As demonstrated in our previous work, UiO-67(I2) MOF is formulated as Zr6O4(OH)4(BPDC)6(I)12 (BPDC = 4,4'-biphenyldicarboxylate) including hexanuclear [Zr6O4(OH)4] cluster (Norouzi and Khavasi 2020). UiO-67(I2) containing two iodine groups into the BPDC linker was synthesized under solvothermal reaction (Scheme 1). The phase purity of the MOF was illustrated by powder X-ray diffraction (PXRD) experiments. The PXRD experiment of UiO-67(I2) also exhibited the similar pattern to UiO-67 MOF. Furthermore, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and N2 adsorption/desorption experiments confirmed the synthesis and characterization of UiO-67(I2) MOF. Brunauer–Emmett–Teller (BET) and Langmuir surface area were recorded 603.03 and 972.45 m2g− 1, respectively. In addition, the UiO-67(I2) exhibited spherical nanoparticles morphology. Also, the thermal stability of the MOF was recorded up to 600°C (Norouzi and Khavasi 2020).
As shown in Fig. 1, crystal structure of diiodo-BPDC ligand shows that packing structure is generated by iodo interactions. According to the σ-hole concept, the electron density distribution around halogen atom is anisotropic and the atomic radius along the C-X is smaller than the perpendicular direction to this bond (Lim and Beer 2018). Since the iodo-group is a halogen with high polarizability and radius, the positive nature of σ-hole increases, so the halogen bonding interaction is more possible and stronger than the other halo-groups (F, Cl, and Br) (Gilday et al. 2015; Lim and Beer 2018). The crystal packing of diiodo-BPDC ligand in bc direction is generated by C-I…O-C and Ccarboxy…I-C with bond distances of 3.405 and 3.661Å.
To investigate the sensing properties of UiO-67(I2) and UiO-67 MOFs towards DBPs, the luminescence experiments of the MOFs performed at room temperature. In this sense, 1 mg of the nano-MOFs was dispersed in 3 ml DMF in the fluorescent cell, then the samples were ultrasonicated for 5 minutes and after that immersed for 1 h to form a stable emulsion before fluorescence experiments. Upon excitation at 320 nm wavelength, intense peaks were appeared at 656 and 650 nm for UiO-67 and UiO-67(I)2, respectively (Fig. 2 and Figure S1, ESI†). The fluorescence data of the MOFs were obtained upon titration of 10− 4 M of DBPs into the MOF cell. The photo-luminescent (PL) response of UiO-67 and UiO-67(I)2 towards five classes of DBPs including 2-X-aniline, 4-X-aniline, 5-amino-2-X-pyridine, pyridine, and aniline were investigated, where X is halogen atom.
The photoluminescence (PL) response of UiO-67 and UiO-67(I)2 upon addition of DBPs was evaluated by quenching efficiency (QE). According to our experiments, PL of the UiO-67 and UiO-67(I)2 in pyridine exhibits turn off response with QE of 13% and 20%, respectively. While PL of UiO-67 and UiO-67(I)2 toward aniline quenched with a yield of 56% and 70%, respectively (Figures S2-5, ESI†). It’s noteworthy that, among the evaluated DBPs (2-X-aniline, 4-X-aniline, and 5-amino-2-X-pyridine), 5-amino-2-X-pyridine derivatives at exposure to MOFs showed better sensing in comparison to 2-X-aniline and 4-X-aniline. As illustrated in Figures (2a-c), upon gradual increase up to 400 µL, the PL response of UiO-67 respecting 2-F-aniline, 2-Cl-aniline, and 2-Br-aniline exhibited QE of 6%, 17%, and 65%. Strikingly, the PL response of UiO-67 with gradual addition of 2-I-aniline up to 40 µL decreased significantly with QE of 100% (Fig. 2d).
The QE of UiO-67(I)2 with regard to 2-F-aniline, 2-Cl-aniline, and 2-Br-aniline calculated 5%, 35%, and 70% upon gradual increase up to 400 µL, while with addition of 2-I-aniline up to 40 µL, the PL spectra demonstrated QE of 100% (Figs. 2e-h). Interestingly, we observed a drastic decrease in the PL response of the MOFs in a low amount of 4-X-anilines rather than 2-X-anilines derivatives. Furthermore, a red shift clearly appears in the PL response of the two MOFs towards 4-X-aniline. As shown in Fig. 3, the PL response of two MOFs respecting 4-X-anilines exhibit complete quenching (100%) with different sensitivities. The quenching amounts of 4-F-aniline, 4-Cl-aniline, 4-Br-aniline, and 4-I-aniline towards UiO-67 cells are 300, 300, 100, and 40 µL, respectively (Figs. 3a-d). In contrast, as expected, the required amounts for quenching of 4-X-anilines in UiO-67(I)2 are fewer than UiO-67. Accordingly, 180, 180, 100, and 40 µL of 4-F-aniline, 4-Cl-aniline, 4-Br-aniline, and 4-I-aniline, respectively, are needed for quenching towards UiO-67(I)2 MOF (Figs. 3e-h).
As can be deduced from the above sensing data, the desired MOFs did not show a good PL response to pyridine and aniline compounds, while by introducing the halogen moieties into the DBPs, the PL sensing response increased significantly. Compared to 2-X-aniline, 4-X-aniline compounds showed a better PL response in fewer amounts of the absorbed compounds. This improvement is related to spatial hindrance, as 2-X-aniline compounds have two functional groups in the ortho position. Whereas, the two pendent groups in 4-X-anilines located in the para position with less hindrance. Therefore, less congestion in 4-X-anilines causes easier encapsulation with the cavity of the MOFs, which results in a better sensing response.
Next, the sensing behavior of 5-amino-2-X-pyridines investigated. The PL response of the MOFs towards this class of compounds is significant. Similar to 4-X-aniline series, the PL response of the MOFs demonstrated complete quenching upon addition of 5-amino-2-X-pyridine series in the least amounts and also the PL emission showed a red shift to the higher wavelength. The PL response of UiO-67 for complete quenching towards addition of 5-amino-2-F-pyridine, 5-amino-2-Cl-pyridine, 5-amino-2-Br-pyridine, and 5-amino-2-I-pyridine are 100, 60, 40, and 20 µL, respectively, while the requiring amounts for UiO-67(I)2 are 40, 10, 6, and 4 µL, respectively. (Figs. 4a-e).
Interestingly, 5-amino-2-X-pyridine compounds were quickly sensed by the MOFs in the least amounts of microliter. The reason for this excellent PL response can be attributed to the presence of aniline, aromatic nitrogen, and halogen functional groups. Each of these functional groups individually can establish potent XB and HB interactions with the MOFs. Furthermore, in each sensing series, the sensing response increased significantly by moving from fluoro to iodo. This phenomenon is attributed to the fact that the sensing process is mainly conducted by XB interactions.