Two-Dimensional Cu-Based MOF for Selective Staining of the Cellular Nucleus through Fluorescence Imaging and Selective Sorption of Dye Molecules in Aqueous Medium.

A two-dimensional copper-based metal-organic framework, [Cu(C23H14O6)(C10H8N2)2]·H2O·DMSO, 1, was synthesized using pamoic acid (C23H16O6) and 4,4'-bipyridine (C10H8N2) as an organic ligand and Cu(II) as a metal ion. Single-crystal structure X-ray diffraction studies of the as-synthesized compound showed a two- dimensional structure with free hydroxyl groups. Upon excitation at 370 nm, the aqueous dispersion of [Cu(C23H14O6)(C10H8N2)2]·H2O·DMSO, 1, showed emission centered at 525 nm resulting from the intraligand energy transfer. Fluorescence microscopic experiments using a human epithelioid cervix carcinoma HeLa cell line were carried out, clearly showing that our compound selectively stained the cellular nucleus. To utilize the porous nature of [Cu(C23H14O6)(C10H8N2)2]·H2O·DMSO, 1, its dye sorption behavior in aqueous solution was determined, and a high affinity for methylene blue (MB) dye was confirmed. Our synthesized compound sorbed 88% MB dye with an initial concentration of 32 mg L-1, and its sorption capacity for MB was found to be 29.79 mg g-1. The possible mechanism of the dye sorption behavior was discussed in terms of the size and charge of dye molecules with respect to molecular-level interactions between the framework and the dye molecules.

Selective Luminescence Turn-On-Based Sensing of Phosphate in the Presence of Other Interfering Anions Using a Heterobimetallic (3d-4d) MOF with an Acidic Pocket.

A luminescent metal-organic framework with the molecular formula [YMn1.5(C7N1H3O5)3(H2O)6]·11H2O, 1 {where C7N1H3O5 = chelidamate}, was synthesized by a hydrothermal method by employing chelidamic acid as an organic ligand and Y(III) and Mn(II) as metal ions. A two-dimensional heterobimetallic structure with phenolic hydroxyl-functionalized pockets was revealed by single-crystal X-ray diffraction analysis of compound 1. PXRD, TGA, IR, BET analysis, and UV-vis spectroscopy were used for the thorough characterization of compound 1. Upon excitation at 280 nm, compound 1 shows bright blue emission, which was utilized for the selective and sensitive turn-on detection of the PO43- ion. Based on Bronsted-Lowry acid-base interactions, the photoluminescence of compound 1 was enhanced in the presence of very low concentrations of the aforementioned anion. The mechanism behind the detection of the phosphate ion has been explored in detail. It was seen that the PO43- anion entered the hydroxyl-functionalized pockets of compound 1 and stabilized the aromatic portion of compound 1 via molecular-level interactions through acid-base interactions. These molecular-level interactions are responsible for the enhancement of the photoluminescence intensity of compound 1 after the incorporation of phosphate ions by reducing the nonradiative transitions. These phenomena were also confirmed by time-correlated single photon counting (TCSPC) measurement, which shows that the excited-state lifetime increased with the increase in addition of phosphate anions. The calculated limit of detection (LOD) of 1 was 19.55 ppb for phosphate (PO43-), which was significantly lesser than the recommended level for the PO43-anion toward the human body. The luminescence enhancement coefficient, KSV, value was also much higher than those of other reported metal-organic frameworks.

Response of a Zn(II)-based metal-organic coordination polymer towards trivalent metal ions (Al3+, Fe3+ and Cr3+) probed by spectroscopic methods.

A new zinc-based two-dimensional coordination polymer, [Zn(5-AIP)(Ald-4)]·H2O (5-AIP = 5-amino isophthalate, Ald-4 = aldrithiol-4), 1, has been synthesized at room temperature by the layer diffusion technique. Single-crystal X-ray diffraction analysis of 1 showed a two-dimensional bilayer structure. An aqueous suspension of 1 upon excitation at 300 nm displayed an intense blue emission at 403 nm. The luminescence spectra were interestingly responsive and selective to Al3+, Cr3+ and Fe3+ ions even in the presence of other interfering ions. The calculated detection limits for Al3+, Cr3+ and Fe3+ were 0.35 µM ([triple bond, length as m-dash]8.43 ppb), 0.46 µM ([triple bond, length as m-dash]22.6 ppb) and 0.30 µM ([triple bond, length as m-dash]15.85 ppb), respectively. Notably, with the cumulative addition of Al3+ ions, the luminescence intensity at 403 nm decreased steadily with a gradual red shift up to 427 nm. Afterward, this red shifted peak showed a turn-on effect upon further addition of Al3+ ions. On the other hand, for Cr3+ and Fe3+ ions, there was only drastic luminescence quenching and a large red shift up to 434 nm. This indicated the formation of a complex between 1 and these metal ions, which was also supported by the UV-Visible absorption spectra of 1 that showed the appearance of a new band at 280 nm in the presence of these three metal ions. The FTIR spectra revealed that these ions interacted with the carboxylate oxygen atom of 5-AIP and the nitrogen atom of the Ald-4 ligand in the structure. The luminescence lifetime decay analysis manifested that a charge-transfer type complex was formed between 1 and Cr3+ and Fe3+ ions that resulted in huge luminescence quenching due to the efficient charge transfer involving the vacant d-orbitals, whereas for Al3+ ions having no vacant d-orbital, turn-on of luminescence occurred because of the increased rigidity of 1 upon complexation.

Luminescent metal-organic framework-based phosphor for the detection of toxic oxoanions in an aqueous medium.

A terbium-doped yttrium-based metal-organic framework, [Tb0.2Y0.8(FDA)(Ox)0.5(H2O)2]·H2O, 1 {where H2FDA = furan-2,5-dicarboxylic acid and Ox = oxalate}, was successfully synthesized using the hydrothermal technique as a phosphor material along with a large Stokes shift and low self-quenching of luminescence for the rapid visible detection of toxic anions in an aqueous medium. To confirm the structure and phase purity of compound 1, single crystals of the isomorphous pure yttrium-based compound [Y(FDA)(Ox)0.5(H2O)2]·H2O, 1a, were synthesized under similar experimental conditions. The single crystal X-ray data of compound 1a confirmed the three-dimensional metal-organic framework formed by the connectivity of the Y3+ ion with furan-2,5-dicarboxylate and the oxalate moiety. The phase purity of compounds 1 and 1a was confirmed by powder X-ray diffraction. Compound 1 was systematically characterized via TGA, SEM and EDX elemental mapping analysis. The aqueous dispersion of compound 1 showed highly intense visible green emission upon excitation at 265 nm. The emissions of compound 1 were utilized for the luminescence-based visible detection of toxic anions in the aqueous medium through luminescence quenching. The observed limit of detection (LOD) was 1.1 nM, 2.2 nM and 6.5 nM for chromate (CrO42-), permanganate (MnO4-) and phosphates (PO43-, H2PO4- and HPO42-), respectively, and the observed KSV values were superior to those of all other metal-organic frameworks previously reported. More importantly, the LODs are significantly lower than the level recommended for these anions towards human life.