*5.1. Characterization of Nanodiamond*

Diamond is an allotrope of carbon, consisting of carbon atoms arranged in a cubic crystal structure covalently bonded in sp3 hybridization (Figure 4a). Like all nanostructured materials, nanodiamonds or diamond nanoparticles are defined as features not exceeding 100 nm in at least one dimension, although some larger diamond particles, i.e., 125–210 nm, are still referred to as nanodiamonds (for a review, see [152]). At the same time, the size of ultrananocrystalline diamond particles is 3–5 nm [153,154]. Diamond nanoparticles can be prepared by various methods. The most widely used techniques are detonation of carbon-containing explosives in an oxygen-deficit environment and microwave-enhanced plasma chemical vapor deposition (MECVD). Other techniques include the radiofrequency plasma-assisted chemical vapor deposition (PACVD) method, milling of diamond microcrystals, hydrothermal synthesis, ion bombardment, laser bombardment, ultrasound synthesis and electrochemical synthesis (for a review, see [10,152–156]). Nanodiamonds are considered to be the most advanced carbon materials in the world. This is due to their excellent mechanical, optical, electrical, thermal and chemical properties. The mechanical properties of nanodiamonds include the highest hardness of all materials on earth, a high Young's modulus, high fracture toughness, high pressure resistance and a low friction coefficient. Their optical properties include transparency, high optical dispersion, and their ability to display various colors and to emit intrinsic luminescence (fluorescence), which is due to defects in the diamond lattice or contamination of the lattice with foreign atoms, such as N, B, H, Ni, Co, Cr or Si. Regarding their electrical properties, nanodiamonds can act as good insulators in their pristine state and as semiconductors after doping, usually with boron. Their thermal properties include superior thermal conductivity and low thermal expansion. The chemical properties of nanodiamonds include low chemical reactivity and resistance to liquid- and gas-phase oxidations. However, nanodiamonds can be doped with various atoms, and their surface can be functionalized by various atoms, chemical groups and (bio)molecules ([152,153,157,158]; for a review, see [10–15,155]).

**Figure 4.** Scheme of a nanodiamond (**a**) and of the preparation and structure of nanocellulose/nanodiamond composites (**b**).
