Dual-emitting barium based metal-organic nanosheets as a potential sensor for temperature and anthrax biomarkers

A new three-dimensional barium(II) coordination polymer constructed from N,N'-bis(glycinyl)pyromellitic diimide: microwave-assisted synthesis, structure, Hirshfeld surface analysis and properties

A new three-dimensional (3D) coordination polymer, namely, poly[diaqua[μ5-2,2'-(1,3,5,7-tetraoxo-1,2,3,5,6,7-hexahydropyrrolo[3,4-f]isoindole-2,6-diyl)diacetato]barium(II)], [Ba(C14H6N2O8)(H2O)2]n, (I), has been synthesized by the microwave-irradiated reaction of Ba(NO3)2 with N,N'-bis(glycinyl)pyromellitic diimide {BGPD, namely, 2,2'-(1,3,5,7-tetraoxo-1,2,3,5,6,7-hexahydropyrrolo[3,4-f]isoindole-2,6-diyl)diacetatic acid, H2L}. The title compound was structurally characterized by single-crystal X-ray diffraction analysis and powder X-ray diffraction analysis, as well as IR spectroscopy. In the crystal structure of (I), the BaII ion is nine-coordinated by six carboxylate O atoms from five symmetry-related L2- dianions and one imide O atom, as well as two water O atoms. The coordination geometry of the central BaII ion can be described as a spherical capped square antiprism. One carboxylate group of the ligand serves as a μ3-bridge linking the BaII cations into a one-dimensional polynuclear secondary building unit (SBU). Another carboxylate group of the ligand acts as a μ2-bridge connecting the 1D SBUs, thereby forming a two-dimensional (2D) SBU. The resulting 2D SBUs are extended into a 3D framework via the pyromellitic diimide moiety of the ligand as a spacer. The 3D Ba framework can be simplified as a 5-connected hexagonal boron nitride net (bnn) topology. The intermolecular interactions in the 3D framework were further investigated by Hirshfeld surface analysis and the results show that the prominent interactions are H...O (45.1%), Ba...O (11.1%) and C...H (11.1%), as well as H...H (11.1%) contacts. The thermal stability, photoluminescence properties and UV-Vis absorption spectra of (I) were also investigated. The coordination polymer exhibits a fluorescence emission with a quantum yield of 0.071 and high thermal stability.

Multi-analyte fluorescence sensing based on a post-synthetically functionalized two-dimensional Zn-MOF nanosheets featuring excited-state proton transfer process

Covalent post-synthetic modification of metal-organic frameworks (MOFs) represents an underexplored but promising avenue for allowing the addition of specific fluorescent recognition elements to produce the novel MOF-based sensory materials with multiple-analyte detection capability. Here, an excited-state proton transfer (ESPT) active sensor 2D-Zn-NS-P was designed and constructed by covalent post-synthetic incorporation of the excited-state tautomeric 2-hydroxypyridine moiety into the ultrasonically exfoliated amino-tagged 2D Zn-MOF nanosheets (2D-Zn-NS). The water-mediated ESPT process facilitates the highly accessible active sites incorporated on the surface of 2D-Zn-NS-P to specifically respond to the presence of water in common organic solvents via fluorescence turn-on behavior, and accurate quantification of trace amount of water in acetonitrile, acetone and ethanol was established using the as-synthesized nanosheet sensor with the detection sensitivity (<0.01% v/v) superior to the conventional Karl Fischer titration. Upon exposure to Fe3+ or Cr2O72-, the intense blue emission of the aqueous colloidal dispersion of 2D-Zn-NS-P was selectively quenched even in the coexistence of common inorganic interferents. The prohibition of the water-mediated ESPT process and local emission, induced by the coordination of ESPT fluorophore with Fe3+ or by Cr2O72- competitively absorbs the excitation energy, was proposed to responsible for the fluorescence turn-off sensing of the respective analytes. The present study offers the attractive prospect to develop the ESPT-based fluorescent MOF nanosheets by covalent post-synthetic modification strategy as multi-functional sensors for detection of target analytes.

2D Metal-Organic Frameworks: Properties, Synthesis, and Applications in Electrochemical and Optical Biosensors

Two-dimensional (2D) nanomaterials like graphene, layered double hydroxides, etc., have received increasing attention owing to their unique properties imparted by their 2D structure. The newest member in this family is based on metal-organic frameworks (MOFs), which have been long known for their exceptional physicochemical properties-high surface area, tunable pore size, catalytic properties, etc., to list a few. 2D MOFs are promising materials for various applications as they combine the exciting properties of 2D materials and MOFs. Recently, they have been extensively used in biosensors by virtue of their enormous surface area and abundant, accessible active sites. In this review, we provide a synopsis of the recent progress in the field of 2D MOFs for sensor applications. Initially, the properties and synthesis techniques of 2D MOFs are briefly outlined with examples. Further, electrochemical and optical biosensors based on 2D MOFs are summarized, and the associated challenges are outlined.

