Preparation of D-NCCDs and its application in fluorescent/colorimetric dual-mode discrimination of glutamine enantiomers

A new type of carbon dots (D-NCCDs) was synthesized by 3, 5-diaminobenzoic acid, N,N-dimethyl-o-phenylenediamine, and D-cysteine. The morphology and structure of D-NCCDs were investigated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and FT-IR spectra, and the chirality was characterized by circular dichroism. In the presence of hydrogen peroxide, the fluorescence of D-NCCDs at 487 nm (λ_ex = 410 nm) showed great discrimination ability towards glutamine enantiomers. The ratio of fluorescence intensity (F/F₀) to the concentration of D-Gln showed good linearity in the range 0.5-10 mM, with a detection limits of 0.11 mM. Meanwhile, the color of the solution gradually changed from light yellow to yellow-brown. The UV-Vis absorption ratio (A/A₀) at 410 nm showed good linearity with the concentration of D-Gln in the range 0.5 to 20 mM; the detection limit is 7.7 μM. But the fluorescence and absorbance of D-NCCDs showed no significant change after the addition of L-glutamine. Thus, fluorescence and colorimetry dual-mode discrimination of glutamine enantiomers was achieved. The fluorescence enantioselectivity of Gln (F_L-Gln/F_D-Gln) is 1.62, and the colorimetric enantioselectivity of Gln (A_D-Gln/A_L-Gln) is 2.14. The chiral discrimination mechanism of D-NCCDs to Gln enantiomers was also investigated systematically. This work not only can discriminate glutamine enantiomers with high sensitivity and convenience, but also offers a new strategy for preparing new dual mode chiral nanoprobes.