Immobilization of Silver(I) Ions on Amino-Functionalized Chromium(III) Terephthalate with Organophosphine and its C-H Carboxylation of a Heteroaromatic Compound

A newly designed heterogenized catalyst that incorporates silver(I) ions with 2-(dicyclohexylphosphaneyl)acetaldehyde (PCy2 aldehyde) into amino-functionalized chromium(III) terephthalate is developed. Silver(I) ions were robustly immobilized on the amino-functionalized chromium(III) terephthalate, which contains an imine bond formed by the reaction with PCy2 aldehyde. The Ag(I) ion is coordinated with the phosphine in the imine group to create MIL-101-AP(Ag). Characterizations were carefully carried out according to the synthetic steps. The catalytic performance of MIL-101-AP(Ag) was evaluated through the C-H carboxylation of thiophene-2-carbonitrile, achieving a 10 % yield with a turnover number of 1.0. The recyclability of the MIL-101-AP(Ag) catalyst was successfully demonstrated with five cycle, with no loss in activity and selectivity observed. This approach, which involves the formation of an imine bond to facilitate silver loading with phosphine on amino-functionalized MIL-101(Cr), exhibits significant potential for both CO2 fixation and C-H carboxylation, thereby highlighting the modified material's promise as a sustainable catalyst.

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.

Schiff base-functionalized metal-organic frameworks as an efficient adsorbent for the decontamination of heavy metal ions in water

Adsorptive removal of heavy metal ions from water is an energy- and cost-effective water decontamination technology. Schiff base functionalities can be incorporated into the pore cages of metal-organic frameworks (MOFs) via direct synthesis, post-synthetic modification, and composite formation. Such incorporation can efficiently enhance the interactions between the MOF adsorbent and target heavy metal ions to promote the selective adsorption of the latter. Accordingly, Schiff base-functionalized MOFs have great potential to selectively remove a particular metal ion from the aqueous solutions in the presence of coexisting (interfering) metal ions through the binding sites within their pore cages. Schiff base-functionalized MOFs can bind divalent metal ions (e.g., Pb(II), Co(II), Cu(II), Cd (II), and Hg (II)) more strongly than trivalent metal ions (e.g., Cr(III)). The adsorption capacity range of Schiff base-functionalized MOFs for divalent ions is thus much more broad (22.4-713 mg g-1) than that of trivalent metal ions (118-127 mg g-1). To evaluate the adsorption performance between different adsorbents, the two parameters (i.e., adsorption capacity and partition coefficient (PC)) are derived and used for comparison. Further, the possible interactions between the Schiff base sites and the target heavy metal ions are discussed to help understand the associated removal mechanisms. This review delivers actionable knowledge for developing Schiff-base functionalized MOFs toward the adsorptive removal of heavy metal ions in water in line with their performance evaluation and associated removal mechanisms. Finally, this review highlights the challenges and forthcoming research and development needs of Schiff base-functionalized MOFs for diverse fields of operations.

Multivariate covalent organic frameworks guided carboxyl functionalized magnetic adsorbent for enrichment of fluoroquinolones in milk prior to high performance liquid chromatographic analysis

Herein, we prepared a carboxyl functionalized magnetic covalent organic framework (Fe3O4@iCOF-COOH) by combining multivariate synthetic strategy with post-synthetic modification. It was used as an adsorbent for magnetic solid phase extraction (MSPE) of six fluoroquinolones (FQs), and showed good absorption performance at neutral pH. Carboxyl groups are found to be crucial for the adsorption of fluoroquinolones. The adsorption mechanism was primarily attributed to strong hydrogen bonding, π-π interaction as well as potential hydrophobic effect. The optimal extraction conditions are sample pH at 6.0, adsorbent dosage of 3 mg, eluent of 1.0 mL methanol solution containing 7.5% ammonia, and extraction/desorption time of 30 min. Under the optimized conditions, the Fe3O4@iCOF-COOH was used as an adsorbent for MSPE of FQs in milk, an analytical method was established by combining with high-performance liquid chromatography-ultraviolet detection (HPLC-UV). The limits of detection (LODs) and limit of quantification (LOQs) were 1.24-4.58 ng⋅mL-1 and 4.12-15.3 ng⋅mL-1, respectively. The recoveries of target FQs in spiked milk were 68.4-105%. This work provides a new way to prepare covalent organic framework based adsorbents for solid phase extraction, and can be readily extended to other type of adsorbents.

