Abstract
Electrostatic forces in solutions are highly relevant to a variety of fields, ranging from electrochemical energy storage to biology. However, their manifestation in concentrated electrolytes is not fully understood, as exemplified by counterintuitive observations of colloidal stability and long-ranged repulsions in molten salts. Highly charged bio-molecules, such as DNA, respond sensitively to ions in dilute solutions. Here, we use non-base-pairing DNA-coated nanoparticles (DNA-NP) to analyze electrostatic interactions in concentrated salt solutions. Despite their negative charge, these conjugates form colloidal crystals in solutions of sufficient divalent cation concentration. We utilize small-angle X-ray scattering (SAXS) to study such DNA-NP assemblies across the full accessible concentration ranges of aqueous CaCl2, MgCl2, and SrCl2 solutions. SAXS shows that the crystallinity and phases of the assembled structures vary with cation type. For all tested salts, the aggregates contract with added ions at low salinities and then begin expanding above a cation-dependent threshold salt concentration. Wide-angle X-ray scattering (WAXS) reveals enhanced positional correlations between ions in the solution at high salt concentrations. Complementary molecular dynamics simulations show that these ion–ion interactions reduce the favorability of dense ion configurations within the DNA brushes below that of the bulk solution. Measurements in solutions with lowered permittivity demonstrate a simultaneous increase in ion coupling and decrease in the concentration at which aggregate expansion begins, thus confirming the connection between these phenomena. Our work demonstrates that interactions between charged objects continue to evolve considerably into the high-concentration regime, where classical theories project electrostatics to be of negligible consequence.
Original language | English (US) |
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Journal | Proceedings of the National Academy of Sciences of the United States of America |
Volume | 121 |
Issue number | 6 |
DOIs | |
State | Published - 2024 |
Funding
This work was primarily supported by Department of Energy, Office of Science, Basic Energy Sciences award number DE-FG02-08ER46539. The SAXS/WAXS experiments were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS) and at APS Sector 12. The APS, an Office of Science User Facility operated for DOE by Argonne National Laboratory, is supported by DOE under Contract DE-AC02-06CH11357. DND-CAT is supported by Northwestern University, The Dow Chemical Company, and DuPont de Nemours, Inc. This work made use of the Integrated Molecular Structure Education and Research Center facility at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental Resource (NSF ECCS-2025633), the State of Illinois, and the International Institute for Nanotechnology. We thank Dr. Trung Dac Nguyen of UChicago, for helpful discussions, Soenke Seifert of Argonne National lab for assistance with SAXS measurements, and the Mirkin Group of Northwestern University for assistance in nucleic acid synthesis and characterization, in particular Jennifer Delgado, Kaitlin Landy, and Matthew Vasher. ACKNOWLEDGMENTS. This work was primarily supported by Department of Energy, Office of Science, Basic Energy Sciences award number DE-FG02-08ER46539. The SAXS/WAXS experiments were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS) and at APS Sector 12.The APS,an Office of Science User Facility operated for DOE by Argonne National Laboratory,is supported by DOE under Contract DE-AC02-06CH11357. DND-CAT is supported by Northwestern University, The Dow Chemical Company, and DuPont de Nemours, Inc. This work made use of the Integrated Molecular Structure Education and Research Center facility at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental Resource (NSF ECCS-2025633),the State of Illinois,and the International Institute for Nanotechnology.We thank Dr.Trung Dac Nguyen of UChicago, for helpful discussions, Soenke Seifert of Argonne National lab for assistance with SAXS measurements,and the Mirkin Group of Northwestern University for assistance in nucleic acid synthesis and characterization,in particular Jennifer Delgado, Kaitlin Landy, and Matthew Vasher.
Keywords
- DNA
- SAXS
- electrolyte
- self-assembly
- underscreening
ASJC Scopus subject areas
- General