materials Article

Preparation of Luminescent Metal-Organic Framework Films by Soft-Imprinting for 2,4-Dinitrotoluene Sensing Javier Roales 1, * ID , Francisco G. Moscoso 1 , Francisco Gámez 1 ID , Tânia Lopes-Costa 1 , Ahmad Sousaraei 2 , Santiago Casado 2 , Jose R. Castro-Smirnov 2 , Juan Cabanillas-Gonzalez 2 ID , José Almeida 3 , Carla Queirós 3 , Luís Cunha-Silva 3 , Ana M. G. Silva 3 ID and José M. Pedrosa 1,* ID 1

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Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Ctra. Utrera Km. 1, 41013 Sevilla, Spain; [email protected] (F.G.M.); [email protected] (F.G.); [email protected] (T.L.-C.) Madrid Institute for Advanced Studies in Nanoscience, IMDEA Nanociencia, Calle Faraday 9, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain; [email protected] (A.S.); [email protected] (S.C.); [email protected] (J.R.C.-S.); [email protected] (J.C.-G.) REQUIMTE-LAQV & Department of Chemistry and Biochemistry, Faculty of Sciences, University of Porto, 4169-007 Porto, Portugal; [email protected] (J.A.); [email protected] (C.Q.); [email protected] (L.C.-S.); [email protected] (A.M.G.S.) Correspondence: [email protected] (J.R.); [email protected] (J.M.P.); Tel.: +34-954349537 (J.M.P.)

Received: 14 July 2017; Accepted: 21 August 2017; Published: 25 August 2017

Abstract: A novel technique for the creation of metal-organic framework (MOF) films based on soft-imprinting and their use as gas sensors was developed. The microporous MOF material [Zn2 (bpdc)2 (bpee)] (bpdc = 4,40 -biphenyldicarboxylate; bpee = 1,2-bipyridylethene) was synthesized solvothermally and activated by removing the occluded solvent molecules from its inner channels. MOF particles were characterized by powder X-ray diffraction and fluorescence spectroscopy, showing high crystallinity and intense photoluminescence. Scanning electron microscope images revealed that MOF crystals were mainly in the form of microneedles with a high surface-to-volume ratio, which together with the high porosity of the material enhances its interaction with gas molecules. MOF crystals were soft-imprinted into cellulose acetate (CA) films on quartz at different pressures. Atomic force microscope images of soft-imprinted films showed that MOF crystals were partially embedded into the CA. With this procedure, mechanically stable films were created, with crystals protruding from the CA surface and therefore available for incoming gas molecules. The sensing properties of the films were assessed by exposing them to saturated atmospheres of 2,4-dinitrotoluene, which resulted in a substantial quenching of the fluorescence after few seconds. The soft-imprinted MOF films on CA/quartz exhibit good sensing capabilities for the detection of nitroaromatics, which was attributed to the MOF sensitivity and to the novel and more efficient film processing method based on soft-imprinting. Keywords: metal-organic frameworks; gas sensors; soft-imprinting; nitroaromatics; explosive detection

1. Introduction Metal-organic frameworks (MOFs) are crystalline organic-inorganic porous materials comprised of metal atoms or clusters coordinated by organic ligands to form extended coordination networks. The stability, high porosity, and easy à la carte synthesis of these materials are some of their key properties, warranting extensive use in many technological areas [1]. Gas storage and separation [2–4], catalysis [5,6], and chemical sensing [7–9] are just a few of the increasing number of applications benefitting from the versatility of these materials. Materials 2017, 10, 992; doi:10.3390/ma10090992

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Owing to their optical properties and internal channels that provide them with an elevated porosity, microporous luminescent MOFs represent an ideal choice for the fabrication of gas-sensing devices based on fluorescence [7,10]. Explosives sensing is one of the most promising applications of this family of materials, which has attracted great attention for anti-terrorism operations, homeland security, and environmental protection in contaminated areas [11]. In this way, a number of works have studied the interaction between MOFs and nitroaromatics, such as 2,4-dinitrotoluene (DNT), precursor of 2,4,6-trinitrotoluene and part of the composition of explosives based in the latter [12–14]. In the search for solid-state sensors, researchers have deposited MOFs onto solid substrates in different ways over the last few years. MOFs cannot be easily dissolved and recrystallized without losing their particular structure, hence some authors report the suspension of the material for its later deposition as dip-coated [15] or spin-coated [16] films. These techniques have the main advantage of being fast and straightforward, but often do not offer accurate control over the preparation of the films and may inherently result in substantial waste of material. The Langmuir-Blodgett technique has also been reported as an option for the creation of MOF films [17,18], with the drawback of requiring either the presence of specific functional groups in the material for the formation of films at the air-water interface, or the addition of host molecules such as polymers. MOFs can also be grown on solid substrates following specific and adapted synthetic routes, which is a rather laborious process [19]. To our knowledge, one of the most common methods for the creation of MOF films involves, surprisingly, the use of adhesive tape [12,13,20]. Double-sided or electrical tape are among the preferred materials for the adhesion of MOF particles to solid substrates, usually glass or quartz. Although straightforward and affordable, this method present two main drawbacks: (i) first, there is a lack of control over the film and (ii) second, adhesive tape is not optically passive, but generates fluorescence upon UV photoexcitation, which is also responsible for solvent release which could potentially interact with the MOF. Here, we use the soft-imprinting technique for the deposition of a microporous MOF onto thin films. Cellulose acetate (CA) films with controlled thickness previously prepared by spin-coating on quartz are used as substrates. This technique allows the replication of nanometric features in thin films [21], into which MOF particles can be embedded with an improved film stability [22]. We aim to incorporate microcrystals of the MOF material [Zn2 (bpdc)2 (bpee)] (Figure 1; bpdc = 4,40 -biphenyldicarboxylate; bpee = 1,2-bipyridylethene) into CA films by embedding them only partially and leaving a portion on top of the surface in order to favor their interaction with gas molecules in the proximity of the film. With these films, we will assess the sensing capabilities of the microporous MOF towards DNT gas. This MOF has been reported to be sensitive to DNT and other nitroaromatics through fluorescence quenching [12,20], showing promising behavior for its implementation as an explosives sensor. We synthesize and activate [Zn2 (bpdc)2 (bpee)] for gas sensing by liberating its internal channels from solvent molecules following an improved method. Furthermore, we compare the soft-imprinting technique with the frequently used strategy based on the application of adhesive tape, aiming to improve the existing deposition methods, with a focus on the creation of MOF-based sensors for the detection of nitroaromatic explosives.

