Nanodiamonds or Ultra Dispersed Diamonds (UDD) or Ultra-Nano-Crystalline Diamonds (UNCD) belong to the carbon nanomaterials, which, along with graphene, fullerene and nanotubes, have attracted great interest in recent years due to their unique physical and chemical properties. Nanodiamond is a carbon nano-crystallite with a cubic diamond structure and a size of less than 100 nm.
Two types of nanodiamonds produced in commercial scale are available today in the market: detonation nanodiamonds (DND) having an average size of 4-5 nm and nanodiamonds obtained by crashing of microdiamonds produced by high pressure high temperature (HPHT) having an average size of more than 30 nm.
Each ND particle has cubic diamond lattice when each carbon atom is bonded covalently with four other carbon atoms inside the crystalline (Fig. 1), while surface atoms, in contrast to internal ones, have less than four neighbor atoms; unpaired electrons provide bonding with diverse functional groups on crystalline surface or form non-diamond bonds with neighbor surface atoms.
In small (4-5 nm) ND crystallites, unlike big once, around 15-18 % of carbon atoms are located on the surface, have unpaired electrons responsible for a high and variable chemical activity of crystallites surface. Controlling the surface chemistry of nanodiamonds defines the possibility to provide their desired interaction with molecules of other materials and objects.