Fluorescence Turn-on Detection of Perfluorooctanoic Acid (PFOA) by Perylene Diimide-Based Metal-Organic Framework.

A novel, water-stable, perylene diimide (PDI) based metal-organic framework (MOF), namely, U-1, has been synthesized for selective and sensitive detection of perfluorooctanoic acid (PFOA) in mixed aqueous solutions. The MOF shows highly selective fluorescence turn-on detection via the formation of a PFOA-MOF complex. This PFOA-MOF complex formation was confirmed by various spectroscopic techniques. The detection limit of the MOF for PFOA was found to be 1.68 µM in an aqueous suspension. Upon coating onto cellulose paper, the MOF demonstrated a significantly lower detection limit, down to 3.1 nM, which is mainly due to the concentrative effect of solid phase extraction (SPE). This detection limit is lower than the fluorescence sensors based on MOFs previously reported for PFAS detection. The MOF sensor is regenerable and capable of detecting PFOA in drinking and tap water samples. The PDI-MOF-based sensor reported herein represents a novel approach, relying on fluorescence turn-on response, that has not yet been thoroughly investigated for detecting per- and polyfluoroalkyl substances (PFAS) until now.

Fluorescent sensor based on solid-phase extraction with negligible depletion: A proof-of-concept study with amines as analytes.

This paper describes the development and proof-of-concept testing of an easy-to-use trace analysis technique, namely F-SPE, by coupling fluorescent sensor with solid phase extraction (SPE). F-SPE is a two-step methodology that concentrates an analyte from a liquid sample onto a fluorophore-modified membrane and measures the amount of analyte from the extent the extracted analyte quenches the emission of the fluorophore. By applying the principle of negligible depletion (ND) intrinsic to SPE, the procedure of F-SPE for analyzing a sample can be markedly simplified while maintaining the ability to detect analytes at low limits of detection (LOD). The merits of this approach are demonstrated by impregnating a SPE membrane with a perylene diimide (PDI) fluorophore, N,N'-di(nonyldecyl)-perylene-3,4,9,10-tetracarboxylic diimide (C9/9-PDI), for the low-level detection of organic amines (e.g., aniline) and amine-containing drugs (e.g., Kanamycin). The sensing mechanism is based on the donor-acceptor quenching of PDI by amines, which, when coupled with the concentrative nature of SPE, yields LODs for aniline and Kanamycin of 67 nM (∼6 ppb) and 32 nM (∼16 ppb), respectively.

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.

Post-synthetic modification of a metal-organic framework with a chemodosimeter for the rapid detection of lethal cyanide via dual emission.

A new Hf(iv) based metal-organic framework with the UiO-66 (UiO = University of Oslo) topology was prepared via a standard modulated solvothermal reaction. The weakly emissive MOF material was well characterized via various analytical techniques. The parent MOF was then post-synthetically modified with a highly emissive pyrene based chemodosimeter probe without altering the parent framework structure. The incorporation of the chemodosimeter in the modified MOF material was successfully confirmed by using FT-IR and 1H NMR spectroscopy. The emission behaviour of the parent MOF was changed drastically with a noticeable colour change after post-synthetic modification. After modification, the MOF material showed a very selective colorimetric and fluorometric dual-emissive response towards lethal cyanide ions. A large blue shift (Δλ = 127 nm) was observed in the emission spectra which can be also visualized with the naked eye under a hand held UV lamp. The presence of a vinyl functional group in the chemodosimeter can serve as a potential nucleophilic reaction centre, which was also confirmed by the DFT calculations. The reaction based sensing mechanism of the MOF material was successfully confirmed by 1H NMR spectroscopy. Furthermore, to apply the modified MOF material in real life, the test strip based sensing of cyanide in drinking water was demonstrated.

Structural Diversity in Supramolecular Organization of Anionic Phosphate Monoesters: Role of Cations.

Syntheses and structures of anionic arylphosphate monoesters [ArOP(O)2(OH)]- (Ar = 2,6-CHPh2-4-R-C6H2; R = Me/Et/iPr/tBu) with different counter cations are reported. The counter cations were varied systematically: imidazolium cation, 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N'-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, 4,4'-bipyridinium dication, and magnesium(II) dication. The objective was to examine if the supramolecular structure of anionic arylphosphate monoesters could be modulated by varying the cation. It was found that an eight-membered P2O4H2-hydrogen-bonded dimeric motif involving intermolecular H-bonding between the [P(O)(OH)] unit of the anionic phosphate monoester along with the counter cation is formed with 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N'-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, and magnesium(II) dication; both discrete and polymeric H-bonded structures are observed. In the case of imidazolium cations and 1,4-diazabicyclo[2.2.2]octan-1-ium cation, the formation of one-dimensional polymers (single lane/double lane) was observed. On the other hand, two types of phosphate motifs, intermolecular H-bonded dimer and an open-form, were observed in the case of 4,4'-bipyridinium dication.

