Real-time analysis of avidin adsorption with an integrated-optical output grating coupler: adsorption kinetics and optical anisotropy of adsorbed monomolecular layers

Abstract The adsorption of the protein avidin on the surface of an oxide (SiO 2 TiO 2 ) waveguide was observed in real time with an integrated-optical output grating coupler working at wavelength λ = 633 nm. From the simultaneously measured effective refractive index changes ΔN TE 0 ( t ) and ΔN TM 0 ( t ) of the TE 0 and TM 0 modes, both the thickness d F′ ( t ) and the refractive index n F′ ( t ) of the (sub-)monomolecular avidin adlayer F′, and from these data its surface mass density Γ ′( t ), were determined as functions of time t . The adsorption kinetics were found to follow not the Langmuirian but the random-sequential-adsorption theory. In the above evaluation, the adlayer F′ was assumed to be optically isotropic. However, an adlayer F′ consisting of oriented, non-spherical protein molecules is actually optically anisotropic. For such an adlayer F′ of oriented ellipsoidal protein molecules, we calculated its ordinary and extraordinary refractive indices from Maxwell-Garnett theory. Thus, we proved that the ‘isotropic’ evaluation of the experimental data yields correct values for the surface mass density Γ ′( t ). We also found that the avidin molecules adsorb as oblate ellipsoids with their axes of rotation perpendicular to the surface. They actually change their shape during the formation of a complete monolayer; being more oblate at low surface coverages, they have, in a complete monolayer, nearly the same form factor as in solution.