Highly orange fluorescence emission by water soluble gold nanoclusters for turn off sensing of Hg2+ ion

Herein, a simple, facile and biocompatible fluorescent sensor using L-cysteine assisted chitosan reduced gold nanocluster (L-cys/CS-AuNC) is prepared by one pot synthetic protocol for ultrasensitive detection of Hg2+ ion which produces a strong orange fluorescence emission at 575 nm. The quantum yield of L-cys/CS-AuNC is calculated to be 5.2%. The surface functionality and morphology of nanocomposites are studied by Fourier transform infrared spectroscopy and transmission electron microscopy respectively. Zeta potential measurement shows the stability of as-synthesized L-cys/CS-AuNC. With addition of Hg2+ ion to the gold nanoclusters, the orange fluorescence emission is quenched which is due to the strong metallophilic interactions i.e. interaction of 5d(10) of Hg2+ and 5d(10) of Au+ on the gold surface. High selectivity and sensitivity for sensing of Hg2+ are achieved through present fluorescent assay and a linear correlation is obtained with correlation coefficient of 0.99 by plotting the graph between the relative fluorescent intensity and concentration of Hg2+ ion (0-1000 nM). Further, the detection limit (LOD) for mercury (II) is found to be 3 nM. Moreover, the L-cys/CS-AuNC can be applied to real water samples such as pond, river and tap water for detection of Hg2+ ion.