Synthesis of hemicellulose/deep eutectic solvent based carbon quantum dots for ultrasensitive detection of Ag and L-cysteine with "off-on" pattern.

The hypersensitive detection of metal ions and amino acid becomes more important for environmental and human physical conditions monitor but remains still a great challenge. In this work, we have reported a green and straight forward procedure (solvothermal treatment) for the synthesis of N-doped hemicellulose-based carbon quantum dots (N-H-CQDs) used as an "off-on" fluorescence sensor for ultrasensitive detecting Ag ion and L-Cysteine (L-Cys). During this process, hemicellulose used as carbon source and nitrogen-rich deep eutectic solvent (DES) of choline chloride/urea acted as both solvent and doping agent. The prepared N-H-CQDs displayed narrow diameter ranging from 2 to 8 nm with relatively higher quantum yield (QY) of 23.45%. This probe was able to selectively detect Ag ion on account of the formation of N-H-CQDs- Ag complex and possessed extremely low detection limit of 21 nM. Additionally, the fluorescence of N-H-CQDs-Ag complex was restored by the addition of L-Cysteine as Ag ion would assemble a more stable Ag-L-Cysteine complex through Ag-thiol bonds. Accordingly, the N-H-CQDs-Ag solution behaved as secondary fluorescent probe to detect L-Cys in the dynamic range of 0-100 μM with a LOD of 242 nM. The prepared N-H-CQDs are promising probes for hypersensitive detection of Ag and L-Cys.