Bulk δ¹³C tracks UV-induced aging of 200-nm polystyrene particles: effects of formulation and particle concentration

Submicrometric plastic particles (hereafter referred to as nanoplastics following the operational environmental definition) are emerging contaminants whose degradation state influences their mobility, reactivity, and biological interactions, yet robust metrics to quantify aging remain limited. Here, we evaluate bulk carbon stable isotopes (δ¹³C), measured by elemental analysis-isotope ratio mass spectrometry (EA-IRMS), as a complementary integrative proxy of UV-induced aging in 200-nm carboxylated polystyrene particles. Three colored formulations (white, orange, blue) were irradiated in ultrapure water under UV-C (254 nm) in two configurations: dilute suspensions (20 mg L^-1, 6 h) to resolve early isotopic and morphological trajectories, and a concentrated white suspension (500 mg L^-1, 30 h) to evaluate responses at higher particle concentration and to provide sufficient material for pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). Under dilute conditions, all PS particles showed systematic ¹³C enrichment, with maximum Δδ¹³C of +2.92 ‰ (orange), +2.78 ‰ (white), and +2.02 ‰ (blue), well above analytical uncertainty, following quasi-first-order kinetics (R² = 0.92-0.98). An empirical Rayleigh-type analysis yielded apparent enrichment coefficient (ε ≈ -1.3 to -2.5 ‰). Estimated from DLS-derived particle-volume proxy rather than a direct carbon mass balance, these coefficients are semi-quantitative and describe the direction and magnitude of the observed isotopic change rather than a specific carbon-loss mechanism. These shifts were accompanied by particle shrinkage (∼31-40%) and progressive loss of spherical morphology. In concentrated suspensions, isotopic enrichment was strongly attenuated (+0.54 to +0.57 ‰), consistent with reduced effective photon availability, although aggregation and other concentration-dependent processes may also contribute. Py-GC-MS showed decreasing styrene-related signals and increasing oxygenated aromatics (benzaldehyde, acetophenone), supporting photo-oxidative transformation. Complementary TEM-EDS observations were consistent with physicochemical modifications of particle surfaces following irradiation. Overall, bulk δ¹³C provides a sensitive, reproducible, integrative descriptor of accelerated UV-induced aging in polystyrene particles. Future applications will require validation under solar irradiation, phase-resolved carbon mass balances, and more complex environmental matrices.