Nanodiamond

Nanodiamonds, or diamond nanoparticles, are diamonds with a size below 100 nanometers. They can be produced by impact events such as an explosion or meteoritic impacts. Because of their inexpensive, large-scale synthesis, potential for surface functionalization, and high biocompatibility, nanodiamonds are widely investigated as a potential material in biological and electronic applications and quantum engineering.

History

In 1963, Soviet scientists at the All-Union Research Institute of Technical Physics noticed that nanodiamonds were created by nuclear explosions that used carbon-based trigger explosives.

Structure and composition There are three main aspects in the structure of diamond nanoparticles to be considered: the overall shape, the core, and the surface. Through multiple diffraction experiments, it has been determined that the overall shape of diamond nanoparticles is either spherical or elliptical. At the core of diamond nanoparticles lies a diamond cage, which is composed mainly of carbons. While the core closely resemble the structure of a diamond, the surface of diamond nanoparticles actually resemble the structure of graphite. A recent study shows that the surface consists mainly of carbons, with high amounts of phenols, pyrones, and sulfonic acid, as well as carboxylic acid groups, hydroxyl groups, and epoxide groups, though in lesser amounts. Occasionally, defects such as nitrogen-vacancy centers can be found in the structure of diamond nanoparticles. 15N NMR research confirms presence of such defects. A recent study shows that the frequency of nitrogen-vacancy centers decreases with the size of diamond nanoparticles.

Production methods

Detonation synthesis of non or weakly fluorescent nanodiamonds has become the industry standard in the commercial production of nanodiamonds: the most commonly utilized explosives being mixtures of trinitrotoluene and hexogen or octogen. Detonation is often performed in a sealed, oxygen-free, stainless steel chamber and yields a mixture of nanodiamonds averaging 5 nm and other graphitic compounds. In detonation synthesis, nanodiamonds form under pressures greater than 15 GPa and temperatures greater than 3000K in the absence of oxygen to prevent the oxidation of diamond nanoparticles. The rapid cooling of the system increases nanodiamond yields as diamond remains the most stable phase under such conditions. Detonation synthesis utilizes gas-based and liquid-based coolants such as argon and water, water-based foams, and ice. Because detonation synthesis results in a mix of nanodiamond particles and other graphitic carbon forms, extensive cleaning methods must be employed to rid the mixture of impurities. In general, gaseous ozone treatment or solution-phase nitric acid oxidation is utilized to remove sp2 carbons and metal impurities. Other than explosions, methods of production include hydrothermal synthesis, ion bombardment, laser heating, microwave plasma chemical vapor deposition techniques, ultrasound synthesis, and electrochemical synthesis. In addition, high-yield synthesis of fluorescent nanodiamonds can be achieved by grinding electron-irradiated cubic crystalline diamond obtained from nitrogen-containing or nitrogen-free carbon precursors. Another method is the decomposition of graphitic C3N4 under high pressure and high temperature which yields large quantities of high purity diamond nanoparticles. Nanodiamonds are also formed by dissociation of ethanol vapour. and via ultrafast laser filamentation in ethanol.

Potential applications

The N-V center defect consists of a nitrogen atom in place of a carbon atom next to a vacancy (empty space instead of an atom) within the diamond's lattice structure. Recent advances (up to 2019) in the field of nanodiamonds in quantum sensing applications using NVs have been summarized in the following review. Applying a microwave pulse to such a defect switches the direction of its electron spin. Applying a series of such pulses (Walsh decoupling sequences) causes them to act as filters. Varying the number of pulses in a series switched the spin direction a different number of times. They efficiently extract spectral coefficients while suppressing decoherence, thus improving sensitivity. Signal-processing techniques were used to reconstruct the entire magnetic field. The prototype used a 3 mm-diameter square diamond, but the technique can scale down to tens of nanometers.

Nano-abrasive Nanodiamonds share the hardness and chemical stability of visible-scale diamonds, making them candidates for applications such as polishes and engine oil additives for improved lubrication.

Medical Diamond nanoparticles have the potential to be used in myriad biological applications and due to their unique properties such as inertness and hardness, nanodiamonds may prove to be a better alternative to the traditional nanomaterials currently utilized to carry drugs, coat implantable materials, and synthesize biosensors and biomedical robots. The low cytotoxicity of diamond nanoparticles affirms their utilization as biologically compatible materials. In vitro studies exploring the dispersion of diamond nanoparticles in cells have revealed that most diamond nanoparticles exhibit fluorescence and are uniformly distributed. Fluorescent nanodiamond particles can be mass produced through irradiating diamond nanocrystallites with helium ions. Fluorescent nanodiamond is photostable, chemically inert, and has extended fluorescent lifetime, making it a great candidate for many biological applications. Studies have shown that small photoluminescent diamond nanoparticles that remain free in the cytosol are excellent contenders for the transport of biomolecules.

In-vitro diagnostics Nanodiamonds containing nitrogen-vacancy defects have been used as an ultrasensitive label for in vitro diagnostics, using a microwave field to modulate emission intensity and frequency-domain analysis to separate the signal from background autofluorescence. Combined with recombinase polymerase amplification, nanodiamonds enable single-copy detection of HIV-1 RNA on a low-cost lateral flow test format.

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