M.K. was supported by a grant from the National Science Foundation, Division of Chemistry under Award No. CHE-2153240. Additional support for materials and supplies was provided by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering under award no. DE-FG02-08ER46519. SEM imaging was performed in part at the San Diego Nano-Technology Infrastructure (SDNI) of U.C. San Diego, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (ECCS-1542148).

1. Surface modification of UiO-66 with cat-DDMAT
<ol>
	<li>Exchange the solvent of UiO-66 from methanol with water.
	<ol>
		<li>Prepare UiO-66 in methanol at a concentration of 20 mg/mL. 
		NOTE: According to Wang et al.20, homogeneous UiO-66 is washed with DMF and methanol after synthesis and then stored in a dispersed state in methanol.</li>
		<li>Transfer the 10 mL of UiO-66 suspension to a 15 mL conical centrifuge tube using a pipette.</li>
		<li>Perform centrifugation at approximately 10,000 &#215; g for 10 min, remove the supernatant, and add 10 mL of deionized (DI) water. 
		NOTE: In cases where the particles do not completely settle under these conditions, an additional 10 min centrifugation step can be performed.</li>
		<li>Redisperse the UiO-66 particles in water. 
		NOTE: For adequate dispersion, the use of sonication and vortexing (maximum speed can be used) at room temperature may be necessary.</li>
		<li>Centrifuge again under the same condition as in step 1.1.3., add 10 mL of fresh DI water, and redisperse.</li>
	</ol>
	</li>
	<li>Dissolve 10 mg of cat-DDMAT in 5 mL of chloroform.
	<ol>
		<li>Weigh 10 mg of cat-DDMAT in a 20 mL vial.</li>
		<li>Add 5 mL of chloroform to the vial using a graduated cylinder.</li>
		<li>Seal the vial and sonicate at room temperature until a clear solution is achieved.</li>
	</ol>
	</li>
	<li>Vortex the cat-DDMAT solution with the UiO-66 solution for 3 min.
	<ol>
		<li>Sequentially transfer the UiO-66 water dispersion from step 1.1 and the cat-DDMAT solution from step 1.2 to a 40 mL conical centrifuge tube.</li>
		<li>Ensure sufficient mixing by vortexing for 3 min.</li>
	</ol>
	</li>
	<li>Add 20 mL of ethanol to the mixture using a cylinder and shake for thorough mixing. 
	NOTE: Some additional vortexing may be required for proper mixing.</li>
	<li>Subject the mixture to centrifugation, wash with 40 mL of ethanol, and disperse in 10 mL of DMSO for storage.
	<ol>
		<li>Centrifuge at approximately 10,000 &#215; g for 10 min, remove the supernatant, and add 40 mL of fresh ethanol. 
		NOTE: In cases where the particles do not completely settle under these conditions, an additional 10 min centrifugation step can be performed.</li>
		<li>Sonicate to ensure proper redispersion.</li>
		<li>Centrifuge again under the same condition as in step 1.5.1, add 40 mL of fresh ethanol, and redisperse. 
		NOTE: Upon centrifugation, it can be observed that the originally white UiO-66 turns into a pale yellow, confirming the functionalization with DDMAT.</li>
		<li>After another round of centrifugation, add 5 mL of DMSO to the tube.</li>
		<li>Sonicate the particles to achieve maximum dispersion and transfer to a 15 mL conical centrifuge tube.</li>
		<li>Add 5 mL of fresh DMSO to a 50 mL tube to disperse the remaining particles.</li>
		<li>Transfer the DMSO dispersion in the 50 mL tube to the 15 mL tube to combine with the original DMSO dispersion.</li>
		<li>Mix through vortexing and sonication, then store the sample. 
		NOTE: The yellow color of cat-DDMAT allows for the observation of surface functionalization through a change in particle color (Figure 3)</li>
	</ol>
	</li>
</ol>
2. Polymerization of methyl acrylate from UiO-66-DDMAT
<ol>
	<li>Transfer 2 mL of the UiO-66-DDMAT dispersion in DMSO to a 10 mL round-bottom flask (RBF) using a pipette. 
	NOTE: If the dispersion has been stored for an extended period, it can be made uniform again through additional vortexing and sonication before transferring to the RBF.</li>
	<li>Add the Ir(ppy)3 catalyst stock solution and the DDMAT stock solution while stirring.
	<ol>
		<li>Place a stirring bar inside the RBF and secure it on a stirring plate.</li>
		<li>Initiate stirring and add 12 &#181;L of the Ir(ppy)3 stock solution (1 mg/mL in DMF) using a micropipette.</li>
		<li>Add 0.45 mL of the DDMAT stock solution (10 mg/mL in DMF) using a micropipette. 
		NOTE: Ir(ppy)3 and DDMAT stock solutions were prepared in advance at their respective concentrations and stored in the refrigerator on a 1-3 mL scale.</li>
	</ol>
	</li>
	<li>Add 1.7 mL of methyl acrylate to a 20 mL vial and dissolve it in 2 mL of fresh DMSO using a micropipette. 
	NOTE: Sonication can be used to achieve the formation of a homogeneous solution.</li>
	<li>Slowly add the solution dropwise to the reaction flask.</li>
	<li>Stop stirring, seal the RBF with a septum, and degas for 15 min.
	<ol>
		<li>Stop stirring and securely seal the RBF with a septum.</li>