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Figure features of of [Zn [Zn22(bpdc) Figure 1. 1. Selected Selected structural structural features (bpdc)22(bpee)]: (bpee)]: the the secondary secondary building building unit unit showing showing the the coordination environment of the Zn(II) centers (top) and crystalline packing arrangement coordination environment of the Zn(II) centers (top) and crystalline packing arrangement revealing revealing channels the H-atoms H-atoms are are omitted. omitted. The channels along along the the b-axis b-axis of of the the unit unit cell cell (bottom). (bottom). For For clarity, clarity, the The images images were prepared filefile deposited in the Cambridge Structural Database (entry(entry code prepared from fromthe theoriginal originalCIF CIF deposited in the Cambridge Structural Database COWPOU). code COWPOU).

2. 2. Results Results and and Discussion Discussion 2.1. Synthesis Synthesis and Characterization The microporous microporousMOF MOFwas wasprepared preparedbyby a solvothermal reaction involving coordination of a solvothermal reaction involving the the coordination of two two highly conjugated ligands, H2bpdc and bpee, with the Zn(II) metal can be seenpowder in the highly conjugated ligands, H2 bpdc and bpee, with the Zn(II) metal ion. Asion. can As be seen in the powder X-ray diffraction (PXRD)(Figure analysis (Figure 2) the as-synthesized PXRD pattern (red line) X-ray diffraction (PXRD) analysis 2) the as-synthesized PXRD pattern (red line) showed high showed high similarities withbased the simulation basedX-ray on single crystal X-ray diffraction (SCXRD) similarities with the simulation on single crystal diffraction (SCXRD) database (black line), database line), indicating theof successful synthesis ofin the material. with This is inresults agreement with the indicating(black the successful synthesis the material. This is agreement the published by results published by Lan et al. [12]. Lan et al. [12].

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Figure 2.2. Powder Powder X-ray X-ray diffraction diffraction patterns patterns of of the the simulated simulated pattern pattern based based on on the the single single crystal crystal Figure 2 (bpdc) 2 (bpee)] (black), the as-synthesized metal-organic framework (MOF) (red), structure of [Zn structure of [Zn2 (bpdc)2 (bpee)] (black), the as-synthesized metal-organic framework (MOF) (red), a MOF sample after the first process—solvent exchange (blue)—and a MOF sample obtained a MOF sample after the first process—solvent exchange (blue)—and a MOF sample obtained afterafter the the second process—drying under vacuum at K 493 5 h (green). second process—drying under vacuum at 493 forK5for h (green).

