Achieving Sub-ppm Sensitivity in SO2 Detection with a Chemically Stable Covalent Organic Framework.

We report the inaugural experimental investigation of covalent organic frameworks (COFs) to address the formidable challenge of SO2 detection. Specifically, an imine-functionalized COF (SonoCOF-9) demonstrated a modest and reversible SO2 sorption of 3.5 mmol g-1 at 1 bar and 298 K. At 0.1 bar (and 298 K), the total SO2 uptake reached 0.91 mmol g-1 with excellent reversibility for at least 50 adsorption-desorption cycles. An isosteric enthalpy of adsorption (ΔHads) for SO2 equaled -42.3 kJ mol-1, indicating a relatively strong interaction of SO2 molecules with the COF material. Also, molecular dynamics simulations and Møller-Plesset perturbation theory calculations showed the interaction of SO2 with π density of the rings and lone pairs of the N atoms of SonoCOF-9. The combination of experimental data and theoretical calculations corroborated the potential use of this COF for the selective detection and sensing of SO2 at the sub-ppm level (0.0064 ppm of SO2).

SO2 Capture at Low Pressure in a Prototypical MIL-53 Aluminum MOF Family: The Influence of Pore Expansion.

Not only is excellent performance in SO2 capture by porous materials (uptake above 17 mmol g-1) relevant, but also finding a correlation between the architecture changes into a family and their SO2 adsorption is very useful. In this contribution, we studied the SO2 adsorption behavior (at very low pressure) of an Al(III)-MOF family that shares the pore architecture of MIL-53. The results indicate an inversely proportional trend for the SO2 capture and pore expansion, since by increasing the length of the channel pore, the SO2 uptake gradually decreases. In addition, this trend is clearly observed in the heat of adsorption, which describes the interaction between the SO2 molecule and the µ-OH functional group. These finding are supported by experimental analysis and computational studies.

Assembling metal-organic cages as porous materials.

There is growing interest in metal-organic cages (MOCs) as porous materials owing to their processability in solution. The discrete molecular character and surface features of MOCs have a direct impact on the interactions between cages, enabling the final physical state of the materials to be tuned. In this tutorial review, we discuss how to use MOCs as core building units, highlighting the role played by surface functionalisation of MOCs in leading to porous materials in a range of states covering crystalline solids, soft matter, liquids and composites. We finish by providing an outlook on the opportunities for this work to serve as a foundation for the development of increasingly complex functional porous materials structured over various length scales.

Mn-CUK-1: A Flexible MOF for SO2, H2O, and H2S Capture.

The environmentally benign metal-organic framework (MOF) CUK-1 based on 2,4-pyridine dicarboxylate has been prepared for the first time using Mn(II) as the inorganic node and water as the only solvent. Mn-CUK-1 shows reversible and efficient capture of H2O, SO2, and H2S. Compared to previously studied Co(II) and Mg(II) versions of the same MOF, Mn-CUK-1 also exhibited unique temperature-induced structural flexibility due to organic linker torsion, as detailed by variable-temperature single-crystal X-ray diffraction studies. Owing to this inherent solid-state flexibility, Mn-CUK-1 showed stepwise adsorption for polar gases, which induce structural deformations upon adsorption, while the nonpolar guest adsorbates were reversibly sorbed in a more classical manner. Notably, Mn-CUK-1 demonstrates the highest reported H2S capacity-to-surface area ratio among MOFs that are chemically stable toward this reactive acidic molecule. Moreover, Mn-CUK-1 displays exceptional structural stability in the presence of high relative humidity and corrosive gases and shows soft crystalline behavior triggered by changes in both the adsorption temperature and guest molecule identity.

Transient Porosity in Densely Packed Crystalline Carbazole-(p-Diethynylphenylene)-Carbazole Rotors: CO2 and Acetone Sorption Properties.

