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Electrolyte-Mediated Assembly of Charged Nanoparticles

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Journal Article - Open Access

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Northwestern University Evanston United States

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Solutions at high salt concentrations are used to crystallize or segregate charged colloids, including proteins and polyelectrolytes via a complex mechanism referred to as salting-out. Here, we combine small-angle X-ray scattering SAXS, molecular dynamics MD simulations, and liquid state theory to show that salting-out is a long-range interaction, which is controlled by electrolyte concentration and colloid charge density. As a model system, we analyze Au nanoparticles coated with noncomplementary DNA designed to prevent interparticle assembly via WatsonCrick hybridization. SAXS shows that these highly charged nanoparticles undergo gas to face-centered cubic FCC to glass-like transitions with increasing NaCl or CaCl2 concentration. MD simulations reveal that the crystallization is concomitant with interparticle interactions changing from purely repulsive to a long range potential well condition. Liquid-state theory explains this attraction as a sum of cohesive and depletion forces that originate from the interelectrolyte ion and electrolyteionnanoparticle positional correlations. Our work provides fundamental insights into the effect of ionic correlations in the salting-out mechanism and suggests new routes for the crystallization of colloids and proteins using concentrated salts.

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  • Physical Chemistry

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