ScienceWeek

MATERIALS SCIENCE: ON THE TOXIC POTENTIAL OF NANOMATERIALS

The following points are made by Andre Nel et al (Science 2006 311:622):

1) By some estimates, nanotechnology promises to far exceed the impact of the Industrial Revolution and is projected to become a $1 trillion market by 2015. Engineered nanomaterials (NM) are already being used in sporting goods, tires, stain-resistant clothing, sunscreens, cosmetics, and electronics and will also be increasingly utilized in medicine for purposes of diagnosis, imaging, and drug delivery. Mihail Roco of the U.S. National Nanotechnology Institute envisages four generations of nanotechnology. The current era is that of passive nanostructures, materials designed to perform one task. The second phase will introduce active nanostructures for multitasking, for example, actuators, drug delivery devices, and sensors. The third generation is expected to emerge around 2010 and feature nanosystems with thousands of interacting components. A few years after that, the first integrated nanosystems, functioning much like a mammalian cell with hierarchical systems within systems, are expected to evolve.

2) The unusual physicochemical properties of engineered NM are attributable to their small size (surface area and size distribution), chemical composition (purity, crystallinity, electronic properties, etc.), surface structure (surface reactivity, surface groups, inorganic or organic coatings, etc.), solubility, shape, and aggregation. Although impressive from a physicochemical viewpoint, the novel properties of NM raise concerns about adverse effects on biological systems, which at the cellular level include structural arrangements that resemble NM in terms of their function. Indeed, some studies suggest that NM are not inherently benign and that they affect biological behaviors at the cellular, subcellular, and protein levels [1-5]. Moreover, some nanoparticles readily travel throughout the body, deposit in target organs, penetrate cell membranes, lodge in mitochondria, and may trigger injurious responses.

3) There is almost unanimous opinion among proponents and skeptics alike that the full potential of nanotechnology requires attention to safety issues. Already there are outcries from environmental activists calling for a worldwide moratorium on NM research and marketing until protocols are in place to ensure worker safety. Science fiction novels and news media reports have also perpetuated a scary scenario in which self-replicating nanoscale robots consume all available materials, ultimately strangling the planet in a "gray goo." Although this scenario is implausible from an energy as well as a structural assembly viewpoint, it points to the need to develop a rational, science-based approach to nanotoxicology. The authors believe that such an approach is feasible and should be implemented to ensure the safe manufacturing and marketing of engineered nanoproducts.

4) In summary: Nanomaterials are engineered structures with at least one dimension of 100 nanometers or less. These materials are increasingly being used for commercial purposes such as fillers, opacifiers, catalysts, semiconductors, cosmetics, microelectronics, and drug carriers. Materials in this size range may approach the length scale at which some specific physical or chemical interactions with their environment can occur. As a result, their properties differ substantially from those bulk materials of the same composition, allowing them to perform exceptional feats of conductivity, reactivity, and optical sensitivity. Possible undesirable results of these capabilities are harmful interactions with biological systems and the environment, with the potential to generate toxicity. The authors believe the establishment of principles and test procedures to ensure safe manufacture and use of nanomaterials in the marketplace is urgently required and achievable.

References (abridged):

1. R. F. Service, Science 304, 1732 (2004)

2. V. L. Colvin, Nat. Biotechnol. 21, 1166 (2003)

3. K. Donaldson et al., Occup. Environ. Med. 58, 211 (2001)

4. G. Oberdörster et al., Environ. Health Perspect. 113, 823 (2005)

5. Royal Society, "Nanoscience and nanotechnologies: Opportunities and uncertainties" (Royal Society, London, 2004); available online at www.nanotec.org.uk/finalReport.htm.

