Surface-mounted metal-organic frameworks for applications in sensing and separation

Microporous and Mesoporous Materials xxx (2015) 1e16

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Surface-mounted metal-organic frameworks for applications in sensing and separation € ll a, * Lars Heinke a, Min Tu b, Suttipong Wannapaiboon b, Roland A. Fischer b, *, Christof Wo a b

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany €t Bochum, D-44780 Bochum, Germany Chair of Inorganic Chemistry II e Organometallics and Materials Chemistry, Ruhr Universita

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 December 2014 Received in revised form 4 March 2015 Accepted 18 March 2015 Available online xxx

Thin films of metal-organic frameworks (MOFs) enable various applications ranging from membrane separations over sensor techniques to potential (micro-) electronic uses. Recent progresses of thin and homogenous surface-mounted MOF (SURMOF) films, which are prepared in a well-defined layer-by-layer fashion on a solid substrate surface, are highlighted in this review. Various substrate surfaces, ranging from plain metal, metal-oxide and polymer surfaces over metal-oxide membranes to magnetic nanoparticles, can be coated with SURMOFs. Multilayered SURMOF films with either identical or different MOF lattice constants or even different MOF structures were prepared, enabling the preparation of functional surface coatings. This allows, by incorporating photoswitchable azobenzene in the MOF structure, the preparation of multilayered, nanoporous films with remote-controllable properties. By means of crosslinking the SURMOF structure employing post-synthetic modifications, water stable thin films, SURGELs, can be prepared. Their thin and homogenous morphology also makes SURMOFs perfectly suited as coatings for electrochemical and electronic applications, where the small dielectric constant k as well as the option to tune the conductivity by loading the pores are very promising features of these porous solids. Furthermore, SURMOFs are very well suited for investigations of MOF-specific properties, since e.g. photoelectron spectroscopies can be applied to these thin films in a straightforward fashion. In additions, mass transfer and diffusion properties in MOFs can be studied for such thin films with high precision using a quartz-crystal microbalance (QCM). © 2015 Published by Elsevier Inc.

Keywords: Metal-organic frameworks Thin films

1. 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 [2e4]. 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

* Corresponding authors. E-mail addresses: roland.fi[email protected] (R.A. Fischer), [email protected] €ll). (C. Wo

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 € ll in 2007 [5,6]. Since the last reviews of SURby Fischer and Wo MOFs 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

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container. Crosslinking the SURMOF structure by employing postsynthetic 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 selfassembled 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 eCOOH 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

Table 1 Summary of SURMOF structures described in this review. SURMOFs

Substrate

Investigations/Applications

Ref.

Cu3(btc)2 or HKUST-1

eCOOH and eOH terminated SAM on Au eCOOH terminated SAM on Au Rough surface of flexible synthetic polymers with eCOOH and eNH2 terminated surface eCOOH terminated magnetic beads eCOOH terminated SAM on Au Modified Si substrate eCOOH terminated SAM on Au eCOOH terminated SAM on Au

Growth process and mechanism

[19]

Defect density in SURMOF Growth process

[22] [30]

MOF magnetic composites (magMOF) Mass transfer studies Measuring dielectric constant Mechanical studies Ferrocene-loaded SURMOF, conductivity investigation Europium-complex-loaded SURMOF as photonic antennas Structure different than structure obtained from solvothermal synthesis Measuring dielectric constant Isoreticular SURMOF-2 series with structures different than structures obtained from solvothermal syntheses Suppression of framework interpenetration by LPE Crystal orientation control

[17] [75] [86] [87] [88]

[20]

Membrane-based separation

[33]

External surface modification by PSM Diffusion coefficient and activation energy of ferrocene Membrane-based separation

[40] [57]

eCOOH terminated SAM on Au SURMOF-2 Zn(bdc) (H2O) and Cu(bdc) (H2O)

eCOOH terminated SAM on Au

Cu(bdc) Isoreticular SURMOF-2: Cu(bdc), Cu(bpdc), Cu(tpdc), Cu(qpdc), Cu(p(ep)2dc), Cu(ppdc)

Modified Si substrate eCOOH terminated SAM on Au

Zn2(bdc)2(bipy)

-Pyridyl terminated SAM on Au

Cu2(ndc)2(dabco)

eCOOH, -pyridyl terminated SAM on Au Macroporous alumina and titania membrane -Pyridyl terminated SAM on Au -Pyridyl terminated SAM on Au

Cu2(BME-bdc)2(dabco) Cu2(NH2-bdc)2(dabco) Zn2(N3-bdc)2(dabco)

Macroporous alumina and titania membrane eOH terminated SAM on Au eOH terminated SAM on Au

Zn2(Dcam)2(dabco) and Zn2(Lcam)2(dabco) Cu2(Dcam)2(dabco)

-Pyridyl terminated SAM on Au -Pyridyl terminated SAM on Au

Cu(DA-SBDC)

eCOOH terminated SAM on Au eCOOH terminated SAM on Au

Zn4O(dmcapz)3, its derivatives and heterostructures Pillared-layer hetero-SURMOFs
Zn2(ndc)2(dabco) on Cu2(ndc)2(dabco)
Cu2(F4bdc)2(dabco) on Cu2(ndc)2(dabco)
Zn2(BME-bdc)2(dabco) on Cu2(ndc)2(dabco)
Cu2(NH2-bdc)2(dabco) on Cu2(bdc)2(dabco); Cu2(NH2-bdc)2(dabco) on Cu2(ndc)2(dabco); Cu2(ndc)2(dabco) on Cu2(NH2-bdc)2(dabco) on Cu2(bdc)2(dabco) Hetero-SURMOF: Cu2(Azo-bpdc)2(bipy) on Cu2(bpdc)2(bipy)

eCOOH terminated SAM on Au -Pyridyl terminated SAM on Au

eOH terminated SAM on Au

Hetero-SURMOF: Cu3(btc)2 on Cu2(ndc)2(dabco)

-Pyridyl terminated SAM on Au

Isoreticular hetero-SURMOF-2

eCOOH terminated SAM on Au

[89] [26] [86] [90]

[24]

[33]

Framework modification by PSM Framework modification by click chemistry PSM Enantioselective adsorption Enantiomer separation from a racemic mixture and circular dichroism Conversion of SURMOF to SURGEL SURGEL functionalization by thiol-yne click chemistry reaction Selective adsorption Intermetallic heterostructures Hetero-SURMOFs with layers of various dicarboxylate linkers Selective PSM in hetero-SURMOFs, Filter effects by spatial arrangement of SURMOF layers

[36] [39]

Photoswitchable azobenzene-containing SURMOF Filter effects by spatial arrangement of SURMOF layers, different structures Hetero-SURMOF with large lattice mismatch

[85]

[79] [80] [41] [42] [49,50] [14] [20] [51e53]

[55]

[56]

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Fig. 1. Sketch of the SURMOF synthesis. SURMOFs are prepared by consecutively immersing the substrate surfaces in the solutions of the metal ions and of the organic linker molecules in a layer-by-layer fashion.

