Raman spectra characterization of size-dependent aggregation and dispersion of polystyrene particles in aquatic environments

Aqueous environments are generally thought to be a source of pooling and re-distribution for both micro-plastics (MPs) and nano-plastics (NPs); however, significantly less data on NPs than MPs have been reported. The occurrence of salts, proteins, and other organic matter may promote or inhibit the aggregation of NPs to form agglomeration particles, making their detection more difficult. In this study, 80 and 500 nm polystyrene nano-plastics ( P S-NPs) modified by four different functional groups ( P S-Bare, P S-COOH, P S-NH 2 , and P S–CHO–500 nm) were selected to mimic the flocculation and/or sedimentation of NPs in salts (NaCl, CaCl 2 , and Na 2 SO 4 ) and protein solutions. The results showed that the 80 nm P S-NPs are only colloidal in pure water. All four strong electrolyte solutions that were tested significantly promoted the aggregation of P S-NPs, including those that were protein-coated. In addition, 500 nm P S-CHO did not flocculate but gradually settled into sedimentation. Therefore, Raman spectrometry can be used to analyze assembled P S-NPs, but is not suitable for analyzing normal P S-NPs. By combining fractal morphology, this study provides insight into the comprehensive analysis of P S-NPs in water solutions, including the digestion of biological samples.