Covalently-linked Adducts of Single-walled Nanotubes with Biomolecules: Synthesis, Hybridization, and Biologically-Direc
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Covalently-linked Adducts of Single-walled Nanotubes with Biomolecules: Synthesis, Hybridization, and Biologically-Directed Surface Assembly Sarah E. Baker, Tami L. Lasseter, Lloyd M. Smith, and Robert J. Hamers Department of Chemistry University of Wisconsin-Madison 1101 University Avenue Madison, WI 53706 ABSTRACT Covalently-linked adducts of single-walled carbon nanotubes (SWNTs) with biomolecules have been fabricated. Results are presented here for DNA-SWNT and for biotin-SWNT adducts. DNA-SWNT adducts are shown to be biochemically accessible and to exhibit high selectivity, favoring hybridization with complementary vs. noncomplementary DNA sequences. Biotin-SWNT adducts were also prepared and used to direct the assembly of nanotubes onto a biotinylated glass surface. INTRODUCTION Covalently-linked adducts of nanotubes with biomolecules have great potential in biosensing and as a possible means of implementing nanoscale assembly, using the selectivity of biomolecular interactions to control assembly of nanometer-sized objects [1,2] Previous studies have focused primarily on the use of non-covalent interaction.[36] However, the use of covalently-linked adducts has a number of potential advantages. For example, covalent attachment disrupts the CNTs structure only at the attachment site, whereas noncovalent functionalization typically involves coating the nanotube with various large molecules/polymers. Indeed, a recent report showed that oxidation of “defect-free” HipCO nanotubes (Carbon Nanotechnologies, Inc.) retained the van Hove features, thereby indicating that the electronic properties are relatively unpertubed by formation of oxidized surface sites. Additionally, covalently-linked adducts are expected to be more thermally and chemically stable. While DNA hybridization involves a large number of very weak interactions (hydrogen bonds), the interaction of biotin with avidin has one of the largest known binding constants of 1015 Molar-1. This large binding constant makes the biotin-avidin interaction potentially useful for the fabrication of robust nanoscale structures. Here, we report recent results aimed at preparing and characterizing covalentlylinked adducts of single-walled carbon nanotubes with biomolecules.[7,8] We show that covalently-linked adducts of DNA with single-walled nanotubes retain the high accessibility and selectivity associated with DNA hybridization.[7] We also extend these methods to prepare biotin-modified nanotubes,[8] and we show that the biotin-avidin interaction can also be used to as a means of assembling nanotubes onto selected locations on a surface. EXPERIMENTAL
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Experiments were performed using two different sources of single-walled carbon nanotubes. Single-wall nanotubes (Carbolex, Lexington, Ky) were first purified by refluxing the as-received nanotubes in 3 M nitric acid for 24 hours and then washing the SWNTs with water using a 0.6 micron polycarbonate membrane filter (Millipore). HipCO Tubes were also prepared by oxidation in 9:1 H2SO4:30% H202 solution.[9]
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