Science http://www.sciencemag.org

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Related Material:

NANOTECHNOLOGY: DNA TEMPLATING OF SUPERCONDUCTING NANOWIRES

The following points are made by D.S. Hopkins et al (Science 2005 308:1762):

1) DNA has recently been considered as a "backbone" for the fabrication of information-processing devices, chemical and biological sensors, and molecular transistors at the nanometer-size scale [1,2]. By taking advantage of DNA self-assembly possibilities [3], one can envision using single DNA and self-assembled DNA constructs as scaffolds for precise nanometer-scale positioning of other molecules and nanoscale objects. Recently, electronic devices with features that have molecular-scale dimensions have been assembled on such molecular-scale scaffolds [4]. One of the simplest practical realizations of this approach lies in the metallization of DNA molecules [5]. Previously, a wet-chemistry approach was used to metallize DNA [5], which tends to yield rather granular wires that become highly resistive at low temperatures.

2) The authors used a physical method of metallization that involves sputter deposition of metallic films over suspended DNA molecules to fabricate wires as thin as 3 to 4 molecular diameters (as thin as 5 to 15 nm). These nanowires are homogeneous, make seamless contacts with the leads, and become superconducting at low temperatures. The authors fabricated structures with pairs of such DNA-templated nanowires to study quantum interference effects and the effect of thermal fluctuations at the nanoscale. Well-known examples of quantum interference include critical-current oscillations in conventional superconducting quantum interference devices (SQUIDs) and Little-Parks resistance oscillations in thin-walled cylinders. The measurements of the authors on two-nanowire devices show a strong discrepancy with the usual behavior, and the authors provide a quantitative theoretical explanation for the observed period and amplitude of the oscillations.

3) In summary: The application of single molecules as templates for nanodevices is a promising direction for nanotechnology. The authors used a pair of suspended DNA molecules as templates for superconducting two-nanowire devices. Because the resulting wires are very thin, comparable to the DNA molecules themselves, they are susceptible to thermal fluctuations typical for one-dimensional superconductors and exhibit a nonzero resistance over a broad temperature range. The authors observed resistance oscillations in these two-nanowire structures that are different from the usual Little-Parks oscillations. The authors provide a quantitative explanation for the observed quantum interference phenomenon, an explanation that takes into account strong phase gradients created in the leads by the applied magnetic field.

References (abridged):

1. E. Braun, K. Keren, Adv. Phys. 53, 441 (2004)

2. H. Watanabe, C. Manabe, T. Shigematsu, K. Shimotani, M. Shimizu, Appl. Phys. Lett. 79, 2462 (2001)

3. N. C. Seeman, Angew. Chem. Int. Ed. 37, 3220 (1998)

4. H. Yan, S. H. Park, G. Finkelstein, J. H. Reif, T. H. LaBean, Science 301, 1882 (2003)

5. E. Braun, Y. Eichen, U. Sivan, G. Ben-Yoseph, Nature 391, 775 (1998)

Science http://www.sciencemag.org

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Related Material:

SELF-ASSEMBLY OF METAL NANOSTRUCTURES ON POLYMER SCAFFOLDS

The following points are made by W.A. Lopes and H.M. Jaeger (Nature 2001 414:735):

1) Self-assembly is emerging as an elegant "bottom-up" method for fabricating nanostructured materials. This approach becomes particularly powerful when the ease and control offered by the self-assembly of organic components is combined with the electronic, magnetic, or photonic properties of inorganic components.

2) The authors report a demonstration of a versatile hierarchical approach for the assembly of organic-inorganic copolymer-metal nanostructures in which one level of self-assembly guides the next. In a first step, ultrathin diblock copolymer films form a regular scaffold of highly anisotropic stripe-like domains. During a second assembly step, differential wetting guides diffusing metal ions to aggregate selectively along the scaffold, producing highly organized metal nanostructures.

3) The authors report that in contrast to the usual requirement of near-equilibrium conditions for ordering, the metal arranged on the copolymer scaffold produces the most highly ordered configurations when the system is far from equilibrium. The authors delineate two distinct assembly modes of the metal component -- each mode characterized by different ordering kinetics and strikingly different current-voltage characteristics. The authors suggest these results therefore demonstrate the possibility of guided large-scale assembly of laterally nanostructured systems.

Nature http://www.nature.com/nature

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