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Raman scattering spots nanoplastic particles for environmental monitoring

19 Jul 2023

TU Wien project could be important step for development of future sensors.

Detecting nanoscale plastic pollutants in the environment is a continuing challenge, and their presence is commonly only estimated rather than measured directly.

A project at Vienna's TU Wien has now developed a measurement method which can detect individual nanoplastic particles, representing an orders-of-magnitude improvement over previous techniques.

Described in Scientific Reports, the method could become the basis for new measurement devices employed in environmental analysis.

The technique is based on surface-enhanced Raman scattering (SERS), extending further the application of Raman methods and their molecular specificity to the problem of microplastics monitoring. The TU Wien project combined SERS with additional confocal microscopy, so as to image and identify single plastic particles down to sizes of 100 nanometers.

Other Raman variants such as stimulated Raman scattering (SRS), in which a Raman signal is generated when the energy difference between two laser beams matches a vibrational state of the molecules being examined, have already helped to reduce signal acquisition times and make the technique attractive for environmental monitoring in several scenarios.

While surface-enhanced Raman methods have shown great benefits in the detection of small molecules, it is only very recently that it has been applied for the detection of nanoparticles, aerosols, and in particular nanoplastics, according to the researchers.

In the TU Wien device, gold nanowires 40 nanometers thick and 60 nanometers apart are used to amplify the interaction between the molecules being analyzed and the incident laser light, amplifying the Raman signal being produced.

Detect nanoplastics in biological systems

The project also sought to accelerate the overall imaging process, by filtering the strongest Raman band for analysis rather than capturing a broad spectrum and then breaking that down into smaller bands.

"We know what the characteristic wavelengths of the nanoplastic particles are, and so we look very specifically for signals at precisely these wavelengths," commented Sarah Skoff from TU Wien's Solid State Quantum Optics and Nanophotonics group. "We were able to show that this can improve the measurement speed by several orders of magnitude. Previously you had to measure for ten seconds to get a single pixel of the image you were looking for. With us, it takes just a few milliseconds."

Filtering the strongest Raman band and simultaneously imaging the particles by recording the scattered laser light led to the image acquisition time for spatially-resolved nanoplastic detection being shortened by a factor of 107, according to the team's publioshed paper.

Experiments with polystyrene nanoparticles and agglomerates in model systems showed that they could be imaged and identified down to sizes of 100 nanometers. For particles in the 300 to 800 nanometer range, the project found that its technique yielded surface-enhancement of the Raman signal by more than three orders of magnitude, "to our knowledge the largest enhancement factors seen so far for individually imaged nanoplastic particles."

The next steps will include assessing how the technique might be applied to nanoplastics in real-world environmental samples and in biological fluids such as blood, but the project believes it has already made an important step towards development of the first in situ detector to monitor nanoplastics in the environment.

"We have been able to show that the basic physical principle works," said Sarah Skoff. "This lays the foundation for development of new measurement devices that could be used to examine samples directly in nature outside the laboratory in the future."

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