Communication pubs.acs.org/cm
Miniaturized Layer-by-Layer Deposition of Metal−Organic Framework Coatings through Digital Microfluidics Daan Witters,‡ Steven Vermeir,‡ Robert Puers,§ Bert F. Sels,† Dirk E. De Vos,† Jeroen Lammertyn,*,‡ and Rob Ameloot*,† †
Center for Surface Chemistry and Catalysis, KU Leuven − University of Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium BIOSYST-MeBioS, KU Leuven − University of Leuven, Willem de Croylaan 42, B-3001 Leuven, Belgium § MICAS-ESAT, KU Leuven − University of Leuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium ‡
S Supporting Information *
KEYWORDS: metal−organic frameworks, microfluidics, microfabrication, coating, layer-by-layer
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desired positions, leaving the rest of the substrate untouched.13−15 Here we present an approach that combines the advantages of both such localized deposition approaches and one of the most powerful techniques for the deposition of MOF coatings, namely, layer-by-layer deposition (LBL). LBL deposition, also referred to as liquid phase epitaxy, is an in situ growth method for MOF deposition that has been pioneered by Fischer, Wöll and co-workers.11,16,17 The technique relies on the sequential deposition of monolayers of metal salts, typically acetates, and organic linkers on a functionalized substrate. Since the presence of both MOF building blocks is separated in time, the growth that occurs during each cycle is self-limiting, which results in extremely flat and uniform layers. Additional advantages are the control over crystallographic orientation and interpenetration18 and the possibility to grow multilayer heteroepitaxial structures by using a different metal or differently functionalized organic linker.17,19 The approach we present here is a digital microfluidic adaptation of the LBL process. Digital microfluidics (DMF) based on electrowetting-on-dielectric (EWOD) actuation is an emerging technology capable of controlling individual μL to nL-sized droplets on hydrophobic surfaces.20 This technique, based on tuning of the interfacial tension between a liquid droplet and an actuation electrode coated with a dielectric layer, enables operations such as droplet dispensing, merging, and transport on-chip on reconfigurable paths of actuation electrodes. By interfacing a DMF chip with a computer, droplets can be manipulated in parallel in a highly automated fashion. The suitability of microfluidics for controlling MOF deposition has recently been demonstrated.14,21 In the LBL process the surface to be coated with a MOF film is repeatedly contacted with three different liquids: a metal salt solution, an organic ligand solution, and clean rinsing solvent. Repeated contact with each of these solutions is achieved either manually,16 by flowing the liquids over the surface,17 or by spraying.22 Figure 1 schematically illustrates how this task is
etal−organic frameworks (MOFs) are a class of microporous hybrid materials consisting of metal ion nodes held together by multitopic organic ligands. MOFs have been studied extensively in recent years for their record breaking surface area and highly functionalizable pore interior, which in turn sparked interest in a wide range of potential applications. Most research attention in this context has been devoted to gas storage and separation,1,2 while the number of reports on liquid phase separations and catalysis is increasing rapidly.3,4 To realize the potential of MOFs beyond these applications, in which MOFs are typically employed as bulk material, shaping and deposition methods are needed.5,6 For instance, while the potential for highly selective and sensitive MOF-based sensors is often suggested in the discussion of adsorption behavior observed for MOF powders, integration of such materials in an actual device is rarely reported.7,8 One of the first steps in the systematic fabrication of MOFbased devices will be the deposition of patterned thin films of MOFs on a substrate.7 Several ways to deposit thin films of MOFs have been reported and can be classified into in situ, ex situ, and seeding methods.9−11 In ex situ and seeding methods, film formation is based on the deposition of previously prepared (nano)crystals on a surface, followed by a secondary growth step in the case of seeding. In contrast, in situ methods rely on the surface chemistry of the substrate to promote preferential nucleation and/or attachment of nuclei on the surface to achieve film growth. Interestingly, while all of the aforementioned techniques can result in the deposition of patterned thin films of MOF, typically by selective surface functionalization or postsynthesis patterning,6,12 in most approaches the entire substrate is exposed to the deposition and/or synthesis conditions. In the case of sensors and other devices where MOFs will have to be integrated with electronics, exposure of preformed circuitry to a solvothermal synthesis mixture or vice versa adding circuitry after MOF deposition will more often than not be unacceptable because of contamination and circuit corrosion on the one hand and the thermal and chemical limitations of MOFs on the other.7,8 Different from the methods outlined above are techniques aimed at depositing arrays of crystallites rather than thin films by precisely positioning droplets of synthesis solution at the © XXXX American Chemical Society
Received: January 18, 2013 Revised: March 17, 2013
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Figure 2. Electron micrographs of a MOF coating deposited by the microfluidic LBL process. (A) Surface detail of HKUST-1 coating showing densely packed intergrown crystallites after 40 LBL cycles. (B) Cross-sectional view of a freestanding fragment of HKUST-1 film removed from the coating to demonstrate the uniform thickness of the MOF layer. The left-hand side is the side of the film that was previously attached to the support. Arrows indicate the cross-sectional thickness of the film fragment.
