An -OH group functionalized MOF for ratiometric Fe³⁺ sensing

Iron is one of the most important elements in the biochemical processes in all living system, both deficiency or excess of iron will lead to metabolism disorder diseases. However, Fe³⁺ is one of the most efficient fluorescence quenchers among the transition-metal ions because of its paramagnetic nature. The realization of Fe³⁺ ratiometric and self-calibrated fluorescent sensor is highly-challenging. We synthesized a novel luminescent -OH functionalized EuOHBDC (Eu₂(OH-BDC)₃, OH-BDC=2-hydroxyterephthalic acid) by hydrothermal reaction and in situ ligand synthesis, and used it as a rare ratiomatric luminescent sensor for Fe³⁺ ions. The -OH functional group facilitates both electron transfer and binding interaction between EuOHBDC and Fe³⁺, which lead to luminescent quenching of ligand-based emission while enhancement of a new peak emission, and thus enables ratiometric detection of Fe³⁺. The relative fluorescent intensity ratio (I₃₇₅/I₄₂₇) increased linearly with increasing Fe³⁺ concentration in the 10–50 μM range with 1.17 μM (65 ppb) detection limit. The EuOHBDC also shows excellent selectivity towards different metal ions, particularly can discriminate Fe³⁺ and Fe²⁺ through different luminescent responses. This result clearly demonstrates the superiority of -OH functionalized MOF for Fe³⁺ detection, which can contribute to develop high performance luminescent probe for detection of metal ions in environmental and biomedical applications.