An Excited-State Intramolecular Proton Transfer (ESIPT) Plus Aggregation Induced Emission (AIE) Phenanthro[9,10-d]imidazole-Based Fluorescence Probe for Detection of Fe3+ in Living Cells

Selective detection of Fe3+ has considerable importance due to its active involvement in various biological processes. Based on the mechanism of excited-state intramolecular proton transfer (ESIPT) plus aggregation induced emission (AIE), a fluorescence probe of phenanthro[9,10-d]imidazole modified by the phenolic hydroxyl (PIP-o-OH) had been designed, synthesized and applied in the detection of Fe3+. The structure of PIP-o-OH was characterized by H-1 NMR, C-13 NMR, IR, HRMS and X-ray single diffraction. Furthermore, a clear intramolecular hydrogen bond was observed between hydroxyl O-H and imidazole N atom in X-ray single structure, which improved the impossibility of ESIPT activity. ESIPT and AIE activities of PIP-o-OH were adequately determined by absorption, emission spectra and scanning electron microscope (SEM). The aggregated PIP-o-OH in MeOH/H2O (V:V=1:9, Hepes 10 mu mol/L, pH=7.4) exhibited a good sensitivity towards Fe3+ with turn-off fluorescence response just after 20 s. The limit of detection (LOD) was calculated as low as 0.49 mu mol/L. So it could be utilized to detect Fe3+ in biology and environmental samples. In addition, the calculation of the density functional theory (DFT) confirmed the formation of PIP-o-OH-Fe3+ complex. Also, PIP-o-OH was successfully applied to monitor Fe3+ in HeLa cells by the fluorescence change and quantificationally detect Fe3+ in water samples.