High-speed Raman Method Detects Micro- and Nanoplastics

Raman spectroscopy is a useful tool for identifying microplastics — plastic particles smaller than 5 mm — although conventional point confocal Raman methods are limited to single-point detection, limiting the detection speed. A line-scan Raman spectroscopy method developed by the Chinese Academy of Sciences and Cardiff University significantly boosts detection speed for both micro- and nanoplastics.

The researchers reported a line-scan Raman microspectroscopy system capable of rapid imaging and chemical identification of microplastics down to 2000 nm in size, and capable of imaging a 40- × 10-µm particle in 10 s, representing a speed improvement by about two orders of magnitude compared to confocal imaging.

Based on the fundamental principles of confocal Raman spectroscopy, the focused excitation spot expands from a convergent point into a convergent line with diffraction-limited width. The optical setup employs a conjugate imaging design. In the two-dimensional image recorded by the charge-coupled devices, the vertical dimension maps the vertical dimension of the sample along the excitation line, while the spectrum is dispersed along the horizontal dimension. In this way, a single acquisition provides the spectra for all spatial positions along the excitation line.

Courtesy of Talanta (2023) DOI: 10.1016/j.talanta.2023.125067.

Researchers developed a confocal line-scan Raman microspectroscopy system, established a preprocessing workflow for line-scan Raman spectral data, and applied the factorization into susceptibilities and concentrations algorithm to obtain Raman hyperspectral images. They employed a concave cylindrical lens to generate the excitation line and improved the uniformity of energy distribution using a Powell lens.

Plastic beads of various sizes were used to test size and composition identification. The detection of beads with a diameter of 200 nm, which is smaller than the diffraction limit, was realized, demonstrating the exceptional sensitivity of the line-scan Raman spectroscopy system.

Four types of plastic powder samples were used for a large-scale area of 1.2 mm in length and 40 µm in height. Notably, the imaging time is 20 min to obtain a 240,000-pixel Raman image. Compared with point confocal Raman imaging, the line-scan confocal Raman technology increases the imaging speed by two orders of magnitude.

Line-scan Raman microspectroscopy offers nondestructive analysis with high sensitivity and high throughput. By employing appropriate sampling techniques such as filtration or sedimentation, environmental samples from various sources, including water, soil, and air, are accessible.

The research was published in Talanta (www.doi.org/10.1016/j.talanta.2023.125067).