Ernest M. Hotze

Sulfidation of Silver Nanoparticles: Natural Antidote to Their Toxicity

Nanomaterials are highly dynamic in biological and environmental media. A critical need for advancing environmental health and safety research for nanomaterials is to identify physical and chemical transformations that affect the nanomaterial properties and their toxicity. Silver nanoparticles, one of the most toxic and well-studied nanomaterials, readily react with sulfide to form Ag(0)/Ag2S core-shell particles. Here, we show that sulfidation decreased silver nanoparticle toxicity to four diverse types of aquatic and terrestrial eukaryotic organisms (Danio rerio (zebrafish), Fundulus heteroclitus (killifish), Caenorhabditis elegans (nematode worm), and the aquatic plant Lemna minuta (least duckweed)). Toxicity reduction, which was dramatic in killifish and duckweed even for low extents of sulfidation (about 2 mol % S), is primarily associated with a decrease in Ag(+) concentration after sulfidation due to the lower solubility of Ag2S relative to elemental Ag (Ag(0)). These results suggest that even partial sulfidation of AgNP will decrease the toxicity of AgNPs relative to their pristine counterparts. We also show that, for a given organism, the presence of chloride in the exposure media strongly affects the toxicity results by affecting Ag speciation. These results highlight the need to consider environmental transformations of NPs in assessing their toxicity to accurately portray their potential environmental risks.

Theoretical Framework for Nanoparticle Reactivity as a Function of Aggregation State

Theory is developed that relates the reactivity of nanoparticles to the structure of aggregates they may form in suspensions. This theory is applied to consider the case of reactive oxygen species (ROS) generation by photosensitization of C(60) fullerenes. Variations in aggregate structure and size appear to account for an apparent paradox in ROS generation as calculated using values for the photochemical kinetics of fullerene (C(60)) and its hydroxylated derivative, fullerol (C(60)(OH)(22-24)) and assuming that structure varies between compact and fractal objects. A region of aggregation-suppressed ROS production is identified where interactions between the particles in compact aggregates dominate the singlet oxygen production. Intrinsic kinetic properties dominate when aggregates are small and/or are characterized by low fractal dimensions. Pseudoglobal sensitivity analysis of model input variables verifies that fractal dimension, and by extension aggregation state, is the most sensitive model parameter when kinetics are well-known. This theoretical framework qualitatively predicts ROS production by fullerol suspensions 2 orders of magnitude higher compared with aggregates of largely undifferentiated C(60) despite nearly an order of magnitude higher quantum yield for the undifferentiated C(60) based on measurements for single molecules. Similar to C(60), other primary nanoparticles will exist as aggregates in many environmental and laboratory suspensions. This work provides a theoretical basis for understanding how the structure of nanoparticle aggregates may affect their reactivity.

Nanomaterials as possible contaminants: the fullerene example

An assessment of the potential risks posed by nanomaterials will require case-by-case evaluations of the processes controlling exposure and hazards such as toxicity. Factors that control fullerene transport and transformation in aqueous environments and their relationship to toxicity are discussed. Natural organic matter is observed to either increase or decrease nanoparticle stability while trends in reactive oxygen generation run counter to proposed mechanisms of possible fullerene toxicity.

Environmental implications and applications of carbon nanomaterials in water treatment

Carbon nanomaterials have been proposed as a basis for developing new technologies for photocatalytic oxidation and disinfection, improved membrane processes, adsorbents, and biofilm-resistant surfaces. This study details recent progress towards the development of these proposed applications. We explored the use of carbon nanomaterials such as fullerene C60, single-wall carbon nanotubes (SWCNTs), and multi-wall carbon nanotubes (MWCNTs) for a range of new technologies including, degradation of a probe organic compound by in situ generation of reactive oxygen species (ROS), new strategies for microbial disinfection, and the inhibition of biofilm development on membrane surfaces. The results show that the degradation of 2-chlorophenol by ROS produced microbial inactivation, and the mobility of the nanoparticle aggregates of the carbon nanomaterials all increased as suspensions were fractionated to enrich with smaller aggregates with sonication followed by successive membrane filtration.

Possible Applications of Fullerene Nanomaterials in Water Treatment and Reuse

Publisher Summary This chapter details recent progress toward the development of the proposed applications. It examines the development of fullerene composite materials using CNTs to strengthen membranes and modify membrane surface chemistry. It explores the use of fullerene nanomaterials to generate reactive oxygen species (ROS) as the basis for a range of new technologies including in situ generation of oxidants to destroy trace organic compounds, new strategies for disinfection, the inhibition of biofilm development, and reduced biofouling. The use of fullerenes in conjunction with ultraviolet (UV) irradiation is considered as an advanced disinfection process (ADP) for viral inactivation. Fullerenes are a class of molecules composed entirely of carbon. The first of these molecules, Buckminsterfiillerene, was discovered in 1985 and contains 60 carbons in the form of a hollow spherical cagc consisting of 12 pentagonal and 20 hexagonal faces.

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.

Chapter 21 – Possible Applications of Fullerene Nanomaterials in Water Treatment and Reuse

Fullerenes are a class of molecules composed entirely of carbon. The first of these molecules, Buckminsterfullerene, was discovered in 1985 and contains 60 carbons in the form of a hollow spherical cage consisting of 12 pentagonal and 20 hexagonal faces. Other spherical fullerenes or “buckyballs” have since been synthesized as well as nonspherical fullerenes that include cylinders (carbon nanotubes-CNTs), lobed structures, and bowls to name a few. Further variations on fullerenes include the addition of an almost infinite variety of functionalities ranging from simple hydroxylation to the grafting of deoxyribonucleic acid (DNA) molecules. Driven by immediate applications and the utility of undifferentiated material for subsequent modification, there has been a significant commercial emphasis placed on the production of buckyballs and CNTs. In environmental engineering, fullerenes have been proposed as a basis for developing new technologies for nanomaterial-enabled oxidation and disinfection, improved membrane processes, adsorbents, and biofilm-resistant surfaces. This chapter details recent progress toward the development of these proposed applications. We examine the development of fullerene composite materials using CNTs to strengthen membranes and modify membrane surface chemistry. We also explore the use of fullerene nanomaterials to generate reactive oxygen species (ROS) as the basis for a range of new technologies including in situ generation of oxidants to destroy trace organic compounds, new strategics for disinfection, the inhibition of biofilm development, and reduced biofouling. The use of fullerenes in conjunction with ultraviolet (UV) irradiation is considered as an advanced disinfection process (ADP) for viral inactivation.