Published: October 19, 2011 r2011 American Chemical Society 14295 dx.doi.org/10.1021/jp205387y | J. Phys. Chem. B 2011, 115, 14295–14300 ARTICLE pubs.acs.org/JPCB Unusually High Dispersion of Nitrogen-Doped Carbon Nanotubes in DNA Solution Jin Hee Kim, † Masakazu Kataoka, † Kazunori Fujisawa, † Tomohiro Tojo, † Hiroyuki Muramatsu, ‡ Sofía M. Vega-Díaz, § F. Trist  an-L opez, § Takuya Hayashi, † Yoong Ahm Kim,* ,† Morinobu Endo, †,‡,§ Mauricio Terrones, §,|| and Mildred S. Dresselhaus* ,^ † Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi 380-853, Japan ‡ Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan § Research Center for Exotic Nanocarbons (JST), Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan ) Department of Physics, Department of Materials Science & Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802-6300, United States ^ Department of Electrical Engineering and Computer Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, United States b S Supporting Information 1. INTRODUCTION Carbon nanotubes (CNTs) can be viewed as graphene that has been rolled into a cylinder with a nanoscale diameter. 1 The excellent mechanical and electrical properties of CNTs and their flexibility make them potentially useful in a wide variety of applications. 2,3 At present, several hundred tons of CNTs are commercially produced each year. However, most of these CNTs are in a strongly bundled state due to the strong van der Waals interactions established between adjacent CNTs. For this reason, improving the processability of CNTs via dispersion (i.e., indivi- dualization) is of great importance in order to fully exploit their intrinsic physical and chemical properties at the molecular level. The uniform dispersion of individual CNTs without a marked change in either their length or crystallinity remains a critical challenge in CNT processing. In fact, the conversion rate from noncovalently bundled CNTs to individual CNTs, when using strong sonication and subsequent ultracentrifugation, is quite low. Among the many types of surfactants, DNA, a flexible amphi- philic biopolymer, has been found to be well-suited for dispersing and sorting CNTs. 4À7 Theoretical and experimental studies have found that the van der Waals interactions between the nucleo- base backbone of DNA and the hydrophobic CNT sidewall are the main driving force behind the dispersion of CNTs in DNA solution, where DNA is helically wrapped around the CNT sidewall. 8À17 Possible applications of such a hybrid DNA/CNT system have been examined, including biosensors, 18,19 DNA transporters, 20 and field-effect transistors; 21 in these applications, the reproducibility of the hybrid DNA/CNT structure is essen- tial. From the viewpoint of tube crystallinity, intrinsic structural defects in CNTs have been reported to provide an additional driving force behind the dispersion of CNTs in DNA solution. 22 In other words, the surface properties of the CNT sidewall govern the DNA/CNT interaction. Functional groups covalently attached to the sidewall of CNTs are sometimes the byproduct of Special Issue: H. Eugene Stanley Festschrift Received: June 8, 2011 Revised: September 7, 2011 ABSTRACT: The dispersibility in a DNA solution of bundled multiwalled carbon nanotubes (MWCNTs), having different chemical functional groups on the CNT sidewall, was investi- gated by optical spectroscopy. We observed that the dispersi- bility of nitrogen (N)-doped MWCNTs was significantly higher than that of pure MWCNTs and MWCNTs synthesized in the presence of ethanol. This result is supported by the larger amount of adsorbed DNA on N-doped MWCNTs, as well as by the higher binding energy established between nucleobases and the N-doped CNTs. Pure MWCNTs are dispersed in DNA solution via van der Waals and hydrophobic interactions; in contrast, the nitrogenated sites within N-doped MWCNTs provided additional sites for interactions that are important to disperse nanotubes in DNA solutions.