A turn-on fluorescent rhodamine-acyl hydrazide for selective detection of Cu²⁺ ions

We present a rhodamine-based probe that can detect Cu²⁺ ions via Cu²⁺-promoted hydrolysis in aqueous solutions. In solution, the probe is non-fluorescent and colorless, but gives a strong fluorescence at 586 nm and shows a color change to pink on the addition of Cu²⁺ ions. We demonstrate that the rhodamine-based probe undergoes Cu²⁺-promoted hydrolysis with a moderate reaction time (within 3 min) and provides a fluorescence off–on change, even in the presence of other competitive metal ions. The rhodamine-based probe shows a linear correlation between increasing fluorescence at 586 nm and the Cu²⁺ concentration, and can detect Cu²⁺ at nanomolar levels (10 nM) in CH₃CN:H₂O (v/v, 90:10)