A multilayer model for gold nanoparticle bioconjugates: Application to study of gelatin and human IgG adsorption using extinction and light scattering spectra and the dynamic light scattering method

A new model of colloidal gold (CG) bioconjugates is proposed. The model consists of a gold core and a primary polymer shell formed during conjugate synthesis. Additionally, the conjugate includes a secondary shell formed during its interaction with target molecules. Each of the inhomogeneous shells is modeled by the arbitrary number of discrete layers. Using Mie theory for multilayered spheres, we calculated the extinction and static light scattering (SLS, at 90degrees) spectra, as well as differential spectra DeltaA(lambda), DeltaI(lambda) describing the effects of primary and secondary shells. Our calculations are performed for the conjugates with gold particle diameters d = 10-160 nm and two 5-nm shells. The primary shell consists of two 2.5-nm layers with the refractive indices of 1.50 and 1.45; the secondary shell, of two 2- and 3-nm layers with the refractive indices of 1.45 and 1.40. The differential spectra are related to the adsorption of target molecules and possess a characteristic resonance that is shifted to the red region of spectra compared to the usual localized plasmon resonances of gold particles. The maximal values of differential resonances DeltaA(max), and DeltaI(max) are observed for gold particles with diameters about 40-60 nm (extinction spectra) or 70-90 nm (the SLS spectra). The adsorption of human gamma-globulin (hIgG) and gelatin onto 15- and 34-nm gold particles was studied using the SLS and extinction spectra in combination with the dynamic light scattering measurements. It is shown that the thickness of adsorbed layer is equal to 5-6 nm for hIgG and to 15-18 nm for gelatin. The experimental extinction and SLS spectra for CG + hIgG conjugates are well explained by a simple model with the gold core and homogeneous polymer coating. For the CG + gelatin conjugates, we used the new model with inhomogeneous polymer coating, which is modeled by 10 discrete layers with the total thickness of 16-18 nm and exponential spatial profile of shell refractive index.