Nanodiamond

A nanodiamond is a carbon nanoparticle with truncated octahedral architecture—differing from ordinary diamonds  which feature a standard tetrahedral network structure—and they are significantly smaller, measuring about two to eight nanometers (nm) in diameter. Nanoparticles have many applications in fields such as transportation, energy, food safety, information technology, and medicine. Nanodiamonds have qualities that make them good candidates for treating infectious diseases and for delivering drugs.

Some commercial applications of nanodiamonds include use in lubricants, polishing materials, and polymer coatings, which benefit from ultraviolet resistance. Nanodiamonds are also used in antibacterial and antifungal coatings and as oil and fuel additives. While nanodiamonds have the hardness of diamonds, they also have flexible bonds. This allows the nanodiamonds to both stretch and deform—the underlying subsurface layer acts as a microscopic shock absorber while the core and outer layer remain rigid, allowing the particle to shift under pressure without breaking.

Background

Nanotechnology is engineering, science, and technology that takes place at the nanoscale. One nanometer is one billionth of a meter (one inch is about 25.4 million nanometers). The nanoscale is between 1 and about 100 nanometers.

Humans have been manipulating nanoparticles—often unknowingly—for centuries. Craftspeople frequently used high heat to produce glassware, metalwork, and pottery with unique qualities that emerged because of nanoparticles. For example, nanoparticles of gold and other metal oxides in stained glass produced from the sixth through the fifteenth centuries give cathedral windows brilliant colors, while carbon nanotubes produced by heating and hammering the metal provided strength and sharp edges to Damascus saber blades made from the thirteenth through the eighteenth centuries.

The roots of modern nanotechnology reach to 1959 on the campus of the California Institute of Technology (CalTech). Physicist Richard Feynman gave a talk at the American Physical Society meeting: “There’s Plenty of Room at the Bottom.” He theorized that in the future, scientists would be able to control individual atoms and molecules. Professor Norio Taniguchi coined the term nanotechnology in 1974 in his paper “On the Basic Concept of ‘Nanotechnology,’” which he presented at the International Conference on Production Engineering.

In 1996, Mike Roco formed an interagency coalition of federal staff that would lay the groundwork for the National Nanotechnology Initiative program, in the United States. It comprised representatives from multiple federal agencies who integrated input from academics, industrialists, and scientists from laboratories around the country tasked with developing a national strategy for nanotechnology. He pitched his ideas to President Bill Clinton in 1999 and received federal funding.

Nanotechnology has been studied for its potential benefits, but researchers have acknowledged a number of concerns that needed to be addressed. These include fears that self-replicating nanomachines could get out of control and the potential dangers of accidental ingestion or inhalation of nanoparticles.

Nanodiamond synthesis, or production, was first discovered in 1963 in the Soviet Union. Different teams of researchers inside the Soviet Union discovered nanodiamonds over the course of several decades, all while studying diamond synthesis by shock compression of nondiamond carbon modifications in blast chambers.

Overview

Nanodiamonds can be synthesized in a number of ways. These include ion and laser bombarding, ultrasonic, electrochemical, and detonation methods. The method used is chosen based on the form of nanodiamond needed. Just like diamonds on a larger scale, nanodiamonds are extremely hard and chemically stable, and at high magnification are radiant as well. While bulk diamond excels here, nanodiamonds aggregates have lower conductivity and scattering transparency.

The first method of synthesizing nanodiamonds, the detonation method, helped lead to their discovery. The detonation method produces nanodiamonds with commercially desirable diameters of from 4 to 6 nm. The method involves detonating explosives with a negative oxygen balance in an environment with inert cooling media. This prevents carbon oxidation from occurring. The explosion lasts for only a fraction of a second. This is not long enough to produce large nanodiamonds, but the nanodiamonds produced will collide and tightly aggregate to one another during synthesis. These detonation nanodiamonds form tight aggregates of primary particles that may measure from 200 to 300 nm. Separating the primary particles within the detonation nanodiamonds is difficult, and a successful method was not found until 2005. With that discovery, the availability of nanodiamond particles smaller than 10 nm opened up a range of applications.

Nanodiamonds have many properties that make them well-suited to a variety of uses. They are nontoxic, so they are good candidates for biomedical applications. They have high surface areas as well. Some uses include bioimaging, tissue engineering, and drug delivery. Some nanoparticles are not appropriate for drug delivery due to potential toxicity, although the reason for this is not yet well understood. Certain carbon-based nanoparticles possess inherent toxicity, but nanodiamonds are exceptionally non-toxic and therefore better suited to use for drug delivery. Nanodiamonds smaller than 10 nm, so-called single digit particles, have been found to be able to cross the blood-brain barrier, which makes them ideal for medical use. For example, a number of chemotherapy drugs have been tried against brain cancer, but have been unable to cross the blood-brain barrier. Drugs such as loperamide and doxorubicin have been bound successfully to nanodiamonds—while other substances like loperamide have been carried by polymer nanoparticles—enabling them to cross the intact barrier to reach brain tumors in sufficient quantities to have a beneficial effect. A further benefit of using nanodiamonds is that they possess high biocompatibility that avoids triggering adverse inflammatory clearance responses, allowing them to be engineered with surface coatings to function for longer than other types of nanoparticles.

The nanodiamonds’ small size and nearly spherical shape make them useful in lubrication applications, called nanolubricants. Detonation nanodiamonds are chemically and mechanically stable. Their surface chemistry is electronegative and has catalytic properties, which means they can increase the rate of chemical reactions. Their crystallographic lattice structure contains strong bonds between carbon atoms and the highest known atomic density. Because they have high chemical stability, they can withstand harsh conditions, making them useful in applications such as diamond films applied using chemical vapor deposition processes. This process involves high-temperature-heating of substrates, the surfaces on which the film is to be applied. Then they are exposed to gaseous precursor materials, such as a methane and hydrogen mixture, which grow carbon structures upon the solid nanodiamond seeds. The gaseous plasma disassociates, causing carbon atoms to chemically deposit and crystallize onto the seeded substrate to form a continuous diamond film.

Because nanodiamonds have the potential to pass through the blood-brain barrier, they can be used for various medical treatments. This includes the neurological imaging and the treatment of glioblastomas.

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