Profile optimization of tapered waveguide sensors by fluorescence imaging

Evanescent field sensors provide unique analytical features. For fiber-optic fluorescence sensors the sensing zone geometry plays an important role in coupling excitation energy to the evanescent field and reciprocally collecting the fluorescence signal. In the past a tapered geometry was introduced to overcome V-number mismatch by mode conversion in the taper zone. It was later developed into a combination-taper profile to improve signal reproducibility. The spectroscopic throughput of fiber-optic evanescent-field sensors, however, is limited by the intrinsically small penetration depth and the small fiber format. In this study it was improved by dimension scale-up and cladding elimination using tapered waveguides fabricated from 6-mm quartz rods. Optimization of their profile was aided by fluorescence imaging, a technique that visually revealed the locations and intensity of the light coupled to evanescent field, as well as the light leaking to the bulk of the solution due to violation of conditions for total internal reflection. Based on this technique, a taper-cylinder profile was selected that provided the best performance among tested 2 geometries in time-resolved luminescence. A 0.64 ppb limit of detection and a 0-500 ppb linear dynamic range (r(2) = 0.9996) were achieved using tetracycline as a model analyte.