Single-walled carbon nanotubes (SWNTs) have emerged as high-performance nanomaterials with numerous applications, including electronic, optical, medical, and structural composite technologies. Because of their anticipated role in large-scale industrial production, there is little doubt that SWNTs will ultimately find their way into our aquatic environment. The unusual physicochemical characteristics of SWNTs compared to other nanoparticles -- particularly their very large aspect ratio and complex colloidal behavior (e.g. aggregation) in aqueous solutions -- preclude meaningful theoretical predictions of their colloidal stability and transport behavior. Furthermore, there is a complete lack of fundamental studies on the effects of SWNT structural properties (e.g. diameter and electronic structure) on their fate, transport and biological interactions in aquatic systems. Consequently, there exist no reliable methods to predict the fate and implications of SWNTs in natural aquatic environments based on measurable physical properties of these nanomaterials.
The purpose of this proposal is to fundamentally understand the effect of structural properties of SWNTs on their fate and biological behavior in the natural aquatic environment. The study design involves systematic evaluation of aggregation, deposition, organic compound sorption, and uptake/toxicity in fish (Japanese medaka) for semiconductive SWNTs for a range of electronic structures (with consequent variation in diameter and chirality). These studies will be fully novel, as there are no fundamental studies reported in the literature examining the effect of basic structural properties on environmental fate and implications of SWNTs. The research proposed will address the following aims. (1) Fractionation of semiconductive SWNTs by diameter/chirality using density gradient ultracentrifugation; (2) examination of aggregation and deposition kinetics of diameter/chirality-sorted SWNT fractions as a function of organic matter concentration and ionic strength using state-of-the-art dynamic light scattering, quartz crystal microbalance, and conventional column-flow transport experiments; (3) determination of the effect of SWNT diameter/chirality on organic contaminant adsorption using common headspace-partitioning methods; and (4) assessment of the uptake, bio-distribution, and toxic effects of diameter/chirality-sorted SWNTs in Japanese medaka fish after waterborne- or dietary exposure using near-IR fluorescence spectroscopy and microscopy.
The proposed research will fill a critical gap in the scientific literature by providing a systematic understanding of the effects of structural properties on SWNT fate, transport and biological behavior. Results of this study may lead to structure-property relations for SWNTs in aquatic environments, allowing a priori predictions of their fate, transport and effects and therefore meets the definition set forth by the NSF for transformative research.
The expected research outcomes and benefits include implementation of techniques to systematically separate chiral SWNTs; a complete understanding of the effects of electronic structure on colloidal stability, deposition, and sorptive properties; role of SWNT electronic structure on biological uptake and toxic effects; and finally an increased knowledge-base on the influence of surface structure on the behavior and effects of nanomaterials in the aquatic environment. This increase in basic scientific understanding will ultimately lead to structure-activity relationships from which we may build strategies to assess risks of nanomaterials in the ambient environment, as is currently possible for molecular environmental contaminants.
The proposed activity will generate critical knowledge to better understand the environmental implication of a commercially important class of nanomaterial. The majority of requested funds are directed toward the training of doctoral students in an emerging and interdisciplinary research topic. This research will lead to discovery and understanding through teaching and exploration. It provides for student education, mentoring, and research in a novel and highly relevant area that is of immediate and critical importance to our society. This project has potential to involve minority students through student exchange activities with two minority institutions. Dissemination of the research results is planned through conference presentations and peer-reviewed publications.
