Optical models for conjugates of gold and silver nanoparticles with biomacromolecules

We discuss optical models for gold and silver nanoparticle bioconjugates that consist of a metal core and a polymer shell formed by recognizing and target molecules. A universal dependence of the normalized spectral shift of the extinction maximum on the shell/core thickness ratio has been found in the dipole approximation. Using Mie's theory for multilayer spheres, we have calculated the extinction and static light-scattering spectra (at 90∘), as well as the differential spectra related to the adsorption of recognizing and target molecules. The differential spectra possess a characteristic resonance that is red-shifted as compared with the known resonance of gold particles. Having in mind an optimization of conjugate-nanosensors, we have studied the following problem: what particle size is optimal for the transduction of polymer adsorption events into variations of recorded optical signals? The maximal values of the differential resonance are observed for gold particles with diameters of about 40–60nm (extinction) or 70–90nm (scattering). In the case of silver particles, the corresponding optimal diameters are equal to 20–40nm for extinction and 30–50nm for scattering.