Diamond carbons : from macro to nano

The ability to generate nano-sized diamonds opens up exciting new prospects. Nano-sized diamond particles were first produced by detonation in the USSR in the 1960s, but remained essentially unknown to the rest of the world until the late 1980s. From the late 1990s onwards, a number of important breakthroughs led to increased interest in these nanoparticles, which are now known as nanodiamonds. Numerous routes for synthesizing nanodiamonds are now available. The most widely used are explosion technology, laser ablation, grinding of micrometric diamond powders produced by high-pressure, high-temperature static synthesis in hydraulic presses, plasma-assisted chemical vapor deposition, autoclave synthesis using supercritical fluids, chlorination of carbides, ion irradiation of graphite, electron irradiation of carbon onions and ultrasonic cavitation. Through some of these processes, colloidal suspensions of individual diamond particles - with diameters of 4-5 nm - have become available, enabling easy shaping of these nanomaterials. New, environmentally-friendly purification techniques have also been developed. They enable the low-cost production of large volumes of high-purity nanodiamond powders with controlled surface chemistry. Finally, nanodiamond has been shown to be less toxic than other carbon nanoparticles, and exhibits unique photoluminescence properties associated with crystallographic defects in the diamond-structured crystallographic lattice. As a result, nanodiamonds are currently being considered for medical applications.

Nanodiamonds have excellent mechanical and optical properties, high surface areas and are easily functionalized. We reviewed the synthesis, structure, properties, surface chemistry and phase transformations of nanodiamonds. In particular, we have discussed the rational control of the mechanical, chemical, electronic and optical properties of nanodiamonds through bulk or surface doping, or the introduction of functional groups. These nanodiamonds have a wide range of potential applications in tribology (ultimate polishing), quantum information processing, heterogeneous catalysis, electrocatalysis, magnetic sensors, active ingredient delivery, bio-imaging and protein separation and purification. In particular, recent advances in nanodiamond-based drug delivery in the orthopedic, dental and ophthalmic sectors underscore their therapeutic applicability. In the form of nanocomposites with polymers, these materials also offer interesting prospects in tissue engineering. This wide range of potential applications for nanodiamonds continues to stimulate research in this field. A better understanding of their structure and surface chemistry should lead to better control of their properties, and will also help to increase manufacturing volumes, possibly to levels that will surpass those of fullerenes and other carbon nanomaterials.