A novel rhodamine-based Hg2+ sensor with a simple structure and fine performance. 
An excellent spectral sensor for Hg2+ named 2-(2-((2-aminoethyl)thio)ethyl)-3',6'-bis(diethylamino)spiro[isoindoline-1,9'-xanthen]-3-one (RMTE) was achieved by a one-step reaction between rhodamine B and thiobisethylamine.  RMTE could detect Hg2+ in nearly pure water reversibly and highly selectively, indicated by a new increasing absorption peak at 561 nm and 170-fold enhanced fluorescence at 578 nm coupled with remarkable visual and fluorescence color changes.  When the Hg2+ concentration ([Hg2+]) varied from 0 to 120 μM, the absorbance and fluorescence intensity of the RMTE solution responded linearly to [Hg2+] with the detection limits of 2.08 and 0.14 μM, respectively.  RMTE could work in ecologically and biologically favorable pH values of 6.41-8.33.  The binding mode of RMTE toward Hg2+ was 1 : 1.  RMTE could monitor Hg2+ in environmental water and living cells effectively with low cytotoxicity.