		<li>Connect the long needle (in the range of 21 to 22G) to a nitrogen supply manifold.</li>
		<li>Insert the long needle through the septum to reach the internal air layer of the RBF.</li>
		<li>Insert a short needle (in the range of 21 to 22 G) through the septum to create an outlet.</li>
		<li>Open the nitrogen valve and lower the long needle to the bottom of the RBF.</li>
		<li>Raise the long needle after 15 min to the internal air layer of the RBF.</li>
		<li>Remove the short needle first, followed by the long needle, ensuring no external air enters the RBF.</li>
		<li>Close the nitrogen valve.</li>
	</ol>
	</li>
	<li>Initiate the reaction under a blue LED light (&#955; = 455 nm) source.
	<ol>
		<li>Place an in-house built blue light LED photoreactor on a stirring plate. 
		NOTE: The photoreactor was built using a 12 V Waterproof Flexible LED Strip Light (Figure 2).</li>
		<li>Connect the power, verify light emission, and cover the upper part with aluminum foil to prevent excessive exposure to blue light. 
		NOTE: Do not seal it completely to allow the heat generated during polymerization to escape.</li>
		<li>Restart stirring.</li>
	</ol>
	</li>
	<li>Turn off the LED when the viscosity of the reaction solution increases to the point where it can no longer be stirred.</li>
	<li>Add excess acetone to the RBF, dilute the mixture, and transfer to a 50 mL tube.
	<ol>
		<li>Remove the septum from the RBF and add acetone to fill the space.</li>
		<li>Stir for approximately 1 h to achieve a homogeneous mixture.</li>
		<li>Transfer the mixture to a 50 mL conical centrifuge tube.</li>
		<li>Add fresh acetone to the RBF and stir for another hour to collect any remaining products.</li>
		<li>Transfer the mixture in the RBF to the 50 mL tube to combine with the mixture collected at first.</li>
	</ol>
	</li>
	<li>Fill up to 40 mL with acetone and mix to dissolve free polymer.</li>
	<li>Wash the mixture until no more free polymer dissolves in acetone, then change the solvent to toluene.
	<ol>
		<li>After centrifugation and supernatant removal, adjust the volume to 40 mL with fresh acetone.</li>
		<li>Disperse particles through sonication and vortex, followed by another round of centrifugation.</li>
		<li>Repeat until no more free polymer dissolves in the supernatant. 
		NOTE: Overnight soaking in fresh acetone after dispersing may be an effective method to facilitate the release of long-chain polymers.</li>
	</ol>
	</li>
	<li>Disperse the product particles in 10 mL of toluene.
	<ol>
		<li>Add 5 mL of toluene to the product after supernatant removal.</li>
		<li>Disperse particles through sonication and vortex, then transfer to a new 15 mL conical centrifuge tube.</li>
		<li>Add fresh toluene (5 mL) to the 50 mL tube to disperse any remaining particles.</li>
		<li>Transfer the toluene dispersion from the 50 mL tube to the 15 mL tube to combine with the original toluene dispersion.</li>
		<li>Mix through vortex and sonication, then store the sample. 
		NOTE: Homogeneously dispersed UiO-66-pMA is obtained as a translucent suspension. (Figure 3)</li>
	</ol>
	</li>
</ol>
3. Particle self-assembly
<ol>
	<li>After centrifugation, disperse the particles in toluene to a volume less than 20% of the original amount.
	<ol>
		<li>Centrifuge the toluene-dispersed particle suspension under the same condition as in step 1.1.3.</li>
		<li>Remove the supernatant and add toluene to achieve a total suspension volume of 1-2 mL. 
		NOTE: The appropriate concentration may vary depending on the size of the Petri dish used, particle size, and polymer molecular weight.</li>
		<li>Completely disperse the particles through sonication.</li>
	</ol>
	</li>
	<li>Prepare a Petri dish (60 mm diameter) by adding DI water.</li>
	<li>Carefully drop approximately 10 &#181;L of the toluene dispersion (equivalent to a single droplet) onto the DI water&#39;s surface. 
	NOTE: Do not add more than one droplet. When adding the dispersion, the tool used can be adjusted to control the droplet&#39;s size. The suitable droplet size may vary depending on the petri dish size, particle size, and polymer molecular weight.</li>
	<li>Place the dish cover over it to allow the toluene to evaporate slowly.</li>
	<li>After all toluene on the surface has evaporated and a monolayer membrane has formed, remove the cover.</li>
	<li>Use a loop made of copper wire to carefully remove part of the monolayer. 
	NOTE: The copper wire used for scooping the monolayer should be prepared to have a flat, approximately circular shape.</li>
	<li>After evaporating the remaining co-associated water, a free-standing monolayer could be observed.</li>
	<li>To prepare for scanning electron microscopy (SEM) image measurements, carefully scoop up the monolayer formed on the water&#39;s surface using a piece of thin glass.</li>
</ol>

Self-Assembly of Polymer-Grafted Metal Organic Framework Monolayers

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The authors have no conflicts of interest to disclose.

