Zr-Based Metal-Organic Frameworks with Intrinsic Peroxidase-Like Activity for Ultradeep Oxidative Desulfurization: Mechanism of H2O2 Decomposition.

The restriction of sulfur content in fuels has become increasingly stringent as a result of the growing environmental concerns. Although several MOF-derived materials like POM@MOF composites have shown the ability to catalyze oxidative desulfurization (ODS), their catalytic activities inevitably obstructed by the encapsulated catalytic sites like POM due to the blockage of cavities. Therefore, MOFs with intrinsic and accessible catalytic sites are highly desirable for their applications in ultradeep ODS. Herein, four representative Zr-based MOFs (Zr-MOFs), namely, UiO-66, UiO-67, NU-1000, and MOF-808, were assessed for catalytic ODS. These MOFs were confirmed that they have peroxidase-like activity and can catalyze ODS with H2O2 as oxidant. Among them, MOF-808 showed the highest catalytic activity and it can fully desulfurize dibenzothiophene (DBT) in a model gasoline with a S concentration of 1000 ppm under 40 °C within 5 min. An extremely low apparent Arrhenius activation energy (22.0 KJ·mol-1) and an extraordinarily high TOF value (42.7 h-1) were obtained, ranking MOF-808 among the best catalysts for the catalytic DBT oxidation. Further studies confirmed that the excellent catalytic activity is mainly responsible for the high concentration of the accessible Zr-OH(H2O) catalytic sites decorated in MOF-808. The superoxide radicals (•O2-) and hydroxyl radicals (•OH) were identified and were proved to involve in the DBT oxidation. Besides, the effects of Brönsted and lewis acidity to the catalytic efficiency were also discussed. Based on the experimental results, a plausible mechanism concerning on Zr-OH(H2O) groups promoting the H2O2 decomposion in to both •O2- and •OH was first proposed. Moreover, MOF-808 can be facilely reused for at least eight runs without significant loss of its catalytic activity. By the integration of facile synthesis, high catalytic efficiency, and good stability, MOF-808 thus represents a new benchmark catalyst for catalytic oxidative desulfurization.

Dual-Emissive Metal-Organic Framework as a Fluorescent "Switch" for Ratiometric Sensing of Hypochlorite and Ascorbic Acid.

The detection of hypochlorite (ClO-) content in tap water is extremely important because excess amounts of hypochlorite can convert into highly toxic species and inadequate amounts of hypochlorite cannot fully kill bacteria and viruses. Although several metal-organic frameworks (MOFs) have been successfully employed as fluorescent sensors for hypochlorite detection, all these sensors are based on single emission that responds to the dose of hypochlorite. Ratiometric sensors are highly desirable, which can improve the sensitivity, accuracy, and reliability via self-calibration. Herein, a nanoscale dual-emission multivariate 5-5-Eu/BPyDC@MOF-253-NH2 was synthesized by sequential mixed-ligand self-assembly and postsynthesis method. Among the two emission bands of 5-5-Eu/BPyDC@MOF-253-NH2, the strong blue emitting derived from ligands is sensitive to hypochlorite, while the red emitting derived from Eu(III) almost keeps invariable. Therefore, 5-5-Eu/BPyDC@MOF-253-NH2 was exploited as a fluorescent ratiometric nanosensor for "on-off" sensing of hypochlorite. Notably, the proposed sensing system showed an excellent performance including fast response (within 15 s), relative high specificity, wide linear range (0.1-30 µM), and low detection limit (0.094 µM). Besides, the suppressed blue emitting was recovered after the addition of ascorbic acid (AA) that consumes ClO- via the redox reaction. Therefore, 5-5-Eu/BPyDC@MOF-253-NH2 was further employed as a fluorescent ratiometric nanosensor for the "on-off-on" sensing of AA. This work represents the first MOF-based fluorescent "switch" for the ratiometric sensing of hypochlorite and the second for ratiometric sensing of AA.

Encapsulation of Phosphotungstic Acid into Metal-Organic Frameworks with Tunable Window Sizes: Screening of PTA@MOF Catalysts for Efficient Oxidative Desulfurization.

