Surface functionalization of multi-walled carbon nanotubes (MWCNTs), with amino groups via chemical modification of carboxyl groups introduced on the nanotube surface, using O-(7-azabenzotriazol-1-yl)-N, N,N',N'-tetramethyluronium hexafluorophosphate (N-HATU) and N,N-diisopropylethylamine (DIEA) is reported. The N-HATU coupling agent provides faster reaction rate and the reaction occurs at lower temperature compared to amidation and acylation-amidation chemistry. The amines, 1,6-hexanediamine (HDA), diethylenetriamine (DETA), triethylenetetramine (TETA) and 1,4-phenylenediamine (PDA) were used. The resulting materials were characterized with different techniques such as FTIR, XRD, elemental analysis, TGA, TEM, UV-vis spectroscopy and cyclic voltammetry. MWCNTs functionalized with PDA posses the best dispersibility and electron transfer properties in comparison to the others amines. Functionalized MWCNTs, at the concentrations between 1 and 50 mu g ml(-1), were not cytotoxic for the fibroblast L929 cell line. However, the concentrations of MWCNTs higher of 10 mu g ml(-1) reduced cell growth and this effect correlated positively with the degree of their uptake by L929 cells. (C) 2009 Elsevier B.V. All rights reserved.