Infrared Waveguide Sensor with Functionalized Monolayer for Detection of Airborne Pollutants

We report the development of a tubular infrared waveguide sensor incorporating a functionalized self-assembled monolayer (SAM) on its metallized interior surface. The SAM used in this application was 11-mercaptoundecanoic acid (MUA) assembled from a 5-mM solution in ethanol onto the inner surfaces of silvered Pyrex(R) glass tubing (4-mm inside diameter x 21-cm length). The sensor was adapted withminor modifications to fit standard fiber-launching and detection optics, which incorporate a remote cryogenic detector and mate to a Fourier transform infrared (FTIR) spectrophotometer. The fl.5 optics allow a sufficient number of multiple reflections inside the sensor to achieve spectroscopic sensitivity to the SAM within a 15 minute measurement time at 2 cm-1 resolution. The overall opticalconfiguration is similar to that of the waveguides used for gas chromatography (GC)-FTIR spectroscopy, except that the inner surface of the sensor, rather than the gas contained in the GC-FTIR waveguide, is sampled. After the FTIR spectrum of a freshly prepared sensor is recorded, the inner surface (functionalized with carboxylic acid) is exposed to anambient or flowing atmosphere. The terminalacid groups exposed at the surface of the MUA SAM react with basic pollutants, such as amines, which can be indoor air pollutants. The spectrum of the exposed sensor is then recorded and ratioed to the original sensor spectrum to produce an absorbance spectrum characteristic of the reacted, surface-bound pollutant. Our sensors exhibited sensitivity to 15 ppm of octylamine in an ambientatmosphere after 2 hours of exposure at 36 deg. C.