Cu2+-Mediated turn-on fluorescence assay for sulfide ions using glutathione-protected gold nanoclusters: enhanced sensitivity, good reusability, and cell imaging

We develop a simple assay for highly selective and sensitive fluorescence detection of sulfide ions (S2-) using glutathione-protected gold nanoclusters (GSH-Au NCs). This approach was based on the much higher affinity of CuS than that of the Cu-GSH complex, which makes the fluorescence of the NCs quenched by Cu2+ recover upon addition of S2-. Importantly, the assistance of Cu2+ enables S2- detection in the "turn-on" mode, eventually enhancing detection sensitivity toward S2- about 15 times more compared to that in the "turn-off" mode. Under the optimal conditions, the linear fluorescence response toward S2- was given within the concentration range of 2 to 24 mu M with a low detection limit of 0.7 mu M. Also, the GSH-Au NCs show good reusability for S2- detection due to effective removal of the bridging reagent Cu2+ by S2-. This phenomenon is further exploited as an integrated logic gate and successfully employed for imaging S2- in living cells.