Dongxu Zhou

Stability and Aggregation of Metal Oxide Nanoparticles in Natural Aqueous Matrices

There is a pressing need for information on the mobility of nanoparticles in the complex aqueous matrices found in realistic environmental conditions. We dispersed three different metal oxide nanoparticles (TiO(2), ZnO and CeO(2)) in samples taken from eight different aqueous media associated with seawater, lagoon, river, and groundwater, and measured their electrophoretic mobility, state of aggregation, and rate of sedimentation. The electrophoretic mobility of the particles in a given aqueous media was dominated by the presence of natural organic matter (NOM) and ionic strength, and independent of pH. NOM adsorbed onto these nanoparticles significantly reduces their aggregation, stabilizing them under many conditions. The transition from reaction to diffusion limited aggregation occurs at an electrophoretic mobility from around -2 to -0.8 microm s(-1) V(-1) cm. These results are key for designing and interpreting nanoparticle ecotoxicity studies in various environmental conditions.

Influence of natural organic matter on the aggregation and deposition of titanium dioxide nanoparticles.

The aggregation kinetics of TiO(2) nanoparticles was studied in the absence and presence of Suwanee River humic acid (SRHA) in either NaCl or CaCl(2) electrolytes. The CCC[Ca(2+)]/CCC[Na(+)] ratios were found to yield a proportionality fraction of z(-7.2) (in the absence of SRHA) and z(-5.6) (in the presence of SRHA), near the theoretical prediction of z(-6), where z is the cations valence. SRHA drastically increased the stability of TiO(2) nanoparticles under most conditions, due to the combined effect of increased electrostatic and steric repulsions. Deposition rates of TiO(2) nanoparticles onto a silica surface were quantitatively measured using a quartz crystal microbalance with dissipation (QCM-D) over a broad range of solution (pH and ionic strength, IS) conditions, and the effects of the SRHA on particle deposition behavior were evaluated. In general, zeta potential can be used to predict the interaction energies between particles or particles and surfaces, and from there an inference can be made as to the potential for aggregation and deposition. The presence of SRHA significantly hinders TiO(2) deposition onto silica surfaces via steric repulsion in addition to repulsive electrostatics even under high ionic strength, which has important implications for the mobility of these nanoparticles.

Role of morphology in the aggregation kinetics of ZnO nanoparticles

The aggregation kinetics of two types of ZnO nanoparticles were investigated under various conditions. Distinct differences in aggregation kinetics were observed between the two ZnO particles. The aggregation of the nearly spherical ZnO (denoted as Me ZnO) exhibited strong dependence on the ionic strength (IS) of the solution; while minimal influence of IS was seen on the irregularly shaped ZnO (mixture of slab-like and rod-shaped particles, denoted as Mk ZnO) in the IS ranged tested. It is postulated that Mk ZnO possesses a critical coagulation concentration (CCC) below the lowest electrolyte concentration tested (1 mM NaCl) due to the interactions between various surfaces. The CCC of ZnO was found to be a function of pH; the CCC increased significantly as the pH was further away from the point of zero charge. Natural organic matter (NOM) was found to substantially hinder the aggregation of both types of ZnO particles (above 10 mg/L for Me ZnO and above 1 mg/L for Mk ZnO). A Langmuir adsorption model was used to describe the NOM to ZnO nanoparticle adsorption isotherms. To our knowledge, this is the first study to report the effect of particle morphology on nanoparticle aggregation, which outlines the importance of accounting morphology into environmental transport assessment of nanoparticles.

