Nanodiamonds, or diamond nanoparticles, are diamonds that are smaller than 100 nanometers. They can be created by events such as explosions or meteorite crashes. Because they are inexpensive to make in large amounts, can have their surfaces changed, and are safe for use in living things, nanodiamonds are being studied a lot for use in biological, electronic, and quantum applications.
History
In 1963, scientists from the Soviet Union working at the All-Union Research Institute of Technical Physics discovered that nuclear explosions using carbon-based trigger explosives can create nanodiamonds.
Structure and composition
Diamond nanoparticles have three main features: their overall shape, their core, and their surface. Scientific experiments show that these particles are either round or oval in shape. At the center of each nanoparticle is a diamond cage made mostly of carbon. This core looks similar to the structure of a diamond. However, the surface of the nanoparticles is more like the structure of graphite. Recent studies show that the surface contains mostly carbon, along with smaller amounts of chemicals such as phenols, pyrones, sulfonic acid, carboxylic acid, hydroxyl groups, and epoxide groups. Sometimes, small imperfections called nitrogen-vacancy centers can be found in the structure. NMR studies confirm that these imperfections are less common in larger diamond nanoparticles.
Production methods
Detonation synthesis is the most common method for making non or weakly fluorescent nanodiamonds in industry. This process often uses mixtures of trinitrotoluene and hexogen or octogen as explosives. Detonation happens in a sealed, oxygen-free stainless steel chamber and produces a mix of nanodiamonds, which average about 5 nm in size, along with other graphitic compounds. During detonation, nanodiamonds form under pressures greater than 15 GPa and temperatures greater than 3000K in the absence of oxygen, which prevents diamond nanoparticles from being oxidized. Rapid cooling of the system increases nanodiamond yields because diamond is the most stable material under these conditions. Coolants like argon gas, water, water-based foams, and ice are used during the process. Because detonation synthesis creates a mix of nanodiamonds and other graphitic carbon forms, cleaning steps are needed to remove impurities. These steps often include gaseous ozone treatment or solution-phase nitric acid oxidation to eliminate sp2 carbons and metal impurities.
Other production methods include hydrothermal synthesis, ion bombardment, laser heating, microwave plasma chemical vapor deposition, ultrasound synthesis, and electrochemical synthesis. Fluorescent nanodiamonds can also be made by grinding electron-irradiated cubic crystalline diamond from nitrogen-containing or nitrogen-free carbon sources. Another method involves breaking down graphitic C3N4 under high pressure and high temperature, which produces large amounts of high-purity diamond nanoparticles. Nanodiamonds are also formed by breaking down ethanol vapor or using ultrafast laser filamentation in ethanol.
Potential applications
The N-V center defect is formed when a nitrogen atom replaces a carbon atom in a diamond's structure, and there is an empty space (a vacancy) next to it. Recent research (up to 2019) has reviewed how nanodiamonds with N-V defects are used in quantum sensing applications.
When a microwave pulse is applied to an N-V defect, it changes the direction of the electron spin. Using a series of these pulses (called Walsh decoupling sequences) helps filter the signal. Changing the number of pulses in a sequence alters how many times the spin direction changes. These methods help extract information from the signal while reducing interference, which improves the accuracy of measurements. Scientists used signal-processing tools to map the full magnetic field.
A prototype used a diamond square with a 3 mm diameter, but the method can be adapted to much smaller sizes, such as tens of nanometers.
Nanodiamonds have the same hardness and chemical stability as regular diamonds, making them useful for tasks like polishing and improving engine oil performance.
Diamond nanoparticles have unique properties, such as being non-reactive and very hard, which make them suitable for biological uses. They could replace traditional materials for drug delivery, coating implants, and creating biosensors or medical robots. These particles are safe for use in the body because they are not harmful to cells.
Studies show that diamond nanoparticles can be evenly spread inside cells and often glow under light. Fluorescent nanodiamonds can be made by exposing diamond particles to helium ions. These particles remain bright under light, do not react chemically, and last a long time, making them useful in biology. Research suggests that small, glowing diamond nanoparticles can help transport important molecules in the body.
Nanodiamonds with N-V defects have been used to detect tiny amounts of HIV-1 RNA in lab tests. By using microwaves to control light emission and analyzing the signal, scientists can distinguish the target from background noise. When combined with a method called recombinase polymerase amplification, this approach allows for detecting a single copy of HIV-1 RNA using a simple, low-cost test.
Diamond nanoparticles about 5 nm in size have a large surface area and can be modified to suit different needs. They have special optical, mechanical, and thermal properties and are not toxic. While their use in drug delivery has been tested, scientists still need to learn more about how drugs attach to them and how this process is affected by factors like surface chemistry, temperature, and the chemical environment.
Nanodiamonds can cross the blood-brain barrier, which normally blocks most substances from entering the brain. In 2013, scientists attached a cancer drug called doxorubicin to nanodiamonds, creating a new drug called ND-DOX. Tests showed that this drug remained in tumors longer, making it more effective and reducing side effects.
Larger nanodiamonds can be used to label cells because they are taken up efficiently. Studies show that nanodiamonds behave similarly to carbon nanotubes, and both become more stable and safe in solutions when treated with surfactants. Smaller nanodiamonds can be modified on their surfaces, making them useful as biolabels with low harm to cells.
Reducing the size of nanodiamonds and changing their surfaces may allow them to carry proteins, which could replace traditional catalysts in chemical processes.
Nanodiamonds are easily absorbed by human skin and can help skin care ingredients penetrate deeper. They also bond strongly with water, which helps keep the skin hydrated.
During jaw and tooth repair, doctors often use surgery to place a sponge with bone-growth proteins near damaged areas. However, nanodiamonds can bind to proteins that help rebuild bone and cartilage, and these can be taken orally. Nanodiamonds have also been used in root canal treatments by mixing them with a material called gutta percha.
Nanodiamonds with N-V defects can measure the strength of magnetic fields by detecting the direction of electron spins. They can also attach to proteins like ferritin, allowing scientists to count the number of iron atoms in the protein.
Natural defects in nanodiamonds called N-V centers can detect changes in weak magnetic fields, similar to how a compass detects Earth's magnetic field. These sensors work at room temperature and can be safely injected into living cells because they are made entirely of carbon. Nanodiamonds can also act as sensors for specific substances, such as dopamine. However, some types of nanodiamonds need further treatment to improve their selectivity. Recent studies suggest that modifying the surface of diamond-based materials before use can improve their performance.
Nanodiamonds can change the properties of materials they are mixed with, such as increasing the ability of the material to conduct ions. This is likely due to chemical groups on the nanodiamond surface that interact with the material.
Recent studies show that nanoscale diamonds can be stretched to more than 9% of their length without breaking, reaching a strength of about 100 gigapascals. This makes them ideal for use in high-performance sensors and tiny mechanical devices.
Nanodiamonds can replace photonic metamaterials in optical computing. The same nanodiamonds used for sensing magnetic fields can control light transmission using green and infrared light, enabling the creation of transistors and other computing components.
Nanodiamonds with N-V defects can be used in quantum computing at room temperature, similar to how trapped ions are used in other systems.
Fluorescent nanodiamonds provide a reliable standard for checking the quality of imaging systems that use fluorescence or multiple light wavelengths.