Imali A. Mudunkotuwa

Aggregation and Dissolution of 4 nm ZnO Nanoparticles in Aqueous Environments: Influence of pH, Ionic Strength, Size, and Adsorption of Humic Acid

Metal oxide nanoparticles are used in a wide range of commercial products, leading to an increased interest in the behavior of these materials in the aquatic environment. The current study focuses on the stability of some of the smallest ZnO nanomaterials, 4 ± 1 nm in diameter nanoparticles, in aqueous solutions as a function of pH and ionic strength as well as upon the adsorption of humic acid. Measurements of nanoparticle aggregation due to attractive particle-particle interactions show that ionic strength, pH, and adsorption of humic acid affect the aggregation of ZnO nanoparticles in aqueous solutions, which are consistent with the trends expected from Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Measurements of nanoparticle dissolution at both low and high pH show that zinc ions can be released into the aqueous phase and that humic acid under certain, but not all, conditions can increase Zn(2+)(aq) concentrations. Comparison of the dissolution of ZnO nanoparticles of different nanoparticle diameters, including those near 15 and 240 nm, shows that the smallest nanoparticles dissolve more readily. Although qualitatively this enhancement in dissolution can be predicted by classical thermodynamics, quantitatively it does not describe the dissolution behavior very well.

Dissolution of ZnO nanoparticles at circumneutral pH: a study of size effects in the presence and absence of citric acid.

Understanding size-dependent processes, including dissolution, of engineered nanoparticles is essential in addressing the potential environmental and health impacts of these materials as well as their long-term stability. In this study, experimental measurements of size-dependent dissolution of well-characterized zinc oxide (ZnO) nanoparticles with particle diameters in the range of 4 to 130 nm have been measured at circumneutral pH (pH 7.5) and compared. Dissolution was found to be enhanced with smaller ZnO nanoparticles compared to larger-sized particles, even though the nanoparticles were present in solution as aggregates with hydrodynamic diameters on the order of 1-3 μm in size. The presence of citric acid significantly enhanced the extent of ZnO dissolution for all sizes, and the greatest enhancement was observed for the 4 nm particles. Although these results are found to be in qualitative agreement with theoretical predictions, a linearized form of the Kelvin equation to calculate a surface free energy yielded quantities inconsistent with expected values from the literature. Reasons for this inconsistency are discussed and include potential deviations of solubility behavior from classical thermodynamics as a result of a lack of detailed knowledge of surface structure and surface properties, including the presence of different surface crystal facets, and the aggregation state.

Citric Acid Adsorption on TiO2 Nanoparticles in Aqueous Suspensions at Acidic and Circumneutral pH: Surface Coverage, Surface Speciation, and Its Impact on Nanoparticle−Nanoparticle Interactions

Citric acid plays an important role as a stabilizer in several nanomaterial syntheses and is a common organic acid found in nature. Here, the adsorption of citric acid onto TiO(2) anatase nanoparticles with a particle diameter of ca. 4 nm is investigated at circumneutral and acidic pHs. This study focuses on both the details of the surface chemistry of citric acid on TiO(2), including measurements of surface coverage and speciation, and its impact on nanoparticle behavior. Using macroscopic and molecular-based probes, citric acid adsorption and nanoparticle interactions are measured with quantitative solution phase adsorption measurements, attenuated total reflection-FTIR spectroscopy, dynamic light scattering techniques, and zeta-potential measurements as a function of solution pH. The results show that surface coverage is a function of pH and decreases with increasing pH. Surface speciation differs from the bulk solution and is time dependent. After equilibration, the fully deprotonated citrate ion is present on the surface regardless of the highly acidic solution pH indicating pK(a) values of surface adsorbed species are lower than those in solution. Nanoparticle interactions are also probed through measurements of aggregation and the data show that these interactions are complex and depend on the detailed interplay between bulk solution pH and surface chemistry.

Environmental implications of nanoparticle aging in the processing and fate of copper-based nanomaterials.

