New Optical Sieve Detects Nanoplastics with an Ordinary Microscope

Researchers have developed a simple test strip that makes invisible nanoplastics visible under a basic microscope. It's fast, cheap, and extremely accessible. The study's findings have been reported in Nature Photonics. 

The innovation, a joint effort by scientists at the University of Stuttgart in Germany and the University of Melbourne in Australia, offers a low-cost solution for identifying tiny plastic particles in environmental samples. 

Nanoplastics, as well as damaging ecosystems, are thought to have a negative impact on human health. These tiny particles, infinitely smaller than human hairs, are formed by the decomposition of larger plastic particles. In fact, they are so small they cannot be seen with the naked eye. As a result, these sub-micrometer particles can easily pass through organic barriers like the skin or the blood-brain barrier.

The researcher's test strip is an easy-to-use tool that provides a solution for their analysis. 

The test strip can serve as a simple analysis tool in environmental and health 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.

Harald Giessen, Professor and Head, 4th Physics Institute, University of Stuttgart

Through color changes on a specialized test strip, researchers can see nanoplastics under an optical microscope, count the number of particles, and measure their size.

Compared with 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.

Dr. Mario Hentschel, Head, Microstructure Laboratory, 4th Physics Institute

The strip, thought of as an “optical sieve,” takes advantage of resonance effects in microscopic holes to reveal nanoplastic particles. These holes are known as Mie voids and can be etched into a semiconductor surface. When exposed to light, these voids reflect vivid colors, depending on their size and depth.

The holes interact with incident light in a certain way depending on their size and depth. This produces a vivid color reflection that can be viewed using an optical microscope. If a particle falls into one of the indentations, its color changes significantly. The shifting color of the void can thus be utilized to determine whether or not a particle exists.

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 can be detected. This allows us to determine whether the voids are filled or empty. If a particle is too large, it won’t fit into the void and will be simply flushed away during the cleaning process. If a particle is too small, it will adhere poorly to the well and will be washed away during cleaning

Dominik Ludescher, Study First Author and PhD Student, University of Stuttgart

Particles from 0.2 to one micrometer can be easily examined with the new detection technique. Only one particle of an appropriate size will collect in each hole if the sieve has voids of varying sizes. This allows the test strips to be modified so that the reflected color may be used to determine the size and quantity of particles in each hole.

The team measured spherical particles of varying size, which they collected from aqueous solutions of particular nanoparticles. Since genuine samples from bodies of water with known nanoparticle concentrations are currently unavailable, the researchers created their own sample.

The scientists took a water sample from a lake that contains sand and other organic components and added spherical particles in predetermined proportions. The concentration of plastic particles was 150 µg/mL. The quantity and size distribution of the nanoplastic particles in this sample were also assessed using the “optical sieve.”

In the long term, the optical sieve will be used as a simple analysis tool in environmental and health research. 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.

Dr. Mario Hentschel, Head, Microstructure Laboratory, 4th Physics Institute

The team is currently preparing experiments with non-spherical nanoplastic particles. The researchers also intend to test if the method can discriminate between particles of various plastics. They are also particularly interested in working with research groups that specialize in processing real samples from bodies of water.

Journal Reference:

Ludescher, D., et al. (2025) Optical sieve for nanoplastic detection, sizing and counting. Nature Photonics. doi.org/10.1038/s41566-025-01733-x