New Fluorescent Raman Technique Enhances Detection of Microplastics in Seawater

Microplastic debris floating in ocean water © Arsenii - stock.adobe.com

As plastic pollution in oceans escalates, microplastics—tiny particles less than 5 mm in size—are now found in every corner of the marine environment, from coastlines to deep seas. These resilient pollutants absorb toxic chemicals, are ingested by marine organisms, and accumulate through food chains, reaching humans. In a bid to tackle this, a team led by Qingsheng Xue, Guiting Yu, Fengqin Lu, and Yang Dong at the Ocean University of China has developed a novel detection system. By combining fluorescence labeling with confocal Raman spectroscopy, they have introduced a method that enables faster, more precise detection of microplastics, crucial for pollution monitoring and marine ecosystem protection. The research published in the journal Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, represents an important development in environmental science, offering a practical solution for rapid, non-destructive microplastic analysis (1).

Details of the Study
Microplastic pollution poses challenges due to the particles’ small size and resistance to traditional sampling and analysis methods. The team focused on developing a system that could overcome these obstacles in detecting microplastics in near-shore seawater. Their approach included refining sample preparation, integrating advanced fluorescence labeling, and optimizing Raman spectroscopic analysis. Using a dual-wavelength laser at 784 nm and 785 nm, they targeted common microplastics such as polyethylene (PE), polypropylene (PP), and polystyrene (PS), achieving precise identification of particles as small as 60 μm (1).

Fluorescence labeling, achieved with Nile Red dye, plays a critical role in this method. Nile Red binds to microplastics due to their hydrophobic properties, causing them to fluoresce under a microscope and allowing for straightforward identification from other marine materials. The researchers optimized the staining procedure by adjusting Nile Red concentration and incubation times, achieving bright fluorescence that distinguishes microplastics, facilitating faster and more reliable initial screening (1).

Read More: Raman in microplastic analysis

Innovative Techniques in Spectroscopy
While Raman spectroscopy has long been valued for its accuracy and non-destructive nature, fluorescence background interference often complicates the analysis, particularly in detecting weak microplastic signals (1,2). The team addressed this issue with a differential Raman spectroscopy technique that effectively eliminates fluorescence interference, producing high signal-to-noise ratio spectra. By removing background noise and refining spectral clarity, this method enhances both the reliability and efficiency of microplastic detection in complex seawater samples (1).

Advancements in Sample Preparation and Analysis
One critical advancement was in sample pretreatment. The researchers combined hydrogen peroxide (H2O2) and nitric acid (HNO3) for an oxidative digestion process, improving the removal of organic matter that commonly interferes with microplastic detection. Using a multi-layered stainless steel mesh filter, the team accelerated filtration while capturing a comprehensive range of particle sizes, ensuring a representative sample. Ultrasonic treatment was then applied, further breaking down residual organic compounds and enhancing the detection accuracy (1).

Implications for Marine Ecosystem Protection
Microplastic detection technology, especially one as effective as this, is vital for understanding the distribution and density of microplastics in marine environments. By creating a Raman spectral library of common microplastic standards, the team established a reference framework for future identification and classification efforts. This innovation provides the technical groundwork for effective monitoring and control of microplastic pollution, especially in coastal areas where human activity significantly impacts plastic accumulation (1).

References

(1) Xue, Q.; Yu, G.; Lu, F.; Dong, Y. 2Fluorescent Labelling Combined With Confocal Differential Raman Spectroscopy to Detect Microplastics in Seawater. Spectrochim Acta A Mol Biomol Spectrosc. 2024,124591. DOI: 10.1016/j.saa.2024.124591

(2) Pasteris, J. D.; Wopenka, B.; Freeman, J. J.; Brewer, P. G.; White, S. N.; Peltzer, E. T.; Malby, G. E. Raman Spectroscopy in the Deep Ocean: Successes and Challenges. Appl. Spectroscopy, 2004, 58 (7), 95A–208A. DOI: 10.1366/00037020413893