Clay Particles Destabilize Engineered Nanoparticles in Aqueous Environments

Given the ubiquity of natural clay minerals, the most likely interaction of nanoparticles released into an aquatic environment will be with suspended clay minerals. Thus, the transport of engineered nanoparticles in the subsurface and the water column will most likely be altered by their interaction with these minerals. We studied the interactions of two of the most produced nanoparticles, Ag and TiO(2), and montmorillonite to determine how heteroaggregation can alter the stability of nanoparticle/clay mineral mixtures. Since at low pH montmorillonite has a negatively charged basal plane and positively charged edges, its interaction with these nanoparticles at different pH lead to unusual behaviors. There are six different interactions for each clay-nanoparticle pair. At pH values below the IEP of montmorillonite edge site, montmorillonite reduced the stability of both negatively charged Ag and positively charged TiO(2) nanoparticles. Surprisingly this enhanced coagulation only occurs within an intermediate ionic strength range. The spillover of the montmorillonite basal plane electric double layer to the montmorillonite edge may screen the electrostatic attraction between Ag and the montmorillonite edge at low ionic strength, whereas a repulsion between TiO(2) and montmorillonite face sites may restabilize the mixture.

Mobility of Capped Silver Nanoparticles under Environmentally Relevant Conditions

The mobility and deposition of capped silver (Ag) nanoparticles (NPs) on silica surfaces were characterized over a wide range of pH and ionic strength (IS) conditions, including seawater and freshwater. Two common organic capping agents (citrate and PVP) were evaluated. Both the capped Ag NPs and the silica surfaces were negatively charged under these environmentally relevant conditions, resulting in net repulsive electrostatics under most conditions. The steric repulsion introduced by the capping agents significantly reduced aggregation and deposition. In addition, the presence of natural organic matter in solution further decreased the deposition of either Ag NP on silica. Ag NPs were found to be highly mobile under these environmentally relevant conditions, with little or no deposition.

Metal oxide nanomaterials in seawater: Linking physicochemical characteristics with biological response in sea urchin development

The fate and behavior of nanomaterials (NMs) in environmental media has important consequences for toxicity. The majority of aquatic research to date has focused on NM behavior in freshwater systems. However, pH and salinity differences of seawater affect dissolution and aggregation of NMs. In this study, physical characteristics of metal oxide NMs in seawater were linked with their toxicity to developing sea urchins. The metal oxide NMs TiO(2) and CeO(2) up to 10mg/L were not toxic to the embryos of the white sea urchin (Lytechinus pictus). In contrast, ZnO NM was highly toxic to these embryos (EC(50) = 99.5 μg/L). The toxicity of ZnO NM was not significantly different from bulk ZnO or soluble Zn(2+) (from ZnSO(4) · 7H(2)O), suggesting that the toxicity of ZnO NM can be attributed to soluble Zn(2+). Furthermore, solubility data indicate that at the concentrations used in our sea urchin embryo experiments, ZnO NM was rapidly and completely solubilized in seawater. The present study also demonstrated that Fe-doped NMs were less soluble in seawater compared to pure ZnO NMs, but there was no concomitant reduction in toxicity.

Accumulation and toxicity of metal oxide nanoparticles in a soft-sediment estuarine amphipod

Estuarine and marine sediments are a probable end point for many engineered nanoparticles (ENPs) due to enhanced aggregation and sedimentation in marine waters, as well as uptake and deposition by suspension-feeding organisms on the seafloor. Benthic infaunal organisms living in sediments encounter relatively high concentrations of pollutants and may also suffer toxic effects of ENPs. We tested whether three heavily used metal oxide ENPs, zinc oxide (ZnO), copper oxide (CuO), and nickel oxide (NiO) were toxic to an estuarine amphipod, Leptocheirus plumulosus. We used results from 10-day laboratory bioassays to estimate potential demographic impacts of ENP exposure. We also evaluated fate and transport pathways of the ENPs in the experiments to elucidate routes of uptake and exposure. Dissolved Zn was found in sediment pore water and overlying water samples at 10 fold the concentrations of Cu or Ni, a pattern indicative of the relatively high dissolution rate of ZnO ENPs compared with CuO and NiO ENPs. Accumulation of metals in amphipod tissues increased with exposure concentrations for all three ENPs, suggesting possible exposure pathways to higher taxa. Amphipods accumulated ≤600 μg Zn and Cu g(-1) and 1000 μg Ni g(-1). Amphipod mortality increased with ZnO and CuO concentrations, but showed no significant increase with NiO to concentrations as high as 2000 μg g(-1). The median lethal concentration in sediment (LC50) of ZnO was 763 μg g(-1) and 868 μg g(-1) for CuO ENPs. Our results indicate that ZnO and CuO ENPs, but not NiO ENPs, are toxic to L. plumulosus and that ZnO toxicity primarily results from Zn ion exposure while CuO toxicity is due to nanoparticle exposure.

