Real-time detection of label-free submicron-sized plastics using flow-channeled differential interference contrast microscopy

Abstract Owing to the surge in plastic waste generated during the COVID-19 pandemic, concern regarding microplastic pollution in aqueous environments is increasing. Since sub-micron plastics can accumulate in the human body, their real-time detection in water is necessary. However, the decrease in the scattering cross-section of small polystyrene (PS) particles in aqueous media precludes detection by bright-field microscopy. To address this problem, we propose and demonstrate a differential interference contrast (DIC) system that incorporates a magnification-enhancing system to detect PS beads in aqueous samples. To detect micron-scale and sub-micron-scale PS beads in the stationary and mobile phases, a microfluidic chip was designed considering the imaging depth of focus and flow resistance. PS beads of various sizes flowing in deionized and tap water at varying speeds were quantitatively observed under Static and Flow conditions. The real-time detection and quantification of PS beads of size ≥ 200 nm at a constant flow rate in water was successful. It was also possible to classify two different sizes, 2 µm and 750 nm, in a mixed solution. Thus, the proposed novel method can significantly reduce the analysis time and improve the size-detection limit. The proposed DIC microscopy system can be coupled with Raman or IR spectroscopy in future study for chemical composition analysis.