A new turn-on fluorescence probe for Zn2+ in aqueous solution and imaging application in living cells

• A new pyrazoline-based turn-on fluorescence probe toward Zn2+ was synthesized.
• Upon addition of Zn2+, the fluorescence intensity was enhanced up to 80-fold.
• The fast response to Zn2+ makes the probe suitable to monitor Zn2+ in living cell.

We designed and synthesized a new pyrazoline-based turn-on fluorescence probe for Zn2+ 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 Zn2+ 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 Zn2+ ions. The association constant estimated by the Benesi–Hildebrand method and the detection limit were 3.92 × 103 M−1 and 5.2 × 10−7 M, respectively. A proposed binding mode was confirmed by 1H 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 Zn2+, the probe can monitor Zn2+ in living cells. Moreover, the selective binding of L and Zn2+ was reversible with the addition of EDTA in buffered EtOH/HEPES solution and Zn2+ could be imaged in SH-SY5Y neuron cells.

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