Aggregation and deposition of gold nanoparticles in singular and binary particle systems : role of size, shape, and environmental characteristics
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Engineered nanomaterials (ENMs) are used in numerous applications due to their unique advantages. Gold nanomaterials (AuNMs) are one such functional ENM, which are utilized as model nanoparticles due to their controlled properties. AuNMs can be synthesized with highly uniform size and shape, where increase in the anisotropy gives rise to novel properties; e.g., unique Plasmon resonance, shape dependent physico-chemical properties, and quantum confinement, etc. These advantages have popularized the use of AuNMs in a wide range of applications in sensing, drug delivery, and photodynamic therapies. Increased use of these ENMs calls for systematic evaluation of the AuNMs to assess the effect of their size and shape on fate and transport in aquatic environment. The natural environment, due to the presence of natural colloids and potential presence of secondary ENMs, presents additional system complexities in studying ENM fate and transport. This dissertation was designed to address two major data gaps: i.e., roles of i) anisotropy and ii) presence of secondary particles on aggregation and deposition of AuNMs. Poly-acrylic acid (PAA) coated uniform-sized gold nanospheres (AuNS) and nanorods (AuNR) are utilized as model nanoparticles. The first part of this research investigated the effect of shape of AuNMs on their aggregation and deposition in singular particulate system under relevant ionic strengths. The second part of this dissertation focused on investigation of the effects of a secondary nanoparticle, i.e., pluronic acid modified single-walled carbon nanotubes (PA-SWNTs), on aggregation and filtration of AuNSs under a wide range of ionic strength. Effects of shape of AuNMs on their aggregation kinetics were investigated by employing time resolved dynamic light scattering (TRDLS) in a wide range of mono- and di-valent electrolyte conditions. Aggregation histories were obtained for the AuNSs and AuNRs in presence of a wide range of NaCl and CaCl2 conditions. The critical coagulation concentrations (CCCs), computed from the stability plots of the AuNMs, were used to compare the aggregation propensity of the PAA coated AuNSs with the AuNRs. The deposition kinetics was monitored using the quartz crystal microbalance with dissipation (QCM-D) technique for a range of NaCl concentrations. Deposition rates determined were used to understand the effects of shape of the AuNMs on their deposition kinetics. Experimental findings suggest that the shape of nanomaterials can influence interfacial properties and result in unique aggregation and deposition behavior under typical aquatic conditions Effects of AuNS size on their aggregation behavior was investigated using citrate stabilized 30 nm and 60 nm sized National Institute of Standards and Technology (NIST) standard particles in biological media. Continuous size evolution of AuNS aggregates was monitored over a 24 h period employing DLS and static light scattering (SLS). Electrokinetic measurements, UV-vis spectroscopy, and electron microscopy were performed for material characterization. The findings from this study indicate that initial differences in AuNS sizes diminish over time as the particles aggregate under the influence of elevated ionic strength in the media. The results from this study will likely influence characterization and experimental design considerations for in vitro nanotoxicity studies. The effects of the presence of a secondary particle, PA-SWNTs, on hetero-aggregation behavior of PAA coated AuNS were systematically studied over a wide range of mono- and divalent electrolyte conditions. PA modification of SWNTs made them stable in the entire range of electrolyte concentration where SWNT-SWNT aggregation has been eliminated via steric interaction. Hetero-aggregation mechanisms of AuNS were deciphered utilizing electron microscopy and electrokinetics. Experimental results suggest that the AuNS aggregates faster in hetero-dispersion in presence of PA-SWNT than in homo-disperstion at elevated ionic strength. However, hetero-rates are slower in case of low ionic strength cases. Techniques developed in this study can be can be adopted elsewhere to assess hetero-aggregation of a wider set of ENMs, hence achieving reliability in nanoparticle environmental exposure and risk. Co-transport of AuNS in presence of PA-SWNTs through saturated porous media was also assessed using bench-scale column experiments under a wide range of aquatic conditions (1-100 mM NaCl). Homogenous AuNS suspensions were utilized as control to compare their breakthrough properties with those of the AuNS hetero-dispersions (in presence of PA-SWNTs). This study also assessed the role of pre-coating of the collectors (with PA-SWNTs) on AuNS' mobility to understand the order of introduction of the secondary particles. The study results demonstrate that the presence of secondary particles and the order in which these are introduced to the experimental system strongly influence AuNS mobility. Thus ENM can be highly mobile or can get strongly filtered out, depending on the secondary PA-SWNT and background environmental conditions. This research not only evaluates the role of material attributes in their fate and transport but also extends to assess the role of environmental complexities on the same. The research findings enhance our current understanding of environmental interaction of the ENMs and call for further studies, incorporating additional complexities (i.e., material and system), to assess the fate, transport, and effects of the ENMs in the natural environment.