fabrication and noncovalent.pdf

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COMMUNICATION View Article OnlineView Journal

This journal is © The Royal Society of Chemistry 2014

Functional Materials Design, Discovery & Development (FMD3), Advanced Membranes

and Porous Materials Center (AMPM), Division of Physical Sciences and Engineering,

King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900,

Kingdom of Saudi Arabia. E-mail: [email protected]

Cite this: DOI: 10.1039/c4ce01402b

Received 8th July 2014,Accepted 11th September 2014

DOI: 10.1039/c4ce01402b

www.rsc.org/crystengcomm

Fabrication and non-covalent modification of highlyoriented thin films of a zeolite-like metal–organicframework (ZMOF) with rho topology

O. Shekhah, A. Cadiau and M. Eddaoudi*

Here we report the fabrication of the first thin film of a zeolite-like

metal–organic framework (ZMOF) with rho topology IJrho-ZMOF-1,

([In48(HImDC)96]48−)n) in a highly oriented fashion on a gold-

functionalized substrate. The oriented rho-ZMOF-1 film was

functionalized by non-covalent modification via post-synthetic

exchange of different probe molecules, such as acridine yellow,

methylene blue, and Nile red. In addition, encapsulation of a por-

phyrin moiety was achieved via in situ synthesis and construction

of the rho-ZMOF. Adsorption kinetics of volatile organic com-

pounds on rho-ZMOF-1 thin films was also investigated. This

study suggests that rho-ZMOF-1 thin films can be regarded as a

promising platform for various applications such as sensing and

catalysis.

Introduction

Zeolites are an important class of porous crystalline materialswith homogenous sized and shaped openings and pores.Although zeolites are porous, it has proven difficult to intro-duce functionalities for desired applications.1 On the otherhand, metal–organic frameworks (MOFs) are a relatively newclass of porous crystalline materials composed of metals(i.e., metal ions, metal clusters, chains, or layers) inter-connected by polytopic organic linkers that are readily func-tionalized.2 The unique properties of these crystallinematerials (i.e., porosity, diverse functionality, and structuraldesign) have made them attractive candidates for severalapplications, such as gas storage, separation, catalysis, drugdelivery, and chemical sensing.2–4 Although MOFs possesssome of the highest reported porosities, one drawback toMOFs with large pores is the occurrence of the self-interpenetration phenomenon.4 In the past decade, new strat-egies have been developed for the synthesis of open MOFs,such as the single-metal-ion-based molecular building block

(MBB) approach, which has shown potential for targetingMOF structures with zeolite-like topologies (i.e., ZMOFs).5

Many of the resultant ZMOFs (e.g., rho-ZMOF) possess charac-teristics of both zeolites (e.g., absence of interpenetration, ionexchange capability) and MOFs (e.g., readily tailorable poresand cavities) as well as synergistic properties (e.g., large andextra-large cavities).

One significant challenge in this area is the developmentof large-pore MOFs with adequate chemical and thermalstability whose internal surface can be appropriately func-tionalized. One of the 3-periodic anionic ZMOFs reported byEddaoudi and co-workers in 2006,5 i.e., the (indium-HImDC)-based rho-ZMOF (rho-ZMOF-1), is of particular interest forvarious applications due to its extra-large cavities (~1.82 nmdiameter), windows (eight-membered ring (8MR) window =~0.86 nm aperture), and ion exchange potential. Therho-ZMOF-1 unit cell contains 48 indium metal ions and96 4,5-imidazoledicarboxylate (HImDC) ligands; the nega-tively charged framework is balanced by cationic dimethyl-ammonium (DMA) guest molecules to give an overall frameworkformula of ([In48(HImDC)96]

48− |(DMA+)48|) (see Scheme 1).5

The functionalization of MOFs, including ZMOFs, for vari-ous applications can be achieved by different approaches,and one of them is through the encapsulation of functionalmoieties within the internal cavities. These functional moie-ties can impart additional properties to the ZMOF platformfor various applications such as sensing, separation, orcatalysis.6 In addition to bulk MOF materials, many attemptshave been reported recently for the functionalization of MOFthin films.7,8 In general, the diffusional access of guest moleculesto the interior of the MOF structures can be assisted by fabri-cating them as thin films, especially in an oriented fashion.9

Here we report for the first time the growth of rho-ZMOF-1thin films in a highly oriented fashion on gold substratesfunctionalized by a self-assembled monolayer (SAM). Then,the modification of the oriented thin film is demonstratedby incorporation of targeted molecules, such as protonatedacridine yellow (AY), fluorescent cationic dye; methylene

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Scheme 1 Schematic representation of the fabrication of rho-ZMOF-1thin films on an MHDA SAM starting from 4,5-imidazoledicarboxylate(HImDC) and In(III). (A) The post-synthetic exchange of acridine yellow(AY), and (B) the in situ inclusion of porphyrin during thin film fabrication,followed by metallation.

