Viruses are constantly on the move. And they are so small – the virus particles of SARS-CoV-2 measure just under 100 nanometers on average – that they cannot be seen under the microscope. To characterize the tiny nanoparticles and observe their dynamics, Markus Schmidt locks them up – in a fiber. To do this, he and his team, together with experts from the Competence Center for Special Fiber Optics at Leibniz IPHT, have developed a sophisticated microstructured fiber with special optical properties. It has a hollow core and can be used like an analysis vessel. „We fill a solution with the nano-objects to be examined – viruses, for example – into the fiber, couple light into it and look at how the viruses diffuse,“ Markus Schmidt explains the method.

The unique microstructure of the fiber, which is permeated by ultra-thin glass membranes, offers a decisive advantage: it limits the lateral diffusion of the particles. In other words, the viruses in the hollow core cannot float away. This gives researchers the ability to observe the light scattering of individual particles over long periods of time. „We can look at the viruses in up to 100,000 images,“ says Markus Schmidt. That’s significantly more images than researchers achieve with conventional methods, which typically yield 200 to 500 images.

„This enormously improves the informative value about the size distribution of the particles in the sample,“ Schmidt says. He and his team analyze about 1,000 particles in a sample. They use the size distribution as a benchmark to find out whether the virus is changing – whether it is decomposing under the influence of UV radiation, for example.

„By measuring the elastic scattering, we can make statements about individual nanoparticles without having to bleach or stain beforehand – and more precisely than is possible with previous non-invasive methods,“ Schmidt sums up the advantage of the method. In collaboration with the research department ­“Optical-Molecular Diagnostics and Systems Technology,“ he and his team have already successfully demonstrated this in experiments with lambda phages. These are even smaller than SARS-CoV-2 viruses.

For research on the coronavirus, the fiber photonics experts are now collaborating with the virology department of the University Hospital. They have already conducted initial successful experiments with SARS-CoV-2 viruses. Now they are elaborating the method further. „Our goal is to detect SARS-CoV-2 reliably and marker-free and to gain information about the long observation time of single viruses,“ Markus Schmidt looks ahead.