Nanoplastic particles made visible: the newly developed test strip allows nanoplastic particles to be detected under a light microscope. [Image: University of Stuttgart / 4th Physics Institute]
Microplastics, which are synthetic plastic particles smaller than 5 mm, have been discovered in ecosystems ranging from the depths of the ocean to the top of Mount Everest. Even human tissues such as the blood, lungs, liver and kidneys harbor microplastics, with concerning effects to human health. Nanoplastics—smaller than 1 micrometer—have the potential to cause even more harm, since they can penetrate the blood–brain barrier and other biological borders.
Now, researchers based in Germany and Australia say they have developed a fast, portable way to detect nanoplastics in environmental samples (Nat. Photon., doi: 10.1038/s41566-025-01733-x). The technique involves only an ordinary light microscope and the team’s optical sieve, a test strip that leverages the concept of Mie void resonances to sort and filter particles.
“The test strip can serve as a simple analysis tool in environmental and health research,” said study author Harald Giessen, University of Stuttgart, Germany, in a press release accompanying the research. “In the near future, we will be working toward analyzing nanoplastic concentrations directly on site. But our new method could also be used to test blood or tissue for nanoplastic particles.”
Leveraging Mie void resonators
Nanoplastic detection is challenging, so actual data on environmental and biological samples remain limited. Due to the particles’ small size, samples require precise separation and filtration, along with complex characterization with a technique such as scanning electron microscopy. Ultimately, Giessen and his colleagues wanted to create an all-optical method that can quickly and affordably obtain data about the size, size distribution and concentration of nanoplastic particles in real-world samples.
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In the optical sieve, nanoplastic particles fall into holes of the appropriate size in the test strip. The color of the holes changes, and the new color provides information about the size and number of particles. [Image: University of Stuttgart / 4th Physics Institute]
The optical sieve consists of a high-refractive-index material such as gallium arsenide or silicon with arrays of Mie void resonators—essentially, open cylindrical holes—in its surface. Each array has holes of different diameters (300 nm, 350 nm, 400 nm and 450 nm), which serve to sort the particles according to their size. Detection is based on a resonance shift of the localized Mie mode confined in the holes, induced by an effective refractive-index change and resulting in a bright color reflection that can be seen in an optical microscope.
“The test strip works like a classic sieve,” said study author Dominik Ludescher, University of Stuttgart. “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 subsequently by the resulting color change [they] can be detected. This allows us to determine whether the voids are filled or empty.”
A mobile test strip
The researchers tested the optical sieve with a sample containing unfiltered and untreated lake water mixed with clean sand and nanoplastic particles of 350 nm, 550 nm and 1 μm diameter. Notably, the approach eliminates the need to preclean biological material from liquid samples, unlike other methods such as dynamic light scattering.
“In the long term, the optical sieve will be used as a simple analysis tool in environmental and health research,” said Hentschel. “The technology could serve as a mobile test strip that would provide information on the content of nanoplastics in water or soil directly on site.”