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blue (MB), photosensitizer; and Nile red (NR), a phenoxazonefluorescent dye, in the rho-ZMOF-1 cavities (Scheme 1) bypost-synthesis exchange. The adsorption kinetics of volatileorganic compounds (VOCs), such as alcohols of differentmolecular weights, at the corresponding vapor pressure and298 K were also studied by growing rho-ZMOF-1 on a quartzcrystal microbalance (QCM) gold substrate. The in situship-in-a-bottle approach for the incorporation of porphyrinsin the rho-ZMOF-1 thin film cavities (Scheme 1) was alsodemonstrated. Finally, metallation experiments provideevidence for their successful inclusion and the accessibility tothe porphyrin and the internal surfaces of the functionalizedMOF oriented thin films.

ExperimentalThin film fabrication procedure

In order to achieve the preferred crystallographic orientationof rho-ZMOF-1 crystals, Au (111) substrates were modifiedwith 16-mercaptohexadecanoic acid (MHDA) SAMs that act asa two-dimensional nucleation template that exhibit terminalcarboxyl groups.7a The original synthesis conditions ofrho-ZMOF-1 were modified slightly to allow immersion of themodified gold substrates in the crystallization solution. First,a mixture of 0.21 mmol of 4,5-imidazoledicarboxylic acid(H3ImDC) and 0.1 mmol of In(NO3)3·2(H2O) in 10 mL of N,N′-dimethylformamide (DMF) was heated at 105 °C for 12 h.5

The reaction supernatant was separated and cooled to 85 °C,and then the SAM-modified gold substrate was immersed inthe cooled solution for an additional 6 hours. The resultantthin film was then removed from the solution, washed thor-oughly with DMF, and then soaked in acetonitrile solvent for48 hours.

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Functional moiety encapsulation in thin films

The encapsulation of various dyes has been performed viadirect soaking of the thin films in a solution of targetedmolecules. In the case of acridine yellow (AY), rho-ZMOF-1was soaked in a 12.5 mM solution of AY (in a mixture ofmethanol–acetonitrile with a volumetric ratio of 3 : 1) for24 hours. In the cases of methylene blue (MB) and Nile red(NR), rho-ZMOF-1 was soaked in a 3.0 mM solution each ofMB and NR (in methanol) for 24 hours.

In contrast, the inclusion of 5,10,15,20-tetrakis-IJ1-methyl4-pyridinio)porphyrin tetra( p-toluenesulfonate)([H2TMPyP][ p-tosyl]4) in rho-ZMOF-1 was completed byadding it to the reaction mixture during the aforementionedsynthesis (ship-in-a-bottle approach).6a

Sample characterization

The crystal structure and orientation properties of theobtained film were studied by X-ray diffraction measure-ments using a high-resolution PANalytical X'Pert MPD-PROdiffractometer with Cu-Kα radiation (λ = 1.5406 Å).Ultraviolet-visible spectra were collected using a PerkinElmerLambda 950 UV/Vis spectrometer. Fluorescence imaging wasperformed using an LSM 710 laser scanning confocal micro-scope from Carl Zeiss. SEM characterization was performedusing an FEI Quanta 600 field emission scanning electronmicroscope (accelerating voltage: 30 kV). Quartz crystalmicrobalance (QCM) measurements were completed using aQ-Sense E4 instrument from LOT Quantum Design.

