OPTICAL-ABSORPTION STUDY OF THE BIOTIN-AVIDIN INTERACTION ON COLLOIDAL SILVER AND GOLD PARTICLES

The biotin-avidin reaction is well studied and is often used as a prot otypical interaction in the development of immunoassays. In this paper , this reaction is studied on the surface of colloidal silver and gold particles as a first step in the development of a sol-based assay. Mo re specifically, silver and gold colloidal particles were biotinylated by self-assembly of a biotin disulfide molecule, and the reaction of the surface-modified colloidal particles with avidin molecules was fol lowed using optical absorption spectroscopy. The specific interaction of avidin, a tetrameric protein, with biotin leads to cross-linking of the colloidal particles (''flocculation'') and a consequent growth of a long wavelength absorption peak. The degree of flocculation was qua ntified using a semiempirical flocculation parameter, and the dependen ce of this parameter on the extent of biotinylation of the colloidal p article surface as well as the concentration of avidin in solution was studied to determine the optimum working conditions of the sol. The s ilver sol required electrostatic stabilization of the biotin-capped pa rticles through the simultaneous incorporation of a charged bifunction al molecule, 4-carboxythiophenol, in the capping monolayer while the g old sol was stable with biotin capping. Both biotinylated silver and g old sols showed a visible color change on addition of avidin.;However, changes in the optical absorption spectra were more marked for the si lver sol.