Nanodiamonds Doped with Silicon Could be Useful for Cell and Tissue Imaging

Researchers are very enthusiastic about diamonds—not the kinds that decorate jewelry, but the microscopic type that is below the thickness of a strand of human hair. These so-called “nanodiamonds” are nearly completely formed of carbon.

However, by adding other elements into the nanodiamond’s crystal lattice by a technique called “doping,” scientists could develop attributes valuable in medical research, computation, and more.

In a paper published on in Science Advances on May 3^rd, 2019, scientists at the University of Washington, the U.S. Naval Research Laboratory, and the Pacific Northwest National Laboratory reported that they can dope nanodiamonds by using very high pressure and temperature. The group employed this technique to dope nanodiamonds with silicon, making the diamonds to glow deep red—a property that would hold promise in cell and tissue imaging.

The group found out that the technique could also be used to dope nanodiamonds with argon, a noble gas and nonreactive element corresponding to helium present in balloons. Nanodiamonds doped with such elements could be used in quantum information science—a fast growing domain that involves quantum computing and quantum communication.

Our approach lets us intentionally dope other elements within diamond nanocrystals by carefully selecting the molecular starting materials used during their synthesis.

Peter Pauzauskie, Study Corresponding Author and Associate Professor, Materials Science and Engineering, University of Washington

Pauzauskie is also a researcher at the Pacific Northwest National Laboratory.

Nanodiamonds can be doped with other techniques like ion implantation, but this process usually causes damage to the crystal structure and the added elements are positioned in a random manner, which restricts performance and applications. In this case, the scientists decided that the nanodiamonds are not doped after they had been produced. Rather, they doped the molecular elements to produce nanodiamonds with the element they wished to add, and later, they used high temperature and pressure to produce nanodiamonds with the introduced elements.

Broadly speaking, it can be compared to cake making: It is easier and more effective to mix sugar to the batter, instead of trying to add sugar to the cake after baking.

The basis for nanodiamonds was a carbon-rich material—analogous to charcoal, reported Pauzauskie—which the scientists turned into a lightweight, porous matrix called aerogel. Later, they doped the carbon aerogel with a silicon-containing molecule known as tetraethyl orthosilicate, which was chemically incorporated within the carbon aerogel. The scientists closed the reactants tightly within the gasket of a diamond anvil cell, which could create pressures as high as 15 gigapascals within the gasket. For reference, 1 gigapascal is around 10,000 atmospheres of pressure or 10 times the pressure at the deepest part of the ocean.

They used argon, which turns solid at 1.8 gigapascals, as a pressure medium, so that the aerogel is prevented from being crushed at such high pressures. After exposing the material to extreme pressure, the scientists heated the cell above 3,100 °F, more than one-third of the surface temperature of the sun, using a laser. Working with E. James Davis, a UW professor emeritus of chemical engineering, they observed that at these temperatures, the solid argon melts to produce a supercritical fluid.

This process converted the carbon aerogel into nanodiamonds having luminescent point defects created by the silicon-based dopant molecules. The nanodiamonds radiated a deep-red light at a wavelength of about 740 nm, which is useful in medical imaging. Nanodiamonds doped with other elements could radiate other colors.

“We can throw a dart at the periodic table and—so long as the element we hit is soluble in diamond—we could incorporate it deliberately into the nanodiamond using this method,” stated Pauzauskie, who is also a researcher with the UW Institute for Nano-Engineered Systems and the Molecular Engineering and Sciences Institute. “You could make a wide spectrum of nanodiamonds that emit different colors for imaging purposes. We may also be able to use this molecular doping approach to make more complex point defects with two or more different dopant atoms, including completely new defects that have not been created before.”

Unexpectedly, the scientists found that their nanodiamonds also included two other ingredients that they didn’t plan to add—the argon used as a pressure medium and nitrogen from the air. As with the silicon that the scientists had planned to add, the argon and nitrogen atoms had been completely incorporated into the nanodiamond’s crystal structure.

This is the first time researchers have used high-temperature, high-pressure assembly to incorporate a noble gas element—argon—into a nanodiamond lattice structure. It is difficult to force nonreactive atoms to combine with other materials in a compound.

This was serendipitous, a complete surprise. But the fact that argon was incorporated into the nanodiamonds means that this method is potentially useful to create other point defects that have potential for use in quantum information science research.

Peter Pauzauskie, Study Corresponding Author and Associate Professor, Materials Science and Engineering, University of Washington

Next, scientists wish to dope nanodiamonds deliberately with xenon, another noble gas, for potential use in fields like quantum sensing and quantum communications.

Ultimately, the team’s approach could also help unravel a cosmic mystery: Nanodiamonds have been discovered in outer space, and something out there—for example, supernovae or high-energy collisions—dopes them with noble gases. While the techniques created by Pauzauskie and his team are for doping nanodiamonds here on Earth, their findings could help researchers learn which types of extraterrestrial events prompt cosmic doping far from home.

Matthew Crane, a former doctoral student in Pauzauskie’s laboratory and now a postdoctoral researcher in the UW Department of Chemistry is the lead author of the paper. Co-authors are former UW postdoctoral researcher Alessio Petrone, now at the University of Napoli Federico II in Italy; doctoral student Ryan Beck and professor Xiaosong Li in the UW Department of Chemistry; former Department of Materials Science & Engineering doctoral students Matthew Lim, now a postdoctoral researcher at Sandia National Laboratories, and Xuezhe Zhou, now a hardware system reliability engineer at Apple; and Rhonda Stroud, head of the Nanoscale Materials Section at the Naval Research Laboratory.

The study was funded by the National Science Foundation, the University of Washington, the U.S. Office of Naval Research, the Microanalysis Society of America, and the Pacific Northwest National Laboratory.