Highly Selective Chloromethanes Detection Based on Quartz Crystal Microbalance Gas Sensors with Ba-MOFs

Three new metal-organic frameworks (MOFs), {(CH₃NH₃)₃[Ba₂(TTHA)(NO₃)(H₂O)₂]}·2H₂O (1), {(CH₃NH₃)₄[Ba₃(HTTHA)₂(H₂O)₇]}·3H₂O (2), and [Ba₇(TTHA)₂(NO₃)₂(H₂O)₁₀]·2H₂O (3) (H₆TTHA = 1,3,5-triazine-2,4,6-triamineh-exaacetic acid) have been synthesized and characterized. The sensing properties of 1-3 were explored with regard to volatile organic compounds (VOCs) by the quartz crystal microbalance (QCM) technique. The results indicated that 1 and 2 have a much higher selectivity and response to chloromethanes (CH₂Cl₂, CHCl₃, and CCl₄) compared with H₂O, CH₃OH, CH₃CH₂OH, CH₃CN, (CH₃)₂CO, C₆H₆, C₆H₅CH₃, C₆H₅CH₂CH₃, and C₆H₅Cl at room temperature. Furthermore, 1 and 2 sensing film also exhibits excellent reversibility and stability, and the response and recovery times are almost within 10 s. 3 displays a lower response and poor selectivity to the above VOCs. The significant difference may be caused by their different structural characteristics. The Ba²⁺ ions are all decacoordinated in 1 and 2, while Ba²⁺ ions have more open metal sites in 3. So, the high selectivity and response of 1 and 2 may be due to the exchange of coordination water molecules with chloromethanes and possible electrostatic effects between (CH₃NH₃)⁺ cations and chloromethanes containing more electronegative Cl atoms. DFT calculation results show that the bond energy of Ba-Cl and Ba-O is not much different, so chloromethanes at high concentrations may exchange coordination water to form weak Ba···Cl interactions and show higher response values. 3 has no obvious VOCs selectivity and higher response due to more open sites of Ba²⁺ ions and smaller pore size. This work develops a fast and effective method to detect chloromethanes, providing a new opportunity for designing QCM gas sensors coated with different MOF materials.

Monolayer nanosheets formed by liquid exfoliation of charge-assisted hydrogen-bonded frameworks

Hydrogen-bonded organic frameworks (HOFs) are a diverse and tunable class of materials, but their potential as free-standing two-dimensional nanomaterials has yet to be explored. Here we report the self-assembly of two layered hydrogen-bonded frameworks based on strong, charge-assisted hydrogen-bonding between carboxylate and amidinium groups. Ultrasound-assisted liquid exfoliation of both materials readily produces monolayer hydrogen-bonded organic nanosheets (HONs) with micron-sized lateral dimensions. The HONs show remarkable stability and maintain their extended crystallinity and monolayer structures even after being suspended in water at 80 °C for three days. These systems also exhibit efficient fluorescence quenching of an organic dye in organic solvents, superior to the quenching ability of the bulk frameworks. We anticipate that this approach will provide a route towards a diverse new family of molecular two-dimensional materials.

0D to 3D PrIII metal-organic networks crystal engineered for optimal iodine adsorption

Four new praseodymium(III) metal-organic compounds varying in dimensionality from 0D to 3D have been designed and synthesized based on N-heterocyclic polycarboxylic acids, including pyridine-2,6-dicarboxylic acid (H₂pydc) and pyrazine-2,3-dicarboxylic acid (H₂pzdc). Altering the concentration of piperazine (pip, ancillary ligand) enables control over the dimensionality of the compound by switching between the 0D [H₂pip][Hpip][Pr(pydc)₃]·4H₂O (I) and the 1D {[Pr(pydc)(Hpydc)(H₂O)₂]·4H₂O}_n (II) coordination polymer (CP). Upon replacing H₂pydc with H₂pzdc, CP II is converted to the 2D CP [Pr(pzdc)(Hpzdc)(H₂O)₃]_n (III) and using the metalloligand [Zn(Hpzdc)₂(H₂O)₂]^2-, the 3D heterometallic CP {[Pr₂Zn(pzdc)₄(H₂O)₆]·2H₂O}_n (IV) is formed. Compound IV shows high stability in the absence of uncoordinated solvent molecules and is stable up to 400°C, even in the presence of humidity. Therefore, IV was utilized for iodine adsorption in the vapour phase and in the presence of humidity. The results confirm the remarkable potential of IV for reversible adsorption of iodine vapour.