Behavior and Potential Impacts of Metal-Based Engineered Nanoparticles in Aquatic Environments.

The specific properties of metal-based nanoparticles (NPs) have not only led to rapidly increasing applications in various industrial and commercial products, but also caused environmental concerns due to the inevitable release of NPs and their unpredictable biological/ecological impacts. This review discusses the environmental behavior of metal-based NPs with an in-depth analysis of the mechanisms and kinetics. The focus is on knowledge gaps in the interaction of NPs with aquatic organisms, which can influence the fate, transport and toxicity of NPs in the aquatic environment. Aggregation transforms NPs into micrometer-sized clusters in the aqueous environment, whereas dissolution also alters the size distribution and surface reactivity of metal-based NPs. A unique toxicity mechanism of metal-based NPs is related to the generation of reactive oxygen species (ROS) and the subsequent ROS-induced oxidative stress. Furthermore, aggregation, dissolution and ROS generation could influence each other and also be influenced by many factors, including the sizes, shapes and surface charge of NPs, as well as the pH, ionic strength, natural organic matter and experimental conditions. Bioaccumulation of NPs in single organism species, such as aquatic plants, zooplankton, fish and benthos, is summarized and compared. Moreover, the trophic transfer and/or biomagnification of metal-based NPs in an aquatic ecosystem are discussed. In addition, genetic effects could result from direct or indirect interactions between DNA and NPs. Finally, several challenges facing us are put forward in the review.

Atomic force microscopy study of the interaction of DNA and nanoparticles.

The interaction between nanoparticles (NPs) and DNA plays an important role in the genotoxicity of NPs, and it is imperative to characterize the nano/DNA interactions and explore the underlying chemical mechanisms. In this chapter, we demonstrated systematic experimental approaches based on atomic force microscope (AFM), coupled with modeling computation to probe the binding activity of NPs with DNA and the putative genotoxicity. Using quantum dots (QDs) as a model NP, we examined the binding kinetics, binding isotherm, binding specificity, and binding mechanisms of NPs to DNA with the application of AFM. We further assessed the binding affinity between NPs and DNA by calculating their interaction energy on the basis of Derjaguin-Landau-Verwey-Overbeek (DLVO) models. The modeling results of binding affinity were validated by the NPs/DNA binding images experimentally derived by AFM. The investigation of the relationship between the binding affinity of five NPs ((QDs (+), QDs (-), silver NPs, hematite NPs, and gold NPs) for DNA with their inhibition effects on DNA replication indicated that NPs with a high binding affinity for DNA molecules exhibited higher inhibition on DNA replication. The methodology employed in this study can be extended to study the interaction of other NPs with DNA, which is anticipated to benefit the future design of safe NPs, as well as the toxicological investigations of NPs.

Nanoparticles inhibit DNA replication by binding to DNA: modeling and experimental validation.

Predictive models are beneficial tools for researchers to use in prioritizing nanoparticles (NPs) for toxicological tests, but experimental evaluation can be time-consuming and expensive, and thus, priority should be given to tests that identify the NPs most likely to be harmful. For characterization of NPs, the physical binding of NPs to DNA molecules is important to measure, as interference with DNA function may be one cause of toxicity. Here, we determined the interaction energy between 12 types of NPs and DNA based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) model and then predicted the affinity of the NPs for DNA. Using the single-molecule imaging technique known as atomic force microscopy (AFM), we experimentally determined the binding affinity of those NPs for DNA. Theoretical predictions and experimental observations of the binding affinity agreed well. Furthermore, the effect of NPs on DNA replication in vitro was investigated with the polymerase chain reaction (PCR) technique. The results showed that NPs with a high affinity for DNA strongly inhibited DNA replication, whereas NPs with low affinity had no or minimal effects on DNA replication. The methodology here is expected to benefit the genotoxicological testing of NPs as well as the design of safe NPs.

Phytotoxicity, accumulation and transport of silver nanoparticles by Arabidopsis thaliana.