Hexagonal rings or carbon when coiled to form one-dimensional tubular structures are known as single-walled carbon nanotubes (SWNTs); the variation in helicity (or coil angle) alongside with tube diameter gives unique atomic configuration, known as chirality. Change in chirality is known to alter electronic properties of SWNTs. This project is the first to-date to evaluate the role of chirality of SWNTs on their environmental behavior. Aggregation kinetics, fractal dimension, transport in complex landfill systems, sorption of synthetic organics, SWNT detection in aquatic species, and their toxicity behavior were tested systematically. New advancements have been made in methods for determining fractal dimension (packing density of SWNT clusters) and detection of SWNTs in aquatic species using near infra-red fluorescence (NIRF) spectroscopy. Intellectual Merit Outcomes: --Differences in chirality, i.e., (6,5) and (7,6) SWNTs, showed significant differences in aggregation kinetics. Larger diameter (7,6) tubes showed ~8 fold higher aggregation compared to (6,5) ones. The mechanism of aggregation was determined as enhanced van der Waals (or tube-tube) interaction for higher diameter (7,6) SWNTs, resulting in increased aggregation. The mechanistic analysis was backed up with ab initio calculations (Khan et al., Environ Sci Technol, 2013, 47, 1844-1852). --Role of selected anionic surfactants on aggregation kinetics was also evaluated. Both surfactant molecular structure and SWNTs’ chiral identity played a significant role in controlling their aggregation behavior. Sodium dodecyl benzene sulfonate showed most effectiveness in SWNT stability followed by sodium deoxycholate and sodium dodecyl sulfate. However, (7,6) tubes compromised the surfactant stability, compared to (6,5) ones (Khan et al., Nanotechnology, 2013, in review). --Fractal dimension Df, a key indicator of aggregate structure and packing density, was determined by angle-dependent light scattering. Results showed chiral specific dependence; i.e., (6,5) were more fractal or loosely bound compared to the (7,6) tubes. Effect of biological medium type showed lower sensitivity. Moreover, presence of fetal bovine serum (FBS) and bovine serum albumin (BSA), used to mimic the in-vitro cell culture condition, reduced the Df values, i.e., created more fractal structures. (Khan et al., Chemosphere, 2013, 93, 1997-2003). --Aggregation behavior differences occurred between metallic and semiconducting SWNTs (such classification is based on their chiral differences as the electronic properties get altered due to such differences). Metallic SWNTs were found to be significantly more stable (less aggregating) compared to semiconducting ones. The differences collapsed in presence of divalent electrolytes. It is hypothesized that the metallic SWNTs, due to their high electrical conductivity (or low resistivity), contributes in ion mobilities, hence caused differences in aggregation (Khan et al., 2013, in preparation). --Transport of SWNTs in complex landfill conditions (high humic acid and salt content) was performed. Results showed that SWNT transport is limited by the type of landfill leachate (higher in older humic-type leachate compared to younger acetic acid-type ones). Also landfill media type showed strong influence on transport; paper showed highest retention compared to glass, plastic, and metal (Khan et al., Environ Sci Technol, 2013, 47, 8425-8433). --Comparative sorption isotherm studies of 14C-naphthalene and 14C-hexachlorobenzene were performed on chiral SWNTs. Results indicate that chirality will not be a primary determining factor in the association of hydrophobic, aromatic organic contaminants with SWNT in the aquatic environment. Instead, the most important determining factor for HOC sorption to SWNTs is the degree of "defect" modification on the hydrophobic surface of the tubes (Keira et al., 2013, in preparation). --Chiral specific SWNTs detection in aquatic species (fathead minnows) was performed using NIRF. Results indicate that exposure to SWNTs did not cause any overt toxicity; no impact on viability and qualitative health was observed. Results from this aim demonstrate that NIRF can provide qualitative and quantitative assessment of SWNT distribution in tissues with many advantages over other techniques used in environmental matrices (Bisesi et al., Environ Sci Technol, 2013, revised manuscript in review). Broader Impact Outcomes: --The evaluated role of chirality on SWNT fate, transport, adsorption, detection, and toxicity will help determine a more informed and accurate environmental safety and risk. --At least 5 graduate students were mentored and trained to pursue chiral-specific SWNT related research. They have published a number of journal articles and also have presented in national and international conferences. --High School students (9th Graders at Dutch Fork High, Irmo, SC) were exposed to nanomaterial and SWNTs related concepts. Summary Statements: Chirality will likely play a significant role in aggregation behavior of SWNTs. SWNT transport in landfill conditions will be affected by leachate type and media conditions. Adsorption of contaminants (HOCs) are less likely to be influenced by chiral differences. NIRF can be used to detect chiral-specific SWNTs, which showed no observed toxicity on fathead minnows.