There are several critical steps where specific attention to detail is required to successfully synthesize polymer-grafted MOFs that will produce SAMMs. First, the monomers utilized in RAFT polymerization are supplemented with inhibitors or stabilizers during storage to prevent undesired polymerization (e.g., hydroquinone or monomethyl ether of hydroquinone, MEHQ). To remove these additives, purification through distillation is required before use22. In protocol step 2.4, it is essential to dilute...

Metal-organic frameworks (MOFs) are crystalline, porous materials that offer large surface areas while being readily tunable through modifications of the organic ligands or metal nodes1,2. MOFs are constructed from two components: an organic ligand and metal ions (or metal ion clusters referred to as secondary building units, SBUs). MOFs have been investigated for chemical (e.g., gas) storage, separations, catalysis, sensing, and drug delivery. Generally, MOFs are synthesized in the form of crystalline powders; however, for ease of handling in many applications, formulation into other form factors is desirable if not necessary3,4. For example, mixed matrix membranes (MMMs) of MOFs with polymers have been reported as one particularly useful composite of MOFs and polymers5. However, in some cases, MMMs may have limitations due to the incompatibility/immiscibility between MOF and polymer components5,6. Therefore, strategies have been explored to incorporate polymer grafting directly onto MOF particles to form polymer-grafted MOFs.
Inorganic and metallic nanoparticles exhibit unique behavior in terms of optical, magnetic, catalytic, and mechanical properties7,8. However, they tend to aggregate easily after synthesis, which can hinder their processability. To enhance their processability, polymer chains can be grafted onto the particle surface9. Nanoparticles with high grafting density offer excellent dispersion and stability due to favorable enthalpic interactions between surface polymers and the solvent and entropic repulsion interactions between the particles10. Grafting of polymers onto particle surfaces can be achieved through a variety of strategies11. The most straightforward approach is the &#39;grafting to&#39; particle strategy, where functional groups, such as thiols or carboxylic acids, are introduced at the ends of polymer chains to directly bind to the nanoparticle. When complementary chemical groups, such as hydroxyls or epoxides, are present on the particle surface, polymer chains can be grafted onto these groups via covalent chemical approaches12,13. The &#39;grafting from&#39; particle or surface-initiated polymerization method involves anchoring initiators or chain transfer agents (CTAs) to the surface of nanoparticles and then growing polymer chains on the particle surface through surface-initiated polymerization. This method often achieves higher grafting density than the &#39;grafting to&#39; approach. Furthermore, grafting from enables the synthesis of block copolymers, thereby expanding the diversity of polymer structures that can be immobilized on a particle surface.
Examples of polymer grafting onto MOF particles have begun to emerge, largely focused on installing polymerization sites on the organic ligands of the MOF. In a recent study published by Shojaei and coworkers, vinyl groups were covalently attached to the ligands of Zr(IV)-based MOF UiO-66-NH2 (UiO = Universitetet i Oslo, where the terephthalic acid ligand contains an amino substituent), followed by methyl methacrylate (MMA) polymerization to create polymer-grafted MOFs with a high grafting density (Figure 1A)14. Similarly, Matzger and coworkers functionalized the amine groups on a core-shell MOF-5 (a.k.a., IRMOF-3@MOF-5) particles with 2-bromo-iso-butyl groups. Using polymerization initiated by the 2-bromo-iso-butyl groups, they created poly(methyl methacrylate) (PMMA)-grafted PMMA@IRMOF-3@MOF-515.
In addition to functionalizing the ligand of the MOF for grafting from polymerization, new methods that create sites for polymer grafting via coordination to the metal centers (a.k.a., SBUs) of the MOF have also been explored. For example, a ligand that can bind to the MOF metal centers, such as catechol (Figure 1B), can be used to coordinate to exposed metal sites on the MOF surface. Using a catechol-functionalized chain-transfer agent (cat-CTA, Figure 1B) the MOF surface can be functionalized and made suitable for a grafting from polymerization.
Recently, the aforementioned strategy for synthesizing MOFs-polymer composites has also been used for the creation of free-standing MOF monolayers16,17,18. MOFs such as UiO-66 and MIL-88B-NH2 (MIL = Materials of Institute Lavoisier) were surface-functionalized with pMMA using a ligand-CTA strategy (Figure 1B). The polymer-grafted MOF particles were self-assembled at an air-water interface to form self-supporting, self-assembled metal-organic framework monolayers (SAMMs) with a thickness of ~250 nm. The polymer content in these composites was ~20 wt%, indicating that SAMMs contained ~80 wt% MOF loading. Followup studies showed that different vinyl polymers could be grafted onto UiO-66 to produce SAMMs with different characteristics19. Analytical techniques such as thermogravimetric analysis (TGA), dynamic light scattering (DLS), and gel permeation chromatography (GPC) were used to calculate polymer brush height and grafting density of the surface-grafted MOF-polymer composites.
