Quaternary Ammonium-Mediated Delamination of Europium-Based Metal-Organic Framework into Ultrathin Nanosheets for the Selective Photoelectrochemical Sensing of Fe3

Structural delamination of bulk layered metal-organic frameworks (MOFs) remains a great challenge, largely owing to a lack of general synthetic strategies. Here, we reported a simple solvent-free intercalation strategy for the delamination of rare-earth-based MOF (RE-MOF) with a topology structure of MIL-78 by tuning the chain length of quaternary ammonium salts. Four types of quaternary ammonium salts, involving tetraethylammonium bromide (TEAB), tetrapropylammonium bromide (TPAB), tetrabutylammonium bromide (TBAB), and hexadecyl trimethyl ammonium bromide (CTAB) were introduced to investigate their intercalation capabilities. It is evident in our case that the interruption/intercalation behavior of quaternary ammonium salts differs with their steric structures, and the chain-like CTAB can induce obvious delamination of MIL-78 crystals. Particularly, the CTAB-intercalated ultrathin Eu-based MIL-78 nanosheets exhibited unique selective photoelectrochemical sensing property toward trace amounts of Fe³⁺ ions in aqueous solution with a detection limit of 0.0899 μM at a signal-to-noise ratio of 3. These results demonstrated a green bottom-up strategy to obtain high-quality RE-MOF nanosheets for potential photocurrent response applications.

A cationic thorium-organic framework with triple single-crystal-to-single-crystal transformation peculiarities for ultrasensitive anion recognition

Single-crystal-to-single-crystal transformation of metal-organic frameworks has been met with great interest, as it allows for the creation of new materials in a stepwise manner and direct visualization of structural transitions when subjected to external stimuli. However, it remains a peculiarity among numerous metal-organic frameworks, particularly for the ones constructed from tetravalent metal cations. Herein, we present a cationic thorium-organic framework displaying unprecedented triple single-crystal-to-single-crystal transformations in organic solvents, water, and NaIO3 solution. Notably, both the interpenetration conversion and topological change driven by the SC-SC transformation have remained elusive for thorium-organic frameworks. Moreover, the single-crystal-to-single-crystal transition in NaIO3 solution can efficiently and selectively turn the ligand-based emission off, leading to the lowest limit of detection (0.107 μg kg-1) of iodate, one of the primary species of long-lived fission product 129I in aqueous medium, among all luminescent sensors.

A dye encapsulated zinc-based metal-organic framework as a dual-emission sensor for highly sensitive detection of antibiotics

Self-assembly of two Zn-MOFs, [Zn₂L(DMF)₃]·H₂O·5DMF (1) and [Zn₂L(H₂O)₂]·4H₂O·3DMF (2), was achieved with an amide-functionalized tetracarboxylate ligand under similar conditions. Incorporated amide groups make the tetratopic linkers exhibit different configurations, tetrahedron and square, and subsequently combine tetrahedral [Zn₂(CO₂)₄] clusters or square paddle-well [Zn₂(CO₂)₄] clusters to afford a lon net for 1 and a nbo net for 2. Remarkably, 2 demonstrated high porosity and amide group decorated cages, and thereby proved to be a good capturing agent for a fluorescent dye molecule (DMASM). Consequently, a dual-emitting DMASM@2 sensor was successfully fabricated based on effective energy transfer from the host framework to DMASM with the variable luminescent color being visible to the naked eye. DMASM@2 could be used for the detection of metronidazole (MDZ) and dimetridazole (DTZ) with high sensitivity and remarkable recyclability.

Enhancement of the fluorescence properties via introducing the tetraphenylethylene chromophores into a novel Mn-organic framework with a rare [Mn4(μ3-OH)2] cluster

By employing a tetraphenylethylene (TPE)-based tetracarboxylate linker, tetrakis(4-carboxyphenyl)ethylene (H₄TCPE), we herein constructed a novel luminescent Mn-MOF based on a rare [Mn₄(μ₃-OH)₂] cluster (SQNU-55). Interestingly, the TPE-based SQNU-55 not only provides a good material for the blue LED device, but also has a better luminescent molecular thermometer for low-temperature detection.

