A new turn-on fluorescence probe for Zn(2+) in aqueous solution and imaging application in living cells

We designed and synthesized a new pyrazoline-based turn-on fluorescence probe for Zn(2+) by the reaction of chalcone and thiosemicarbazide. The structure of the probe was characterized by IR, NMR and HRMS spectroscopy. The probe (L) exhibits high selectivity and sensitivity for detecting Zn(2+) in buffered EtOH/HEPES solution (EtOH/HEPES=1/1, pH 7.2) with 80-fold fluorescence enhancement, which is superior to previous reports. Job's plot analysis revealed 1:1 stoichiometry between probe L and Zn(2+) ions. The association constant estimated by the Benesi-Hildebrand method and the detection limit were 3.92×10(3)M(-1) and 5.2×10(-7)M, respectively. A proposed binding mode was confirmed by (1)H NMR titration experiments and density functional theory (DFT) calculations. The probe is cell-permeable and stable at the physiological pH range in biological systems. Because of its fast response to Zn(2+), the probe can monitor Zn(2+) in living cells. Moreover, the selective binding of L and Zn(2+) was reversible with the addition of EDTA in buffered EtOH/HEPES solution and Zn(2+) could be imaged in SH-SY5Y neuron cells.