In situ coffee ground-derived carbon dots as a sustainable antenna for photosynthesis enhancement: Experimental and DFT mechanistic insights

Developing sustainable, scalable, and safe nanomaterial-based strategies to enhance crop photosynthesis under environmental stress is critical for improving agricultural productivity. However, most reported nanomaterials are derived from synthetic precursors or require additives, leaving a gap for simple, additive-free, waste-derived systems with mechanistic clarity. Here, a green strategy for the synthesis of fluorescent carbon dots (CDs) from coffee-derived materials is presented, and their photophysical, physiological, and mechanistic roles are systematically evaluated. The roasting history strongly influenced in situ CD formation, with darker roasts producing larger and more emissive particles. Mixed post-consumer coffee grounds yielded optically robust and colloidally stable CDs at practical levels, validating a scalable waste-to-nanomaterial route. Applied as foliar sprays to Thai jasmine rice (Oryza sativa L. cv. KDML105), CDs enhanced photosynthesis in a dose-dependent manner, with 250 µg·mL⁻¹ identified as optimal. This treatment increased the net photosynthetic rate by 32.66% at 15 days post treatment (DPT) and 26.98% at 25 DPT, doubled soluble sugars (100.18%), and improved shoot biomass, while stomatal conductance and transpiration were unchanged and water-use efficiency improved. Pigment analysis showed that balanced enrichment of chlorophyll b and carotenoids, alongside moderate chlorophyll a, underpinned these gains. Experimentally, CD fluorescence (440–460 nm) overlapped strongly with chlorophyll absorption, particularly chlorophyll b, providing evidence for Förster resonance energy transfer (FRET). Computational analyses (time-dependent density functional theory (TD-DFT) and transition density matrix (TDM)) further revealed favorable energy-level alignment and electron density redistribution, indicating that electron transfer from the CD core to chlorophylls also contributes. This dual mechanism, FRET and electron transfer, creates a synergistic pathway that strengthens photosynthetic electron transport. Together, experiment and computation establish in situ coffee-derived CDs as artificial antennae that improve photosynthesis through sustainable and scalable waste-to-resource conversion.