Novel sensor can detect ‘ever smaller’ nanoparticles

08 Nov 2021

KIT, Karlsruhe, develops new type of optical resonator to track movement of nanoparticles in space.

Nanoparticles are omnipresent in our environment: viruses in the air in the room, proteins in the body, or as building blocks of new materials for electronics, for example, or in surface coatings. However making these tiny particles visible can be problematic: They are so small that they usually cannot be viewed under a conventional optical microscope.

Researchers at Karlsruhe Institute of Technology (KIT), Germany, have developed a sensor with which they can not only detect nanoparticles, but also determine their properties and track their spatial movement. They describe their extremely sensitive and very compact detector, a novel Fabry-Pérot resonator, in Nature Communications.

Conventional microscopes produce greatly enlarged images of small structures or objects. However, because the nanoparticles, due to their tiny size, hardly absorb or scatter light, they remain invisible. Optical resonators, on the other hand, reinforce the interaction between light and nanoparticles: they keep light trapped in a small space by reflecting it thousands of times between two mirrors.

If there is a nanoparticle in the trapped light field, the particle interacts with the light thousands of times, enabling the change in light intensity to be measured. “Because the light field has different intensities at different points in space, we can draw conclusions about the position of the nanoparticle in three-dimensional space,” commented Dr. Larissa Kohler from the Physics Institute at KIT.

Visible movement

The resonator makes the movements of the nanoparticles visible. And not only that: “When a nanoparticle is in water, it collides with the water molecules, which move in arbitrary directions due to thermal energy,” she added.

“As a result of the impact, the nanoparticle executes a kind of trembling movement. We can now also understand this Brownian movement. Up until now, it was not possible to track the spatial movement of a nanoparticle with an optical resonator, but only to say whether a particle was in the light field or not,” she said.

Further to that, the novel fiber-based Fabry-Pérot resonator, in which the highly reflective mirrors are located on the end faces of glass fibers, opens up the possibility of deriving the hydrodynamic radius of the particle, i.e. the thickness of the surrounding shell of water, from the three-dimensional movement.

This is significant because it changes the properties of the nanoparticle. “For example, due to the hydration shell, nanoparticles can still be detected that would be too small without this shell,” said Kohler. The hydration shell around proteins or other biological nanoparticles could also have an influence on biological processes.

The researchers see possible uses for their resonator in the future detection of three-dimensional movement with high temporal resolution and the characterization of the optical properties of biological nanoparticles such as proteins, DNA origami or viruses. The sensor could thus enable insights into not yet understood biological processes.