Herein, the preparation of SAMMs from UiO-66-pMA (pMA = poly(methyl acrylate)) is presented. For the polymerization of methyl acrylate (MA), 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT, Figure 1B) is used as the CTA19. The functionalization of the UiO-66 particles with cat-DDMAT is essential for the grafting of pMA. Cat-DDMAT can be synthesized through a two-step acylation procedure from a commercially available CTA&#160;and dopamine hydrochloride19. It is also crucial to use UiO-66 particles of uniform size for the successful formation of SAMMs19; therefore, the UiO-66 used in this study was prepared using the continuous addition method20. The polymerization method employed for forming the polymer-grafted MOF particles is photoinduced reversible addition-fragmentation chain transfer (RAFT) conducted under blue LED light (using an in-house-built photoreactor, Figure 2) with a tris(2-phenylpyridine)iridium (Ir(ppy)3) photocatalyst. RAFT polymerization gives exceptionally narrow polymer dispersity that can be finely controlled. Free CTA is included during the polymerization reaction because the ratio of transfer agent to monomer allows for control over the molecular weight during polymerization. The amount of cat-DDMAT transfer agent on the surface of the MOF particles is small; therefore, excess free CTA is added and the amount of monomer to be used is calculated based off of the amount of free CTA present21. After polymerization, the free polymer produced from the free CTA is removed by washing, leaving only the polymer-grafted UiO-66-pMA. Subsequently, this composite is dispersed in toluene at a high concentration and used to form SAMMs at an air-water interface.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT)</td>
			<td>Sigma-Aldrich</td>
			<td>723010</td>
			<td>98%</td>
		</tr>
		<tr>
			<td>10 mL Single Neck RBF</td>
			<td>Chemglass</td>
			<td>CG-1506-82</td>
			<td>14/20 Outer Joint</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>Fisher Chemical</td>
			<td>A18-20</td>
			<td>ACS Grade</td>
		</tr>
		<tr>
			<td>Allegra X-30R Centrifuge</td>
			<td>BECKMAN COULTER</td>
			<td>B06320</td>
			<td>1.6 L max capacity, 18,000 RPM, 29,756 x g</td>
		</tr>
		<tr>
			<td>Analog Vortex Mixer</td>
			<td>VWR</td>
			<td>10153-838</td>
			<td>300 - 3,200 rpm</td>
		</tr>
		<tr>
			<td>cat-DDMAT</td>
			<td></td>
			<td></td>
			<td>Prepared according to literature procedure (ref. 17).</td>
		</tr>
		<tr>
			<td>Centrifuge Tube, 50 mL / 15 mL</td>
			<td>CORNING</td>
			<td>430291 / 430766</td>
			<td>Conical Bottom with plug seal cap, polypropylene</td>
		</tr>
		<tr>
			<td>Chloroform</td>
			<td>Fisher Chemical</td>
			<td>AC423550040</td>
			<td>99.8%</td>
		</tr>
		<tr>
			<td>Conventional needles</td>
			<td>Becton Dickinson</td>
			<td>382903051670</td>
			<td>21 G x 1 1/2</td>
		</tr>
		<tr>
			<td>Copper wire</td>
			<td>Malin Co.</td>
			<td></td>
			<td>No. 30 B &#38; S GAUGE</td>
		</tr>
		<tr>
			<td>Dimethyl Sulfoxide (DMSO)</td>
			<td>Fisher Bioreagents</td>
			<td>BP231-1</td>
			<td>&#62;=99.7%</td>
		</tr>
		<tr>
			<td>Disposable Pasteur Pipets</td>
			<td>Fisher Scientific</td>
			<td>13-678-20C</td>
			<td>Borosilicate Glass</td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>KOPTEC</td>
			<td>V1001</td>
			<td>200 proof ethanol</td>
		</tr>
		<tr>
			<td>Glass Scintillation Vial, 20 mL</td>
			<td>KIMBIL</td>
			<td>74508-20</td>
			<td></td>
		</tr>
		<tr>
			<td>Graduated Cylinder, 10 mL</td>
			<td>KIMBIL</td>
			<td>20024-10</td>
			<td></td>
		</tr>
		<tr>
			<td>Hypodermic Needles</td>
			<td>Air-Tite</td>
			<td>N224</td>
			<td>22 G x 4&#39;&#39;</td>
		</tr>
		<tr>
			<td>Methanol</td>
			<td>Fisher Chemical</td>
			<td>A412-20</td>
			<td>99.8%</td>
		</tr>
		<tr>
			<td>Methyl Acrylate</td>
			<td>Aldrich Chemistry</td>
			<td>M27301</td>
			<td>99%, contains =&#60; 100 ppm monomethyl ether hydroquinone as inhibitor</td>
		</tr>
		<tr>
			<td>Micropipette P10 (1 - 10 &#181;L)</td>
			<td>GILSON</td>
			<td>F144055M</td>
			<td>PIPETMAN, Metal Ejector</td>
		</tr>
		<tr>
			<td>Micropipette P1000 (100 - 1,000 &#181;L)</td>
			<td>GILSON</td>
			<td>F144059M</td>
			<td>PIPETMAN, Metal Ejector</td>
		</tr>
		<tr>
			<td>Micropipette P20 (2 - 20 &#181;L)</td>
			<td>GILSON</td>
			<td>F144056M</td>
			<td>PIPETMAN, Metal Ejector</td>
		</tr>
		<tr>
			<td>Microscope cover glass</td>
			<td>Fisher Scientific</td>
			<td>12542A</td>
			<td>18 mm x 18 mm</td>
		</tr>
		<tr>
			<td>NN-Dimerhylformamide (DMF)</td>
			<td>Fisher Chemical</td>
			<td>D119-4</td>
			<td>99.8%</td>
		</tr>
		<tr>
			<td>Petri Dish, Stackable Lid</td>
			<td>Fisher Scientific</td>
			<td>FB0875713A</td>
			<td>60 mm x 15 mm</td>
		</tr>
		<tr>
			<td>Septum Stopper</td>
			<td>Chemglass</td>
			<td>CG302401</td>
			<td>14/20 - 14/35</td>
		</tr>
		<tr>
			<td>Stir Bar</td>
			<td>Chemglass</td>
			<td>CG-2005T-01</td>
			<td>Magnetic, PTFE, Turbo, Rare Earth, Elliptical, 10 x 6mm</td>
		</tr>
		<tr>
			<td>SuperNuova+ Stirring Hot Plate</td>
			<td>Thermo Scientific</td>
			<td>SP88857190</td>
			<td>50 - 1,500 rpm, 30 - 450 &#176;C</td>
		</tr>
		<tr>
			<td>Toluene</td>
			<td>Fisher Chemical</td>
			<td>T324-4</td>
			<td>99.5%</td>
		</tr>
		<tr>
			<td>Tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3)</td>
			<td>Sigma-Aldrich</td>
			<td>688096</td>
			<td>97%</td>
		</tr>
		<tr>
			<td>UiO-66 (120 nm edge length)</td>
			<td></td>
			<td></td>
			<td>Prepared according to literature procedure (ref. 18).</td>
		</tr>
		<tr>
			<td>Ultrasonic Cleaner CPX3800H</td>
			<td>EMERSON / BRANSON</td>
			<td>CPX-952-318R</td>
			<td>40 kHz, 5.7 L</td>
		</tr>
		<tr>
			<td>Waterproof Flexible LED Strip Light</td>
			<td>ALITOVE</td>
			<td>ALT-5B300WPBK</td>
			<td>16.4 ft 5050 Blue LED</td>
		</tr>
	</tbody>
</table>

