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Express It in Numbers: Efforts to Quantify Engineered Nanoparticles in Environmental Matrices Advance Desiree L. Plata,*,† P. Lee Ferguson,†,‡ and Paul Westerhoff§ †
Department of Civil and Environmental Engineering, ‡Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, United States § School of Sustainable Engineering and The Built Environment, Arizona State University, Tempe, Arizona 85287-5306, United States structures. CNTs can be produced as powders (from freefloating catalysts) or as aligned structures with organized architectures (from patterned, substrate-affixed catalysts). To add to the variability, as-produced, bulk CNT powders can contain much more than nanotubes; by weight, they may be up to 30% metal catalyst and/or 30% non-CNT carbonaceous impurities (i.e., amorphous or noncrystalline carbon).1 The type of metal associated with the powders varies according to the synthesis (e.g., Fe, Co, Mo, Ni, and/or Y), while substrateaffixed nanotubes can be relatively free of metals if the substrate is removed. Postfabrication processing further alters CNT composition, where (1) impurities can be removed via thermal and/or acid treatment, (2) the nanotubes can be functionalized with heteroatoms, and/or (3) CNTs can be coated with surfactants or fully encapsulated in polymer resins. Further modifications to CNTs will occur following their release to the environment, but these are not yet fully understood.2 Possible processes include agglomeration, interaction with natural organic matter, and photochemical alterations. Whether engineered or natural, surface modifications will affect CNT charge, mobility, reactivity, and toxicity. Clearly, CNTs are not a single entity, but rather, a suite of distinct but related compounds. ithout the ability to quantify a novel chemical or In this issue of ES&T, several articles target distinct material in the environment, we are blind to the study of environmental matrixes and account for variability in its transport, ignorant in the prediction of biological exposure commercially available materials. Each analysis presents unique to it, and unarmed in both the pursuit and enforcement of challenges, briefly highlighted here: meaningful regulation. Typically, analytical methods for new materials in complex matrices (e.g., air, water, sediment, and BIOLOGICAL MATRICES tissue) require years to decades of research before useful techniques are available. This delay between production and Quantifying CNT exposure to organisms requires detection in ability to study a chemical/material in the natural world has led the presence of tissues (e.g., fats and muscle) and fluids (e.g., to unfortunate consequences for the environment, public heath, blood, serum, milk, and excrement). CNTs may be physically and economic longevity of commercial products. Aware of this and/or chemical entrapped within tissues, or may be coated pattern, environmental chemists have made significant and with biological materials that influence their extraction early efforts to avoid such a tragedy associated with the efficiency and pose significant interferences to CNT measurement. For example, thermal combustion processes that have developing nanomaterial industry. In particular, several research been used for CNT measurement can transform biological groups endeavored to quantify carbon nanotubes (CNTs, a organic matter into thermally stable “charred” material that class of nanomaterials with great industrial promise inorganic elicits a similar instrumental response as CNTs. Methods to nanoparticles) in complex natural systems. In this issue of selectively reduce or account for such interferences should be Environmental Science and Technology (ES&T), several articles enumerated (e.g., extraction). Following CNT isolation, it may present marked advances toward this time-sensitive, critical be necessary to confirm composition using spectroscopy and/ goal. or electron microscopy techniques. The Doudrick et al. Generically, CNTs are hollow cylinders of sp2-hybridized manuscript (doi:10.1021/es300804f) investigates techniques carbon atoms with characteristically high aspect (length-todiameter) ratios. Generalities end here; beyond this, CNTs represent a very diverse class of materials. They can be either Received: December 12, 2011 single-walled or multiwalled (Figure 1), and the former can be Accepted: July 16, 2012 semiconducting or metallic as a result of unique primary Published: November 20, 2012
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Figure 1. Graphitic nanoparticles and representative pre- and postmanufacturing modifications. Representative transformation processes are shown for individual NPs, but apply to several types of NPs (e.g., agglomeration is shown for SWCNTs, but applies to all NPs). SWCNTs = single-walled CNTs; MWCNT = multiwalled CNTs.
