In Situ Spectroscopic Probing of Naphthalene Enrichment at the Polystyrene Microplastic-Water Interface

Microplastics (MPs) can act as carriers for polycyclic aromatic hydrocarbons (PAHs), yet direct evidence of pollutant enrichment at the microplastic-water interface remains limited. Here, we developed an in situ spectroscopic approach for probing naphthalene (Nap) enrichment at the interface of polystyrene microplastics (PS MPs) across diverse aqueous matrices. A sponge-assisted surface-enhanced Raman scattering (SERS) platform with interfacial confinement enabled simultaneous identification of PS backbone and sorbed Nap signals within the same composite system, without chemical extraction or chromatographic separation. The resulting spectra provided direct evidence of PS-Nap interfacial association. Spectroscopic observations supported by theoretical calculations suggest that this association may involve hydrophobic interactions, π-π interactions, and van der Waals forces. Characteristic PS and Nap signals remained detectable in deionized water, tap water, river water, and seawater. A principal component analysis-one-dimensional convolutional neural network framework achieved 90% accuracy on an external validation set, while Gradient-weighted Class Activation Mapping identified the spectral regions contributing to classification. This study establishes a SERS-based strategy for distinguishing polymer backbone signals from sorbed contaminant signals and provides molecular-level information on interfacial association within the specific PS-Nap model system.