11-Mercaptoundecanoic acid directed one-pot synthesis of water-soluble fluorescent gold nanoclusters and their use as probes for sensitive and selective detection of Cr3+ and Cr6+

We report a one-pot approach to prepare fluorescent gold nanoclusters (AuNCs) from HAuCl4 by simply using 11-mercaptoundecanoic acid (11-MUA) as a reducing and capping agent, in an aqueous solution of NaOH at room temperature. The as-prepared water-soluble AuNCs with average diameters of 1.8 +/- 0.4 nm exhibit a unique fluorescence excitation at 285 nm, a maximum emission at 608 nm, and a quantum yield of 2.4%. We find that the fluorescence of 11-MUA-AuNCs can be quenched by several metal ions but selectively by Cr3+ ions when using EDTA as the masking agents for other metal ions. This phenomenon is further exploited as a "turn-off" fluorescent sensor for sensitive and selective detection of Cr3+ ions. Upon the quantitative addition of Cr3+ in the presence of EDTA, the fluorescence intensity quenches linearly within the range of 25 nM to 10 mu M with high sensitivity (LOD = 26 nM, S/N = 3). Furthermore, the 11-MUA-AuNCs could be used to detect Cr6+ indirectly by using ascorbic acid to reduce Cr6+ into Cr3+ in aqueous solution