Charged Nanoparticle Assemblies in Extremely Saline Solutions*

Electrostatic interactions manifest in counterintuitive ways in highly concentrated salt solutions.  In this study, we utilize assemblies of highly charged nanoparticles as responsive probes for the changes in the strength and spatial extent of electrostatic interactions over a very wide range of ionic conditions. Gold nanoparticles functionalized with monodisperse, non-base-pairing DNA are aggregated in solutions containing calcium ions. Our small-angle X-ray scattering (SAXS) measurements reveal that raising the solution ionic strength induces transitions from FCC, to body-centered cubic (BCC), to amorphous structures. Furthermore, the nearest-neighbor distances in the lattices begin to increase above a threshold salt concentration, suggesting an increase in screening length. This anomalous lattice expansion is coincident with the appearance of short-ranged correlations between ions in the bulk solution, as revealed by in situ wide-angle X-ray scattering. The generality of the phase transitions and the lattice expansion will be discussed. This work demonstrates how interactions between charged objects continue to evolve past the regime where classical theory predicts electrostatics to become negligible.