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Posted: Apr 27, 2010

Meaningful nanotechnology EHS research requires independent nanomaterial characterization

(Nanowerk Spotlight) One of the key issues in the young field of nanotoxicology is the lack of standards and definitions. Although there have been some international efforts like the International Alliance for NanoEHS Harmonization (which since its inception in 2008 and an initial January 2009 newsletter, hasn't posted much on its website, though) there still is no coherent international approach to determining if and what risks are posed by what kind of nanotechnology materials. At the core of the problem are the serious challenges that are created when comparing test results and drawing conclusions without adequate standardization and nanomaterial characterization.
What we are currently seeing is that individual research groups are picking certain areas of toxicological concern and forge ahead with – often highly specific – toxicology studies. Unfortunately, for lack of a common standard system (see: "Nanotechnology risk assessment could benefit from nanoparticle categorization framework "), these studies are difficult to compare and sometimes they even appear to contradict each other; a situation that is especially confusing in risk assessments of carbon nanotubes (see: "Comparing apples with oranges – the problem of nanotubes risk assessment").
Exemplifying this set of problems further, a new study shows that even the most basic set of data, the nanomaterial characterization information provided by the manufacturer, can't be relied on – something which shouldn't come as a complete surprise given the existing problems with characterization data. Nevertheless, the study drives home the fact that there is an important need for independent characterization data in environmental health and safety (EHS) studies of purchased nanomaterials.
" What we found in our work is that nanomaterials purchased from commercial sources may not be as well characterized as indicated by the manufacturer," Vicki H. Grassian, a professor in the Department of Chemistry at the University of Iowa, tells Nanowerk. "For example, it might be stated that a certain nanoparticle is being sold as 30 nm in diameter and, although '30 nm' might be close to the average diameter, there is usually a range of particle sizes that can extend from as much as small as 5 nm to as large as 300 nm."
Transmission electron microscopy (TEM) images of commercial aluminum oxide nanoparticles and size distribution
(A) Transmission electron microscopy (TEM) images of commercial aluminum oxide nanoparticles. (B) Size distribution obtained from TEM images. (Reprinted with permission from Wiley-VCH Verlag)
Reporting their findings in a recent paper in Environmental Toxicology and Chemistry ("Commercially manufactured engineered nanomaterials for environmental and health studies: Important insights provided by independent characterization"), among other problems Grassian and first author Heaweon Park also discuss the issue of batch-to-batch variability during the production of nanoparticles and that some nanomaterials which were being sold as having spherical morphology could contain mixed morphologies such as spheres and rods.
Grassian points out that among scientists in fields that have traditionally been involved in nanotechnology, for example physics, chemistry, chemical engineering and materials science, it is well known that materials characterization is really important. However, in fields of study that are more new to the field of nanoscience and nanotechnology, for example scientists who are focused on the environmental and health impacts of nanomaterials, this may not be as well known and understood. This paper was written for that group of scientist. It also underscores many of the reports and recommendations that have been made on the importance of nanomaterials characterization as a component of any environmental and health study.
"Over the past few years, I have heard talks from a number of researchers investigating the environmental and health implications of nanomaterials that had often times not been fully characterized or the characterization was taken off of the vendor's website without verification," says Grassian. "Our paper underscores several points and provides case studies of select examples of the need to provide an independent characterization of nanomaterials. The results discussed in this paper will provide an understanding of why independent characterization is important."
The study's results show that the information provided by the manufacturer may be incomplete and nonrepresentative of the entire sample and, in some cases, the data provided are incorrect. Grassian and Park describe three basic areas that can be the cause for these problems:
1) Nanomaterials can be inhomogeneous and unlike molecules with exact formulae, nanomaterials are typically made up of a sample that contains a distribution of sizes.
2) There are no standard methods for measuring properties such as particle size and surface area; thus different methods are used to report these physical properties, some of which are more accurate than others. In fact, much of the equipment needed to characterize nanoscale materials is expensive and/or specialized and therefore not readily available. This is especially true for small startup companies.
3) There is batch-to-batch variability in the manufacturing of nanomaterials and not all batches are characterized.
The conclusion from this is that it is important that independent characterization of commercially manufactured materials be performed in EHS studies of nanomaterials if the results are to be of use to the scientific community.
Grassian notes that in her own work, her group is using an integrated approach of utilizing methods and techniques in surface chemistry, surface science, colloid chemistry, aerosol science, and materials science to characterize nanomaterials so as to better understand the behavior of metal and metal oxide nanoparticles in air and water.
"Through EPA funding, we are investigating the 'state' of metal and metal oxide nanoparticles in water. We are interested in whether metal and metal oxide nanoparticles will be present as isolated nanoparticles, will they aggregate to form larger particles or will they dissolve into ions. This is an important question for a science that is "all about size" as the size regime that needs to be considered or modeled e.g. in water transport models is very different for ions, isolated nanoparticles or aggregates of nanoparticles." The group has published a paper in Langmuir which shows that dissolution of isolated iron-containing nanoparticles was enhanced on the nanoscale relative to larger particles but when the nanoscale particles were aggregated the dissolution was quenched ("Nanorod Dissolution Quenched in the Aggregated State").
"In NIOSH-funded research, we are trying to better understand the toxicity of inhaled metal and metal oxide nanoparticles, to determine if 'nanodust', i.e. dust from the production of engineered nanomaterials, is particularly hazardous relative to other types of dust that people in general and workers in production facilities in particular are exposed to. In these NIOSH studies, we have partnered with our colleagues in public health to combine careful nanomaterials characterization with inhalation toxicity studies." For instance, the group has published several papers on nanoscale titanium dioxide and shows that the smallest particles (ca. 5 nm) are not the most toxic (see: "Inhalation Exposure Study of Titanium Dioxide Nanoparticles with a Primary Particle Size of 2 to 5 nm" and "Inflammatory response of mice to manufactured titanium dioxide nanoparticles: comparison of size effects through different exposure routes").
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