Green Synthesis of a Reversible "ON-OFF-ON" Fluorescent Sensor for Fe³⁺ using Licorice-Derived N-Doped Carbon Dots

The detection of ferric ions (Fe³⁺) is of crucial importance in environmental monitoring and biomedical diagnostics. However, developing highly selective, sensitive, and environmentally friendly detection methods remains an important challenge. In this study, nitrogen-doped carbon quantum dots (N-CQDs) were green-synthesized as a novel and sustainable fluorescent sensor for Fe³⁺ detection. The synthesis was achieved via a one-pot hydrothermal method using licorice powder as a renewable carbon source and p-phenylenediamine as a nitrogen dopant. The synthesized N-CQDs display bright blue fluorescence, with a maximum emission at 436 nm when excited at 320 nm. They serve as a highly selective and sensitive fluorescent probe for Fe³⁺, showing a distinct fluorescence "turn-OFF" response. The sensor offers a linear range of 0 to 50 µM for Fe³⁺ detection, with a calculated limit of detection (LOD) of 0.346 µM. A notable aspect of this work is the demonstration of an "ON-OFF-ON" sensing paradigm. The fluorescence quenched by Fe³⁺ can be effectively restored ("turn-ON") by adding ascorbic acid, which reduces Fe³⁺. This "ON-OFF-ON" behavior emphasizes the specificity of N-CQDs towards Fe³⁺ to Fe²⁺. The practical applicability of the sensor was confirmed through successful detection of Fe³⁺ in complex real-world samples, including beer and human blood serum, achieving excellent recovery percentages (97.6% - 101.7%) and low relative standard deviations (RSD, 0.9% - 1.8%). Overall, this research presents an environmentally friendly N-CQD-based fluorescent sensor with a unique reversible fluorescence response "ON-OFF-ON" for Fe³⁺, holding great potential applications in environmental monitoring and biomedical diagnostics.