Carbon Dots Derived from Polyphenols by Hydrothermal Carbonization: Spectral, Antioxidant, and Antimicrobial Properties and Cytotoxicity Assessment

Carbon dots (CDs) represent an emerging class of bioactive nanomaterials with significant potential in material science and food preservation. This study systematically investigated the structural, colloidal, and interfacial properties of polyphenol–derived CDs synthesized via hydrothermal carbonization using gallic acid (GA), epigallocatechin gallate (EG), caffeic acid (CA), and quercetin (QC). Comprehensive characterization revealed blue–fluorescent CDs with negative surface charge (− 5.81 to − 11.51 mV) and distinct optical properties (emission: 310–380 nm; bandgap: 4.98–5.74 eV). Spectroscopic analyses (FTIR, ¹³C–NMR, ¹H–NMR) demonstrated preservation of aromatic cores with surface–functionalized polar groups (–OH, C = O), while transmission electron microscopy confirmed spherical morphology (4–8 nm diameter). Antimicrobial assays demonstrated concentration–dependent activity, in which EG^CD exhibited the highest antibacterial inhibition zones (30.5 ± 0.8 mm against E. coli at 3000 ppm) and CA^CD showed potent antifungal effects (85% reduction in A. flavus growth at 3000 ppm). Antioxidant assays revealed the superior radical scavenging activities by EG^CD (ABTS–RSA: 1.1 mmol Trolox eq/mg), and QC^CD (DPPH–RSA: 0.7 mmol Trolox eq/mg) at a concentration of 40 ppm. Cytotoxicity assessment on human dermal cells (HDFa, BJ) indicated biocompatibility at low concentrations (≥ 80% viability at 25 ppm) but dose–dependent toxicity was observed (≥ 50% viability at 200 ppm after 48 h). These findings highlight the dual functionality of polyphenol–derived CDs as antimicrobial/antioxidant agents with tunable biocompatibility for food and biomedical applications. These findings emphasize the structural, colloidal, and interfacial chemistry of CDs, reinforcing their relevance as sustainable biomaterials for material technology, and food applications.