Organochlorine pesticide residuals in chickens and eggs at a poultry farm in Beijing, China

Chicken organs, animal feed, droppings, and ambient air were sampled at a farm in Beijing to determine the concentrations of hexachlorocyclohexane isomers (HCHs) and dichlorodiphenyltrichloroethane and metabolites (DDTs). Mean fresh weight concentrations of HCHs and DDTs were 0.122+/-0.061 ng/g and 0.051+/-0.038 ng/g in the muscles. These values are 1-2 orders of magnitude lower than those reported in China in 1980. Contaminated feed was the main source of HCHs and DDTs. Only 12.8% of HCH and 3.3% of DDT of the amount consumed were excreted. Accumulated quantities of HCHs and DDTs increased during growth. However, concentrations of HCHs and DDTs did not increase because of dilution from rapid growth. Based on the observed residual levels in mature chicken and the average diet of residents of China, the contributions from chicken and egg consumption to per capita daily intake of HCHs and DDTs were 487% and 88% of those of fish consumption.

Increased Mobility of Metal Oxide Nanoparticles Due to Photo and Thermal Induced Disagglomeration

Significant advances have been made on our understanding of the fate and transport of engineered nanomaterials. One unexplored aspect of nanoparticle aggregation is how environmental stimuli such as light exposure and temperature variations affect the mobility of engineered nanoparticles. In this study, TiO2, ZnO, and CeO2 were chosen as model materials for investigating the mobility of nanoparticles under three external stimuli: heat, light and sonication. Sunlight and high power sonication were able to partially disagglomerate metal oxide clusters, but primary particles bonded by solid state necks were left intact. A cycle of temperature increase from 25°C to 65°C and then decrease back was found to disagglomerate the compact clusters in the heating phase and reagglomerate them as more open fractal structures during the cooling phase. A fractal model summing the pair-wise DLVO interactions between primary particles within two fractal agglomerates predicts weak attractions on the order of a few kT. Our study shows that common environmental stimuli such as light exposure or temperature variation can disagglomerate nanoparticle clusters and enhance their mobility in open waters. This phenomenon warrants attention since it is likely that metal oxide nanoparticles will experience these natural stimuli during their transport in the environment.

Photoinduced Disaggregation of TiO2 Nanoparticles Enables Transdermal Penetration

Under many aqueous conditions, metal oxide nanoparticles attract other nanoparticles and grow into fractal aggregates as the result of a balance between electrostatic and Van Der Waals interactions. Although particle coagulation has been studied for over a century, the effect of light on the state of aggregation is not well understood. Since nanoparticle mobility and toxicity have been shown to be a function of aggregate size, and generally increase as size decreases, photo-induced disaggregation may have significant effects. We show that ambient light and other light sources can partially disaggregate nanoparticles from the aggregates and increase the dermal transport of nanoparticles, such that small nanoparticle clusters can readily diffuse into and through the dermal profile, likely via the interstitial spaces. The discovery of photoinduced disaggregation presents a new phenomenon that has not been previously reported or considered in coagulation theory or transdermal toxicological paradigms. Our results show that after just a few minutes of light, the hydrodynamic diameter of TiO2 aggregates is reduced from ∼280 nm to ∼230 nm. We exposed pigskin to the nanoparticle suspension and found 200 mg kg−1 of TiO2 for skin that was exposed to nanoparticles in the presence of natural sunlight and only 75 mg kg−1 for skin exposed to dark conditions, indicating the influence of light on NP penetration. These results suggest that photoinduced disaggregation may have important health implications.