Carbon Nanotubes
Toxicity
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The toxicity of carbon nanotubes has been an important question in nanotechnology. The data are still fragmentary and subject to criticism. Preliminary results highlight the difficulties in evaluating the toxicity of this heterogeneous material. Parameters such as structure, size distribution, surface area, surface chemistry, surface charge, and agglomeration state as well as purity of the samples, have considerable impact on the reactivity of carbon nanotubes. However, available data clearly show that, under some conditions, nanotubes can cross membrane barriers, which suggests that, if raw materials reach the organs, they can induce harmful effects such as inflammatory and fibrotic reactions.[citeration 39]
Under certain conditions CNTs can enter human cells and accumulate in the cytoplasm, causing cell death.[citeration 40]
Realibility [citeration 41]
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Recent tests at the National Institute of Standards and Technology (NIST) suggest carbon nanotube device reliability is a major issue. As a rough comparison, NIST researchers fabricated and tested numerous nanotube interconnections between metal electrodes. NIST test results show that nanotubes can sustain extremely high current densities (tens to hundreds of times larger than that in a typical semiconductor circuit) for several hours but slowly degrade under constant current. Of greater concern is that the metal electrodes fail—the edges recede and clump—when currents rise above a certain threshold. The circuits fail in about 40 hours.
Defects [citerarion 42], [citeration 43]
As with any material, the existence of a crystallographic defect affects the material properties. Defects can occur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to 85%. An important example is the Stone Wales defect, which creates a pentagon and heptagon pair by rearrangement of the bonds. Because of the minuscule structure of CNTs, the tensile strength of the tube is dependent on its weakest segment in a similar manner to a chain, where the strength of the weakest link becomes the maximum strength of the chain. Crystallographic defects also affect the tube's electrical properties. A common result is lowered conductivity through the defective region of the tube. A defect in armchair-type tubes (which can conduct electricity) can cause the surrounding region to become semiconducting, and single monatomic vacancies induce magnetic properties.
Crystallographic defects strongly affect the tube's thermal properties. Such defects lead to photon scattering, which in turn increases the relaxation rate of the phonons. This reduces the mean free path and reduces the thermal conductivity of nanotube structures. Photon transport simulations indicate that substitutional defects, such as the addition nitrogen or boron, will lead primarily to scattering of high-frequency optical phonons. However, larger-scale defects, such as Stone Wales defects, cause photon scattering over a wide range of frequencies, leading to a greater reduction in thermal conductivity.
Nanopollution [citerarion 44], [citeration 45]
Ecotoxicological impacts of nanoparticles and the potential for bioaccumulation in plants and microorganisms is a subject of current research, as carbon nanotubes are considered to present novel environmental impacts. The capacity for nanoparticles to function as a transport mechanism also raises concern about the transport of heavy metals and other environmental contaminants.
Whether carbon nanotubes merits special government regulation is a controversial issue. Regulatory bodies, such as the United States Environmental Protection Agency and the Health & Consumer Protection Directorate of the European Commission, have started dealing with the potential risks of nanoparticles. The organic food sector has been the first to act, with the regulated exclusion of engineered nanoparticles from certified organic produce, firstly in Australia and the UK,[1] and more recently in Canada, as well as for all food certified to Demeter International Standards. [citeration 37],[citeration 38 ]
The impact of carbon nanotubes extends from its medical, ethical, mental, legal and environmental applications, to fields such as engineering, biology, chemistry, computing, materials science, and communications. There are lots of major benefits of carbon nanotubes, However, potential risks include environmental, health, and safety issues.
Also, transitional effects, such as displacement of traditional industries as the products of nanotechnology become dominant, are of concern to privacy rights advocates. These problems may become particularly important, if potential negative effects of nanoparticles are resolved.
Regulation [citerarion 46], [citeration 47], [citeration 48]
Significant debate exists relating to the question of whether carbon nanotubes based products merit special government regulation. This debate is related to the circumstances in which it is necessary and appropriate to assess new substances prior to their release into the market, community, and environment.
Regulatory bodies in the U.S., as well as in the EU, have concluded that nanoparticles carry the potential for an entirely new risk and that it is necessary to carry out an extensive analysis of the risk. The challenge for regulators is whether a system can be developed which would identify nanoparticles and more complex nanoformulations which are likely to have special toxicological properties or whether it is more reasonable for each particle or formulation to be tested separately.
