Diamino group-functionalized Zr-based metal-organic framework for fluorescence sensing of free chlorine in the aqueous phase and Knoevenagel condensation.

We developed a porous diamino group-functionalized Zr(IV) metal-organic framework (MOF). The synthesized MOF has a similar structure to DUT-52 (DUT = Dresden University of Technology), which has a face-centered cubic structure with an Fm3̄m space group. The synthesized material (DUT-52-(NH2)2-1) was solvent exchanged with methanol (MeOH) and activated at 100 °C overnight. Both the as-synthesized and activated materials (DUT-52-(NH2)2-1') are thermally stable until 300 °C. The Brunauer-Emmett-Teller (BET) surface area of DUT-52-(NH2)2-1' was found to be 413 m2 g-1. DUT-52-(NH2)2-1' showed a significant quenching of fluorescence response after coming in contact with free chlorine (ClO-) in an aqueous medium. The selectivity of DUT-52-(NH2)2-1' towards ClO- was not significantly hampered in the presence of any competitive ion. The limit of detection (LOD) value was found to be 0.08 µM in phosphate-buffered saline (PBS, pH = 7.4). DUT-52-(NH2)2-1' is recyclable and very sensitive towards ClO-. Moreover, the paper strip method was developed for onsite identification of ClO-. Furthermore, the catalytic activity of DUT-52-(NH2)2-1' was tested in the Knoevenagel condensation between benzaldehyde and cyanoacetamide. The experimental results clearly indicate that DUT-52-(NH2)2-1' exhibits high activity with very high selectivity towards condensation products. The solid was reusable three times with no decay in its activity, as evidenced by powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM) and fourier transform infrared (FT-IR).

A Zr-Based Metal-Organic Framework with a DUT-52 Structure Containing a Trifluoroacetamido-Functionalized Linker for Aqueous Phase Fluorescence Sensing of the Cyanide Ion and Aerobic Oxidation of Cyclohexane.

A zirconium (Zr) metal-organic framework having a DUT-52 (DUT stands for Dresden University of Technology) structure with face-centered cubic topology and bearing the rigid 1-(2,2,2-trifluoroacetamido) naphthalene-3,7-dicarboxylic acid (H2NDC-NHCOCF3) ligand was prepared, and its solid structure was characterized with the help of the X-ray powder diffraction (XRPD) technique. Other characterization methods like thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy were applied to verify the phase purity of the compound. In order to get the solvent-free compound (1'), 1 was stirred with methanol for overnight and subsequently heated at 100 °C overnight under vacuum. As-synthesized (1) and activated (1') compounds are thermally stable up to 300 °C. The Brunsuer Emmett-Teller (BET) surface area of 1' was found to be 1105 m2 g-1. Fluorescence titration experiments showed that 1' exhibits highly selective and sensitive fluorescence turn-on behavior toward cyanide (CN-) anion. The interference experiments suggested that other anions did not interfere in the detection of CN-. Moreover, a very short response time (2 min) was shown by probe 1' for CN- detection. The detection limit was found to be 0.23 µM. 1' can also be effectively used for CN- detection in real water samples. The mechanism for the selective detection of CN- was investigated systematically. Furthermore, the aerobic oxidation of cyclohexane was performed with 1' under mild reaction conditions, observing higher activity than the analogous DUT-52 solid under identical conditions. These experiments clearly indicate the benefits of hydrophobic cavities of 1' in achieving higher conversion of cyclohexane and cyclohexanol/cyclohexanone selectivity. Catalyst stability was proved by two consecutive reuses and comparing the structural integrity of 1' before and after reuses by the XRPD study.

Metal-Organic Framework (MOF) Derived Recyclable, Superhydrophobic Composite of Cotton Fabrics for the Facile Removal of Oil Spills.

Marine oil spill cleanup is one of the major challenges in recent years due to its detrimental effect on our ecosystem. Hence, the development of new superhydrophobic oil absorbent materials is in high demand. The third-generation porous materials, namely metal-organic frameworks (MOFs), have drawn great attention due to their fascinating properties. In this work, a superhydrophobic MOF with UiO-66 (SH-UiO-66) topology was synthesized strategically with a new fluorinated dicarboxylate linker to absorb oil selectively from water. The fully characterized superhydrophobic MOF showed extreme water repellency with an advancing water contact angle (WCA) of 160° with a contact angle hysteresis (CAH) of 8°. The newly synthesized porous MOF (SBET = 873 m2 g-1) material with high WCA found its promising application in oil/water separation. The superhydrophobic SH-UiO-66 MOF was further used for the in-situ coating on naturally abundant cotton fiber to make a superhydrophobic MOF@cotton composite material. The MOF-coated cotton fiber composite (SH-UiO-66@CFs) showed water repellency with a WCA of 163° and a low CAH of 4°. The flexible superhydrophobic SH-UiO-66@CFs showed an oil absorption capacity more than 2500 wt % for both heavy and light oils at room temperature. The superoleophilicity of SH-UiO-66@CFs was further exploited to separate light floating oil as well as sedimentary heavy oil from water. SH-UiO-66@CFs material can also separate oil from the oil/water mixture by gravity-directed active filtration. Hence, the newly developed MOF-based composite material has high potential as an oil absorbent material for marine oil spill cleanup.

