In a similar approach to the noncovalent modification of carbon nanotubes with detergent molecules, Kam et al. (2005) used phospholipid derivatives to coat SWNTs for photo- therapeutic agents against tumor cells in vivo. A phospholipid containing a PEG-NH2 group was used to couple folic acid as an affinity ligand (using EDC to form an amide bond), which preferentially can be taken up by cancer cells. SWNTs were modified by the lipid derivative in aqueous solution using sonication for 1 hour and centrifuged to remove insoluble material. The aqueous fraction contained modified nanotubes, which contained the surface adsorbed lipid derivative. After the modified SWNTs were incubated with cells, the solution was irradiated using an 808 nm laser. The nanotubes absorb light in this region of the spectrum and heat up to the point of causing cell death.