We report for the first time the high sorption properties of a molecular rotor with no permanent voids or channels in its crystal structure. Such crystalline phase originates from THF, DCM, or the irreversible desolvation of entrapped benzene molecules. From these, the benzene in its solvate form acts as rotation stopper, as supported by dynamic characterization using solid-state 2H NMR experiments. In the solvent-free form, the diffusion of small quantities of iodine vapors caused a significant change in the intramolecular rotation, increasing the known activation energy to rotation from 8.5 to 10.6 kcal mol-1. Notably, those results paved the way for the discovery of the high CO2 uptake (201.6 cm3 g-1 at 196 K, under 1 atm) and acetone (5 wt %), a sorption property that was attributed to both, the restriction of the molecular rotation at low temperatures and the flexibility of the molecular axle made of conjugated p-(ethynylphenylene), surrounded by carbazole.

Bottleneck Effect of N,N-Dimethylformamide in InOF-1: Increasing CO2 Capture in Porous Coordination Polymers.

The bottleneck effect of confined N,N-dimethylformamide (DMF) molecules was observed in InOF-1 for the first time: CO2 capture was remarkably enhanced in samples of as-synthesized InOF-1, thermally activated in such a way that a small residual amount of DMF molecules remained confined within the pores (DMF@InOF-1). Dynamic CO2 adsorption experiments on DMF@InOF-1 exhibited a CO2 capture of 8.06 wt % [1.5-fold higher than that of a fully activated InOF-1 (5.24%)]. DMF@InOF-1 can reversibly adsorb/desorb 8.09% CO2 with no loss of CO2 capacity after 10 cycles, and the desorption is accomplished by only turning the CO2 flow off. Static CO2 adsorption experiments (at 196 K) demonstrated a 1.4-fold CO2 capture increase (from 5.5 mmol·g-1, fully activated InOF-1, to 7.5 mmol·g-1, DMF@InOF-1). Therefore, these CO2 capture properties are the result of the presence of residual-confined DMF molecules within the InOF-1 framework and their interactions via a very strong hydrogen bond with the In2(µ-OH) groups, which prevent DMF leaching. The stability of this hydrogen bond is given by a perfect fit of the DMF molecule in the "dent" around the OH group that allows a nearly ideal orientation of the DMF molecule towards the OH group.

SO2 capture and detection with carbon microfibers (CMFs) synthesised from polyacrylonitrile.

SO2 emissions not only affect local air quality but can also contribute to other environmental issues. Developing low-cost and robust adsorbents with high uptake and selectivity is needed to reduce SO2 emissions. Here, we show the SO2 adsorption-desorption capacity of carbon microfibers (CMFs) at 298 K. CMFs showed a reversible SO2 uptake capacity (5 mmol g-1), cyclability over ten adsorption cycles with fast kinetics and good selectivity towards SO2/CO2 at low-pressure values. Additionally, CMFs' photoluminescence response to SO2 and CO2 was evaluated.

Dual photoresponsive & water-triggered nitric oxide releasing materials based on rhodium-based metal-organic polyhedra.

The exogenous administration of nitric oxide (NO) is considered a potential therapeutic treatment against a great variety of diseases due to its significant role in multiple physiological functions. Due to the gaseous nature, short lifetime and dose- and tissue-dependent activity of this molecule, the development of new administration procedures is required to control the NO delivery in terms of dosage, timing, and location. In this work, we propose a new molecular material based on robust metal-organic polyhedra (MOPs) for controlled NO release. We select dirhodium paddlewheel complex-based cuboctahedral MOPs (RhMOP), in which NO can chemically coordinate to the open-metal sites at the axial sites of dirhodium paddlewheel moieties. We further prepare amorphous coordination polymer particles (CPPs) by connecting RhMOP with bis(imidazole) linkers at the external axial sites. Both molecular MOPs and polymeric CPPs show relevant NO payloads and the release of NO can be triggered by two different stimuli: light and humidity. We show that imidazole ligands coordinating to the external axial sites of the paddlewheel moieties tune the light-triggered NO release property. We further demonstrate that the size and the extrinsic pores of CPPs are important for enhanced NO release.

MFM-300(Sc): a chemically stable Sc(III)-based MOF material for multiple applications.

