On-chip absorption measurements using an integrated waveguide
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Square hollow waveguides are used to integrate measurement of absorption with chip-based electrophoresis. The 50×50 µm liquid channel and 50×50 µm waveguide are etched as a negative pattern into a silicon master and replicated as a positive in poly-dimethylsiloxane (PDMS). The uniform refractive index of the chip prevents guiding by total internal reflection. Instead, light at 488 nm is guided by reflection at the air-PDMS interface. The waveguide has a 60% efficiency over a distance of 3.2 cm. Separation of fluorescein and the dye BODIPY is demonstrated. A detection limit (S/N=3) of 200 µM fluorescein is obtained using a 50 µm pathlength and a simple photocell detector.
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