synthesis and characterization of self-assembled metal-organic framework monolayers using polymer-coated particles

Metal-organic frameworks (MOFs) are materials with potential applications in fields such as gas adsorption and separation, catalysis, and biomedicine. Attempts to enhance the utility of MOFs have involved the preparation of various composites, including polymer-grafted MOFs. By directly grafting polymers to the external surface of MOFs, issues of incompatibility between polymers and MOFs can be overcome. Polymer brushes grafted from the surface of MOFs can serve to stabilize the MOF while enabling particle assembly into self-assembled metal-organic framework monolayers (SAMMs) via polymer-polymer interactions.
Control over the chemical composition and molecular weight of the grafted polymer can allow for tuning of the SAMM characteristics. In this work, instructions are provided on how to immobilize a chain transfer agent (CTA) onto the surface of the MOF UiO-66 (UiO = Universitetet i Oslo). The CTA&#160;serves as initiation sites for the growth of polymers. Once polymer chains are grown from the MOF surface, the formation of SAMMs is achieved through self-assembly at an air-water interface. The resulting SAMMs are characterized and shown to be freestanding by scanning electron microscopy imaging. The methods presented in this paper are expected to make the preparation of SAMMs more accessible to the research community and thereby expand their potential use as a&#160;MOF-polymer composite.

When the polymer-grafted MOFs are gently dropped onto water from a concentrated toluene dispersion (as illustrated in Figure 4A), a monolayer is formed in a few seconds with an iridescent appearance. Furthermore, using a mold made from copper wire to lift this monolayer and subsequently drying the obtained water allows for the formation of free-standing SAMMs (Figure 4B). After transferring the monolayer to a glass microscope cover slip and drying it, SEM imagin...

Author Spotlight: Exploring Self-Assembled MOF-Polymer Composites

A protocol for the synthesis and characterization of self-assembled metal-organic framework monolayers is provided using polymer-grafted, metal-organic framework (MOF) crystals. The procedure shows that polymer-grafted MOF particles can be self-assembled at an air-water interface resulting in well-formed, free-standing, monolayer structures as evidenced by scanning electron microscopy imaging.

