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Monday, 23 February 2015

Are Nanomaterials Toxic?

There's a lot of concern about the potential toxicity of nanomaterials, intensified by the absence of regulatory standards. This means they aren’t currently required to be safety tested before being used in commercial products. So are nanomaterials toxic, what limits our understanding and how big are the risks to our health?

What are nanomaterials?

Nanomaterials are defined purely in terms of size - they are between 1 and 100 nm in at least one dimension. 1 nm is one millionth of a mm in length, and may be occupied by as few as three atoms, depending on their kind. Nanomaterials can be sheets, wires, rods, particles or platelets, and can be made of any material and have any other properties. Natural nanomaterials include spiders' silk and cotton, and manufactured nanomaterials include carbon nanotubes, metal and metal oxide particles and soots. They occur in paints, fabrics and cosmetics, food packaging and drug delivery medicines. More than a dozen new consumer products containing nanomaterials enter the market every month[1].

Why does the “nano” category exist?

Nanomaterials differ from particles with larger dimensions because their surface area to mass ratio is so high that their chemistry is governed primarily by surface rather than bulk properties: i.e. most atoms can easily reach or directly influence the surface. The chemistry of surface atoms is different because they are not surrounded on all sides to provide a stable environment: they behave like something between bulk atoms and free atoms.

Diagram distinguishing surface atoms
The first few layers of atoms influence the surface, and so contribute to the material's 'surface properties'.
Image credit: ©TWDK

Even across the nanoscale, transitions may occur between fundamental physical and chemical properties rendering new behaviours and unique properties; for example, copper is highly malleable and ductile because it forms layers 50 nm thick which can slide past each other; but when copper is less than 50 nm thick it exists only as one layer, and so loses these bulk properties. When gold nanoparticles are suspended in solution, they have a variety of colours depending upon particle sizes - but none of them gold. The colour and shininess of gold is due to light absorption by fast-moving electrons used in metallic bonding. In gold nanoparticles, the motion of electrons is limited because of the small size, so bulk properties are suppressed. Trapped electrons produce quantum effects which dominate bonding, making the relatively inactive metal highly reactive, and changing the colour we see.

Selection of different solutions of gold nanoparticles, each with a different colour
As the size of the gold nanoparticle increases, the colour of the solution in which it is suspended changes: here you can see that the smaller nanoparticle gives a red tone, whilst the larger nanoparticle produces lilac.
Photograph by Aleksandar Kondinski, via Wikimedia Commons (CC BY-SA)

So what about toxicity? Toxicity is already hard to measure because it depends upon dose and the specific animal, plant or cells being tested; toxicity may vary between species. But importantly, properties of a bulk material may not be a good indication of the toxic behaviour of the nanomaterial. New chemical behaviour at the nanoscale may mean nanomaterials readily transform into new, unpredictable materials that vary in toxicity further. So far there is no clear trend to suggest nanomaterials are in general more toxic than bulk materials. In fact, there are no clear trends at all. We just don't know.

The risk...

Nanomaterials are consistently being released into the environment via spillages, wear, washing and disposal at a rate proportional to their level of use. We don't know where they go or what happens to them after they are released: they may accumulate on land or water and enter the food chain via plants and aquatic life, where they could exist harmlessly or react. They might enter the human body: routes include inhalation, ingestion or even possibly through our skin.

It has been suggested that once inside the body, the smallness of nanomaterials allows them to easily could allow them to penetrate living cells, where they can amass, disrupt cell activity or corrupt genes[3]. They can also bypass the usual transportation channels, cross the blood-brain barrier and enter the central nervous system. The high surface area and surface activity of nanomaterials means they may have amplified effects; tests on human cells grown in laboratories have shown immune reactions and inflammations in lung tissue[4]. However, it is important to remember that many of these dangers are still speculative: man-made nanoparticles are relatively simple, and existing materials are not considered to have the complexity of systems like viruses that themselves struggle to enter and corrupt cells, and are of a similar size to nanomaterials[5].

There is only one way to find out whether these risks are realistic or speculative at normal nanomaterial concentrations – testing. But this is rather difficult...

Problems with Testing

Toxicity is not only idiosyncratic to the chemical composition and structure of each nanomaterial, but is also dependent upon other factors - in particular, shape and solubility[2]. Shape may affect interactions with binding sites on enzymes, and sphericity or regularity may affect surface reactivity and mobility. Surface charge fundamentally changes reactivity: negatively charged membranes interact with positively charged particles, but not negative ones. Even size matters: after all, a 100 nm diameter particle is one million times bigger (in volume) than a 1 nm diameter particle – and even across the nanoscale, transitions may occur between fundamental properties...

Whether the tests are carried out on plants, cells, or organisms can also affect the conclusions of testing, as can the materials used during nanomaterial synthesis to direct the nano size and shape – since most of these are toxic themselves and might contaminate products. These factors all depend on the experimental conditions under which the nanomaterial was synthesised and tested, making comparison between research groups almost impossible.

For the toxicity of nanomaterials to be meaningful, they need to be compared with the toxicity of the bulk material, but not all materials can exist with the same chemical structure for nano and bulk types. Even if they do, we can't be sure nanomaterials retain their dimensions under testing: they might form chains, aggregate lumps, or break down into smaller nanoparticles.

Without accounting for these variations, we can't be sure how the “nano” factor affects toxicity, and whether nanomaterials pose us any risk. This is why no regulatory guidelines exist, and why nanomaterials are so freely used. But precisely because they are being freely used, we really need to know the answers.

This article was written by TWDK chemistry editor Rowena Fletcher-Wood, who is completing a PhD in environmental materials chemistry at the University of Birmingham. Rowena can be found on twitter as @RowenaFW.

Edit: 26 Mar 2015
The original version of this article incorrectly stated the smallness of nanomaterials allows them to easily penetrate living cells which has been clarified as speculative.

why don't all references have links?

[1] Ray, Paresh Chandra, Hongtao Yu, and Peter P. Fu. "Toxicity and Environmental Risks of Nanomaterials: Challenges and Future Needs." Journal of environmental science and health. Part C, Environmental carcinogenesis & ecotoxicology reviews 27.1 (2009): 1–35. PMC. Web. 5 Jan. 2015. doi: 10.1080/10590500802708267
[2] Warheit, David B. "How meaningful are the results of nanotoxicity studies in the absence of adequate material characterization?." Toxicological Sciences 101.2 (2008): 183-185. doi: 10.1093/toxsci/kfm279
[3] Karlsson HL, Cronholm P, Gustafsson J, Moller L. "Copper oxide nanoparticles are highly toxic: A comparison between metal oxide nanoparticles and carbon nanotubes." Chem. Res. Toxicol. 2008;21:1726–1732 doi: 10.1021/tx800064j.
[4] Patra HK, Banerjee S, Chaudhuri U, Lahiri P, Dasgupta AK. "Cell-selective response to gold nanoparticles." Nanomedicine. 2007;3:111–119 PMID: 17572353
[5] Levy, R. "We are doomed..." Rapha-z-lab June 5, 2014