Stacey M. Louie

Effects of molecular weight distribution and chemical properties of natural organic matter on gold nanoparticle aggregation.

The complexity of natural organic matter (NOM) motivates determination of how specific components in a NOM mixture interact with and affect nanoparticle (NP) behavior. The effects of two Suwannee River NOM fractions (separated by a 100,000 g/mol ultrafiltration membrane) on gold NP aggregation are compared. The weight-average molecular weight, Mw, for the unfractionated NOM was 23,300 g/mol, determined by size exclusion chromatography with multiangle light scattering. The NOM was comprised of ~1.8 wt % of >100,000 g/mol retentate (NOMr, Mw = 691,000 g/mol) and 98 wt % of filtrate (NOMf, Mw = 12,800 g/mol). Ten ppm of NOMr provided significantly better NP stability against aggregation than 10 ppm of NOMf in 100 mM NaCl due to steric effects. In the unfractionated NOM, the relative importance of the two components was concentration-dependent. For a low concentration of unfractionated NOM (10 ppm), both fractions contributed to the NOM effects; for a high concentration (560 ppm), NP stability was controlled by the small amount (10 ppm) of NOMr present, rather than the higher amount (550 ppm) of NOMf. Therefore, large humic aggregates in a heterogeneous NOM sample can have disproportionately strong effects, and characterization of Mw distributions (rather than average Mw) may be required to explain NOM effects on NP behavior.

Comparative study of polymeric stabilizers for magnetite nanoparticles using ATRP.

A series of polyelectrolytes with controlled molecular weight, a narrow chain-length distribution, and systematic structural differences were synthesized using atom-transfer radical polymerization and investigated as stabilizers for magnetite nanoparticles in aqueous suspensions. Structural differences include the degree of polymerization, the chain architecture, and the identity of the charged functional unit. The synthesized polymers are sulfonated poly(2-hydroxyethyl methacrylate), a block copolymer of the former with poly(n-butyl methacrylate), poly(sodium styrene sulfonate), poly(sodium acrylate), and poly(sodium vinylphosphonate). The colloidal stability is assessed by measuring the fraction of particles, based on turbidity, that sediment after a period of time at increasing ionic strength. Sedimentation results are complimented by dynamic light scattering determinations of the hydrodynamic diameter of the particles that remain suspended. When adsorption and sedimentation are conducted at high pH, poly(sodium acrylate) and poly(sodium vinylphosphonate) yield the most stable suspensions because of their strong coordinative interactions with the iron oxide surface. At low pH, the polymers that retain pendant negative charges (each of the sulfonated polymers) yield high stable fractions at all ionic strengths investigated up to 100 mM (NaCl), whereas polyelectrolytes that become protonated with decreasing pH, poly(sodium acrylate) and poly(sodium vinylphosphonate), lose their stabilizing capacity even at low ionic strengths. The chain-length distribution profoundly alters a polymers stabilization tendencies. Two poly(sodium acrylate) samples with the same number-average molecular weight but widely different chain-length distributions proved to have opposite tendencies, with the polydisperse sample being a good stabilizer and the low polydispersity one being a strong flocculant. This investigation provides guidelines for the design of polymeric stabilizers for magnetite nanoparticles according to the pH and ionic strength of the intended application.

Correlation of the Physicochemical Properties of Natural Organic Matter Samples from Different Sources to Their Effects on Gold Nanoparticle Aggregation in Monovalent Electrolyte

Engineered nanoparticles (NPs) released into natural environments will interact with natural organic matter (NOM) or humic substances, which will change their fate and transport behavior. Quantitative predictions of the effects of NOM are difficult because of its heterogeneity and variability. Here, the effects of six types of NOM and molecular weight fractions of each on the aggregation of citrate-stabilized gold NPs are investigated. Correlations of NP aggregation rates with electrophoretic mobility and the molecular weight distribution and chemical attributes of NOM (including UV absorptivity or aromaticity, functional group content, and fluorescence) are assessed. In general, the >100 kg/mol components provide better stability than lower molecular weight components for each type of NOM, and they contribute to the stabilizing effect of the unfractionated NOM even in small proportions. In many cases, unfractionated NOM provided better stability than its separated components, indicating a synergistic effect between the high and low molecular weight fractions for NP stabilization. Weight-averaged molecular weight was the best single explanatory variable for NP aggregation rates across all NOM types and molecular weight fractions. NP aggregation showed poorer correlation with UV absorptivity, but the exponential slope of the UV-vis absorbance spectrum was a better surrogate for molecular weight. Functional group data (including reduced sulfur and total nitrogen content) were explored as possible secondary parameters to explain the strong stabilizing effect of a low molecular weight Pony Lake fulvic acid sample to the gold NPs. These results can inform future correlations and measurement requirements to predict NP attachment in the presence of NOM.

