Fluorescence Correlation Spectroscopy Studies of Dye Diffusion on Fresh and Aged Polyethylene Terephthalate

Microplastics accumulate a wide varietyof organic pollutantsandthus may serve as efficient vectors for the transport of toxic substances.Much remains to be learned about how organic molecules interact withthe surfaces of plastics and how these properties evolve as the microplasticsare weathered. In this Letter, we report, for the first time, theapplication of confocal fluorescence correlation spectroscopy (FCS)to studies of organic molecules adsorbed from aqueous solution ontothe surfaces of synthetic secondary microplastics. Both fresh andartificially aged poly(ethylene terephthalate) (PET) plastics areemployed. The plastics are artificially aged in a UV-ozone chamber.Raman and infrared spectra confirm the composition of the PET microplastics.Water contact angle and surface roughness measurements reveal, respectively,an increase in wettability and a change in the nature of roughnesswith aging, consistent with surface oxidation. Rhodamine B (RhB) dyeis used as a fluorescent probe in FCS studies and serves as an analoguefor organic pollutants commonly found on microplastics. The FCS resultsreveal the accumulation of dye on the PET surfaces as they age. Dyemotion is significantly slower on the plastics than in bulk aqueoussolution and occurs by anomalous subdiffusion. The rate of diffusionbecomes dramatically slower and more anomalous as the plastics areaged. Surface diffusion is likely slowed by either ionic interactionsor hydrogen bonding between the dye and plastic. These results providenew insights critical to the understanding of how microplastics accumulateand transport organic pollutants as they weather in the environment.