This study elucidates a sustainable nanobiotechnological intervention using Carbon Quantum Dots (CQDs)
synthesized from the red macroalga Gracilaria corticata via a facile one-step hydrothermal carbonization method.
The synthesized CQDs, optimized using response surface methodology, exhibited a mean particle size of 2.65 ±
0.6 nm and distinct optical properties, characterized by strong UV absorption (~283 nm) and blue-violet fluorescence (~425 nm). This Stokes shift enables the CQDs to function as light-harvesting nanoantennae, converting
photosynthetically inactive UV radiation into useful visible light. In a controlled hydroponic trial with basil
(Ocimum basilicum), the biological impact of these nanomaterials was evaluated across a concentration gradient
(0–75 mg.L− 1
) via foliar and root application pathways. Foliar application at 50 mg L
− 1 emerged as the optimal
biostimulation strategy, eliciting a profound 44.65% increase in net photosynthetic rate (A) and a ~30% gain in
total biomass compared to the controls. Physiological profiling revealed that this enhancement was driven by a
synergistic mechanism involving accelerated electron transport, upregulated chlorophyll biosynthesis, and
optimized stomatal conductance, without compromising water use efficiency. Conversely, high-concentration
root applications (>50 mg.L− 1
) induced non-stomatal limitations to photosynthesis, highlighting the critical
importance of the application interface. These findings establish red algae-derived CQDs as potent, multifunctional agents capable of simultaneously expanding the light-harvesting capacity of crops and modulating
metabolic flows, offering a scalable, green solution for high-value industrial crop production.