Synthesis and Characterization of Self-Assembled Metal-Organic Framework Monolayers Using Polymer-Coated Particles

Metal-organic frameworks (MOFs) are materials with potential applications in fields such as gas adsorption and separation, catalysis, and biomedicine. ...

synthesis-characterization-self-assembled-metal-organic-framework

Research

JoVE Journal

Chemistry

1.4K Views.  University of California, San Diego. A protocol for the synthesis and characterization of self-assembled metal-organic framework monolayers is provided using polymer-grafted, metal-organic framework (MOF) crystals. The procedure shows that polymer-grafted MOF particles can be self-assembled at an air-water interface resulting in well-formed, free-standing, monolayer structures as evidenced by scanning electron microscopy imaging.

Video: Author Spotlight: Exploring Self-Assembled MOF-Polymer Composites

Covalent Binding of BMP-2 on Surfaces Using a Self-assembled Monolayer Approach

Bone morphogenetic protein 2 (BMP-2) is a growth factor embedded in the extracellular matrix of bone tissue. BMP-2 acts as trigger of mesenchymal cell differentiation into osteoblasts, thus stimulating healing and de novo bone formation. The clinical use of recombinant human BMP-2 (rhBMP-2) in conjunction with scaffolds has raised recent controversies, based on the mode of presentation and the amount to be delivered. The protocol presented here provides a simple and efficient way to deliver BMP-2 for in vitro studies on cells. We describe how to form a self-assembled monolayer consisting of a heterobifunctional linker, and show the subsequent binding step to obtain covalent immobilization of rhBMP-2. With this approach it is possible to achieve a sustained presentation of BMP-2 while maintaining the biological activity of the protein. In fact, the surface immobilization of BMP-2 allows targeted investigations by preventing unspecific adsorption, while reducing the amount of growth factor and, most notably, hindering uncontrolled release from the surface. Both short- and long-term signaling events triggered by BMP-2 are taking place when cells are exposed to surfaces presenting covalently immobilized rhBMP-2, making this approach suitable for in vitro studies on cell responses to BMP-2 stimulation.

We describe a method for accomplishing efficient immobilization of BMP-2 on surfaces. Our approach is based on the formation of a self-assembled monolayer to achieve the covalent binding of BMP-2 via its free amine residues. This method is a useful tool to study signaling at the cell membrane.

Bone morphogenetic protein 2 (BMP-2) is a growth factor embedded in the extracellular matrix of bone tissue. BMP-2 acts as trigger of mesenchymal cell ...

Synthesis and Characterization of Functionalized Metal-organic Frameworks

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and technological applications. Their signature property is their ultrahigh porosity, which however imparts a series of challenges when it comes to both constructing them and working with them. Securing desired MOF chemical and physical functionality by linker/node assembly into a highly porous framework of choice can pose difficulties, as less porous and more thermodynamically stable congeners (e.g., other crystalline polymorphs, catenated analogues) are often preferentially obtained by conventional synthesis methods. Once the desired product is obtained, its characterization often requires specialized techniques that address complications potentially arising from, for example, guest-molecule loss or preferential orientation of microcrystallites. Finally, accessing the large voids inside the MOFs for use in applications that involve gases can be problematic, as frameworks may be subject to collapse during removal of solvent molecules (remnants of solvothermal synthesis). In this paper, we describe synthesis and characterization methods routinely utilized in our lab either to solve or circumvent these issues. The methods include solvent-assisted linker exchange, powder X-ray diffraction in capillaries, and materials activation (cavity evacuation) by supercritical CO2 drying. Finally, we provide a protocol for determining a suitable pressure region for applying the Brunauer-Emmett-Teller analysis to nitrogen isotherms, so as to estimate surface area of MOFs with good accuracy.

Synthesis, activation, and characterization of intentionally designed metal-organic framework materials is challenging, especially when building blocks are incompatible or unwanted polymorphs are thermodynamically favored over desired forms. We describe how applications of solvent-assisted linker exchange, powder X-ray diffraction in capillaries and activation via supercritical CO2 drying, can address some of these challenges.

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and ...

Facile Preparation of Internally Self-assembled Lipid Particles Stabilized by Carbon Nanotubes

We present a facile method to prepare nanostructured lipid particles stabilized by carbon nanotubes (CNTs). Single-walled (pristine) and multi-walled (functionalized) CNTs are used as stabilizers to produce Pickering type oil-in-water (O/W) emulsions. Lipids namely, Dimodan U and Phytantriol are used as emulsifiers, which in excess water self-assemble into the bicontinuous cubic Pn3m phase. This highly viscous phase is fragmented into smaller particles using a probe ultrasonicator in presence of conventional surfactant stabilizers or CNTs as done here. Initially, the CNTs (powder form) are dispersed in water followed by further ultrasonication with the molten lipid to form the final emulsion. During this process the CNTs get coated with lipid molecules, which in turn are presumed to surround the lipid droplets to form a particulate emulsion that is stable for months. The average size of CNT-stabilized nanostructured lipid particles is in the submicron range, which compares well with the particles stabilized using conventional surfactants. Small angle X-ray scattering data confirms the retention of the original Pn3m cubic phase in the CNT-stabilized lipid dispersions as compared to the pure lipid phase (bulk state). Blue shift and lowering of the intensities in characteristic G and G&#39; bands of CNTs observed in Raman spectroscopy characterize the interaction between CNT surface and lipid molecules. These results suggest that the interactions between the CNTs and lipids are responsible for their mutual stabilization in aqueous solutions. As the concentrations of CNTs employed for stabilization are very low and lipid molecules are able to functionalize the CNTs, the toxicity of CNTs is expected to be insignificant while their biocompatibility is greatly enhanced. Hence the present approach finds a great potential in various biomedical applications, for instance, for developing hybrid nanocarrier systems for the delivery of multiple functional molecules as in combination therapy or polytherapy.

