Ionothermal synthesis of carbon dots from cellulose in deep eutectic solvent: A sensitive probe for detecting Cu2+ and glutathione with “off-on” pattern

Ultrasensitive and stable carbon dots with “off–on” pattern were prepared from microcrystalline cellulose by one-step ionothermal method in deep eutectic solvents. • Carbon dots (CDs) were prepared from microcrystalline cellulose by one-step ionothermal method in deep eutectic solvents (DESs). • The urea and sulfamic acid-based DES acted both solvent and dopant for making CDs with the best fluorescence intensity. • CDs acted as a sensitive probe for detecting Cu 2+ and glutathione with “Off-on” pattern. • The fluorescence intensity of CDs can still retain 85% after 50 days. Highly sensitive detection of metal ions and biothiols is very important for environment-friendly and human health but under great challenges of high-cost and complex experimental operation. In this work, we reported a simple and effective ionothermal method for the synthesis of nitrogen and sulfur doped carbon dots (N, S-CDs) from cellulose, which can be used as an on–off fluorescent sensor for the sensitive detection of copper ions (Cu 2+ ) and glutathione (GSH). In this process, cellulose was used as a renewable carbon source, and deep eutectic solvent (DES) prepared by urea/sulfamic acid was used as a green solvent and nitrogen sulfur dopant. The as-prepared CDs have excellent optical properties, pH resistance, excellent photostability and hydrophilicity, with a quantum yield (QY) of 7.17% and the fluorescence intensity of the CDs still reached 85% of the initial intensity after 50 days. The fluorescent probe can selectively detect Cu 2+ with a high sensitivity of 23.4 nM. In addition, the addition of GSH can restore the fluorescence of CDs by forming a more stable Cu 2+ -GSH complex. Therefore, the CDs-Cu 2+ complex can be used as a secondary sensor with a detection range of 5.98 μM.