A colorimetric sensor for Cu²⁺ in aqueous solution based on metal ion-induced deprotonation: Deprotonation/protonation mediated by Cu ²⁺-ligand interactions

A Cu²⁺-specific colorimetric sensor 3 of a 1,8-naphthalimide chromophore integrated with a 2-aminodiphenylamine (2-adpa)-based receptor was designed and synthesized. Sensor 3 had high selectivity and sensitivity for Cu²⁺ in neutral aqueous buffer solution. The analytical detection limit (ADL) was 3.0 × 10^-7 M. Based on the Cu ²⁺-induced deprotonation of the diarylamino "NH" moiety, the coordination mode was proposed with 1:1 stoichiometry between 3 and Cu ²⁺. These results opened up possibilities for the construction of novel colorimetric sensors with a red-shift absorption in a D-π-A system.