Use of a Chondroitin Sulfate Isomer as an Effective and Removable Dispersant of Single-Walled Carbon Nanotubes

Three isomers of chondroitin sulfate (CS), i.e., CS-A, CS-B, and CS-C, are investigated as nanotube dispersants and are found to have vastly different abilities to disperse single-walled carbon nanotubes (SWNTs) in water due to their different intramolecular interactions. Only CS-A and CS-C effectively disperse SWNTs into small bundles or individual tubes while CS-B disperses SWNTs poorly. Computer simulation and circular dichrosim show that neat CS-A and CS-C have weak intramolecular hydrogen bonding and extended conformations in solution resulting in energetically more favorable interactions with nanotubes. CS-B has relatively strong intramolecular Coulombic interaction and more alpha-helical secondary structure in solution resulting in energetically less favorable interaction with the nanotubes. Atomic force microscopy images show helical wrappings of CS-A and CS-C around the SWNTs. Transmission electron microscopy corroborates the helical wrapping of CS-A. Different isomeric forms of a polymer can have vastly different dispersing power because of their different intramolecular interactions and conformations. The easy removability of CS-A from nanotubes is confirmed with X-ray photoelectron spectroscopy showing almost no detectable sulphur content after washing with water and by application of washed CS-A dispersed SWNTs in field-effect transistors.