Treating single-walled carbon nanotubes (SWNTs) with sec-butyllithium and subsequently with carbon dioxide leads to the SWNTs functionalized with both alkyl and carboxyl groups (named as c-a-SWNTs for simplicity). The resultant c-a-SWNTs are largely with lengths between 100 and 200 nm, which can be individually dispersed in water at the concentration of 0.5 mg/mL and the solution is nearly transparent. Zeta potentiometric titration indicates that c-a-SWNTs have a feature similar to a zwitter-ionic polyelectrolyte. (C) 2004 Elsevier B.V. All rights reserved.