DNA-Functionalized Nanoparticles in Mixed Electrolytes: Salting In, Out, and Beyond

Dissolved ions mediate interactions between charged biomolecules. While general principles for the effects of added salts on proteins have been established, comprehensive descriptions have yet to be achieved for DNA-based nanomaterials. Here, we analyze how interactions between nanoparticles grafted with non-base-pairing DNA change with the composition of the surrounding electrolyte. Small-angle X-ray scattering measurements reveal that specific cations (e.g., Ca²⁺) drive the assembly of colloidal crystals. At low salinity, the addition of NaCl suppresses this crystallization, consistent with “salting in” mechanism, where NaCl screens attractive electrostatic interactions. At higher salt concentrations, crystallization instead becomes increasingly favorable, suggesting a transition to a “salting out” regime, where added salt dehydrates the DNA. At extremely high salinity, DNA-NP aggregates swell with added salt, reflecting an additional transition to a concentrated electrolyte regime. The roles of solvent, anions, and temperature will be discussed. These results demonstrate analogous behavior of proteins and DNA-NPs, and also outline additional design considerations for DNA-based materials.