Nitrogen-doped graphene quantum dots tune the mineral, acid, and sugar profile of raisins in a cultivar-specific manner.

BACKGROUND: Carbon dots are emerging as low-toxicity nano-biostimulants, but their effects on the integrated mineral, organic-acid, and sugar profiles of grape berries remain poorly characterized, especially in indigenous Vitis vinifera L. cultivars. Understanding how such treatments modulate the composition of mature berries is important for producing raisins of consistent quality. RESULTS: Nitrogen-doped graphene quantum dots (N-GQDs) (control, 0.10, 0.25, 0.50, and 1.00 g L-1) were applied as a pre-harvest foliar spray to two Anatolian V. vinifera L. cultivars, 'Ekşi Kara' and 'Gök Üzüm', and 27 response variables were quantified in the resulting raisins. Composition was altered in a cultivar- and dose-dependent manner, with a significant application effect for every variable except glucose. Most responses were biphasic: N (56.4 g kg-1 DW), Mn, Fe and the entire organic-acid block peaked at 0.50 g L-1, whereas K (35.7 g kg-1 DW), Ca (11.2 g kg-1 DW), and Cu peaked at 0.25 g L-1 in 'Gök Üzüm' and declined thereafter. Tartaric acid, the dominant acid, increased from 12.5 to 15.8 g kg-1 DW in 'Ekşi Kara' and from 13.4 to 17.8 g kg-1 DW in 'Gök Üzüm'; malic and citric acids followed the same pattern. Fructose showed the strongest sugar response (196.9-333.4 g kg-1 DW), and xylose and arabinose increased up to sixfold. CONCLUSION: A dose of 0.50 g L-1 maximized most compositional variables in both cultivars, with the response declining at 1.00 g L-1, a pattern consistent with hormesis. Yield, sensory quality, and nanoparticle residues were not assessed, so these doses are compositional optima that require validation before practical recommendation. © 2026 The Author(s). Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.