engineered nanoparticles, CNTs may be the most difficult to analyze at trace levels in environmental samples. In the case of metal nanoparticles, the analyst is looking for the proverbial “needle in a haystack”. Recent success in this area was achieved using a novel single-particle ICP-MS technique; the subject of the Ranville et al. contribution. With CNTs in carbon-rich sediments, the problem is more like searching for unique wheat within the same haystack. However, single-walled carbon nanotubes do have distinguishing electronic structures that can be interrogated via a novel and powerful spectroscopy, near-infrared fluorescence spectroscopy. The Schierz et al. study (doi:10.1021/es301856a) describes the application of this technique for part-per-billion CNT quantification. In the words of Lord Kelvin, “When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge of it is a meager and unsatisfactory kind.” Fundamentally, our ability to understand any process is limited by our ability to interrogate it. The nanomaterial revolution has become a commercial reality, and fluxes of engineered nanomaterials to the environment will increase as the industry grows. Thus, the development of more precise and sensitive analytical tools for engineered nanoparticles is both crucial and urgent. The studies presented in this issue lay the necessary foundations and represent significant advances toward reaching this vital goal.
to quantify CNTs in biological matrices, and these will prove indispensible for meaningful exposure, toxicity, bioaccumulation, and biological fate studies.
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AEROSOL CONCENTRATES AND SEDIMENTS CNTs are prized for their exceptional electronic, mechanical, and thermal properties, and these unique properties might offer routes toward quantification in the environment. Thermal methods have been used to isolate other recalcitrant carbon forms (e.g., black carbon)3 from aerosol concentrates and sediments, where carbonaceous interferences are great. However, due to the very low anticipated environmental levels of CNTs (e.g., circa pg to ng m−3air or pg to ng g−1sediment),4 thermal methods alone may not be sufficient to quantify CNTs. Tandem approaches, where thermal separation is paired with secondary detection methods that interrogate the unique chemical composition of CNTs (e.g., high C-to-O/H/N ratios by mass spectrometry or elemental analysis), may prove valuable to distinguish CNTs at lower detection limits. The Plata et al. work (doi:10.1021/es203198x) represents an important advance in such techniques and may someday overcome the challenge of isolating and quantifying CNTs from the critical air, water, and sediment compartments.
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AQUEOUS SAMPLES AND SEDIMENTS Analysis of CNTs and inorganic nanopartilces in aqueous systems presents unique challenges. In water, the insoluble nature and variable colloidal stability of nanomaterials necessitates high sensitivity for quantification in dilute solutions. In sediments, nanomaterials may exist within a complex matrix of organic and inorganic components. Thus, analytical methods must be highly selective in order to effectively discriminate against the background. Among
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 12244
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REFERENCES
(1) Plata, D. L.; Gschwend, P. M.; Reddy, C. M. Industrially synthesized singlewalled carbon nanotubes: compositional data for users, environmental risk assessments, and source apportionment. Nanotechnology 2008, 19, 185706. (2) Petersen, E. J.; Zhang, L.; Mattison, N. T.; O’Carroll, D. M.; Whelton, A. J.; Uddin, N.; Nguyen, T.; Huang, Q.; Henry, T. B.; Holbrook, R. D.; Chen, K. L. Potential release pathways, environmental fate, and ecological risks of carbon nanotubes. Envrion. Sci. Technol. 2011, 45, 9837−9856. (3) Goldberg, E. D. Black Carbon in the Environment: Properties and Distribution; John Wiley & Sons: New York, NY. 1985. (4) Koelmans, A. A.; Nowack, B.; Wiesner, M. R. Comparison of manufactured and black carbon nanoparticle concentration in aquatic sediments. Environ. Pollut. 2009, 157, 1110−1116.
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