Overview: The overall purpose of the proposal is to elucidate the ionic correlations underlying nonintuitive like-charged attractions and long-ranged repulsions between nanoparticles in very saline solutions containing mono- and di-valent ions. As a model system, DNA-coated nanoparticles (DNA-NPs), which exclude the possibility of assembly by Watson-Crick hybridization, will be synthesized. These versatile constructs allow independent tuning of the NP charge and charge density. Furthermore, NPs of different types (metals, proteins) can be functionalized with DNA. This is advantageous for X-ray experiments. The proposed research is divided into three sets of experiments. (1) Ionic correlations in nanoparticle assembly, (2) Ionic correlations in extremely saline solutions, and (3) Ionic correlations: effects of macromolecular crowding. These studies aim to (1) gain insight into the origin of like-charged attractions that lead to the formation of ordered assemblies of nanoparticles in saline solutions, (2) correlate the nanoscopic structure of the very concentrated electrolyte solution and the range and strength of interactions between highly charged nanoparticles dispersed in these solutions, and (3) understand how the collective electrostatic effects of charged colloids/macroions alter their ionic environment, and lead to novel properties. The proposed research will utilize state of the art in situ X-ray scattering methods [anomalous small- and wide-angle X-ray scattering (ASAXS/AWAXS) and X-ray scattering]. The X-ray studies will be coupled with numerical methods (e.g. Molecular dynamics simulations), which take into account the shape and the size of the nanoparticles, the conjugated ligands, and the ions in the solution to deduce the distribution of ions surrounding charged nanoparticles (NPs). Such a holistic approach is necessary in general because of the complexity of the nanoconstructs being used nowadays in various applications (including the specific case of DNA-coated nanoparticles proposed here). Intellectual Merit: Charged colloids and macromolecules dispersed in aqueous environments are ubiquitous in biology and chemistry. Their interactions are strongly modulated by the salt concentration and the valence of the ions constituting these salts. The focus of the present proposal is interactions and assembly in moderate-to-extremely high concentrations of salts with monovalent or divalent ions. Recent theory and simulations show that at moderate concentrations, mono- and di-valent ions can induce attractions between like-charged particles. Counter to common intuition, these attractions have purely electrostatic origins, and arise due to positional correlations between the counterions condensed onto the neighboring particles and due to depletion-like forces induced by ionic clusters (positional correlations between ions) formed in these solutions. By contrast, in extremely saline environments (> 1M), ionic correlations in the bulk solution are expected to be so strong that the ions are no longer “free” to screen the charge of the colloids. In such environments, strong, long-ranged repulsive interactions are expected for like-charged particles. The proposed studies aim to test these modern theories and hypotheses by in-situ synchrotron X-ray measurements of the distribution of ions surrounding the particles and the structure of salt solutions and then connecting these findings to nanoparticle interactions and assembly. Broader Impacts: This study will show how complex correlations between ions and charged colloids/macrom
|Effective start/end date||10/1/21 → 9/30/25|
- National Science Foundation (CNS-2107392-001)
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