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 eCOOHeterminated surfaces, like MHDA SAMs (ndc ¼ naphthalene-1,4-dicarboxylate and dabco ¼ 1,4diazabicyclo-(2.2.2)-octane). On the other hand, HKUST 1 and Cu2(ndc)2(dabco) grow in [111] and [001] orientation, respectively, on eOHe or epyridyl-terminated surfaces, like 11-mercapto-1undecanol (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]. 2. 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

3

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, where two identical lattices interpenetrate each other leading to the same lattice constant but with a much smaller (free) pore volume [23]. As reported by Shekhah et al. [24], interpenetration can be suppressed in the SURMOF fabrication by using LPE. SURMOFs of type Zn2(bdc)2(bipy) (bdc ¼ benzene-1,4-dicarboxylate, bipy ¼ 4,40 bipyridine) were grown on a pyridyl-terminated SAM (PBMT). Conventionally in bulk synthesis, two structures of Zn2(bdc)2(bipy) were obtained depending on the presence or absence of guest solvent molecules, both of which consist of two-fold interpenetrating networks, termed as MOF-508a and MOF-508b [25]. The absence of non-interpenetration was confirmed by the agreement of the out-of-plane XRD patterns of SURMOF Zn2(bdc)2(bipy) with the simulated diffraction pattern of noninterpenetrated MOF-508a. Moreover, the surface area of SURMOF Zn2(bdc)2(bipy), 1010 m2 g1, is substantially higher than the value of 660 m2 g1 for the interpenetrated MOF-508a and MOF508b. These findings confirmed that LPE suppresses the interpenetration during MOF growth. Another example of SAM-templated (i.e. induced by the surface termination) effects is that the structure of SURMOF2 (Zn(bdc)(H2O) and Cu(bdc)(H2O)) grown on carboxylateterminated surface is different from the conventional solvothermally synthesized MOF-2 (Zn(bdc)(H2O)(dmf) or Cu(bdc)(dmf)) [26e28]. MOF-2 consists of planar, quadratic twodimensional sheets formed by attaching 4 dicarboxylate groups to Cu2þ- or Zn2þ-paddle-wheel units. Conventional solvothermal synthesis yields stacks of paddle-wheel planes shifted relative to each other, with a corresponding reduction in symmetry to yield a P2 or C2 symmetry [27,28]. Interestingly, when the corresponding SURMOF (termed SURMOF-2) is grown on a carboxylate-terminated substrate, the sheets are not distorted and provide an array of perfectly stacked lamellae with water molecules that are hydrogenbonded to the axial positions of the paddle wheel [26]. A series of isoreticular SURMOF-2s is shown in Fig. 2. 3. 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 eCOOH or eNH2 groups [30]. Combining the selective adsorption properties of MOFs, MOF/ polymer composites open a wide field of possible applications, such as protection layers for working clothes and gas separation materials in the textile industry. MOF membranes have been successfully used for the separation of gases and liquids [31,32]. A major challenge in this field is that the separation performances of MOF membranes strongly rely on the ability to achieve continuous, homogeneous and low-defect MOF films. Two SURMOFs of type Cu2(BME-bdc)2(dabco) (BMEbdc ¼ 2,5-bis(2-methoxyethoxy)-benzene-1,4-dicarboxylate) and Cu2(ndc)2(dabco) were deposited on macroporous alumina and titania membrane discs for the separation of CO2 from a CO2/CH4 mixture [33]. Due to the porous nature of the macroporous substrates, the growth of MOFs could occur not only on the external surface but also into the channels. Therefore, after a certain number of deposition cycles, intergrown MOF layers were grown on the top of the macroporous substrates together with foam-like MOFs inside

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which approaches the value of bulk HKUST-1. Although the saturated magnetization of MagPrep silica decreases from 72 to 34 A m2 kg1 after 60 cycles of HKUST-1 growth, the remaining magnetic property of magMOF particles indicates no significant change in the magnetic properties of the magnetic core of the particles during the MOF growth. The synthesized magMOF particles were tested as chromatographic media towards the separation of two environmentally problematic compounds, toluene and pyridine. The retention times of pure toluene and pyridine were found to be 25.2 and 28.3 min, respectively. Despite its smaller size, the retention time of pyridine is longer than that of toluene due to its interaction with the coordinatively unsaturated sites (CUSs) in the framework of HKUST-1. In the case of the separation in a mixture of the analytes, the differences in the retention times remained largely unaffected, showing the potential of magMOFs to serve as chromatographic materials for the separation of small molecules. 4. Post-synthetic modifications of SURMOFs 4.1. Internal surface modifications of SURMOFs

Fig. 2. Out-of-plane XRD patterns of Cu(bdc), Cu(2,6-ndc), Cu(bpdc), Cu(tpdc), Cu(qpdc), Cu(p(ep)2dc) and Cu(ppdc) (upper left), schematic representation (upper right) and structure of SURMOF-2 analogues (lower panel). The SURMOF-2 series is grown on eCOOH terminated SAM surfaces. (bdc ¼ benzene-1,4-dicarboxylate; 2,6ndc ¼ naphthalene-2,6-dicarboxylate; bpdc ¼ 4,40 -biphenyldicarboxylate; tpdc ¼ para-terphenyldicarboxylate; qpdc ¼ para-quaterphenyldicarboxylate; p(ep)2dc; ppdc ¼ para-pentaphenyl dicarboxylate). Reprint with permission from Ref. [90]. Copyright 2012 Macmillan publisher Limited: Scientific Report.

(Fig. 3). The separation performances of obtained MOF membranes were investigated by permeation experiments of CO2/CH4 mixture (50/50) using the WickeeKallenbach method. The CO2/CH4 separation factor of the membrane of type of Cu2(BME-bdc)2(dabco) is ~4e5. The high separation factor is caused by the presence of the two polar ether side chains of BME-bdc [34]. The results support the concepts of introducing functional groups in the framework to improve the efficiency of MOF. The incorporation of nano- and microparticles into porous MOFs provides additional functionalities, such as magnetic, luminescent and catalytic properties [2,4]. MOFs incorporating magnetic particles allow the positioning in precise locations by an external magnetic field. By using LPE, HKUST-1 can be grown on COOH-terminated magnetic beads to prepare functionalized MOF magnetic composites (magMOFs) [17]. As shown in Fig. 4, after 40 growth cycles, the observed thickness is between 20 and 25 nm, corresponding to 0.5e0.6 nm per growth cycle. With increasing numbers of growth cycles, the surface area of the magMOF particles increases from 17 m2 g1 to around 1150 m2 g1 (200 cycles),

