Blue-emitting carbon quantum dots: Ultrafast microwave synthesis, purification and strong fluorescence in organic solvents

The impurity of unpurified carbon quantum dots (CQDs) severely limits their application. In this paper, CQDs synthesized by ultrafast microwave hydrothermal method from L-cystine were purified by both dialysis and direct phase-transfer. The as-obtained CQDs in basic aqueous reaction mixture show excitation-independent fluorescence around 505 nm with a maximum excitation at 421 nm and pH-responsive emission intensity. However, after purification by dialysis the CQDs display excitation-dependent blue fluorescence from 410 to 500 nm with respect to the excitation from 310 to 430 nm. Particularly, the CQDs purified by direct phase-transfer into various organic solvents show two strong blue fluorescence peaks respectively focusing on 422–434 nm and 400–410 nm with a maximum excitation at 370–372 nm. The quantum yield of the CQDs decreases to 45.1% after a purification by dialysis from the original 82.7%, while it decreases to 57.5% after a purification of phase-transfer. The CQDs extracted in organic solvents are demonstrated to be pure small crystalline-carbon nanoparticles with chemically surface-grafted groups, while the dialyzed CQDs will physically adsorb additional L-cystine, sodium and by-product nitrate ions. The difference of surface state and solvent environment of these CQDs shall contribute to the different fluorescence property. Such strong luminescent, pure CQDs in organic solvents should have obvious advantages in further applications.