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Posted: Jun 04, 2007
DNA wrappers for carbon nanotubes
(Nanowerk Spotlight) To achieve the full benefits of the amazing properties of carbon nanotubes (CNTs) researchers are exploring all kinds of CNT composite materials. Material engineers are interested because this will lead to lighter,stronger and tougher materials. Another fascinating area involves CNT/polymer composite structures that will lead to a vast range of improved and novel applications, from antistatic and EMI shielding to more efficient fuel and solar cells, to nanoelectronic devices. One particular area of CNT/polymer composites is dealing with DNA-CNTs hybrids. Although researchers expect a plethora of new applications, the fact that even the formation mechanism of these complexes is not yet clear shows how early in the game this research still is. This might be due to the fact that in spite of the quite large number of experimental investigations on the interaction between DNA and CNTs, the number of theoretical studies is limited. Researchers in Germany now present, for the first time, the results of a systematic quantum mechanical modeling of the stability and the electronic properties of complexes based on single-walled carbon nanotubes, which are helically wrapped by DNA molecules.
One example of a current DNA-CNT research project involves the development of new methods of purification and separation of CNTs. Several years ago, a research team found that DNA strands could be used to separate CNTs according to their electronic characteristics. The discovery was reported in various articles (e.g. "DNA-assisted dispersion and separation of carbon nanotubes") and cited later by Forbes magazine as one of the top five nanotechnology breakthroughs of 2003. The reason DNA wrap methods are so promising for CNT production and development is that, in contrast to most other methods of purification and separation, they do not destroy the initially produced structure of the tube walls.
Based on these findings, the National Science Foundation in the U.S. last year 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.
"Beside CNTs wrapped by DNAs, other kinds of CNT–DNA hybrids also exhibit a big potential for applications" Dr. Gotthard Seifert explains to Nanowerk. "Other noncovalent complexes of CNTs and DNA consist of hydrophobic and hydrophilic parts and can be useful in stabilizing emulsions and in producing water-based liquid crystals. DNA-decorated carbon nanotubes may be used in analytical chemistry and biochemistry in sensor applications. DNA-wrapped CNTs will also play an important role in the development of miniaturized field-effect transistors (FETs) where DNA molecules can be templates to provide a precise localization of a semiconducting CNT in the production of a FET. Last but not least, covalently linked adducts of the DNA and carbon nanotubes are able to self-assemble via DNA-modified gold nanoparticles into structures such as CNT-based nanocircuits."
This list of potential applications makes clear that there is a number of different ways of interaction between DNA and CNTs that can be very useful for integration of the CNTs as building blocks of nano-and microelectronic devices.
Seifert, a professor in the department of Physical Chemistry at the Technical University of Dresden/Germany, together with collaborators from the Institute of Solid State Chemistry in Ekaterinburg/Russia and the Forschungszentrum Rossendorf in Dresden/Germany, published a paper in which the scientists explore the stability and electronic properties of complexes of single-walled carbon nanotubes wrapped by homopolymeric single-stranded DNA molecules by using a quantum mechanical density-functional tight-binding method ("DNA-wrapped carbon nanotubes").
The structural models of the (5, 5) CNT composed of 48 unit cells (a), distorted chain of the single-stranded polyG-DNA composed of 36 nucleotides (b) and their complex (5, 5) CNT@polyG-DNA (c). (Reprinted with permission from IOP Publishing Ltd.)
"We constructed atomic models of DNA-wrapped single-walled CNTs and then developed a phenomenological model of their stability" says Seifert. "This model gives the interaction energy as a function of the nanotube radii and the number of chains of homopolymeric DNAs wrapped around a CNT. As a result of the quantum mechanical calculations we have found a charge transfer between the DNA bases and the CNT, which is sensitively dependent on the type of the DNA base and the chirality of the CNT."
This work shows that even in the case of weakly bonded DNA one could find an essential change in the electronic and conductive properties of the CNTs
within DNA-wrapped CNT. Seifert notes that a more detailed study of this phenomenon is already in progress and will be discussed in a forthcoming paper.
"We believe that the investigations of the CNTs wrapped by DNA and other CNT-DNA complexes are of big interest not only from a fundamental point of view, but
also have great perspectives for various practical applications" says Seifert. "Furthermore, the DNA-wrapped CNT complexes are, probably, only one example of the great variety of CNTs wrapped by other types of polymers – polysugars, polyimides, polyhydrocarbons etc. Research on these complexes in general can also considerably extend the areas of CNT technology and applications.