Mixed Metal-Organic Framework Mixed-Matrix Membranes: Insights into Simultaneous Moisture-Triggered and Catalytic Delivery of Nitric Oxide using Cryo-scanning Electron Microscopy.

The fundamental chemical and structural diversity of metal-organic frameworks (MOFs) is vast, but there is a lack of industrial adoption of these extremely versatile compounds. To bridge the gap between basic research and industry, MOF powders must be formulated into more application-relevant shapes and/or composites. Successful incorporation of varying ratios of two different MOFs, CPO-27-Ni and CuBTTri, in a thin polymer film represents an important step toward the development of mixed MOF mixed-matrix membranes. To gain insight into the distribution of the two different MOFs in the polymer, we report their investigation by Cryo-scanning electron microscopy (Cryo-SEM) tomography, which minimizes surface charging and electron beam-induced damage. Because the MOFs are based on two different metal ions, Ni and Cu, the elemental maps of the MOF composite cross sections clearly identify the size and location of each MOF in the reconstructed 3D model. The tomography run was about six times faster than conventional focused ion beam (FIB)-SEM and the first insights to image segmentation combined with machine learning could be achieved. To verify that the MOF composites combined the benefits of rapid moisture-triggered release of nitric oxide (NO) from CPO-27-Ni with the continuous catalytic generation of NO from CuBTTri, we characterized their ability to deliver NO individually and simultaneously. These MOF composites show great promise to achieve optimal dual NO delivery in real-world medical applications.

Computational Insights into the Mechanism of Nitric Oxide Generation from S-Nitrosoglutathione Catalyzed by a Copper Metal-Organic Framework.

The controlled generation of nitric oxide (NO) from endogenous sources, such as S-nitrosoglutathione (GSNO), has significant implications for biomedical implants due to the vasodilatory and other beneficial properties of NO. The water-stable metal-organic framework (MOF) Cu-1,3,5-tris[1H-1,2,3-triazol-5-yl]benzene has been shown to catalyze the production of NO and glutathione disulfide (GSSG) from GSNO in aqueous solution as well as in blood. Previous experimental work provided kinetic data for the catalysis of the 2GSNO → 2NO + GSSG reaction, leading to various proposed mechanisms. Herein, this catalytic process is examined using density functional theory. Minimal functional models of the Cu-MOF cluster and glutathione moieties are established, and three distinct catalytic mechanisms are explored. The most thermodynamically favorable mechanism studied is consistent with prior experimental findings. This mechanism involves coordination of GSNO to copper via sulfur rather than nitrogen and requires a reductive elimination that produces a Cu(I) intermediate, implicating a redox-active copper site. The experimentally observed inhibition of reactivity at high pH values is explained in terms of deprotonation of a triazole linker, which decreases the structural stability of the Cu(I) intermediate. These fundamental mechanistic insights may be generally applicable to other MOF catalysts for NO generation.

Surface Modification of Oxygenator Fibers with a Catalytically Active Metal-Organic Framework to Generate Nitric Oxide: An Ex Vivo Pilot Study.

Coating all portions of an extracorporeal membrane oxygenation (ECMO) circuit with materials exhibiting inherent, permanent antithrombotic properties is an essential step to prevent thrombus-induced complications. However, developing antithrombotic coatings for oxygenator fibers within membrane oxygenators of ECMO systems has proven challenging. We have used polydopamine (PDA) to coat oxygenator fibers and immobilize a Cu-based metal-organic framework (MOF) on the surface to act as a nitric oxide (NO) catalyst. Importantly, the PDA/MOF coating will produce NO indefinitely from endogenous S-nitrosothiols and it has not previously been applied to ECMO oxygenator fibers.

Monitoring a MOF Catalyzed Reaction Directly in Blood Plasma.

Herein, we establish a method to quantitatively monitor a metal-organic framework (MOF)-catalyzed, biomedically relevant reaction directly in blood plasma, specifically, the generation of nitric oxide (NO) from the endogenous substrate S-nitrosoglutathione (GSNO) catalyzed by H3[(Cu4Cl)3-(BTTri)8] (CuBTTri). The reaction monitoring method uses UV-vis and 1H NMR spectroscopies along with a nitric oxide analyzer (NOA) to yield the reaction stoichiometry and catalytic rate for GSNO to NO conversion catalyzed by CuBTTri in blood plasma. The results show 100% loss of GSNO within 16 h and production of 1 equiv. of glutathione disulfide (GSSG) per 2 equiv. of GSNO. Only 78 ± 10% recovery of NO(g) was observed, indicating that blood plasma can scavenge the generated NO before it can escape the reaction vessel. Significantly, to best apply and understand reaction systems with biomedical importance, such as NO release catalyzed by CuBTTri, methods to study the reaction directly in biological solvents must be developed.

