Integrated optical input grating couplers as direct affinity sensors

Abstract

An input grating coupler instrument with planar monomode waveguides as sensors was used for real-time monitoring of protein adsorption and of affinity- and immunoreactions. The adsorbed proteins studied were avidin and immunoglobulins (h-IgG). In affinity- and immunosensing, adsorbed monolayers of these proteins formed the receptors on the waveguide surface for the corresponding ligand in the sample. The formation of complexes between avidin and biotinylated protein A (prot A) and biotinylated bovine serum albumin (BSA), between h-IgG and anti-hIgG, and between h-IgG and prot A were investigated. With an improved sensor instrument, changes of the effective refractive indices N TE 0 and N TM 0 of the TE 0 and TM 0 guided modes at two laser wavelengths λ 1 = 632·8 nm and λ 2 = 514·5 nm were measured. From these data not only the thickness d F′, but also the refractive index n F′, and the surface coverage Γ′ of an adsorbed or bound single isotropic adlayer F′ were determined at both wavelengths. This model was extended to include (i) a possible optical anisotropy a F′ of an (adsorbed single) adlayer and (ii) the formation of double adlayers (F′ + F″) in affinity reactions, adlayer F′ describing the immobilized receptors and adlayer F″ the bound ligands. The SiO 2-TiO 2 waveguides, fabricated by a sol-gel process, are microporous and, in aqueous buffer solutions, show a slow but persistent increase of the measured effective refractive indices with time. This drift effect is greatly diminished in compact SiO 2-TiO 2 waveguides annealed at a temperature of T = 900°C (instead of T = 500°C).