In 1963, in Chelyabinsk-70, one of the ‘closed cities’ of the former Soviet Union, now called Snezhinsk, three scientists – K.V. Volkov, V.V. Danilenko and V.I. Elin – were doing nuclear research, and found a substance that is now attracting worldwide scientific interest.1 They had discovered nanodiamonds. These tiny diamonds, between 5 and 10nm wide, had formed from the carbon present in the conventional explosives surrounding the nuclear material required to compress it to critical mass. They are still sometimes known as detonation nanodiamonds (DNDs).
Nanodiamonds have multi-faceted surfaces, allowing them to bond to a variety of different molecules; in addition, they often have a gap in their crystal lattice. In jewellery-grade diamonds, the presence of any flaw reduces its value, but in nanodiamonds, the isolated colour centre functions as a single, trapped atom. These two properties, combined with their small size, make nanodiamonds useful in a range of applications that extend from engineering, particularly lubrication and heat transfer, through computing and communications, to medical applications.
Lubrication and polishing
Nanodiamonds are being investigated as a potential additive to lubricants such as engine oil. In theory, their small size and hardness could cleanse an engine while it is operating. Qiao Yu-Lin and colleagues at the National Key Laboratory for Remanufacturing in Beijing point out that nanodiamond particles improve the antiwear and friction reducing properties of the engine oil at high temperature. In essence, they say, the nanodiamonds operate as sub-microscopic ball-bearings that prolong engine life and increasing fuel efficiency.2
However, other engineers are more cautious about this application. Boris Zhmud at Applied Nano Surfaces in Sweden and Bogdan Pasalskiy in the department of commodity science and technology at Kyiv National University of Trade and Economics, Ukraine, point out that nanodiamond-based lubricants can, after a while, lead to increased wear on piston rings and other delicate components of engines.3 They suggest that nanodiamond-doped oils should only be used during the running-in phase and then replaced with more conventional oils.
An extension of this application would be the use of nanodiamonds in polishing compounds. In particular, they could be used to polish semiconductors – including, for example, sapphire, SiC, GaAs, and GaN – where an especially smooth surface is required. The conventional process for doing this, known as advanced chemical mechanical planarisation (CMP), involves smoothing and planing surfaces with a combination of chemical and mechanical forces, a hybrid of chemical etching and abrasive polishing. Nanodiamonds could potentially be used to improve surface integrity by removing sub-surface damage, cracks or micro-cracks, resulting in surface finishes of less than 2nm. Similar finishes could be achieved on silicon carbide wafers for integrated circuit memory devices.
In short, nanodiamonds could be used in any application where superb finishes are required, such as in the polishing of optics that are used in telescopes, laser windows, acoustic wave guides and sapphire wafers. Next-generation diamond cutting tools are another application. With a more uniform and ordered microstructure, nanodiamonds could be used to increase the wear resistance of drills so cutting tools would be sharper and produce better finishes.
Computing and communications
Transistors and integrated circuits based on photons rather than electrons are central to the development of faster computers and telecommunications. Photons transfer data at much higher rates than electrons and generate much less heat in the process.
In 2009, a research group, led by Romana Schirhagi at ETH Zurich, showed that supercooled dye molecules could be used as the basis for an optical transistor - crudely, a valve for controlling the flow of data. Unfortunately, these optical transistors operate only at superconducting - ie extremely low - temperatures and so applications outside the lab have been impractical. Since optical signal processing is significantly faster than traditional transistors, there is demand for it to become available at room temperature.
This is where nanodiamonds might be useful. In 2013, a research team led by Michael Geiselmann, at the Institute of Photonic Sciences in Barcelona, Spain, found a way to make an optical transistor based on nanodiamonds that would work at room temperature.4 They used a nanodiamond where the ‘flaw’ was a missing nitrogen atom, ie a nitrogen vacancy centre. A green laser (532 nm) focused onto the single nitrogen vacancy switched the nanodiamond to an ‘on’ state, while illumination with an infrared laser switched it off. By alternating the lasers, they were able to switch the nanodiamond on and off at speeds far in excess of that possible with electron switching.
By stacking nanodiamonds together in clusters, the researchers speculate that it will be possible to create logic gates and, ultimately, full integrated circuits that run on light instead of electricity.
An exciting application for this technology, they suggest, is quantum computing. By harnessing the superior speed of optical processors, computers where the binary code is based on the superfast switching of nanodiamonds could mean that room temperature quantum computing is one step closer.
Despite major advances in recent years, chemotherapy of cancers is rarely fully effective. This is because cancer cells can quickly remove the drugs from the cell’s cytoplasm before they have time to act. Indeed, 90% of anticancer drug resistance is because of the cancer cells’ aggressive self-protection mechanisms. But nanodiamonds cannot be transported by the tumour’s cell transport protein systems. So doping drugs with nanodiamonds could keep them inside the cell.
In a proof-of-concept experiment, Dean Ho, professor of oral biology and medicine at the University of California, Los Angeles (UCLA), and colleagues combined nanodiamonds with the anti-cancer drug doxorubicin and tested the drug – ND-DOX – on rodents with liver tumours. As a control, they used doxorubicin on its own.5
Checking levels of the drug in the tumours two days later they found that doxorubicin levels were 10x higher in mice treated with ND-DOX, compared with those given doxorubicin alone, and the level remained high for seven days. The tumours of mice receiving ND-DOX also shrank more and the mice survived longer than those treated with doxorubicin.
