Aggregation and Toxicology of Titanium Dioxide Nanoparticles
Environ Health Perspect. doi:10.1289/ehp.10915 available via http://dx.doi.org [Online 27 March 2008]
Referencing: Inhalation Exposure Study of Titanium Dioxide Nanoparticles with a Primary Particle Size of 2 to 5 nm
In their study of inhalation exposure of titanium dioxide particles, Grassian et al. (2007) presented a transmission electron micrograph (TEM) (their Figure 2A) as an image of "dispersed" TiO2 nanoparticles. Yet, the TiO2 nanoparticles in this TEM do not appear to be dispersed. There is clear evidence of self-organization of the nanoparticles into distinct assemblages, separated by relatively large regions devoid of any particle. This spatial pattern, very unlikely to occur randomly, is even more apparent when Grassian et al.'s TEM is contrast-enhanced, sharpened, and thresholded (Figure 1A) to eliminate the initial grainy background. With this image, one can demonstrate quantitatively the extent of clustering by calculating the radial distribution function (Torquato 2002), defined as the probability of finding a nanoparticle, in any direction, at various distances away from the center of a given nanoparticle. We compared the values obtained for this function with those associated with an image in which the same nanoparticles have been artificially dispersed (with image processing software). In the dispersed case (Figure 1B), the probability of finding a black pixel drops precipitously when the distance exceeds the apparent radius of nanoparticles, and then stays close to zero thereafter. In the "original" case (Grassian et al.'s Figure 2A), there is also a drop, but the radial distribution function never gets to zero. It progressively increases again as the radial distance increases. This quantitative difference between the curves in Figure 1B leads to the conclusion that the nanoparticles in Figure 1A are clustered.
Figure 1. (A) Contrast-enhanced, sharpened, and segmented version of a TEM of a TiO2 nanoparticle suspension (modified from Grassian et al. 2007). (B) Radial distribution function versus radial distance for a representative point in a nanoparticle in (A); the dashed line indicates values for the "original image" [Figure 2A from Grassian et al. (2007)] and the solid line represents a similar point in an image where the nanoparticles are artificially dispersed.
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However, this conclusion is intriguing in itself. Indeed, before obtaining their TEM, Grassian et al. (2007) suspended the TiO2 nanoparticles in methanol and sonicated the suspension for an unspecified, but presumably appreciable "period of time." Given this strongly dispersive treatment, it is remarkable that aggregation still occurred to the extent it did. This observation suggests that the 2- to 5-nm size of the primary TiO2 "nano"-particles may be somewhat irrelevant to environmental and toxicologic concerns because in nature, under conditions far more conducive to aggregation than those imposed by Grassian et al. (2007), nanoparticles may never be found alone, but are part of significantly larger-sized aggregates. In a recent study, French et al. (French RA, Jacobson AR, Kim B, Isley SL, Penn RL, Baveye PC, unpublished data) observed that in aqueous suspensions under a range of environmentally relevant conditions of pH and ionic strength, TiO2 nanoparticles form aggregates of several hundred nanometers to several micrometers in diameter within minutes.
This aggregation may have toxicologic implications. In any given system (e.g., aerosols), it is possible that even a slight change in pH or ionic strength may cause TiO2 nanoparticles to cluster differently, and therefore to have very dissimilar biological activity. In general, this might explain mixed results found in the literature on the toxicity of TiO2 nanoparticles to environmentally relevant species. Until now, these inconclusive results have been explained (Oberdörster et al. 2005) by arguing that the high biological activity of TiO2 nanoparticles, caused by their large specific surface area, creates a high potential for inflammatory, pro-oxidant, and antioxidant activity. Yet, conflicting observations may perhaps be imputable instead to compounding factors due to nanoparticle aggregation, which so far has not been given serious consideration.
The authors declare they have no competing financial interests.
