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

Summary

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.

Abstract

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 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 MOF-polymer composite.

Introduction

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 nodes^1,2. 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 necessary^3,4. 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 components^5,6. 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 properties^7,8. 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 'grafting to' 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 approaches^12,13. The 'grafting from' 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 'grafting to' 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 monolayers^16,17,18. 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 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.

Protocol

1. Surface modification of UiO-66 with cat-DDMAT

1. Exchange the solvent of UiO-66 from methanol with water.
   1. 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.
   2. Transfer the 10 mL of UiO-66 suspension to a 15 mL conical centrifuge tube using a pipette.
   3. Perform centrifugation at approximately 10,000 × 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.
   4. 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.
   5. Centrifuge again under the same condition as in step 1.1.3., add 10 mL of fresh DI water, and redisperse.
2. Dissolve 10 mg of cat-DDMAT in 5 mL of chloroform.
   1. Weigh 10 mg of cat-DDMAT in a 20 mL vial.
   2. Add 5 mL of chloroform to the vial using a graduated cylinder.
   3. Seal the vial and sonicate at room temperature until a clear solution is achieved.
3. Vortex the cat-DDMAT solution with the UiO-66 solution for 3 min.
   1. 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.
   2. Ensure sufficient mixing by vortexing for 3 min.
4. 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.
5. Subject the mixture to centrifugation, wash with 40 mL of ethanol, and disperse in 10 mL of DMSO for storage.
   1. Centrifuge at approximately 10,000 × 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.
   2. Sonicate to ensure proper redispersion.
   3. 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.
   4. After another round of centrifugation, add 5 mL of DMSO to the tube.
   5. Sonicate the particles to achieve maximum dispersion and transfer to a 15 mL conical centrifuge tube.
   6. Add 5 mL of fresh DMSO to a 50 mL tube to disperse the remaining particles.
   7. Transfer the DMSO dispersion in the 50 mL tube to the 15 mL tube to combine with the original DMSO dispersion.
   8. 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)

2. Polymerization of methyl acrylate from UiO-66-DDMAT

1. 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.
2. Add the Ir(ppy)₃ catalyst stock solution and the DDMAT stock solution while stirring.
   1. Place a stirring bar inside the RBF and secure it on a stirring plate.
   2. Initiate stirring and add 12 µL of the Ir(ppy)₃ stock solution (1 mg/mL in DMF) using a micropipette.
   3. 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.
3. 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.
4. Slowly add the solution dropwise to the reaction flask.
5. Stop stirring, seal the RBF with a septum, and degas for 15 min.
   1. Stop stirring and securely seal the RBF with a septum.
   2. Connect the long needle (in the range of 21 to 22G) to a nitrogen supply manifold.
   3. Insert the long needle through the septum to reach the internal air layer of the RBF.
   4. Insert a short needle (in the range of 21 to 22 G) through the septum to create an outlet.
   5. Open the nitrogen valve and lower the long needle to the bottom of the RBF.
   6. Raise the long needle after 15 min to the internal air layer of the RBF.
   7. Remove the short needle first, followed by the long needle, ensuring no external air enters the RBF.
   8. Close the nitrogen valve.
6. Initiate the reaction under a blue LED light (λ = 455 nm) source.
   1. 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).
   2. 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.
   3. Restart stirring.
7. Turn off the LED when the viscosity of the reaction solution increases to the point where it can no longer be stirred.
8. Add excess acetone to the RBF, dilute the mixture, and transfer to a 50 mL tube.
   1. Remove the septum from the RBF and add acetone to fill the space.
   2. Stir for approximately 1 h to achieve a homogeneous mixture.
   3. Transfer the mixture to a 50 mL conical centrifuge tube.
   4. Add fresh acetone to the RBF and stir for another hour to collect any remaining products.
   5. Transfer the mixture in the RBF to the 50 mL tube to combine with the mixture collected at first.
9. Fill up to 40 mL with acetone and mix to dissolve free polymer.
10. Wash the mixture until no more free polymer dissolves in acetone, then change the solvent to toluene.
    1. After centrifugation and supernatant removal, adjust the volume to 40 mL with fresh acetone.
    2. Disperse particles through sonication and vortex, followed by another round of centrifugation.
    3. 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.
11. Disperse the product particles in 10 mL of toluene.
    1. Add 5 mL of toluene to the product after supernatant removal.
    2. Disperse particles through sonication and vortex, then transfer to a new 15 mL conical centrifuge tube.
    3. Add fresh toluene (5 mL) to the 50 mL tube to disperse any remaining particles.
    4. Transfer the toluene dispersion from the 50 mL tube to the 15 mL tube to combine with the original toluene dispersion.
    5. Mix through vortex and sonication, then store the sample.
       NOTE: Homogeneously dispersed UiO-66-pMA is obtained as a translucent suspension. (Figure 3)

