The environmental behaviour of SWCNTs is almost completely unexplored, and current toxicological reports give apparently conflicting results. In order to reconcile these differences and begin to make sound material risk assessments, one must understand the chemical content of industrially produced SWCNTs.

A recent study published in Nanotechnology presents a first effort to provide such data. It showed that the materials had a wide variety of metal and amorphous carbon contents; and that no two SWCNT products were alike. These results indicate that risk and toxicological assessments must consider these differences explicitly, as they will affect environmental reactivity and toxicity. Risk assessments for a “generic” SWCNT will be flawed.

The ostensibly carbonaceous materials contained metals at weight-percent, even after manufacturer purification. The metals and their purification history influenced the apparent reactivity of the SWCNTs in halogenated solvents, as well as the calculated oxygen content. Oxygen functionality will dramatically affect surface properties of these nanoparticles, which are important for applications that rely on SWCNT adsorptivity. Futhermore, oxygen contained in the SWCNT-lattice will confer pH-dependent charge on the molecules, affecting their transport and interaction with biological membranes.

To date, there are no established methods to measure SWCNTs after their release to the environment, and unique metal contents will allow their quantification at microgram per cubic meter levels in air samples. In addition, distinct metal ratios may be used to apportion natural and industrial sources to the atmosphere.

At this critical juncture, when the nanotube industry is poised for growth and the public searches for conscientious product development, we must rely on chemical and physical understanding to avoid unintended consequences and secure the beneficial future of nanomaterials.