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Label-free biosensors consisting of polymeric waveguides with a Bragg reflection grating are demonstrated for the purpose of highly sensitive protein detectors. Due to the unique nanoimprinting and injection molding processes, polymer waveguide devices have the potential to provide cost-effective disposable biosensors as long as a robust compatible immunoassay process is developed. A fluorinated polymer material with the lowest refractive index, which is close to that of the saline solution, is used for the sensor waveguide, in order to increase the effective index modulation efficiency. The large contrast waveguide exhibits a 1.9 times higher sensitivity for the detection of the variation of the cover index compared to low-contrast sensors. Bragg reflectors with a 3-dB bandwidth of 0.9 nm are fabricated using laser interferometry and dry etching of the core layer by 50 nm. An immunoassay experiment to detect the concentration of antibiotin is performed by measuring the peak wavelength shift of the Bragg reflection sensors. By using a blocking buffer, it is possible to obtain a stable saturated output signal by preventing the nonspecific binding. In the experiment to measure the antibiotin concentration, the sensor exhibits linear correspondence of peak wavelength shift for the concentrations.