We report on a smart application of carbon nanotubes for kinetic stabilization of lipid particles that contain self-assembled nanostructures in their cores. The preparation of lipid particles requires rather low concentrations of carbon nanotubes permitting their use in biomedical applications such as drug delivery.

We present a facile method to prepare nanostructured lipid particles stabilized by carbon nanotubes (CNTs). Single-walled (pristine) and multi-walled ...

Highly Stable, Functional Hairy Nanoparticles and Biopolymers from Wood Fibers: Towards Sustainable Nanotechnology

Nanoparticles, as one of the key materials in nanotechnology and nanomedicine, have gained significant importance during the past decade. While metal-based nanoparticles are associated with synthetic and environmental hassles, cellulose introduces a green, sustainable alternative for nanoparticle synthesis. Here, we present the chemical synthesis and separation procedures to produce new classes of hairy nanoparticles (bearing both amorphous and crystalline regions) and biopolymers based on wood fibers. Through periodate oxidation of soft wood pulp, the glucose ring of cellulose is opened at the C2-C3 bond to form 2,3-dialdehyde groups. Further heating of the partially oxidized fibers (e.g., T = 80 &#176;C) results in three products, namely fibrous oxidized cellulose, sterically stabilized nanocrystalline cellulose (SNCC), and dissolved dialdehyde modified cellulose (DAMC), which are well separated by intermittent centrifugation and co-solvent addition. The partially oxidized fibers (without heating) were used as a highly reactive intermediate to react with chlorite for converting almost all aldehyde to carboxyl groups. Co-solvent precipitation and centrifugation resulted in electrosterically stabilized nanocrystalline cellulose (ENCC) and dicarboxylated cellulose (DCC). The aldehyde content of SNCC and consequently surface charge of ENCC (carboxyl content) were precisely controlled by controlling the periodate oxidation reaction time, resulting in highly stable nanoparticles bearing more than 7 mmol functional groups per gram of nanoparticles (e.g., as compared to conventional NCC bearing &#60;&#60; 1 mmol functional group/g). Atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) attested to the rod-like morphology. Conductometric titration, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), dynamic light scattering (DLS), electrokinetic-sonic-amplitude (ESA) and acoustic attenuation spectroscopy shed light on the superior properties of these nanomaterials.

Synthesis schemes to prepare highly stable wood fiber-based hairy nanoparticles and functional cellulose-based biopolymers have been detailed.

Nanoparticles, as one of the key materials in nanotechnology and nanomedicine, have gained significant importance during the past decade. While ...

Synthesis of a Water-soluble Metal&#8211;Organic Complex Array

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as Ru(II), Pt(II), and Rh(III). The compound, named as a water-soluble metal-organic complex array (WSMOCA), is obtained through 1) the conventional solution-chemistry-based preparation of the corresponding metal complex monomers having a 9-fluorenylmethyloxycarbonyl (Fmoc)-protected amino acid moiety and 2) their sequential coupling together with other water-soluble organic building units on the surface-functionalized polymeric resin by following the procedures originally developed for the solid-phase synthesis of polypeptides, with proper modifications. Traces of reactions determined by mass spectrometric analysis at the representative coupling steps in stage 2 confirm the selective construction of a predetermined sequence of metal centers along with the peptide backbone. The WSMOCA cleaved from the resin at the end of stage 2 has a certain level of solubility in aqueous media dependent on the pH value and/or salt content, which is useful for the purification of the compound.

Synthesis of a Water-soluble Metal&amp;#8211;Organic Complex Array

A potential general method for the synthesis of water-soluble multimetallic peptidic arrays containing a predetermined sequence of metal centers is presented.

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as ...

Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and non-porous supports (polymer, ceramic, metal, carbon, and graphene). We developed a novel crystallization technique, which is termed the ENACT approach: the electrophoretic nuclei assembly for the crystallization of highly intergrown thin films (ENACT). This approach allows for a high density of heterogeneous nucleation of MOFs on a chosen substrate via the electrophoretic deposition (EPD) directly from the precursor sol. The growth of well-packed MOF nuclei leads to a highly intergrown polycrystalline MOF film. We show that this simple approach can be used for the synthesis of thin, intergrown zeolite imidazole framework (ZIF)-7 and ZIF-8 films. The resulting 500 nm-thick ZIF-8 membranes show a considerably high H2 permeance (8.3 x 10-6 mol m-2 s-1 Pa-1) and ideal gas selectivities (7.3 for H2/CO2, 15.5 for H2/N2, 16.2 for H2/CH4, and 2655 for H2/C3H8). An attractive performance for C3H6/C3H8 separation is also achieved (a C3H6 permeance of 9.9 x 10-8 mol m-2 s-1 Pa-1 and a C3H6/C3H8 ideal selectivity of 31.6 at 25 &#176;C). Overall, the ENACT process, owing to its simplicity, can be extended to synthesize intergrown thin films of a wide range of nanoporous crystalline materials.

