Determination of Cu2+ in drinking water using a hydroxyjulolidine-dihydroperimidine colorimetric sensor.

A new and highly efficient colorimetric Cu²⁺ chemosensor HL, synthesized by condensation between 8-hydroxyjulolidine-9-carboxaldehyde and 1,8-diaminonaphthalene, has been rationally designed and thoroughly studied. In a buffered aqueous methanol mixture, interactions between HL and Cu²⁺ produce an intense visible band at 421 nm, considerably red-shifted (~ 100 nm) from the peak maxima of HL (320 nm). Absorbance spectrophotometry experiments pointed to an exceptional 2.3 nM limit of detection (LoD) calculated from the ratiometric response upon Cu²⁺ binding. Furthermore, without the aid of instrumentation an impressive 0.5 µM LoD is possible by naked-eye observations, far below the 31.5 µM (2 mg/L) guidelines for drinking water established by the World Health Organization. Spectrophotometric pH titrations allowed the determination of the equilibrium constants and speciation plots for the formation of the various chemical species of HL in the absence and presence of Cu²⁺, with only mononuclear complexes being found. Additional studies highlighted the selectivity of HL to Cu²⁺ when in the presence of other metal ions, and a 1:1 (M:L) binding stoichiometry has also been confirmed with results from Cu²⁺ titrations, Job's plot and ESI-HRMS in good agreement. The Cu²⁺ sensing mechanism was also found to be reversible by cycling with H₂Na₂EDTA. [ABSTRACT FROM AUTHOR]