Fluorescence sensing of microplastics on surfaces

Microplastics, nanoplastics and related products have been recently found in marine ecosystems worldwide, calling for new analytical methods for rapid detection and risk assessment. Fluorescence is a sensitive technique that when used with polarity probes can potentially detect low-polarity plastic particles in environments. Here, we evaluated the fluorescence technique to sense polystyrene microparticles directly on salt, silica and sand surfaces, using Nile Red and pyrene as polarity probes. Results show that all probes displayed fluorescence on silica and sand, whereas strong fluorescence quenching was observed on NaCl. Polystyrene particles increase the fluorescence intensity due to probe migration into their nonpolar microenvironment. In the presence of polystyrene, the spectra of Nile Red are shifted to shorter wavelengths, while the ratio of vibronic bands I1/I3 of fluorescence of pyrene decreases to about 1. Pyrene showed similar sensitivity toward surface-modified carboxyl polystyrene particles. On NaCl, the emission of pyrene increases linearly with polystyrene content for concentrations from 0.5 to 20 µg/g. The detection limit of polystyrene microparticles on natural sea salt using pyrene as probe is about 0.2 µg/g, while on sand, the sensitivity is about one order of magnitude lower. Overall, although being of relatively low selectivity, the fluorescence technique can be used to determine a maximum content of plastic particles of few micrometers size with little sample preparation. Fluorescence, when used in conjunction with pyrene probe, allows for detection and quantification of microplastic particles in the sub-ppm range.