Environmental gaseous factors govern fluorescence characteristics of photoaged biodegradable and non-biodegradable microplastics.

Microplastics (MPs) readily undergo natural aging in the environment, which can alter their intrinsic fluorescence, an emerging property with potential for their identification and tracing. Environmental gaseous factors, water (H2O) and dioxygen (O2), strongly influence aging, but their roles in shaping the fluorescence characteristics of photoaged MPs remain poorly understood. Here, we investigated how H2O and O2 affect the fluorescence characteristics of biodegradable and non-biodegradable MPs during photoaging. All MPs generated new fluorophores after photoaging, exhibiting stronger and more readily excited fluorescence under low H2O and normal O2 volume fraction (21%). Lower H2O and normal O2 favored greater accumulation of conjugated CC- and CO-containing structures, facilitating π-π* transitions and enhancing fluorescence. For non-biodegradable polyvinyl chloride (PVC) and polystyrene (PS), fluorescent chromophores were primarily polyene-based conjugated structures, whereas those in biodegradable poly(butylene adipate-co-terephthalate) (PBAT) and polylactic acid (PLA) involved polyene- and carbonyl-containing conjugated systems. MPs photoaged under low H2O and normal O2 volume fraction also showed stronger fluorescence for tracing ingested particles within model-organism digestive tracts. Overall, this study reveals a previously overlooked regulatory mechanism of MP fluorescence and provides mechanistic insight into how atmospheric aging conditions shape fluorescence characteristics, supporting the potential application of intrinsic fluorescence for biological tracing.