handbook of modern sensors

17.5 Complex Sensors 515
Fig. 17.12. Simplified configuration of an optical CO 2 sensor.
The slots allow CO 2 to diffuse into the chamber. The bottom parts of the chambers
are made of glass. Both wafers, A and B, form optical waveguides. The test chamber
is filled with a reagent, and the reference chamber is not. The sample part of the
sensor monitors the optical absorbency of a pH indicator in a dilute solution, where
the optical absorbency changes in accordance with the Beer–Lambert law:
I = I 0 exp [−α (λ, pH) dC] , (17.12)
where I is the transmitted intensity, I 0 is the source intensity, a is the molar absorptivity,
λ is the wavelength, C is the concentration, and d is the optical path length.
Ambient CO 2 equilibrates with the bicarbonate ion buffer system in the reagent,
as it is done in the traditional Severinghaus–Stow CO 2 electrode. Equilibrium among
CO 2 ,H 2 CO 3 , and HCO 3 produces a change in the pH of the solution. The solution
contains a pH indicator Chlorophenol Red, which exhibits a sharp, nearly linear
change in the optical absorbency at 560 nm from pH 5 to pH 7. The buffer concentration
can be selected to exhibit pH changes in the range for partial CO 2 pressures from
0 to 140 torr. Because the buffer pH varies linearly with the log of the partial pressure
of carbon dioxide (pCO 2 ), changes in optical absorbency can also be expected to vary
linearly with the log of pCO 2 .
The LED common for both halves of the sensor transmits light through the pHsensitive
sample to a test photodiode (PD1). The second photodiode (PD2) is for
reference purposes to negate variations in the light intensity of the LED. For temperature
stability, the sensor should operate in a thermally stable environment.
Fiber-optic chemical sensors (Fig. 17.13) use a chemical reagent phase to alter
the amount or wavelength of light reflected by, absorbed by, or transmitted through
a fiber waveguide (see also Fig. 4.17A of Chapter 4). A fiber-optic sensor typically
contains three parts: a source of incident (pilot) light, an optrode, and a transducer
(detector), to convert the changing photonic signal to an electrical signal. It is the optrode
that contains the reagent phase membrane or indicator whose optical properties
are affected by the analyte [19].
The location of the reagent, and the specific optical characteristic that is affected
by it, vary from one type of optical sensor to another. Simple polymer-coated fibers