Sarah C. Larsen

Effect of Crystal Size and Surface Functionalization on the Cytotoxicity of Silicalite-1 Nanoparticles

In this report, we describe the synthesis and characterization of nanocrystalline silicalite (the purely siliceous form of the zeolite, ZSM-5) of defined crystal size and surface functionalization and determine the effect on the type and degree of cytotoxicity induced in two distinct model cell lines. The silicalite materials were characterized by powder X-ray diffraction, dynamic light scattering and zeta potential, solid state NMR, thermal gravimetric analysis, and nitrogen adsorption using the BET method to determine specific surface area. The silicalite samples were functionalized with amino, thiol, and carboxy groups and had crystal sizes of approximately 30, 150, and 500 nm. The cytotoxicities of the silicalite samples with different crystal sizes and different surface functional groups were investigated using human embryonic kidney 293 (HEK-293) cells and RAW264.7 macrophage cell lines. We used the lactic dehydrogenase release assay to measure damage to the cell membrane, the caspase 3/7 activity assay to measure key molecules involved in apoptosis, and the Annexin V-propidium iodide staining method to provide visual confirmation of the types of cell death induced. We have shown that the impact of size and surface functionalization of silicalite nanoparticles on cell toxicity and mechanism of cell death is cell type-dependent. Thirty nanometer silicalite nanoparticles were nontoxic in RAW264.7 cells relative to untreated controls but caused necrosis in HEK293 cells. Carboxy-functionalized 500 nm silicalite nanoparticles resulted in apoptosis and necrosis in RAW264.7 cells and predominantly activated apoptosis in HEK293 cells.

Carbon dioxide (C16O2 and C18O2) adsorption in zeolite Y materials: effect of cation, adsorbed water and particle size

In this study, CO2 adsorption in the presence and absence of co-adsorbed H2O was investigated in zeolite Y. Several different zeolite Y materials were investigated including commercial NaY, commercial NaY ion-exchanged with Ba2+ and nanocrystalline NaY; herein referred to as NaY, BaY and nano-NaY. Following heating of these zeolites to 573 K and cooling to room temperature, CO2 was adsorbed as a function of pressure. FTIR spectra show that a majority of CO2 adsorbs in the pores of these three zeolites (NaY, BaY and nano-NaY) in a linear complex with the exchangeable cation, as indicated by the intense absorption band near 2350 cm−1, assigned to the ν3 asymmetric stretch of adsorbed CO2. Most interestingly is the formation of carbonate and bicarbonate on the external surface of nano-NaY zeolites as indicated by the presence of several broad absorption bands in the 1200–1800 cm−1 region, suggesting unique sites for CO2 adsorption on the surface of the nanomaterial. For the other two zeolite materials investigated, bicarbonate formation is only evident in BaY zeolite in the presence of co-adsorbed water. Adsorption of 18O-labeled carbon dioxide and theoretical quantum chemical calculations confirm these assignments and conclusions.

Chemical Insight into the Adsorption of Chromium(III) on Iron Oxide/Mesoporous Silica Nanocomposites

Magnetic iron oxide/mesoporous silica nanocomposites consisting of iron oxide nanoparticles embedded within mesoporous silica (MCM-41) and modified with aminopropyl functional groups were prepared for application to Cr(III) adsorption followed by magnetic recovery of the nanocomposite materials from aqueous solution. The composite materials were extensively characterized using physicochemical techniques, such as powder X-ray diffraction, thermogravimetric and elemental analysis, nitrogen adsorption, and zeta potential measurements. For aqueous Cr(III) at pH 5.4, the iron oxide/mesoporous silica nanocomposite exhibited a superior equilibrium adsorption capacity of 0.71 mmol/g, relative to 0.17 mmol/g for unmodified mesoporous silica. The aminopropyl-functionalized iron oxide/mesoporous silica nanocomposites displayed an equilibrium adsorption capacity of 2.08 mmol/g, the highest adsorption capacity for Cr(III) of all the materials evaluated in this study. Energy-dispersive spectroscopy (EDS) with transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) experiments provided insight into the chemical nature of the adsorbed chromium species.

Hexagonal, hollow, aluminium-containing ZSM-5 tubes prepared from mesoporous silica templatesElectronic supplementary information (ESI) available: XRD patterns for MS and MS coated with silicalite-1 during the hydrothermal synthesis procedure. See http://www.rsc.org/suppdata/cc/b4/b406753c/

Hexagonal hollow ZSM-5 tubes were synthesized using mesoporous silica with a worm-like morphology as the template. A new method for aluminium incorporation during the hydrothermal synthesis step was developed.

