Measurement Challenges Make Nanomaterials Assessments Difficult
3 May, 2007 12:05 pm
Single wall carbon nanotubes (SWNTs) have been highlighted repeatedly as a unique and versatile material that could revolutionize numerous industries in the coming decades. Potential applications in electronic, mechanical and thermal devices as well as composites have led to great excitement in the basic and applied research communities.
Many of the deficiencies in addressing these issues arise because materials on the nanometer (1×10-9) length scales are very difficult to measure precisely. These classes of materials in general reside in a gray area between small molecules and bulk materials. A detailed assessment is even more difficult because many “nanomaterials” have heterogeneous distributions in physical properties such as size, charge, mass, and density. In addition, subtle structural, chiral, or compositional differences may have little effect on the actual solution properties but make the common spectroscopic techniques utilized by most synthetic laboratories to measure these and other phenomena much more complicated. SWNTs, for example possess metal or semiconductor-like properties, depending on their chirality or degree of “twisting”. Our recent efforts have shown the importance of detailed measurements on accurate assessments of cell-material interactions.
The team first chose to isolate the effects of nanotube length. They adsorbed short DNA molecules onto the nanotubes because this renders them soluble in water and allows them to be sorted and separated by length. This portion of the study was designed to call attention to the importance of dispersion and concentration on cellular toxicity. Wide variations can largely be attributed to two causes: first the wide variability in sample preparation, “purification” and dispersion methods, and second, the use of non-uniform or incomplete characterization methods and materials with different preparative protocols, viability assessment methods and cell/species populations. The researchers exposed human lung fibroblasts to solutions containing dispersed but not length sorted nanotubes. Regardless of the concentration levels, the cells did not absorb between about one-fourth and one-third of the SWCNTs in the solutions. Further examination of the results revealed that only short nanotubes made it into the cellular interior.
In our most recent study, we demonstrated that only length fractions of DNA-wrapped SWNTs shorter than about 200 nanometers enter readily into human lung cells. We chose to pursue these data because the published data citing in vitro toxicity of SWNT are inconsistent, widely disputed, and have ignored length as a possible contributing effect.[5-15] While this study is not definitive, it does demonstrate that establishing a definitive toxicity framework is impossible without implementation of precise measurements, complete characterization, and the use of well-defined materials. In addition, caution should be exercised when trying to extrapolate these results to conclusively assess or predict the long term health effects that acute or continuous exposure could present. How cell effects are measured against cells or tissues grown in labs (in vitro) and how toxic they actually are to live animals (in vivo) are very different. Often overlooked in these reports is the method of exposure, which is perhaps the most difficult hypothesis to design an experiment around. The results of inhalation versus ingestion or skin exposure can be expected to vary significantly. Warheit et. al. have shown that in vitro studies could show opposite effects from those observed in in vivo studies.
Our goals moving forward are not to conclusively map a material toxicity framework that covers multiple or even single classes of nanomaterials. This endeavor is bigger than any single entity or government agency. Our goal is to provide guidance and methods to correctly measure important physical properties on broad classes of nanometer scale materials from which others may draw to make their own assessments.
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