Interactions of Small Molecules and Au Nanoparticles with Solubilized Single-Wall Carbon Nanotubes Jian Zhang, † Gangli Wang, † Young-Seok Shon, †,‡ Otto Zhou, § Richard Superfine, § and Royce W. Murray* ,† Kenan Laboratories of Chemistry and Department of Physics and Astronomy, UniVersity of North Carolina, Chapel Hill, North Carolina 27599 ReceiVed: October 24, 2002; In Final Form: February 12, 2003 Oxidatively end-cut single-wall carbon nanotube (SWNT) has been solubilized (sol-SWNT) in organic solvents by refluxing in aniline and purified by silica gel chromatography. NMR analysis of sol-SWNT indicates that the ratio of nanotube carbons to aniline sites is 360:1. A hydroxy-stilbene fluorophore (trans-4-nitro-4′- hydroxy stilbene, 1) adsorbs onto sol-SWNT, but can also be demonstrably ester-coupled to the nanotube material, based on comparisons of absorbance spectra. In the combinations of 1 and sol-SWNT made by ester-coupling and adsorption, the fluorescences of hydroxy-stilbene and of sol-SWNT could be excited selectively and separately. When exciting (500 nm) the sol-SWNT in the presence of adsorbed hydroxy- stilbene, the nanotube’s emission intensity decreases, but the energetics are unchanged, whereas ester-coupled hydroxy-stilbene diminishes both the sol-SWNT energy and intensity. When exciting either adsorbed (at 372 nm) or ester-coupled stilbene (at 347 nm), the stilbene emission intensities are substantially reduced, and a weak, photosensitized sol-SWNT emission could be seen. Energy transfer quenching of the stilbene by the nanotube structure produces some quantity of excited states that relax by emission. Mixed monolayer-protected Au clusters adsorb strongly onto both end-opened SWNT and sol-SWNT. Adsorption of MPCs with hydroxyl groupings in the mixed monolayer onto end-opened SWNTs occurred without change in shape of the nanotube bundles but could effect the rolling up of the more flexible sol-SWNTs. Amine-labeled MPCs were less aggressive adsorbers and the 15-20 nm nanotube bundles of sol-SWNT could be imaged, outlined with the adsorbed nanoparticles. Introduction Carbon nanotubes have attracted considerable research at- tention since their introduction in the 1990s. Single-wall carbon nanotubes (SWNTs) have interesting chemical and physical properties, 1-5 and for chemists they offer interesting challenges for functionalization with other chemical entities and combining into useful nanostructures. 6,7 A problem in functionalizing SWNT is the persistent van der Waals aggregation of individual nanotubes into nanotube bundles that are poorly soluble in organic solvents. 8 Dispersing bundles and increasing SWNT solubility in organic solvents is a significant issue, in which there has been progress by attaching solubilizing moieties to the ends of nanotubes, 7,9-11 and by reducing the nanotube lengths and oxidatively opening the nanotube ends (“end- opening”) with oxidants such as H 2 SO 4 /HNO 3 . 8 A recent contribution by Wilson et al. 12a was reaction and dispersal of SWNT bundles by refluxing in aniline. The product, an aniline- derivatized SWNT, is quite soluble in organic solvents. 12a Other solubilizing procedures have also been reported. 12b,c Here, we combine the oxidative 8 “end-opening” of SWNT and the anilination reaction of Wilson et al. 12a to prepare solubilized SWNT (abbreviated sol-SWNT) that can be chro- matographically purified of excess and weakly side-wall- adsorbed aniline. Several experiments have been conducted to advance the existing characterization of this material. The solubilized SWNTs are examined spectroscopically and the attachment of small molecules and metal nanoparticles to them is investigated. The small molecule probe is the fluorophore trans-4-nitro-4′-hydroxy stilbene (1). Stilbene derivatives are known to be quite sensitive to their molecular environment both in solution and in solid state. 13-15 The stilbene derivative 1 adsorbs onto the sol-SWNT. We are also able to spectroscopi- cally show that use of an ester-coupling reagent yields a different product in which 1 has been covalently linked to sol-SWNT. Interactions of metal nanoparticles with the surfaces of bulk nanotube 16 bundles and with C60 fullerene 17 have been de- scribed. Because nanoparticles, in particular the monolayer- protected cluster (MPC) variety, can be flexibly function- alized, 18-20 it would appear that attaching MPCs to solubilized SWNTs offers avenues to combining a wide range of chemical systems with the sol-SWNTs. Their strong adsorptive inter- actions overwhelm, however, observations of covalent nano- particle-sol-SWNT linking. We describe the adsorptive inter- actions of sol-SWNTs with MPCs bearing mixed monolayers of hexanethiolate (C6) and ω-hydroxyhexanethiolate (HOC6), ω-hydroxybutanethiolate (HOC4), or ω-aminehexanethiolate (NH 2 C6). The mixed monolayer MPCs were prepared from C6 MPCs using place exchange reactions, 18 and the adsorption was detected by transmission electron microscopy (TEM). Experimental Section Chemicals. All reagents (Aldrich), spectroscopic grade solvents (Fisher, Aldrich), and deuterated solvents (Isotech, † Kenan Laboratories of Chemistry. ‡ Present address: Department of Chemistry, Western Kentucky Uni- versity, 1 Big Red Way, Bowling Green, KY 42101. § Department of Physics and Astronomy. 3726 J. Phys. Chem. B 2003, 107, 3726-3732 10.1021/jp0273068 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/28/2003