Aggregation-induced Emission Active Naphthalimide Derived Schiff Base for Detecting Cu²⁺ and Its Applications

Herein, an aggregation-induced emission (AIE) active Schiff base (NHS) was synthesized by condensing naphthalimide hydrazide with salicylaldehyde. The non-fluorescent solution of NHS in DMSO turned to emissive NHS upon increasing the HEPES fraction in DMSO from 70 to 95%. The UV-Vis absorption and DLS studies supported the self-aggregation of NHS that restricted the intramolecular rotation and activated the ESIPT process. The blue fluorescence of AIE luminogen NHS in DMSO:HEPES (5:95, v/v, pH = 7.4) was examined by adding different metal ions (Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Cr²⁺, Fe²⁺, Fe³⁺, Hg²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺ and Zn²⁺). NHS showed a selective fluorescence switch-off response for Cu²⁺ due to the chelation enhancement quenching effect (CHEQ). The quenching of NHS by Cu²⁺ was explored by using density functional theory (DFT) and Stern-Volmer plot. The practical utility of NHS was examined by quantitative and qualitative analysis of Cu²⁺ in real water samples.