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 ¼ 2-amino-benzene-1,4dicarboxylate) 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 SURMOF to n-butylisocyanates and/or 4-fluorophenylisothiocyanate vapor at room temperature, the amino groups can react with isocynate and isothiocyanate groups to form strong covalent urea and thiourea bonds with a reaction yield of ~50%. Even a larger compound, such as 1-ferrocenylmethylisocyanate, can be used as a PSM reagent to graft ferrocene inside SURMOFs and thus provide the opportunity for employing SURMOFs for electrochemical application. The XRD investigations indicate that the crystallinity and orientation of the SURMOFs is not affected by performing the PSM reaction. Another approach to realize PSM is via click chemistry, where the reaction can be carried out with high yields under mild conditions [37,38]. However, triggering the click reaction with conventional CuI catalysts exhibits some drawbacks. For example, the cytotoxic CuI metal ions may remain in the framework and cause unwanted effects in the context of biological applications. Wang et al. [39] reported the PSM of the SURMOF Zn2(N3-bdc)2(dabco) (N3-bdc ¼ 2-azido-benzene-1,4-dicarboxylate) with phenylacetylene using conventional CuI-catalyst-based click chemistry. The triple bond reacts with the eN3 group in the framework to yield a new SURMOF with a phenyl-modified pore surface. The maximum number of reacted eN3 groups were found to be slightly more than 90%. In the same work, PSM with cyclooctyne derivatives using metal-free azide-alkyne cycloadditions is shown, see Fig. 5. Importantly, in both cases of metal-free click reactions, the total conversion yields (~92%) are substantially higher than the CuIcatalyzed reaction with phenylacetylene, despite the much larger molecular sizes of the two synthons compared to the phenylacetylene. 4.2. External surface modifications of SURMOFs Besides the functionalization of the internal pore surface of MOFs, the external surface can also be modified by PSM. External surface functionalization is of great interest to the field of MOF

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Fig. 3. Representative examples of SEM images of the cross-sections of Cu2(ndc)2(dabco) on alumina (a and b) and Cu2(BME-bdc)2(dabco) on titania substrates (c and d) at different magnifications. Reprint with permission from Ref. [33]. Copyright 2011 Elsevier.

Fig. 4. TEM images of pure MagPrep Silica nanoparticles (a,b) and HKUST-1 grown on COOH-terminated MagPrep Silica after 40 cycles of the LPE method (c,d). It is noteworthy that the small black particles are most likely CuO particles resulting from electron induced beam damage. Reprint with permission from Ref. [17]. Copyright 2013 Wiley-VCH.

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Fig. 5. Click reaction of N3-bdc ligands in the Zn2(N3-bdc)2(dabco) SURMOFs using azide-alkyne cycloadditions. Reprint with permission from Ref. [39]. Copyright 2013 American Chemical Society.

research because it alters the surface properties, such as hydrophilicity/hydrophobicity and affinity, while keeping the porosity and the inner pore space unchanged [35]. It is well recognized that LPE offers the ability to precisely control the location of functional groups within the deposited MOF thin films. Therefore, the reactive sites can be selectively located at the external surface by using the functional linkers during the last step of the SURMOF growth. Subsequently, PSM is performed at the reactive sites, leading to a selective functionalization of the outer layer without any change of the internal surface. The related research has been applied on SURMOFs of type Cu2(ndc)2(dabco), which is schematically described in Fig. 6 [40]. First, on top of the terminating ndc and dabco groups on the [001]-oriented Cu2(ndc)2(dabco), CuAc complexes were attached. The terminating acetate groups are located on all crystal surfaces, and thus, subsequent binding of NH2-bdc leads to single monolayer amino-functionalization. Afterwards, the surface-attached amino groups allow fluorescence labeling by fluoresceinisothiocyanate (FITC) based on the reaction between amino and isothiocyanate groups. A permanent mass uptake was observed from the FITC labeling on the aminofunctionalized SURMOF Cu2(ndc)2(dabco) which was monitored in situ by using a quartz crystal microbalance (QCM). In contrast, FITC ethanol solution flowing over blank and bdc-modified SURMOF Cu2(ndc)2(dabco) did not lead to any uptake at the surface. The molecular size of FITC is much larger than the pore size, thus, the adsorption can only occur on the external surface. The control experiments demonstrated that the introduction of the amino groups on the SURMOF surface enables the attachment of FITC labeling molecules. Because FITC is a highly fluorescent molecule, the surface-labeled FITC exhibits a dramatic change in the fluorescence intensity of the SURMOF sample as shown by fluorescence microscopy. A side-by-side comparison of fluorescence microscopy images suggests noticeable fluorescence intensity enhancements before and after FITC labeling. This is consistent with the QCM data. 4.3. Surface-mounted gels (SURGELs) derived from SURMOFs Beyond functionalization at either the internal or the external surface, a further opportunity of PSM is the conversion of the fabricated SURMOFs to metal-free, water-stable coatings (called surface-mounted gels, SURGELs) [41]. The conversion process is

Fig. 6. Stepwise approach for external surface functionalization of SURMOF of type Cu2(ndc)2(dabco) (top) and the reaction of NH2-bdc with FITC (bottom). Reprint with permission from Ref. [40]. Copyright 2011 American Chemical Society.

based on a covalent cross-linking of SURMOF ligands via copperfree click chemistry and subsequent removal the metal ions to yield robust gels with pronounced stability under biological conditions. First, as shown in Fig. 7, the fabricated SURMOF Cu(DASBDC) (DA-SBDC ¼ diazido-stilbenedicarboxylate) was immersed in a solution containing an electron-deficient alkyne cross-linker; the spontaneous coupling of alkyne groups with the azido groups in the framework takes place at room temperature. Afterwards, the Cu ions located in the framework were removed from the crosslinked SURMOF by immersion in a solution of ethylenediaminetetraacetic acid (EDTA), resulting in a homogeneous, amorphous, surface-mounted gel (SURGEL). The SURGELs were studied as a substrate for the delivery of biomolecules to the interior of adhering cells. The fabricated SURGEL was first loaded with arabinose and subsequently exposed to bacterial cells that were gene-modified with an arabinose-triggered GFP switch. The induction of GFP expression has proved to be highly site-specific; it occurred only for those bacteria in direct contact with the SURGEL substrate. As shown in Fig. 7, after 24 h of incubation of bacteria cells of type Pseudomonas putida pJN::GFP, only bacterial cells adhering to the surface of arabinose-loaded SURGEL exhibited GFP fluorescence. In contrast, non-adhering bacteria in the broth supernatant and in bacterial cells adhered to SURGEL surfaces without previous arabinose loading, showed only marginal GFP fluorescence. These results demonstrate that arabinose-induced GFP gene expression is highly localized in space and occurs only at the water/SURGEL interface. As shown Fig. 7, the residual alkyne group in the SURGELs allows further PSM to introduce other functional groups. Mugnaini et al. [42] reported the introduction of ferrocenyl thiol derivative (6-

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Fig. 7. (a) Copper-free click reaction between the diazido-stilbenedicarboxylic acid (DA-SBDC) and the cross-linker trimethylolethane tripropiolate and the corresponding schematic representation. (b) Schematic representation and fluorescence microscopy images of P. putida pJN::GFP bacteria after 24 h of incubation in the presence of SURGEL substrates. b-left: bacteria settled on the SURGEL substrate containing arabinose. b-right: bacteria settled on an “empty” SURGEL substrate. Reprint with permission from Ref. [41]. Copyright 2014 American Chemical Society.

