A fluorescent probe for copper and hypochlorite based on rhodamine hydrazide framework

A rhodamine derivative (1) was synthesized as a fluorescence turn-on probe for copper (Cu²⁺) and hypochlorite (ClO⁻). The probe gave a fluorescence turn-on change at 587 nm with a color change from colorless to pink in the presence of Cu²⁺ or ClO⁻ ions in aqueous solution. It was found that the Cu²⁺ ion bound to 1 in a 1:1 stoichiometry and induced a spirolactam ring opening of the rhodamine moiety leading to a fluorescence turn-on, confirmed by Job's plot, ESI-Mass, and ¹H NMR analyses. In the presence of ClO⁻ ion, probe 1 underwent a hypochlorite-mediated oxidation and hydrolysis to produce a ring-opened rhodamine B with a fluorescence enhancement. However, these changes were not monitored in case of other metal ions, anions, and reactive redox species. In addition, probe 1 can readily react with the ClO⁻ to provide a distinct fluorescence enhancement along with a pink color even in the presence of various competitive species.