Optical Biosensing Using Localized Surface Plasmon Resonance of Gold Nanoparticles

Abstract

This thesis describes some experiments developed to probe the fundamental aspects of the interfacial behaviour of proteins. The contents of this thesis can be broadly divided into two parts. In the first part, we studied how the size of the nanoparticles and other variables such as pH and bulk protein concentration affect the structure of the adsorbed protein layers. We also probed how these factors can influence the binding activity of adsorbed proteins. Study on the adsorption of IgG, Protein A and streptavidin on gold nanoparticles reveals that not all proteins are similarly affected by the size of the adsorbing surface. We found that though the optical properties of all the proteins vary with the size of the nanoparticle, their functionalities are not similarly affected by nanoparticle curvature. Protein A and streptavidin retain their binding capacity to IgG and biotin, respectively, irrespective of the size of the gold nanoparticle that they are attached to. On the other hand, a reduction/ loss in binding of adsorbed IgG to Protein A molecules is observed. The reduction in biological activity further depends on the radius of curvature of the adsorbing surface. The second part of the thesis describes how nanoparticles can used as a probe to study the complex interfacial behaviour of proteins. We have utilized the extreme sensitivity of localized surface plasmon resonance (LSPR) of gold nanoparticles to local refractive index to determine the optical properties of BSA adsorbed on various polymer surfaces. The dielectric properties of the adsorbed protein depend on the nature of the substrate. Further, we have developed a model to determine the refractive index profile of adsorbed protein as a function of the distance from the substrate.