An “off–on” fluorescent and colorimetric probe bearing fluorescein moiety for Mg2 + and Ca2 + via a controlled supramolecular approach

• A new kind of fluorescein-based ion sensor was synthesized.
• The sensor could be a good probe for Ca2 + and Mg2 + determination.
• Two-step binding events were observed in the sensing process.

In the present work, a new kind of fluorescein-based chemosensor L was designed and synthesized to selectively recognize Ca 2+ or Mg 2+ over other competing ions. The chemosensor showed “off–on” fluorescent and color changes upon the addition of Ca2 + and Mg2 +. The dynamic binding events with the formation of 1:1(L/M) and 1:2(L/M) complexes were examined. The cation-driven conformation changes of L were understood and proposed rationally by the UV–vis, FL, and 1HNMR titrations. By this allosteric effect, Ca2 + and Mg2 + could be selectively recognized with the 1:2 stoichiometry by fluorescent changes, which were different from other known reports on chemosensors that the host-cation complexion was exploited for controlling chromophore interaction as the model of signaling.

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