Zeolite and mesoporous silica nanomaterials: greener syntheses, environmental applications and biological toxicity

Zeolites and mesoporous silica nanoparticles are silicate or aluminosilicate nanomaterials with well-defined pore networks. Zeolites are widely used in industry for applications such as catalysis, separations and gas adsorption. Mesoporous silica nanomaterials have not been as extensively applied relative to zeolites due to the cost and reduced thermal stability, but are being intensively investigated for potential environmental and biomedical applications. In this article, zeolite and mesoporous silica nanomaterials are reviewed with emphasis on connections to the environment. Specifically, the topics of greener syntheses, environmental applications and biological toxicity will be addressed. Both of these materials are typically synthesized with a template, which is then removed to produce the pore volume. Synthetic strategies for the “greening” of the syntheses of zeolites and mesoporous silica will be discussed. Environmental applications including the adsorption of environmental contaminants and environmental catalysis will also be presented. Finally, the toxicity of zeolite and mesoporous silica nanomaterials will be considered.

High yield method for nanocrystalline zeolite synthesis

Nanocrystalline zeolites, such as silicalite-1 and zeolite Y, were synthesized in high yield by periodically removing nanocrystals from the synthesis solution and recycling the unused reagents, including the template and T-atom sources.

Silica Nanoparticle-Generated ROS as a Predictor of Cellular Toxicity: Mechanistic Insights and Safety by Design.

Evaluating toxicological responses of engineered nanomaterials such as silica nanoparticles is critical in assessing health risks and exposure limits. Biological assays can be used to evaluate cytotoxicity of individual materials, but specific nano-bio interactions-which govern its physiological response-cannot currently be predicted from materials characterization and physicochemical properties. Understanding the role of free radical generation from nanomaterial surfaces facilitates understanding of a potential toxicity mechanism and provides insight into how toxic effects can be assessed. Size-matched mesoporous and nonporous silica nanoparticles in aminopropyl-functionalized and native forms were investigated to analyze the effects of porosity and surface functionalization on the observed cytotoxicity. In vitro cell viability data in a murine macrophage cell line (RAW 264.7) provides a model for what might be observed in terms of cellular toxicity upon an environmental or industrial exposure to silica nanoparticles. Electron paramagnetic resonance spectroscopy was implemented to study free radical species generated from the surface of these nanomaterials and the signal intensity was correlated with cellular toxicity. In addition, in vitro assay of intracellular reactive oxygen species (ROS) matched well with both the EPR and cell viability data. Overall, spectroscopic and in vitro studies correlate well and implicate production of ROS from a surface-catalyzed reaction as a predictor of cellular toxicity. The data demonstrate that mesoporous materials are intrinsically less toxic than nonporous materials, and that surface functionalization can mitigate toxicity in nonporous materials by reducing free radical production. The broader implications are in terms of safety by design of nanomaterials, which can only be extracted by mechanistic studies such as the ones reported here.

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.

Density Functional Theory Investigation of EPR Parameters for Tetragonal Cu(II) Model Complexes with Oxygen Ligands

Density functional theory (DFT) calculations of the electron paramagnetic resonance (EPR) parameters for a series of tetragonal Cu(II) model complexes were conducted. Model complexes containing four oxygen atoms directly coordinated to a Cu(II) metal center were chosen because of their importance in the Peisach-Blumberg truth tables and their frequent use in the interpretation of EPR spectra of Cu(II) proteins and copper-containing catalysts. Molecular g- and copper A-tensors were calculated using the BP86 and B3LYP density functionals. The DFT calculations reproduce the experimentally observed trends in the parallel components of the A- and g-tensors. Important insight into the structural basis for the empirical trends in g( parallel) and A( parallel) was obtained from the DFT calculations. Notably, g( parallel) systematically increases and A( parallel) systematically decreases with increasing Cu-O equatorial bond length. These results have been used to provide structural insight into copper EPR data for copper-exchanged zeolites.

Framework Stability of Nanocrystalline NaY in Aqueous Solution at Varying pH

Nanocrystalline zeolites are emerging as important materials for a variety of potential applications in industry and medicine. Reducing the particle size to less than 100 nm results in advantages for nanocrystalline zeolites relative to micrometer-sized zeolite crystals, such as very large total and external specific surface areas and reduced diffusion path lengths. Understanding the physical and chemical properties of zeolite nanocrystals is imperative for further development and application of nanocrystalline zeolites. In this study, the framework stability of nanocrystalline NaY zeolite with a crystal size of 66 nm and Si/Al = 1.74 was investigated at pH 7.4, 4, 2, and 1. The solids and solutions were analyzed using several different analytical techniques. The relative crystallinity and crystal size and morphology of the solids were examined by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The aluminum content, Si/Al, and coordination were monitored by inductively coupled plasma/optical emission spectroscopy (ICP/OES), X-ray photoelectron spectroscopy (XPS), and aluminum-27 solid-state magic-angle spinning NMR. As the acidity of the medium increased, the framework stability of nanocrystalline NaY decreased. Treatment of the zeolite samples at pH 1 resulted in complete degradation of the zeolite framework after 1 h. An increase in Si/Al was also observed, suggesting the selective removal of aluminum at low pH.