06 Feb 2024
Lasers help concentrate nanoparticles on substrate for Raman spectroscopy analysis.University of Notre Dame has made direct observations of nanoplastics in samples of global ocean water, revealing their diverse shapes and compositions.
These nanoplastics may profoundly affect marine organisms, said the Notre Dame team, and the results can provide critical information for designing appropriate toxicity studies.
"Nanoplastics are potentially more toxic than larger plastic particles," commented Tengfei Luo from the University of Notre Dame. "Their small size makes them better able to penetrate the tissues of living organisms."
Monitoring nanoplastic particles has been a challenge, with data about their spread through the environment only now starting to become available. The Notre Dame project described nanoplastics research as "an emerging field."
A recent project at Columbia University employed hyperspectral stimulated Raman scattering (SRS) imaging to detect nanoplastics in bottled water, revealing the presence not only of substantial amounts of common nanoplastics but even larger quantities of nanoparticles that it could not readily identify.
The Notre Dame project employs Raman spectroscopy in its analysis operation, exploiting the technique's ability to detect contaminants in low concentrations, but also uses lasers in a unique bubble deposition technique previously developed to find traces of DNA molecules for early detection of cancers.
The shrinking surface bubble deposition (SSBD) method involves optically heating silver nanoparticles mixed with ocean water samples, so that plasmonic heating caused by the laser creates a thermal bubble. Variations in surface tension cause the nanoplastic particles to accumulate on the bubble’s exterior.
Real-world complexity compared to standard toxicity tests
When the bubble shrinks and then vanishes, a thermofluidic flow effect concentrates the suspended particles and deposits them in one localized spot on a suitable substrate. They can then be imaged through surface-enhanced Raman spectroscopy for chemical identification of trace amounts of nanoplastics, alongside direct observation by electron microscopy.
In trials, the SSBD technique was used to assess samples of seawater collected on the coastlines of China, South Korea, the United States, and in the Gulf of Mexico - seven different locations across two oceans.
Tengfei Luo's team found nanoplastics made of nylon and polystyrene in the seawater samples, plus particles of polyethylene terephthalate (PET) used in food packaging, water bottles, clothing and fish nets.
The morphologies were highly diverse, with nanofibers, flakes, ball-sticks and other irregular shapes all being seen, according to the project's published paper - distinctly different from the spherical plastic nanoparticles used in laboratory toxicity studies. PET nanoparticles were found in water samples collected approximately 300 meters deep in the Gulf of Mexico, suggesting nanoplastic contamination is not restricted to the ocean surface.
According to Tengfei Luo, who runs Notre Dame's MONSTER (Molecular/Nano-Scale Transport & Energy Research) laboratory, the value of the SSBD technique is in how it efficiently concentrates the substances from samples. This not only allows them to be detected in relatively small volumes of water, but also considerably enhances the sensitivity of the downstream analyses, including Raman spectroscopy.
"The nanoplastics we found in the ocean were distinctively different from laboratory-synthesized ones," Luo said. "Understanding the shape and chemistry of the actual nanoplastics is an essential first step in determining their toxicity and devising ways to mitigate it."