Developing robust multifunctional metal-organic frameworks (MOFs) is the key to advancing the further deployment of MOFs into relevant applications. Since the first report of MFM-300(Sc) (MFM = Manchester Framework Material, formerly known as NOTT-400), the development of applications of this robust microporous MOF has only grown. In this review, a summary of the applications of MFM-300(Sc), as well as some emerging advanced applications, have been discussed. The adsorption properties of MFM-300(Sc) are presented systematically. Particularly, this contribution is focused on acid and corrosive gas adsorption. In addition, recent applications for catalysis based on the outstanding hemilabile Sc-O bond character are highlighted. Finally, some new research areas are introduced, such as host-guest chemistry and biomedical applications. This highlight aims to showcase the recent advances and the potential for developing new applications of this promising material.

Modulated self-assembly of three flexible Cr(III) PCPs for SO2 adsorption and detection.

Modulated self-assembly protocols are used to develop facile, HF-free syntheses of the archetypal flexible PCP, MIL-53(Cr), and novel isoreticular analogues MIL-53(Cr)-Br and MIL-53(Cr)-NO2. All three PCPs show good SO2 uptake (298 K, 1 bar) and high chemical stabilities against dry and wet SO2. Solid-state photoluminescence spectroscopy indicates all three PCPs exhibit turn-off sensing of SO2, in particular MIL-53(Cr)-Br, which shows a 2.7-fold decrease in emission on exposure to SO2 at room temperature, indicating potential sensing applications.

Detection of SO2 using a chemically stable Ni(II)-MOF.

The MOF-type Ni2(dobpdc) shows a high chemical stability towards SO2, high capacity for SO2 capture at low pressure (4.3 mmol g-1 at 298 K and up to 0.05 bar), and exceptional cycling performance. Fluorescence experiments demonstrated the SO2 detection properties of Ni2(dobpdc) with a remarkable SO2 detection selectivity. Finally, time-resolved photoluminescence experiments provided a plausible mechanism of SO2 detection by this Ni(II)-based MOF material.

Control of Extrinsic Porosities in Linked Metal-Organic Polyhedra Gels by Imparting Coordination-Driven Self-Assembly with Electrostatic Repulsion.

The linkage of metal-organic polyhedra (MOPs) to synthesize porous soft materials is one of the promising strategies to combine processability with permanent porosity. Compared to the defined internal cavity of MOPs, it is still difficult to control the extrinsic porosities generated between crosslinked MOPs because of their random arrangements in the networks. Herein, we report a method to form linked MOP gels with controllable extrinsic porosities by introducing negative charges on the surface of MOPs that facilitates electrostatic repulsion between them. A hydrophilic rhodium-based cuboctahedral MOP (OHRhMOP) with 24 hydroxyl groups on its outer periphery can be controllably deprotonated to impart the MOP with tunable electrostatic repulsion in solution. This electrostatic repulsion between MOPs stabilizes the kinetically trapped state, in which an MOP is coordinated with various bisimidazole linkers in a monodentate fashion at a controllable linker/MOP ratio. Heating of the kinetically trapped molecules leads to the formation of gels with similar colloidal networks but different extrinsic porosities. This strategy allows us to design the molecular-level networks and the resulting porosities even in the amorphous state.

Gold nanoparticles: current and upcoming biomedical applications in sensing, drug, and gene delivery.

Gold nanoparticles (AuNPs) present unique physicochemical characteristics, low cytotoxicity, chemical stability, size/morphology tunability, surface functionalization capability, and optical properties which can be exploited for detection applications (colorimetry, surface-enhanced Raman scattering, and photoluminescence). The current challenge for AuNPs is incorporating these properties in developing more sensible and selective sensing methods and multifunctional platforms capable of controlled and precise drug or gene delivery. This review briefly highlights the recent progress of AuNPs in biomedicine as bio-sensors and targeted nano vehicles.

SO2 Capture by Two Aluminum-Based MOFs: Rigid-like MIL-53(Al)-TDC versus Breathing MIL-53(Al)-BDC.

