Nanoplastics are tiny, undetectable polymer-based particles that are hazardous for human health. Compared to their more well-known parents, microplastics, nanoplastics are not well understood by scientists.
Bottled water has shown to have extremely large quantities of microplastics, and further broken down nanoplastics. A group of scientists from Columbia University’s Climate School are hoping to develop a better method for analyzing nanoplastics in water bottles (1). The research team, which is led by Naixin Qian, a Columbia University graduate student in chemistry, found more than 240,000 detectable plastic fragments in plastic water bottles using a new technique called stimulated Raman scattering (SRS) microscopy.
SRS microscopy speeds up Raman imaging by more than 1,000 times. The technique uses stimulated Raman spectroscopy as the imaging contrast mechanism, the researchers wrote in the study (1). SRS is commonly used in medical imaging.
Spectroscopy spoke with Qian and authors Beizhan Yan and Wei Min to learn more about the technique and their research.
Q: Why is stimulated Raman scattering (SRS) microscopy useful for analyzing nanoplastics in water bottles?
A: SRS microscopy excels in sensitivity and specificity, making it appropriate for analyzing nanoplastics in bottled water. Its high sensitivity ensures that small nanoplastics as small as 100 nm can be detected, while specificity ensures that nanoplastics found under the microscope are indeed plastics, confirmed by their polymer composition. This capacity is vital for us to measure plastic contamination in a quantitative manner.
Q: What techniques are currently being used to analyze nanoplastics in water bottles? Why is SRS microscopy better?
A: In contrast to microplastics, which can be effectively analyzed using well-developed techniques like Fourier transform infrared (FT-IR) and spontaneous Raman, nanoplastics, due to their small size, have fewer established quantification techniques. Researchers have used transmission electron microscopy (TEM) or scanning electron microscopy (SEM) for imaging particles for morphological characterization and element quantification, yet these methods do not reveal chemical bonds to accurately distinguish plastic types or, even worse, determine if the particle is plastic at all. Surface-enhanced Raman spectroscopy has been applied to identify nanoplastics down to about 400 nm, but tedious sample preparation and lack of real automation restrict its wide usage. Pyrolysis gas chromatography-mass spectrometry (GC–MS) can merely offer the ensemble composition information, not at the individual particle resolution. Stimulated Raman microscopy, presented in this study, provides rich information regarding chemical composition, particle size, and shape, all at single particle resolution and with the required throughput to provide the quantitative result in particle counts.
Q: Why do we need better technology to analyze nanoplastics?
A: Chemical composition, particle size, and shape are all critical information for finding the plastic sources, characterizing exposure levels, and examining possible associated health outcomes. Nanoplastics have been found everywhere. Whether we like it or not, we live with them as fact, which is likely to last for a long time. Better analytical techniques lay the foundation for us to fully understand our situation with nanoplastics, which we believe is what people deserve to know. The second natural question would be whether they are harmful. The consequences of exposure require prudent examination, which cannot be carried out well if there are no suitable analytical tools to quantify the amount of exposure.
Q: What did you find most surprising about your research? What are you planning on investigating next?
A: The most surprising finding to us is the variety of the plastic types and different size distributions of different plastics. Previous studies found the major chemical composition to be polyethylene terephthalate (PET), which is expected since the bottle is made of PET. Therefore, for a long time, we had a misimpression that most plastic particles inside bottled water would be PET particles of a few hundred nanometers in size. This was incorrect. PET particles are found to be mostly in micron size, contributing only a limited portion of the particle population. There are many smaller particles other than PET, such as polystyrene (PS), and polyvinyl chloride (PVC), that have greater particle numbers. In the future, we plan to apply our platform to analyze more environmental samples, such as tap water and air samples as well as biological tissue, to uncover more about the hidden world of nanoplastics.
Q: What challenges did you experience during this study and how did you address them?
A: One of the biggest challenges in this study was that in each sample there can be thousands of particles awaiting chemical identification, which is nearly impossible to do manually. Some sort of algorithms for automated analysis need to be employed. However, we quickly realized that conventional spectral identification algorithms for FT-IR imaging or micro-Raman (µ-Raman) imaging cannot be directly applied in the scenario of SRS imaging. Eventually, inspired by the advice from Tingran Wang and Mariam Avagyan from the Data Science department at Columbia University, we came up with a data-driven SRS-tailored spectral matching algorithm to facilitate automated particle analysis and eventually reveal the multidimensional statistical profiles for micro-nano plastics in bottled water.
Q: Is there anything else you think our readers should know about your research?
A: Appropriate sample processing methods are critical, including the selection of the filter type and the instrument parameter settings. Another thing we would like to emphasize is the extraordinary particle heterogeneity and nonorthogonality between composition and morphologies for nanoplastics in bottled water.
The accurate analysis of micro-nano plastics really requires high throughput single-particle imaging with chemical specificity. One needs to be cautious when drawing a conclusion by merely combining the two characterization results from different analytical tools. As the discussed example shows, a sample with a majority 200 nm particle size and a major component of PET does not necessarily mean the sample is mainly 200 nm PET particles.
This interview has been lightly edited for length and clarity.
Qian, N.; Gao, X.; Lang, X. et al. Rapid Single-Particle Chemical Imaging of Nanoplastics by SRS Microscopy. Proc. Natl. Acad. Sci.2024, 121 (3). DOI:10.1073/pnas.2300582121.