近日，美国西北大学的Monica Olvera de la Cruz及其研究团队取得一项新进展。经过不懈努力，他们揭示浓缩电解质中带电纳米粒子组装的再膨胀过程。相关研究成果已于2024年1月30日在国际知名学术期刊《美国科学院院刊》上发表。

该研究团队通过使用非碱基配对DNA包覆纳米颗粒（DNA-NP）来分析浓盐溶液中的静电相互作用。这些DNA-NP在足够的二价阳离子浓度的溶液中能够形成胶体晶体，尽管它们自身带负电荷。研究人员利用小角X射线散射（SAXS）在CaCl₂、MgCl₂和SrCl₂水溶液的整个可达浓度范围内研究了这种DNA-NP的组装过程。SAXS结果表明，组装结构的结晶度和物相随阳离子类型的不同而变化。在低盐度下，聚集体与添加的离子收缩，然后在高于阳离子依赖的阈值盐浓度时开始膨胀。广角X射线散射（WAXS）揭示了高盐浓度下溶液中离子之间增强的位置相关性。

通过分子动力学模拟，研究人员发现这些离子-离子相互作用降低了DNA刷内密集离子构型的有利性，低于体溶液的有利性。在介电常数较低的溶液中进行的测量表明，离子耦合同时增加，聚集体膨胀开始时的浓度降低，从而证实了这些现象之间的联系。这项研究工作表明，带电物体之间的相互作用在浓盐溶液中继续不断演化，这一现象在经典理论中通常被忽略不计。

据悉，溶液中的静电力在许多领域都有着重要的应用，从电化学能量存储到生物学。然而，在浓缩电解质中的表现尚不完全清楚，例如对熔盐中胶体稳定性和长程排斥力的反直觉观察。高电荷的生物分子，如DNA，对稀溶液中的离子反应敏感。

附：英文原文

Title: Reexpansion of charged nanoparticle assemblies in concentrated electrolytes

Author: Reinertsen, Roger J. E., Kewalramani, Sumit, Jiménez-ángeles, Felipe, Weigand, Steven J., Bedzyk, Michael J., Olvera de la Cruz, Monica

Issue&Volume: 2024-1-30

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 biomolecules, 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.

DOI: 10.1073/pnas.2316537121

Source: https://www.pnas.org/doi/abs/10.1073/pnas.2316537121