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Lignin-derived carbon dots made from leaf waste offer a sustainable tool for plant protection by converting agricultural residues into functional nanomaterials. High-purity lignin was extracted from Norway maple (Acer platanoides) leaves, and two classes of lignin carbon dots were synthesized by hydrothermal treatment at 200 °C (partially carbonized, pLCD-200) and 350 °C (fully carbonized, fLCD-350). Microscopy confirmed spherical nanoscale structures with mean diameters of 12.3 ± 3.8 nm for pLCD-200 and 9.3 ± 1.9 nm for fLCD-350, while zeta potential values of −13.7 and −19.9 mV indicated good colloidal stability. Spectroscopic analyses revealed distinct optical behaviors: pLCD-200 showed strong UV absorption (200–350 nm) and blue emission at 440–460 nm with excitation-dependent shifts, converting part of the absorbed UV light into photosynthetically active radiation (PAR). In contrast, fLCD-350 displayed broader UV-visible absorption and near-infrared emission around 710–730 nm with minimal excitation dependence. Under natural sunlight, pLCD-200 exhibited emission decay consistent with first-order kinetics (k ≈ 0.010 h−1; half-life ≈3 days), while fLCD-350 demonstrated persistent emission and gradual photo-brightening modeled by first-order association (kg ≈ 0.002 h−1; doubling time ≈15 days). FTIR confirmed that fLCD-350 stability arises from decarbonylation, ether bond cleavage, and condensation of aromatic domains into a carbonized core with few oxygen groups. Formulation with sodium alginate produced uniform films that retained strong leaf-surface fluorescence for at least 10 days outdoors, ensuring durable adhesion and UV shielding. Short-term photosynthesis measurements (0–48 h) and greenhouse growth assays with foliar exposure to pLCD and fLCD at 300 and 600 mg L−1 revealed no adverse effects on photosystem function or plant development. These findings demonstrate that lignin carbon dots, particularly fLCD-350, are durable, optically versatile, and plant-compatible nanomaterials functioning as long-lasting sunscreens and UV-protective foliar coatings that convert harmful UV into photosynthetically useful light. This journal is © The Royal Society of Chemistry, 2026