Europium chelate-anionic exchange functionalized covalent organic frameworks for the sensing of aristolochic acid a in humans and sulfamethoxazole/trimethoprim in surface water

The application of covalent organic frameworks (COFs) in fluorescence detection is of great interest. Herein, we have synthesized the ionic covalent organic framework TGH⁺·PD: Eu(TTA)₄ with the characteristic emission of lanthanides by a straightforward ion-exchange method. This is the first time that aristolochic acid A (AA), a key biomarker for absorption and metabolism in the body for early diagnosis of diseases, has been detected by using COF as a fluorescent probe, which exhibits a good linear correlation with the AA concentration over a range from 5.0 to 1000 μM with a detection limit of 0.0808 μM. In addition, the selective response to sulfamethoxazole (SMZ)/trimethoprim (TMP) is achieved by varying the excitation wavelength with detection lines of 30.2 nM and 2.898 μM, respectively. It is worth mentioning that BNPP has been developed for the accurate determination of SMZ in uncertain samples. In a word, the prepared TGH⁺·PD: Eu(TTA)₄-based sensor can be used for the quantitative detection of AA and SMZ/TMP, separately, effectively extending the application of COFs in the field of fluorescence sensing.

Room Temperature Hydroxyl Group-Assisted Preparation of Hydrophobicity-Adjustable Metal-Organic Framework UiO-66 Composites: Towards Continuous Oil Collection and Emulsion Separation

Developing a straightforward and effective hydrophobic modification for metal-organic frameworks (MOFs) under mild conditions is meaningful for MOF applications. Here, a post-synthetic modification approach assisted with metal hydroxyl groups at room temperature is reported to induce hydrophobicity in the hydrophilic UiO-66. The bonding between Zr-OH in UiO-66 and n-tetradecylphosphonic acid (TPA) is the essential force for the modifier TPA. Superhydrophobic and superoleophilic composites were constructed for efficient oil-water separation by coating TPA-modified UiO-66 (P-UiO-66) on commercial melamine sponges (MS) and filter papers (FP). The P-UiO-66/MS composite could quickly and selectively absorb oily liquids up to 43 times its weight from water. P-UiO-66/FP and P-UiO-66/MS showed high separation efficiencies for water-in-oil emulsions and oil-in-water emulsions, respectively, with high resistance to low/high temperatures and acid/base conditions. The metal hydroxyl group-assisted post-synthetic modification strategy offers a facile and broad way to prepare hydrophobic MOFs for promising applications in environmental fields.

Novel strategies for the formulation and processing of aluminum metal-organic framework-based sensing systems toward environmental monitoring of metal ions

Aluminum is a relatively inexpensive and abundant metal for the mass production of metal-organic frameworks (MOFs). Aluminum-based MOFs (Al-MOFs) have drawn a good deal of research interest due to their unique properties for diverse applications (e.g., excellent chemical and structural stability). This review has been organized to highlight the current progress achieved in the synthesis/functionalization of Al-MOF materials with the special emphasis on their sensing application, especially toward metal ion pollutants in the liquid phase. To learn more about the utility of Al-MOF-based sensing systems, their performances have been evaluated for diverse metallic components in reference to many other types of sensing systems (in terms of the key quality assurance (QA) criteria such as limit of detection (LOD)). Finally, the challenges and outlook for Al-MOF-based sensing systems are discussed to help expand their real-world applications.

Sulfonate-modified calixarene-based porous organic polymers for electrostatic enhancement and efficient rapid removal of cationic dyes in water

