The Nanotube Metrology project is dedicated to improving the measurement methods for, and fundamental understanding of, single-wall carbon nanotubes (SWCNTs). We accomplish this goal through a strategy of population purification (by length, nanotube species, chiral handedness, purity) followed by intensive characterization of intrinsic and extrinsic properties of the isolated materials. These results are then used to produce research publications, documentary standards on how to measure important characteristics, and NIST reference materials.
Collectively these outputs contribute to the foundation of commerce for SWCNT, and SWCNT containing, materials. SRM 2483, a certified homogenous reference material for SWCNT soot, has been released as of December 2011.
The certificate of characterization and purchase of the material are available here.
We recently held a workshop to capture advancing measurement needs related to the use of single species and other purified SWCNT populations.
The website for the workshop and a summary can be found here.
Exciting postdoctoral research opportunities are available in this project through National Research Council Fellowships. Please contact us for more details.
Single-wall carbon nanotubes (SWCNTs) are a tubular form of carbon consisting of a single shell of sp2 bonded carbon with a nanometer scale diameter that have many predicted properties superior to other available materials. However, every production technique for SWCNTs produces many different species of SWCNTs (called chiralities, and defined by the vector with which a graphene sheet would be rolled to form the tube structure), with different lengths of those SWCNTs agglomerated in varied nanoscale to mesocale structures containing impurities both carbonaceous and from the catalyst. This fundamental polydispersity of SWCNT materials from production has vastly complicated the application of SWCNT materials for industrial use. To underlie technology development, measurement techniques and standards are necessary (so that everyone is speaking the same language), and technology for the isolation of purified nanotubes by the structural parameters that define their properties. This project pursues both of these directions.
The properties of any SWCNT species are strongly linked to both the structure of the SWCNT, which defines whether a SWCNT is metallic or semiconducting (and its band gap), and its length, which strongly affects transport properties and collective behavior. Furthermore the properties can be dramatically affected by the local environment surrounding the SWCNT, and its aggregation state. Thus to characterize the intrinsic properties of any specific SWCNT species or length distribution, an initially polydisperse nanotube sample must be first purified of non-SWCNT carbon and metallic catalyst, as well as separated by diameter or length (or both) to achieve robust results. Characterization by optical methods (absorption, fluorescence, Raman scattering), conductivity measurements, microscopy or other techniques then yields specific results.
Figure 1. A photograph of SWCNTs at different stages of processing. A. Raw SWCNT soot. B. Dispersed and purified SWCNTs. C. Isolated, water-filled SWCNTs (1.3 nm average diameter). D. Metallic (blue), and semiconducting (yellow) SWCNTs separated from the green colored fraction shown in panel C.
We achieve separation and purification of SWCNTs thorough the use of dispersants such as small molecule surfactants or biomolecules such as DNA, that modify the surface of the nanotubes and enable them to be individually isolated in an aqueous environment. We can then use a variety of different methods to separate SWCNTs non-destructively, utilizing handles such as differential bouyancy or differential adsorption determined by the dispersant-SWCNT combination, or fundamental colloidal physics. Measurement of the separated populations can then be used for fundamental studies.
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Our systematic focus on controlling the nanotube population in a sample through use and development of separation techniques differentiates the NIST effort in SWCNTs. Using well controlled populations ensures that we can accurately develop measurement techniques and characterize intrinsic SWCNT properties with greater confidence. Two main classes of separations are currently being used in the project, chromatography based techniques and ultracentrifugation based techniques.
DNA-SWCNT Separation through Chromatography
Ming Zheng is the developer of, and world leader in, the separation of DNA dispersed SWCNTs through chromatography. Separation of the DNA dispersed SWCNTs by length is achievable through size exclusion chromatography, resulting in narrow and reproducible length fractions. Exciting current work however is focused on the direct separation of single SWCNT species from mixed populations using specific DNA sequences and ion exchange chromatography.
Recent results of this work have been pulished in Nature and the Journal of the American Chemical Society, highlighting specific extraction of individual semiconducting and metallic nanotube species respectively.
SWCNT Separation through Ultracentrifugation
Jeff Fagan is among the world leaders in ultracentrifugation based separations of SWCNTs. Through controlling parameters of the dispersed nanotubes and the medium, particularly their densities, separations can be driven for different SWCNT species, SWCNT length, or whether the SWCNTs are empty or are water-filled. He has developed separations utilizing rate-zonal methods that allow mass throughput well beyond earlier methods. These results have been published in journals such as Advanced Materials, Langmuir, and ACS Nano.
SWCNT Uptake and Toxicity
Separation of SWCNT populations with different lengths enabled investigation of this parameter's effect on cellular uptake. DNA dispersed and length separated SWCNTs were exposed to multiple cell lines, including human lung fibroblasts, and the SWCNT concentration monitored through fluorescence and absorbance methods, and effects to the cell metabolism through a WST-1 assay. Our results found that only the shortest SWCNTs, those less than 200 nm, were uptaken at a significant rate by the cells. This has substantial implications both for the potential toxicity of SWCNTs materials in inadvertant exposure, as well as for SWCNT based nano-medicine, and underlines the importance of using well separated and characterized SWCNT materials.
- The global market for nanotubes has the potential to reach $5 billion by 2012 in electrical, mechanical, health and medical applications.We are developing the measurements infrastructure that underlies this new material.
- Four biannual workshops on single-walled carbon nanotubes have led the way for dissemination of the NIST effort to both the industrial and academic communities. Outputs include a Recommended Practice Guide which launched the extensive ISO activities in SWCNTs. Eleven documentary standards are in preparation by five countries.
- Reference Materials (RMs), are physical standards with specified values that can be purchased for comparative measurements. A SWCNT soot SRM, with certified values for its elemental composition will be available in late 2011.
- We collaborate with industrial, academic and government researchers at many institutions. By providing samples with unprecedented quality we drive innovation beyond what we can accomplish alone.
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