An apparatus for enhancing the collection of optical signals comprises a waveguide, which comprises a structure in a channel adapted for the flow of assay fluid. In one embodiment, fluorophore is coupled to a surface of the structure, where the fluorophore has a receptor for an analyte (ligand) in the assay fluid and the structure is adapted to transmit optical signals from the fluorophore to a detector that is adapted to detect optical signals from the fluorophore coupled to the waveguide.