Studies On The Fluorescence Sensing And Adsorption Performance Of Metal-Organic Frameworks For Ions In Aqueous Solutions

Recently, microporous/mesoporous functional materials have experienced large development in chemical engineering and material science. Particularly, metal-organic frameworks (MOFs) attract wide attentions in many fields such as adsorption/separation, sensing, catalysis and biological medicine, due to their special advantages including pore size adjustability, structural designability and chemical functionality. Ions are widespread in humans and environment and concentration controlling is significantly valuable for human health and sustainable development of environment. This work mainly focuses on the removal and on-line monitoring for ions with MOFs. The main contents and findings are given as follows:1. The stability of metal-organic frameworks (MOFs) in fluoride solutions was studied based on eleven water-stable MOFs, and several factors were found to have large influence. Furthermore, the defluoridation performance of the stable UiO-66(Zr) was investigated comprehensively. As the active sites, the μ-OH groups in UiO-66(Zr) play siginificant role on the defluoridation performance of the MOF. UiO-66(Zr) shows a Langmuir adsorption capacity of 44.92 mg g-1 that is higher than most of the reported adsorbents. In conclusion, this work provides the theoretical basis for the construction of MOFs adsorbents for defluoridation in waste water.2. Based on hard-soft acid-base theory, silver(Ⅰ) is classified as soft acid, which can strongly interact with soft bases such as iodide. As a proof-of-concept demonstration, we successfully synthesized the MIL-101(Cr)-SO3Ag based on the water-stable MOF, MIL-101(Cr)-SO3H, via ion exchange process. The existence and uniform distribution of Ag(Ⅰ) in the porous MOF were demonstrated by XPS patterns and EDS maps. As excepted, MIL-101(Cr)-SO3Ag shows a high iodide adsorption capacity of 244.2 mg g-1, much higher than that of the unmodified sample (94.1 mg g-1) and most of other adsorbents. In addition, the removal efficiency of MIL-101(Cr)-SO3Ag can reach almost 100% at lower concentrations (< 80 mg L-1) and the value is nearly zero with respect to MIL-101(Cr)-SO3H. This improvement is mainly attributed to the strong interaction between iodide and silver ion.3. Eu3+ was successfully incorporated into two isostructural MOFs with different distributions of carboxyl groups. The resulting Eu3+@UiO-66-COOH and Eu3+@UiO-66-2COOH combine the excellent stability of UiOs and luminescence properties of the rare-earth element. Utilizing the specific binding site between two free carboxyl groups for efficiently capturing Cu2+, Eu3+@UiO-66-2COOH shows highly selective and sensitive luminescence quenching effect for Cu2+in aqueous solution, while Eu3+@UiO-66-COOH with single free carboxyl group on the ligand possesses relatively poor performance. Meanwhile, such immobilization behavior of Cu2+ makes quenching is pronounced even with a nanomolar concentration of Cu2+, superior to the majority of reported MOF sensors.4. In this work, we synthesized six MOFs:Zn2Zn(bpdc)3(bipy), Zn2Mg(bpdc)3(bipy), Zn2Cd(bpdc)3(bipy), Zn2Ni(bpdc)3(bipy), Zn2Mn(bpdc)3(bipy) and Zn2Co(bpdc)3(bipy). These MOFs are isostructural and the difference is the species of metal ions in the organic clusters based on XRD and elements analysis. These six MOFs exhibit different luminescence intensities and quantum yields, and the order is as follows:Zn> Mg> Cd> Ni> Mn> Co. Furthermore, as a turn-on fluorescence sensor, Zn2Co(bpdc)3(bipy) with lowest quantum yield shows high selectivity in the sensing of Zn2+; Zn2Zn(bpdc)3(bipy) with highest quantum yield as a turn-off fluorescence probe exhibits high selectivity and potential in sensing of low concentration of nitrobenzene (NB). This work indicates that metal ions replacement is an efficient approach to modify not only the luminescence property but also the fluorescence sensing performance of MOFs.5. A new fluorescent MOF containing the dye fluorescein sodium (FS), FS@UiO-66 was prepared via in-situ synthesis method. From the characterization of FS@UiO-66, it is deduced that FS molecule can bond to Zr in the UiO-66 structure and parts of FS molecule may be encapsulated in the MOF. The fluorescence intensity of FS@UiO-66 after immersed in pure water for 24 h stays almost invariable, indicating the excellent immobilization of FS molecule in the MOF. In fluoride aqueous solution, the FS in FS@UiO-66 can be released and the solution shows bright green fluorescence in only 4 s, and similar phenomenon does not happen in other anions solutions. Meanwhile, co-existing anions in solutions have negligible effect on the sensing performance of FS@UiO-66. This excellent selectivity and rapidity is mainly due to the interaction between Zr in the organic cluster of the MOF and fluoride, which can lead to the collapse of FS@UiO-66 framework and the release of fluorescent FS.