Three-layer-structure polymer optical fiber with a rough inter-layer surface as a highly sensitive evanescent wave sensor

We present a high-sensitivity and high-accuracy D-shaped polymer fiber-optic evanescent wave (FOEW) sensor with a three-layer structure (i.e., bottom, inter-, and surface layers) and a rough inter-layer surface in the sensing region. The surface layer is made of dilute Canada balsam in xylene, and the longitudinal section is coated with light-absorbing (LA) film. To obtain the maximum sensitivity and accuracy, the morphologies of the inter-layer and surface layer are determined. The effects of coating thickness and the refractive index of the surface layer and LA film on the performance of FOEW sensors are investigated both theoretically and experimentally. The sensitivity, accuracy and repeatability of the sensors are evaluated using glucose solutions; the response of the sensors, which are self-assembled from the polycation (tris[2-(4-phenyldiazenyl) phenylaminoethoxy] cyclotriveratrylene (TPC) plus poly dimethyl diallyl ammonium chloride (PDDAC)) and polyanion (TPC plus polyacrylic acid (PAA)), to Hg(II) in aqueous solutions is examined. The results highlighted the high sensitivity of the FOEW sensor with a three-layer structure and appropriate roughness of the inter-layer surface, which showed a 6.5-fold improvement in the detection of the target Hg(II) ions compared to the conventional FOEW sensor with a core-cladding structure, and the novel sensor was validated with a lower limit of detection of 0.1 mg L−1.