Selective turn-on fluorescence for Zn(2+) and Zn(2+)+Cd(2+) metal ions by single Schiff base chemosensor

Chemosensor based on Schiff base molecules (1, 2) were synthesized and demonstrated the selective fluoro/colorimetric sensing of multiple metal ions (Mn(2+), Zn(2+) and Cd(2+)) in acetonitrile-aqueous solution. Both 1 and 2 showed a highly selective naked-eye detectable colorimetric change for Mn(2+) ions at 10(-7) M. Fluorescence sensing studies of 1 and 2 exhibited a strong fluorescence enhancement (36 fold) selectively upon addition of Zn(2+) (10(-7) M, λmax=488 nm). Fluorescence titration and single crystal X-ray analysis confirmed the formation of 1:1 molecular coordination complex between 1 and Zn(2+). Interestingly, a rare phenomenon of strong second turn-on fluorescence (190 fold, λmax=466 nm) was observed by the addition of Cd(2+) (10(-7) M) into 1+Zn(2+) or Zn(2+) (10(-7) M) into 1+Cd(2+). Importantly both 1 and 2 exhibited different fluorescence λmax with clearly distinguishable color for both Zn(2+) and Cd(2+).