Fenton-Mediated Surface Modulation of Microplastics: Effects and Mechanisms on the Fluorescence Response of Nile Red Staining

Nile Red (NR) fluorescence staining enables rapid microplastic (MP) screening, but weak dye affinity and matrix-derived false positives limit quantitative reliability. This study investigated whether controlled Fenton digestion can modulate microplastic surfaces to enhance NR fluorescence and detection accuracy. Using polystyrene (PS) as a model MP, we found that the Fenton digestion significantly increased the fluorescence intensity of NR-stained PS, which was strongly correlated with the surface carbonyl index (R2 = 0.90, p < 0.05). Oxygen-containing functional groups, particularly carbonyl groups, reached their maximum abundance after 90 min digestion, coinciding with the lowest crystallinity and the highest NR adsorption capacity. Density functional theory calculations further showed that PS surfaces bearing C═O and −OH groups exhibited stronger adsorption toward NR than pristine PS, whereas the formation of carboxyl groups at prolonged digestion times weakened the interaction. In real water samples, NR-Fenton counts agreed with laser direct infrared (LDIR) spectroscopy and achieved 99.1% recovery. These results demonstrate that Fenton digestion is a cost-effective pretreatment strategy to improve NR-based MP detection, and provide a mechanistic basis for understanding how the surface modulation of MPs affects the adsorption and fluorescence response.