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 (Hg2+) 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 Hg2+, 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 Hg2+, Hg2+ was reduced to Hg-0 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 Hg2+ stimulating oxidase-like properties of Au-AgNCs, a fluorometric assay for determination of Hg2+ was developed in this study. The proposed sensing strategy showed a linear range from 10 nM to 500 nM, with ultralow LOD of similar to 0.7 nM for Hg2+. 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 Hg2+ in tap water samples. The fluorescent assay reported here, for sensitive and selective determination of Hg2+, may find great application in multiple areas, such as environmental and pharmaceutical analysis.