As Lan et et al. al. [12], [12],the theMOF MOFstructure structurecontains containsroughly roughlyrectangular rectangular channels As described by Lan 1D1D channels in in which dimethylformamide (DMF) solvent molecules encapsulated. To activate material, which dimethylformamide (DMF) solvent molecules areare encapsulated. To activate thisthis material, the the DMF molecules need to removed, be removed, releasing channels (pores) thus making it suitable DMF molecules need to be releasing the the channels (pores) andand thus making it suitable for for detection purposes. Our initial attempt activatethe thematerial materialinvolved involved the the treatment in detection purposes. Our initial attempt totoactivate in CH CH33OH OH (three (threedays) days)and andCH CH22Cl Cl22 (four days) for solvent exchange, followed followed by by a drying period period under under vacuum vacuum at at room room temperature. temperature. Following this protocol, the the PXRD PXRD analysis analysis revealed revealed that that the the intended intended MOF MOF was (Figure2,2,blue blueline). line).However, However, initial studies in detection gas detection showed a much was obtained (Figure ourour initial studies in gas showed a much lower lower sensitivity than reported, we attributed to the material notproperly being properly sensitivity than reported, which which we attributed to the material not being activatedactivated because the pores the MOF not been from from the DMF molecules, hindering the the gas because theofpores of thehad MOF had not fully been released fully released the DMF molecules, hindering diffusion through the sensing material. Considering this, anthis, alternative activation strategystrategy to remove gas diffusion through the sensing material. Considering an alternative activation to the uncoordinated DMF was which involved the the solvent exchange drying remove the uncoordinated DMFtested, was tested, which involved solvent exchangeand andaathermal thermal drying procedure at at 493 K under terms of of PXRD analysis, Figure 2 presents the procedure under vacuum vacuum(30 (30mbar) mbar)for for5 5h.h.InIn terms PXRD analysis, Figure 2 presents comparison for the MOF obtained from the first process of DMF removal (solvent exchange) and the the comparison for the MOF obtained from the first process of DMF removal (solvent exchange) and second oneone (drying under vacuum at 493 K). K). TheThe crystallinity andand main structural features of the the second (drying under vacuum at 493 crystallinity main structural features of material were maintained after both processes, which correspond the material were maintained after both processes, which correspondtotothe theactivation activationof of the the MOF. Apparently,the theactivation activationof ofthe theMOF MOFby bythe theremoval removalof of DMF DMF molecules molecules from from the the pores pores causes causes small small Apparently, structural changes, changes, as as reflected reflected by by the the few few alterations alterations verified verified in in the the PXRD PXRD patterns. patterns. However, itit is is structural evident that that the the MOF MOF material material maintains maintains considerable considerable permanent permanent porosity, porosity, allowing allowing its its potential potential evident activity in in sensing. sensing. activity The incomplete incomplete elimination elimination of of DMF DMF from from the the MOF MOF after after the the solvent solvent exchange exchange was was confirmed confirmed by by The − 1 −1 FTIR spectroscopy. The presence of a characteristic C=O stretching peak at ~1670 cm proved the FTIR spectroscopy. The presence of a characteristic C=O stretching peak at ~1670 cm proved the existence of of DMF DMF molecules molecules occluded occluded into into the the material material (Figure (Figure 3) 3) [23]. [23]. Upon Upon drying drying the the MOF MOF under under existence vacuumat at493 493K, K,DMF DMFininthe theMOF MOF was efficiently removed, as supported by the disappearance of vacuum was efficiently removed, as supported by the disappearance of the the DMF stretching peakthe from thespectrum FTIR spectrum of the desolvated sample 3). This DMF C=O C=O stretching peak from FTIR of the desolvated sample (Figure 3).(Figure This procedure procedure for DMF removal (porepresent activation) present advantage a significant comparison withone, the for DMF removal (pore activation) a significant in advantage comparisoninwith the reported reported as it three took less three days to obtain the of MOF, instead of the seven by days reported by as it took one, less than daysthan to obtain the MOF, instead the seven days reported Lan et al. [12]. Lan material et al. [12]. The material using the second was for the gas sensing The prepared usingprepared the second process was theprocess one used forthe theone gasused sensing tests due to its tests due to its better in terms of gas sensitivity and gas diffusion. better performance inperformance terms of sensitivity and diffusion.

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Figure 3. spectra FTIR spectra [Zn2(bpdc)2(bpee)] obtained from different conditions: as-synthesized Figure 3. FTIR of [Znof 2 (bpdc)2 (bpee)] obtained from different conditions: as-synthesized (black), (black), after the first process—solvent exchange (red)—and after the second—drying under vacuum after Figure the first3.process—solvent exchange after from the second—drying underas-synthesized vacuum at 493 K FTIR spectra of [Zn 2(bpdc)2(red)—and (bpee)] obtained different conditions: at 493 K for 5 h (blue). after the first process—solvent exchange (red)—and after the second—drying under vacuum for 5 (black), h (blue). at 493 K for 5 h (blue).

SEM images of MOF showed the material in the form of microneedles alongside bigger aggregates

SEM images of MOF showed the material in the form of microneedles alongside bigger aggregates with no particular shape (Figure 4). Such an arrangement leads to a high surface-to-volume ratio due SEM images of MOF showed material in the form ofleads microneedles alongside bigger aggregates due with no particular shape (Figure 4). the Such an arrangement to a capabilities high surface-to-volume to the small diameter of the microneedles, favoring the gas-sensing of our materialratio due with no particular shape (Figure 4). Such an arrangement leads to a high surface-to-volume due to thetosmall diameter of the microneedles, favoring gas-sensing capabilities our ratio material the increase in exposed surface area. This adds to the the fact that the MOF is highly of porous owing to due to the small diameter of the microneedles, favoring the gas-sensing capabilities of our material due to theitsincrease exposed surface area. This adds the factofthat MOF is highlythe porous owing internalin channels [12,20], hence allowing easy to diffusion gas the molecules through internal to the increase in exposed surface area. This adds to the fact that the MOF is highly porous owing to to its structures internal channels [12,20], hence allowing easy diffusion of sites. gas molecules through the internal and facilitating their accessibility to the material active its internal channels [12,20], hence allowing easy diffusion of gas molecules through the internal structures and and facilitating their accessibility activesites. sites. structures facilitating their accessibilityto tothe the material material active

Figure 4. Scanning electron microscope images of [Zn2(bpdc)2(bpee)] powder. (A,B) [Zn2(bpdc)2(bpee)] microneedles; (C) [Zn2(bpdc)2(bpee)] aggregates. Figure 4. Scanning electron microscope images of [Zn2(bpdc)2(bpee)] powder. (A,B) [Zn2(bpdc)2(bpee)] Figure 4. Scanning electron microscope images of [Zn2 (bpdc)2 (bpee)] powder. (A,B) [Zn2 (bpdc)2 (bpee)] microneedles; (C) [Zn2(bpdc)2(bpee)] aggregates.

microneedles; (C) [Zn2 (bpdc)2 (bpee)] aggregates.

