Synthesis, spectroscopic and DFT investigation of a dual-fluorophore methylene blue–naphthalimide probe for selective Cu2+ ions sensing and HeLa cell imaging

Cu2+ ions are essential trace elements in biological systems. Any imbalance in their level leads to serious neurodegenerative diseases like Wilson's disease, Parkinson's disease, and Alzheimer's disease. In addition, Cu2+ ion pollution from the environment can cause further health problems. Therefore, developing sensitive and selective detection approaches for Cu2+ ions is of particular significance. In this work, we designed, synthesized, and utilized a new fluorescent probe (MB-M) based on fluorophores methylene blue (MB) and 1,8-naphthalimide (M) for the specific detection of Cu2+ ions in physiological conditions. As a result of interaction with Cu2+, amide bond cleavage occurs without coordination of the probe to the ion, and both fluorophores are released simultaneously resulting in dual fluorescence signal generation in green and red regions. The probe showed high selectivity and sensitivity (LOD 0.19 μM for MB and 2.0 μM for M) towards Cu2+ ions in PBS buffer solution at pH 7.4 with 30% ethanol content and fast response time. DFT calculations supported the proposed sensing mechanism, revealing a small HOMO-LUMO energy gap (2.215 eV) and high softness (0.451 eV-1) for MB-M. Fluorescent imaging studies and HeLa cell imaging further demonstrated the probes' possible applications as instant recognition and the real-time detection of Cu2+ ions in biological systems. These findings demonstrate that MB-M is a promising fluorescent probe for Cu2+ ions ion detection.