Figure 1. Microfluidic chip design and LBL MOF deposition method. (A) Top view and (B) side view of the DMF chip. The actuation electrodes in the bottom plate are used to direct droplets sandwiched between top and bottom plates toward an array of SAM-functionalized Au hydrophilic patches (250 or 500 μm diameter) in the top plate. The individual actuation electrodes are 1.4 × 1.4 mm2 in size. (C) Schematic representation of the bottom plate during sequential movement of droplets containing MOF building blocks in the LBL deposition process. The area directly underneath the section of the top plate containing the hydrophilic patches to be coated with MOF is highlighted yellow. Fluid reservoirs on the chip contain pure solvent (blue), metal salt solution (green), and ligand solution (pink). After each passage of a droplet containing MOF building blocks, pure solvent droplets are used to remove unreacted species.
removing parts of the coating, it can easily be seen that the coating has a uniform thickness of approximately 550 nm after 40 cycles (Figure 2b and Supporting Information Figure S2). Interestingly, this implies that the increase in HKUST-1 layer thickness per cycle is significantly larger than the 1.3 nm per cycle observed in the original reports, where substrates were manually submerged in the appropriate solutions. A similar enhanced increase in layer thickness has been observed in the preparation of HKUST-1 LBL films by spraying the different solutions onto the substrate.22 In that study, the enhanced growth has been attributed to a less effective rinsing, resulting in reactants that remain in the already formed layer, most likely in intracrystalline pores as well as in interstitial voids, and contributing to a larger than expected increase in layer thickness during subsequent cycles. To study the evolution of layer thickness, the increase in absorbance in the 1200−1800 cm−1 region (Supporting Information Figure S3), which is related to carboxylate groups bound to metal ions, was monitored by FTIR microscopy operating in reflection mode.16 As can be seen clearly from Figure 3a, the increase in absorption intensity seems to show an
completed by droplets dispensed from different fluid reservoirs on the microfluidic chip. The modular chip design consists of two plates. The reusable bottom plate, containing all electronics, ensures droplet transport (Supporting Information Figure S1). The removable top plate contains the substrate for MOF deposition, in this case a pattern of small gold patches (250 or 500 μm diameter) functionalized with a self-assembled monolayer (SAM) of carboxylic acid terminated thiols and surrounded by a hydrophobic coating of amorphous fluoropolymer (Teflon-AF). Droplets sandwiched between both plates are repeatedly dispensed from on-chip fluid reservoirs and transported toward the functionalized patches in the top plate by the actuation electrodes in the bottom plate.23 Patches in the top plate will be coated with MOF in the region where the paths of droplets containing the MOF building blocks overlap, highlighted in yellow in Figure 1c. These droplet paths are highly configurable,14 thus enabling different deposition patterns for a series of identical top plates and possibly even a different number of LBL cycles and resulting coating thicknesses for patches within the same top plate. The use of DMF thus introduces an extra level of flexibility in comparison with other methods for patterned MOF deposition, in which a change in deposition pattern requires a different surface functionalization pattern. HKUST-1, a MOF material consisting of Cu(II) ions and trimesate (1,3,5-benzenetricarboxylate; BTC) linkers,24 was chosen as an exemplary case given its suitability for LBL processing.16,22,25 Electron micrographs of the coatings obtained after running the process described above with ethanolic solutions of copper acetate and H3BTC are shown in Figure 2. In each step, reactant droplets were kept in contact with the substrate to be coated for a few minutes before several washing droplets were used to remove leftover reactants. As could be expected from previous LBL studies, highly uniform dense HKUST-1 coatings are formed that cover the whole surface of the functionalized gold patches. By mechanically
Figure 3. (A) Evolution of the characteristic HKUST-1 absorption peaks in the 1200−1800 cm−1 region with cycle number. (B) FTIR image of a single HKUST-1 coated patch visualizing infrared absorption in the OH-vibration band region (3000−3750 cm−1) due to the presence of ethanol. In the dark region, the MOF coating has been mechanically removed.
