A functionalized UiO-66 MOF for turn-on fluorescence sensing of superoxide in water and efficient catalysis for Knoevenagel condensation

Abstract

In the present work, a new MOF material of the UiO-family called Zr-UiO-66-NH-CH₂-Py (1) has been obtained by the solvothermal technique and successfully characterized. The MOF structure was assembled with 2-((pyridin-4-ylmethyl) amino) terephthalic acid (H₂BDC-NH-CH₂-Py) as linker and Zr⁴⁺ ion. The activated form of 1 (called 1′) exhibits considerable thermal and chemical stability. Compound 1′ showed a very rapid and selective response for the fluorometric sensing of superoxide (O₂^·−) in aqueous medium even in the presence of the potentially competitive reactive oxygen species (ROS). The limit of detection value for O₂^·− sensing is 0.21 μM, which is comparable with those of the reported O₂^·− sensors. This is the first MOF based fluorescent sensor for the detection of O₂^·−. The response time of this MOF sensor for O₂^·− is very short (240 s). On the other hand, 1′ was employed as a solid heterogeneous catalyst for Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate at 80 °C in ethanol resulting in a very high yield of the desired product. The effects of the esterified linker ((CH₃)₂BDC-NH-CH₂-Py) and the corresponding metal salt (ZrCl₄) on this catalytic reaction were examined separately. We have also tested the substrate scope elaborately for the catalytic reaction promoted by catalyst 1′.