Clean fuels with extremely low sulfur content are highly desirable due to environmental concerns. Herein, three water-stable and eco-friendly metal-organic frameworks with tunable window diameters, denoted as MOF-808X, have been employed as PTA solid supports. An array of PTA@MOF-808X composites were facilely synthesized via the encapsulation strategy. With tunable window sizes and adjustable PTA loading amounts, the obtained PTA@MOF-808X composites were screened for catalytic oxidative desulfurization (ODS) with H2O2 serving as oxidant. The experiments found that 42%PTA@MOF-808A had the highest catalytic ODS activity and could completely remove dibenzothiophene (DBT) in a model fuel with an initial sulfur content of 1000 ppm within 30 min, which falls far below the acceptable limits for fuel standards (10 ppm). Further investigations revealed that this high catalytic activity could be attributed to the cooperative catalysis of metal clusters in the host framework and the guest PTA molecules. Moreover, 42%PTA@MOF-808A could be facilely recovered and reused for at least five runs without loss of catalytic activity. Having a combination of eco-sustainability, high stability, high catalytic activity, and good recyclability, 42%PTA@MOF-808A therefore represents a new benchmark material for catalytic ODS and provides a new perspective for ultradeep desulfurization.

MOF-808: A Metal-Organic Framework with Intrinsic Peroxidase-Like Catalytic Activity at Neutral pH for Colorimetric Biosensing.

Natural enzyme mimetics with high catalytic activity at nearly neutral pH values are highly desired for their applications in biological systems. Herein for the first time a stable MOF, namely MOF-808, has been shown to possess high intrinsic peroxidase-like catalytic activity under acidic, neutral, and alkaline conditions. As a novel peroxidase mimetic, MOF-808 can effectively catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine when H2O2 serves as oxidant, accompanied by a significant color variation in the solution. The catalytic activity and the color variation were greatly dependent on H2O2 concentration, and thus MOF-808 can be applied to the colorimetric sensing of H2O2. The H2O2 detection limit is 4.5 µM, and the linear range is 10 µM to 15 mM. In view of the significant inhibition effect produced by ascorbic acid, a facile and sensitive approach for colorimetric sensing of ascorbic acid was successfully established. The AA detection limit is 15 µM, and the linear range is 30-1030 µM. Further investigation found that the catalytic activity of MOF-808 could be mainly ascribed to the Zr-OH(OH2) groups. Such active Zr-OH(OH2) groups can be effectively shielded by gluconic acid, and subsequently the catalytic activity of MOF-808 was significantly suppressed. With these findings, a facile and selective colorimetric assay for glucose sensing has been successfully explored via combination of the glucose oxidation with the TMB oxidation. The glucose detection limit is 5.7 µM, and the linear range is 5.7-1700 µM. MOF-808 is one of the best colorimetric biosensors among the peroxidase mimics reported for H2O2, AA, and glucose detection.

Efficient Capture and Effective Sensing of Cr2O72- from Water Using a Zirconium Metal-Organic Framework.

Highly efficient decontamination of heavily toxic Cr2O72- from water remains a serious task for public health and ecosystem protection. An easily regenerative and reused sorbent with suitable porosity may address this task. Herein, a series of water-stable and ecofriendly metal-organic frameworks (MOFs) with large surface areas were assessed for their ability to adsorb and separate Cr2O72- from aqueous solutions. Among these tested MOFs, NU-1000 shows an extraordinary capability to efficiently capture (within 3 min) Cr2O72- with a sorption capacity of up to 76.8 mg/g, which is the largest one for the neutral MOF-based Cr2O72- sorbents. NU-1000 also shows remarkable selectivity for Cr2O72- capture and can effectively reduce the Cr(VI) concentration from 24 ppm to 60 ppb, which is below the acceptable limit for the drinking water standard (100 ppb by the U.S. Environmental Protection Agency). Moreover, this adsorbent can be easily regenerated by Soxhlet extraction with an acidic methanol solution (2.5 M HCl) and can be reused at least three times without a significant loss of it adsorption ability. More intriguingly, NU-1000 can also serve as an efficient photoluminescent probe for the selective detection of Cr2O72- in aqueous media. The Cr2O72- detection limit is as low as 1.8 µM, and the linear range is from 1.8 to 340 µM. Our work shows that NU-1000 is a unique material combining both efficient sorption and exceptional fluorescent sensing of Cr2O72- in aqueous media.

Photo-Stimuli-Responsive Dual-Emitting Luminescence of a Spiropyran-Encapsulating Metal-Organic Framework for Dynamic Information Encryption.