Critical review: impacts of macromolecular coatings on critical physicochemical processes controlling environmental fate of nanomaterials

Attachment of engineered and naturally occurring macromolecules greatly affects the environmental fate and toxicity of engineered nanomaterials (ENMs). A better understanding of macromolecule–ENM interactions at the nanoscale will improve the ability to predict the effects of macromolecular coatings, e.g. natural organic matter (NOM), on ENM fate, reactivity, and toxicity. This review briefly discusses relevant theory from colloid and polymer science for highly idealized polymers on surfaces that can be used to describe ENM environmental behaviors and introduces classes of macromolecules of interest in the field of environmental nanotechnology. Methods to characterize adsorbed macromolecules on ENMs are presented along with their limitations for ENMs in natural systems. Finally, the current state of knowledge regarding the effects of attached organic macromolecules, both engineered and incidental, on the environmental fate and reactivity of ENMs is critically reviewed. These concepts in whole are synthesized to identify the fundamental gaps in understanding and metrology that must be addressed to improve our mechanistic understanding of the effects of organic macromolecules on ENM environmental fate, and approaches to correlate the properties of coated ENMs to their environmental fate are discussed. We postulate that a first principles approach to modeling ENM–macromolecule interactions is not warranted, particularly for complex and heterogeneous natural macromolecules. On the other hand, a mechanistic understanding is needed to inform parameter selection for empirical correlations, which may offer tractable alternatives to predicting the behavior of macromolecule–coated ENMs. Development of these empirical correlations and prediction of the long-term fate of ENMs is currently hampered by incomplete characterization of the adsorbed macromolecule layer properties and their evolution over time in natural systems.

Parameter Identifiability in Application of Soft Particle Electrokinetic Theory To Determine Polymer and Polyelectrolyte Coating Thicknesses on Colloids

Soft particle electrokinetic models have been used to determine adsorbed nonionic polymer and polyelectrolyte layer properties on nanoparticles or colloids by fitting electrophoretic mobility data. Ohshima first established the formalism for these models and provided analytical approximations ( Ohshima, H. Adv. Colloid Interface Sci.1995, 62, 189 ). More recently, exact numerical solutions have been developed, which account for polarization and relaxation effects and require fewer assumptions on the particle and soft layer properties. This paper characterizes statistical uncertainty in the polyelectrolyte layer charge density, layer thickness, and permeability (Brinkman screening length) obtained from fitting data to either the analytical or numerical electrokinetic models. Various combinations of particle core and polymer layer properties are investigated to determine the range of systems for which this analysis can provide a solution with reasonably small uncertainty bounds, particularly for layer thickness. Identifiability of layer thickness in the analytical model ranges from poor confidence for cases with thick, highly charged coatings, to good confidence for cases with thin, low-charged coatings. Identifiability is similar for the numerical model, except that sensitivity is improved at very high charge and permeability, where polarization and relaxation effects are significant. For some poorly identifiable cases, parameter reduction can reduce collinearity to improve identifiability. Analysis of experimental data yielded results consistent with expectations from the simulated theoretical cases. Identifiability of layer charge density and permeability is also evaluated. Guidelines are suggested for evaluation of statistical confidence in polymer and polyelectrolyte layer parameters determined by application of the soft particle electrokinetic theory.

Chapter 2 – Transformations of Nanomaterials in the Environment

Nanomaterials (NMs) will undergo a variety of transformations in the natural environment that can significantly change their transport behavior, ultimate fate, and toxicity. Chemical transformations include oxidation–reduction reactions, ligation, and dissolution (which are often associated with a redox reaction). Physical transformations include aggregation or disaggregation and adsorption of naturally occurring macromolecules. Biological interactions and biouptake of NMs can result in further physicochemical transformations. The current state of knowledge on the effects of these transformations on NM properties and hence their behavior, persistence, and risk in the environment is presented. The implications of these transformations for detection and characterization of NMs in biological and environmental matrices are also discussed. Finally, challenges for risk assessment of NMs due to their environmental transformations are identified.

Nanoparticle core properties affect attachment of macromolecule-coated nanoparticles to silica surfaces

Environmental context The increasing use of engineered nanoparticles has led to concerns over potential exposure to these novel materials. Predictions of nanoparticle transport in the environment and exposure risks could be simplified if all nanoparticles showed similar deposition behaviour when coated with macromolecules used in production or encountered in the environment. We show, however, that each nanoparticle in this study exhibited distinct deposition behaviour even when coated, and hence risk assessments may need to be specifically tailored to each type of nanoparticle. Abstract Transport, toxicity, and therefore risks of engineered nanoparticles (ENPs) are unquestionably tied to interactions between those particles and surfaces. In this study, we proposed the simple and untested hypothesis that coating type can be the predominant factor affecting attachment of ENPs to silica surfaces across a range of ENP and coating types, effectively masking the contribution of the particle core to deposition behaviour. To test this hypothesis, TiO2, Ag0 and C60 nanoparticles with either no coating or one of three types of adsorbed macromolecules (poly(acrylic acid), humic acid and bovine serum albumin) were prepared. The particle size and adsorbed layer thicknesses were characterised using dynamic light scattering and soft particle electrokinetic modelling. The attachment efficiencies of the nanoparticles to silica surfaces (glass beads) were measured in column experiments and compared with predictions from a semi-empirical correlation between attachment efficiency and coated particle properties that included particle size and layer thickness. For the nanoparticles and adsorbed macromolecules in this study, the attachment efficiencies could not be explained solely by the coating type. Therefore, the hypothesis that adsorbed macromolecules will mask the particle core and control attachment was disproved, and information on the properties of both the nanoparticle surface (e.g. charge and hydrophobicity) and adsorbed macromolecule (e.g. molecular weight, charge density extended layer thickness) will be required to explain or predict interactions of coated nanoparticles with surfaces in the environment.

