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

Abstract: A new and highly efficient colorimetric Cu ²⁺ 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 Cu ²⁺ 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 Cu ²⁺ 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 Cu ²⁺, with only mononuclear complexes being found. Additional studies highlighted the selectivity of HL to Cu ²⁺ when in the presence of other metal ions, and a 1:1 (M:L) binding stoichiometry has also been confirmed with results from Cu ²⁺ titrations, Job’s plot and ESI-HRMS in good agreement. The Cu ²⁺ sensing mechanism was also found to be reversible by cycling with H ₂Na ₂EDTA. Graphical abstract: [Figure not available: see fulltext.].