The widespread availability of nano-enabled products in the global market may lead to the release of a substantial amount of engineered nanoparticles in the environment, which frequently display drastically different physiochemical properties than their bulk counterparts. The purpose of the study was to evaluate the impact of citrate-stabilised silver nanoparticles (AgNPs) on the plant Arabidopsis thaliana at three levels, physiological phytotoxicity, cellular accumulation and subcellular transport of AgNPs. The monodisperse AgNPs of three different sizes (20, 40 and 80 nm) aggregated into much larger sizes after mixing with quarter-strength Hoagland solution and became polydisperse. Immersion in AgNP suspension inhibited seedling root elongation and demonstrated a linear dose-response relationship within the tested concentration range. The phytotoxic effect of AgNPs could not be fully explained by the released silver ions. Plants exposed to AgNP suspensions bioaccumulated higher silver content than plants exposed to AgNO3 solutions (Ag(+) representative), indicating AgNP uptake by plants. AgNP toxicity was size and concentration dependent. AgNPs accumulated progressively in this sequence: border cells, root cap, columella and columella initials. AgNPs were apoplastically transported in the cell wall and found aggregated at plasmodesmata. In all the three levels studied, AgNP impacts differed from equivalent dosages of AgNO3.

Quantum dot binding to DNA: single-molecule imaging with atomic force microscopy.

The interaction between nanoparticles (NPs) and DNA is of significance for both application and implication research of NPs. In this study, a single-molecule imaging technique based on atomic force microscopy (AFM) was employed to probe the NP-DNA interactions with quantum dots (QDs) as model NPs. Reproducible high-quality images of single DNA molecules in air and in liquids were acquired on mica by optimizing sample preparation conditions. Furthermore, the binding of QDs to DNA was explored using AFM. The DNA concentration was found to be a key factor influencing AFM imaging quality. In air and liquids, the optimal DNA concentration for imaging DNA molecules was approximately 2.5 and 0.25 µg/mL, and that for imaging DNA binding with QDs was 0.5 and 0.25 µg/mL, respectively. In the presence of QDs, the DNA conformation was altered with the formation of DNA condensates. Finally, the fine conformation of QD-DNA binding sites was examined to analyze the binding mechanisms. This work will benefit investigations of NP-DNA interactions and the understanding of the structure of NP-DNA bioconjugates. See accompanying article by Wang DOI: 10.1002/biot.201200309.

Surface interactions affect the toxicity of engineered metal oxide nanoparticles toward Paramecium.

To better understand the potential impacts of engineered metal oxide nanoparticles (NPs) in the ecosystem, we investigated the acute toxicity of seven different types of engineered metal oxide NPs against Paramecium multimicronucleatum, a ciliated protozoan, using the 48 h LC(50) (lethal concentration, 50%) test. Our results showed that the 48 h LC(50) values of these NPs to Paramecium ranged from 0.81 (Fe(2)O(3) NPs) to 9269 mg/L (Al(2)O(3) NPs); their toxicity to Paramecium increased as follows: Al(2)O(3) < TiO(2) < CeO(2) < ZnO < SiO(2) < CuO < Fe(2)O(3) NPs. On the basis of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, interfacial interactions between NPs and cell membrane were evaluated, and the magnitude of interaction energy barrier correlated well with the 48 h LC(50) data of NPs to Paramecium; this implies that metal oxide NPs with strong association with the cell surface might induce more severe cytotoxicity in unicellular organisms.

Oxidative dissolution of polymer-coated CdSe/ZnS quantum dots under UV irradiation: mechanisms and kinetics.

To advance the knowledge of environmental fate of nanomaterials, we systematically investigated the dissolution of polymer-coated CdSe/ZnS quantum dots (QDs) under UV (254 nm) irradiation. The environmental effects (i.e., irradiation intensity, dissolved oxygen, temperature, and humic acid), as well as the coating effects on dissolution kinetics of QDs were investigated. Our results showed that higher irradiation intensity and temperature increased ion release rates (Cd(2+), SeO(4)(2-), and Zn(2+)), whereas the different polymer coatings varied the dissolution rates. The absence of dissolved oxygen inhibited the dissolution of QDs, and we further demonstrated that the dissolution was a photo-oxidative process involved superoxide radical formation. Humic acid had a twofold effect on dissolution due to its photosensitization and photoabsorption for UV irradiation. Finally, an empirical kinetic law was proposed to interpret the above environmental effects. This study lays groundwork to better understand the environmental fate of QDs.

Effect of natural organic matter on the aggregation kinetics of CeO2 nanoparticles in KCl and CaCl2 solutions: measurements and modeling.

