Synthesis and Applications of a Novel Naphthalimide‐Based Fluorescent Probe for Relay Recognition of Cu ²⁺ and Cysteine

Developing multifunctional probes capable of detecting two or more analytes under same test conditions is important for improving assay efficiency, simplifying experimental procedures, and providing more comprehensive analytical results in complex systems. This study employed 1,8-naphthalimide as a fluorescent group to design and synthesizes a naphthalimide-based Schiff base fluorescent probe, ANQ. In a CH₃CN/HEPES buffer solution with a volume ratio of 3:7, ANQ enabled the continuous and rapid detection of Cu²⁺ and cysteine under the same detection conditions. Of these, ANQ coordinated with Cu²⁺ in a 1:1 ratio, allowing for specific fluorescence "on-off" detection of Cu²⁺, with a complexation constant of 4.41 × 10⁴ M^-1 and a detection limit of 3.6 × 10^-8 mol/L. In addition, under the same conditions, complex ANQ-Cu²⁺ had a fluorescence turn-on response to cysteine and was unaffected by other amino acids and glutathione, with a complexation constant of 5.2 × 10⁴ M^-1 and a limit of detection as low as 4.2 × 10^-8 mol/L. Moreover, ANQ could be prepared as a test strip to detect copper ions, and ANQ-Cu²⁺ complex could be used to detect cysteine in milk, demonstrating excellent potential for practical applications.