Spectral down-conversion of carbon quantum dot-PVA films for photosynthesis and growth augmentation of Brassica juncea in indoor hydroponic systems

In controlled-environment agriculture, there is a limitation of spectral mismatch between incident solar radiation and the photosynthetically active radiation (PAR). This study employed solid Stokes shift down-conversion in carbon quantum dots (CQD) incorporated in polyvinyl alcohol (PVA) films to augment PAR delivery for indoor hydroponic Brassica juncea cultivation. The CQD of different sizes obtained commercially (CQD-4 and CQD-9), were incorporated into PVA matrices, and their structure-photophysics-performance relationships were systematically studied. CQD-4 were characterized by superior graphitic ordering (89.5% sp² carbon) and excitation-independent photoluminescence. Meanwhile, CQD-9 exhibited a defect-rich carbon matrix (80.2% sp² carbon) with excitation-dependent emission. Well-defined structured CQD-4 Stokes shift emission with a greater contribution from core states that translated into efficient and concentration-robust spectral conversion. In contrast, a multiple state emission by CQD-9 demonstrated a defect-mediated Stokes shift performance. CQD-4/PVA recorded higher PPFD (278.50 μmol m^-2 s^-1 at 50% v/v) over CQD-9/PVA (266.80 μmol m^-2 s^-1 at 30% v/v), despite comparable apparent optical absorption-edge energies (E_g) of 5.20 eV and 5.30 eV, due to a well-defined Stokes shift that minimized reabsorption losses and maintained conversion efficiency albeit at higher film concentrations. Further assessment of CQD/PVA film's down-conversion efficiency was conducted by wrapping the films around the LED-light sources in an indoor hydroponic system to evaluate the effects on cultivating B. juncea over a five-week cultivation period. Over the cultivation period, B. juncea grown under CQD-4/PVA 50% v/v demonstrated significant growth improvements relative to controls, by 50% increase in fresh biomass, 78% in leaf area, 38% in leaf number, and 46% in plant height, better than CQD-9/PVA treatments across all parameters. The photosynthesis light utilization efficiency was confirmed by physiological properties and validated by gas-exchange analysis and intrinsic water use efficiency (iWUE). These results establish that CQD structure affects the Stokes shift efficiency for the spectral conversion performance in CQD/polymer films.