In situ imaging of the spatial and temporal penetration of organic pollutants into microplastics via surface-enhanced Raman spectroscopy.

Understanding the spatial and temporal penetration patterns of organic pollutants in microplastics (μP) is important for evaluating their environmental and biological impacts, such as the "Trojan Horse" effect. However, there is a lack of an effective method to monitor the penetration processes and patterns in situ. This study aimed to develop a simple and sensitive approach for in situ imaging of organic pollutant penetration into μP. The novel method was developed using surface-enhanced Raman spectroscopy (SERS) coupled with gold nanoparticles as nanoprobes that could sensitively detect organic pollutants in low-density polyethylene (LDPE) μP spatially and temporally. The detection limit of this SERS-based method was 0.36 and 0.02 ng/mm for ferbam (pesticide) and methylene blue (synthetic dye), respectively. The results showed that both ferbam and methylene blue could penetrate LDPE μP. The penetration depth and amount increased as the interaction time increased. Most of the absorbed organic pollutants accumulated within the top 90 μm layer of the tested μP. Compared to methylene blue, ferbam was more quickly absorbed and achieved higher accumulation in μP with a maximum of 32.57 ng/mm after 168 h interaction. This pioneering study clearly demonstrated that SERS mapping is a sensitive and in situ approach to visualize and quantify the penetration patterns of organic pollutants in μP. The new approach developed here can advance our understanding of μP as pollutant carriers and their influence on the environmental fate, behavior, and biological impacts of organic pollutants.