“O-PTIR overcomes several key limitations of conventional methods by achieving submicron IR resolution and exhibiting minimal susceptibility to fluorescence interference commonly encountered in Raman-based methods.“
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Reporting in Environmental Science & Technology, researchers at McGill University and the National Research Council Canada have demonstrated a label-free approach for detecting and identifying microplastics and nanoplastics in whole organisms using optical photothermal infrared (O-PTIR) microspectroscopy. Traditional methods for studying plastic particle uptake in organisms face significant limitations: fluorescence microscopy requires labeled particles that may leach dyes and produce false positives, while conventional μFTIR cannot detect particles smaller than 20 μm, and μRaman suffers from weak signals overpowered by background fluorescence. The research team developed a label-free approach combining O-PTIR with histological sectioning to overcome these challenges and achieve detection of submicron plastic particles in biological tissues.
The researchers exposed three model organisms—Daphnia magna (water flea), Drosophila melanogaster (fruit fly), and Eisenia andrei (earthworm)—to four different plastic types at 1 ppm concentration: 750 nm polystyrene beads, 2-30 μm polyethylene, 2-30 μm polypropylene, and 1-10 μm poly(methyl methacrylate). Following exposure, organisms were fixed, embedded in paraffin, sectioned, and deparaffinized for analysis. The O-PTIR technique successfully identified larger microplastics (PE, PP, and PMMA) in the gut regions based on characteristic spectral peaks, with PE identified by peaks at 2915 and 2850 cm⁻¹, PP by peaks at 2955 and 1378 cm⁻¹, and PMMA by its carbonyl peak at 1725 cm⁻¹. Importantly, the team detected 750 nm polystyrene nanoplastics within gut tissue, though spectra from submicron particles exhibited elevated biomass signals alongside characteristic PS peaks at 2920, 2846, 1601, 1492, and 1451 cm⁻¹.
The methodology proved particularly valuable for analyzing larger organisms like earthworms, where the researchers were able to distinguish plastic particles from ingested soil particles without efforts to remove the soil particles. O-PTIR mapping at 1465 cm⁻¹ confirmed the presence of PE particles within earthworm tissue, demonstrating the technique’s capability to visualize plastics and distinguish them from ingested soil particles. When organisms were exposed to mixtures of all four plastic types simultaneously, O-PTIR mapping successfully located and identified the larger particles using wavenumbers at 1725 cm⁻¹ (PMMA) and 1465 cm⁻¹ (PP/PE), though nanoplastic detection remained more challenging due to spectral overlap with biomass components.
O-PTIR microspectroscopy emerges as a powerful tool for microplastics and nanoplastics research, overcoming several key limitations of conventional methods by achieving submicron IR resolution and exhibiting minimal susceptibility to fluorescence interference commonly encountered in Raman-based methods. The label-free approach eliminates concerns about dye leaching and surface alteration while providing both chemical identification and spatial localization of plastic particles in complex biological matrices. By establishing proof-of-concept detection down to 750 nm particles across diverse terrestrial and aquatic organisms, this methodology opens new avenues for understanding plastic particle biouptake, distribution, and potential impacts across different biota, representing a promising advance for environmental plastics research.
Authors:
Jun-Ray Macairan, Arav Saherwala, Frank Li, Fanny Monteil-Rivera, and Nathalie Tufenkji
Department of Chemical Engineering, McGill University