The development of photo-stimuli-responsive luminescent materials is essential to address emerging demands in encryption security. Here, a novel photo-stimuli-responsive dual-emitting luminescent material ZJU-128⊃SP (SP = spiropyran) is reported, which is obtained by encapsulating spiropyran molecules into a cadmium-based metal-organic framework (MOF), [Cd3 (TCPP)2 ]·4DMF·4H2 O (ZJU-128, H4 TCPP = 2,3,5,6-tetrakis (4-carboxyphenyl)pyrazine). This MOF/dye composite ZJU-128⊃SP exhibits a blue emission from the ligand of ZJU-128 at 447 nm and a red emission around 650 nm from spiropyran. Utilizing the photoisomerization of spiropyran from ring-closed to ring-open form under UV-light irradiation, significant fluorescence resonance energy transfer (FRET) process between ZJU-128 and spiropyran is achieved. As a result, the blue emission of ZJU-128 is gradually decreased while the red emission of spiropyran increases. This dynamic fluorescent behavior can fully recover to the original state following exposure to visible light (>405 nm). By taking advantage of the time-dependent fluorescence, dynamical anti-counterfeiting patterns and multiplexed coding are successfully developed based on ZJU-128⊃SP film. This work provides an inspiring point for the design of information encryption materials with higher security requirements.

Precise Design and Deliberate Tuning of Turn-On Fluorescence in Tetraphenylpyrazine-Based Metal-Organic Frameworks.

The manipulation on turn-on fluorescence in solid state materials attracts increasing interests owing to their widespread applications. Herein we report how the nonradiative pathways of tetraphenylpyrazine (TPP) units in metal-organic frameworks (MOFs) systems could be hindered through a topological design approach. Two MOFs single crystals of different topology were constructed via the solvothermal reaction of a TPP-based 4,4',4″,4‴-(pyrazine-2,3,5,6-tetrayl) tetrabenzoic acid (H4TCPP) ligand and metal cations, and their mechanisms of formation have been explored. Compared with the innate low-frequency vibrational modes of flu net Tb-TCPP-1, such as phenyl ring torsions and pyrazine twists, Tb-TCPP-2 adopts a shp net, so the dihedral angle of pyrazine ring and phenyl arms is larger, and the center pyrazine ring in TPP unit is coplanar, which hinders the radiationless decay of TPP moieties in Tb-TCPP-2. Thereby Tb-TCPP-2 exhibits a larger blue-shifted fluorescence and a higher fluorescence quantum yield than Tb-TCPP-1, which is consistent with the reduced nonradiative pathways. Furthermore, Density functional theory (DFT) studies also confirmed aforementioned tunable turn-on fluorescence mechanism. Our work constructed TPP-type MOFs based on a deliberately topological design approach, and the precise design of turn-on fluorescence holds promise as a strategy for controlling nonradiative pathways.

Dual-emissive metal-organic framework: a novel turn-on and ratiometric fluorescent sensor for highly efficient and specific detection of hypochlorite.

Hypochlorite (ClO-) is widely used as a disinfectant, whose residue content in water should be strictly controlled due to the potential threat to human health in an inappropriate concentration. Herein, dual-emissive metal-organic frameworks with a UiO-66 prototype structure, PDA/Eu/PDA-UiO-66-NH2(x), were elegantly designed and prepared by a mixed ligand assembly and sequential post-synthesis strategy. Since blue emission is sensitive to ClO-, PDA/Eu/PDA-UiO-66-NH2(40) was selected as a model nanosensor for ratiometric and turn-on sensing of ClO- while red emission acts as a reference signal. Remarkably, PDA/Eu/PDA-UiO-66-NH2(40) shows high efficiency and specificity toward ClO- detection, as verified by a very short response time of 15 s, a wide linear range of 0.1-60 µM, a low detection limit of 0.10 µM, and excellent selectivity toward common competing ions. The recovery experiments show that the recoveries of spiking ClO- in tap water range from 96 to 103%. The rigidification of the coordinated H2N-BDC2- ligands should be responsible for the turn-on fluorescence of PDA/Eu/PDA-UiO-66-NH2(40). This work not only shows a highly efficient and specific fluorescent nanosensor for ClO- detection but also presents the first MOF-based fluorescent probe for turn-on and ratiometric sensing of ClO-.