A novel dual-excitation and dual-emission fluorescent probe (CQDs-O-NBD) based on carbon quantum dots for detection and discrimination of Cys/Hcy and GSH/H2S in living cells

Cysteine (Cys), homocysteine (Hcy), glutathione (GSH) and hydrogen sulfide (H2S) are essential active substance in human body. The identification and detection of biothiols and H2S are of great significance for the early diagnosis of certain diseases. In this paper, a new dual excitation and dual emission fluorescent probe, carbon quantum dot-O-7-nitrobenzo-2-oxa-1,3-diazole (CQDs-O-NBD), for the detection of biothiols and H2S was synthesized. The carbon quantum dots (CQDs-OH) (lambda ex/em maxima = 377 nm/467 nm) were synthesized by a microwave-assisted method using hydroquinone as the carbon source. Through the reaction of the phenolic hydroxyl group of the carbon quantum dots and 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl), the CQDs-ONBD was synthesized. Due to the static quenching (SQ) and the photo-induced electron transfer (PET) from CQDs to NBD, the fluorescence of the CQDs-O-NBD is very weak. After adding biothiols and H2S, the ether bond of the CQDs-O-NBD was broken through nucleophilic substitution reaction, which led to the release of the CQDs-OH (channel 1 is turned on) and generation of a new compound NBD-thiols (channel 2). The CQDs-O-NBD as a probe shows the high selectivity and sensitivity for Cys/Hcy/GSH/NaHS (LOD of 0.24 mu M/0.21 mu M/0.19 mu M/ 0.18 mu M) over other aminos and ions by channel 1. The new formed compounds from NBD and Cys/Hcy have strong fluorescence under 470 nm wavelength light excitation (channel 2 is turned on), while the formed compounds from NBD and GSH/NaHS have no fluorescence (no channel 2), so the CQDs-O-NBD can discriminate Cys/Hcy (LOD: 0.07 mu M/0.06 mu M) from GSH/NaHS. In addition, the CQDs-O-NBD shows low toxicity and can be applied to the detection of Cys in fetal bovine serum and imaging in living cells.