Cal Baier-Anderson, Ph.D., is a Health Scientist.
The manufacture of carbon nanotubes (CNTs) is a very complicated business. Different production processes leave behind different kinds of metal catalysts, which yield differences in physical and chemical – as well as toxicological – properties of the CNTs.
Removal of metals can alter the surface and other properties. These properties affect both the performance and the potential toxicity or other biological activity of the CNTs.
While a given material’s intended use often dictates the means of manufacturing and subsequent processing, the challenge is to integrate safety considerations up front rather than as an afterthought to be considered only at the end of the design process. But to do so, we need to develop a much better understanding of how properties and structural features correlate with safety risks.
A pair of interesting papers just published in the journal Chemical Research in Toxicology examines the relationship between these physicochemical properties and potential hazard.
In these papers, a group of European researchers created and characterized five kinds of CNTs, and then explored the question of what features are the most important in determining hazard: residual metals, the presence of oxygenated functional groups, or surface defects. As measures of hazard, the authors examined the ability of each CNT to scavenge free radicals (a protective function since free radicals can damage cell membranes), cause lung inflammation in rats, and damage DNA. The results are intriguing:
- The presence of oxygenated functional groups on the surface of the CNTs appears to be most closely associated with genotoxicity.
- Lung inflammation increased with the number of surface defects or oxy groups, and also with increasing metal concentration.
- Somewhat counter-intuitively, CNTs that lacked both residual metals and surface defects were the only ones not effective as scavengers of free radicals. Introduction of surface defects into these CNTs enhanced their ability to scavenge free radicals, supporting the hypothesis that such scavenging is associated with surface defects.
It’s more complicated than I can describe here and it’s likely that multiple features contribute to most or all of the observed effects. But these papers begin the difficult task of teasing out which are the most important properties in determining hazard potential.
The findings inch us closer to the day when designers of nanomaterials can incorporate specific properties that meet the dual goals of performance and low toxicity.