Detection of Lysosomal Hg2+ Using a pH-Independent Naphthalene Monoimide-Based Fluoroprobe

The development of fluorometric detection methods for toxic metal ions in real samples and inside cellular environments using fluorescent dyes has gained tremendous research interest. This work represents the design and synthesis of a 1,8-naphthalimide-based visible light absorbing fluorescence probe His-NMI-Bu showing an intramolecular charge transfer (ICT) feature. Photophysical properties of the fluoroprobe are investigated in-depth through a combination of steady-state, time-resolved spectroscopic techniques, and DFT calculation. The probe displays outstanding pH tolerance in the pH range of 5–10 as evident from UV–Vis. and fluorescence measurements. The fluoroprobe exhibits chelation with Hg2+-induced fluorescence attenuation via PET in the solution, thus acting as a suitable fluorescence sensor for mercury ions with LOD 0.52 µM. The high sensitivity and selectivity of the probe towards Hg2+ are validated from fluorescence titration with various metal ions. Banking on its intriguing solid-state emissive properties, dye-loaded filter paper-based sensing of Hg2+ is also developed demonstrating the sensitivity in the micromolar range. Finally, His-NMI-Bu fluorophore depicts its selective localization inside the lysosomal compartment of live cells which assists further to monitor the presence of mercury ions inside the lysosome showing similar Hg2+-induced fluorescence depletion.