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.