09 Jan 2024
Further breakdown of microplastics into even smaller particles can be tracked by Raman spectroscopy.
Recent research into how microplastics can break down further into nanoplastics below 1 micron in size has led to new concerns, since nanoparticles are able to enter the human body more easily and cause more disruption to natural metabolic processes.
A project at New York's Columbia University has now analyzed bottled water and discovered that on average 240,000 nanofragments of seven identifiable common plastics were present in each liter. The results have been published in PNAS.
Plastics in bottled water became a public issue after a 2018 study detected an average of 325 particles per liter, noted the project, although even at the time scientists suspected the true number to be much higher.
"Previously this was just a dark area, uncharted,” said Beizhan Yan from Columbia Climate School’s Lamont-Doherty Earth Observatory. "Toxicity studies were just guessing what's in there. This opens a window where we can look into a world that was not exposed to us before."
The project developed a hyperspectral stimulated Raman scattering (SRS) imaging platform for its work, building on the increasing use of related Raman techniques to detect and quantify the presence of environmental pollutants. Previous examples include a 2023 study at TU Wien that combined surface-enhanced Raman with additional confocal microscopy, to image and identify single nanoplastic particles.
"While SRS is often credited for speeding up regular Raman imaging which enables fast identification of microplastics, the utility for it to analyze nanoplastic remains to be explored," commented the Columbia University project in its published paper.
Many nanoparticles of unknown type
The team's approach was to employ a narrowband SRS imaging scheme, focusing all the energy of the stimulating beam to target characteristic vibrational modes with the largest Raman cross-sections in its target molecules, and combine that with a bespoke algorithm designed to allow fast throughput analysis rates.
"To address the fundamental sensitivity-specificity trade-off and unleash the full potential of hyperspectral SRS imaging, we devised a data-driven SRS-tailored spectral matching algorithm based on the spectral library of seven common plastic standards," wrote the project.
In trials on three popular brands of bottled water sold in the US, the team's SRS method spotted 110,000 to 370,000 fragments of the plastics it was looking for in each liter, 90 percent of which were nanoplastics with the remainder being microplastics.
The project's spectral database included PET, polystyrene, PVC, and also polyamide, which in bottled water probably came from the plastic filters ironically intended to purify the water during bottling.
However, the team also detected SRS spectra from nanoparticles which did not match any of the standards. Indeed the seven typical plastics could only account for 10 percent of the total particles imaged under SRS microscopy. If all the rest are also nanoplastics then that class of contaminant could number in the tens of millions per liter; but at present "they could be almost anything," commented Columbia University.
Future work will include clarifying this question, and also analyzing tap water and other water sources, partly as a route to investigating the amount of nanoplastics accumulating in wastewater.
“There is a huge world of nanoplastics to be studied,” said Columbia's Wei Min, recognized as an original developer of the SRS technique, adding that although nanoplastics comprise far less than microplastics in terms of mass, that's not what matters. "It's the size, because the smaller things are, the more easily they can get inside us."