Covalent and non-Covalent Functionalization of Single Walled Carbon Nanotubes (SWNTs) Paul S. Engel, Vittal B. Gudimetla, David W. Abmayr, Runtang Wang, W.E. Billups, Ravi Singh, and Suzy Torti

As discovered by W. E. Billups and coworkers, SWNT radical anions can react with a variety of organic compounds to functionalize the SWNT sidewalls. We find that sulfides react as well and can be used to introduce thiol groups onto SWNTs. Thiol groups are potentially useful for covalent bonding to polymers, and attaching fluorescent labels and biologically active compounds under mild conditions.

The evidence for SWNT functionalization is a large increase in the Raman D:G ratio, a sizeable weight loss on TGA and the appearance of “worms” of 1.5 to 4.3 nm height in AFM.

In contrast to forming chemical bonds to SWNTs, noncovalent functionalization has been used to render SWNTs soluble in water and has the advantage that the fluorescence properties of SWNTs are preserved. However, SWNTs can easily become detached from surfactants like SDBS and poloxomers like pluronic. A possible solution is to encapsulate SWNTs in a water-soluble polymer from which escape is difficult. This might be achieved by first complexing SWNTs with an appropriate polymer and then crosslinking the backbone.

The crosslinking concept is illustrated here. A triblock copolymer is selected consisting of an inner hydrophobic core (gray), a center crosslinkable block (red) and an outer, water-soluble segment (blue).

The very hydrophobic SWNT is surrounded by a polystyrene (PS) inner block. The outer block is a hydrophilic polyethylene glycol (PEG or PEO). The central block is polyacrylic acid (PAA), which is crosslinked by addition of a carbodiimide (EDC) and a water soluble diamine. The resulting species was termed a “PEG-egg” by Richard Smalley. PEO(17)-PAA(35)-PS(14) suspending SWNT in water EDC Diamine crosslinker

The black suspension of PEG-eggs (right hand vial above) fluoresces as intensely as SWNTs suspended in SDBS and remains unchanged for years. PEG-eggs can be isolated, lyophilized, and readily re-suspended in water. The aqueous suspension is stable to bovine serum albumin and shows no acute toxicity at 33 ug/mL towards macrophage and kidney cells nm NIR irradiation of aqueous PEG- eggs produces a large temperature rise potentially useful for killing cancer cells.

concentration (ug/mL) Temperature increase ( o C) Temperature of aqueous PEG-eggs after 30 s exposure to a 3W/cm 2 YAG laser. Starting temp. was 22 o C. A higher concentration produces a greater temperature rise but multi-walled carbon nanotubes (MWNTs) are very likely to give an even greater response if MWNT PEG-eggs can be made. Control experiments are required to verify that the observed heating is not due merely to the blackness of the suspension. Eggs only SWNT PEG-egg, 1 mg/mL