One-pot synthesis of two-sized clusters for ratiometric sensing of Hg2+

This paper presents a discussion of a one-pot approach for preparing lyszoyme type VI (Lys VI) stabilized clusters, including small (Au7Ag and Au8) and large (Au24Ag) clusters, for ratiometric fluorescence sensing of Hg2+. Our previous study (Chen and Tseng, Small 8 (2012) 1912) showed the formation of intermediate Au8 clusters in the conversion of Au+–Lys VI protein complexes to Au25 clusters. The presence of Ag+ in the precursor solution slowed this conversion, thereby forming two-sized clusters. With an increase in Ag+ content, a systematic blue shift in the first exciton absorption and fluorescence peaks indicated the formation of Au–Ag bimetallic clusters. The prepared Ag+/Au3+ molar ratio of 2:8 resulted in the formation of two-sized clusters, with dual emission bands centered at 471 and 613 nm. After these clusters are separated by a membrane filter, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to determine the composition of Au24Ag clusters. By monitoring the intensity ratio of the two emission wavelengths, the solution consisting of Hg2+-insensitive small clusters (Au7Ag and Au8) and Hg2+-sensitive Au24Ag clusters exhibited a ratiometric fluorescence response toward Hg2+, and provided a built-in correction for photobleaching; the limit of detection at a signal-to-noise ratio of three for Hg2+ was estimated to be 1 nM. This probe was successfully applied to ratiometric fluorescence sensing of Hg2+ in tap water.