Analysis of excitation and collection geometries for planar waveguide immunosensors

We demonstrate the use of a two-channel flowcell for fluorescent immunoassays. The flowcell contains a planar silica waveguide for evanescent excitation of the fluorophores, and the planar waveguide surface provides the solid support for immobilization of the antibodies. The detection system is composed of a grating spectrometer and a CCD camera for spectral characterization of the emitted signals. Two methods of sensing have been studied: a displacement-type technique and a sandwich-type assay. The sensitivity achieved for measuring concentrations of HCG by the sandwich method is sub-picomolar. Also, we have experimentally compared the signal strengths for two alternative ways of excitation and collection, and determine that waveguide excitation/side collection has some practical advantages over side excitation/waveguide collection.