(ferrocenyl)-hexanethiol) using thiol-yne click chemistry reaction. The cyclic voltammetry (CV) experiments performed on pristine SURGELs in ferrocene dissolved [BMIM][NTf2] ionic liquid revealed that the SURGEL formed a closed and non-porous insulating film for ferrocene sized molecules. However, the CV curves of ferrocenyl modified SURGEL showed reversible waves which can be assigned to the ferrocene/ferrocenium redox couple. Thus, the SURGEL was altered from being completely blocked to permeable for the ferrocene molecules after the introduction of the ferrocenyl groups. Considering that the SURGEL lacks any crystalline order, it was speculated that the ferrocenyl groups decorating in the SURGEL pores form an unordered array, thus excluding an electron hopping mechanism. 5. 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 functionalities into MOFs. However, some functional groups may hinder the desired MOF formation. Post-synthetic modification (PSM) provides an alternative, since the desired functionalization of the inner and/or the outer surfaces occurs after the MOF fabrication. The reactive moieties within the frameworks are modified selectively by applying appropriate organic reactions (as discussed in the previous section). Besides PSM, another complementary method to functionalize the MOF outer surface relies on growing layers of different but structurally related MOFs on top of each other. This form of heteroepitaxy has been applied successfully in a number of cases. Due to the good lattice matching between both MOF structures, heterometallic core-shell pillared-layer MOF crystals [45] and core-shell IRMOF crystals with different linkers [46] have been successfully synthesized. In the case of pillared-layer MOFs, the use of two pillar linkers with different lengths allow heteroepitaxial growth of the overlaying MOFs at the lattice-matching facets of the seeding MOFs only [47]. In this example, the epitaxy occurs through the connection of the metal-dicarboxylate layered structures with the extended pillar linkers along the c-axis, which results in the hybrid BAB-type crystals. The matching of the lattice parameters and the coordination chemistry is critical for the

heteroepitaxial growth to succeed under solvothermal conditions [47]. Multi-component MOFs offer functionalities resulting either from the simple addition of the components' features or from a synergistic combination, which is achieved through a controlled spatial arrangement of the components. This enlarges the field of potential MOF applications [48]. The concept of multi-component MOFs has been transferred from bulk crystal fabrications to thin film methodologies, especially to LPE-based procedures. Since LPE relies on the sequential and iterative dosing of the individual building-blocks to the substrate, the procedure can be adapted for hetero-epitaxial growth by changing structurally related reactants at a defined step. Unlike the solvothermal syntheses of bulk crystals, LPE provides a straightforward control of location and distribution of the added functionalities through the variation of deposition sequences and cycles. Hence, hetero-SURMOF films can be fabricated with a controlled distribution of the functionalities along the growth direction. 5.1. Stepwise liquid-phase heteroepitaxial growth of SURMOFs Zacher et al. [20] have used LPE-based methods to fabricate heterostructured pillared-layer M2(L)2(P) SURMOFs with layers of various dicarboxylate linkers and metals, i.e. Cu2(F4bdc)2(dabco)on-Cu2(ndc)2(dabco) and Zn2(BME-bdc)2(dabco)-on-Cu2(ndc)2 (dabco) (F4bdc ¼ tetrafluorobenzene-1,4-dicarboxylate). The XRD patterns of such hetero-SURMOFs show only the peaks related to the [001] orientation, like in the pre-deposited template layer. These observations imply that the heteroepitaxial growth maintains the crystal orientation of the pre-formed SURMOF (Fig. 8). The existence of the fluorinated, thus hydrophobic, top layer results in an increase of the water contact angle. Though X-ray photoelectron spectroscopy (XPS) does not reveal the in-depth distribution of each element, it confirms the presence of F, Zn and Cu elements in these hetero-SURMOFs [20]. Shekhah et al. have also reported the successful heteroepitaxial growth of the intermetallic Zn2(ndc)2(dabco)-on-Cu2(ndc)2(dabco) hetero-SURMOF. An increase of intensity of the (001) XRD peak and an increase of refractive index unit (RIU) based on in situ surface plasmon resonance (SPR) spectroscopy indicates the presence of the Zn-based SURMOF on the Cu-based SURMOF. The expected thickness increase for the hetero-film formation is also revealed by infrared reflection absorption spectroscopy (IRRAS) and atomic force microscopy (AFM). Moreover, by using a four-circle synchrotron X-ray diffractometer for scanning the hetero-SURMOF perpendicular to the sample stage (c scan), the perfect

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by using 3-methyl-5-isopropyl-4-carboxypyrazolate (mipcapz) linkers. When the Zn4O(mipcapz)3 (MIP) films are grown on top of a DM film, the resulting heterostructured SURMOF films show a high crystallinity and a size selectivity of the shell MIP layer with the high storage capacity of the core DM film. Hence, size-selective vapor adsorption (methanol and ethanol over isopropanol) with relatively high total adsorption capacities has been observed (Fig. 9). Moreover, being moisture-tolerant, these MOF films could be potentially integrated in devices, which operate under ambient conditions [50]. 5.2. Post-synthetic modification on heterostructured SURMOFs

Fig. 8. Schematic illustration of liquid-phase epitaxial (LPE) growth of Cu2(ndc)2 (dabco) (top) using the 2-steps-deposition on pyridyl-terminated substrates leading to the [001] orientation; (middle) using the 3-steps-deposition on carboxylateterminated substrates leading to the [100] orientation in the growth direction; and (bottom) heteroepitaxial growth of Zn2(BME-bdc)2(dabco)-on-Cu2(ndc)2(dabco) illustrating the retention of crystallographic orientation of the pre-deposited SURMOF. Reprint with permission from Ref. [20], Copyright 2011 Wiley-VCH.

orientation along the substrate surface normal could be shown. These studies have demonstrated that LPE allows fabricating hetero-SURMOFs with a control on a mesoscopic and microscopic scale to obtain well-defined thin film topologies with specific crystallographic orientations [14]. The hydrophobic MOF-5 isotype Zn4O(dmcapz)3, (DM, dmcapz ¼ 3,5-dimethyl-4-carboxypyrazolate) and its derivatives have been fabricated as homo- and heterostructured thin films by LPE deposition [49,50]. DM films show size-selective adsorption of alcohol molecules influenced by the size of the pore openings. DM films show a preferential adsorption of non-polar analytes over polar ones of similar size due to hydrophobic interactions. The adsorption of the films can be modulated by extending the alkyl side chain length of the linker, hence reducing the pore openings [49]. Wannapaiboon et al. have managed to significantly improve the crystallinity and total adsorption capacity of such derivatives