Developing of fast and efficient adsorbents for removal of low concentration refractory organics in water is significant. Herein, a novel calix[4]arene-based porous organic polymer CaPy is constructed through Sonogashira-Hagihara cross-coupling polycondensation. The strong polar sulfonate is further anchored onto the polymer skeleton of CaPy and three sulfonate-modified anionic polymers CaPy-S1, CaPy-S2, and CaPy-S3 were obtained and fully characterized. The adsorption isotherms showed that the maximum adsorption capacities of CaPy, CaPy-S1, CaPy-S2, and CaPy-S3 toward methylene blue (MB) were 270, 1454, 558 and 1381 mg g^-1, whereas those for Rhodamine B (RhB) were 183, 2653, 1132, and 1796 mg g^-1, respectively. The maximum adsorption capacity toward RhB was the highest reported vale among the currently used synthetic adsorbents. In addition, the pseudo-second-order rate constants of CaPy, CaPy-S1, CaPy-S2, and CaPy-S3 toward MB were 0.00572, 0.488, 2.24, and 0.192 g mg^-1 min^-1, respectively, and those toward RhB were 0.000234, 0.138, 0.0819, and 0.203 g mg^-1 min^-1, respectively. The pseudo-second-order rate constant of CaPy-S2 toward MB was 2.24 g mg^-1 min^-1 indicating one of the highest adsorption speeds. The activation energy of CaPy-S1 for RhB and MB were 121 and 109 kJ mol^-1, respectively, demonstrating that the adsorption of both dyes on CaPy-S1 was chemisorption process. Further, the obtained values of Gibbs free energy were negative, revealing that the adsorption process was spontaneous. This work provides an effective approach for improving adsorption performance via post-modification.

Enhancement of aqueous stability of NH2-MIL-101(Fe) by hydrophobic grafting post-synthetic modification

The development of water-stable metal-organic frameworks is a critical issue for their photocatalysis applications in water treatment. A phenyl-ethyl side chain with low surface energy was grafted into NH₂-MIL-101(Fe) through a post-synthetic modification (PSM) method. As a result, a novel MIL-101(Fe)-1-(4-(ethyl)phenyl)urea (named MIL-101(Fe)-EPU) was synthesized. Basic morphology, crystal structure, and chemical bond features of MIL-101(Fe)-EPU were retained after PSM. Nitrogen X-ray photoelectron spectroscopy analysis confirmed the successful introduction of the phenyl-ethyl side chain, and this transformation increased its hydrophobicity and water stability. Contact angles of MIL-101(Fe)-EPU to water raised from 59.6 to 140.4°. And its structure maintained intact after 72 h water exposure, indicating higher stability than parent NH₂-MIL-101(Fe). In the photocatalysis reaction with visible light and oxidant donor (H₂O₂), MIL-101(Fe)-EPU demonstrated a degradation efficiency of tetrabromobisphenol A with a reaction rate at 0.0313 min^-1. The predominant reaction mechanism was OH·oxidation. The acid condition was beneficial for this photocatalysis reaction and high stability was observed. Besides, photocatalysis efficiency, crystal structure, and chemical structures were all retained in different actual water mediums, suggesting high adaptability of MIL-101(Fe)-EPU. In general, hydrophobic group grafting using a PSM method endows MIL-101(Fe)-EPU the potentiality as photocatalyst for organic contaminant elimination from water.

Ag-exchanged mesoporous chromium terephthalate with sulfonate for removing radioactive methyl iodide at extremely low concentrations in humid environments

The development of efficient adsorbents to remove radioactive methyl iodide (CH₃I) in humid environments is crucial for air purification after pollution by nuclear power plant waste. In this work, we successfully prepared a post-synthetic covalent modified MIL-101 with a sulfonate group followed by the ion-exchange of Ag (I), which is well characterized by diffuse reflectance FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS) and the hydrophobic index (HI). After modification of the MOFs, we applied functionalized MIL-101 obtained by either one-pot synthesis (MIL-101-SO₃Ag) or a post-synthetic modification process (MIL-101-RSO₃Ag, R = NH(CH₂)₃) to remove the CH₃I at an extremely low concentration (0.31 ppm) in an environment with very high relative humidity (RH 95%). Enhanced hydrophobicity of the surface-modified MIL-101 was evaluated by examining the HI with the competitive adsorption of water and cyclohexane vapor, with a high surface area maintained, as confirmed by Ar physisorption. Interestingly, the post-synthetically modified MIL-101-RSO₃Ag showed exceptional adsorption performance as determined by its decontamination factor (DF = 195,350) at 303 K and RH 95%. This performance was in comparison to Ag (I)-exchanged 13X zeolite and MIL-101-SO₃Ag, which include much higher amounts of Ag. Furthermore, MIL-101-RSO₃Ag retained ~94-100% of its fresh adsorbent performance during five cycle repetitions.

