Tomato-derived carbon quantum dots: composition-driven structure and copper sensing potential

Carbon quantum dots (CQDs) were synthesized via a green, bottom-up approach from tomato biomass, using stems and fruits as distinct carbon precursors. To tailor their surface chemistry and optical response, m-aminobenzoic acid was used as a nitrogen-containing dopant during synthesis. The resulting CQDs form colloidally stable, nanoscale aggregates in aqueous media and exhibit strong excitation-dependent photoluminescence, with maximum emission under excitation at 300 and 370 nm. Comprehensive structural and surface analyses reveal that precursor origin significantly influences defect density, hybridization states, and functional group distribution, directly governing luminescent behavior. Both fruit-derived and stem-derived CQDs display efficient fluorescence quenching upon coordination with Cu²⁺ ions, enabling sensitive detection with limits of detection of 0.059 µM and 0.048 µM, respectively. The enhanced sensing performance is attributed to surface-accessible oxygen- and nitrogen-containing functional groups that promote strong metal-ligand interactions. This work demonstrates a sustainable and low-cost strategy for converting agricultural waste into functional nanomaterials, while highlighting the critical role of biomass composition and surface chemistry in tuning CQD photophysics and metal-ion sensing capabilities.