Photochromism and photoswitchable luminescence in a Zn7 cluster-based metal-organic framework with an organic guest

Photochromic materials coupled with photoswitchable luminescence functionalities are of particular interest due to their potential applications in switches and optical memory devices. However, the construction of such materials, especially those with two-color emission states, is still challenging. In this context, a rare Zn₇ cluster-based host-guest MOF material, (bbmp)[Zn₇(IPA)₆(OH)₄(H₂O)₂] (1) (H₂IPA = isophthalic acid, bbmp·2I = 4,4'-([1,1'-biphenyl]-4,4'-diyl)bis(1-methylpyridin-1-ium) diiodide), was prepared by encapsulating an organic cation into an anionic MOF produced from zinc cations and isophthalic acid ligands, which exhibits reversible naked detectable photochromic properties varying from yellow to green upon UV-Vis light irradiation. The photoactive guest bbmp²⁺ and the short O⋯N⁺ distances between the oxygen atoms of the carboxylate groups and the pyridine ring play a crucial role in the photochromism of this compound. More interestingly, the luminescence color of this cluster-based host-guest material can be reversibly switched from green to blue upon irradiation, exhibiting photoswitchable luminescence properties.

Metal-Organic Frameworks with Novel Catenane-like Interlocking: Metal-Determined Photoresponse and Uranyl Sensing

The assembly of two tripyridinium-tricarboxylate ligands and different metal ions leads to seven isostructural MOFs, which show novel 2D→2D supramolecular entanglement featuring catenane-like interlocking of tricyclic cages. The MOFs show tripyridinium-afforded and metal-modulated photoresponsive properties. The MOFs with d¹⁰ metal centers (1-Cd, 1-Zn, 2-Cd, 2-Zn) show fast and reversible photochromism and concomitant fluorescence quenching, 1-Ni displays slower photochromism but does not fluoresce, and 1-Co and 2-Co are neither photochromic nor fluorescent. It is shown here that the network entanglement dictates donor-acceptor close contacts, which enable fluorescence originated from interligand charge transfer. The contacts also allow photoinduced electron transfer, which underlies photochromism and concomitant fluorescence response. The metal dependence in fluorescence and photochromism can be related to energy transfer through metal-centered d-d transitions. In addition, 1-Cd is demonstrated to be a potential fluorescence sensor for sensitive and selective detection of UO₂ ²⁺ in water.

Luminescent Metal-Phenolic Networks for Multicolor Particle Labeling

The development of fluorescence labeling techniques has attracted widespread interest in various fields, including biomedical science as it can facilitate high-resolution imaging and the spatiotemporal understanding of various biological processes. We report a supramolecular fluorescence labeling strategy using luminescent metal-phenolic networks (MPNs) constructed from metal ions, phenolic ligands, and common and commercially available dyes. The rapid labeling process (<5 min) produces ultrathin coatings (≈10 nm) on diverse particles (e.g., organic, inorganic, and biological entities) with customized luminescence (e.g., red, blue, multichromatic, and white light) simply through the selection of fluorophores. The fluorescent coatings are stable at pH values from 1 to 8 and in complex biological media owing to the dominant π interactions between the dyes and MPNs. These coatings exhibit negligible cytotoxicity and their strong fluorescence is retained even when internalized into intracellular compartments. This strategy is expected to provide a versatile approach for fluorescence labeling with potential in diverse fields across the physical and life sciences.

Ultrasensitive and Selective Detection of Uranium by a Luminescent Terbium-Organic Framework

Detection and remediation of radioactive components have become the focus of worldwide research interest due to the ever-increasing generation of nuclear waste and the concerns on nuclear accidents. Among the numerous radionuclides, uranium and its isotopes receive the most attention because of their high proportion in nuclear waste and long half-life. Herein, a highly luminescent terbium-organic framework, formulated as [Tb₄(C₂₉O₈H₁₇)₂(NO₃)₄(DMF)₄(H₂O)₄]·4H₂O·8.5DMF (YTU-100), with exceptional sensitivity and selectivity toward uranium was successfully prepared. The material exhibits fast adsorption kinetics and moderate sorption capacity. Interestingly, the luminescence intensity variation highly correlates to the amount of adsorbed uranium, which results in a quantitative, accurate, and selective uranium detection manner. The detection limits in deionized water and tap water were determined to be 1.07 and 0.75 ppb, respectively, which are lower than the US Environmental Protection Agency standard of the maximum contamination of uranium in drinking water. YTU-100 offers an alternative approach for building multifunctional MOFs used for simultaneous detection and removal of uranium from aqueous solutions.

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.

