Single-walled carbon nanotubes (SWNTs) are molecular wires that exhibit interesting structural, mechanical, electrical, and electromechanical properties. 1-3 A SWNT is unique among solidstate materials in that every atom is on the surface. Surface chemistry could therefore be critical to the physical properties of SWNTs and their applications. 3-10 SWNT sidewall functionalization is important to soluble nanotubes, 4-6 self-assembly on surfaces, and chemical sensors. 8-10 For these purposes, it is imperative to functionalize the sidewalls of SWNTs in noncovalent ways to preserve the sp2 nanotube structure and thus their electronic characteristics.

Immobilization of biomolecules on carbon nanotubes has been pursued in the past, motivated by the prospects of using nanotubes as new types of biosensor materials. 11-15 The electronic properties of nanotubes coupled with the specific recognition properties of the immobilized biosystems would indeed make for an ideal miniaturized sensor. A prerequisite for research in this area is the development of chemical methods to immobilize biological molecules onto carbon nanotubes in a reliable manner. Thus far, only limited work has been carried out with multiwalled carbon nanotubes (MWNTs). 11-15 Metallothionein proteins were trapped inside and placed onto the outer surfaces of open-ended MWNTs. 11-14 Streptavidin was found to adsorb on MWNTs presumably via hydrophobic interactions between the nanotubes and hydrophobic domains of the proteins. 15 DNA molecules adsorbed on MWNTs via nonspecific interactions were also observed. 12-14 In this communication, we report a simple and general approach to noncovalent functionalization of the sidewalls of single-walled carbon nanotubes, and subsequent immobilization of various biological molecules onto nanotubes with a high degree of control and specificity. The noncovalent functionalization involves a bifunctional molecule, 1-pyrenebutanoic acid, succinimidyl ester (1)(Molecular Probes, Inc., USA), irreversibly adsorbed onto the inherently hydrophobic surfaces of SWNTs in an organic solvent