Assembling carbon nanotubes The Tube or the Helix? This is the Question: Towards the Fully Controlled DNA-Directed Assembly of Carbon Nanotubes** Giampaolo Zuccheri,* Marco Brucale, and Bruno Samorì Keywords: · carbon nanotubes · DNA · nanoparticles · nanostructures · nanotechnology The remarkable characteristics of carbon nanotubes (CNTs) have elicited an escalating amount of research in recent years, gradually but firmly establishing CNTs in the scientific community as a keystone, potentially capable of bringing novel and diverse technologies into existence, with nanoelectronics forging ahead of its many competitors. [1] Some applications are ready for the market, [2] and the cur- rent research is both focused and vibrant. Analogously, DNA and other nucleic acids are at the center of extremely vivacious areas of research for synthetic chemists, biophysi- cists, and nanotechnologists. The reasons for the excitement caused by DNA in the nanotech field is at least partly due to the high intrinsic informational content of DNA mole- cules: Information that drives the highly specific and struc- turally predictable self-assembly of just two molecules in a complex mixture, thus allowing the facile realization of arbi- trarily complex nanostructures. [3] Other unique features of DNA make it a desirable building block for nanotechnolo- gy; it is an extremely stiff polymer that can be tailored with single-atom accuracy thanks to the exploitation of tools (en- zymes) that nature has evolved through the ages. Very re- cently, the two apparently disparate research fields of CNTs and nucleic acids have crossed paths as several research groups have joined DNA (or its analogues) and CNTs cova- lently, [4] in an effort to drive their self-assembly into desired structures or, simply, to confer upon them novel properties (such as an increased water solubility [5] or the ability to act as a sensor for the presence of DNA molecules, as required for diagnostics [6] ). Herein, we will highlight some of the ef- forts in using DNA for driving the self-assembly of CNTs. CNTs can be chemically derivatized in many ways, and a number of these methods are viable for their functionaliza- tion with biomolecules such as DNA oligodeoxynucleotides (ODNs). [7] Some of the methods lead to a chemical modifi- cation mainly located at the tips of the CNT, while others bring the functional group and the ensuing biomolecule to the sidewalls. The preference for this type of localization is one of the hottest issues in CNT chemistry, and its precise control is a prerequisite for the production of univocally de- fined CNT–DNA superstructures. Taft et al. have recently shown their approach towards selective apical and side func- tionalizations of CNTs. [8] They obtain apical functionaliza- tion by amide coupling of an amine-modified ODN with carboxylic groups created by nitric-acid etching, and side- wall functionalization by the adsorption on the CNT of a hy- drophobic pyrene moiety linked to a different ODN. While the chemistry they employ might not be original, [7] they report that performing both apical and side functionaliza- tions simultaneously enhances the selectivity of both, to the point of almost completely eliminating cross-reactivity. The ODNs anchored to the tips and sidewalls of the CNTs have been demonstrated to be able to hybridize with their com- plementary ODNs either via the attachment of ODN-func- tionalized gold nanoparticles, which are easy to detect, [8] or via the attachment of the CNT onto ODN-functionalized electrodes. [9] Using the same chemistry, Li et al. have recently shown that it is possible to use ODN-functionalized gold nanopar- ticles as cornerstones of a CNT assembly: Several ODN- functionalized CNTs can bind to a central gold nanoparticle opportunely functionalized with thiol-modified ODNs com- plementary to that residing on the CNTs. [10] While the ge- ometry and the stoichiometry of the adduct is not under control, the reported strategy is a simple way towards the directed assembly of multi-nanotube, multimaterial con- [*] Dr. G. Zuccheri, Dr. M. Brucale, Prof. B. Samorì Department of Biochemistry “G. Moruzzi” National Institute for the Physics of the Matter (INFM), and The National Consortium of Materials Science and Technology (INSTM) University of Bologna Via Irnerio, 48—Bologna 40126 (Italy) Fax: (+ 39) 051-209-4387 E-mail: giampaolo.zuccheri@unibo.it [**] The authors wish to acknowledge support to their experimental work by FISR D.M. 16/10/20-1999, ESF Eurocores SONS “BION- ICS,” FP6-STRP “NUCAN.” 590  2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim DOI: 10.1002/smll.200500088 small 2005, 1, No. 6, 590 –592 highlights