Characterization of long pathlength capillary waveguides for evanescent fluorescence sensing applications

The optical properties of a novel fused silica fiber-optic capillary (FOCap) waveguide for fluorescence chemical sensing are evaluated. Evanescent fluorescence from samples contained in the FOCap is measured by coupling the FOCap to a light source and a fluorescence spectrophotometer. The FOCap has negligible excitation light loss over long lengths (tested to 20 m). Evanescent fluorescence is linear in FOCaps up to 20 m long when filled with a fluorophore solution and up to 15 m when a fluorophore is covalently attached to the inner surface. Evanescent fluorescence measurements in 50, 150, and 250 μm inner diameter FOCaps indicate that greater sensitivity is achieved with thinner-walled capillaries which have more internal reflections. The ability of the waveguide to function as an indirect chemical sensor for nitroaromatic compounds is demonstrated via fluorescence quenching of a pyrene-labeled FOCap.