Photochemical transformations of thiolated polyethylene glycol coatings on gold nanoparticles

Photochemical reactions can cause significant transformations of manufactured nanomaterials in sunlit environments. While transformations of inorganic nanoparticles (NPs) have been investigated extensively, less attention has been focused on the direct impact of aqueous photochemical reactions on adsorbed organic macromolecules that form the NP corona and strongly influence the surface interactions and reactivity that affect NP transport, fate, and toxicity. Here, we assess the transformations of methoxy polyethylene glycol thiol (mPEGSH) coatings on gold NPs (AuNPs) under controlled UV irradiation. A decrease in the adsorbed layer thickness of polymer was observed within 24 h of UV irradiation, resulting in increased susceptibility of the transformed NPs to aggregation. Surface chemistry analyses, including X-ray photoelectron spectroscopy (XPS), showed loss of the ether groups but persistence of reduced S on the AuNP surface, indicative of a chain scission mechanism yielding different NP surface properties from that of either the initial PEGylated AuNP or the citrate-stabilized AuNP prior to coating with mPEGSH. The transformation of the chemisorbed polymer was compared to that of dissolved mPEGSH in the presence and absence of the Au NPs, as evaluated by liquid chromatography-mass spectrometry (LC-MS). In contrast to the NP-adsorbed coating, the primary observed transformation of the dissolved mPEGSH was thiol oxidation to disulfides without extensive chain scission. This study demonstrates that transformations of adsorbed macromolecular coatings must be considered to accurately predict NP attachment behavior, and hence transport, in environmental systems. Because the corona transformation was not predictable from that of the dissolved polymer, direct NP surface characterization is required to discern the fundamental reactions involved in the photochemical transformation of coatings after sorption to the NP surface.

Ultraviolet photo-oxidation of polyvinylpyrrolidone (PVP) coatings on gold nanoparticles

Polymeric coatings are commonly applied to impart functionality and colloidal stability to engineered nanoparticles. In natural environments, transformations of the coating can modify the particle transport behavior, but the mechanisms and outcomes of these transformations have not yet been thoroughly evaluated. This study investigates the photo-transformations of polyvinylpyrrolidone (PVP) coatings on gold nanoparticles (AuNPs) under ultraviolet (UV) irradiation, representing light exposure in surface waters or other sunlit environments, and the impact on the AuNP colloidal stability. Multiple orthogonal characterization methods were applied to interrogate UV-induced transformations and their consequences. Rapid oxidation of the PVP coating occurred upon UV exposure. The transformed PVP largely persisted on the AuNP surface, albeit in a collapsed polymer layer around the AuNP surface. This transformation resulted in drastically diminished colloidal stability of the AuNPs, consistent with loss of steric stabilization. While the residual coating modified the interaction of the AuNPs with calcium counterions, it did not prevent subsequent stabilization by humic acid. This study demonstrates the importance of both chemical and physical coating transformations on nanoparticles, and hence the need for orthogonal and complementary characterization methods to fully characterize the coating transformations. Finally, the specific transformations of the PVP-coated AuNPs investigated here are discussed more broadly with respect to generalizability to other polymer-coated NPs and the implications for their fate in sunlit or other reactive environments.

Research highlights: engineering nanomaterial-based technologies for environmental applications

Nanomaterials are currently of interest for water treatment and remediation applications because they can exhibit high adsorption capacities and high reactivity to degrade or transform contaminants. Research is ongoing to further increase the adsorption capacity of the nanomaterials and to engineer nanomaterial-based treatment systems for contaminant removal. Here, we highlight three articles that advance this field by devising and testing approaches to improve the design of nanomaterials as well as their implementation in water treatment applications. One study demonstrates a method for non-covalent surface functionalization to produce silica and magnetite nanoparticles exhibiting thiol ligands for heavy metal removal. In another study, the surface coating chemistry of manganese oxide nanoparticles is optimized to enhance their uranyl sorption capacity. Finally, we highlight research that evaluates the overall implementation of magnetite nanoparticles for removal of hexavalent chromium (Cr(VI)) from water, including the production of the nanoparticles, their efficiency in removing Cr(VI) in a reactor, and the recovery of the used NPs in a magnetic separation system.