exposed to sol-
−
H 2 CO 3 ⇐⇒ H + HCO ,
+
3 vents with increasing polarity. A single solvatochromic
HCO − ⇐⇒ H + CO . dye can be immobilized within a polymer matrix, hav-
+
2−
3 3
ing its own baseline polarity. The sorption of organic va-
These reactions are governed by the following equilibrium pors into the polymer changes the microenvironmental po-
constants: larity of the dye, which changes its emission/absorption
−
+
K 1 = [H ][HCO ]/[CO 2 ], (16) maximum. Different organic vapors change the polarity,
3
hydrophobicity, and swelling tendency of the polymer,
+

−
K 2 = [H ] CO 2− [HCO ]. (17)
3 3 generating different fluorescence responses of the immo-
bilized dye.
By considering the above two equilibria, the equation used
to relate the external [CO 2 ] to the internal [H ]is
+
c. Ion sensing. Several different schemes can be ap-
+ 2
+ 3
+
[H ] + N[H ] − (K 1 [CO 2 ] + K W )[H ] plied to fiber-optic chemical sensors for detecting ions
other than hydrogen. One approach is to design a system,
− 2K 1 K 2 [CO 2 ] = 0, (18)
similar to pH fiber-optic chemical sensors, in which a dye
where N is the bicarbonate ion concentration in the in- that selectively binds a metal ion of interest is immobilized
ternal solution, [CO 2 ] is the total analytical concentration in an ion-permeable polymer such as cellulose or a hydro-
of carbon dioxide, i.e., [CO 2 ] = [CO 2 ] (aq) + [H 2 CO 3 ], and gel at the tip of an optical fiber. The reaction between the
−
+
K W = [H ][OH ]. As carbon dioxide crosses the mem- dye and the ion changes the absorbance or fluorescence of
brane, the microenvironmental pH of the entrapped buffer such dyes. Absorbance or fluorescence intensity changes
changes, changing the absorbance or fluorescence inten- are measured as a function of ion concentration, but this