Nanodiamonds (NDs)

　Diamond, an allotrope of carbon, has excellent hardness, coefficient of friction, thermal conductivity, insulation characteristics, and refractive index. Large and highly pure diamond is good for use as jewelry. Furthermore, the major industrial application of diamond is for cutting and polishing tools, because it is the hardest of natural products. However, diamond is not workable enough because of its hardness so there is a limitation for industrial use of a large diamond. Nanodiamond (ND) is a nanoparticle having the crystal structure of diamond, and it has excellent properties of normal diamond. ND is artificially synthesized and is useful for polishing tools and additives of engine oil.

　We can modify the surface of ND by introducing carboxyl and amino groups. These groups are further converted by a chemical modification to functionalize the ND particle.^1-3) Non-modified ND can be dispersed in water, but coheres in organic solvents. On the other hand, a functionalized ND particle with alkyl groups can be dispersed in organic solvents. We can modify a glass surface with ND that is functionalized with a silane coupling reagent.⁴⁾

　An application of ND is extended for biology and medical use^5,6) because it is a harmless nanoparticle in-vivo. Since a diamond with a complex defect (NV) containing nitrogen (N) and vacancy (V) shows fluorescence,⁷⁾ we can monitor movements and structural changes of a biological molecule on a microscope using the NV diamond as a fluorescent labeling reagent.^8-10) We can easily detect fluorescent behavior in a complex atmosphere in-vivo, because ND is chemically stable. A publication described that functionalization of ND also showed fluorescent behavior without NV defects.¹¹⁾ Protein- and biotin-supported ND were reported in order to enhance affinity toward a biological molecule.^12,13) These modified NDs are expected to be used for a drug delivery system.