Facile preparation of N, S co-doped carbon dots and their application to a novel off-on fluorescent probe for selective determination of Hg²⁺

In this work, citric acid (CA) and glutathione (GSH) were selected as the raw materials for preparation carbon dote co-doped with N and S (NS-CDs) via microwave heating method. The obtained NS-CDs displayed stable and strong blue luminescence with a maximum emission (λem) wavelength of 442 nm and a maximum excitation (λex) wavelength of 371 nm, and their fluorescence quantum yield (FLQY) was 10.9%. The fluorescence of NS-CDs was quenched by silver nanoparticles (AgNPs) through a combination of static quenching effect and inner filter effect (IFE), and could be recovered by Hg2+ even at nanomolar levels ascribing to redox reaction between Hg2+ and AgNPs. Based upon this phenomenon, a novel off-on fluorescent probe (NS-CDs - AgNPs) for determination trace Hg2+ in aqueous solution was constructed using the obtained NS-CDs and AgNPs. Under optimal conditions, NS-CDs - AgNPs exhibited a wide linearity range of 10–90 nM (R2 = 0.993) for Hg2+and a low LOD of 4.2 nM. NS-CDs - AgNPs was successfully utilized for monitoring Hg2+ in real water samples with the recoveries of 91.4–108.6% (RSD <5.84%), which indicated that the fluorescent probe has enormous application potential to detect trace Hg2+ in real water samples.