Copper nanomaterials are being used in a large number of commercial products because these materials exhibit unique optical, magnetic, and electronic properties. Metallic copper nanoparticles, which often have a thin surface oxide layer, can age in the ambient environment and become even more oxidized over time. These aged nanoparticles will then have different properties compared to the original nanoparticles. In this study, we have characterized three different types of copper-based nanoparticle (NP) samples designated as Cu(new) NPs, Cu(aged) NPs, and CuO NPs that differ in the level of oxidation. The solution phase behavior of these three copper-based nanoparticle samples is investigated as a function of pH and in the presence and absence of two common, complexing organic acids, citric and oxalic acid. The behavior of these three copper-based NP types shows interesting differences. In particular, Cu(aged) NPs exhibit unique chemistry including oxide phases that form and surface adsorption properties. Overall, the current study provides some insights into the impacts of nanoparticle aging and how the physicochemical characteristics and reactivity of nanomaterials can change upon aging.

The devil is in the details (or the surface): impact of surface structure and surface energetics on understanding the behavior of nanomaterials in the environment.

Metal and metal oxide nanomaterials are found in many consumer products for use in a wide range of applications including catalysis, sensors and contaminant remediation. Because of the extensive use of metal-based nanomaterials, there are concerns that these materials have the potential to get into the environment sometime during production, distribution, use and/or disposal. In particular, there exists the potential that they will make their way into water systems, e.g. drinking water systems, ground water systems, estuaries and lakes. In this review, some of the uncertainties in understanding nanoparticle behavior, which is often due to a lack of fundamental knowledge of the surface structure and surface energetics for very small particles, are discussed. Although classical models may provide guidance for understanding dissolution and aggregation of nanoparticles in water, it is the detailed surface structure and surface chemistry that are needed to accurately describe the surface free energy, a large component of the total free energy, in order to fully understand these processes. Without this information, it is difficult to develop a conceptual framework for understanding the fate, transport and potential toxicity of nanomaterials. Needed research areas to fill this void are discussed.

Histidine Adsorption on TiO2 Nanoparticles: An Integrated Spectroscopic, Thermodynamic, and Molecular-Based Approach toward Understanding Nano–Bio Interactions

Nanoparticles in biological media form dynamic entities as a result of competitive adsorption of proteins on nanoparticle surfaces called protein coronas. The protein affinity toward nanoparticle surfaces potentially depends on the constituent amino acid side chains which are on the protein exterior and thus exposed to the solution and available for interaction. Therefore, studying the adsorption of individual amino acids on nanoparticle surfaces can provide valuable insights into the overall evolution of nanoparticles in solution and the protein corona that forms. In the current study, the surface adsorption of l-histidine on TiO2 nanoparticles with a diameter of 5 nm at pH 7.4 (physiological pH) is studied from both macroscopic and molecular perspectives. Quantitative adsorption measurements of l-histidine on 5 nm TiO2 particles yield maximum adsorption coverage of 6.2 ± 0.3 × 10(13) molecules cm(-2) at 293 K and pH 7.4. These quantitative adsorption measurements also yield values for the equilibrium constant and free energy of adsorption of K = 4.3 ± 0.5 × 10(2) L mol(-1) and ΔG = -14.8 ± 0.3 kJ mol(-1), respectively. Detailed analysis of the adsorption between histidine and 5 nm TiO2 nanoparticle surfaces with attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy indicates both the imidazole side chain and the amine group interacting with the nanoparticle surface and the adsorption to be reversible. The adsorption results in no change in surface charge and therefore does not change nanoparticle-nanoparticle interactions and thus aggregation behavior of these 5 nm TiO2 nanoparticles in aqueous solution.

Effects of Eyjafjallajökull volcanic ash on innate immune system responses and bacterial growth in vitro.

Background: On 20 March 2010, the Icelandic volcano Eyjafjallajökull erupted for the first time in 190 years. Despite many epidemiological reports showing effects of volcanic ash on the respiratory system, there are limited data evaluating cellular mechanisms involved in the response to ash. Epidemiological studies have observed an increase in respiratory infections in subjects and populations exposed to volcanic eruptions. Methods: We physicochemically characterized volcanic ash, finding various sizes of particles, as well as the presence of several transition metals, including iron. We examined the effect of Eyjafjallajökull ash on primary rat alveolar epithelial cells and human airway epithelial cells (20–100 µg/cm2), primary rat and human alveolar macrophages (5–20 µg/cm2), and Pseudomonas aeruginosa (PAO1) growth (3 µg/104 bacteria). Results: Volcanic ash had minimal effect on alveolar and airway epithelial cell integrity. In alveolar macrophages, volcanic ash disrupted pathogen-killing and inflammatory responses. In in vitro bacterial growth models, volcanic ash increased bacterial replication and decreased bacterial killing by antimicrobial peptides. Conclusions: These results provide potential biological plausibility for epidemiological data that show an association between air pollution exposure and the development of respiratory infections. These data suggest that volcanic ash exposure, while not seriously compromising lung cell function, may be able to impair innate immunity responses in exposed individuals.

