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

The adsorption of the protein avidin on the surface of an oxide (SiO2-TiO2) waveguide was observed in real time with an integrated-optical output grating coupler working at wavelength lambda = 633 nm. From the simultaneously measured effective refractive index changes Delta N-TE0(t) and Delta N-TM0(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 Gamma'(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 Gamma'(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. (C) 1997 Elsevier Science Limited.