Heterostructured SURMOFs have multiple functionalities and are of interest for a large field of promising applications, especially in sensing and separation. However, some functional building blocks can hinder the fabrication when LPE-based methods are used. In this case, PSM can allow a specific tuning of LPE fabricated heterostructured SURMOFs to obtain the desired functionalities. Liu et al. incorporated reactive eNH2 functional groups at a controlled depth in highly oriented hetero-SURMOFs resulting in core-shell (A-B) and core-shell-shell (A-B-C) SURMOF structures (A: Cu2(bdc)2(dabco), B: Cu2(NH2-bdc)2(dabco) and C: Cu2(ndc)2(dabco)). The presence of eNH2 groups within the hetero-SURMOFs is identified by the NeH vibration bands using IRRAS (at 3500 and 3380 cm1). The eNH2 groups allow a localized PSM by reacting 4-fluorophenyl isothiocyanate (FPI) specifically within the B layer. The isothiocyanate groups of FPI molecules react with the amino groups to form robust thiourea bonds and hence modify the frameworks. By in situ QCM measurements during the FPI loading, complex kinetic uptake behaviors by the hetero-SURMOFs could be revealed. The fine tuning of the pore structure within the material by the combined hetero-LPE-PSM methodology can have great effects on the kinetic uptake of guest molecules [51]. Extending this work, Tu et al. have used the LPE-PSM method to functionalize hetero-SURMOFs for selective adsorptions. B@A hetero-SURMOF of Cu2(NH2-bdc)2(dabco)@Cu2(bdc)2(dabco) have been fabricated and subsequently modified with tert-butyl isothiocyanate (tBITC). Here, the PSM leads to the chemical modification of the outer layer (B) of the hetero-SURMOF (Fig. 10), and induces the size-selective adsorption of methanol and hexane over cyclohexane. Moreover, the total uptake of hexane in the tBITC@B@A SURMOF is higher than for a tBITC@B SURMOF of comparable thickness, owing the contribution of the larger pore volume of the unmodified core component (A). This emphasizes again the advantages of heterostructured SURMOFs [52]. The influence of the spatial arrangement of each component within the hetero-SURMOFs on the integrated properties of materials has been highlighted by the work of Meilikhov et al. [53]. Two SURMOF-on-SURMOF-structures, both fabricated by the same

Fig. 9. Adsorption isotherms in Zn4O(mipcapz)3 (20 cycles) on Zn4O(dmcapz)3 (20 cycles) in comparison with the corresponding homostructured MOF films of (a) methanol, (b) ethanol and (c) isopropanol at 25  C using an environmentally controlled quartz crystal microbalance (QCM). The isotherms illustrate the size selective adsorption property of the core-shell heterostructured MOF films. Adapted from Ref. [50] with permission, Copyright 2013 Wiley-VCH.

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Fig. 10. Schematic illustration of programmed functionalization of SURMOFs via LPE-PSM: initially, SURMOF Cu2(bdc)2(dabco) (A) was deposited on pyridyl-terminated SAM on Au coated QCM sensors. Sequentially, SURMOF Cu2(NH2-bdc)2(dabco) (B) was deposited on A (B@A). Finally, SURMOF B@A was modified by PSM with tert-butyl isothiocyanate (tBITC). Reprint with permission from Ref. [52], Copyright 2013 The Royal Society of Chemistry.

combined LPE-PSM method but with inversed spatial ordering, have been prepared. The PSM with succinic acid anhydride limits the pore openings when reacting with the eNH2 containing SURMOF block. The C@A and A@C SURMOFs-on-SURMOFs have the modified MOF with smaller pores as the shell and as the core, respectively (Fig. 11). Both films have been probed for single and multiple volatile organic vapors (VOCs) adsorptions by QCM. The C@A SURMOF shows a typical type I isotherm with similar overall MeOH uptake in both the single-component MeOH and the MeOH/ hexane adsorption experiments. The MeOH molecules are selectively adsorbed from the mixture and stored within the pore of the C@A SURMOF. However, no selectivity is observed with the A@C SURMOF. This stresses the role of the film spatial arrangement within the hetero-SURMOFs on the adsorption properties [54]. 5.3. Heterostructured SURMOFs with different lattice parameters The aforementioned heterostructured SURMOFs are based on lattice matching between the deposited layers. In recent reports, the LPE technique has been also used to successfully grow MOF films with different lattice parameters and topologies on top of each other to form coherent heterostructured SURMOFs. Tu and Fischer have fabricated a Cu3(btc)2 on Cu2(ndc)2(dabco) heteroSURMOFs [55]. Cu3(btc)2 (also known as HKUST-1) has a facecentred-cubic structure (space group Fm-3m) whereas Cu2(ndc)2(dabco) has a tetragonal symmetry (space group P4/ mmm). Cu2(ndc)2(dabco) was grown on a pyridyl-terminated substrate along the [001] direction and terminated with a dabco pillar layer. The top layer acts like a pyridyl-terminated surface, which is used to seed Cu3(btc)2 films and direct their growth along the [111] direction. Hence Cu3(btc)2 can be fabricated on Cu2(ndc)2(dabco). The presence of the two crystalline MOF films has been confirmed by cross-sectional SEM images and by XRD measurements that show peaks related to the [001] orientation of Cu2(ndc)2(dabco) and to the [111] orientation of Cu3(btc)2. Both films contribute to the gas adsorption properties of the material, albeit the top film is exposed first and may block any uptake by the core layer, as shown by QCM. Concurrently, Wang et al. [56] used the LPE spraying procedure to fabricate heterostructured SURMOFs that are comprised of three layers of distinct isoreticular SURMOF-2 structures e despite the large lattice mismatches of about 20%. The XRD patterns from this material show a perfect [001]-related crystal orientation for all

SURMOF layers. The selective loading of Eu(bzac)3bipy compound (bzac ¼ 1-benzoylacetone) allows to differentiate the different layers by the contrast in the SEM images (Fig. 12). Quantum theoretical analysis has provided justification for the unexpected formation of hetero-SURMOFs with large lattice mismatch. Possible vacancies at the MOFeMOF interface that result from lattice mismatch can be capped by coordination with residual acetate groups. Moreover, the low elastic constant of the MOF materials can facilitate the delocalization of the mismatch, resulting in stress distribution over a large area to produce a small local stress within the material [56]. 6. 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 Xray and ultraviolet photoelectron spectroscopy (XPS and UPS). The application of these photoelectron spectroscopies to powder materials is often affected by charging problems. In addition to this, other surface sensitive techniques like atomic force microscopy (AFM), scanning electron microscopy (SEM), infrared reflectionabsorption microscopy (IRRAS) as well as in-plane and out-ofplane X-ray diffraction (XRD) can be applied. By means of XPS, for instance, the defect density in SURMOFs of type HKUST-1 was investigated [22]. In addition to this, the thinness results in very small uptake and release times of guest molecules, making SURMOFs ideally suited for investigating very slow diffusion processes. For instance, the diffusion coefficient of ferrocene molecules, which are solid at room temperature, in MOFs of type Cu2(1,4-ndc)2(dabco) was determined to be in the order of 1018 m2 s1 with an activation energy for diffusion of ~90 kJ mol1 [57]. 6.1. Investigation of the surface barrier phenomenon affecting the loading of MOFs Due to their thinness, SURMOF films are a unique model system and can be used to investigate certain aspects of the mass transfer

