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A Window of Opportunity: Designing Carbon Nanomaterials for Environmental Safety and Health

April 30, 2008
The nanotechnology movement has been given a unique "window of opportunity" to systematically investigate the toxicity of nanotechnology products and to develop ways to manage health risks before large scale manufacturing becomes widespread. 

 

Reviewed by Christie M. Sayes, Ph.D., Assistant Professor, Department of Veterinary Physiology & Pharmacology, Texas A&M University.

Background

Carbon nanomaterials are among the best known and most promising products of the nanotechnology movement.  Some early studies suggest that fullerenes and nanotubes may pose significant health risks, and this has given rise to an emerging literature on carbon nanotoxicology.  However, because of the various conflicting reports in the literature, further study is needed.

Objective

This review focuses on the various properties specific to carbon-based nanostructures (nanotubes and bucky balls) that may provide insight into their observed toxicity in both in vitro and in vivo models.  As the emerging field of nanotoxicology progresses, there will be increasing focus on the how nanoscale materials interact with biology and what properties greatly influence a nanomaterial’s toxicity. The goal of understanding the contributions of specific material features, such as size, shape, surface chemistry, and metals content.

Participants/Methods

The review includes discussions of three potential causes of carbon nanostructure toxicity: the role of catalytic metals, the role of carbon surface chemistry, and the role of aggregation state.

Evidence

The Role of Catalytic Metals. Of the various carbon nanotube production methods, the catalytic routes have become dominant, and as a result almost all of the CNT samples produced commercially contain metals. The most abundant CNT metals are iron and nickel, but yttrium, cobalt, and molybdenum are also common. A key material science question raised by the reviewers is whether sufficient metal catalyst or impurity can be released from CNT samples into the environment. They also point out that because the molecular mechanisms of metal toxicity involve soluble forms of the compound (as in particle dissolution), metal particles should only be active if they are accessible to the surrounding physiological fluids. Bioavailability is the single most important issue governing metal-CNT toxicity. If residual metal impurities in the nanotube sample are not accessible to cells, then there is no potential route of exposure; thus CNT risk is low. The authors cite their previous work as examples (Hurt, et al, Carbon (2006) 44 1028–1033).

The Role of Carbon Surface Chemistry.
Recent evidence indicates that surface chemistry is an important factor in the toxicity of carbon nanomaterials. Carbon surfaces are often described as “hydrophobic” – not soluble in water -- but in fact their surfaces vary greatly in hydrophobicity depending on the number and form of surface functional groups, that is, compounds added to the surface of the nanomaterial -- introduced by air or acids used in purification schemes. Several studies have shown that surface functionalization to increase hydrophilicity (water solubility) tends to also decrease the toxicity of carbon nanomaterials.

The Role of Aggregation State. Some have speculated that the risk to human health may depend on likelihood of carbon nanomaterials to aggregate. The hydrophobic nature of many carbon nanomaterials, coupled with the fibrous nature of CNTs that allow entanglement, give rise to aggregates both in solution and in the dry state. This entanglement greatly minimizes nanotube toxicity because exposure to entangled mats or polymer matrices limits aerosol formation and cellular uptake.

Results

The review emphasizes that further study is needed. The paper ends with a schematic drawing that depicts the potential carbon-based nanostructure exposure routes during various stages of the material’s life cycle.  Step 1 includes the exposure to laboratory personnel (graduate students, research technicians, etc.) in the initial research and development phase. Step 2 includes the occupational exposure during fabrication, purification & processing, and device manufacturing.  Step 3 includes exposures to the market place (consumers and the environment).

 

Last updated: July 12, 2010
 

DOI: 10.4053/er83-080430

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This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported.

Tags: Carbon nanotubes, Fullerenes, toxicity, purification, surface chemistry

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