Rational design of a functionalized aluminum metal-organic framework as a turn-off fluorescence sensor for α-ketoglutaric acid.

A 3D metal-organic framework (MOF) called Al-DUT-5-N2H3 (1) (DUT: Dresden University of Technology) was prepared hydrothermally using Al(iii) salt and a hydrazinyl functionalized linker called 2-hydrazinyl-[1,1'-biphenyl]-4,4'-dicarboxylic acid (BPDC-N2H3). Material 1 was successfully characterized by X-ray powder diffraction (XRPD), FT-IR spectroscopy, N2 sorption (BET) experiment, thermogravimetric analysis (TGA), EDX and FE-SEM analyses. The activated form of material 1 (called 1') was achieved by a direct heating process. Material 1' was successfully employed for the solution-phase fluorescence detection of α-ketoglutaric acid (α-KG). It showed high detection performance even when there were other competitive analytes present in the mixture. Material 1' is the first MOF-based fluorescent turn-off sensor for the detection of α-KG. The response time for α-KG is exceptionally low (60 s) as compared to any other reported α-KG sensor. The limit of detection (LOD) was found to be 0.61 µM, which is far better as compared to any other reported sensor for α-KG to date. The mechanism for α-KG sensing was thoroughly investigated and proposed to be PET (photoinduced electron transfer) process by TD-DFT (time-dependent DFT) calculations.

A new 3D luminescent Zn(ii)-organic framework containing a quinoline-2,6-dicarboxylate linker for the highly selective sensing of Fe(iii) ions.

A new 3D zinc-organic framework [Zn(QDA)]·0.5H2O·0.7DMF (1, H2QDA = quinoline-2,6-dicarboxylic acid, DMF = N,N-dimethylformamide) was synthesized under solvothermal conditions. The single crystal X-ray diffraction analysis reveals that the 3D framework structure of 1 has a PtS topology and contains Zn(ii) ions having distorted square pyramidal geometry with ZnO4N configuration. The phase purity of the bulk sample was characterized by X-ray powder diffraction (XRPD), thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy. The as-synthesized sample (1) was activated by stirring with acetone for 24 h, followed by heating under vacuum for 24 h at 120 °C. The TGA experiment indicated that both 1 and its activated form (1') are stable up to 380 °C. The crystalline structure of the compound was retained after immersion in water, 1 M HCl, acetic acid and NaOH (at pH = 10) solutions. Compound 1' exhibited very quick fluorescence quenching response after the addition of Fe3+ solution. This quenching was not affected by the presence of other competitive metal cations. A very low detection limit of 9.2 ppb was observed for Fe3+ ions, which is among the lowest values documented in the literature for MOF based fluorescent probes. Both fluorescence resonance energy transfer (FRET) and photo-induced electron transfer (PET) processes play major roles in the selective detection of Fe3+ ions. The recyclability experiment suggested that 1' can be used for the long-term detection of Fe3+ ions.

A new quinoline based luminescent Zr(iv) metal-organic framework for the ultrasensitive recognition of 4-nitrophenol and Fe(iii) ions.

A Zr(iv) based luminescent metal-organic framework (MOF; 1) was synthesized by a solvothermal method using a mixture of ZrCl4, quinoline-2,6-dicarboxylic acid (H2QDA) ligand and trifluoroacetic acid (modulator) having a 1 : 1 : 10 molar ratio in N,N-dimethylformamide (DMF). For activation, the methanol-exchanged sample was heated under high vacuum at 120 °C for 1 day. Different techniques like X-ray powder diffraction (XRPD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy and energy dispersive X-ray (EDX) analysis were applied to fully characterize 1. The activated form of 1 (called 1') has the formula [Zr6O6(OH)2(CF3COO)2(C11H5NO4)4(H2O)4]. It exhibited quick response and great selectivity for the fluorimetric sensing of 4-nitrophenol (4-NP) in acetonitrile and Fe3+ ions in water. The probe maintained its high selectivity for 4-NP and Fe3+ ions even in the presence of potentially intrusive nitroaromatics and metal ions, respectively. The detection limits for 4-NP and Fe3+ were found to be 1.40 ppt and 0.71 ppb, respectively. These values are the lowest among the existing MOF probes for Fe3+ and 4-NP. The electron and energy transfer processes along with the electrostatic interactions of 4-NP with the MOF can be the possible reasons for the selectivity for 4-NP. The fluorescence resonance energy transfer (FRET) process can be the major reason behind the quenching mechanism for Fe3+ ions. The recyclability test indicates that 1' is a promising probe for the long-term detection of 4-NP and Fe3+ ions.