Understanding the Toxicity Effects of Nanostructures: I. Effect of Physico-chemical Properties on Environmentally Relevant Organismal Toxicity. II. Design of Novel Nanostructures to Reduce Environmental Toxicity while Retaining Functional Properties
Nanomaterials are increasingly being used in consumer products due to their unique optical, electronic, and antibacterial properties. However, proliferated use of nanomaterials will lead to increased entrance into the environment where toxicity can occur to plants and other organisms. The focus of this research is to understand the origin of nanomaterial organismal toxicity and methods for its reduction through engineering. Nano silver, nano carbon, and nano copper were chosen as the subjects of study due to their prevalent use in consumer products, and likely entrance into the environment during use and disposal.
The majority of the research focuses on silver nanomaterials (AgNMs). In the first part of this work, Chapters 2-5, the toxicity of AgNMs with three different shapes (i.e. silver nanoparticles, silver nanocubes, and silver nanowires) was examined in environmentally relevant and model organisms including Lolium multiflorum, Danio rerio, Caenorhabditis elegans, and the bacterial strains Escherichia coli, Pseudomonas aeruginosa, and Bacillus cereus to determine if the shape of the AgNM affected its toxicity. Shape was shown to affect the toxicity of silver nanomaterials to L. multiflorum; silver nanoparticles (AgNPs) were more toxic than silver nanocubes (AgNCs) and silver nanowires (AgNWs). However, this shape-based toxic effect was not found in the other species examined.
To further understand the shape-specific toxicity, dissolution and physical contract were studied. While dissolution is often assumed to be the cause of silver nanomaterial toxicity, we concluded that dissolution alone did not account for all of the toxicity shown. Instead, physical contact between the nanomaterial and plant was found to be necessary for shape-based phytotoxicity. The surface reactivity was experimentally calculated for each nanomaterial to determine if different surface reactivity between different AgNM shapes could further explain phytotoxicity. It was shown that this value correlated to phytotoxicity, suggesting surface reactivity might be related to nanomaterial toxicity in environmentally relevant organisms.
Since AgNMs are likely to undergo chemical and physical transformations after entrance into the environment, the effect of sulfidation on shape-based phytotoxicity was studied. Sulfidizing the AgNMs resulted in an increase in size due to the formation of a silver sulfide shell. Sulfidation was found to decrease AgNM toxicity toward L. multiflorum, D. rerio, and C. elegans. However, it was determined that shape still affected the AgNM toxicity after sulfidation.
Due to their lack of phytotoxicity as compared to bacterial toxicity, AgNCs were chosen as a precursor for a novel silver-antibiotic hybrid surface coating. Silver nanocubes were functionalized with a sub-monolayer of gold (Au@AgNC) to increase the ease of covalent attachment of functional moieties. The Au@AgNCs were then covalently functionalized with gentamicin, a commonly used antibiotic. The gentamicin-Au@AgNCs showed increased bacterial toxicity to Staphylococcus aureus compared to a mixed gentamicin and Au@AgNC composite. Phytotoxicity of the gentamicin-Au@AgNC compound was found to be minimal in L. multiflorum, Lactuca sativa, and Solanum lycopersicum. In this work the physical and chemical properties of nanostructures were tuned to reduce environmental toxicity while retaining desired functional properties.
The research was also extended to other nanomaterials. In Chapter 7, the effect of copper nanowire (CuNW) diameter on phytotoxicity was studied in the plant species L. multiflorum, L. sativa, and S. lycopersicum. L. multiflorum and S. lycopersicum did not show any diameter-based toxicity, though L. sativa did. Smaller diameters correlated to increased toxicity in L. sativa, and all CuNWs were more toxic than ionic copper. CuNWs were aged to simulate more realistic plant exposures. After aging, CuNWs were less toxic, though the diameter-based toxicity trend was still present. To further understand the diameter-based toxicity, dissolution was studied. It was determined that dissolution alone could not account for the phytotoxicity shown.
In Chapter 8, the recently synthesized material soluble graphitic nanofiber (SGNF) was studied for its phytotoxicity to plants using the environmentally relevant and model plant species L. multiflorum, L. sativa, and S. lycopersicum. SGNFs consist of a carbon nanotube (CNT) core wrapped with sheets of graphene oxide (GO). As both of these materials are toxic, it was hypothesized that their toxicity would be additive in SGNFs. In plant growth assays, GO was the most phytotoxic and CNTs were the least phytotoxic. SGNFs were between the GO and CNTs for toxicity. It was determined that pH of the solution played a minor effect on the decreased growth shown. The majority of toxicity could be attributed to agglomeration of SGNFs onto the surface of the plant root.
Additionally, four zero-dimensional carbon nanomaterials (CNMs) were studied to determine if chemical bonding properties affected their phytotoxicity to the plant species L. multiflorum, L. sativa, and S. lycopersicum. Nanodiamond, onion-like carbon, partially graphitized nanodiamond, and carbon dots were all studied to determine if exposure resulted in decreased germination, root, or shoot growth. Generally, all four CNMS resulted in similar or increased growth compared to the control. This data suggests that differences in chemical bonding are unlikely to cause toxicity at environmentally relevant concentrations.
Finally, graphene oxide was studied as a surface coating for ceramic pot water filtration devices (CPWFDs). GO was covalently attached to the ceramic membrane surface using linker chemistry. The ceramic membranes were challenged with Esherichia coli to determine their bacterial removal efficiency. Hydraulic conductivity was also measured to determine the water flux of the ceramic membranes as a proxy for measuring membrane fouling.
The results from these studies show that a greater understanding of how physico-chemical properties affect toxicity toward environmentally relevant organisms is necessary to better engineer nanomaterials with increased desired properties while reducing unintended consequences. Additionally, this work showed collaborations among scientists from different disciplines were necessary to better understand the complex scientific question of environmental fate and toxicity of nanomaterials.
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