▎ 摘　　要

Folic acid (FA or vitamin B-9) and Methyl Cobalamin (MeCob or vitamin B-12) are vital controllers of homocysteine metabolism. Increased intake of FA and vitamin B-12 plays a crucial role in the prevention of cardiovascular disease and the reduction of mortality by lowering serum homocysteine levels. Therefore, fluorescent probes for monitoring FA and MeCob level alternation are crucial and greatly preferred. Reported herein is a new fluorescent probe, 1,8-diaminonaphthalene (DAN) functionalized graphene quantum dots (DAN-GQD), which sensitively respond to FA and MeCob at a very low acceptable concentration in H2O. The sensitive detection behavior of DAN-GQD towards FA and MeCob was examined by absorption and fluorescence techniques. The addition of FA (100 x 10(-12) M) and MeCob (25 x 10(-9) M) gradually increases the absorption intensity of DANGQD (5 x 10(-9) M) owing to static interaction and dynamic equilibrium, respectively. The fluorescence intensity of aqueous solution of DAN-GQD (5 x 10(-9) M) shows ca. 2-fold quenching upon the addition of FA (Phi(F)0.54 -> 0.31) and MeCob (Phi(F)0.54 -> 0.30), which is mainly due to inner filter effect (IFE). Job's plot analysis divulges that DAN-GQD bind with FA in 1:2 and with MeCob in 1:1 stoichiometry. DAN-GQD showed an excellent sensitivity towards FA and MeCob with a limit of detection (LoD; (S/N = 3)) of 1.73 x 10(-15) M and 6.37 x 10(-12) M, respectively. The time-resolved fluorescence decay profile confirms that quenching of DAN-GQD upon the addition of FA and MeCob follows non-radiative relaxation of excited electrons. The morphology change, interference studies, the effect of time on FA and MeCob detection of DAN-GQD, and the stability of DAN-GQD center dot FA and DAN-GQD center dot MeCob complexes were also investigated.