A new fluoropolymer is proposed as the basis of a novel class of sensors. The devices are based on selective chromogenic reactions and in situ long-path optical absorbance measurement. The polymer is transparent from 200 to 2000 nm and has the lowest known refractive index (RI) of any synthetic polymer. The RI is less than that of water. A tube of this material, filled with an aqueous solution (or virtually any other liquid), behaves as a liquid core optical fiber. As a result, long-path length optical cells are possible without significant loss of light. The material is highly permeable to a number of trace gases of environmental interest. Relative to common poly(tetrafluoroethylene) (PTFE)-type Teflon, the new amorphous fluoropolymer (Teflon AF 2400) is more than 3 orders of magnitude more permeable to many gases. If a Teflon AF tube is filled with a reagent that responds to a gaseous analyte by undergoing a change that is spectroscopically detectable, an unusually versatile, sensitive, and inexpensive ...