Ho suggests that the surface chemistry of the nanodiamonds is the crucial factor. They have an octahedral shape and the various facet surfaces possess different properties, such as electrical charge. So a drug can be attached to one neutral surface, for example, while another facet retains an electrostatic charge, allowing the nanodiamond to disperse in fluids.
Ho’s group has also used the nanodiamond-doped doxorubicin to treat breast cancer. On its own, this drug has little effect on breast cancer but in combination with nanodiamonds, the drug showed an improved effect. An additional finding was that the nanodiamonds also reduced the toxicity of the drug by releasing it more slowly. Doses that would have killed mice if given as free drug did not even cause them to lose weight when the drug was carried on nanodiamonds.
Nanodiamonds also hold promise for the treatment of brain tumours, which are especially difficult to treat because chemotherapy drugs find it difficult to cross the blood–brain barrier. By combining chemotherapy with nanodiamonds, a new generation of drugs is being developed that can not only cross the blood–brain barrier but are retained within the tumour.6
Ho is also exploring the use of nanodiamonds in his own specialist field of oral surgery. During jaw and tooth repair operations, doctors normally use surgery to implant bone-growth-stimulating proteins in the target area. Such procedures tend to be expensive, time consuming and painful. Ho and colleagues found if nanodiamonds were doped with both bone morphogenetic protein and fibroblast growth factor, the nanodiamond vector stimulated bone and cartilage to rebuild themselves more effectively than with conventional drugs. The proteins could be delivered over a longer period of time via a simple injection or oral rinse. No invasive surgery was required.
Nanodiamonds are also proving useful in blood tests.7 Current methods for testing iron levels in the blood involve measuring the concentration of the proteins responsible for the storage and transport of iron such as ferritin, which can contain up to 4500 magnetic iron ions. These proteins are counted using immunological techniques, which estimate iron concentrations on the basis of the abundance of various iron-related biochemical markers. A team of researchers led by Tanja Weil, director the Institute for Organic Chemistry at the University of Ulm, Germany, has developed a method for measuring ferritin directly, allowing illnesses to be detected sooner and more accurately.
The flaws in nanodiamonds – their colour centres – allow their detection in an electron field in a similar way to electron spin resonance imaging. By absorbing ferritin onto the surface of the diamonds, the Ulm team was able to calibrate the difference between untreated nanodiamonds and ferritin-treated diamonds. From this modelling, the researchers can measure the concentration of ferritin directly, and therefore iron, in the blood.
Meanwhile, back at UCLA, Dean Ho has been expanding the repertoire of nanodiamonds in medical applications. In 2014, his research group announced that they have found a way of using nanodiamonds to manage glaucoma, a disease of the eye that afflicts a growing number of people worldwide. Glaucoma is usually treated with the drug Timolol via eye drops but its application is uncomfortable and can have side effects, such as irregular heartbeat, since the dosage is hard to manage. By binding Timolol to nanodiamond-impregnated contact lenses, this ‘burst-release’ can be avoided and a controlled dosage applied. ‘This discovery represents the pipeline of innovation that is coming from Ho’s team,’ said No-Hee Park, dean of the school of dentistry at UCLA. ‘Ho is a visionary in his field and his advances continue to generate significant excitement regarding the use of nanodiamonds in biology and medicine.’
Finally, Israeli company Ray Technologies, which makes nanodiamonds by proprietary technology, is also investigating the possibility of using nanodiamonds in cosmetics. Unlike monocrystalline diamond, nanodiamonds are hydrophilic owing to their high surface-to-volume ratio. Thus they have an unusually high adsorption rate, so adding them to skincare products could enable active ingredients to work at maximum potential. The nanodiamonds adsorb more of the active ingredient than the cream can carry on its own, and since nanodiamonds penetrate the deeper layers of the skin, the active ingredient is carried with it. In addition to being fully and rapidly absorbed by skin, the high water absorption of nanodiamonds mean that skin remains hydrated for longer.
Nanodiamonds are not just limited to skin cream. According to submissions made to the US patent office in recent years, other uses include everything from increasing the abrasiveness of toothpastes to DNA test strips.
In the museum at Chelyabinsk-70 lie some of the casings made for several hydrogen bombs – maybe nanodiamonds, with their legion of uses, may eventually go some way to wipe out such bleak origins.
1 V. V. Danileko, Phys. Solid State, 2004, 46(4), 595.
2 Q. Y-Lin et al, J. Central South Uni. Technol., 2005, 12(2), 181.
3 B. Zhmud and B. Pasalskiy, Lubricants, 2013, 1, 95.
4 M. Geiselmann et al, Nature Physics, 2013, 9, 785.
5 D. Ho et al, Sci. Trans. Med., 2011, 3(73), 73ra21.
6 D. Ho et al, Nanomedicine, 2014, 10(2), 381.
7 T. Well et al, Nano Letters, doi: 10.1021/nl4015233.
Richard Corfield is a science writer based in Oxford, UK