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Fig. 11. Schematic illustration of two different Janus SURMOF-on-SURMOF C@A and A@C coatings fabricated by combined LPE-PSM; A: Cu2(ndc)2(dabco), B: Cu2(NH2-bdc)2(dabco) and C: Cu2(HOOC(CH2)2OCNH-bdc)2(dabco) and the comparison of single-VOC and multiple-VOCs adsorption isotherms: a) MeOH single-VOC (black) and multiple-VOCs adsorption isotherm (red) with constant hexane relative vapor pressure (p/p0) of 47.5% and variable MeOH P/P0 of 0e47.5%, b) MeOH single-VOC (black) and multiple-VOCs adsorption isotherm (red) with constant MeOH p/p0 of 47.5% and variable hexane p/p0 of 0e47.5% for C@A, c) and d) the corresponding measurements for A@C as in a) and b), respectively (the multipleVOCs isotherms are in blue). Adapted from Ref. [54] with permission, Copyright 2013 Wiley-VCH. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

in MOFs with very high precision. For this purpose, a quartz crystal microbalance (QCM) can be employed, which allows to measurement of the mass changes of the sample with an accuracy of up to ~10 ng cm2 and a time resolution of up to ~0.01 s. SURMOFs can be grown on the gold-coated surfaces of QCM sensors in a straightforward fashion. The QCM cell can be connected to a gas flow system, where vapor of the investigated guest molecules can be instantly added to the constant carrier gas flow [57,58]. The uptake rate and uptake amount of a guest molecule can be investigated from the increase of the total sample mass. Surface barriers are additional mass transfer resistances for the guest molecules at the external crystal surface, meaning that these barriers hinder the guest molecules from entering and leaving the pore space [59,60]. By detailed analyses of the intracrystalline concentration profiles [61,62] measured by means of interference microscopy [63] and infrared micro-imaging [64,65],

it was shown in several publications that these surface barriers influence or even dominate the uptake rate by most investigated MOFs [66e70]. Due to these surface barriers, the fraction of molecules which enter the MOF pore space after hitting the external crystal surface is often vanishingly small, typically in the order of 107 [68,71]. This means only one out of 10 million molecules which hits the external MOF surface from the gas or liquid phase also enters the MOF. It could be shown for MOFs of type Zn(tbip) (tbip ¼ 5-tert-butyl isophthalate), that the surface barriers are caused by a total blockage of the majority of pore entrances, while only a few pore entrances are accessible [64,72e74]. Although these surface barriers seem to be omnipresent, the detailed structure as well as the reason and origin of these transport barriers are not understood. This means it was unknown whether these surface barriers are an intrinsic feature of MOFs.

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Fig. 12. (Top) Scheme of size-selected loading of nanoparticles in hierarchically porous SURMOFs. (Down): (A) SEM image recorded for a hierarchically porous SURMOF (Cu(bpdc) þ Cu(ndc) þ Cu(bdc)) with 5 nm gold films coated on the surface. (B) SEM image recorded for a hierarchically SURMOF (Cu(bpdc) þ Cu(ndc) þ Cu(bdc)) after loading Eu(bzac)3bipy compound without gold coating on the surface. Reprint with permission from Ref. [56]. Copyright 2014 American Chemistry Society.

To study the surface barrier phenomenon in more detail, SURMOFs of type HKUST-1 were prepared in a closed QCM flow cell [75]. Apart from the option that the mass (and thus the thickness) of the SURMOF is recorded during the preparation, the SURMOF synthesis in the closed QCM cell is very clean, meaning that the thin film is never exposed to the atmosphere, i.e. to (wet) air, which might influence the SURMOF growth. After a certain number of cycles, the synthesis was interrupted, the sample was carefully activated in pure argon and the uptake of a probe molecule, cyclohexane, was investigated, see Fig. 13. It was found that the uptake time increases quadratically with the film thickness (Fig. 13b), which is a clear indication for the diffusion limitation of the mass transfer. In contrast, the uptake times would increase linearly with the film thickness for barrier-limited uptake processes [59,60]. Furthermore, the plots of the normalized uptake (i.e. the uptake divided by the equilibrium uptake) versus the normalized time (i.e. the time divided by the square of the film thickness) are coinciding (Fig. 13c), which further proves the diffusion limitation. This clearly demonstrates that surface barriers are no intrinsic features of MOFs. To understand what may cause the additional transport barriers at the external crystal surface, pristine SURMOFs, which show no indication for surface barriers, were exposed to different substances, see Fig. 14. By exposing the SURMOF to the solvothermal MOF synthesis solution (mixture of ethanolic 0.15 M CuAc and 0.1 M btc solutions) for 10 min, small MOF crystallites were grown on top of the SURMOF resulting in the increase in the total cyclohexane uptake. However, a significant reduction of the uptake time, which would be a clear signal for the formation of surface barriers, is not observed. By exposing the SURMOF to air with 30% humidity for 30 min and 1 day, respectively, the uptake amount was reduced and the

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Fig. 13. Investigating surface barriers in HKUST-1. (a) Layer-by-layer growth of the HKUST-1 SURMOF on the QCM sensor. The synthesis was interrupted after 30, 70, 125, 180, 240, 340 and 480 cycles to perform uptake experiments with the probe molecule cyclohexane. The inset is a magnification of the layer-by-layer growth of the SURMOF. The left scale shows the quadratic increase in the uptake time of the probe molecule with increasing film thickness. (b) Time constants for cyclohexane uptake. The time constant for cyclohexane uptake increased quadratically with increasing SURMOF thickness. The black line is t ¼ l2/D with D ¼ 6  1013 m2 s1. (c) Normalized uptake as a function of normalized time (that is, time divided by the square of the film thickness). Reprint with permission from Ref. [75]. Copyright 2014 Nature Publishing Group.

uptake rate was significantly slowed down. By exposing a pristine HKUST-1 SURMOF to water vapor for only 10 s and 30 s, the uptake process is dramatically slowed down, similar to the exposure to air. Since the XRD patterns are not significantly influenced by the exposure to air or water vapor [75], it was concluded that the changes in the uptake behavior are caused by surface effects, i.e. surface barriers. This means the surface barriers are a collapse of the crystal structure at the external surface due to corrosion with water. It should be noted that the samples were carefully activated prior each uptake experiment, so that remaining water adsorbed in the MOF structure can be excluded. This study, using SURMOFs as a model system to study the mass transfer in MOFs, demonstrates that the surface barriers are easily produced by exposure to some humidity, explaining also why surface barriers in MOFs are almost omnipresent although they are no intrinsic features of MOFs. 6.2. Chiral SURMOFs Enantiomeric separation of chiral molecules is a vital field of science and has various important applications in pharmaceutical, chemical and agricultural engineering [76]. Many pharmaceutical molecules are chiral and often times only one enantiomer has the desired effect, whereas the other enantiomer often has undesired side effects. An efficient enantiomer separation is therefore crucial for many applications. Due to their large specific surface area and their crystalline structure, homochiral MOFs are very promising candidates for an efficient enantiomer separation [77,78]. Chiral SURMOFs enable not only the option to study a well-defined chiral MOF model system, but also to prepare thin porous coatings with a chiral and crystalline framework.