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2.2. Soft-Imprinted Soft-Imprinted MOF MOF Films Films 2.2. Soft-imprinting of 200 nm) nm) with with an an applied applied Soft-imprinting of MOF MOF on on CA-coated CA-coated quartz quartz substrates substrates (CA (CA thickness: thickness: 200 pressure of 2, 4, and 6 bar resulted in MOF particles partially immersed into the CA film, as can seen pressure of 2, 4, and 6 bar resulted in MOF particles partially immersed into the CA film, asbe can be in atomic force force microscope (AFM)(AFM) images (Figure 5). In general, MOF crystals were dispersed on the seen in atomic microscope images (Figure 5). In general, MOF crystals were dispersed surface and embedded but notbut covered by the by CA, clearly protruding from the film. to the on the surface and embedded not covered the CA, clearly protruding from theAccording film. According images, individual microneedles were found, lying flat on the surface, along with bigger aggregates. to the images, individual microneedles were found, lying flat on the surface, along with bigger In the analyzed regions, the regions, average the height of theheight microneedles from the CAfrom surface found towas be aggregates. In the analyzed average of the microneedles the was CA surface around 5 nm, while that of the aggregates was approximately 95 nm. Further characterization of the found to be around 5 nm, while that of the aggregates was approximately 95 nm. Further films was performed analyzing the roughness of the AFM Root square roughness characterization of thebyfilms was performed by analyzing theimages. roughness ofmean the AFM images. Root for all samples was similar and in the order of 5 nm, indicating on one hand that the three different mean square roughness for all samples was similar and in the order of 5 nm, indicating on one hand pressures applied to imprint the MOF led totofilms with the similar surface On the other that the three different pressures applied imprint MOF led tocharacteristics. films with similar surface hand, the roughness of the films points the inhomogeneity of their surfaces, which can be beneficial characteristics. On the other hand, theatroughness of the films points at the inhomogeneity of their for gas-sensing purposes due to a high surface area to volume ratio and improved access gas surfaces, which can be beneficial for gas-sensing purposes due to a high surface area to volumeofratio molecules towards the active sites of the films, i.e., MOF particles. For comparison, AFM images of a and improved access of gas molecules towards the active sites of the films, i.e., MOF particles. For pure CA filmAFM wereimages also analyzed, revealing smooth even surface prior to the soft-imprinting of comparison, of a pure CA film awere alsoand analyzed, revealing a smooth and even surface MOF (Figure S1). The CA layer was found to efficently retain the MOF particles, providing mechanical prior to the soft-imprinting of MOF (Figure S1). The CA layer was found to efficently retain the MOF stability toproviding the sensor.mechanical By using a stability thin CA film, wesensor. aimed By to only embed thewe MOF particles in particles, to the usingpartially a thin CA film, aimed to only order to leave part of MOF them available gases the environment, facilitating diffusion partially embed the particles to in the order to in leave part of them hence available to the gas gases in the through them into the portion submerged into the CA, whilst assuring MOF immobilization onCA, the environment, hence facilitating gas diffusion through them into the portion submerged into the substrate. In thisMOF way,immobilization a compromise between adhesionInofthis the way, MOFatocompromise the substrate and accessibility whilst assuring on the substrate. between adhesion of gas molecules was achieved. Total immersion of MOF particles into CA would probably further of the MOF to the substrate and accessibility of gas molecules was achieved. Total immersion of MOF increase film stability, but with the undesired outcome of hindered access of gas molecules towards particles into CA would probably further increase film stability, but with the undesired outcome of the interior of theofsensing material. hindered access gas molecules towards the interior of the sensing material.

Figure images of of soft-imprinted soft-imprinted[Zn [Zn2(bpdc) (bpdc)2(bpee)] (bpee)] powder powder on on CA/quartz CA/quartz Figure 5. 5. Atomic Atomic force force microscope microscope images 2 2 substrates prepared at (A) 2; (B) 4; and (C) 6 bar. substrates prepared at (A) 2; (B) 4; and (C) 6 bar.

To analyze analyze whether the pressures applied applied during the soft-imprinting process affected the crystallinity of the MOF, we compared the PXRD patterns of MOF activated (desolvated) at 493 K under vacuum for 5 h with its corresponding films at 2, 4, and 6 bar. In Figure 6, extreme cases of low (2 bar) and high (6 bar) pressure are shown. shown. As can be seen, in both cases the PXRD patterns were similar, despite despite the the peaks shift verified verified in the diffractograms very similar, diffractograms of the two films relative to the MOF powder, which corresponds to an experimental misalignment due to the distinct morphology of the analyzed samples samples(films (films and powder). These observations indicating that the crystallinity was and powder). These observations indicating that the crystallinity was retained retained the soft-imprinting process, and no observable differences occur in the preparations after the after soft-imprinting process, and no observable differences occur in the preparations between between 2 and bar.outcome, With thisthe outcome, theofsuitability of soft-imprinting asfor a technique for the 2 and 6 bar. With6 this suitability soft-imprinting as a technique the processing of processing of MOFs onto solid substrates is confirmed. MOFs onto solid substrates is confirmed.

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Figure diffraction patterns of [Zn 2(bpdc) 2(bpee)] (black) and films prepared by softFigure 6.6.Powder PowderX-ray X-ray diffraction patterns of [Zn (black) and films prepared by 2 (bpdc) 2 (bpee)] imprinting on cellulose acetate at 2 (red) and 6and bar6(blue). soft-imprinting on cellulose acetate at 2 (red) bar (blue).

