Development of a novel resonator to aid nanoparticle detection

A researcher from the Karlsruhe Institute of Technology has developed a technique utilising optical resonators to advance the field of nanoparticle detection.

Nanoparticles are ubiquitous in the environment around us: viruses in ambient air, proteins in the body, as building blocks of new materials for electronics, or in surface coatings.

However, we currently face difficulties with visualising these tiny particles. They are so tiny that they can hardly be seen under a conventional optical microscope.

Overcoming current limitations

While traditional microscopes – with the assistance of light – are capable of producing in-depth images of small structures or objects, using these methods to study nanoparticles poses a challenge. Nanoparticles are so tiny that they can barely absorb or scatter light; thus, it has not been possible to study them proficiently with the use of a microscope.

Now, a novel technique utilising optical resonators to enhance the interaction between light and nanoparticles has been developed by Dr Larissa Kohler, a physicist from Karlsruhe Institute of Technology (KIT).

This sensor is not only proficient at nanoparticle detection, but also determines their condition and tracks their movements in space.

The findings of this groundbreaking study have been published in Nature Communications.

New method enhances nanoparticle detection

The optical resonators capture light in the smallest space by reflecting it thousands of times between two mirrors.

Using this method, when a nanoparticle is located in the captured light field, it interacts thousands of times with the light such that the change in light intensity can be measured.

“The light field has various intensities at different points in space. This allows conclusions to be drawn with respect to the position of the nanoparticle in the three-dimensional space,” said Dr Larissa Kohler from KIT’s Physikalisches Institut.

“If a nanoparticle is located in water, it collides with water molecules that move in arbitrary directions due to thermal energy. These collisions cause the nanoparticle to move randomly. This Brownian motion can now also be detected,” she added.

“So far, it has been impossible for an optical resonator to track the motion of a nanoparticle in space. It was only possible to state whether or not the particle is located in the light field,” Kohler explained.

Fibre-based optical resonator

In the novel fibre-based Fabry-Pérot resonator, highly reflecting mirrors are situated on the ends of glass fibres. It enabled the physicist to derive the hydrodynamic radius of the particle – the thickness of the water surrounding the particle – from its three-dimensional movement.

This is a significant step because this thickness changes the properties of the nanoparticle. “As a result of the hydrate shell, it is possible to detect nanoparticles that would have been too small without it,” Kohler explained.

Gaining an insight into biological processes

Moreover, the hydrate shell surrounding proteins or other biological nanoparticles could have an impact on biological processes.

A possible application of the resonator could be the detection of three-dimensional motion with high temporal resolution and characterisation of optical properties of biological nanoparticles, such as proteins, DNA origami, or viruses. Therefore, the sensor could offer insights into not yet understood biological processes.

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