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A simple and highly sensitive biosensor based on a fiber-optic coupler is developed. The change of refractive index due to biomolecular interaction on the surface of the coupler can be detected as the change of the transmission power. The sensitivity of the sensor is evaluated to be a noise level equivalent to a refractive index variation of 4×10−6. The binding of streptavidin is detected to be concentration dependent over a range of 0.5–2μg∕ml, by immobilizing biotin on the coupler surface via aminosilan treatment. This sensor allows the construction of a low-cost, portable, and label-free biosensing system.

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