Surface-mounted metal-organic frameworks for applications in sensing and separation
Graphical abstract
Introduction
Thin films of metal-organic frameworks (MOFs), a nanoporous, crystalline hybrid material composed of metal complexes and organic linker molecules [1], are a rapidly developing field with many potential applications [2]. These films can be prepared in many different ways. The preparation by spin-coating, dip-coating and hydro/solvothermal growth results in MOF layers composed of crystallites with a fairly large size distribution bound to the substrate surface [2], [3], [4]. These crystallites can also be overgrown to prepare pinhole-free films. A different approach is when the synthesis is directly on the surface, which is done by successively depositing the different components of this hybrid material. In this review article, we focus on thin, homogenous MOF films which are prepared directly on the substrate surface in a layer-by-layer (LBL) fashion, employing liquid-phase epitaxy (LPE). These thin MOF films, called surface-mounted MOFs (SURMOFs), were introduced by Fischer and Wöll in 2007 [5], [6]. Since the last reviews of SURMOFs in 2011 and 2012 [3], [7], [8], many new and interesting properties of the SURMOFs have been investigated. In the meantime, the LBL-MOF-synthesis (i.e. SURMOF) principle has also been successfully applied by many other research groups worldwide, e.g. by M. Allendorf et al. [9], J. Hupp et al. [10] and H. Kitagawa et al. [11], resulting in a number of very interesting new applications. Here, we concentrate on the SURMOF preparation on various substrate surfaces, ranging from plain metal, metal-oxide and polymer surfaces over metal-oxide membranes to magnetic nanoparticles, as well as on multilayered SURMOF films, which are composed of different SURMOF layers with either identical or different MOF lattice constants or even different MOF structures. By incorporating photoswitchable azobenzene in the MOF structure, functional, multilayered SURMOFs with remote-controllable properties can be prepared, allowing the remote-controlled release from a porous container. Crosslinking the SURMOF structure by employing post-synthetic modifications results in water-stable thin films, referred to as SURGELs. In addition to this, SURMOFs are a unique model system for MOFs, for instance enabling detailed investigations of the mass transfer in MOFs and unveiling the almost omnipresent phenomenon of surface barriers in MOFs. Chiral separation by using homochiral SURMOFs, i.e. SURMOFs with homochiral linkers, can also be studied. The option of tuning the electronic properties of MOFs in combination with the thin and homogenous morphology of SURMOFs also enables interesting electronic applications. A summary of all the SURMOF structures and substrates reviewed in this article is provided in Table 1.
First, we will briefly explain the synthesis of SURMOFs using the liquid-phase epitaxy method. For more detailed descriptions, we refer to previous review articles [7], [8], [12]. The modified substrate is alternatively immersed in the solution of the metal ions and of the organic linker molecules, see Fig. 1. In between, the substrate surface is rinsed with pure solvent to remove any unreacted compounds from the surface. Surface modification can be carried out by using a number of different approaches. In the case of Au, the most appropriate method is to functionalize the Au surface with a self-assembled monolayer (SAM) made from organothiol monomers with a functional end group [13].
An example is the synthesis of HKUST-1 SURMOFs grown in [100] orientation on a 16-mercaptohexadecanoic acid (MHDA) SAM: The gold surface functionalized with the MHDA SAM, which has a functional –COOH head group, is immersed in the ethanolic copper(II) acetate (CuAc) solution, where the paddle-wheel-like CuAc compound chemically binds to the SAM. Subsequently, the sample surface is rinsed by ethanol and then the sample is immersed in the benzene-1,3,5-tricarboxylate (btc) solution, so that the btc-linker molecules can bind to the CuAc, resulting in the first MOF layer. Afterwards, any unreacted molecules are removed by rinsing the sample with ethanol again. These processes are repeated until the desired number of cycles is reached.
So far, several variants of this layer-by-layer technique have been established for the SURMOF preparation, namely the pump [14], the spray [15] and the manual-dipping [16] methods. This method has also been successfully used to coat small magnetite particles with SURMOFs [17]. Recently, an automated dipping robot has been introduced to prepare very homogenous SURMOFs with a very low surface roughness [18]. Furthermore, a quartz crystal microbalance (QCM) or a surface plasmon resonance (SPR) cell connected with an autosampler can also be used to prepare SURMOFs in an automated and continuous way while the growth is monitored in situ [19].
