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

816 Fiber-Optic Chemical Sensors

polymers, e.g., polyaniline, substituted polyaniline, and ports used for immobilizing these indicators are silicone
polythiophene, vary in color as a function of pH. A thin rubber, plasticized PVC, polystyrene, poly(hydroxyethyl
coating of a conducting polymer on the end of the fiber methacrylate) (pHEMA), or porous glass. The sensitivity
surface can be used to detect the pH of a solution. A of the sensor depends on the dye interaction with oxygen
fiber-optic pH sensor has been developed based on ab- and the gas permeability of the polymer film. Sensor se-
sorbance changes of the polymers both in the visible and lectivity is provided both by the molecular specificity of
near-IR regions. These pH sensors have the advantage that the quenching phenomenon and the molecular restrictions
the polymers can be used over a wide pH range since due to selective permeability of the polymer membrane.
they are essentially electrolytes with multiple pK a val- Oxygen affects not only the intensity of the indicator
ues. These polymers have certain disadvantages such as fluorescence, but also its decay time. Organometallic dyes
interference from other ions and the need for recondition- have longer fluorescence decay times and can be used to
ing (with HCl) before each measurement, which is nec- prepare oxygen sensors based on the measurement of this
essary to nullify the conformational changes occurring in decay time. In comparison to fluorescence-intensity-based
the polymer on pH changes. sensor types, decay-time-based sensors have less signal
Fiber-optic chemical sensors for different analytes can drift due to leaching or bleaching because the decay time
be developed based on pH sensors. Gases such as CO 2 is not dependent on fluorophore concentration.
and NH 3 react with water and change the pH of a solution Fiber-optic chemical sensors for acidic or basic gases
when solvated, which can be detected by a pH sensor. Gas such as ammonia, carbon dioxide, hydrogen cyanide, and
fiber-optic chemical sensors will be discussed in the next nitrogen oxide can be constructed by coupling simple
section. Any chemical or biological species that produces acid–base chemistry with pH sensors. First, pH-sensitive
either acids or bases during a chemical or enzymatic reac- dyes or indicators are immobilized using polymers or sol–
tion can be detected by measuring the pH of the medium. gel glasses and subsequently covered by a gas-permeable
membrane on the distal end of a fiber. The gas-permeable
b. Gas and vapor sensing. Gas sensing fiber-optic membrane separates the sample solution from the immo-
chemical sensors are mainly based on fluorescence bilized dye. The acidic or basic gases cross the membrane,
quenching and acid–base chemistry. Optical oxygen enter into the indicator layer, and undergo proton trans-
sensors are primarily designed based on quenching of a fer with the dye. The extent of this reaction is monitored
luminescent dye. Fiber-optic chemical sensors for oxy- spectroscopically through the fiber.
gen are constructed by immobilizing oxygen-sensitive For example, a fiber-optic ammonia sensor is prepared
fluorophores, using entrapment or adsorption on the fiber based on
surface since most of these dyes do not have an appropriate
−
+
functional group suitable for covalent immobilization. Dy − H + NH 3 → Dy + NH . (13)
4
The process of dynamic quenching is fully reversible,
Usually the nonprotonated form of the indicator dye is
i.e., the dye is not consumed by reaction with oxygen. Dy-
detected either by an absorbance or fluorescence mea-
namic quenching of the fluorophore is responsible for the
surement. As the ammonia concentration in the sample
decrease in fluorescence, and the Stern–Volmer quench-
increases, the concentration of the nonprotonated dye in-
ing model can be used to express the extent of oxygen
creases, which causes an increase in the measured fluo-
quenching:
rescence or absorbance. The measured absorbance A is
I 0 /I = 1 + K SV [O 2 ], (12) related to the ammonia concentration in the sample solu-
tion by the following expression:
where I is the fluorescence intensity at a particular oxygen
concentration, I 0 is the value in the absence of oxygen, εbK eq C Dy [NH 3 ] s
K SV is the Stern–Volmer quenching constant, and [O 2 ]is A = , (14)
C NH 3 + K eq [NH 3 ] s
theconcentrationofoxygen.Alinearcalibrationisthereby
obtained by plotting the intensity ratio as a function of the where ε is the molar absorptivity of the chromophore,
oxygen concentration. At higher oxygen concentrations, b is the effective path length at the sensor end, K eq is
this plot deviates from linearity. the equilibrium constant of the above reaction, C Dy and
Any dye whose fluorescence intensity is quenched by C NH 3 correspond to total dye and total ammonia concen-
+
oxygen can be used to construct a fiber-optic oxygen tration ([NH ] + [NH 3 ]) in the dye solution, respectively,
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sensor. Commonly used dyes include polyaromatic hy- and [NH 3 ] s is the ammonia concentration in the sample
drocarbons (PAH) such as pyrene, fluoranthene, or ben- solution.
zoperylene, and organometallic complexes of ruthenium, Two different methods can be used to prepare a fiber-
osmium, palladium, and platinum. Typical polymeric sup- optic carbon dioxide sensor.Inthe first method, the