Researchers solve nanodiamond mystery

A team of researchers led by chemists at San Jose State University has solved the puzzle of how silica-coated nanodiamonds are formed and acquire properties that make them useful in an increasing number of scientific applications.

In an article published in the Sept. 15 edition of ACS Nanoscience Au, the researchers described the results of their investigation into the chemistry of silica coatings on nanodiamonds. The study was aimed at helping scientists improve the shells and experiment with coatings made of other materials.

"Surface chemistry of materials that host quantum bits such as diamond is an important avenue of exploration as quantum computation and quantum sensing platforms mature," wrote Perla Sandoval, lead author of the paper titled, "Quantum Diamonds at the Beach: Chemical Insights into Silica Growth on Nanoscale Diamond using Multimodal Characterization and Simulation."

While silica growth on nanodiamond cores has been used successfully for quantum sensing and biolabeling, the surface mechanism to initiate growth was unknown.

"Interfacing diamond, in general, and nanoscale diamond, in particular, with silica is a potential route to integrate room temperature quantum bits into photonic devices, fiber optics, cells, or tissues with flexible functionalization chemistry. While silica growth on (nanodiamond) cores has been used successfully for quantum sensing and biolabeling, the surface mechanism to initiate growth was unknown," Sandoval continued.

Shattered into pieces so tiny that you'd need thousands of them to span the width of a single human hair, the synthetic structures are wrapped in coatings made of silica, the main ingredient in sand.

While potentially useful, uncoated nanodiamonds cannot be easily programmed to go where they are directed, and the sharp points on their jagged edges can rupture cell membranes, the researchers explained.

Coating the edges with silica, however, solves both problems by creating a smooth, uniform shell that covers the sharp edges and creates a surface to which scientists can attach tags to direct the particles toward specific cells, like cancer cells or neurons.

"The diamond with (a) silica shell becomes a controllable system," said Abraham Wolcott, Ph. D., a physical chemistry professor at San Jose State.

Until now, no one understood how the silica shells bonded to the tiny pieces of diamonds, and scientists had different theories on how the coatings formed.

It was Wolcott who led the research team on a quest to unravel the mystery.

"Nobody was able to explain it for over 10 years," Wolcott told a reporter recently. "But we were able to tease out that information."

Microscopic scrutiny

After studying the particles with transmission electron microscopes at the U.S. Department of Energy's Lawrence Berkeley National Laboratory Molecular Foundry, the San Jose State team turned to Stanford University's Synchrotron Radiation Lightsource at DOE's SLAC National Accelerator Laboratory in Menlo Park, Calif., for help.

The researchers turned the powerful SSRL X-rays on the nanodiamonds to examine surfaces hidden below the silica coating.

Aided by the resulting images, they demonstrated that ammonium hydroxide with ethanol, normally included in the coating process, produces alcohol groups on the nanodiamonds' surfaces, and the alcohols enable the silica coatings to form and bond with the minuscule pieces of diamond.

Using the Stanford lab's transition edge sensor – a super-sensitive thermometer that collects temperature changes and converts them to X-ray energies – the researchers also figured out which chemical groups were present on the nanodiamonds' surfaces.

Wide-ranging potential

Theoretically, nanodiamonds have perfect carbon lattices, but occasionally a nitrogen atom sneaks in and replaces a carbon atom.

Technically a defect, the resulting structure, nonetheless, has proven useful to researchers because it responds to magnetic fields, electric fields, and light – all at room temperature. This means nanodiamonds can emit signals that scientists could use in many applications.

A diamond's carbon matrix makes it safe to use in living cells and tissues, which are largely composed of carbon. Additionally, diamonds are chemically inert, good at transferring heat, and optically transparent – meaning that light can easily pass through them, Wolcott said.

In short, chemical properties of the coated nanodiamonds make them useful for a wide range of applications, from detecting protein synthesis in real time to quantum computing – although Wolcott notes the technology is still in its infancy.

If examined under a green light, the nanodiamonds will glow red, so biologists could inject them into living cells, and the light they emit can be used to track their movement inside a living organism. The tiny particles can also be used for qubits, the basic unit for a quantum computer.

Using X-ray absorption spectroscopy, the researchers stimulated electrons to move on the surface of nanodiamonds and then monitored the microscopic particles as they traveled through the silica shell and escaped.

"XAS is powerful because you can detect something that is submerged, that's hidden – like diamond underneath a silica shell," Wolcott explained.

"Folks have never done this with nanodiamonds before, so in addition to figuring out the bonding mechanism, we've also shown that XAS is useful for material scientists and chemists," he said.

The research team also observed that the thicker the silica coating, the fewer electrons manage to escape a nanodiamond's surface.

The signals the electrons emit act like a tiny measuring tape, showing the thickness of the silica coatings.

New research opportunities

Wolcott, who is known for providing hands-on research opportunities usually reserved for doctoral candidates, post-doctoral researchers, and full-time lab technicians to undergraduate and master's degree students as early as in their freshman year, wants to put students to work in the future studying the effects of coating nanodiamonds with other materials.

Titanium, zinc, and other metal oxides, for example, could open new avenues in quantum sensing, biological labeling and other applications, he said.

The research project was funded in part by DOE's Office of Science and the Laboratory Directed Research and Development program, with additional contributions from other public and private sources.