Metal-organic frameworks MIL-53(Al)-TDC and MIL-53(Al)-BDC were explored in the SO2 adsorption process. MIL-53(Al)-TDC was shown to behave as a rigid-like material upon SO2 adsorption. On the other hand, MIL-53(Al)-BDC exhibits guest-induced flexibility of the framework with the presence of multiple steps in the SO2 adsorption isotherm that was related through molecular simulations to the existence of three different pore opening phases narrow pore, intermediate pore, and large pore. Both materials proved to be exceptional candidates for SO2 capture, even under wet conditions, with excellent SO2 adsorption, good cycling, chemical stability, and easy regeneration. Further, we propose MIL-53(Al)-TDC and MIL-53(A)-BDC of potential interest for SO2 sensing and SO2 storage/transportation, respectively.

Bottleneck Effect Explained by Le Bail Refinements: Structure Transformation of Mg-CUK-1 by Confining H2O Molecules.

The structure transformation of Mg-CUK-1 due to the confinement of H2O molecules was investigated. Powder X-ray diffraction (PXRD) patterns were collected at different H2O loadings and the cell parameters of the H2O-loaded Mg-CUK-1 material were determined by the Le Bail strategy refinements. A bottleneck effect was observed when one hydrogen-bonded H2O molecule per unit cell (18% relative humidity (RH)) was confined within Mg-CUK-1, confirming the increase in the CO2 capture for Mg-CUK-1.

Reversible and efficient SO2 capture by a chemically stable MOF CAU-10: experiments and simulations.

The adsorption of sulphur dioxide (SO2) in CAU-10 is obtained with the use of advanced experimental and computational tools to gain insight into the molecular mechanisms responsible for the adsorption of SO2. It is shown that the adsorption by CAU-10 is highly energy efficient and that van der Waals interactions are the driving force that controls adsorption in this system.

Partially Reversible H2S Adsorption by MFM-300(Sc): Formation of Polysulfides.

The metal-organic framework (MOF)-type MFM-300(Sc) exhibits a combined physisorption and chemisorption capture of H2S, leading to a high uptake (16.55 mmol g-1) associated with high structural stability. The irreversible chemisorbed sulfur species were identified as low-order polysulfide (n = 2) species. The isostructural MFM-300(In) was demonstrated to promote the formation of different polysulfide species, paving the way toward a new methodology to incorporate polysulfides within MOFs for the generation of novel MOF-lithium/sulfur batteries.

Synthesis and Characterization of an SWCNT@HKUST-1 Composite: Enhancing the CO2 Adsorption Properties of HKUST-1.

Synthesis of a new HKUST-1 composite based on single-walled carbon nanotubes (SWCNTs) was successfully achieved (SWCNT@HKUST-1). SWCNTs were used as templates to grow rod-like HKUST-1 crystals over the surface of the nanotubes. N2 adsorption properties showed an increment on the surface area and pore volume for the SWCNT@HKUST-1 composite. Furthermore, the CO2 capture increased, from 7.92 to 8.75 mmol g-1 at 196 K up to 100 kPa, for the SWCNT@HKUST-1 composite. This enhancement was directly associated with the increase of the surface area of the composite. Additionally, an increase in the CO2 heat of adsorption was estimated, from 30 to 39.1 kJ mol-1 for the SWCNT@HKUST-1 composite. In situ Raman experiments corroborated the favored CO2 adsorption for the composite and provided an insight into the augmented hydrophobicity of the SWCNT@HKUST-1. Ethanol adsorption isotherms corroborated an increase in the hydrophobicity of the material upon the incorporation of carbon nanotubes.

Outstanding reversible H2S capture by an Al(iii)-based MOF.

The MOF-type MIL-53(Al)-TDC was demonstrated to be an optimal adsorbent for H2S capture combining an unprecedented uptake at room temperature, excellent cyclability and low-temperature regeneration.

Confinement of H2O and EtOH to enhance CO2 capture in MIL-53(Al)-TDC.

EtOH sorption properties were investigated in MIL-53(Al)-TDC and found a strong interaction between EtOH and the MOF material (ΔHads = 69.6 kJ mol-1). CO2 capture was enhanced upon confining small amounts of H2O. Upon confining small amounts of EtOH however, the CO2 uptake was not improved. The difference in CO2 uptake with EtOH and H2O was rationalised using computational calculations. The analysis of the quantum theory of atoms in molecules (QTAIM) showed a covalent interaction between a MOF model and confined molecules (EtOH and H2O), and no difference in the hydrogen bonds between confined molecules and CO2.