2.3. 2.3. Explosive Explosive Vapor Vapor Sensing Sensing MOF MOF powder powder was was first first deposited deposited on on glass glass substrates substrates using using either either double-sided double-sided tape tape or or the the adhesive tape. Similar procedures for for the the creation of MOF filmsfilms can adhesive residue residueleft leftafter afterpeeling peelingoff offthe the tape. Similar procedures creation of MOF be in theinliterature [12]. We double-sided tape astape method to adhere the MOF the canfound be found the literature [12].discarded We discarded double-sided as method to adhere theto MOF substrate due todue the to strong photoluminescence in theinregion of 400–500 nm, nm, coinciding withwith the to the substrate the strong photoluminescence the region of 400–500 coinciding emission band of the MOF (Figure S2). Besides, solvents and other compounds contained in the the emission band of the MOF (Figure S2). Besides, solvents and other compounds contained in the adhesive altering itsits response towards thethe analytes to be adhesivetape tapemay mayinteract interactwith withthe thesensing sensingmaterial, material, altering response towards analytes to detected. These drawbacks werewere minimized by using only the adhesive residue left by left the tape, which be detected. These drawbacks minimized by using only the adhesive residue by the tape, showed no photoluminescence (Figure(Figure S2) andS2) allowed the fabrication of uniform MOF thin films. which showed no photoluminescence and allowed the fabrication of uniform MOF thin These films were exposed to DNT, resulting in an important quenching of their fluorescence films. These films were exposed to DNT, resulting in an important quenching of their fluorescence (Figure (Figure S3). S3). This This proves proves the the potential potential of of the thedeveloped developedsamples samplesas asan anexplosives explosivessensor, sensor, in in agreement agreement with the results published by other authors [12]. However, the adhesive-residue method with the results published by other authors [12]. However, the adhesive-residue method for for the the deposition the MOF MOF showed showedvery verylittle littlestability, stability,asasMOF MOF particles were only partially adhered to deposition of of the particles were only partially adhered to the the substrate. Even with extreme care, manipulation of the films led to MOF particles dropping onto substrate. Even with extreme care, manipulation of the films led to MOF particles dropping onto every every surface. Successive introduction and removal of the from the fluorimeter caused a surface. Successive introduction and removal of the films fromfilms the fluorimeter caused a progressive progressive loss in fluorescence due to decrease inof the amount of surface covered by MOF loss in fluorescence intensity dueintensity to a decrease inathe amount surface covered by MOF particles that particles that was even visible to the naked eye (Figure S4). This instability limits the possibilities was even visible to the naked eye (Figure S4). This instability limits the possibilities of the films to of be the films to be used as sensors in commercial applications. used as sensors in commercial applications. As As an an alternative, alternative, soft-imprinted soft-imprinted MOF MOF films films on on CA-coated CA-coated quartz quartz were were used used for for the the detection detection of of DNT. Films prepared at 4 bar were chosen for this purpose. Photoluminescence spectra of DNT. Films prepared at 4 bar were chosen for this purpose. Photoluminescence spectra of activated activated MOF MOF powder powder deposited deposited by by soft-imprinting soft-imprinting showed showed an an intense intense emission emission band band at at 460 460 nm nm when when excited excited at 280 nm (Figure 7, red line). This excitation wavelength was chosen based on the excitation spectra at 280 nm (Figure 7, red line). This excitation wavelength was chosen based on the excitation spectra (Figure nm. The (Figure 7, 7, blue blue line), line), which which showed showed an an excitation excitation region region from from 250 250 nm nm to to 370 370 nm. The excitation excitation at at 280 280 nm nm provided provided the the highest highest emission emission among among the the different different wavelengths wavelengths tested, tested, hence hence maximizing maximizing the the intensity intensity of of the the band band to to be be monitored monitored in in subsequent subsequent sensing sensing experiments. experiments. Identical Identical results results were were obtained by depositing MOF powder on glass with the adhesive residue left by a double-sided-tape. obtained by depositing MOF powder on glass with the adhesive residue left by a double-sided-tape. The The high high emission emission intensity intensity observed observed in in the the photoluminescence photoluminescence spectrum spectrum of of the the sample sample suggests suggests its its suitability as a fluorescent probe. suitability as a fluorescent probe.

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Figure 7. 7. Photoluminescence excitation spectra spectra (λ (λem em = nm) of of Figure Photoluminescence spectra spectra (λ (λexc 280 nm) nm) and and excitation = 460 460 nm) exc == 280 [Zn22(bpdc) [Zn (bpdc)22(bpee)] (bpee)] powder powder deposited deposited by by soft-imprinting. soft-imprinting.

