Citric acid-modulated carbon dots derived from black tea waste for multifunctional food packaging: anti-counterfeiting and UV-protective preservation.

The application of biomass-derived carbon dots (CDs) in smart food packaging is often limited by their low photoluminescence quantum yield. Here, we report a one-pot hydrothermal synthesis of high-performance CDs from black tea waste using citric acid as a multifunctional modulator. FTIR, XPS, zeta potential, and TEM measurements collectively reveal that citric acid enhances photoluminescence primarily through surface passivation, accompanied by the formation of smaller and more uniform carbon cores with a narrower size distribution. This synergistic modulation, dominated by surface passivation, leads to a 25.5-fold quantum yield enhancement from 0.4% to 10.2%, a narrowed emission spectrum with full width at half maximum reduced from 105.59 nm to 64.95 nm, a prolonged fluorescence lifetime from 2.86 ns to 6.93 ns, and improved photostability. Leveraging these optimized properties, the as-prepared CDs are applied in two functional systems for food applications. As a fluorescent ink, they enable covert anti-counterfeiting patterns that are invisible under ambient light but emit bright blue fluorescence under UV excitation. Additionally, a nanocomposite coating incorporating these CDs significantly extends the shelf life of strawberries at room temperature, as evidenced by reduced weight loss, maintained firmness, and preserved soluble solids content over a six-day storage period. This work presents a citric acid-mediated strategy for upcycling agricultural waste into value-added carbon nanomaterials, advancing sustainable and intelligent food packaging technologies.