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Surface Immobilization of Engineered Nanomaterials for in Situ Study of their Environmental Transformations and Fate
Citation:
Sekine, R., M. Khaksar, G. Brunetti, E. Donner, K. Scheckel, E. Lombi, AND K. Vasilev. Surface Immobilization of Engineered Nanomaterials for in Situ Study of their Environmental Transformations and Fate. Jerald Schnoor (ed.), ENVIRONMENTAL SCIENCE & TECHNOLOGY. ACS Publications, Washington, DC, 47(16):9308-9316, (2013).
Impact/Purpose:
Rapid progress in nanotechnology over the last two decades has led to the birth of innovative applications and industries that can potentially improve many aspects of life. However, the inevitable release of engineered nanomaterials (ENMs) into the environment is an increasing concern.1,2 A diverse range of ENMs with specifically engineered properties (e.g., optical,3 transport,4 and catalytic5) has been developed for use in consumer products.6 These are generally produced with tailored elemental composition, particle size and shape, surface charge, and functionality. There are unique properties that arise in the nanoscale due to geometrical (e.g., high specific surface area) and quantum mechanical (e.g., optical resonance) effects, which are not observed within the micro- or macro-scopic domains. However, these special properties are also a reason for the significant concern regarding their potential interactions with the environment, as the associated risks (acute and chronic) may differ from that of their micro- or macro-scale equivalents. Uniquely, X-ray absorption spectroscopy (XAS) can be performed within the host matrix (i.e., without separation), enabling the detection and investigation of ENMs deployed in the real environment. For example, XAS has been applied to detect and study the fate of AgNPs and zinc oxide (ZnO) nanoparticles through wastewater and sewage treatment processes in situ17-19 and found them to yield their respective sulfides in the freshly treated sludge. This is in agreement with the previous identification of Ag2S nanoparticles in sewage sludge products.20 However, the concentrations need to be high enough for reliable characterization by XAS (typically tens of mg•kg-1) which may exceed realistic environmental concentrations other than in specific end points such as sludge. Further complications arise in the real, open environment as it is a complex and dynamic system that cannot always be simulated accurately in a closed environment. On the field scale, a major project is underway where AgNPs are being released into the Experimental Lakes Area under the Lake Ecosystem Nanosilver (LENS) project to determine ecosystem level impacts.21 This is a highly valuable investigation, but is a unique approach that cannot be performed repeatedly in different environments. In this work, we present a novel approach designed to track the chemical transformations of ENMs deployed in the environment by attaching them to a solid support via strong surface interactions. This was achieved by plasma polymerization; a technique that can deposit thin polymer films onto solid surfaces and that can immobilize nanoparticles via electrostatic or covalent bonding.28,29 This approach has a unique advantage in that the charge, functionality, and density of the polymer coatings can be easily controlled by varying the monomer composition,30 so that the coated surfaces can accommodate ENMs of different surface charge and concentration. ENMs often adsorb and accumulate at various interfaces (e.g., liquid-solid interface, bacterial/algal cell walls); therefore, this also provides a realistic model design of where they may be found in natural environments.
Description:
The transformation and environmental fate of engineered nanomaterials (ENMs) is the focus of intense research due to concerns about their potential impacts in the environment as a result of their uniquely engineered properties. Many approaches are being applied to investigate the complex interactions and transformation processes ENMs may undergo in aqueous and terrestrial environments. However, major challenges remain due to the difficulties in detecting, separating, and analyzing ENMs from environmental matrices. In this work, a novel technique capable of in situ study of ENMs is presented. By exploiting the functional interactions between surface modified silver nanoparticles (AgNPs) and plasma-deposited polymer films, AgNPs were immobilized on to solid supports that can be deployed in the field and retrieved for analysis. Either negatively charged citrate or polyethylene glycol, or positively charged polyethyleneimine were used to cap the AgNPs, which were deployed in two field sites (lake and marina), two standard ecotoxicity media, and in primary sewage sludge for a period of up to 48 h. The chemical and physical transformations of AgNPs after exposure to different environments were analyzed by a combination of XAS and SEM/EDX, taken directly from the substrates. Cystine- or glutathione-bound Ag were found to be the dominant forms of Ag in transformed ENMs, but different extents of transformation were observed across different exposure conditions and surface charges. These results successfully demonstrate the feasibility of using immobilized ENMs to examine their likely transformations in situ in real environments and provide further insight into the short-term fate of AgNPs in the environment. Both the advantages and the limitations of this approach are discussed.
