Ions Control Assembly of DNA-Functionalized Nanoparticles in Concentrated Electrolytes

Achieving a comprehensive understanding of concentrated electrolytes is challenging due to complex ion-ion and ion-solvent effects. This work utilizes highly charged DNA-functionalized gold nanoparticles to study electrostatic forces in concentrated salt solutions. Small-angle X-ray scattering measurements reveal that divalent cations induce the reversible crystallization of these nanoparticles into face-centered cubic, body-centered cubic, or amorphous structures. The type of structure formed depend on cation type and concentration. Interparticle separations within the assemblies exhibit a non-monotonic dependence on salt concentration; The structures contract with added salt at low salinity, but swell when salt is added at high salinity, where classical theory predicts electrostatic forces to be of negligible range. Observations from wide-angle X-ray scattering and molecular dynamics simulations implicate ion-ion interactions as driving this unexpected swelling at high salt concentration. Changing the solvent mixture to lower the dielectric permittivity increases the electrostatic coupling in the system and enhances these effects. This work demonstrates continuous evolution of interactions between charged objects as salt concentration increases to saturation, providing insight on how ion-ion correlations shape how electrolytes interact with charged materials.