Polymer waveguide backplanes for optical sensor interfaces in microfluidics

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Abstract
A polymer optical backplane capable of generic luminescence detection within microfluidic chips is demonstrated using large core polymer waveguides and vertical couplers. The waveguides are fabricated through a new process combining mechanical machining and vapor polishing with elastomer microtransfer molding. A backplane approach enables general optical integration with planar array microfluidics since optical backplanes can be independently designed but still integrated with planar fluidic circuits. Fabricated large core waveguides exhibit a loss of 0.1 dB cm−1 at 626 nm, a measured numerical aperture of 0.50, and a collection efficiency of 2.86% in an n = 1.459 medium, comparable to a 0.50 NA microscope objective. In addition to vertical couplers for out-of-plane collection and excitation, polymer waveguides are doped with organic dyes to provide wavelength selective filtering within waveguides, further improving optical device integration. With large core low loss waveguides, luminescence collection is improved and measurements can be performed with simple LEDs and photodetectors. Fluorescein detection via fluorescence intensity with a limit of detection (3σ) of 200 nM in a 1 µL volume is demonstrated. Phosphorescence lifetime based oxygen detection in water in an oxygen controllable microbial cell culture chip with a limit of detection (3σ) of 0.08% or 35 ppb is also demonstrated utilizing the waveguide backplane. Single waveguide luminescence collection performance is equivalent to a back collection geometry fiber bundle consisting of nine 500 µm diameter collection fibers.

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