Schiff Base-Derived Chemosensors for Toxic Ions (Al ³⁺ , Cu ²⁺ , and Hg ²⁺ ) Sensing and Cellular Fluorescence Imaging: Trends, Challenges, and Future Directions

Schiff base-derived chemosensors have emerged as highly versatile and tunable platforms for the selective optical detection of environmentally and biologically hazardous metal ions, particularly Al³⁺, Cu²⁺, and Hg²⁺. This review critically consolidates recent progress in imine-based ligand design, highlighting the strategic incorporation of heteroaromatic scaffolds, electron-donating coordination sites (-O, -N, and -S), π-conjugation rigidification, and intramolecular hydrogen-bonding architectures that precisely modulate metal-binding geometry and photophysical response. Special attention is devoted to mechanistic insights underlying signal generation, including chelation-enhanced fluorescence (CHEF), metal-induced quenching, PET/ICT regulation, ESIPT modulation, FRET-assisted ratiometric sensing, and aggregation-induced emission phenomena. Their performance in live-cell imaging is discussed in terms of intracellular stability, pH adaptability, photostability, and cytocompatibility, underscoring the translational potential of Schiff base sensors in environmental monitoring and biomedical diagnostics.