Determination of trace amount of Cu2+ with a multi-responsive colorimetric and reversible chemosensor

Abstract

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 Cu²⁺ 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 Cu²⁺ could make it a suitable ‘naked-eye’ indicator for Cu²⁺. The detection limit (LOD) obtained was down to 2.0 nM and the association constant (K_a) was evaluated as 4.65 × 10⁷ M⁻¹. The accuracy for detecting Cu²⁺ 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 Cu²⁺ binding and a 2 : 1 stoichiometric structure between 1 and Cu²⁺.