Ultrasound-Assisted Synthesis of Luminescent Micro- and Nanocrystalline Eu-Based MOFs as Luminescent Probes for Heavy Metal Ions

The luminescent coarse-, micro- and nanocrystalline europium(III) terephthalate tetrahydrate (Eu(2)bdc(3)·4H(2)O) metal-organic frameworks were synthesized by the ultrasound-assisted wet-chemical method. Electron micrographs show that the europium(III) terephthalate microparticles are 7 μm long leaf-like plates. According to the dynamic light scattering technique, the average size of the Eu(2)bdc(3)·4H(2)O nanoparticles is equal to about 8 ± 2 nm. Thereby, the reported Eu(2)bdc(3)·4H(2)O nanoparticles are the smallest nanosized rare-earth-based MOF crystals, to the best of our knowledge. The synthesized materials demonstrate red emission due to the (5)D(0)–(7)F(J) transitions of Eu(3+) upon 250 nm excitation into (1)ππ* state of the terephthalate ion. Size reduction results in broadened emission bands, an increase in the non-radiative rate constants and a decrease in both the quantum efficiency of the (5)D(0) level and Eu(3+) and the luminescence quantum yields. Cu(2+), Cr(3+), and Fe(3+) ions efficiently and selectively quench the luminescence of nanocrystalline europium(III) terephthalate, which makes it a prospective material for luminescent probes to monitor these ions in waste and drinking water.