A simple, reproducible, and versatile approach for the synthesis of intergrown, polycrystalline metal-organic framework membranes on a wide range of unmodified porous and non-porous supports is presented.

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and ...

Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks (MOFs)

Substrate size discrimination by the pore size and homogeneity of the chiral environment at the reaction sites are important issues in the validation of the reaction site in metal-organic framework (MOF)&#8211;based catalysts in an enantioselective catalytic reaction system. Therefore, a method of validating the reaction site of MOF-based catalysts is necessary to investigate this issue. Substrate size discrimination by pore size was accomplished by comparing the substrate size versus the reaction rate in two different types of carbonyl-ene reactions with two kinds of MOFs. The MOF catalysts were used to compare the performance of the two reaction types (Zn-mediated stoichiometric and Ti-catalyzed carbonyl-ene reactions) in two different media. Using the proposed method, it was observed that the entire MOF crystal participated in the reaction, and the interior of the crystal pore played an important role in exerting chiral control when the reaction was stoichiometric. Homogeneity of the chiral environment of MOF catalysts was established by the size control method for a particle used in the Zn-mediated stoichiometric reaction system. The protocol proposed for the catalytic reaction revealed that the reaction mainly occurred on the catalyst surface regardless of the substrate size, which reveals the actual reaction sites in MOF-based heterogeneous catalysts. This method for reaction site validation of MOF catalysts suggests various considerations for developing heterogeneous enantioselective MOF catalysts.

Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks MOFs

Here, we present a protocol for active site validation of metal-organic framework catalysts by comparing stoichiometric and catalytic carbonyl-ene reactions to find out whether a reaction takes place on the inner or outer surface of metal-organic frameworks.

Substrate size discrimination by the pore size and homogeneity of the chiral environment at the reaction sites are important issues in the validation of ...

Synthesis of Single-Crystalline Core-Shell Metal-Organic Frameworks

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. However, the synthesis of single-crystalline core-shell MOFs is very challenging, and thus a limited number of examples have been reported. Here, we suggest a method of synthesizing single-crystalline HKUST-1@MOF-5 core-shells, which is HKUST-1 at the center of MOF-5. Through the computational algorithm, this pair of MOFs was predicted to have the matched lattice parameters and chemical connection points at the interface. To construct the core-shell structure, we prepared the octahedral- and cubic-shaped HKUST-1 crystals as a core MOF, in which the (111) and (001) facets were mainly exposed, respectively. Via the sequential reaction, the MOF-5 shell was well-grown on the exposed surface, showing a seamless connect interface, which resulted in the successful synthesis of single-crystalline HKUST-1@MOF-5. Their pure phase formation was proved by optical microscopic images and powder X-ray diffraction (PXRD) patterns. This method presents the potential of and insights into the single-crystalline core-shell synthesis with different kinds of MOFs.

Here, we demonstrate a protocol for the two-step synthesis of single-crystalline core-shells using a non-isostructural metal-organic framework (MOF) pair, HKUST-1 and MOF-5, which have well-matched crystal lattices.

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. ...

Synthesis of Low-Valent MOFs from Multitopic Phosphine Linkers

Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, energy, and catalysis. Recently, low-valent MOFs (LVMOFs) have been explored for their potential use as heterogeneous catalysts, and multitopic phosphine linkers have been shown to be a useful building block for the formation of LVMOFs. However, the synthesis of LVMOFs using phosphine linkers requires conditions that are distinct from those in the majority of the MOF synthetic literature, including the exclusion of air and water and the use of unconventional modulators and solvents, making it somewhat more challenging to access these materials. This work serves as a general tutorial for the synthesis of LVMOFs with phosphine linkers, including information on the following: 1) the judicious choice of the metal precursor, modulator, and solvent; 2) the experimental procedures, air-free techniques, and required equipment; 3) the proper storage and handling of the resultant LVMOFs; and 4) useful characterization methods for these materials. The intention of this report is to lower the barrier to this new subfield of MOF research and facilitate advancements toward novel catalytic materials.

Author Spotlight: Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers  

Here, we describe a protocol for the synthesis of low-valent metal-organic frameworks (LVMOFs) from low-valent metals and multitopic phosphine linkers under air-free conditions. The resulting materials have potential applications as heterogeneous catalyst mimics of low-valent metal-based homogeneous catalysts.

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, ...

Magnetometric Characterization of Redox-Active MOFs Solid-State Electrochemistry

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

Author Spotlight: Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks  

Ex situ magnetic surveys can directly provide bulk and local information on a magnetic electrode to reveal its charge storage mechanism step by step. Herein, electron spin resonance (ESR) and magnetic susceptibility are demonstrated to monitor the evaluation of paramagnetic species and their concentration in a redox-active metal-organic framework (MOF).

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs ...