A joint team from the University of Stuttgart and the University of Melbourne in Australia has developed a new method for the simple analysis of tiny nanoplastic particles in environmental samples. All that is needed is an ordinary optical microscope and a newly developed test strip - the so-called optical sieve. The research results have now been published in "Nature Photonics" (doi: 10.1038/s41566-025-01733-x).
In the future, the test strip can serve as a simple analysis tool in environmental and health research," explains Harald Giessen, Head of the 4th Institute of Physics at the University of Stuttgart. in the near future, we are working towards analyzing nanoplastic concentrations directly on site. But our new method could also be used to test blood or tissue for nanoplastic particles."
Nanoplastics as a danger to humans and the environment
Plastic waste is one of the central and acute global problems of the 21st century. It not only pollutes oceans, rivers and beaches, but has now also been detected in living creatures and organisms in the form of microplastics. Until now, environmental researchers have primarily focused their attention on larger plastic residues. However, it has been known for some time that a presumably even greater danger is growing: nanoplastic particles. These tiny particles are much smaller than a human hair and are created by the decomposition of larger plastic particles. They are invisible to the naked eye. These particles in the sub-micrometer range can also easily cross organic barriers such as the skin or the blood-brain barrier.
Color changes make tiny particles visible
Due to the small particle size, their detection poses a particular challenge. As a result, there are not only gaps in our understanding of how particles affect organisms, but also a lack of rapid and reliable detection methods. Researchers at the University of Stuttgart, in collaboration with a working group from Melbourne in Australia, have now developed a novel method that can be used to detect such small particles quickly and cost-effectively. Color changes on a special test strip make nanoplastics visible under a light microscope and allow the number of particles to be counted and their size to be determined. "Compared to conventional, widely used methods such as scanning electron microscopy, the new method is considerably cheaper, does not require specialist staff to operate it and reduces the time required for a detailed analysis," explains Dr. Mario Hentschel, Head of the Microstructure Laboratory at the 4th Institute of Physics.
The "optical sieve" uses resonance effects in small holes to make the nanoplastic particles visible. A study on optical effects in such holes was first published by the research group at the University of Stuttgart in 2023. The method is based on tiny depressions, known as Mie voids, which are produced in a semiconductor. Depending on their diameter and depth, the holes interact characteristically with the incident light. This leads to a clear color effect that can be seen under an optical microscope. If a particle falls into one of the holes, there is a clear change in color. It can therefore be deduced from the color whether a particle is present or not.
"The test strip therefore works like a classic sieve," explains Dominik Ludescher, PhD student and first author of the publication in Nature Photonics. Particles with sizes of 0.2 to 1 micrometer can be examined without any problems. "The particles are filtered out of the liquid using the sieve, in which the size and depth of the holes can be adapted to the nanoplastic particles, and detected using the color change. This allows us to determine whether the wells are filled or empty."
Number, size and size distribution of the particles can be determined
But the system used can do even more. If the sieve is provided with wells of different sizes, only one particle of a suitable size is collected in each hole. "If a particle is too large, it does not fit into the recess and is simply flushed away during the cleaning process," says Ludescher. "If a particle is too small, it adheres poorly to the well and is also rinsed away during the cleaning process." In this way, the test strips can be adapted so that the size and number of particles in a single hole can be determined from the color.
Synthesized environmental samples examined
For their measurements, the researchers used spherical particles of various diameters, which can be bought in known concentrations in water. As real samples from bodies of water with known nanoparticle concentrations do not yet exist, the team produced a suitable sample themselves. The researchers used a water sample from a lake that contained a mixture of sand and other organic components and added spherical particles whose quantity was known. The concentration of plastic particles was 150 micrograms per milliliter. The number and size distribution of the nanoplastic particles could also be determined for this sample using the "optical sieve".
Can be used like a test strip
in the long term, the optical sieve will be used as a simple analysis tool in environmental and health research. The technology has the potential to serve as a mobile test strip, which could then provide information on the content of nanoplastics in water or soil directly on site," explains Mario Hentschel. Next, the team is planning experiments with nanoplastic particles that are not spherical. The researchers also want to investigate whether the method can be used to differentiate between particles of different plastics. The Stuttgart researchers are also very interested in collaborating with working groups that have specific expertise in the preparation of real samples from bodies of water.