exponential rather than linear variation with the number of cycles. Such behavior is what would be mathematically expected in the case where the growth rate is not constant but contains a factor that is proportional to the thickness of the layer already formed. This observation supports the hypothesis that the MOF coating deposited during previous cycles adsorbs reactants that contribute to growth in subsequent cycles. Effective rinsing to remove leftover reactants is possibly complicated in the present study due to partial evaporation of the small amount of liquid that remains on the gold patches after a droplet containing MOF building blocks has passed. The increase in concentration resulting from evaporation would B
dx.doi.org/10.1021/cm400216m | Chem. Mater. XXXX, XXX, XXX−XXX
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Communication
government for Methusalem funding CASAS METH/08/04 and for Research Foundation Flanders (FWO−Vlaanderen) Projects G.0997.11 and G.0337.08. R.A. and S.V. acknowledge Research Foundation Flanders FWO−Vlaanderen for postdoctoral research fellowships.
make it harder to efficiently remove all unreacted building blocks. Future efforts will therefore be aimed at performing the microfluidic LBL process in an immiscible silicone oil environment to prevent evaporation.26 An additional observation that can be made from the FTIR spectra is the very intense absorption in the OH-vibration region from 3000 to 3750 cm−1. As samples were briefly dried in a stream of air and no liquid was visible at the time of measurement, this is most likely due to the presence of ethanol in the pores of the HKUST-1 films, which is in line with the high capacity and affinity of this MOF material for polar solvents. As can be seen from the FTIR image (Figure 3b) in which the absorption intensity of this OH-vibration band is visualized for a single HKUST-1 coated patch, ethanol adsorption capacity is completely lacking in the region where the MOF coating has been removed, thus confirming the negligible adsorption in the underlying layers. While X-ray diffraction confirms that the phase formed is indeed HKUST-1, a preferred crystallographic [100] orientation as would be expected on carboxylic acid terminated SAMs is not observed (Supporting Information Figure S4).27 Most likely this lack of preferred orientation is related to the incomplete removal of leftover reagents in the already formed MOF layer. Indeed, as has been noted earlier, oriented growth is less likely to occur if the two different reactants are present simultaneously on the substrate.22 It is therefore likely that performing the entire LBL process in an oil environment and hence preventing evaporation will close the gap between the properties of coatings prepared by the microfluidic LBL process and the classic immersion method, both in terms of growth rate and crystallographic orientation. In summary, we presented DMF as a unique tool to scale down and automate the LBL process for depositing patterned MOF coatings by making use of the coordinated movement of individual droplets containing the MOF building blocks. The resulting MOF films are dense and highly uniform in thickness. A unique feature of this methodology is the exquisite level of control over which parts of the substrate are contacted with the MOF reactant solutions, thus preventing interference with preformed elements present on the substrate. The high level of process control presented here is likely to play a key role in integrating MOFs as functional components in electronic devices.
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ASSOCIATED CONTENT
S Supporting Information *
DMF fabrication methods, a photo of a DMF chip, SEM images, FTIR spectrum and XRD of HKUST-1 films. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (R.A.); jeroen.
[email protected] (J.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for European funding (EFRO financing Interreg NanoSensEU) and support from the Belgian federal government (Belspo - IAP 7/05) and the Flemish C
dx.doi.org/10.1021/cm400216m | Chem. Mater. XXXX, XXX, XXX−XXX
