Nitro groups (-NO2) have been introduced on the surface of multi-walled carbon nanotubes (MWCNTs) by treatment with a mixture of concentrated H2SO4/HNO3 solution at low temperature (60 degrees C). Such a low-temperature treatment simultaneously can well prevent MWCNTs from the structural damage. From the nitro-modified MWCNTs, MWCNTs can be readily modified with amino groups by reduction of nitro groups. The prepared amino-modified MWCNTs are highly soluble in polar solvents such as dimethylformamide (DMF), alcohol and acetone. Further, as a demonstration, MWCNTs can be functionalized with guest objects, provided by the strong bonding ability of amino groups. (C) 2010 Elsevier B.V. All rights reserved.