Nanoparticles are entities that have one, two or three dimensions of 100 nm or less. In recent years, their applications have expanded exponentially in diverse fields, ranging from medicine to electronics, due to their enhanced or novel physicochemical properties. As particles become smaller, the ratio of surface atoms to inner atoms increases giving a higher surface area to nanoparticles compared to their bulk counterparts. The physicochemical properties of the particles are also different from the bulk form and can even be size dependant. Therefore, nanomaterials can be made with special characteristics for dedicated applications. Whilst they are already present in many commercially available products, doubts have been expressed over their safety, given that the same factors responsible for their novel properties may be the source of their potential hazard. At present, commercially available nanomaterials are characterised only for the purpose of establishing whether a desired property is exhibited; there is no legal requirement for toxicity tests.
Carbon nanotubes (CNTs), cylindrical carbon molecules with uniquely high length to diameter ratios, can exist as single-walled (SWCNT) or multi-walled (MWCNT) entities. Their main technological advantage is their exceptional strength, but they also display other interesting physical properties. For these reasons, they have been studied extensively and are already used in many new commercial products. However, their high length to diameter ratios mean they are fibre shaped, and thus have been compared to asbestos. Some studies have found evidence that at least some types of CNTs can, under certain circumstances, show pathogenicity . A further toxicity concern for CNTs and other carbon-based nanomaterials (e.g. fullerenes) is the potential presence of metal impurities, which are the by-product of the synthesis methods , as well as the use of surfactants to disperse them in aqueous media, which can modify their properties and hence toxicity as well as interfere directly with the toxicity tests .
Here, we make a case for detailed characterisation of CNTs prior to toxicity tests, particularly because manufacturers' characterisation are often incomplete. We demonstrate the presence of carbon based micron-sized structures, effectively structural impurities within MWCNTs, from two different manufacturers; both materials have been used in many previous studies. We emphasis the need to combine characterisation methods and look for impurities at different scales.