Environmental implications of higher order fullerenes and conjugated nanostructures
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In quest of harnessing emergent properties and achieving multifunctionality in the materials realm, synthesis and manipulation at the nano-scale has moved its focus from simple passive nanomaterials (NMs) to hierarchical nanostructures. Such nanostructures include higher order fullerenes (HOFs), carbon allotropes composed of more than 60 carbon atoms per fullerene cage, and conjugated nanohybrids (NHs), prepared from materials of multiple chemical origin. The advantages in their electronic, optical, physicochemical, and magnetic properties have inspired their research and use in photovoltaics, nano-electronics, biomedical imaging and drug delivery, catalysis, energy generation and storage, and environmental remediation and sensing. Not only as research grade materials, a global market of bio-imaging and fuel-cell applications have been integrating use of HOFs, and NHs, respectively. Thus it is an exciting time for materials engineering to expand the spectrum of these ‘horizon materials’ by putting together a variety of chemical ‘building blocks’ and build a wide range of multifunctional hierarchical structures. However, such conjugation leading to complex hierarchical structures also introduces unknown environmental risks. The emergent properties of these hierarchical structures necessitate careful assessment of their environmental health and safety. This dissertation is one of the first organized efforts to identify hierarchical nanostructures and assess their environmental implications. This research, through extensive literature review of these novel nanostructures, proposes a working definition of NH from environmental perspective, classifies a wide array of NHs based on chemical origin, and identifies their emerging and altered physicochemical properties with potential to generate unprecedented environmental fate, transport, transformation, and toxicity. Furthermore, this dissertation makes an effort to address three major data gaps: i.e., a) challenges in aqueous solubilization of HOFs, b) possible correlation of carbon numbers on fullerene molecules with their aggregation behavior, and c) influence of hybridization on aggregation kinetics and antimicrobiality of an important electrocatalyst NH (metal-carbon). To address the first data gap, aqueous suspensions of nC₆₀, nC₇₀, nC₇₆, and nC₈₄ were prepared using a calorimetry-based solvent exchange method. Non-aggregating and size-specific aqueous nC₆₀ and nC₇₀ fullerene clusters also were prepared using a non-ionic polymer, pluronic acid (PA). The environmental processes section of this research assessed aggregation kinetics of nHOFs and NHs as well as antimicrobiality of TiO₂ conjugated oxidized multiwalled carbon nanotube (OMWNT-TiO₂) NH. Aqueous solubilization of C₇₀, C₇₆, and C₈₄ was performed being guided by molecular dynamics (MD) simulations. Increased energy demand reflects favorability of HOF-water interaction. The experimental findings suggest that nHOF clusters obtained via solvent-exchange solubilization method remains stabilized by electrostatic repulsion. Similarly, non-ionic triblock co-polymer PA F-127 stabilized aqueous C₆₀ and C₇₀s were prepared. Experimental results suggest that size uniformity of aqueous fullerenes increased with the increase in PA concentration, yielding optimum 58.8±5.6 and 61.8±5.6 nm nC₆₀s and nC₇₀s, respectively (0.10 %w/v PA). Fullerene aqueous suspensions also manifested colloidal stability even in presence of 1 M NaCl or in biological media, i.e., DMEM and RPMI. MD simulations results indicate encapsulation of fullerene clusters by PA molecules and subsequent steric stabilization. The results from this study may facilitate mechanistic environmental and toxicological studies with size-specific fullerenes that do not aggregate in high ionic strength biological media. Aqueous suspensions of nC₆₀ and three nHOFs (i.e., nC₇₀, nC₇₆, and nC₈₄) obtained via solvent-exchange method were systematically studied to determine their aggregation kinetics in a wide range of mono- (NaCl) and divalent (CaCl₂) electrolytes. Experimentally obtained critical coagulation concentration (CCC) values of nC₆₀ and nHOFs displayed a strong negative correlation with the carbon number in fullerenes. The aggregation mechanism was dominated by van der Waals interaction as enumerated via MD simulation and modified Derjaguin-Landau-Verwey-Overbeek (DLVO) model. Natural macromolecules profoundly stabilized all fullerene clusters, even at 100 mM NaCl concentration. The results from this study can be utilized to predict aggregation kinetics of nHOFs other than the ones studied here. To understand the aggregation behavior of carbon-metal NHs, oxidized MWNTs were hybridized sequentially with undoped or Nb-doped TiO₂ and Pt NPs. OMWNT-TiO₂, OMWNT-TiNbO₂, OMWNT-TiO₂, and OMWNT-TiNbO₂-Pt and the component materials were characterized and their aggregation behavior was studied systematically. Experimental findings show that CCC values OMWNT were reduced by TiO₂ attachment; however, Nb-doping and Pt attachment increased their colloidal stability and CCC values. The aggregation mechanism was elucidated by modified DLVO energy calculations using composition-averaged Hamaker constants for NHs. Natural macromolecules stabilized all the NHs and the component materials. Antimicrobiality of OMWNT-TiO₂ NH was studied via in vitro cell viability tests. Opportunistic pathogen Pseudomonas aeruginosa PAO1 strain was exposed to OMWNT, TiO₂, and OMWNT-TiO₂ NH at different concentrations in dark and UV-irradiated conditions. OMWNT-TiO₂ NH showed higher antimicrobial activity compared to the component materials under UV-irradiation. Extracellular reactive oxygen species (ROS) measurement by using fluorescence molecular probes for H₂O₂ identifies UV-induced enhanced ROS generation by the NH as a likely antimicrobial mechanism. The research presented in this dissertation takes the first attempt toward EHS assessment of complex and hierarchical nanostructures. The research findings present new insights into these ‘horizon materials’ and likely will spark interests on this necessary line of research to better understand the environmental fate, transport, and effects of HOFs and NHs. As nanotechnology is advancing from passive singular nanostructures to active and complex nano-systems; such undertakings become imperative to evaluate implications of material complexity at the environmental interface.