STUDYING NANOPARTICLE AGGREGATE STRUCTURE BY CRYOGENIC ELECTRON TOMOGRAPHY

Nanoparticles in aqueous suspension frequently form complex structures via aggregation. Aggregate morphology is believed to affect transport of nanoparticles in surface and groundwater, and to reduce the accessibility and hence the effective reactivity of nanoparticle surfaces. Characterization of the aggregate structures formed in solution has historically been very difficult. Small-angle scattering techniques can be performed in situ, but data interpretation is typically based upon a predetermined structural model. Traditional methods in electron microscopy require dry samples, which severely alters aggregate morphology. We have adapted techniques in cryogenic electron microscopy (cryo-EM) to circumvent these issues. Using a rapid freezing method, suspended nanoparticle aggregates may be preserved intact within vitreous ice and studied in a transmission electron microscope.

We are using cryo-EM to obtain real space images of iron oxyhydroxide nanoparticles and their aggregates in solution. By applying tomographic techniques, we can reconstruct a three-dimensional representation of the distribution of nanoparticles in suspension. This provides insight into the interaction forces between non-aggregated nanoparticles, and directly reveals the morphology of aggregates when they have formed. We will show how the aqueous environment, particularly ionic strength, affects aggregate structure. In complementary experiments we are studying the transport of nanoparticle aggregates through a saturated silica column, and we will discuss the relationship between aggregate structure and transport.