An ICT-FRET Fluorescent Probe Derived from Coumarin-Naphthalimide for Ratiometric Detection of Copper Ions

This research presents a new ratiometric fluorescent probe designed for Cu²⁺ detection, utilizing a dual mechanism of intramolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET). In this probe, coumarin acts as the energy donor while naphthalimide serves as the acceptor, with a 2-picolinic ester moiety functioning as the Cu²⁺ binding site. In the absence of Cu²⁺, the 2-picolinic ester effectively suppresses electron transfer, inhibiting both ICT and FRET, resulting in strong blue fluorescence (λem = 478 nm) from the coumarin moiety. However, upon Cu²⁺ binding, the Cu²⁺-triggered hydrolysis of the 2-picolinic ester activates the ICT process and enables FRET, leading to a distinct yellow fluorescence (λem = 550 nm). Furthermore, the fluorescence intensity ratio (I_550nm/I_478nm) exhibited a strong linear correlation with Cu²⁺ concentrations (0.1-4 µM), achieving a detection limit of 14 nM. Remarkable selectivity and sensitivity toward Cu²⁺ were observed with this probe over an extensive pH range. The probe exhibited excellent biocompatibility in cell viability tests and performed effectively in ratiometric imaging of intracellular Cu²⁺ in A549 cells.