Preparation of tryptophan-derived chiral carbon quantum dots by hydrothermal carbonization and their chiral recognition properties

The key to preparing chiral carbon quantum dots (CCQDs) is to preserve the properties of carbon quantum dots while endowing them with chiral characteristics. Herein, chiral carbon quantum dots (L-/D-NCQDs) were synthesized by hydrothermal carbonization method using glucosamine (N-Glu) as the carbon source and tryptophan enantiomer (L-/D-Trp) as the chiral source, emitting bright blue fluorescence under UV lamp. It is showed that L-/D-NCQDs have an average diameter of 4.0−4.5 nm, a maximum excitation wavelength of 310 and 368 nm, and a maximum emission wavelength of 450 nm. Circular dichroism spectra confirms that L-NCQDs and D-NCQDs are pairs of enantiomers with opposite and symmetric chiral signals at 220, 280 and 350 nm. The stronger chiral signal at 220 nm was attributed to the retention of surface chiral residues, and the chiral centers at 280 and 350 nm were attributed to the aggregates of chiral molecules and optical transitions in carbon nucleus. L-NCQDs exhibited excellent chiral recognition for L-/D-asparagine (L-/D-Asn) and L-/D-glutamine (L-/D-Gln). The fluorescence intensity of L-NCQDs had no observable change in the presence of left-handed enantiomers, but enhanced obviously in the presence of right-handed enantiomers. This method displayed a linear range of 0–6.25 mM with the detection limit of 0.06 and 0.29 mM. The enantioselectivity (ID−I0)/(IL−I0) can reach up to 8.7 and 4.0, respectively.