Copper Metal-Organic Framework Surface Catalysis: Catalyst Poisoning, IR Spectroscopic, and Kinetic Evidence Addressing the Nature and Number of the Catalytically Active Sites En Route to Improved Applications.

The metal-organic framework (MOF) H3[(Cu4Cl)3-(BTTri)8, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene] (CuBTTri) is a precatalyst for biomedically relevant nitric oxide (NO) release from S-nitrosoglutathione (GSNO). The questions of the number and nature of the catalytically most active, kinetically dominant sites are addressed. Also addressed is whether or not the well-defined structural geometry of MOFs (as solid-state analogues of molecular compounds) can be used to generate specific, testable hypotheses about, for example, if intrapore vs exterior surface metal sites are more catalytically active. Studies of the initial catalytic rate vs CuBTTri particle external surface area to interior volume ratio show that intrapore copper sites are inactive within the experimental error (≤1.7 × 10-5% of the observed catalytic activity)-restated, the traditional MOF intrapore metal site catalysis hypothesis is disproven for the current system. All observed catalysis occurs at exterior surface Cu sites, within the experimental error. Fourier transform infrared (FT-IR) analysis of CN--poisoned CuBTTri reveals just two detectable Cu sites at a ca. ≥0.5% detection limit, those that bind three or one CN- ("Cu(CN)3" and "CuCN"), corresponding to the CN- binding expected for exterior surface, 3-coordinate (Cusurface) and intrapore, 5-coordinate (Cupore) sites predicted by the idealized, metal-terminated crystal structure. Two-coordinate Cu defect sites are ruled out at the ≥0.5% FT-IR detection limit as such defect sites would have been detectable by the FT-IR studies of the CN--poisoned catalyst. Size-selective poisoning studies of CuBTTri exterior surface sites reveal that 1.3 (±0.4)% of total copper in 0.6 ± 0.4 µm particles is active. That counting of active sites yields a normalized turnover frequency (TOF), TOFnorm = (4.9 ± 1.2) × 10-2 mol NO (mol Cusurface)-1 s-1 (in water, at 20 min, 25 °C, 1 mM GSNO, 30% loss of GSNO, and 1.3 ± 0.4 mol % Cusurface)-a value ∼100× higher than the TOF calculated without active site counting. Overall, Ockham's razor interpretation of the data is that exterior surface, Cusurface sites are the catalytically most active sites present at a 1.3 (±0.4)% level of total Cu.

Exploring the Parameter Space of p-Cresyl Sulfate Adsorption in Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have high porosity and surface area, making them ideal candidates for adsorption-mediated applications. One high-value application is the removal of uremic toxins from solution for dialysis. Previous studies have reported adsorptive removal of the uremic toxin p-cresyl sulfate from solution via zirconium-based MOFs, but a specific analysis of parameters contributing to adsorptive uptake is needed to clarify differences in uptake performance between MOFs. We synthesized zirconium 1,3,5-benzenetricarboxylate (MOF-808) and an iron-based analog, MIL-100(Fe), and compared their adsorptive uptake with previously reported values of other zirconium-based MOFs. MIL-100(Fe) adsorbed three times more p-cresyl sulfate from solution on a per mass basis than MOF-808 and had a greater adsorption efficiency than 75% of previously reported Zr-based MOFs. We compared p-cresyl sulfate uptake by MOFs as a function of BET surface area, number of aromatic carbons in the organic linker, internal cage diameter, and pore window diameter. There is poor correlation between p-cresyl sulfate uptake and each of the variables considered, but the number of aromatic carbons of the MOF linker was a better predictor of uptake than BET surface area (R2 = 0.7034 and 0.1430, respectively), and pore window aperture was a better predictor of uptake than the pore cage diameter (R2 = 0.4780 and 0.0383, respectively). We hypothesize that the greater adsorptive capacity of MIL-100(Fe) compared to MOF-808 results from direct coordination of p-cresyl sulfate to vacant metal sites in the MOF, and the total adsorption may be accounted for by some combination of adsorptive interactions occurring at both metal and organic linker sites near to the exterior particle surface. The adsorptive uptake of p-cresyl sulfate by MIL-100(Fe) was observed to increase with p-cresyl sulfate content, mass of MIL-100(Fe), and volume of p-cresyl sulfate solution; the mass of MIL-100(Fe) had the greatest effect on total adsorption.

