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

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 (Cu²⁺) 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 Cu²⁺ 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 Cu²⁺-GSH complex. Therefore, the CDs-Cu²⁺ complex can be used as a secondary sensor with a detection range of 5.98 μM.