Simple turn-on fluorescent chemosensor for ultrafast and highly selective trace-level detection of Cu2+ ions in aqueous solutions.

A benzimidazole-based probe, BIPMA (2-(1H-benzo[d]imidazol-2-yl)-N-(pyridin-2-ylmethyl)aniline), was designed and synthesized to detect Cu²⁺ ions. BIPMA exhibited a fluorescent "turn-on" mechanism when bound to Cu²⁺ ions in an acetonitrile/water mixture (5:5, v/v, HEPES 10 mM, pH 7.4) owing to the synergistic effect of the chelation-enhanced fluorescence and internal charge-transfer mechanisms. Moreover, the BIPMA probe effectively detected nanomolar-range concentrations (0-400 nM) of Cu²⁺ ions in an aqueous system with a detection limit of 4.80 nM; this value is significantly lower than that set by the U.S. Environmental Protection Agency (≈20 μM). Additionally, BIPMA showed an ultrafast response to Cu²⁺ ions, with a maximum intensity achieved 25 s after adding Cu²⁺. Furthermore, BIPMA detected Cu²⁺ ions in solutions with a pH range of 5-11, without being influenced by pH, underscoring its applicability under various physiological conditions. Density functional theory studies revealed that internal charge transfer was responsible for emission. Finally, BIPMA effectively detected Cu²⁺ ions in real water samples and living cells.