(Nanowerk Spotlight) Current production methods for carbon nanotubes (CNTs) result in units with different diameter, length, chirality and electronic properties, all packed together in bundles, and often blended with some amount of amorphous carbon. These mixtures are of little practical use since many advanced applications, especially for nanoelectronics, are sensitively dependent on tube structures. Consequently, the separation of metallic and semiconducting CNTs is essential for future applications and studies. The tube diameter of semiconducting nanotubes is also important because the band gap depends on it.
Separation of nanotubes according to desired properties is still proving to be a challenging task, especially single-walled carbon nanotube (SWCNT) sorting, because the composition and chemical properties of SWCNTs of different types are very similar, making conventional separation techniques inefficient.
The separation techniques for SWCNTs explored to date rely on preferential electron transfer on metallic SWCNTs treated with diazonium salts, dielectrophoresis, enhanced chemical affinity of semiconducting SWCNTs with octadecylamine, and wrapping of SWCNTs with single-stranded DNA. The selectivity of these methods can be further enhanced by vigorous centrifugation of prepared dispersions and the use of ion-exchange chromatography (IEX).
A structure model of DNA-wrapped carbon nanotube (Image: Dr. Anand Jagota)
Ming Zheng of DuPont and coworkers have discovered an oligonucleotide sequence that self-assembles into a helical structure around individual nanotubes, creating DNA-CNT hybrids with electrostatic properties that depend upon the tube diameter and electrical properties. CNTs can be separated on the basis of these properties using anion exchange chromatography. The separation of metallic and semiconducting nanotubes is improved compared with other techniques, and separation on the basis of tube diameter has become possible.
The reason DNA wrap method is so interesting for CNT manipulation is that it introduces tools of traditional biochemical science to the nascent nanomaterials science, and promotes new growth points in both fields. DNA wrapping allows not only separation but also controlled alignment of CNTs. Based on these findings, in 2006 the National Science Foundation in the U.S. awarded a $1.25 million, 4-year research grant to develop new methods of manipulating CNTs in solution. Much of the project’s focus will be on the use of single-walled CNTs wrapped with single-stranded DNA. The DNA-CNT hybrid has proven effective in CNT dispersion and researchers hope it will also aid in sorting and placing the tubes (see our Nanotechnology Spotlight: "DNA wrappers for carbon nanotubes ").
In a new review article in Nano Research ("A DNA-Based Approach to the Carbon Nanotube Sorting Problem" – open access article), Zheng presents his team's current understanding of the DNA-carbon nanotube hybrid structure and separation mechanisms, and addresses future developments of the DNA-based approach.
Zheng explains to Nanowerk that different degrees of CNT separation can be classified: "A single-walled CNT can be either metallic or semiconducting, depending solely on its chiral index (n, m). As a consequence of their different electronic structures, metallic and semiconducting tubes have distinct chemical reactivities, and different physical properties such as polarizability."
Exploiting all these differences in order to separate the two types of tube constitutes one degree of separation.
Within the same electronic type, tube diameter determines surface area per unit tube length, affecting such quantities as linear charge density and effective hydrodynamic size for dispersed tubes. Separation by diameter is therefore also conceptually feasible and constitutes another degree of separation.
"The most demanding task, in our opinion, is to separate two species of the same electronic type and same diameter but different chiralities" says Zheng. "To a first order of approximation, these tubes have little difference in their electronic structures. The challenge in this type of separation is how to convert the minute difference into something macroscopically measurable and the DNA-based approach appears to be especially suited for this purpose."
In his review paper, Zheng highlights a few approaches that are representative for the current state of the field. One is Mark Hersam's work at Northwestern University on a gradient ultracentrifugation technique (see also our Nanotechnology Spotlight: "Conductive and tunable transparent coatings made from monodisperse carbon nanotubes "). The review centers on the chromatographic separation of DNA-wrapped carbon nanotubes developed by Zheng at his lab at DuPont. It explains the mechanisms of separation based on IEX and describes the current understanding of this process.
"So far, we have shown the capability of the DNA-based approach in metal/semiconductor CNT separation, and single chirality CNT enrichment for certain small diameter tubes" says Zheng. "The latter demonstrates the exquisite resolution power of the DNA approach. In addition to our own work, others have also employed IEX and SEC (size-exclusion chromatography) methods for DNA-CNT separation. The separated DNA-CNTs have found use in both fundamental studies and applications. In comparison with other separation approaches, chromatographic separation of DNA-CNTs offers higher resolution and requires shorter processing times."
Understanding the mechanisms of the DNA-based separation approach is of both scientific and technological significance. At the heart of the problem is the structure of DNA-CNT. Zheng notes that experimental evidence strongly suggests that the hybrid structure is dependent on both DNA sequence and CNT structure."So far, the structural information comes primarily from low resolution AFM obtained from dried samples. Whether or not it is relevant to the solution state structure is an open question. High resolution TEM and solution-phase techniques such as circular dichroism may provide a more accurate measure of the structure."
Future efforts in the field will include finding better DNA sequences for CNT separation. Given the astronomical size of the single-stranded DNA library available, the researchers are optimistic that such sequences exist.
"What is needed is a rational search strategy, based either on predictions of molecular dynamics modeling, or on some ingenious experimental design, in order to discover these sequences" says Zheng. He points out that some results also indicate that the length of the sequence might play an important role, generating another
whole dimension of opportunity for DNA-CNT interactions.