An “on-off-on” fluorescent system based on the microwave-assisted preparation of copper-functionalized carbon quantum dots for sensitive detection of ascorbic acid

Carbon dots (CDs) are a unique class of fascinating carbon-based fluorescent sensors with strong photoluminescence, chemical stability and good biocompatibility. In this work, new copper-doped carbon dots (Cu-CDs) were successfully synthesized from citric acid, ammonium tartrate and CuCl2 via a household microwave. The Cu-CDs exhibit excitation-independent emissions, high fluorescence quantum yield (~34.74%) and excellent fluorescence stability. The fluorescence of Cu-CDs can be directly quenched by ascorbic acid (AA) linearly in the range of 0.1–3.0 μM based on the inner filter effect (IFE). The limit of detection (LOD) can be determined to 27 nM. Meanwhile, an “on-off-on” sensing system based on the complex of Cu-CDs and Fe3+ is designed and served as highly specific detection probes for AA through the fluorescence resonance energy transfer (FRET) with a lower LOD of 6 nM, which is one of the best reports. The proposed methods provide obvious advantages of convenience, simplicity, rapid response, high selectivity and sensitivity. Besides, good results also have been acquired for AA detection in real medicine samples.