The contribution of capping layer dielectric properties to nanoparticle stability

A surge in research aimed at both synthesis and application of colloidal nanoparticles occurred in recent decades. Many biological and biomedical applications with nanoparticles depend on specifically tailored surface modifications that enable dispersion stability in high salt buffers. To address the impact of the coating layer on nanoparticles, we modify the venerable Derjaguin-Landau-Verwey-Overbeek (DLVO) theory that is typically used to explain colloidal stability. Our free energy functional includes the non-linear Poisson-Boltzmann potential and a modified van der Waals attraction that specifically considers a multi-layer geometry. Our results indicate that even a thin hydrocarbon layer over a gold nanoparticle core can substantially reduce particle-particle attraction. This modification can lead to nanoparticle stability over a wider range of salt concentrations, especially when the particle core radius and shell thickness are similar. Importantly, this effect depends on dielectric contrast and, therefore, cannot be explained by simply considering steric interactions. By comparing with experiments that assess the stability of nanoparticle dispersions, we show that this formulation can account for differences in nanoparticle stability due to various coating layer thicknesses.