The Synthesis and Properties of TIPA-Dominated Porous Metal-Organic Frameworks

Metal-Organic Frameworks (MOFs) as a class of crystalline materials are constructed using metal nodes and organic spacers. Polydentate N-donor ligands play a mainstay-type role in the construction of metal-organic frameworks, especially cationic MOFs. Highly stable cationic MOFs with high porosity and open channels exhibit distinct advantages, they can act as a powerful ion exchange platform for the capture of toxic heavy-metal oxoanions through a Single-Crystal to Single-Crystal (SC-SC) pattern. Porous luminescent MOFs can act as nano-sized containers to encapsulate guest emitters and construct multi-emitter materials for chemical sensing. This feature article reviews the synthesis and application of porous Metal-Organic Frameworks based on tridentate ligand tris (4-(1H-imidazol-1-yl) phenyl) amine (TIPA) and focuses on design strategies for the synthesis of TIPA-dominated Metal-Organic Frameworks with high porosity and stability. The design strategies are integrated into four types: small organic molecule as auxiliaries, inorganic oxyanion as auxiliaries, small organic molecule as secondary linkers, and metal clusters as nodes. The applications of ratiometric sensing, the adsorption of oxyanions contaminants from water, and small molecule gas storage are summarized. We hope to provide experience and inspiration in the design and construction of highly porous MOFs base on polydentate N-donor ligands.

Linker Engineering toward Full-Color Emission of UiO-68 Type Metal-Organic Frameworks

Luminescent metal-organic frameworks (LMOFs) demonstrate strong potential for a broad range of applications due to their tunable compositions and structures. However, the methodical control of the LMOF emission properties remains a great challenge. Herein, we show that linker engineering is a powerful method for systematically tuning the emission behavior of UiO-68 type metal-organic frameworks (MOFs) to achieve full-color emission, using 2,1,3-benzothiadiazole and its derivative-based dicarboxylic acids as luminescent linkers. To address the fluorescence self-quenching issue caused by densely packed linkers in some of the resultant UiO-68 type MOF structures, we apply a mixed-linker strategy by introducing nonfluorescent linkers to diminish the self-quenching effect. Steady-state and time-resolved photoluminescence (PL) experiments reveal that aggregation-caused quenching can indeed be effectively reduced as a result of decreasing the concentration of emissive linkers, thereby leading to significantly enhanced quantum yield and increased lifetime.

Polymer Ligand-Sensitized Lanthanide Metal-Organic Frameworks for an On-Site Analysis of a Radionuclide

Herein, a new strategy to increase the sensitivity of a lanthanide metal-organic framework (Ln-MOF) to UO₂²⁺ was proposed by using polymeric ligands. By utilizing [Tb(1,3,5-benzenetrisbenzoate)]n (Tb-TBT) MOF as the host, preloaded 2-vinyl terephthalic acid (VTP) was polymerized in situ, which produced a novel fluorescent composite denoted as PVTP⊂Tb-TBT. Benefiting from the coordination of PVTP to the Tb nodes, the polymeric chains performed both as molecular scaffolds that improved the water stability of the framework and as additional antennae that sensitized the photoluminescence of the Tb nodes. More importantly, the detection sensitivity and selectivity of PVTP⊂Tb-TBT to UO₂²⁺ were much improved compared to those of Tb-TBT. Detailed characterizations indicated that the incorporation of PVTP efficiently enriched UO₂²⁺ in the probe, which promoted the energy dissipation to UO₂²⁺. Besides, UO₂²⁺ was also supposed to release PVTP from PVTP⊂Tb-TBT and, thus, exposed the open metal sites to water molecules, which interrupted the sensitization effect of PVTP and induced a nonradiative energy dissipation. A limit of detection (LOD) as low as 0.75 nm was recorded by suspending the PVTP⊂Tb-TBT probe in a water sample, far below the limit in drinking water set by the United States Environmental Protection Agency (130 nm). Furthermore, a remotely controlled sampling and an on-site analysis of real water samples were realized by facilely loading PVTP⊂Tb-TBT on thin films (TFs). The LOD for UO₂²⁺ was 2.5 nm by using the TFs. This study reports a new strategy for boosting the sensitivity and selectivity of Ln-MOF to monitor UO₂²⁺ and expands the application of the strategy to an on-site analysis.

A metal–organic framework featuring highly sensitive fluorescence sensing for Al3+ ions

A new fluorescent MOF can detect Al3+ ions with high selectivity and sensitivity via turn-off effect and emission color change.

Assembly of {Co14} nanoclusters from adenine-modified Co4-thiacalix[4]arene units

An adenine-modified Co₄-thiacalix[4]arene unit can serve as a second building unit for fabrication of three Co₁₄ clusters with different structures.