The present invention is directed to an assembly for use in detecting an analyte in a sample based on thin-film spectral interference. The assembly comprises a waveguide, a monolithic substrate optically coupled to the waveguide, and a thin-film layer directly bonded to the sensing side of the monolithic substrate. The refractive index of the monolithic substrate is higher than the refractive index of the transparent material of the thin-film layer. A spectral interference between the light reflected into the waveguide from a first reflecting surface and a second reflecting surface varies as analyte molecules in a sample bind to the analyte binding molecules coated on the thin-film layer.