A Fluorescence Turn-On Probe for Thiols with a Tunable Dynamic Range.

We designed and synthesized a novel probe Cou-Rho-SA-Cu (II) for detection of thiols based on a coumarin-rhodamine FRET system. The compound Cou-Rho-SA exhibited strong blue fluorescence as the rhodamine moiety of the compound Cou-Rho-SA was in a ring-closing form and no FRET process occurred. The probe Cou-Rho-SA-Cu (II) was prepared by addition of Cu(2+) to the solution of Cou-Rho-SA. In the absence of GSH, the complex Cou-Rho-SA-Cu (II) showed almost no fluorescence owing to the FRET process and the paramagnetic nature of Cu(2+) . However, upon addition of GSH, the rhodamine moiety of the probe Cou-Rho-SA-Cu (II) recovered to its ring-closing form, and the FRET process was prohibited to show the blue fluorescence again. Moreover, via tuning the Cu(2+) concentrations, the probe could detect the different levels of GSH.