Room-Temperature Synthesis of a Hollow Microporous Organic Polymer Bearing Activated Alkyne IR Probes for Nonradical Thiol-yne Click-Based Post-Functionalization

This work shows that hollow microporous organic polymer (H-MOP-A) with activated internal alkynes as IR probes can be prepared by template synthesis based on acyl Sonogashira-Hagihara coupling at room temperature. The H-MOP-A is a versatile platform in the main chain PSM based on nonradical thiol-yne click reaction. Moreover, an IR peak of internal alkynes in the H-MOP-A is very intense and could be utilized in the monitoring of thiol-yne click-based main chain PSM. The functionalized H-MOP-A with carboxylic acids (H-MOP-CA) showed efficient adsorption toward Ag⁺ ions. The resultant H-MOP-CA-Ag showed excellent performance in the CO₂ fixation to α-alkylidene cyclic compounds.

Host sensitized lanthanide photoluminescence from post-synthetically modified semiconductor nanoparticles depends on reactant identity

This work investigates the photoluminescence characteristics where cadmium selenide (CdSe) and zinc sulfide (ZnS) nanoparticles are treated post-synthetically by the trivalent lanthanide cations (Ln³⁺) [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] separately to form either CdSe/Ln or ZnS/Ln nanoparticles. Host sensitized Ln³⁺ emission was found to be present only in CdSe/Eu, CdSe/Tb, ZnS/Eu, ZnS/Tb and ZnS/Yb nanoparticles. In all the cases tuning of emission of the nanoparticles has been observed, irrespective of the presence or absence of host sensitization. The elemental compositions of CdSe and ZnS nanoparticles upon post-synthetic treatment show a remarkable difference. Incorporation of lanthanides in the nanoparticles is evident with significant alteration in the anionic content, and complete cation exchange of either Cd²⁺ or Zn²⁺ by Ln³⁺ has not been detected; as evaluated from energy dispersive X-ray spectroscopy. Further evaluation on this comes from considering thermodynamic parameters of inter cation interaction. In cases where the host sensitized Ln³⁺ emission have been observed, luminescence lifetime measurements reveal significant protection of Ln³⁺ in the nanoparticles. Noticeable difference in photophysical properties for a given Ln³⁺ has been realized in the two hosts. The photophysical observations have been rationalized using (i) charge trapping mediated host sensitized dopant emission, (ii) autoionization of excited electrons, and (iii) environment induced photoluminescence quenching. The post-synthetic modification discussed in the present work provides an easy and less synthetically demanding room temperature based protocol to avail lanthanide incorporated (doped) semiconductor nanoparticles that can potentially use the unique emission properties of the lanthanide cations.

Syntheses of prodrug-type 2'-O-methyldithiomethyl oligonucleotides modified at natural four nucleoside residues and their conversions into natural 2'-hydroxy oligonucleotides under reducing condition

We previously reported that reducing-environment-responsive prodrug-type small interfering RNA (siRNA) bearing 2'-O-methyldithiomethyl (2'-O-MDTM) uridine exhibits efficient knockdown activity and nuclease resistance. In this report, we describe the preparation of 2'-O-MDTM oligonucleotides modified not only at uridine but also at adenosine, guanosine and cytidine residues by post-synthetic modification. Precursor oligonucleotides bearing 2'-O-(2,4,6-trimethoxybenzylthiomethyl) (2'-O-TMBTM) adenosine, guanosine, and cytidine were reacted with dimethyl(methylthio)sulfonium tetrafluoroborate to form 2'-O-MDTM oligonucleotides in the same manner as the oligonucleotide bearing 2'-O-TMBTM uridine. Furthermore, the oligonucleotides bearing 2'-O-MDTM adenosine, guanosine, and cytidine were efficiently converted into corresponding natural 2'-hydroxy oligonucleotides under the cytosol-mimetic reducing condition.

MOF the beaten track: unusual structures and uncommon applications of metal-organic frameworks

Over the past few decades, metal-organic frameworks (MOFs) have proved themselves as strong contenders in the world of porous materials, standing alongside established classes of compounds such as zeolites and activated carbons. Following extensive investigation into the porosity of these materials and their gas uptake properties, the MOF community are now branching away from these heavily researched areas, and venturing into unexplored avenues. Ranging from novel synthetic routes to post-synthetic functionalisation of frameworks, host-guest properties to sensing abilities, this review takes a sidestep away from increasingly 'traditional' approaches in the field, and details some of the more curious qualities of this relatively young family of materials.