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Fig. 14. Cyclohexane uptake after treatment of the thin MOF film. a) The SURMOF was exposed to the MOF synthesis solution, where MOF crystallites were grown on the SURMOF. The uptake by the SURMOF with MOF crystallites grown on top (red) is somewhat larger and marginal slower than the uptake by the pristine SURMOF (black). b) The uptake by the SURMOF is significantly decreased and retarded by exposing the SURMOF to air for 30 min and 1 day, respectively. c) Exposing the SURMOF to water vapor with a partial pressure of 23 mbar for 10s and 30 s at 30  C causes a decrease and significant deceleration of the uptake. All three pristine SURMOF samples have a thickness of roughly 700 nm. Reprint with permission from Ref. [75]. Copyright 2014 Nature Publishing Group. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The first chiral SURMOFs had a pillared-layer structure of type Zn2(cam)2(dabco) and were grown in (001) direction on PPMT SAM-terminated gold surfaces [79]. The chiral centers in these MOF structures are the chiral linker molecules D- and L-camphoric acid (Dcam: (1R,3S)-(þ)-camphoric acid and Lcam: (1S,3R)-()-camphoric acid), respectively. SURMOFs of both homochiral structures with either enantiopure Dcam or Lcam linker could be prepared. It was shown by QCM uptake experiments that the uptakes of the enantiopure guest molecules (2R,5R)-2,5-hexandiol (R-HDO) or (2S,5S)-2,5-hexanediol (S-HDO) by the homochiral SURMOF Zn2(Dcam)2(dabco) or Zn2(Lcam)2(dabco), respectively, differ by a factor of about 1.5, see Fig. 15. This shows that homochiral SURMOFs with enantioselective adsorption properties can be prepared and enantioseparation by these nanoporous coatings is possible. These chiral, crystalline thin films may be used as chiral coatings, for instance of magnetic nanoparticles (see chapter 3), and may enable enantioselective adsorption and chiral separation. Chiral SURMOFs also enable the possibility to investigate the enantiomer separation from a racemic mixture by means of (oriented) circular dichroism (CD). The enantiomer excess of

Fig. 15. Enantiopure uptake in homochiral SURMOF of type Zn2(Dcam/Lcam)2(dabco). (a) Schematic illustrations of oriented growth of Zn2(cam)2(dabco) in (001) orientation on pyridyl-terminated (PPMT) and (110) orientation on COOH-terminated (MHDA) SAMs on gold substrates. (b) and (c): QCM uptake data of each enantiomer from the gas phase: R-HDO (black) and S-HDO (red) uptake by Zn2(Dcam)2(dabco) (b) and by Zn2(Lcam)2(dabco) (c). The difference in the absolute adsorption values for the two samples may arise from a small difference in the total amount of SURMOF deposited on the QCM substrate surface. Reprint with permission from Ref. [79]. Copyright 2012 Wiley VCH. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(þ)-ethyl-D-lactate [(þ)EtLt] and ()-ethyl-L-lactate [()EtLt] in SURMOFs of type Cu2(Dcam)2(dabco) was investigated by CD (Fig. 16). By comparing the CD spectra of the pure EtLt and the enantiomer-pure EtLt adsorbed in the SURMOF with the spectrum measured for the racemic EtLt mixture adsorbed in the SURMOF, an enantiomer excess ee(þ) vs. () of 28% was found [80]. In addition, these chiral SURMOFs are a chiral, crystalline model system where the CD can be investigated separately in the different crystal orientations, i.e. by means of oriented CD (OCD). Furthermore, SURMOFs with a chiral planar structure were synthesized and the enantiomer selectivity for the probe molecules (R)- and (S)-limonene was investigated [81].

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Fig. 16. Uptake by homochiral Cu2(Dcam)2(dabco) from racemic mixture. a) CD spectra of the pure enantiomers ()EtLt (black), (þ)EtLt (blue) and racemic mixture (green). b) OCD spectra of Cu2(Dcam)2(dabco) SURMOF before (red) and after (black and blue) loading with the enantiomers and with a racemic mixture (green) of EtLt. The dotted curves represent the difference between the OCD spectrum of the pristine SURMOF and the SURMOF loaded with (þ)EtLt (blue) or ()EtLt (black). The shaded areas represent the relative net amount of the loaded enantiomer. c) OCD spectra of Cu2(Dcam)2(dabco) before (red) and after (green) loading with a racemic mixture of EtLt. The shaded green area represents the relative net amount of the adsorbed (þ)EtLt enantiomer. d) Schematic illustration of the enantioselective separation of a racemic EtLt mixture by the enantiopure chiral Cu2(Dcam)2(dabco). Reprint with permission from Ref. [80]. Copyright 2012 Wiley VCH. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 17. Remote-controlled release from a molecular container. a) Porous films composed of two different layers are synthesized on a solid surface. The bottom layer serves as a reservoir and can be loaded with different molecules, whereas the top layer serves as a valve that can be opened and closed. b) Structure of the two-layered SURMOF with the passive Cu2(bpdc)2(bipy) bottom layer and the photoswitchable Cu2(Azo-bpdc)2(bipy) layer on top. c) View of the pore windows in the [001] direction. The azobenzene embedded in the MOF structure as well as molecular azobenzene can be switched from the trans to the cis state by UV light (n1) and vice versa by visible light (n2). d) Photoinduced release of the probe molecule 1,4-butanediol from the two layered SURMOF initiated by irradiation with red light. Adapted from Ref. [85] with permission. Copyright 2014 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

