The regulation of engineered nanoparticles requires a widely agreed definition of such particles. Nanoparticles are routinely defined as particles with sizes between about 1 and 100 nm that show properties that are not found in bulk samples of the same material. Here we argue that evidence for novel size-dependent properties alone, rather than particle size, should be the primary criterion in any definition of nanoparticles when making decisions about their regulation for environmental, health and safety reasons. We review the size-dependent properties of a variety of inorganic nanoparticles and find that particles larger than about 30 nm do not in general show properties that would require regulatory scrutiny beyond that required for their bulk counterparts.

Iron-based nanoparticles have been proposed for an increasing number of biomedical or environmental applications although in vitro toxicity has been observed. The aim of this study was to understand the relationship between the redox state of iron-based nanoparticles and their cytotoxicity toward a Gram-negative bacterium, Escherichia coli. While chemically stable nanoparticles (gammaFe2O3) have no apparent cytotoxicity, nanoparticles containing ferrous and, particularly, zerovalent iron are cytotoxic. The cytotoxic effects appear to be associated principally with an oxidative stress as demonstrated using a mutant strain of E. coli completely devoid of superoxide dismutase activity. This stress can result from the generation of reactive oxygen species with the interplay of oxygen with reduced iron species (Fe(II) and/or Fe(0)) or from the disturbance of the electronic and/or ionic transport chains due to the strong affinity of the nanoparticles for the cell membrane.

We examined the physical and chemical characteristics of colloidal dispersions of fullerene materials (nC60) produced through several solvent exchange processes and through extended mixing in water only. The nC60 produced via the different methods were unique from each other with respect to size, morphology, charge, and hydrophobicity. The greatest dissimilarities were observed between the nC60 produced by extended mixing in water alone and the nC60 produced by solvent exchange processes. The role of the respective solvents in determining the characteristics of the various nC60 were attributed to differences in the solvent-C60 interactions and the presence of the solvent as a residual in the nC60 structure, indicating the significance of the solvent properties in determining the ultimate characteristics of the colloidal fullerene. Thus, fullerene C60 that may become mobilized through natural processes (agitation in water) may behave in dramatically different ways than those produced through more artificial means. These results highlight the difficulties in generalizing nC60 properties, particularly as they vary in potential toxicity considerations.

The production of significant quantities of engineered nanomaterials will inevitably result in the introduction of these materials to the environment. Mobility in a well-defined porous medium was evaluated for eight particulate products of nanochemistry to assess their potential for migration in porous media such as groundwater aquifers and water treatment plant filters. Contrary to the assertion that nanomaterials present monolithic environmental risks, here we show that these nanomaterials exhibit widely differing transport behaviors. Fullerene-based nanomaterials that had been functionalized to facilitate dispersal in water displayed the highest mobilities, with a calculated potential to migrate approximately 10 m in unfractured sand aquifers. Colloidal aggregates of C60, which have been the focus of recent toxicity studies, were among the least mobile of the nanomaterials evaluated.

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Fe K-edge EXAFS spectroscopy has been used to determine the nucleation mechanisms and to propose a model of colloid and gel formation during the hydrolysis of FeCl3 in the presence of PO4 in acidic solutions. The displacement of OH or O ligands by PO4 anions drastically changes the normal path of Fe(III) hydrolysis in pure water: (i) The Fe(III) nuclei are mainly dimers and linear trimers formed through equatorial edge sharing. (ii) One Cl remains in the first coordination shell of Fe even when gels or precipitates are formed. (iii) Precipitates or gels are obtained through the aggregation of different subunits. In the precipitates, the subunits correspond to three iron dimers associated through one PO 4 tetrahedron. The precipitates are formed by aggregating the subunits through a Fe-Fe corner-sharing link (Fe-Fe ) 4 Å). In gels, the subunits are two dimers associated through one PO4 tetrahedron. The aggregation of subunits through PO4 leads to the formation of chains. Fe monomer bridges link these chains together by corner sharing, thus forming the gel. The growth mechanisms of Fe-PO4 colloids are dependent on the initial P/Fe ratio.