New Scalable Green Protocols for Processing Carbon Nanotubes

NIST News:

Summary: The overall aim of this project is to develop and validate general methods and protocols for purification, size-separation and dispersion of carbon nanotubes (CNTs) that will enable aqueous ‘biological’ applications. New NMR metrologies are also being developed and applied to characterize and compare the physio-chemical properties of various length/diameter sized separated CNTs in aqueous dispersions.

Description: Since the discovery of the C60 “buckyball” by the 1996 Nobel Prize laureates Robert F. Curl, Harold W. Kroto, and Richard E. Smalley, potential applications of CNTs have become the focus of a broad range of academic and industrial research programs in areas ranging from microelectronics to clinical therapeutics with a potential market for CNT nanoparticles in the multi-billion dollar range. Current efforts at manipulating CNTs have typically focused on longer nanotubes (> 500 nm), so that materials studied were “nano” scaled in diameter only. While longer tubes are easier to manipulate with existing methods, these CNTs do not exhibit mechanical, electrical and biological properties of a true nanoscale material (diameter and length) – smaller is better.

For example, longer nanotubes tend to aggregate and fall out of aqueous solution. A 2 to 5 micron long CNT is on the same scale as a typical human cell, which makes these tubes too long for many applications. True nanoscale tubes, ‘nanoblocks’,are as much as 100x shorter making them much easier to mix with composite materials, interact with biological samples, and even align in solution at relatively low magnetic field strengths.

While raw carbon nanotube materials can be generated relatively inexpensively, there still remains a lack of low cost, scalable purification techniques to produce high purity, homogenous CNT fractions. Previous methods to shorten and debulk CNTs, primarily using such procedures as acid treatment and sonication, have tended to result in a conversion of up to 90% of the starting material into carbon “dust” – amorphous carbon particles. Nanoscale length fractionation has also not been routinely achieved since CNTs have been found to rapidly aggregate and clog many separation systems. Drawbacks to existing purification methods also include defect generation in the sidewalls of CNTs. A report from a workshop held at NIST in 2005 entitled “Measurement Issues in Single Wall Carbon Nanotubes” highlighted the lack of consensus on a set of standard methods for preparation and physio-chemical analysis of CNTs.

To address the bottleneck in post-production, scalable CNT processing, NIST researchers and collaborators at the University of Maryland Biotechnology Institute have developed a series of novel aqueous-based methods: (1) mechanical processing of crude CNT materials, (2) continuous micro-scale and nano-scale hydrodynamic size separation of CNTs with self-cleaning mesh filters, (3) magnetic gradient fractionation in a glycerol gradient, and (4) power-dialysis/electrophoresis to remove < 0.5 nm amorphous carbon species. Using these methods, a step-wise enrichment and fractionation of commercially obtained as provided (AP) CNT materials has been achieved as monitored using spectroscopic methods. In addition, protocols for uniformity in sample preparation and measurement have also been explored as these will be essential to enabling a quantitative comparison of low-to-high grade AP CNTs in their raw, intermediate and purified states.

A potential multi-billion dollar market exists for CNTs in everything from basic research to applied nanoscale biotechnology and electronics. On a macroscale CNTs could have wide application in light-weight, strong, corrosion resistant building materials. Realizing these commercial and societal benefits will require standardized, robust metrologies for CNT processing and analysis. Protocols for the generation of standard CNT preparations will also be integral to enabling standard procedures to assay the environmental, health, and safety of these nanoparticles.

Major Accomplishments:

  • A series of novel nanoprocessing and filtering/dialysis devices have been designed and prototypes built for carbon nanotube processing and size sorting.
  • A patent covering the intellectual property associated with the new devices and protocols associated with scalable carbon nanotube processing has been submitted.

The National Institute of Standards and Technology (NIST) is an agency of the U.S. Department of Commerce.