NanoEHS – defining fundamental science needs: no easy feat when the simple itself is complex

Nanotechnology is no longer in its infancy and has made significant advances since the implementation of the National Nanotechnology Initiative (NNI) in 2000. Incorporation of nanotechnology in many fields including information technology, medicine, materials, energy, catalysis and cosmetics has led to an increase in engineered nanomaterial (ENM) production, and consequently, increased nanomaterial use. In comparison, the generation of concrete and consistent evidence related to the environmental health and safety of nanomaterials (NanoEHS) is lacking. The main factors contributing to the slower progress in NanoEHS versus conventional EHS are related to the complexity, property transformations, life cycles and behavior of nanomaterials even in carefully controlled environments. Therefore, new systematic, integrated research approaches in NanoEHS are needed for overcoming this complexity and bridging current knowledge gaps. A workshop on “NanoEHS: Fundamental Science Needs” brought together scientists and engineers to identify current fundamental science challenges and opportunities within NanoEHS. Detailed discussions were conducted on identifying the fundamental properties that are critical in NanoEHS, differentiating between conventional and NanoEHS studies as well as understanding, the effect of dynamic transformations on nanometrology, role of dosimetry and mechanistic data gaps in nanotoxicology. An important realization that even simple nanoscale materials can be complex when considering NanoEHS implications was noted several times during the workshop. Despite this fact, a number of fundamental research areas to further the scientific foundation to address NanoEHS needs are suggested.

Iron oxide nanoparticles induce Pseudomonas aeruginosa growth, induce biofilm formation, and inhibit antimicrobial peptide function

Given the increased use of iron-containing nanoparticles in a number of applications, it is important to understand any effects that iron-containing nanoparticles can have on the environment and human health. Since iron concentrations are extremely low in body fluids, there is potential that iron-containing nanoparticles may influence the ability of bacteria to scavenge iron for growth, affect virulence and inhibit antimicrobial peptide (AMP) function. In this study, Pseudomonas aeruginosa (PA01) and AMPs were exposed to iron oxide nanoparticles, hematite (α-Fe2O3), of different sizes ranging from 2 to 540 nm (2 ± 1, 43 ± 6, 85 ± 25 and 540 ± 90 nm) in diameter. Here we show that the greatest effect on bacterial growth, biofilm formation, and AMP function impairment is found when exposed to the smallest particles. These results are attributed in large part to enhanced dissolution observed for the smallest particles and an increase in the amount of bioavailable iron. Furthermore, AMP function can be additionally impaired by adsorption onto nanoparticle surfaces. In particular, lysozyme readily adsorbs onto the nanoparticle surface which can lead to loss of peptide activity. Thus, this current study shows that co-exposure of nanoparticles and known pathogens can impact host innate immunity. Therefore, it is important that future studies be designed to further understand these types of impacts.

Biological and environmental media control oxide nanoparticle surface composition: the roles of biological components (proteins and amino acids), inorganic oxyanions and humic acid

Current practices of initial nanoparticle characterization with respect to particle size, shape, surface and bulk composition prior to experiments to test, for example, cellular interaction or toxicity, will not accurately describe nanomaterials in a given medium. The use of initial characterization data in subsequent analyses inherently assumes that nanoparticles are static entities. However, nanoparticle characterization, which is crucial in all studies related to their applications and implications, should also include information about the dynamics of the interfacial region between the nanomaterial surface and the surrounding medium. The objective of this tutorial review is to highlight the importance of in situ characterization of metal oxide nanoparticle surfaces in complex media. In particular, several examples of TiO2 (5 nm) and α-Fe2O3 (2 nm) nanoparticles, in different environmental and biological media, are presented so as to show the importance of the milieu to oxide surface composition. The surface composition is shown to be controlled by the adsorption of biological components (proteins and amino acids), inorganic oxyanions (phosphates and carbonates) and environmental ligands (humic acid). The extent of surface adsorption depends on the solution phase composition and the affinity of different components to adsorb to the nanoparticle surface. The examples presented here show that there is a range of possible surface interactions, adsorption energetics and adsorption modes including reversible adsorption, irreversible adsorption and co-adsorption.