Assembly of DNA-Functionalized Nanoparticles in Concentrated Electrolytes*

In concentrated electrolytes, ion-ion correlations and solvent effects modulate electrostatic interactions, potentially producing counterintuitive behaviors.  In this study, we study the manner in which high salt concentrations in solution influence the assembly of gold nanoparticles functionalized with non-base-pairing DNA. Our small-angle X-ray scattering (SAXS) measurements reveal that various divalent cations induce the reversible crystallization of these nanoparticles into various structures, dependent on cation type and concentration. Additionally, interparticle separations within the assemblies are found to vary throughout the full accessible ranges of salt concentrations, even increasing at the highest concentrations, where classical theory predicts electrostatic forces to be of negligible range. Observations from wide-angle X-ray scattering measurements and molecular dynamics simulations will be analyzed in order to elaborate upon the underlying mechanisms governing these behaviors. Additionally, effects associated with solvent composition, ion valency, and temperature will be discussed. This work demonstrates the continuous evolution of interactions between charged objects well above biological salt concentrations.