Highly cysteine-selective fluorescent nanoprobes based on ultrabright and directly synthesized carbon quantum dots.

Strongly green fluorescent carbon dots (CQDs) have been directly synthesized from 2,4-diaminophenylhydrazine and 2-hydroxy-5-methylisophthalaldehyde through a facile solvothermal method. The novel CQDs exhibit high fluorescence quantum yield and excellent water solubility due to the abundant amino and hydroxy groups on their surface. The use of the as-prepared CQDs combined with Cu2+ constructed a "turn-on" switch cysteine-responsive nanoprobe. In the CQDs-Cu2+ assemblies, the binding of Cu2+ to CQDs results in the fluorescence quenching of CQDs by electron transfer mechanism, while the addition of cysteine leads to the fluorescence recovery because of the competitive binding between cysteine and CQDs to Cu2+. The nanoprobes showed high sensitivity to cysteine with the detection limit of 2.6 nmol L-1. The selectivity investigation results demonstrated that the Cu2+-integrated nanoparticles were highly selective toward cysteine over the other amino acids and biologically related metal ions. The proposed nanoprobe was then employed for detecting the recovery of cysteine in rabbit serum and plasma samples and imaging the cysteine in cancer cells, and the recovery was found to be 98.2-104.0%. This "synthesis-modification integration" strategy for the fabrication of CQDs may offer a new sight for the preparation of multifunctional nanostructures and broadening the application of CQDs in bioimaging. Graphical abstract Fluorescent carbon dots (CQDs) were directly synthesized from 2,4-diaminophenylhydrazine and 2-hydroxy-5-methylisophthalaldehyde. CQDs exhibit high fluorescence quantum yield and excellent water solubility due to the abundant amino and hydroxy groups on their surface. The use of CQDs combined with Cu2+ constructed a cysteine-responsive nanoprobe, which showed high sensitivity to cysteine with the detection limit of 2.6 nM.