Correlative SEM-Raman microscopy to reveal nanoplastics in complex environments

Nowadays "microplastics" (MPs) is an already well-known term and results of micro-sized particles found in consumer products or environments are regularly reported. However, studies of native MPs smaller than 1 μm, often referred to as nanoplastics (NPs), in analytically challenging environments are rare. In this study, a correlative approach between scanning electron microscopy and Raman microscopy is tested to meet the challenges of finding and identifying NPs in the 100 nm range in various environments, ranging from ideal (distilled water) to challenging (sea salt, human amniotic fluid). To test the viability of this approach in principle, standardized polystyrene beads (Ø 200 nm) are mixed into the various environments in different concentrations. Promising detection limits of 2 10^-3 μg/L (distilled water), 20 μg/L (sea salt) and 200 μg/L (human amniotic fluid) are found. To test the approach in practices both sea salt and amniotic fluid are analysed for native NPs as well. Interestingly a nylon-NP was found in the amniotic fluid, maybe originating from the sampling device. However, the practical test reveals limitations, especially with regard to the reliable identification of unknown NPs by Raman microscopy, due to strong background signals from the environments. We conclude from this in combination with the excellent performance in distilled water that a combination of this approach with an advanced sample preparation technique would yield a powerful tool for the analysis of NPs in various environments.