Electrically Addressable Biomolecular Functionalization of Carbon Nanotube and Carbon Nanofiber Electrodes Chang-Soo Lee, †,§ Sarah E. Baker, †,§ Matthew S. Marcus, ‡ Wensha Yang, † Mark A. Eriksson, ‡ and Robert J. Hamers* ,† Department of Chemistry, UniVersity of Wisconsin, 1101 UniVersity AVenue, Madison, Wisconsin 53706, and Department of Physics, UniVersity of Wisconsin, 1150 UniVersity AVenue, Madison, Wisconsin 53706 Received June 28, 2004 ABSTRACT We demonstrate the electrically addressable biomolecular functionalization of single-walled carbon nanotube electrodes and vertically aligned carbon nanofiber electrodes. The method uses an electrochemical reaction in which nitro groups on specific nanostructures are reduced to amino groups and then used to covalently link DNA to only these nanostructures. We demonstrate fabrication of a four-element array of distinct DNA oligonucleotides on carbon nanotube electrodes and the addressable functionalization of submicron bundles of <100 nm diameter vertically aligned carbon nanofibers. DNA hybridization shows that the DNA-modified nanoscale structures have excellent biological selectivity. One of the major goals of nanotechnology is the fabrication of high-density biosensor arrays composed of nanotubes, nanowires, or nanofibers, each modified with distinct bio- molecular recognition elements. 1 Carbon nanotubes and nanofibers are especially attractive candidates for nanoscale biosensing systems because of their exceptional chemical and biochemical stability. 2-4 While methods have been developed for growing single-walled carbon nanotubes (SWNTs) 5 and vertically aligned carbon nanofibers (VACNFs) 6 on surfaces, there has remained an unmet need for methods able to chemically modify closely spaced electrodes. Here, we demonstrate how a combination of chemical and electro- chemical reactions can be used to achieve electrically addressable DNA modification of carbon nanotube electrodes and of submicron bundles of VACNFs. Our results provide a general and direct pathway for fabricating high-density arrays of biosensor elements using nanoscale components such as nanotubes and nanofibers tethered to highly specific biomolecular recognition elements such as DNA, proteins, and antibodies. As schematically illustrated in Figure 1, the heart of this scheme is the functionalization of the SWNTs/VACNFs with nitro groups, followed by the “addressable” electrochemical reduction of the nitro groups to amino groups on SWNTs/ VACNFs of interest. The use of an electrochemical reaction to control the functionalization is key, as it provides a way to electrically address individual sensor elements within an array. Since the electron-transfer step in electrochemistry can typically only occur for species within 1 nanometer of an electrode surface, 7 we expect that this process has the potential for extension down to near-atomic length scales. The process we developed is discussed in detail in the Supporting Information. To fabricate an array of distinct biologically modified SWNTs, we started with an oxidized silicon wafer “chip” and used standard lithographic patterning * Corresponding author. E-mail: rjhamers@wisc.edu. ² Department of Chemistry. ‡ Department of Physics. § These authors contributed equally to this work and should both be considered as first authors. Figure 1. Schematic illustration of addressable biomolecular functionalization of carbon nanotubes. The procedure for carbon nanofibers is identical. NANO LETTERS 2004 Vol. 4, No. 9 1713-1716 10.1021/nl048995x CCC: $27.50 © 2004 American Chemical Society Published on Web 08/10/2004