Philippe Baveye
SIMBIOS Centre
University of Abertay Dundee
Dundee, Scotland
Magdeline Laba
Department of Civil and
Environmental Engineering
Cornell University
Ithaca, New York
References
Grassian VH, O'Shaughnessy PT, Adamcakova-Dodd A, Pettibone JM. 2007. Inhalation exposure study of titanium dioxide nanoparticles with a primary particle size of 2 to 5 nm. Environ Health Perspect 115:397–402.
Oberdörster G, Oberdörster E, Oberdörster J. 2005. Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles. Environ Health Perspect 113:823–839.
Torquato S. 2002. Random Heterogeneous Materials: Microstructure and Macroscopic Properties. New York: Springer.
Titanium Dioxide Nanoparticles: Grassian et al. Respond
Environ Health Perspect. doi:10.1289/ehp.10915R available via http://dx.doi.org [Online 27 March 2008]
Baveye and Laba have further analyzed the transmission electron micrograph (TEM) image shown in Figure 2A of our article (Grassian et al. (2007b) to quantitatively determine the extent of titanium dioxide nanoparticle clustering in the image by calculating the radial distribution function. The main point of doing this calculation was to demonstrate that TiO2 nanoparticle aggregates will not completely deaggregate even when subjected to harsh conditions.
We completely agree with the statement of Baveye and Laba that "aggregation may have toxicologic implications." We disagree with their suggestion that "nanoparticle aggregation … so far has not been given serious consideration." There is growing consensus that nanoparticle aggregation is an important factor in understanding the health implications of nanoparticles. This has been described by researchers working in the area of nanoparticle toxicity (Balbus et al. 2007; Powers et al. 2006), as well as by us. In addition to Grassian et al. (2007b), we refer to another study in which we further investigated TiO2 nanoparticle aggregation in inhalation and instillation studies (Grassian et al. 2007a). In that study we demonstrated that the size and nature of nanoparticle aggregates are important factors in evaluating their toxicity, and we suggested that the natural behavior of these particles and the manner in which people are exposed are critical factors in determining risk. If the nanoparticles do not deaggregate when inhaled, then the aggregation size and nature may be significant physicochemical properties in the toxicity of nanoparticles.
Moreover, combining extensive physicochemical characterization studies of nanoparticles with evaluation of their toxicity, as we have done (Grassian et al. (2007a, 2007b), will foster greater understanding of the environmental health impacts of nanotechnology.
V.H.G. is paid a consulting fee as a member of the science advisory board of Nanoscale Materials Inc. (Manhattan, KS) and owns stock shares in that company. In addition, she is a paid member of the scientific advisory board of Northern Nanotechnologies, Inc. (Toronto, Ontario, Canada). The remaining authors declare they have no competing financial interests.
Vicki H. Grassian
Patrick T. O'Shaughnessy
Andrea Adamcakova-Dodd
John M. Pettibone
Peter S. Thorne
University of Iowa
Iowa City, Iowa
References
Balbus JM, Maynard AD, Colvin VL, Castranova V, Daston GP, Denison RA, et al. 2007. Meeting Report: Hazard assessment for nanoparticles—report from an interdisciplinary workshop. Environ Health Perspect 115: 1654–1659.
Grassian VH, Adamcakova-Dodd A, Pettibone JM, O'Shaughnessy PT, Thorne PS. 2007a. Inflammatory response of mice to manufactured titanium dioxide nanoparticles: comparison of size effects through different exposure routes. Nanotoxicology 1(3):211–226.
Grassian VH, O'Shaughnessy PT, Adamcakova-Dodd A, Pettibone JM, Thorne PS. 2007b. Inhalation exposure study of nanoparticulate titanium dioxide with a primary particle size of 2 to 5 nm. Environ Health Perspect 115: 397–402.
Powers KW, Brown SC, Krishna VB, Wasdo SC, Moudgil BM, Roberts SM. 2006. Research strategies for safety evaluation of nanomaterials. Part VI. Characterization of nanoscale particles for toxicological evaluation. Toxicol Sci 90:296–303.