Micro- and Nano-Plastics (MNPs) are significant environmental contaminants directly released during polymer manufacturing or by the degradation of polymeric materials. The monitoring of MNPs in a variety of environmental media requires a complex morphological and chemical analysis. An Electron Scanning Microscopy (SEM) commonly applies for the morphological characterization due to its high spatial resolution of ∼ 5 nm, but the technical limitations in chemical characterization are present. However, an optical method such as Fourier transform infrared (FTIR) spectroscopy is capable to provide chemical compound analysis along with morphology characterization, but it has particle characterization size limitations of around 5 μm.<br/><br/>To overcome these limitations, we propose a technique based on advanced super-resolution infrared spectroscopy, called InfraRed - Photothermal Heterodyne Imaging (IR-PHI). IR-PHI can perform spatially resolved mid-infrared spectroscopy with a resolution below the IR diffraction limit. Thus, it provides chemical classification, morphology, and quantity of MNPs in a single analysis with a 300 nm resolution, allowing us to study spectroscopically crowded environments.<br/><br/>In our work, we first validated our proof-of-concept IR-PHI setup by measuring commercially available PMMA microspheres – a suitable model material for MNPs – of which we successfully identified the chemical nature, size, morphology, and quantity. This was followed by the in-depth analysis of MNPs contamination in two case studies relevant for everyday life exposure. First, we investigated the release of MNPs by nylon teabags in hot water, finding that polymer particles chemically degrade along with morphological changes presented by brake up of fiber-shaped structures to spherically shaped. Second, we analyzed road dust, a complex environmental matrix, detecting the presence of clusters and single particles (&lt; 3 μm) composed of nylon and rubber. Such results were accomplished without the need for any advanced sample preparation, demonstrating the versatility of IR-PHI for routine environmental monitoring.

Infrared photothermal heterodyne imaging for micro- and nano-plastics characterization in environmental matrices

ACS Spring 2022

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