The The exposure exposure of of the the films films to to DNT DNT vapors vapors resulted resulted in in aa substantial substantial quenching quenching of of the the typical typical fluorescence quenching hashas been attributed to the of two fluorescenceofof[Zn [Zn 2(bpdc) 2(bpee)](Figure (Figure8).8).This This quenching been attributed to presence the presence of 2 (bpdc) 2 (bpee)] -NO electron-withdrawing functional groups ingroups the structure DNT. As aofresult, π two2 -NO 2 electron-withdrawing functional in theofstructure DNT.theAsnitroaromatic a result, the system would π besystem electron-depleted and the moleculeand would as would a π acceptor upon nitroaromatic would be electron-depleted the behave molecule behave as a interaction π acceptor with organic ligands of organic the MOFligands [20]. We quantified the quenching of fluorescence (%) as uponthe interaction with the of the MOF [20]. We quantified the quenching of − /I0 is × the 100,maximum in which fluorescence is the maximum intensity the I0 − I )/I0 × (%) 100, as in which intensityfluorescence of the sample and I of is the (fluorescence sample and is the corresponding maximum intensity after exposure to DNT (Figure 8, inset). The corresponding maximum intensity after exposure to DNT (Figure 8, inset). The emission intensity emission intensity was monitoredof at the wavelength maximum emission (460 nm). Mean was monitored at the wavelength maximum emission of (460 nm). Mean fluorescence quenching fluorescence quenching a 10-s was foundthe to fast be 15%, indicating the fast response our for a 10-s exposure was for found to exposure be 15%, indicating response of our sensor towardsof DNT. sensor towards DNT.responded On average, all films responded equally to the nitroaromatic irrespective of the On average, all films equally to the nitroaromatic irrespective of the pressure applied during pressure applied during the (Figure soft-imprinting procedure (Figure S5),reproducibility which suggests the soft-imprinting procedure S5), which suggests an elevated of an the elevated method. reproducibility ofbethe method. However, it must be notedunderestimated that this quenching was possibly However, it must noted that this quenching was possibly due to experimental underestimated to experimental constraints. DNT exhibits extremely lowfor vapor pressures, constraints. DNTdue exhibits extremely low vapor pressures, requiring long times the generation requiring long times for the of saturated environments. In our case, was of saturated environments. In generation our case, saturation was ensured by generating DNT saturation headspace in a ensured generating DNT headspace in adays). smallHowever, vial during a long period of time (four days). small vialby during a long period of time (four the dilution of the saturated headspace However, the dilution of saturated during of the vial the during the manipulation of the the vial for theheadspace introduction of thethe filmmanipulation cannot be discarded. This for would introduction of the film cannot be found discarded. Thisauthors would [12], explain different values found explain different quenching values by other besides otherquenching experimental variations bythe other authors [12], besides other experimental variations or the existence of undisclosed or existence of undisclosed preparation details in other publications that might affect the sensing preparationofdetails in other that DNT. might affect the sensing capabilities of the MOF or its capabilities the MOF or itspublications interaction with interaction withsensor DNT.benefited from the good sensing capabilities shown by the MOF and from the Our DNT Our DNT sensor benefited from theby good sensing capabilities shown confirms by the MOF from the enhanced stability of the films prepared soft-imprinting. This outcome the and suitability of enhanced stability of the films prepared by soft-imprinting. This outcome confirms the suitability of [Zn (bpdc) (bpee)] for the detection of nitroaromatics. Besides, the soft-imprinting technique emerges 2 2 [Znan 2(bpdc) detectionand of nitroaromatics. Besides, theofsoft-imprinting technique emerges as ideal2(bpee)] tool forfor thethe controlled reproducible processing MOFs or similar materials onto as an substrates. ideal tool for and reproducible processing MOFs materials onto solid Inthe ourcontrolled view, these promising results pave theofway foror ansimilar improved method forsolid the substrates. of In MOF our view, these results pave the way for anstill improved method for the fabrication sensors. Topromising do this, the soft-imprinting technique needs to be refined by fabricationall ofvariables MOF sensors. do this, technique needs to be MOFs refinedand by analyzing in the To process andthe by soft-imprinting extending the scope of thisstill study to other analyzing all variables the process and by extending themade scopetoofestablish this study to other MOFs and their exposures to otherinanalytes. Besides, efforts are being a recovery procedure their exposures to other analytes. effortsthe arepotential being made to establish a recovery for the reutilization of the sensors.Besides, Meanwhile, applications of our films areprocedure oriented for the reutilization of the sensors. Meanwhile, thesystem potential films are oriented towards disposable sensors. Last but not least, the for applications the exposureoftoour nitroaromatics needs towards disposable sensors. Last not least,the thetypical systemprocedure for the exposure to the nitroaromatics needs to be perfected. Due to their lowbut volatility, found in literature merely to be perfected. Due tooftheir low volatility, the vial typical procedure found in the literature merely consists of the insertion the sensor into a sealed containing a saturated headspace generated by consists of the insertion[12,14,20]. of the sensor into sealed vial which containing a saturated generated the desired compound With thisa technique, we also followedheadspace in our experiments, by the desired compound [12,14,20]. With be thiscontrolled. technique,Our which we also followed in our experiments, the concentration of nitroaromatics cannot current experimental efforts are working the concentration of nitroaromatics cannot be controlled. Our current experimental efforts are

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workingthe towards the implementation system for the of controlledofatmospheres of towards implementation of a system of forathe generation ofgeneration controlled atmospheres nitroaromatics, nitroaromatics, intended to be able to quantify intended to be able to quantify the responses ofthe ourresponses sensors. of our sensors.

8. Exposure Exposure of of [Zn [Zn22(bpdc)22(bpee)] soft-imprinted on CA/quartz CA/quartz at Figure 8. at 44 bar bar to DNT vapors. Inset: quenching percentages for each exposure time. quenching percentages for each exposure time.

