Au-Ag Nanoclusters/3,3',5,5' Tetramethylbenzidine Complex as a Sensitive "Turn-On" Fluorescent Nanoplatform for Mercury (II) Ions Sensing.

Fluorescent bimetallic Au-Ag nanoclusters (Au-AgNCs) were found to exhibit oxidase-like activity and could catalyze the oxidation of 3,3',5,5' tetramethylbenzidine (TMB) to oxTMB. On the basis of this property, we assembled a fluorescent nanoplatform as a turn-on probe for sensing mercury (II) ions (Hg) through the inner-filter effect (IFE). Au-AgNCs and oxTMB were chosen as IFE absorber and fluorophore pair for the first time. In the absence of Hg, the Au-AgNCs absorption band well. Covered the fluorescence emission band of oxTMB, and as a result, the fluorescence of oxTMB was reduced. In the presence of Hg, Hg was reduced to Hg0 by extra BSA in Au-AgNCs probe system and anchored on the surface of Au-AgNCs. The absorption intensity for Au-AgNCs then decreased at 418 nm, resulting in the recovery of fluorescence from oxTMB. The formed Au-Hg thin amalgam layer obviously enhanced the oxidase-like activity of Au-AgNCs as well as hindered the IFE activity between Au-AgNCs and oxTMB. Therefore, based on the Hg stimulating oxidaselike properties of Au-AgNCs, a fluorometric assay for determination of Hg was developed in this study. The proposed sensing strategy showed a linear range from 10 nM to 500 nM, with ultralow LOD of ~0.7 nM for Hg. Moreover, the detection probe system was stable over a wide pH range, making it able to be applied in complex sample systems. We have successfully demonstrated the detection of Hg in tap water samples. The fluorescent assay reported here, for sensitive and selective determination of Hg, may find great application in multiple areas, such as environmental and pharmaceutical analysis.