Colorimetric and Fluorimetric Polymer Membrane Gas-Sensing Materials
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rating temperature. Sensors fabricated in this way have been called "plnsticfilm sensors."1"3 However, room-temperature elastomers, most notably silicone rubbers, are now being used increasingly in this role. The high segmental mobility and high small-molecule diffusion coefficients of the plasticized and elastomeric polymer membranes mean that sensor response and recovery times of a few seconds are attainable if the membrane thickness is limited to a few tens of microns. The dye-bearing sensor membrane is "read" using an interrogating light beam which is substantially monochromatic and is typically produced using a lightemitting diode (LED), a diode laser, or a filtered/monochromated white light source such as an incandescent filament lamp. Those sensors which operate through a change in optical absorption have been termed colorimetric,1-** and those operating t h r o u g h a c h a n g e in fluorescence intensity or lifetime have been termed fluorinietric.4,5 Sensor mem branes may be self-supporting, but more usually they are coated onto a transpar ent glass or polymer Substrate. The transparent Substrate may be an optical waveguide, such as an optical fiber, which presents an opportunity to exploit evanescent-wave phenomena and lightpiping in reading the sensor.''"1" Indirect optical sensors using a waveguide ap proach have been termed optrodes.6,7 The use of optical fiber technology has obvious implications for device miniaturization," and so-called capillnry wnveguidesv" have been developed to enable evanescent-
wave reading of sensor membranes used for gas analysis in small volumes of liquid. Figure 1 illustrates a number of the possible geometries used for indirect optical gas sensing.
Carbon Dioxide Sensing The indirect optical sensing of CO: re lies on the reaction of G0 2 gas with water to produce carbonic acid (H2CO.i), which then dissociates to produce protons (H + ), bicarbonate anions (HCO? ), and carbonate anions (CO,2"),1'12'13 CO, + H 2 0 = = = H 2 C 0 3 = = H + + H O V = ^ 2H + + CO, 2 - .
(1)
Thus, in a water-containing medium, the presence of C 0 2 will produce an increase in proton concentration [H + ]. Furthermore, it may be shown that when a significant concentration of a bicarbonate buffer salt is present in the m e d i u m , [ H + ] is approximately proportional to the partial pressure of carbon dioxide ^cor 13 ' n t n e indirect optical sensing of C0 2 , changes in proton concentration in the sensor membrane are monitored us ing a pH indicator dye, which is generally a weak acid dissociating as follows: D-H=^=D +H+. acid form basic form
(2)
In early sensors, the pH indicator dye was dissolved in an essentially aqueous bicarbonate buffer Solution which was either liquid 14 or a hydrogel. 15,16 Thus, to prevent displacement or evaporation of the aqueous dye-buffer System, the active part of the sensor was overcoated or laminated with a hydrophobic (Teflon or Silastic) gas diffusion membrane. How ever, the use of a diffusion membrane necessarily increases sensor response times and co
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