3. Materials Materials and and Methods Methods 3. 3.1. Chemicals and General Methods All solvents and starting materials for synthesis were commercially available and were used as received. Fourier Fouriertransform transforminfrared infrared spectra (FTIR) were collected using a Shimadzu IRAffinity-1 received. spectra (FTIR) were collected using a Shimadzu IRAffinity-1 from − 1 −1 from 4000–400 cm . PXRD were collected at room temperature using anPANalytical Empyrean 4000–400 cm . PXRD patternspatterns were collected at room temperature using an Empyrean PANalytical diffractometer (CuKα1,2λradiation, λ 1 = 1.540598 Å and λ 2 = 1.544426 Å) equipped with a diffractometer (CuKα1,2 radiation, = 1.540598 Å and λ = 1.544426 Å) equipped with a PIXcel 1 2 PIXcel 1D detector and a flat-plate sample holder in a Bragg–Brentano para-focusing optics 1D detector and a flat-plate sample holder in a Bragg–Brentano para-focusing optics configuration ◦ configuration kV, 40 mA). Intensity data were by themethod step counting method 0.02°) (45 kV, 40 mA).(45 Intensity data were obtained by theobtained step counting (step: 0.02 ) in (step: continuous in continuous mode in therange approximate ≤ 2θ ≤ 50°. Scanning electron(SEM) microscope mode in the approximate 3.0◦ ≤ 2θrange ≤ 50◦3.0° . Scanning electron microscope images(SEM) were images were acquired using a Philips (200 kV). Fluorescence spectra for the characterization acquired using a Philips CM-200 (200 CM-200 kV). Fluorescence spectra for the characterization of the MOF of the MOFsensing and were for DNT sensing collected using a Hitachi F-7000 Fluorescence and for DNT collected using were a Hitachi F-7000 Fluorescence Spectrophotometer (Hitachi Spectrophotometer (Hitachi Germany). High Technologies, Krefeld, Germany). Fluorescence films High Technologies, Krefeld, Fluorescence spectra of films containing [Znspectra (bpdc) (bpee)] 2 2of containing [Znby 2(bpdc) were obtained by samples. using a AFM sample holder for acquired solid samples. AFM were obtained using2(bpee)] a sample holder for solid images were by means of were acquired by means of a JPK NanoWizard II AFM functioning contact mode,Ticonnected aimages JPK NanoWizard II AFM functioning in contact mode, connected to a in Nikon Eclipse inverted to a Nikon Eclipse MikroMasch Ti inverted optical microscope. MikroMasch HQ:XSC11/Al cantilevers 0.2 optical microscope. HQ:XSC11/Al BS cantilevers of 0.2 N/m spring BS constant and 15ofkHz N/m spring constantwere andused 15 kHz resonant frequency were used in air conditions. resonant frequency in air conditions. 3.2. Synthesis of [Zn2(bpdc)22(bpee)] (bpee)] MOFs MOFs [Zn22(bpdc)2(bpee)] (bpee)] was was synthesized synthesized using solvothermal conditions as reported by Lan et al. [12], with ·6H22O O (0.0921 (0.0921 g, g, 0.31 0.31 mmol), with aa few few slight slight modifications. modifications. Briefly, a mixture mixture of of Zn(NO Zn(NO33))22·6H 0 -biphenyldicarboxylic acid (H bpdc, 0.0734 g, 0.30 mmol), and 1,2-bipyridylethene (bpee, 0.0554 g, 4,4 4,4′-biphenyldicarboxylic 22bpdc, 0.0734 g, 0.30 mmol), 0.30 mmol) in DMF (15 mL) was placed in a 23-mL 23-mL teflon-lined reactor, reactor, which was sealed in a stainless-steel vessel. A total of 20 reactors were prepared and placed in the oven, at 438 K for three days, to afford afford the the MOF MOF material material as as block-shaped block-shaped crystals. crystals. After cooling, the resultant solid was filtered and stirred stirred in in DMF DMF(20 (20mL) mL)atat338 338KKfor for h to remove to unreacted ligands. Afterwards, 2424 h to remove to unreacted ligands. Afterwards, the the precipitant was separated and immersed in CH OH (20 mL), which was stirred at 343 K for 45 min. precipitant was separated and immersed in CH3OH 3 (20 mL), which was stirred at 343 K for 45 min. The (20 mL). mL). The product product was was collected collected by by centrifugation centrifugation (10 (10 min, min, at at 3500 3500 rpm) rpm) and and suspended suspended in in CH CH22Cl2 (20 After filtration, the material was activated at 493 K under vacuum (30 mbar) for 5 h, to give 1.12 g (24% yield) of [Zn2(bpdc)2(bpee)].

