The Effect of Tapering the Optical Fiber on Evanescent Wave Measurements

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Abstract
Optical biosensors are analytical tools employed in environmental, biotechnology and clinical fields. For remote evanescent wave detection with optical fibers, the majority of the fiber remains cladded while only a small portion of the core is exposed to perform the analysis. To improve the sensitivity of this type of fiber optic biosensor, tapering the sensing region of the fiber was investigated. Silica fibers were tapered into two geometries: step and continuous tapers. To determine the effect of tapering on detection levels, a fluoroimmunoassay was performed on fibers with nontapered, step-tapered and continuously tapered geometries. Solutions containing a fluorescent analyte were circulated over an antibody-coated fiber and the fluoresence signal measured The minimum detection limit for fibers with 10 cm sensing region was 1.56 nM, 0.31 nM and 0.16 nM for nontapered, step-tapered and continuously tapered fibers, respectively. Continuous tapering of the sensing region of an optical fiber offered a 10-fold increase in the sensitivity of the optical biosensor.

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