Nanomaterials (NM) exhibit novel physicochemical properties that determine their interaction with biological substrates and processes. Three metal oxides nanoparticles that are currently being produced in high tonnage, TiO2, ZnO and CeO2, were synthesized by flame spray pyrolysis process and compared in a mechanistic study to elucidate the physicochemical characteristics that determine cellular uptake, subcellular localization, and toxic effects based on a test paradigm that was originally developed for oxidative stress and cytotoxicity in RAW 264.7 and BEAS-2B cell lines. ZnO induced toxicity in both cells, leading to the generation of reactive oxygen species (ROS), oxidant injury, excitation of inflammation and cell death. Using ICP-MS and fluorescent-labeled ZnO, it is found that ZnO dissolution could happen in culture medium and endosomes. Non-dissolved ZnO nanoparticles enter caveolae in BEAS-2B, but enter lysosomes in RAW 264.7 cells in which smaller particle remnants dissolve. In contrast, fluorescent-labeled CeO2 nanoparticles were taken up intact into caveolin-1 and LAMP-1 positive endosomal compartments, respectively, in BEAS-2B and RAW 264.7 cells, without inflammation or cytotoxicity. Instead, CeO2 suppressed ROS production and induced cellular resistance to an exogenous source of oxidative stress. Fluorescent-labeled TiO2 was processed by the same uptake pathways as CeO2 but did not elicit any adverse or protective effects. These results demonstrate that metal oxide nanoparticles induce a range of biological responses that vary from cytotoxic to cytoprotective and can only be properly understood by using a tiered test strategy such as we developed for oxidative stress and adapted to study other aspects of nanoparticle toxicity.

Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments.

Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC ⇒ anhydrous ACC ∼ biogenic anhydrous ACC ⇒ vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO 2 sequestration. amorphous calcium carbonate (ACC) | calorimetry | crystallization enthalpy | sea urchin larval spicules | synthetic and biogenic ACC

Sea urchin larval spicules transform amorphous calcium carbonate (ACC) into calcite single crystals. The mechanism of transformation is enigmatic: the transforming spicule displays both amorphous and crystalline properties, with no defined crystallization front. Here, we use X-ray photoelectron emission spectromicroscopy with probing size of 40–200 nm. We resolve 3 distinct mineral phases: An initial short-lived, presumably hydrated ACC phase, followed by an intermediate transient form of ACC, and finally the biogenic crystalline calcite phase. The amorphous and crystalline phases are juxtaposed, often appearing in adjacent sites at a scale of tens of nanometers. We propose that the amorphous-crystal transformation propagates in a tortuous path through preexisting 40- to 100-nm amorphous units, via a secondary nucleation mechanism.

The thermodynamic behaviour of small particles differs from that of the bulk material by the free energy term gammaA--the product of the surface (or interfacial) free energy and the surface (or interfacial) area. When the surfaces of polymorphs of the same material possess different interfacial free energies, a change in phase stability can occur with decreasing particle size. Here we describe a nanoparticle system that undergoes structural changes in response to changes in the surface environment rather than particle size. ZnS nanoparticles (average diameter 3 nm) were synthesized in methanol and found to exhibit a reversible structural transformation accompanying methanol desorption, indicating that the particles readily adopt minimum energy structural configurations. The binding of water to the as-formed particles at room temperature leads to a dramatic structural modification, significantly reducing distortions of the surface and interior to generate a structure close to that of sphalerite (tetrahedrally coordinated cubic ZnS). These findings suggest a route for post-synthesis control of nanoparticle structure and the potential use of the nanoparticle structural state as an environmental sensor. Furthermore, the results imply that the structure and reactivity of nanoparticles at planetary surfaces, in interplanetary dust and in the biosphere, will depend on both particle size and the nature of the surrounding molecules.

Nanoparticles may contain unusual forms of structural disorder that can substantially modify materials properties and thus cannot solely be considered as small pieces of bulk material. We have developed a method to quantify intermediate-range order in 3.4-nanometer-diameter zinc sulfide nanoparticles and show that structural coherence is lost over distances beyond 2nanometers. The zinc-sulfur Einstein vibration frequency in the nanoparticles is substantially higher than that in the bulk zinc sulfide, implying structural stiffening. This cannot be explained by the observed 1% radial compression and must be primarily due to inhomogeneous internal strain caused by competing relaxations from an irregular surface. The methods developed here are generally applicable to the characterization of nanoscale solids, many of which may exhibit complex disorder and strain.

We present near edge X-ray absorption spectra of manganese oxides at the Mn L2,3, Mn K, and O K edges to investigate the relative sensitivity of the edges to bonding and structure. Collectively, the spectra probe local electronic structure and intermediate range crystal structure. Spin independent full multiple scattering calculations of the Mn K edge give good agreement with data above threshold and qualitatively reproduce the prepeak that is observed for each compound. We show that the apparent prepeak for MnO is not due to p−d hybridization at the Mn atom (in accordance with symmetry principles) or quadrupolar transitions but originates from multiple scattering within the fifth shell. We present spin dependent multiple scattering calculations of the O K edge and show that this edge allows for a more direct description of the 3d states than either the Mn L edge or K edge prepeak, which are complicated by multiplet effects.

Enamel, the hardest tissue in the body, begins as a three-dimensional network of nanometer size mineral particles, suspended in a protein gel. This mineral network serves as a template for mature enamel formation. To further understand the mechanisms of enamel formation we characterized the forming enamel mineral at an early secretory stage using x-ray absorption near-edge structure (XANES) spectromicroscopy, transmission electron microscopy (TEM), FTIR microspectroscopy and polarized light microscopy. We show that the newly formed enamel mineral is amorphous calcium phosphate (ACP), which eventually transforms into apatitic crystals. Interestingly, the size, shape and spatial organization of these amorphous mineral particles and older crystals are essentially the same, indicating that the mineral morphology and organization in enamel is determined prior to its crystallization. Mineralization via transient amorphous phases has been previously reported in chiton teeth, mollusk shells, echinoderm spicules and spines, and recent reports strongly suggest the presence transient amorphous mineral in forming vertebrate bones. The present finding of transient ACP in murine tooth enamel suggests that this strategy might be universal.