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A 30 mm-long multimode waveguide, 40 mum wide and 40 mum high, is fabricated on a silicon wafer using polymer SU-8 as the core and liquid buffer as the cladding. Antibodies are successfully immobilized on the SU-8 surface designated for binding target antigens dispersed in the buffer solution. Evanescent-wave spectroscopy is performed by exciting the fluorescently-labeled antigens, bound to the waveguide surface within its evanescence field, and measuring emission light intensity. This evanescent-wave biosensor detects specific molecular interaction; its output depends on antigen concentration with a sensitivity of 40 light counts per 1 mug/ml.