Through an explosive technique, researchers in Japan have produced the smallest nanodiamonds to date, capable of probing microscopic temperature differences in their surrounding environments. With a carefully controlled explosion, followed by a multi-step purification process, Norikazu Mizuochi and a team at Kyoto University fabricated photoluminescent nanodiamonds some 10 times smaller than those produced with existing techniques. The innovation could substantially improve researchers’ ability to study the minute temperature differences found inside living cells.

Recently, silicon-vacancy (SiV) centres in diamond have emerged as a promising tool for measuring variations in temperature across nanoscale regions. These defects form when two neighbouring carbon atoms in diamond’s molecular lattice are replaced with a single silicon atom. When irradiated with a laser, these atoms will brightly fluoresce over a narrow range of visible or near-infrared wavelengths – whose peaks shift linearly with the temperature of the diamond’s surroundings.

These wavelengths are particularly useful for biological investigations as they pose no threat to delicate living structures. This means that when nanodiamonds containing SiV centres are injected into cells, they can probe the microscopic temperature variations of their interiors with sub-kelvin precision – allowing biologists to closely study the biochemical reactions taking place inside.

So far, SiV nanodiamonds have largely been produced through techniques including chemical vapour deposition, and subjecting solid carbon to extreme temperatures and pressures. For now, however, these methods can only fabricate nanodiamonds down to sizes of roughly 200 nm – still large enough to damage delicate cellular structures.

In their study, Mizuochi and team developed an alternative approach, where they first mixed silicon with a carefully selected blend of explosives. After detonating the mixture in a CO₂ atmosphere, they then treated the explosion’s products in a multi-stage process, which included: removing any soot and metal impurities with a mixed acid; diluting and rinsing the products with deionized water; and coating the nanodiamonds that remained with a biocompatible polymer.

Finally, the researchers used a centrifuge to filter out any larger nanodiamonds. The end result was a batch of uniform, spherical SiV nanodiamonds with an average size of roughly 20 nm: the smallest nanodiamonds ever used to demonstrate thermometry using photoluminescent lattice defects. Through a series of experiments, Mizuochi and colleagues observed clear linear shifts in the photoluminescent spectra of their nanodiamonds, over temperatures ranging from 22 to 45 °C – encompassing the variations found in most living systems.

Read more

Nanodiamonds measure thermal conductivity in living cells

The success of this approach now opens the door for far more detailed, non-invasive thermometry from within cellular interiors. Next, the team aims to optimize the number of SiV centres in each nanodiamond, making them even more sensitive to their thermal environments. With these improvements, the researchers hope that these structures could be used to study organelles: the even smaller and more delicate subunits of cells, which are vital to the functioning of all living organisms.

The researchers describe their findings in Carbon.