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Posted: Feb 04, 2008
The detection of carbon nanotubes and workplace safety
(Nanowerk Spotlight) More and more carbon nanotube (CNT) applications are moving from the research lab into commercial products. For example, CNTs can be found already in tennis rackets and bicycles, displays and TV screens, and numerous resins used by aerospace, defense, health care, and electronics companies. Not surprisingly, CNT production is growing by hundreds of metric tons a year. One of the large suppliers alone, Bayer, is talking about having 3,000 metric tons of production capacity in place by 2012. As a result of the increasing supply, prices are dropping fast. While a kilogram of multi-walled CNTs (MWCNTs) sold for tens of thousands of dollars just a few years ago (and single-walled CNTs still do), the price for some types of MWCNTs has fallen to hundreds of dollars per kg. Recent market analyses forecast sales of all nanotubes to reach $1 billion to $2 billion annually within the next four to seven years. In terms of dollar value, electronics devices will be the largest end-use category, although composite materials in automotive applications may account for greater volumes. These volumes are expected to approach several thousand metric tons per year. This means that the exposure to CNTs, especially by factory workers, will increase substantially over the next few years. Since the jury is still out as to the toxicity of nanotubes it appears prudent to at least develop suitable sensor technology to detect CNTs, especially in the workplace.
Over recent years, the rapidly emerging applications of CNTs have made the CNT community increasingly aware of the need for more toxicological studies on the material they were working with. A recent review paper in Nanotoxicology ("The detection of airborne carbon nanotubes in relation to toxicology and workplace safety") introduces a new paradigm in that the community should not only be interested in production, applications and toxicological effects but also to recognize flaws with current tools for detection and emphasize the need to develop an efficient sensor platform.
"In our review paper we have raised the need for a better detection platform in the CNT-affected workplace" Dr. Peter Cumpson tells Nanowerk. "The quickly rising industrial production of carbon nanotubes highlights the ever-increasing need to have an efficient and effective tool for the detection of nanotubes – because right now we don't. This new tool must be improved compared to the general purpose airborne particle counters that are currently employed, to allow better sensitivity and specificity to CNTs."
According to Cumpson, a researcher at the National Physical Laboratory (NPL) and a Visiting Professor at the University of Newcastle-upon-Tyne in the UK, such a platform ideally would allow single strand sensitivity with sufficient selectivity for nanotube detection amongst (other airborne) impurities. "The general consensus is that the sampling and detection combination should be capable of monitoring the concentration of airborne nanotubes and that the detection platform should have sufficient sensitivity to measure exposure well below levels where there is a risk to health" he says.
The paper, first-authored by Ratna Tantra, a researcher at the NPL, is an assessment of current commercial instruments that have the potential to meet certain criteria identified as key components in a CNT detection platform in the workplace.
The tools reviewed are based on instruments that have been previously used as characterization techniques to examine CNTs' quality (diameter, purity, etc) after production. The focus of the paper is evaluating their suitability as an effective detection platform.
"Besides sensitivity and selectivity issues, we have assessed other criteria including the potential for automation, the ability of detecting the different forms of CNTs (aggregates, bundles and individual strands) and the ability to give some indication of CNT concentration" Ratna explains to Nanowerk. "Our review has shown that no single perfect method exists i.e. one that meets all of the requirements against the key criteria identified."
Raman spectroscopy, a tool capable of generating a chemical fingerprint (as displayed in the spectrum of that molecule) was considered as having the most potential. Although the tool provides the basis for single strand detection, it only shows high selectivity to certain types of carbon nanotubes, particularly single-walled CNTs. In addition, the need to incorporate multiple laser lines in the instrument so as to detect nanotubes with different diameters, may prove to be expensive.
A comparison of various technologies, which show potential to be used as tool for the detection of airborne CNTs in the workplace. (Source: Ratna Tantra)
"In the past, scientists have likened some nanotubes to resemble the shape of asbestos fibers, which have been linked to cancer" Ratna says. "This has triggered new concerns about airborne exposure to CNTs. However, it is important to stress that the toxicology of CNTs is largely an unexplored field, particularly with regards to the different levels of toxicological complexity and diversity associated with the many different types of nanotubes. In addition, there is also the issue of exposure frequency and concentration levels."
The two UK scientists stress that their biggest concern is the limited knowledge that exists with regard to CNTs' toxicity and they argue that the minimum criteria regarding detection issues for CNTs, until the facts have been established, should be based on the assumption that all CNTs are hazardous to health.
"Until we are able to fully extract and unravel all the relevant information an immediate implementation of preventive measures should be carried out and having an early detection platform is vital" says Cumpson. "Quite simply put: CNTs are a next generation material and there is a need to develop next generation detection technologies to go with that."
Ratna notes that, with regard to evaluation criteria in relation to sampling, this will follow a similar trend for general nanoparticle sampling: "In the past, a number of key issues have been highlighted, which are related to the general properties of nanoparticles. For example, nanoparticles will not stay localized and have tendency to move rapidly away from the source, potentially exposing many individuals; this poses questions regarding sampling location and frequency. From a workplace safety point of view, this would suggest the need to monitor CNTs from personal rather than fixed workstations on a routine basis. Furthermore, like all nanoparticles, CNTs are prone to aggregation; aggregates can bond to each other to form agglomerates and the speed at which they agglomerate will depend on their numerical concentration and mobility. Careful considerations in choosing how to sample are required so as to ensure that analysis will give a true representation of the different forms of CNTs in which the workers are exposed to. Correspondingly, it is best to implement ‘direct’ sampling activities, which would involve the collection of airborne CNTs on to a suitable substrate. For example, an adhesive coated substrate would be suitable supporting substrate, as the adhesive layer will promote entrapment and fix CNTs."
Cumpson says that this review article marks the starting point of the NPL scientists' research into this subject. In the future they would very much like to be involved in a ‘community-based’ research, which would require more communication with different players of the nanotube community e.g. toxicologists, instrument manufacturers, nanotube factories, users, etc.
"Certainly this in itself is challenging – with different people having different vested interests – but it is important for specialists to work together on an international level" he says. "Through the collection and dissemination of knowledge, this will enable us to firstly further develop the criteria for evaluating an effective detection platform, which will then pave the way to further developing technologies to render them as ‘suitable’; this will be done either through using new technology or through the novel integration of existing platforms."