Investigation of the effect of curvature on sensitivity of bio/chemical sensors based on embedded polymer semicircular waveguides

U-bend optical fiber probes have been shown to be very responsive to refractive index variations around the core, which makes them ideally suited for biosensor applications. Similar U-shaped polymer waveguides have also been fabricated using standard micro-fabrication technology. In the present study, suitable bend radii for such U-shaped embedded polymer waveguide probes have been determined for bio/chemical sensing applications. Although the effect of bend radii have been studied earlier for U-bend optical fiber sensors, such an investigation has not yet been performed for embedded bent waveguides. A single step fabrication procedure was performed to pattern SU-8 photo resist into semicircular C-shaped waveguides (of various bending diameters) interfacing a microchannel network and ending in fiber-to-waveguide coupler structures. The refractive index (RI) sensitivity was found to double as the bending radius was reduced from 2 to 1 mm, however the interaction length did not have any effect on RI sensitivity. The evanescent absorbance per unit length was also found to double, when the bend radius was reduced from 2 to 1 mm. Finally, the efficacy of these 1 mm bending diameter embedded waveguides as evanescent wave absorbance based immuno-biosensors was studied by immobilizing HIgG molecules on the waveguides and allowing FITC tagged GaHIgG molecules to interact with them.