A fluorescent ratiometric Cu2+ probe based on FRET by naphthalimide-appended rhodamine derivatives

Abstract

A fluorophore dyad (L) containing rhodamine B and naphthalimide units linked through CN was constructed as a ratiometric probe for Cu²⁺. The selective fluorescent response of the probe L to Cu²⁺ was triggered by a Cu²⁺ induced ring-opening process of the rhodamine B spirolactam, which was based on an intramolecular fluorescence resonance energy transfer (FRET) mechanism from naphthalimide to rhodamine B. The system exhibited high selectivity for Cu²⁺ as compared to other tested metal ions and anions, and signaled the binding event through occurrence of FRET mediated emission at 580 nm of the rhodamine chromophore with enhancement in the fluorescence intensity, concurrently, the featured emission of the naphthalimide donor at around 525 nm was nearly completely quenched. Furthermore, it has been used for ratiometric imaging of Cu²⁺ in living cells with satisfying results.