an assay element is described including recognition ligands adapted for binding to carcinoembryonic antigen bound to a film on a single mode planar optical waveguide , the film from the group of a membrane , a polymerized bilayer membrane , and a self - assembled monolayer containing polyethylene glycol or polypropylene glycol groups therein and an assay process for detecting the presence of cea is described including injecting a possible cea - containing sample into a sensor cell including the assay element , maintaining the sample within the sensor cell for time sufficient for binding to occur between cea present within the sample and the recognition ligands , injecting a solution including a reporter ligand into the sensor cell ; and , interrogating the sample within the sensor cell with excitation light from the waveguide , the excitation light provided by an evanescent field of the single mode penetrating into the biological target - containing sample to a distance of less than about 200 nanometers from the waveguide thereby exciting any bound reporter ligand within a distance of less than about 200 nanometers from the waveguide and resulting in a detectable signal .