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Materials whose dielectric constant varies spatially with submicrometer periodicity exhibit diffractive optical properties which are potentially valuable in a number of existing and emerging applications. Here, such systems are fabricated by exploiting the spontaneous crystallization of monodisperse silica spheres into close-packed arrays. By reliance on a vertical deposition technique to pack the spherical colloids into close-packed silica-air arrays, high quality samples can be prepared with thicknesses up to 50 µm. These samples are planar and thus suitable for optical characterization. Scanning electron microscopy (SEM) of these materials illustrates the close-packed ordering of the spherical colloids in planes parallel to the substrate; cross-sectional SEM micrographs of the arrays as well as optical methods are used to measure sample thickness and uniformity. Normal-incidence transmission spectra in the visible and near-infrared regions show distinct peaks due to diffraction from the colloidal layers. While these basic optical characteristics are similar to thicker and polycrystalline gravity-sedimented colloidal crystals, the systematic control over the number of colloidal layers allows the effect of sample thickness on the optical spectrum to be studied for the first time.

Human exposure to nanoparticles is inevitable as nanoparticles become more widely used and, as a result, nanotoxicology research is now gaining attention. However, while the number of nanoparticle types and applications continues to increase, studies to characterize their effects after exposure and to address their potential toxicity are few in comparison. In the medical field in particular, nanoparticles are being utilized in diagnostic and therapeutic tools to better understand, detect, and treat human diseases. Exposure to nanoparticles for medical purposes involves intentional contact or administration; therefore, understanding the properties of nanoparticles and their effect on the body is crucial before clinical use can occur. This Review presents a summary of the in vitro cytotoxicity data currently available on three classes of nanoparticles. With each of these nanoparticles, different data has been published about their cytotoxicity due to varying experimental conditions as well as differing nanoparticle physiochemical properties. For nanoparticles to move into the clinical arena, it is important that nanotoxicology research uncovers and understands how these multiple factors influence the toxicity of nanoparticles so that their undesirable properties can be avoided.

For nearly a decade, researchers have debated the mechanisms by which AgNPs exert toxicity to bacteria and other organisms. The most elusive question has been whether the AgNPs exert direct "particle-specific" effects beyond the known antimicrobial activity of released silver ions (Ag(+)). Here, we infer that Ag(+) is the definitive molecular toxicant. We rule out direct particle-specific biological effects by showing the lack of toxicity of AgNPs when synthesized and tested under strictly anaerobic conditions that preclude Ag(0) oxidation and Ag(+) release. Furthermore, we demonstrate that the toxicity of various AgNPs (PEG- or PVP- coated, of three different sizes each) accurately follows the dose-response pattern of E. coli exposed to Ag(+) (added as AgNO(3)). Surprisingly, E. coli survival was stimulated by relatively low (sublethal) concentration of all tested AgNPs and AgNO(3) (at 3-8 μg/L Ag(+), or 12-31% of the minimum lethal concentration (MLC)), suggesting a hormetic response that would be counterproductive to antimicrobial applications. Overall, this work suggests that AgNP morphological properties known to affect antimicrobial activity are indirect effectors that primarily influence Ag(+) release. Accordingly, antibacterial activity could be controlled (and environmental impacts could be mitigated) by modulating Ag(+) release, possibly through manipulation of oxygen availability, particle size, shape, and/or type of coating.

With the increased presence of nanomaterials in commercial products, a growing public debate is emerging on whether the environmental and social costs of nanotechnology outweigh its many benefits. To date, few studies have investigated the toxicological and environmental effects of direct and indirect exposure to nanomaterials and no clear guidelines exist to quantify these effects.

We show that the cytotoxicity of water-soluble fullerene species is a sensitive function of surface derivatization; in two different human cell lines, the lethal dose of fullerene changed over 7 orders of magnitude with relatively minor alterations in fullerene structure. In particular, an aggregated form of C60, the least derivatized of the four materials, was substantially more toxic than highly soluble derivatives such as C3, Na+ 2 - 3[C60O7 - 9(OH)12 - 15](2-3)-, and C60(OH)24. Oxidative damage to the cell membranes was observed in all cases where fullerene exposure led to cell death. We show that under ambient conditions in water fullerenes can generate superoxide anions and postulate that these oxygen radicals are responsible for membrane damage and subsequent cell death. This work demonstrates both a strategy for enhancing the toxicity of fullerenes for certain applications such as cancer therapeutics or bactericides, as well as a remediation for the possible unwarranted biological effects of pristine fullerenes.

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