To characterize the environmental transport and quantify the risk of nanoparticles (NPs), it is important to fundamentally understand the aggregation of NPs and to describe this process quantitatively. This study investigates the aggregation kinetics of CeO(2) NPs in the presence of KCl, CaCl(2) and humic acid (HA) using time-resolved dynamic light scattering. In KCl solutions, regardless of their concentration, HA drastically reduces the aggregation kinetics of CeO(2) NPs. However, the effect of HA was more complicated in CaCl(2) solutions. At low CaCl(2) concentrations, HA inhibited NP aggregation, whereas at high CaCl(2) concentrations, HA promoted aggregation. The critical coagulation concentration (CCC) in KCl in the absence of HA is approximately 36.5mM. In presence of both 1 ppm and 10 ppm HA in KCl solutions, extremely low aggregation kinetics were observed even at very high KCl concentrations (500 mM), implying KCl-CCCs in presence of HA were larger than 500 mM. The CCCs under conditions of no HA, 1 ppm HA and 10 ppm HA in CaCl(2) solutions are approximately 9.5, 8.0 and 12.0mM, respectively. These observations were analyzed in the framework of extended Derjaguin-Landau-Verwey-Overbeek (EDLVO) theory. Moreover, a kinetic model was used to predict the aggregation kinetics of CeO(2) NPs. The model predictions are in close agreement with experimental observations. To the best of our knowledge, this work is the first to model quantitatively the aggregation of NPs in the presence of natural organic matter.

Attachment efficiency of nanoparticle aggregation in aqueous dispersions: modeling and experimental validation.

To describe the aggregation kinetics of nanoparticles (NPs) in aqueous dispersions, a new equation for predicting the attachment efficiency is presented. The rationale is that at nanoscale, random kinetic motion may supersede the role of interaction energy in governing the aggregation kinetics of NPs, and aggregation could occur exclusively among the fraction of NPs with the minimum kinetic energy that exceeds the interaction energy barrier (E(b)). To justify this rationale, we examined the evolution of particle size distribution (PSD) and frequency distribution during aggregation, and further derived the new equation of attachment efficiency on the basis of the Maxwell-Boltzmann distribution and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The new equation was evaluated through aggregation experiments with CeO(2) NPs using time-resolved-dynamic light scattering (TR-DLS). Our results show that the prediction of the attachment efficiencies agreed remarkably well with experimental data and also correctly described the effects of ionic strength, natural organic matter (NOM), and temperature on attachment efficiency. Furthermore, the new equation was used to describe the attachment efficiencies of different types of engineered NPs selected from the literature and most of the fits showed good agreement with the inverse stability ratios (1/W) and experimentally derived results, although some minor discrepancies were present. Overall, the new equation provides an alternative theoretical approach in addition to 1/W for predicting attachment efficiency.

Influence of dissolved oxygen on aggregation kinetics of citrate-coated silver nanoparticles.

Aggregation, an important environmental behavior of silver nanoparticles (AgNPs) influences their bioavailability and cytotoxicity. The work studied the influence of dissolved oxygen (DO) or the redox potential on the stability of AgNPs in aqueous environments. This study employed time-resolved dynamic light scattering (TR-DLS) to investigate the aggregation kinetics of citrate-coated AgNPs. Our results demonstrated that when DO was present, the aggregation rates became much faster (e.g., 3-8 times) than those without DO. The hydrodynamic sizes of AgNPs had a linear growth within the initial 4-6 h and after the linear growth, the hydrodynamic sizes became random for AgNPs in the presence of DO, whereas in the absence of DO the hydrodynamic sizes grew smoothly and steadily. Furthermore, the effects of primary particles sizes (20, 40, and 80 nm) and initial concentrations (300 and 600 µg/L) of AgNPs on aggregation kinetics were also investigated.

Comparative study on the micelle properties of synthetic and dissolved organic matters.

The conductivity of two synthetic surfactants and several natural surfactants, dissolved organic matters (DOMs), as well as their enhancement on phenanthrene solubility were measured in order to compare the formation of micelle by DOMs with synthetic surfactants, and their applicability for promoting hydrophobic organic pollutants' mobility. The DOMs could form micelle structure, similar to the synthetic surfactants. The critical micelle concentration values of the DOMs are lower than those of the synthetic surfactants, and the enhancement of phenanthrene solubility by the DOMs is comparable to or more remarkable than the synthetic surfactants. The partitioning coefficient of phenanthrene to DOM micelles decreased at high DOM concentrations, which is attributed to the structure rearrangement of DOM macromolecules, while no such phenomenon was observed for simple synthetic surfactant micelles. There was an optimum concentration range when applying DOMs to enhance HOCs' solubility and mobility. Synthetic surfactants gave a concentration dependent conductivity plot with two evident regimes, premicellar and postmicellar regimes, whereas the DOMs showed a gradual transition between the two regimes. The degree of counterion dissociation (alpha) of the DOMs was remarkably higher than those of the synthetic ionic surfactants. These results provide insight into DOM micelle structure and micelle forming process with compared to synthetic surfactants, and valuable information on using natural surfactant-enhanced remediation technology.