SCIENCE
A sensor to stick with A new class of sensor for environmental and occupational monitoring of gases, such as carbon dioxide, sulfur dioxide, ozone, and nitrogen oxide (N0 2 ), was described in the November 15 issue of Analytical Chemistry (p. 4661). The sensor is based on a new class of amorphous Teflon polymer. A 20-um-thick tube is filled with a liquid and acts as a liquid-core optical fiber that is highly permeable to various gases, including many of environmental interest. Purnendu "Sandy" Dasgupta and colleagues at Texas Tech University (Lubbock), working with Su-Yi Liu of World Precision Instruments (Sarasota, FL), have exploited a recently developed polymeric material, which has the lowest refractive index of any known polymer. The material—Teflon AF, a copolymer of perfluoro2,2-dimethylT,3-dioxole and tetrafluoroethylene—is transparent from 200 to 2000 nm and highly permeable to numerous gases. To use the new material as a sensor, the team fills Teflon AF tubes with a reagent whose spectroscopic character changes on exposure to a gas of interest. For instance, colorless Griess-Saltzman reagent, which
Sensitive liquid crystals A compact, inexpensive, and portable sensing system based on liquid crystals (LCs)—materials more commonly associated with display technology—has been developed by U.K. chemists. According to team leader Paul Nicholas of Birmingham University (United Kingdom), the development could revolutionize analytical science in areas such as environmental monitoring, biotechnology, and medical testing, as well as home diagnostics. Nicholas and colleague Jim Hay collaborated with David Lacey of Hull University to develop a simple yet highly versatile chemical sensor that can detect and distinguish between a variety of compounds from aromatic hydrocarbons to biomolecules. The devices are based on cyanobiphenyl LCs embedded in porous membranes so that the LCs are held in a spe-
turns purple when exposed to N0 2 , can be used to make measurements down to the low parts-per-billion level with an inexpensive green light-emitting diode (LED) and a photodiode. Dasgupta points out that the path length can be much longer than for other optical absorbance devices because the liquid-filled tube behaves like an optical fiber and there is negligible loss of light. The Beer-Lambert law relates path length to sensitivity, so a longer path length is usually desirable. However longer path lengths generally do not give better detection limits because too much light is lost to the wall and noise overrides sensitivity. Common HPLC or flowinjection analysis detectors for example have optimized path lengths between 4 and 8 mm. In this work Dasgupta describes devices with path lengths between 150 and 300 Light loss in these larger devices is prevented by the internally reflecting surface of the liquid interior Dasgupta's capillary hollow-fiber structure allows a high surface-to-volume ratio. This design improves gas sensitivity, because diffusion is increased, and reduces response times to
