The University of Stuttgart has simplified the detection of nanoplastics

Detecting the presence of nanoscale plastic particles in the environment has become a topic of concern for industrial societies worldwide, not least since particles of that size can evade the body's blood-brain barrier and damage metabolic processes.
Optical technologies have been at the forefront of these monitoring efforts. Recent examples have included the use of stimulated Raman scattering to spot nanoplastics in ocean waters and in popular brands of bottled water, with the latter study finding more extensive contamination than previously suspected.

A project from the University of Stuttgart and the University of Melbourne has now demonstrated a new method for the straightforward analysis of tiny nanoplastic particles in environmental samples, one needing only an ordinary optical microscope.

Enter the void: nanoplastics in nanoscale holes

Described in Nature Photonics the technique could serve as a valuable new tool in environmental and health research.

"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," commented Mario Hentschel from the 4th Physics Institute, the University of Stuttgart's ultrafast nano-optics research center.

The technique is based around an optical sieve, a strip of semiconductor substrate manufactured with arrays of tiny voids of different diameters. Optical resonance effects under incident light cause these holes to act as Mie voids, a phenomenon studied at Stuttgart since 2023 in which light is confined and forced to interact with whatever matter is contained within the holes.

On-site environmental testing of water or soil

Depending on their diameter and depth, the Mie voids catch nanoplastic particles of different diameters, which then interact characteristically with incident light to create a bright color reflection that can be seen in an optical microscope. When a particle falls into one of the indentations its color changes noticeably, allowing an observer to infer from the changing color that a nanoplastic is present in the void.

"The test strip works like a classic sieve," said Stuttgart's Dominik Ludescher. "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 detected by the resulting color change."

For a proof-of-concept trial the team used a real-world water sample taken from a lake which already containined sand and other organic components, and added spherical nanoplastic particles at a concentration on 150 micrograms per ml. When the sample was passed through the optical sieve and illuminated, statistical information on numbers, size and size distribution of the particles was successfully derived from observation of distinct colour changes.

The project team now plans experiments with nanoplastic particles that are not spherical, and to investigate whether the process can be used to distinguish between particles of different plastics.

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

Source: optics.org