6.3. Functionality 6.3.1. Remote controlled release from photoswitchable SURMOFs Remote control is one of the key challenges for intelligent, functional materials. A very promising opportunity for the realization of the remote-controllable switching of physical and chemical properties is the incorporation of photoswitchable molecules, like azobenzene, into the MOFs structure. The first (powder) MOF containing linker molecules with azobenzene side groups was published in 2012 [82] and later, a switching of the chemical and physical properties of azobenzene-containing MOFs could be shown [83,84]. The option to remote-control the adsorption capacity and the diffusion coefficient in the azobenzene-containing MOF can be combined with the unique property of SURMOFs to prepare multi-layered thin films. In this way, a two-layered SURMOF with a passive bottom layer Cu2(bpdc)2(bipy) and a photoswitchable top layer Cu2(azo-bpdc)2(bipy) (azo-bpdc: 3azobenzene-4,40 -biphenyldicarboxylate) was prepared, see Fig. 17 [85]. It was demonstrated that the permeability of the top layer can be changed by switching the azobenzene groups from its basic trans state to the cis state by UV light and back by red light. This means the bottom layer acts as a molecular container and the top

layer acts as a valve which can open and close the container. The release of a probe molecule (1,4-butanediol) could be initiated by illuminating the sample with light and switching the azobenzene side groups in the top layer from cis to trans. 6.4. Electronic properties The homogenous and continuous growth and morphology of SURMOFs make them perfectly suited for applications in electrochemistry and electronics. For instance, materials with low dielectric constant k (k < 2) and good mechanical stability are required for the further rapid development in semiconductor industry, whereas thin MOF films might find an application as insulating layers. By means of ellipsometry, the dielectric constant of the thin MOF film can be determined [86]. It was found that the dielectric constants of SURMOFs of types HKUST-1 and Cu(bdc) are roughly 2.0 and 1.8, respectively. It should be noted, however, that the dielectric constant significantly increases (by ~0.2e0.4) when the SURMOF is not activated and the pores are filled. In another study, the mechanical properties of HKUST-1 (SUR)MOFs were investigated by means of

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nanoindentation [87]. The Young's modulus of HKUST-1 was determined to be about 10 GPa, which means the material is stiff enough to be applied in semiconductor industry. Another very interesting aspect of MOFs and in particular of thin MOF films is that electronic properties can be tuned for instance by changing or functionalizing the linker of the MOF or by loading the framework with various guest molecules. The conductivity of SURMOFs of type HKUST-1 can be significantly increased by loading the thin MOF film with ferrocene, see Fig. 18. While the activated, empty HKUST-1 MOF is not electrically conducting, the conductivity of HKUST-1 increases to 2  109 S cm1 by loading with ferrocene [88]. It was found that the charge transport through the ferrocene-loaded MOF proceeds by a hopping transport between two adjacent ferrocene molecules. A similar increase in conductivity can also be gained by loading the pores with other molecules, for instance with 7,7,8,8-tetracyanoquinododimethane (TCNQ) [9]. SURMOFs can also be applied as photonic antennas. For instance, by loading HKUST-1 SURMOFs with europium b-diketonate

complexes, an energy transfer from the HKUST-1 framework to the europium complex can be observed [89]. It was found that the MOF works as antennas, converting light into electronic excitation and then transferring the excitation to the europium complex. 7. 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 surfacemounted MOFs (SURMOFs). The unique SURMOF properties e like the thin, homogenous morphology, the option to control the crystal orientation and thickness as well as the opportunity to prepare multilayered films by means of heteroepitaxy e make these thin MOF films perfectly suited for various applications, in particular for fabricating sensors in the gas or in the liquid phase

Fig. 18. Redox mediation in HKUST-1 SURMOFs. Schematics of the samples and charge-transfer reactions of the MHDA SAM-terminated Au surface (a), the HKUST-1 SURMOF on MHDA SAM-terminated Au (b) and the HKUST-1 loaded with ferrocene (Fc) (c). d) Cyclic voltammetries for the HKUST-1 SURMOF which was heat-treated at 60  C and loaded with Fc from the gas phase, following desorption of Fc in ethanol solution and following repeated thermal treatment and loading with Fc. Reprint with permission from Ref. [88]. Copyright 2012 Wiley-VCH.

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and for the fabrication of membranes. Furthermore, SURMOFs are well suited for determining MOF properties in a quantitative fashion, for instance they allow detailed mass transfer investigations, which helped to unveil the almost omnipresent phenomenon of surface barriers. Also, diffusion constants can be determined in a rather straightforward fashion. List of abbreviations 2,6-ndc naphthalene-1,6-dicarboxylate AFM atomic force microscopy Azo-bpdc 3-azobenzene-4,4-biphenyldicarboxylate bdc benzene-1,4-dicarboxylate bipy 4,40 -bipyridine BME-bdc 2,5-bis(2-methoxyethoxy)-benzene-1,4-dicarboxylate bptc 4,40 -biphenyl dicarboxylate btc benzene-1,3,5-tricarboxylate bzac 1-benzoylacetone cam camphorate CUS(s) coordinatively unsaturated site(s) dabco 1,4-diazabicyclo-(2.2.2)-octane DA-SBDC diazido-stilbenedicarboxylate dmf N,N-dimethylformamide DXP N,N0 -bis(2,6-dimethylphenyl)-3,4,9,10perylentetracarboxylicdi-imide EDTA ethylenediaminetetraacetic acid EtLt ethyllactate F4bdc tetrafluorobenzene-1,4-dicarboxylate FITC fluoresceinisothiocyanate FPI 4 fluorophenyl isothiocyanate HDO 2,5-hexandiol IRRAS infrared reflection absorption spectroscopy LBL layer-by-layer LPE liquid phase epitaxy MHDA 16-mercaptohexadecanoic acid MOF(s) metal-organic framework(s) MUD 11-mercapto-1-undecanol N3-bdc 2-azido-benzene-1,4-dicarboxylate ndc naphthalene-1,4-dicarboxylate NH2-bdc 2-amino-benzene-1,4-dicarboxylate PBMT 4-(4-pyridyl)phenylmethanethiol ppdc para-pentaphenyl dicarboxylate QCM quartz crystal microbalance qpdc para-quaterphenyldicarboxylate RIU refractive index unit SAM self-assembled monolayer SEM scanning electron microscopy SPR surface plasmon resonance SURGEL surface-mounted gel SURMOF(s) surface-mounted metal-organic framework(s) tbip 5-tert-butyl isophthalate tBITC tert-butyl isothiocyanate TCNQ 7,7,8,8-tetracyanoquinododimethane tpdc para-terphenyldicarboxylate UPS ultraviolet photoelectron spectroscopy VOCs volatile organic vapors XRD X-ray diffraction References [1] H.C. Zhou, J.R. Long, O.M. Yaghi, Chem. Rev. 112 (2012) 673e674. [2] P. Falcaro, R. Ricco, C.M. Doherty, K. Liang, A.J. Hill, M.J. Styles, Chem. Soc. Rev. 43 (2014) 5513e5560. [3] D. Bradshaw, A. Garai, J. Huo, Chem. Soc. Rev. 41 (2012) 2344e2381. [4] S. Furukawa, J. Reboul, S. Diring, K. Sumida, S. Kitagawa, Chem. Soc. Rev. 43 (2014) 5700e5734.

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