Lignin-based fluorescence-switchable graphene quantum dots for Fe³⁺ and ascorbic acid detection

The synthesis of lignin-based graphene quantum dots (GQDs) with excellent fluorescence stability, quantum yield, and biocompatibility for sensitive and selective detection of Fe³⁺ and ascorbic acid (AA) has remained a challenging endeavor. Using an acidolysis process with 17.5% nitric acid followed by hydrothermal treatment at 200 °C, this study provided an improved synthesis route for the production of high-quality GQDs from alkali lignin. The nitrogen-doped GQDs exhibit remarkable fluorescence stability under a wide range of pH (3–10), duration (1–12 h), and [NaCl] (0–1000 mM) conditions, and have a high quantum yield of 28%. The GQDs or GQDs/Fe³⁺ sensing systems ([GQDs] at 50 mg L⁻¹, [Fe³⁺] at 500 μmol L⁻¹, and UV excitation at 370 nm) for fluorescence sensing of Fe³⁺ or AA have excellent sensitivity, selectivity, and reproducibility. For Fe³⁺ and AA, the limit of detection is 1.49 and 1.62 μmol L⁻¹, respectively. Mechanism investigation shows that photoluminescence quenching is caused by the formation of GQDs-Fe³⁺ complexes, whereas fluorescence recovery is due to Fe³⁺ reduction by AA.