Glow and behold: How weathering alters the surface, chemical and fluorescence properties of microplastics

Microplastics (MPs) are now ubiquitous across both built and natural environments. Their environmental fate and transport are governed by chemical and physical properties that evolve with weathering, making accurate characterization of these properties essential for predicting environmental behavior and informing mitigation and remediation strategies. Environmental weathering processes, particularly UV aging, can substantially alter plastic physicochemical properties over time and can also affect the fluorescence behavior of stained MPs, an important consideration given the widespread use of fluorescence microscopy for their detection and identification. Here, we investigated the physicochemical evolution of ten widely used polymers (PP, HDPE, LDPE, EPS, PS, PC, ABS, PVC, PET, and PA) subjected to artificial weathering under UVA (365 nm) irradiation and full-spectrum solar radiation for up to 333 and 150 days, respectively. Changes in surface roughness (RMS), water contact angle (WCA), chemical composition (Hydroxyl and Carbonyl groups) and fluorescence behavior were quantified to characterize degradation-induced transformations and evaluated using correlational analyses, including principal component analysis biplots and Spearman’s rank correlation coefficients. Our results show that physical and chemical degradation followed distinct temporal trajectories. RMS and WCA evolved in non-monotonic, sinusoidal-like patterns characterized by alternating roughening, smoothing, and surface reorganization, whereas chemical oxidation of hydroxyl and carbonyl groups progressed monotonically according to power-law kinetics. The fluorescence response of weathering induced red-shift of Nile Red-stained MPs closely tracked chemical oxidation, particularly carbonyl formation, rather than surface restructuring. Despite convergence in fluorescence color with progressive weathering, multispectral fluorescence imaging (FIMAP) maintained high classification accuracy. Notably, oxidation kinetics varied substantially among polymers and between UVA and full-spectrum exposures, highlighting the strong influence of polymer chemistry, exposure duration, and light source on degradation pathways. Together, these findings demonstrate that MPs weathering is highly polymer- and condition-specific and underscore the need for caution when using artificial aging approaches to represent environmentally relevant weathered plastics.