Naphthalimide-phenanthroimidazole incorporated new fluorescent sensor for “turn-on” Cu²⁺ detection in living cancer cells

In recent years, fluorescent sensors have emerged as attractive imaging probes due to their distinct responses toward bio-relevant metal ions. However, the bioimaging application main barrier is the ‘turn-off’ response toward paramagnetic metal ions such as Cu ²⁺ under physiological conditions. Herein, we report a new sensor (2-methyl(4-bromo-N-ethylpiperazinyl-1,8-naphthalimido)-4-(1H-phenanthro[9,10-d]imidazole-2-yl) phenol) NPP with multifunctional (Naphthalimide, Piperazine, Phenanthroimidazole) units for fluorescent and colourimetric detection of Cu ²⁺ in an aqueous medium. Both absorption and fluorescence spectral titration strategies were used to monitor the Cu ²⁺-sensing property of NPP. The NPP displays a weak emission at ca. 455 nm, which remarkably enhances (⁓3.2-fold) upon selective binding of Cu ²⁺ over a range of metal ions, including other paramagnetic metal ions (Mn ²⁺, Fe ³⁺, Co ²⁺). The stoichiometry, binding constant (K _a) and the LOD (limit of detection) of NPP toward Cu ²⁺ ions were found to be 1:1, 5.0 (± 0.2) × 10 ⁴ M ⁻¹ and 6.5 (± 0.4) × 10 ⁻⁷ M, respectively. We have also used NPP as a fluorescent probe to detect Cu ²⁺ in live (human cervical HeLa) cancer cells. The emission intensity of NPP was almost recovered in HeLa cells by incubating ‘in situ’ the derived Cu ²⁺ complex (NPP-Cu ²⁺) in the presence of a benchmark chelating agent such as EDTA (ethylenediaminetetraacetate). The fluorescent emission of NPP was reverted significantly in each cycle upon sequencial addition of Cu ²⁺ and EDTA to the NPP solution. Overall, NPP is a novel, simple, economic and portable sensor that can detect Cu ²⁺ in biological and environmental scenarios.