Local Ionic Environment around Polyvalent Nucleic Acid-Functionalized Nanoparticles

Polyvalent oligonucleotide-functionalized gold nanoparticles are remarkably stable in a cellular environment against degradation by nucleases, a property that was recently attributed to the local high concentration of mono- and divalent ions. To evaluate this hypothesis in more detail, we investigate the composition of the ion cloud around spherical nanoparticles that are functionalized by stiff, highly charged polyelectrolyte chains by means of classical density functional theory and molecular dynamics simulations. We present a cell model that includes ligands explicitly and both applies over the entire relevant parameter space and is in excellent quantitative agreement with simulations. We study the ion cloud for varying oligonucleotide grafting densities and bulk ionic concentrations, as well as different sizes of nanoparticles and chains, and distinguish a parameter regime where many-body interactions between the ligands have a dominant effect on the local environment. For small particles with high oligonucleotide surface densities, we find strongly enhanced local salt concentrations, a large radial component of the electric field between the ligands, and a pronounced localization of divalent ions near the surface of the nanoparticle, thus providing multiple supporting arguments for the hypothesis.