Dipesh Das

Aggregation Kinetics of Higher-Order Fullerene Clusters in Aquatic Systems

The aggregation kinetics of nC60 and higher-order fullerene (HOF) clusters, i.e., nC70, nC76, and nC84, was systematically studied under a wide range of mono- (NaCl) and divalent (CaCl2) electrolytes and using time-resolved dynamic light scattering. Suwanee River Humic Acid (SRHA) was also used to determine the effect of natural macromolecules on nHOF aggregation. An increase in electrolyte concentration resulted in electrical double-layer compression of the negatively charged fullerene clusters, and the nC60s and nHOFs alike displayed classical Derjaguin-Landau-Verwey-Overbeek (DLVO) type interaction. The critical coagulation concentration (CCC) displayed a strong negative correlation with the carbon number in fullerenes and was estimated as 220, 150, 100, and 70 mM NaCl and 10, 12, 6, and 7.5 mM CaCl2 for nC60, nC70, nC76, and nC84, respectively. The aggregation mechanism (i.e., van der Waals interaction domination) was enumerated via molecular dynamics simulation and modified DLVO model. The presence of SRHA (2.5 mg TOC/L) profoundly influenced the aggregation behavior by stabilizing all fullerene clusters, even at a 100 mM NaCl concentration. The results from this study can be utilized to predict aggregation kinetics of nHOF clusters other than the ones studied here. The scaling factor for van der Waals interaction can also be used to model nHOF cluster interaction.

Influence of the Gastrointestinal Environment on the Bioavailability of Ethinyl Estradiol Sorbed to Single-Walled Carbon Nanotubes

Recent evidence suggests that, because of their sorptive nature, if single-walled carbon nanotubes (SWCNTs) make their way into aquatic environments, they may reduce the toxicity of other waterborne contaminants. However, few studies have examined whether contaminants remain adsorbed following ingestion by aquatic organisms. The objective of this study was to examine the bioavailability and bioactivity of ethinyl estradiol (EE2) sorbed onto SWCNTs in a fish gastrointestinal (GI) tract. Sorption experiments indicated that SWCNTs effectively adsorbed EE2, but the chemical was still able to bind and activate soluble estrogen receptors (ERs) in vitro. However, centrifugation to remove SWCNTs and adsorbed EE2 significantly reduced ER activity compared to that of EE2 alone. Additionally, the presence of SWCNTs did not reduce the extent of EE2-driven induction of vitellogenin 1 in vivo compared to the levels in organisms exposed to EE2 alone. These results suggest that while SWCNTs adsorb EE2 from aqueous solutions, under biological conditions EE2 can desorb and retain bioactivity. Additional results indicate that interactions with gastrointestinal proteins may decrease the level of adsorption of estrogen to SWCNTs by 5%. This study presents valuable data for elucidating how SWCNTs interact with chemicals that are already present in our aquatic environments, which is essential for determining their potential health risk.

An elegant method for large scale synthesis of metal oxide–carbon nanotube nanohybrids for nano-environmental application and implication studies

Nanohybrids (NHs) with synergistic and emergent properties are used as electrocatalysts, photocatalysts, and antimicrobial agents in numerous applications. Carbon nanotube (CNT)–metal oxide NHs are one of the most commercialized heterostructures because of their advantages as catalyst supports in the fuel cell industry. To date, there has been little understanding of their environmental behavior, primarily due to the lack of a robust yet facile synthesis technique. This study presents an elegant synthesis method, which varies the reagent composition and ratio to grow metal oxides of choice onto multiwalled carbon nanotube (MWNT) surfaces. This technique can synthesize heterostructures with elements ranging from transition (Ti and Zn) to lanthanide series (Er and Pr) metals. This modified sol–gel method can provide large material yields (100s of mg) with a high degree of overall homogeneity between synthesized batches. Such a method for preparing complex NHs from component materials can be extremely useful to perform systematic environmental analyses.

Environmental Interactions of Geo- and Bio-Macromolecules with Nanomaterials

Engineered nanomaterials (ENMs) are mostly synthesized with modi- fied surfaces using various surfactants, polymeric, or biomolecule coatings to achieve desired functionality. When exposed to the environment, coatings on the ENMs will undergo the first set of interactions with natural geo- and bio-macro- molecules pre-existing in aqueous and/or soil matrices. Such interfacial interaction will likely alter the conformation and extent of coverage of the synthetic ENM surface coatings via exchange, displacement, and/or overcoating by environmental macromolecules. The exchange kinetics and extent of replacement of the synthetic coatings will profoundly impact environmental fate, transport, transformation, and toxicity of the ENMs. This chapter discusses the state-of-the-art literature to identify key synthetic coating types, their interaction with the environmental and biological macromolecules, and illustrate the existing challenges to determine coating exchange kinetics and its environmental implications on ENMs.

Insights into Metal Oxide and Zero-Valent Metal Nanocrystal Formation on Multiwalled Carbon Nanotube Surfaces during Sol-Gel Process

Carbon nanotubes are hybridized with metal crystals to impart multifunctionality into the nanohybrids (NHs). Simple but effective synthesis techniques are desired to form both zero-valent and oxides of different metal species on carbon nanotube surfaces. Sol-gel technique brings in significant advantages and is a viable technique for such synthesis. This study probes the efficacy of sol-gel process and aims to identify underlying mechanisms of crystal formation. Standard electron potential (SEP) is used as a guiding parameter to choose the metal species; i.e., highly negative SEP (e.g., Zn) with oxide crystal tendency, highly positive SEP (e.g., Ag) with zero-valent crystal-tendency, and intermediate range SEP (e.g., Cu) to probe the oxidation tendency in crystal formation are chosen. Transmission electron microscopy and X-ray diffraction are used to evaluate the synthesized NHs. Results indicate that SEP can be a reliable guide for the resulting crystalline phase of a certain metal species, particularly when the magnitude of this parameter is relatively high. However, for intermediate range SEP-metals, mix phase crystals can be expected. For example, Cu will form Cu2O and zero-valent Cu crystals, unless the synthesis is performed in a reducing environment.