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....

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 .......

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