Modeling of electrodynamic interactions between metal nanoparticles aggregated by electrostatic interactions into closely-packed clusters

This paper is a theoretical and experimental study of the optical properties of binary aqueous suspensions containing metal nanoparticles (NPs) coated with charged organics. If all nanoparticles bear charges of the same polarity, the NPs do not aggregate, and the solutions are stable. Under these circumstances, optical response of the mixture is a linear combination of the optical responses of the individual components. In contrast, when the NPs are oppositely charged, they aggregate into clusters, whose optical properties cannot be understood without taking into account electrodynamic coupling between the constituent NPs. To model such aggregates, we present two theoretical approaches: (i) an exact analytical model accounting for the granularity of the NPs and (ii) an approximation, in which the aggregates are represented as spherosymmetric core-and-shells. Both models reproduce optical spectra recorded experimentally and give physically reasonable estimates of the aggregates' internal structure and composition.