Kevin D. Ausman

Principles for characterizing the potential human health effects from exposure to nanomaterials: Elements of a screening strategy

AbstractThe rapid proliferation of many different engineered nanomaterials (defined as materials designed and produced to have structural features with at least one dimension of 100 nanometers or less) presents a dilemma to regulators regarding hazard identification. The International Life Sciences Institute Research Foundation/Risk Science Institute convened an expert working group to develop a screening strategy for the hazard identification of engineered nanomaterials. The working group report presents the elements of a screening strategy rather than a detailed testing protocol. Based on an evaluation of the limited data currently available, the report presents a broad data gathering strategy applicable to this early stage in the development of a risk assessment process for nanomaterials. Oral, dermal, inhalation, and injection routes of exposure are included recognizing that, depending on use patterns, exposure to nanomaterials may occur by any of these routes. The three key elements of the toxicity screening strategy are: Physicochemical Characteristics, In Vitro Assays (cellular and non-cellular), and In Vivo Assays.There is a strong likelihood that biological activity of nanoparticles will depend on physicochemical parameters not routinely considered in toxicity screening studies. Physicochemical properties that may be important in understanding the toxic effects of test materials include particle size and size distribution, agglomeration state, shape, crystal structure, chemical composition, surface area, surface chemistry, surface charge, and porosity. In vitro techniques allow specific biological and mechanistic pathways to be isolated and tested under controlled conditions, in ways that are not feasible in in vivo tests. Tests are suggested for portal-of-entry toxicity for lungs, skin, and the mucosal membranes, and target organ toxicity for endothelium, blood, spleen, liver, nervous system, heart, and kidney. Non-cellular assessment of nanoparticle durability, protein interactions, complement activation, and pro-oxidant activity is also considered.Tier 1 in vivo assays are proposed for pulmonary, oral, skin and injection exposures, and Tier 2 evaluations for pulmonary exposures are also proposed. Tier 1 evaluations include markers of inflammation, oxidant stress, and cell proliferation in portal-of-entry and selected remote organs and tissues. Tier 2 evaluations for pulmonary exposures could include deposition, translocation, and toxicokinetics and biopersistence studies; effects of multiple exposures; potential effects on the reproductive system, placenta, and fetus; alternative animal models; and mechanistic studies.

Reversible water-solubilization of single-walled carbon nanotubes by polymer wrapping

Abstract Single-walled carbon nanotubes (SWNTs) have been solubilized in water by non-covalently associating them with linear polymers, most successfully with polyvinyl pyrrolidone (PVP) and polystyrene sulfonate (PSS). This association is characterized by tight, uniform association of the polymers with the sides of the nanotubes. A general thermodynamic drive for this wrapping is discussed, wherein the polymer disrupts both the hydrophobic interface with water and the smooth tube–tube interactions in aggregates. The nanotubes can be unwrapped by changing the solvent system. This solubilization process opens the door to solution chemistry on pristine nanotubes, as well as their introduction into biologically relevant systems.

Helical ice-sheets inside carbon nanotubes in the physiological condition

Molecular dynamics simulations were performed, in the physiological condition (300 K and 1 atm), on nanotube segments of various diameters submerged in water. The results show that water molecules can exist inside the nanotube segments, and, most importantly, the water molecules inside the tubes tend to organize themselves into a highly hydrogen-bonded network, i.e., solid-like wrapped-around ice sheets. The disorder-to-order transition of these ice-sheets can be achieved purely by tuning the size of the tubes. The results also suggest that the nanotubes have the potential to be used as proton-conducting pores for a variety of biological applications.

Scanning electron microscopy study of carbon nanotubes heated at high temperatures in air

Multiwalled carbon nanotubes (MWNTs) were dispersed in 2-butanol and dropped onto a V-ridge, lithographically patterned Si substrate that was coated with a thin layer of gold. These MWNTs were shown by scanning electron microscopy (SEM) to conform to the V-ridge surface topology at room temperature, which is thus useful for introducing kinks (at the apex of the V-ridge and the bottom of the trenches between V ridges). The substrate-supported MWNTs were then heated in air at temperatures from 673 to 1173 K for varying exposure times and were monitored with SEM. A 122 kJ mol−1 activation energy for complete oxidation was obtained, and preferential oxidation at kink sites was observed on some MWNTs at high temperatures. The dominant mode of oxidation was either thinning of the walls of the MWNTs or sequential oxidation of the component tubes in bundles. Some MWNTs, which at room temperature conformed to the V-ridge surface topology, detached (“sprang” away) from the substrate surface, demonstrating that the ...

Kinetics of fullerene triplet states

Studies are described whose goal is a quantitative kinetic description of fullerene triplet relaxation. The room-temperature intrinsic lifetimes of solution phase T1 C60 and C70 differ substantially, with values in toluene of 143 μs and 12 ms, respectively. These decay rates exhibit only weak temperature dependence near room temperature. The intrinsic lifetime of T1 C60 has a simple dependence on vibrational energy content up to 1000 K. Efficient triplet-triplet annihilation occurs in C60 and C70 solutions at ca. 50% of the diffusion-limited rate. In mixed solutions, rapid reversible triplet energy exchange was observed between C60 and C70, and between C60 and (CH3)2 C60. A new method for measuring relative triplet enthalpies and entropies in such mixtures has also been applied. Complex kinetics has been uncovered in C70 solutions and modeled by reversible formation of shortlived triplet excimers, accounting for the efficient self-quenching. C60 self-quenching has been found to be highly temperature dependent, but the mechanism remains unresolved.

Roping and wrapping carbon nanotubes

Single-walled carbon nanotubes can be dispersed into solvents by ultrasonication to the point that primarily individual tubes, cut to a few hundred nanometers in length, are present. However, when such dispersions are filtered to a thick mat, or paper, only tangles of uniform, seemingly endless ropes are observed. The factors contributing to this “roping” phenomenon, akin to aggregation or crystallization, will be discussed. We have developed methods for generating “super-ropes” more than twenty times the diameter of those formed by filtration, involving the extraction of nanotube material from an oleum dispersion. Nanotubes have been solubilized in water, largely individually, by non-covalently wrapping them with linear polymers. The general thermodynamic drive for this wrapping involves the polymer disrupting both the hydrophobic interface with water and the smooth tube-tube interaction in aggregates. The nanotubes can be recovered from their polymeric wrapping by changing their solvent system. This sol...