Nanogeoscience at Berkeley

Introduction to Nanogeoscience

Research Highlights

Geochemical reactions are complex, multistep processes in which the elementary steps are too fast for direct observation. However, developments in the field of ultrafast spectroscopy offer new ways to observe chemical reactions at the timescales that they occur. An introduction to how pump-probe studies are able to watch reactions with subnanosecond temporal resolution is given here. We are particularly interested in studying interfacial electron transfer reactions, such as the reductive dissolution of ferric iron oxides and oxyhydroxides by inorganic or protein electron donors. We recently demonstrated a method for light-initiated electron transfer to iron oxide nanoparticles, and used this to perform the first measurements of electron hopping rates in these solid phases.

The nanoscience revolution is leading to increasing release of nanoscale materials into the environment. It is particularly challenging to understand the potential biological impacts of nanomaterials that dissolve in aqueous solutions. Zinc oxide (ZnO) nanoparticles are widely used in sunscreen, and are partially soluble, releasing the toxic divalent zinc ion. Although there is no evidence that the application of ZnO nanoparticles in sun screen to human skin epidermal cells is hazardous, ZnO nanoparticle exposure to other cells, or to organisms in the environment, may have additional consequences. ZnO nanoparticle toxicity has been documented but without directly distinguishing the contributions from aqueous ion and nanoparticle effects. We used two complementary X-ray microscopes at the Advanced Light Source to identify and chemically speciate zinc in human bronchial epithelial cells. Our data strongly support a model of ZnO nanoparticle toxicity that is based upon nanoparticle uptake followed by intracellular dissolution.

Nanoparticle Coastlines: Predictions of water structure around iron oxide nanoparticles
Water is ubiquitous at the Earth's surface, and the interactions of water with mineral surfaces is in an important factor for their stability and reactivity. Dino Spagnoli performed large-scale molecular dynamics simulations using the Geochemistry cluster computer to investigate how the size and shape of iron oxide (hematite) nanoparticles affected the sorption and structure of interfacial water. The simuations predict that the surfaces cause the formation of ordered, layered water but that the ordering descreases with the particle size. Moreover, the dynamic properties of interfacial water are affected, with the residence time of water molecules near the surface being shorter for smaller, less-crystalline nanoparticles than for larger nanoparticles or a bulk hematite surface.

Read this paper in Geochim. Cosmochim. Acta

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Extracellular Proteins Sweep Up Nanoparticles
Certain microorganisms are proficient at precipitating mineral nanoparticles as a by-product of their metabolism. In oxygen-poor and zinc-rich waters, sulfate reducing bacteria can precipitate zinc sulfide nanoparticles, reducing the dissolved concentrations of this toxic metal. However, the effects of ZnS precipitation on the bacteria, and mobility

Nanocluster Formation in Aqueous Nanoparticle Suspensions
We discovered that ~ 6 nm nanoparticles of iron oxyhydroxide (FeOOH) exhibit qualitatively new colloid behavior. In addition to regimes of aqueous chemical conditions for which the nanoparticles are (i) fully suspended and (ii) completely aggregated and settled, there is an additional regime in which the nanoparticles (iii) undergo partial aggregation to form nanoclusters. The cluster size is controlled by solution pH, indicating that it is the charge on the nanoparticle surfaces that is the crucial parameter. Although nanocluster formation can be explained by classical concepts in colloid science, it is a size-dependent phenomenon.

Read this paper in JCIS

Water-Driven Structural Transition in Nanoparticles
An important consequence of small particle size is the high proportion of surface sites at which atoms may be unable to adopt the chemical environment found in the interior. Molecular dynamics simulations indicated that this can lead to high surface energies for nanomaterials, and suggested the possibility that nanoparticle structure may be sensitive to the type and extent of ligand binding. We tested this idea by allowing water molecules to adsorb to ZnS nanoparticle synthesized in anyhydrous conditions, and found the water binding stimulated a profound re-arrangement of the nanoparticle structure. Our results indicated that nanoparticle structure is not kinetically trapped, but responsive to environmental changes. Any nanoscale material or component may be susceptible to unpredictable structural change if exposed to water, or other adsorbates.