Aqueous Phase Sensing of Fe3+ and Ascorbic Acid by a Metal-Organic Framework and Its Implication in the Construction of Multiple Logic Gates.

A new HfIV -based metal-organic framework with UiO-66 topology was synthesized via a one-step solvothermal method by using 3-methyl-4-phenylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2 MPTDC) as a ligand. The MOF material showed a high stability in a broad pH range (from pH 2 to pH 12) in an aqueous medium. The presence of hydrophobic methyl and phenyl substituents in the carboxylic acid ligand and strong Hf-O bond play crucial roles in its stability. The new MOF material was systematically characterized by various techniques such as XRPD, N2 sorption, thermogravimetric analyses and FT-IR spectroscopy. The photophysical properties of the MOF material were also examined by steady-state and time-resolved fluorescence studies. It was observed that the blue fluorescence of the MOF material was selectively quenched in the presence of Fe3+ ion in pure aqueous medium. A mechanistic study disclosed that quenching occurs via a strong inner filter effect (IFE) arising from Fe3+ ion in aqueous medium. Interestingly, the fluorescence of the MOF material can be recovered by elimination of the IFE of Fe3+ ion via reduction of Fe3+ ion by ascorbic acid (AA). Based on the fluorescence recovery by AA, a MOF based on-off-on probe was developed for the sensing of Fe3+ ion and AA in aqueous medium. Inspired by this reversible sensing event, we demonstrate basic (NOT, OR, YES, INHIBIT and IMP) and higher integrated logic operations utilizing this fluorescent MOF. This MOF-based logic systems could be potentially used for next-generation logic-gate based analytical applications as well as for the detection and discrimination of targeted molecules in various complex domains.

A Pyrene-Functionalized Metal-Organic Framework for Nonenzymatic and Ratiometric Detection of Uric Acid in Biological Fluid via Conformational Change.

In this work, a water-stable, pyrene-functionalized Hf-based mixed-ligand metal-organic framework (MOF) with UiO-66 (UiO = University of Oslo) framework topology was designed and synthesized for the fluorimetric detection of uric acid (UA) in aqueous medium. The free pyrene-functionalized ligand changes its conformation (monomer to excimer) upon framework formation, which was confirmed via fluorescence spectral studies. The MOF material showed a fast, selective, sensitive (detection limit 1.4 µM), and ratiometric fluorescence response toward UA in pure aqueous medium. During sensing, a further conformational change (excimer to monomer) of the ligand molecule was observed within the MOF. These dual conformation changes helped us to utilize the MOF material for the sensing of UA. Detailed experimental studies showed that the sensing mechanism involves hydrogen-bonding interactions between UA and the framework ligand, which is responsible for the change in conformation and fluorescence emission properties of the pyrene-functionalized ligand molecule. The MOF probe could be reused up to five cycles of sensing events. The MOF material was also utilized for the detection of UA present in serum and urine samples. Moreover, MOF-coated paper test strips were developed for the on-site fluorimetric detection of UA.

The effect of functional groups in the aqueous-phase selective sensing of Fe(iii) ions by thienothiophene-based zirconium metal-organic frameworks and the design of molecular logic gates.

The synthesis of four isoreticular, water-stable Zr(iv) metal-organic frameworks (MOFs) was conducted under solvothermal conditions by using thienothiophene-based ligands. The MOFs were fully characterized by XRPD analyses, FT-IR spectroscopy and TG analyses. The ligands were systematically modified with methyl and phenyl groups for tuning the fluorescence and hydrophobic behaviour of the MOFs. The photophysical properties of free ligands and the corresponding MOFs were investigated by steady-state as well as time-resolved fluorescence experiments. The MOFs containing π-electron rich, conjugated thieno[3,2-b]thiophene units were employed for the sensing of metal ions. All the MOFs displayed high selectivity and sensitivity towards the sensing of biologically important Fe3+ ions in pure aqueous medium through the fluorescence quenching mechanism. Detailed experimental investigations suggest that the transfer of electrons from the electron-rich frameworks to the half-filled 3d orbitals of Fe3+ ions results in fluorescence quenching. Surprisingly, the electron acceptor methyl viologen (MV2+) ions exhibited a reverse trend in the quenching efficiency compared to the Fe3+ ions. Both the steric and electronic effects of the attached functional groups can be proposed to play determining roles in the fluorescence quenching mechanism. All the MOFs showed high photostability and reusability, which are beneficial for the long-term real-world detection of Fe3+ ions. Interestingly, the MOFs can be utilized to construct molecular logic gates for the efficient discrimination between Fe3+ and Fe2+ ions.