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A multi-responsive sensor 1 was constructed by combining a ferrocene unit and a rhodamine block via a carbohydrazone bond. The sensor showed high selectivity toward Cu(2+) over other common metal ions in a wide pH range with excellent reversibility and rapid response. The obvious color change from colorless to pink upon the addition of Cu(2+) could make it a suitable 'naked-eye' indicator for Cu(2+). The detection limit (LOD) obtained was down to 2.0 nM and the association constant (Ka) was evaluated as 4.65 × 10(7) M(-1). The accuracy for detecting Cu(2+) in environmental river water was compared favorably with the traditional atomic absorption spectroscopy method (AAS). Finally, we proposed a reversible ring-opening mechanism (Off-On) of the rhodamine spirolactam induced by Cu(2+) binding and a 2 : 1 stoichiometric structure between 1 and Cu(2+).

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Azetidin-3-ones are important heterocycles in organic synthesis and pharmaceutical chemistry. With gold-catalyzed alkyne intermolecular oxidation, these versatile heterocycles can be prepared in three steps from readily available propargylic alcohols with high efficiencies.