3. Particle self-assembly

1. After centrifugation, disperse the particles in toluene to a volume less than 20% of the original amount.
   1. Centrifuge the toluene-dispersed particle suspension under the same condition as in step 1.1.3.
   2. 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.
   3. Completely disperse the particles through sonication.
2. Prepare a Petri dish (60 mm diameter) by adding DI water.
3. Carefully drop approximately 10 µL of the toluene dispersion (equivalent to a single droplet) onto the DI water's surface.
   NOTE: Do not add more than one droplet. When adding the dispersion, the tool used can be adjusted to control the droplet's size. The suitable droplet size may vary depending on the petri dish size, particle size, and polymer molecular weight.
4. Place the dish cover over it to allow the toluene to evaporate slowly.
5. After all toluene on the surface has evaporated and a monolayer membrane has formed, remove the cover.
6. 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.
7. After evaporating the remaining co-associated water, a free-standing monolayer could be observed.
8. To prepare for scanning electron microscopy (SEM) image measurements, carefully scoop up the monolayer formed on the water's surface using a piece of thin glass.

Results

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...

Discussion

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...

Materials

Name	Company	Catalog Number	Comments
2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT)	Sigma-Aldrich	723010	98%
10 mL Single Neck RBF	Chemglass	CG-1506-82	14/20 Outer Joint
Acetone	Fisher Chemical	A18-20	ACS Grade
Allegra X-30R Centrifuge	BECKMAN COULTER	B06320	1.6 L max capacity, 18,000 RPM, 29,756 x g
Analog Vortex Mixer	VWR	10153-838	300 - 3,200 rpm
cat-DDMAT			Prepared according to literature procedure (ref. 17).
Centrifuge Tube, 50 mL / 15 mL	CORNING	430291 / 430766	Conical Bottom with plug seal cap, polypropylene
Chloroform	Fisher Chemical	AC423550040	99.8%
Conventional needles	Becton Dickinson	382903051670	21 G x 1 1/2
Copper wire	Malin Co.		No. 30 B & S GAUGE
Dimethyl Sulfoxide (DMSO)	Fisher Bioreagents	BP231-1	>=99.7%
Disposable Pasteur Pipets	Fisher Scientific	13-678-20C	Borosilicate Glass
Ethanol	KOPTEC	V1001	200 proof ethanol
Glass Scintillation Vial, 20 mL	KIMBIL	74508-20
Graduated Cylinder, 10 mL	KIMBIL	20024-10
Hypodermic Needles	Air-Tite	N224	22 G x 4''
Methanol	Fisher Chemical	A412-20	99.8%
Methyl Acrylate	Aldrich Chemistry	M27301	99%, contains =< 100 ppm monomethyl ether hydroquinone as inhibitor
Micropipette P10 (1 - 10 µL)	GILSON	F144055M	PIPETMAN, Metal Ejector
Micropipette P1000 (100 - 1,000 µL)	GILSON	F144059M	PIPETMAN, Metal Ejector
Micropipette P20 (2 - 20 µL)	GILSON	F144056M	PIPETMAN, Metal Ejector
Microscope cover glass	Fisher Scientific	12542A	18 mm x 18 mm
NN-Dimerhylformamide (DMF)	Fisher Chemical	D119-4	99.8%
Petri Dish, Stackable Lid	Fisher Scientific	FB0875713A	60 mm x 15 mm
Septum Stopper	Chemglass	CG302401	14/20 - 14/35
Stir Bar	Chemglass	CG-2005T-01	Magnetic, PTFE, Turbo, Rare Earth, Elliptical, 10 x 6mm
SuperNuova+ Stirring Hot Plate	Thermo Scientific	SP88857190	50 - 1,500 rpm, 30 - 450 °C
Toluene	Fisher Chemical	T324-4	99.5%
Tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3)	Sigma-Aldrich	688096	97%
UiO-66 (120 nm edge length)			Prepared according to literature procedure (ref. 18).
Ultrasonic Cleaner CPX3800H	EMERSON / BRANSON	CPX-952-318R	40 kHz, 5.7 L
Waterproof Flexible LED Strip Light	ALITOVE	ALT-5B300WPBK	16.4 ft 5050 Blue LED