Thermogravimetric Analysis and Mass Spectrometry Allow for Determination of Chemisorbed Reaction Products on Metal Organic Frameworks.

Thermogravimetric analysis (TGA) is a technique which can probe chemisorption of substrates onto metal organic frameworks. A TGA method was developed to examine the catalytic oxidation of S-nitrosoglutathione (GSNO) by the MOF H3[(Cu4Cl)3(BTTri)8] (abbr. Cu-BTTri; H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene), yielding glutathione disulfide (GSSG) and nitric oxide (NO). Thermal analysis of reduced glutathione (GSH), GSSG, GSNO, and Cu-BTTri revealed thermal resolution of all four analytes through different thermal onset temperatures and weight percent changes. Two reaction systems were probed: an aerobic column flow reaction and an anaerobic solution batch reaction with gas agitation. In both systems, Cu-BTTri was reacted with a 1 mM GSH, GSSG, or GSNO solution, copiously rinsed with distilled-deionized water (dd-H2O), dried (25 °C, < 1 Torr), and assessed by TGA. Additionally, stock, effluent or supernatant, and rinse solutions for each glutathione derivative within each reaction system were assessed by mass spectrometry (MS) to inform on chemical transformations promoted by Cu-BTTri as well as relative analyte concentrations. Both reaction systems exhibited chemisorption of glutathione derivatives to the MOF by TGA. Mass spectrometry analyses revealed that in both systems, GSH was oxidized to GSSG, which chemisorbed to the MOF whereas GSSG remained unchanged during chemisorption. For GSNO, chemisorption to the MOF without reaction was observed in the aerobic column setup, whereas conversion to GSSG and subsequent chemisorption was observed in the anaerobic batch setup. These findings suggest that within this reaction system, GSSG is the primary adsorbent of concern with regards to strong binding to Cu-BTTri. Development of similar thermal methods could allow for the probing of MOF reactivity for a wide range of systems, informing on important considerations such as reduced catalytic efficiency from poisoning, recyclability, and loading capacities of contaminants or toxins with MOFs.

Copper ion vs copper metal-organic framework catalyzed NO release from bioavailable S-Nitrosoglutathione en route to biomedical applications: Direct 1H NMR monitoring in water allowing identification of the distinct, true reaction stoichiometries and thiol dependencies.

Copper containing compounds catalyze decomposition of S-Nitrosoglutathione (GSNO) in the presence of glutathione (GSH) yielding glutathione disulfide (GSSG) and nitric oxide (NO). Extended NO generation from an endogenous source is medically desirable to achieve vasodilation, reduction in biofilms on medical devices, and antibacterial activity. Homogeneous and heterogeneous copper species catalyze release of NO from endogenous GSNO. One heterogeneous catalyst used for GSNO decomposition in blood plasma is the metal-organic framework (MOF), H3[(Cu4Cl)3-(BTTri)8, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl) benzene] (CuBTTri). Fundamental questions about these systems remain unanswered, despite their use in biomedical applications, in part because no method previously existed for simultaneous tracking of [GSNO], [GSH], and [GSSG] in water. Tracking these reactions in water is a necessary step towards study in biological media (blood is approximately 80% water) where NO release systems must operate. Even the balanced stoichiometry remains unknown for copper-ion and CuBTTri catalyzed GSNO decomposition. Herein, we report a direct 1H NMR method which: simultaneously monitors [GSNO], [GSH], and [GSSG] in water; provides the experimentally determined stoichiometry for copper-ion vs CuBTTri catalyzed GSNO decomposition; reveals that the CuBTTri-catalyzed reaction reaches 10% GSNO decomposition (16 h) without added GSH, yet the copper-ion catalyzed reaction reaches 100% GSNO decomposition (16 h) without added GSH; and shows 100% GSNO decomposition upon addition of stoichiometric GSH to the CuBTTri catalyzed reaction. These observations provide evidence that copper-ion and CuBTTri catalyzed GSNO decomposition in water operate through different reaction mechanisms, the details of which can now be probed by 1H NMR kinetics and other needed studies.