Surface modifications of undoped nanodiamond (ND) particles were carried out through different annealing treatments. The methods of Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy were used to characterize the ND surface before and after the annealing process. The electrochemical properties of the modified ND powders in aqueous solution were investigated with Fe(CN)(6)(3-/4-) as a redox probe. When the annealing temperature was below 850 degrees C, vacuum annealing removed parts of the oxygen-containing surface functionalities from the ND surface and produced more sp(2) carbon atoms in the shell. The charge transfer of the Fe(CN)(6)(3-/4-) redox couple decreased with increasing annealing temperature. Re-annealing in air restored the original surface conditions: few sp(2)-bonded carbon atoms and similar surface functionalities, and thus the electrochemical activity. When ND was annealed in vacuum at 900-1100 degrees C, more serious graphitization produced a continuous fullerenic shell wrapped around a diamond core, which had a high conductivity and electrochemical activity. This provides a novel nanoparticle with high conductivity and high stability for electrochemical applications. (C) 2012 Elsevier Ltd. All rights reserved.