Green Synthesis of Fluorescent and Bioactive Carbon Dots Derived from Agri-Food Industry Waste

Fluorescent nitrogen-doped carbon dots (TRCDs) were synthesized from tomato residues (TR) by hydrothermal carbonization, and the effects of parameters like temperature, reaction time, nitrogen precursor, and ethylenediamine/tomato residue (EDA/TR) mass ratio on their photophysical and biological properties were investigated. The optimal synthesis conditions were achieved at 250 °C for 6 h using an EDA/TR mass ratio of 0.08, yielding an aqueous carbon dot fraction in 17.6% (w/w). TRCDs exhibit a strong blue fluorescence, with a fluorescence quantum yield of 0.17, at an emission wavelength of 460 nm. AFM and TEM analyses showed well-dispersed quasi-spherical nanoparticles with dimensions characteristic of carbon dots. The synthesized TRCDs, produced with different additives, demonstrate antioxidant activity in both ABTS•+ and DPPH• assays, reaching values of up to 969 and 75 µg Trolox equivalents (TE)/mg TRCDs, respectively. Cytotoxicity studies indicate cell viability in above 80% up to 500 μg/mL in all tested cell lines and above 90% in VERO cells at concentrations up to 2000 μg/mL, while fluorescence microscopy revealed cell-associated fluorescence following TRCD exposure, supporting their potential application as fluorescent cellular markers.