SURMOFs grow usually as continuous films without (visible) cracks or pinholes. The SURMOF thickness is directly controlled by the number of synthesis cycles. The crystal orientation can be determined by the functionalization of the substrate. For instance, HKUST-1 and pillared-layer MOFs, e.g. of type Cu2(ndc)2(dabco), grow in [100] orientation on –COOH–terminated surfaces, like MHDA SAMs (ndc = naphthalene-1,4-dicarboxylate and dabco = 1,4-diazabicyclo-(2.2.2)-octane). On the other hand, HKUST 1 and Cu2(ndc)2(dabco) grow in [111] and [001] orientation, respectively, on –OH– or –pyridyl-terminated surfaces, like 11-mercapto-1-undecanol (MUD) or 4-(4-pyridyl)phenylmethanethiol (PBMT) SAMs [20]. SURMOFs cannot only be grown on modified Au substrates but also on a number of different other materials, see chapter 3. In addition to this, it is also possible to delaminate the thin porous coatings to yield free-standing thin MOF films [21].
Compared to thin MOF films prepared by other techniques, SURMOFs offer several advantages, for instance: (1) smooth and homogeneous morphologies with small surface roughnesses [16]; (2) controllable thickness, obtained by varying the number of deposition cycles [19]; (3) easy scale-up, for instance by spraying the MOF components [15]; (4) perfectly oriented films and the opportunity to control the crystal orientation [6]; and (5) lower defect density than the bulk material synthesized using conventional methods [22].
Section snippets
New MOF structures synthesized using LPE method
Functionalized substrate surfaces (e.g. SAMs on flat gold surfaces) show template effects for the fabrication of SURMOFs, resulting in different growth directions perpendicular to the surfaces. Besides the orientation control, MOF structures that are different from the solvothermally synthesized MOF structures can be obtained by using LPE on SAM-functionalized substrates. For many known isoreticular MOF series, the formation of larger pores is limited by the phenomenon of interpenetration,
Fabrication of SURMOFs on different substrates
Apart from the fabrication on flat substrates, SURMOFs can also be deposited on differently shaped substrates, yielding functional composites which can add additional properties to facilitate other novel applications [29]. For example, continuous HKUST-1 thin layers can be deposited on the rough surface of flexible synthetic polymers with surfaces terminated by –COOH or –NH2 groups [30]. Combining the selective adsorption properties of MOFs, MOF/polymer composites open a wide field of possible
Internal surface modifications of SURMOFs
Post-synthetic modification (PSM) of MOFs opens up the possibility to introduce various functional groups into MOFs [35]. Transferring the concept of PSM from bulk materials to thin MOF films, Shekhah et al. [36] fabricated a pillared-layer SURMOF of type Cu2(NH2-bdc)2(dabco) (NH2-bdc = 2-amino-benzene-1,4-dicarboxylate) on a OH-terminated Au surface. The free amino groups in the framework do not bind to the metals and thereby provide the chemical handle for performing PSM. By exposing the
Heterostructured SURMOFs (hetero-SURMOFs)
The functionalization of MOFs with defined physical and chemical properties has become an important issue for the development of MOF-integrating devices [43]. MOF features, like the pore topology, size, shape as well as the coordination space and the reactive centers within the frameworks, are directly tailored through selecting the appropriate building elements [44]. Hence, the direct incorporation of pre-designed components in a solvothermal synthesis is a straightforward way to introduce
Using SURMOFs to determine MOF properties
Apart from the potential applications as coatings and membranes, SURMOFs are well suited to determine MOF properties in a quantitative and, at least in some cases, time-resolved fashion. These thin films are very homogenous and have a small defect density [22]. Furthermore, the crystal orientation and the film thickness can be directly controlled. The thinness of the SURMOF films allows the application of surface sensitive techniques like X-ray and ultraviolet photoelectron spectroscopy (XPS
Conclusion
With its wide range of applications from membranes over functional coatings to electronic applications, thin films of MOFs show great potential. A very attractive way is the direct fabrication of the thin MOF films on the substrate surface in a layer-by-layer fashion by means of liquid-phase epitaxy, referred to as surface-mounted MOFs (SURMOFs). The unique SURMOF properties – like the thin, homogenous morphology, the option to control the crystal orientation and thickness as well as the
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2021, Microporous and Mesoporous MaterialsCitation Excerpt :It has a very similar structure to the silicon derivative, SIFSIX-Ni, for which a crystal structure has been already published [27] (Fig. 1). However, the shorter length of the Ti-F bond in comparison to Si-F leads to a pore aperture of ~0.35 nm in TIFSIX [42], which is close to the kinetic diameter of CO2 (~0.33 nm [48]). This, in turn, leads to one of the highest known CO2 adsorption capacity (~40 cm³ g−1 at 2 mbar CO2 pressure) for a MOF [46], and selectivity at low CO2 pressures.