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After filtration, the material was activated at 493 K under vacuum (30 mbar) for 5 h, to give 1.12 g (24% yield) of [Zn2 (bpdc)2 (bpee)]. 3.3. Fabrication of Adhesive Tape-Based MOF Films Glass slides were cut to size (26 × 8 mm2 ) and carefully cleaned with acetone and lint-free wipes. Double-sided tape was then applied to the lower part of the glass substrates and manually pressed until complete adhesion was ensured. For the preparation of thin layers, the tape was removed after a few minutes, leaving a small amount of adhesive residue onto the glass substrate. In the case of thick layers, the double-sided tape was not removed at any time during the preparation procedure. Subsequently, for the fabrication of both thin and thick films, the substrates containing either the adhesive residue or the double-sided tape were covered with [Zn2 (bpdc)2 (bpee)] MOF powder, resulting in the substrates coated with a fine layer of crystals. Afterwards, the films were gently tapped while facing down in order to remove any crystals that were not adhered well to the surface. 3.4. Fabrication of Soft-Imprinted MOF/CA Films First, a solution of CA (40 mg/mL, MW = 61,000, resulting refractive index of the film n = 1.48 at 570 nm, Sigma Aldrich, Madrid, Spain) in 4-hydroxy-4-methyl-2-penthanon (diacetone alcohol, Sigma Aldrich), was spin-coated at 3000 rpm for 1 min, with the acceleration set at 12,000 rpm/s, onto a quartz substrate. The thickness of the samples was measured using a mechanical profilometer Bruker Dektak XT with a 2.5-mm radio stylus. The applied strength was the minimum possible (around 1 mg), and the length of analysis was 7500 µm. Scratches were performed on the surface of the sample prior to the scan. All measurements were repeated five times and the results averaged. The CA films used in this work had an average thickness of 200 nm, which was determined by the concentration of CA solution (Figure S6). [Zn2 (bpdc)2 (bpee)] in the form of powder was deposited manually on top of the CA film and an identical CA/quartz system was used to encapsulate the MOFs during the soft-imprint process. The films were imprinted by means of a Compact Nanoimprint Tool (CNI Tool, NILT, Kongens Lyngby, Denmark). During the hot embossing process, the substrates were heated to 413 K with applied pressures of 2, 4, and 6 bar for 1200 s each. The samples were subsequently allowed to cool down to a temperature of 343 K before the pressure was released and the imprinted substrate removed. As a result, two CA/quartz substrates with a similar quantity of partially embedded MOFs were obtained. Given the reduced thickness of the CA film and micron-size of [Zn2 (bpdc)2 (bpee)], most of the material was exposed, allowing the sensing experiments. 3.5. Exposure to Explosives Prior to the sensing experiments, a small open vial containing DNT (Sigma Aldrich, Madrid, Spain) was introduced in another vial hermetically sealed and allowed to evaporate for four days at laboratory temperature until headspace saturation. Such long times were chosen due to the low vapor pressure of DNT. Similar procedures for the creation of saturated explosive atmospheres can be found in the literature [12]. The exposure to explosives was achieved by introducing the sensor film into the bigger vial for the required time. Subsequently, the film was rapidly placed in the sample holder of the fluorimeter and the corresponding measurement was performed immediately. The fluorescence of the films was measured in different positions to ensure reproducibility of the method, and the quenching of fluorescence upon exposure to DNT was calculated with respect to the corresponding initial spectrum for each position. 4. Conclusions A microporous [Zn2 (bpdc)2 (bpee)] MOF was successfully prepared by a solvothermal reaction involving the coordination of the ligands H2 bpdc and bpee with the Zn(II) metal ion. The powder XRD analysis revealed that a highly porous structure with internal 1D channels was obtained. FTIR showed that DMF molecules were occluded into the MOF channels. These uncoordinated solvent molecules

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were removed by solvent exchange and a thermal drying procedure under vacuum that proved to be successful according to FTIR spectra, besides being faster than other reported techniques. SEM images showed that MOF particles were arranged mostly as crystalline microneedles, resulting in a high surface area to volume ratio that enhanced the gas molecules’ interaction with the material. MOF particles were successfully soft-imprinted onto CA films at different pressures, resulting in all cases in partially embedded particles, according to AFM images. This arrangement favored both film stability, due to CA holding MOF particles in place, and gas accessibility to the MOF active sites, owing to the particles partially protruding from the CA film. Fluorescence spectra of soft-imprinted MOF films showed a high emission intensity, indicating the suitability of the MOF to be used as a fluorescent probe. Exposure of the soft-imprinted films to DNT resulted in a substantial quenching of the fluorescence after a few seconds. Overall, it can be concluded that [Zn2 (bpdc)2 (bpee)] soft-imprinted films on CA exhibit good sensing capabilities as candidates for the detection of nitroaromatics, owing to the MOF sensitivity and to the novel and more efficient film processing method based on soft-imprinting. Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/10/9/992/s1, Figure S1: AFM image of a pure CA film on quartz prepared by spin-coating; Figure S2: Photoluminescence spectra of double-sided tape, adhesive residue after peeling off the tape, MOF adhered to the tape and MOF adhered to the adhesive residue; Figure S3: Photoluminescence spectra of an MOF film prepared with the adhesive residue left by an adhesive tape and after its exposure to a DNT saturated environment; Figure S4: Photoluminescence spectra and pictures of an MOF film prepared with the adhesive residue left by an adhesive tape before and after typical experimental manipulation; Figure S5: Quenching percentages for films prepared at 2, 4, and 6 bar; Figure S6: Thickness of CA films prepared from CA solutions at different concentrations. Acknowledgments: Authors are grateful to the Spanish Ministry of Economy and Competitiveness (MINECO) for funding through projects PCIN-2015-169-C02-01/02 (MOFsENS, under the project M-Era-NET/0005/2014), MAT2014-57652-C2-1/2-R (LAPSEN). Funding from the Operative Programme FEDER-Andalucia through project P12 FQM-2310 also contributed to the present research. The Porto group thanks Fundação para a Ciência e a Tecnologia (FCT/Ministério da Ciência, Tecnologia e Ensino Superior) for the financial support to the UID/QUI/50006/2013-POCI/01/0145/FERDER/007265 (LAQV/REQUIMTE) through national funds and co-financed by FEDER under the PT2020 Partnership Agreement, and to the project M-ERA-NET/0005/2014. S.C. thanks projects MAD2D S2013/MIT-3007 and MAT2015-66605-P. The authors also gratefully acknowledge the COST action CM1302 (SIPs). Author Contributions: J.M.P., J.C.-G., A.M.G.S. and L.C.-S. conceived and designed the experiments; J.R., F.G.M., J.A., C.Q., A.S., J.R.C.-S. and S.C. performed the experiments; J.R., F.G., F.G.M., J.M.P., J.C.-G., A.M.G.S., L.C.-S., T.L.-C. and J.R.C.-S. contributed to the scientific discussion; J.R., F.G., C.Q., F.G.M. and J.R.C.-S. analyzed the data; J.R., F.G. and C.Q. wrote the manuscript. All authors read and approved the manuscript. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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