A simple test strip that reveals the invisible nanoplastic threat

Researchers at the University of Stuttgart have created an “optical sieve” capable of detecting tiny nanoplastic particles. Operating as well as a test strip, this innovation is designed to provide a new analytical tool for environmental and healthy research.

Researchers from the University of Stuttgart in Germany and the University of Melbourne in Australia introduced a simple way to analyze very small nanoplastic particles in environmental samples. The approach is based on a standard optical microscope and a newly designed test strip called optical sieve. The results are reported in Nature photonics.

“The test strip can serve as a simple analysis tool in environmental and health research,” said Professor Harald Giessen, chief of 4th Physics Institute at the University of Stuttgart. “In the near future, we will work to analyze nanoplastic concentrations directly on the site. But our new method could also be used to test blood or tissues for nanoplastic particles. ”

Nanoplastics as a danger to humans and the environment

Plastic waste ranks among the most urgent global challenges of the 21st century. It contaminates the oceans, rivers and beaches and microplastics were found in living organisms. Until recently, researchers have mainly examined larger plastic fragments. The evidence now indicates an even more worrying threat: nanoplastic particles.

These particles are much smaller than the width of human hair, form like larger plastic pieces decompose and cannot be seen with the naked eye. With submicrometer sizes, they can also go through organic barriers, including blood-brain skin and barrier.

Color changes make tiny particles visible

Due to the small size of the particles, their detection poses a particular challenge. Consequently, there are not only gaps in our understanding of how particles affect organisms but also a lack of rapid and reliable detection methods.

In collaboration with a research group in Melbourne in Australia, researchers from the University of Stuttgart have now developed a new method that can quickly and affordable such small particles. Color changes on a special test strip make nanoplastics visible under the optical microscope and allow researchers to count the number of particles and determine their size.

“Compared to conventional and widely used methods such as scanning electron microscopy, the new method is considerably less expensive, does not require trained personnel to operate and reduces the time required for detailed analysis,” explains Dr. Mario Hentschel, head of the microstructure laboratory at 4th Physics Institute.

Optical sieve instead of an expensive electron microscope

The “optical sieve” uses resonance effects in small holes to make nanoplastic particles visible. A study on optical effects in such holes was published for the first time by the Research Group of the University of Stuttgart in 2023. The process is based on tiny depressions, known as MIE Voids, which are engraved in a semiconductor substrate.

Depending on their diameter and depth, the holes interact characteristicly with incident light. The result is a bright reflection that can be observed in an optical microscope. If a particle falls into one of the indentations, its color changes significantly. We can therefore deduce from changing color if a particle is present in the void.

“The test strip works like a classic sieve,” explains Dominik Ludecher, doctoral student and first author of the publication in “Nature Photonics”. Particles ranging from 0.2 to 1 µm can thus be examined without difficulty. “The particles are filtered out of the liquid using the sieve in which the size and depth of the holes can be adapted to nanoplastic particles, and thereafter, the resulting color change can be detected. This allows us to determine whether the voids are filled or empty. ”

The number, size and size of the size of the particles can be determined

The new detection method used can do even more. If the sieve is supplied with voids of different sizes, a single particle of an appropriate size will accumulate in each hole.

“If a particle is too large, it will not go into a vacuum and will simply be rinsed during the cleaning process,” explains Ludecher. “If a particle is too small, it will adhere badly to the well and will be swept during cleaning.” In this way, the test strips can be adapted so that the size and number of particles in each individual hole can be determined from the reflected color.

Synthesized environmental samples examined

For their measurements, the researchers used spherical particles of various diameters. These are available in aqueous solutions with specific nanoparticles. Because the real samples of water bodies with known nanoparticles concentrations are not yet available, the team has produced an appropriate sample themselves.

The researchers used a lake water sample which contained a mixture of sand and other organic components and added spherical particles in known quantities. The concentration of plastic particles was 150 µg / ml. The number and distribution of size of nanoplastic particles have also been determined for this sample using the “optical sieve”.

Can be used as a test strip

“In the long term, the optical sieve will be used as a simple analysis tool in environmental and healthy research. The technology could serve as a mobile test strip which would provide information on the content of nanoplastics in water or soil directly on site, ”explains Hentschel.

The team is now planning experiences with nanoplastic particles that are not spherical. Researchers also plan to determine whether the process can be used to distinguish particles from different plastics. They are also particularly interested in collaborating with research groups that have specific expertise in the treatment of real samples of water bodies.

Reference: “Optical sieve for the detection, dimensioning and counting of nanoplastics” by D. Ludecher, L. Wessemann, J. Schwab, J. Karst, SB Sulejman, M. Ubl, Bo Clarke, A. Roberts, H. Giessen and M. Hentschel, September 8, 2025, Nature photonics.
Two: 10.1038 / S41566-025-01733-X

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