Impacts of Ionic Strength on Three-Dimensional Nanoparticle Aggregate Structure and Consequences for Environmental Transport and Deposition

The transport of nanoparticles through aqueous systems is a complex process with important environmental policy ramifications. Ferrihydrite nanoparticles commonly form aggregates, with structures that depend upon solution chemistry. The impact of aggregation state on transport and deposition is not fully understood. In this study, small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) were used to directly observe the aggregate structure of ferrihydrite nanoparticles and show how the aggregate structure responds to changing ionic strength. These results were correlated with complementary studies on ferrihydrite transport through saturated quartz sand columns. Within deionized water, nanoparticles form stable suspensions of low-density fractal aggregates that are resistant to collapse. The particles subsequently show limited deposition on sand grain surfaces. Within sodium nitrate solutions the aggregates collapse into denser clusters, and nanoparticle deposition increases dramatically by forming thick, localized, and mechanically unstable deposits. Such deposits limit nanoparticle transport and make transport less predictable. The action of ionic strength is distinct from simpler models of colloidal stability and transport, in that salt not only drives aggregation or attachment but also alters the behavior of preexisting aggregates by triggering their collapse

Determination of the Three-Dimensional Structure of Ferrihydrite Nanoparticle Aggregates

Aggregation impacts the reactivity, colloidal stability, and transport behavior of nanomaterials, yet methods to characterize basic structural features of aggregates are limited. Here, cryo-transmission electron microscope (cryo-TEM) based tomography is utilized as a method for directly imaging fragile aggregates of nanoparticles in aqueous suspension and an approach for extracting quantitative fractal dimensions from the resulting three-dimensional structural models is introduced. The structural quantification approach is based upon the mass autocorrelation function, and is directly comparable with small-angle X-ray scattering (SAXS) models. This enables accurate characterization of aggregate structure, even in suspensions where the aggregate cluster size is highly polydisperse and traditional SAXS modeling is not reliable. This technique is applied to study real suspensions of ferrihydrite nanoparticles. By comparing tomographic measurements with SAXS-based measurements, we infer that certain suspensions contain polydisperse aggregate size distributions. In other suspensions, fractal-type structures are identified with low intrinsic fractal dimensions. The fractal dimensions are lower than would be predicted by simple models of particle aggregation, and this low dimensionality enables large, low-density aggregates to exist in stable colloidal suspension

Osmotically-Driven Transport in Carbon Nanotube Porins

We report the measurements of transport of ions and uncharged species through carbon nanotube (CNT) porinsshort segments of CNTs inserted into a lipid bilayer membrane. Rejection characteristics of the CNT porins are governed by size exclusion for the uncharged species. In contrast, rejection of ionic species is governed by the electrostatic repulsion and Donnan membrane equilibrium. Permeability of monovalent cations follows the general trend in the hydrated ion size, except in the case of Cs⁺ ions

Self-Assembly of “S-Bilayers”, a Step Toward Expanding the Dimensionality of S‑Layer Assemblies

Protein-based assemblies with ordered nanometer-scale features in three dimensions are of interest as functional nanomaterials but are difficult to generate. Here we report that a truncated S-layer protein assembles into stable bilayers, which we characterized using cryogenic-electron microscopy, tomography, and X-ray spectroscopy. We find that emergence of this supermolecular architecture is the outcome of hierarchical processes; the proteins condense in solution to form 2-D crystals, which then stack parallel to one another to create isotropic bilayered assemblies. Within this bilayered structure, registry between lattices in two layers was disclosed, whereas the intrinsic symmetry in each layer was altered. Comparison of these data to images of wild-type SbpA layers on intact cells gave insight into the interactions responsible for bilayer formation. These results establish a platform for engineering S-layer assemblies with 3-D architecture

Self-Assembly of “S-Bilayers”, a Step Toward Expanding the Dimensionality of S‑Layer Assemblies

Protein-based assemblies with ordered nanometer-scale features in three dimensions are of interest as functional nanomaterials but are difficult to generate. Here we report that a truncated S-layer protein assembles into stable bilayers, which we characterized using cryogenic-electron microscopy, tomography, and X-ray spectroscopy. We find that emergence of this supermolecular architecture is the outcome of hierarchical processes; the proteins condense in solution to form 2-D crystals, which then stack parallel to one another to create isotropic bilayered assemblies. Within this bilayered structure, registry between lattices in two layers was disclosed, whereas the intrinsic symmetry in each layer was altered. Comparison of these data to images of wild-type SbpA layers on intact cells gave insight into the interactions responsible for bilayer formation. These results establish a platform for engineering S-layer assemblies with 3-D architecture