Visualization and Detection of Polystyrene Micro(nano)plastics in PM2.5 by Atomic Force Microscopy–Raman Spectroscopic Imaging

The pollution of micro- and nanoplastics (MNPs) poses a threat to global ecological safety. While the research of MNPs in other environmental matrices (e.g., water and soil) has been conducted for a long time, rare works have studied MNPs in atmospheric fine particulate matter (PM_2.5) due to the lack of satisfactory analytical methods. Previous means of pyrolysis-gas chromatography-tandem mass spectrometry provided quantitative data but failed to retain morphology and size information on MNPs in PM_2.5. Reported spectroscopic methods (e.g., optical microscopy and Raman spectroscopy) cannot effectively obtain both the real morphological and chemical characteristics of very small MNPs in PM_2.5. In the present work, with the assistance of a self-designed simple flotation device, polystyrene (PS) MNPs in PM_2.5 were detected by atomic force microscopy-Raman spectroscopic imaging (AFM-RSI). With the developed strategy, the morphology of PS particles with sizes as small as 300 nm was visually observed and accurately identified. In contrast to the surface enhanced Raman spectroscopy approach, which is the most applied for analysis of tiny PM-bound MNPs currently, the AFM-RSI-based method does not require expensive and synthetic substrates. The operation flow is also uncomplicated. Through this novel means, micro- and nano-PS particles (370 nm-2.4 μm) were detected in PM_2.5 samples collected from Zhengzhou, a megacity in central China, during summer and winter for the first time. Their morphological features were also obtained. The 100% sample detection rates highlighted the broad occurrence and potential residential exposure risks of PS in urban PM_2.5 and also confirmed the developed strategy could conduct clear visualization and accurate analysis of PM_2.5-bound micro- and nano-PS.