Salt-induced aggregation of gold nanoparticles for sensitive SERS-based detection of nanoplastics in water

The presence of micro and nano plastics in the environment and their impact on the various life forms within it are of principle concern around the globe. However, whilst a considerable amount of work has been done on the detection of microplastics, many challenges remain in the development of analytical techniques for nanoplastics due to their inherent ultra-small size and ubiquitous shapes. Here, a simple technique is reported based on surface enhanced Raman spectroscopy (SERS) and salt (NaCl) induced aggregation of gold nanoparticles that has been used to detect 100 nm diameter polystyrene (PS) beads. The gold nanoparticles (AuNPs) were synthesized and stabilized by negatively charged sodium citrate. When the PS beads present in a water sample were introduced into the solution of colloidal AuNPs, they interact to each other via hydrophobic interactions and other weak forces (i.e. hydrogen, ionic, and Van der waals forces). Upon an addition of NaCl, the negatively charged ions around the AuNPs are shielded and disturbed, resulting in their aggregation around the PS beads. As a consequence, strong SERS signal enhancement produced by the aggregated AuNPs was observed, and also demonstrated in numerical modelling. Concentrations of 100 nm PS beads as low as 1 part per million (ppm) were measured, and to the best of the author's knowledge, this is the lowest concentration detected for nanoplastics of that size or smaller by such a simple technique that has been reported.