Results and discussion

In order to achieve the preferred crystallographic orientationof rho-ZMOF-1 crystals, Au (111) substrates were functional-ized with carboxyl groups by using MHDA SAMs.7a The PXRDpatterns of both as-prepared thin films and simulatedrho-ZMOF-1 are presented in Fig. 1; comparison of bothspectra reveals that their similarity and the structure-directedorientation of rho-ZMOF-1 crystal layers along the [110] direc-tion is evident. The crystal orientation is additionally con-firmed by observed reflections of higher-order lattice planes.7

The oriented surface growth of rho-ZMOF-1 crystals is alsoreflected in the SEM images shown in Fig. 2. The SEM imagesshow the 1 μm thick thin film composed of many micrometer-sized rho-ZMOF-1 crystals that are attached to (MHDA-SAM)-functionalized gold substrates in a highly oriented fashion.The thickness of the thin film was controlled by controlling theimmersion time, where “relatively thicker” thin films can beachieved by longer immersion times.

The preferred orientation along the [110] direction isclearly shown by the presence of (110) faces parallel to thesurface, which exposes the 8MR window of the rho-ZMOF-1polyhedral cages, which is in excellent agreement with the110 reflection intensity deduced from the PXRD results(Fig. 1).7 The ability to grow these homogenous mesoporousrho-ZMOF-1 thin films provided the basis to test their

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Fig. 1 PXRD patterns of rho-ZMOF-1: simulated (black), rho-ZMOF-1thin film (red), and after the post-synthetic encapsulation of acridineyellow (green), methylene blue (pink) and Nile red (blue) in the orientedrho-ZMOF-1 thin films.

Fig. 2 SEM images of the rho-ZMOF-1 thin film on the MHDA SAMafter 3 hours (left) and after 12 hours (right).

Fig. 3 Reflectance UV-vis spectra of the rho-ZMOF-1 thin film (black)after the post-synthetic encapsulation of acridine yellow (red), methyleneblue (blue), and Nile red (pink) in the oriented rho-ZMOF-1 thin films.

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post-synthetic functionalization, taking advantage of thecation-exchange properties of the ZMOF for some chargeddyes, and thus the incorporation of functionality into

This journal is © The Royal Society of Chemistry 2014

the internal cavities of rho-ZMOF-1.5 The post-syntheticfunctionalization of bulk samples has been extensivelystudied,8 whereas studies on MOF thin films are still rare.9

This is due to the limited number of addressable MOFcandidates meeting these requirements for post-syntheticfunctionalization (i.e., functional group, chemical stability,and accessible large pores).9c The high stability of therho-ZMOF-1 framework in aqueous solutions and organic sol-vents allowed for further framework functionalization using acation exchange approach. Exposure of the rho-ZMOF-1 thinfilms to a 3 mM solution of MB+ or a 12.5 mM solution of AY+

leads to the rapid uptake of both MB+ and AY+ into the frame-work. After a 30 min exposure, the thin film started to transi-tion in color, light blue in the case of MB+ and light yellow inthe case of AY+, indicating a strong affinity between the dyemolecules and the rho-ZMOF-1 framework. The color of thethin films continued to change with the increase in immersiontime, and finally a deep blue or yellow-colored thin filmwas obtained after immersion overnight. Subsequent inten-sive washing of the functionalized thin film with acetonitrileovernight did not lead to the release of MB+ or AY+ from theframework.5,6b The same loading was tested for HNR+, andthe rho-ZMOF-1 thin film started to exhibit a violet color,which continued with increasing immersion time; however,intensive washing led to some release of NR. The crystallinityand highly oriented morphology were preserved as confirmedby PXRD, proving the chemical stability of the rho-ZMOFstructure (Fig. 1), whereas some changes in the intensitiesprove the presence of the different dyes in the pores. Theimaging of the thin film using laser confocal fluorescencemicroscopy (Fig. 4) shows the fluorescence images of thinfilms after encapsulation of AY, MB, and NR dyes. Bright bluefluorescence (Fig. 4(A)) due to the encapsulation of AY+, redfluorescence due to the encapsulation of MB+ (Fig. 4(B)), andviolet fluorescence due to the encapsulation of HNR+

(Fig. 4(C)) were clearly observed. The presence of these threemolecules in the pores of rho-ZMOF was also demonstratedby differences in the respective UV-vis spectra (Fig. 3), wherethe spectrum shows their characteristic absorption bands at590 nm for AY, 740 nm for MB, and 600 nm for NR.

The inclusion of ijH2TMPyP]ij p-tosyl]4 in the rho-ZMOF-1thin films was also tested by the growth of the thin film inthe presence of porphyrin.6a The highly oriented crystal mor-phology was again observed, as shown by PXRD, proving thechemical stability of the rho-ZMOF structure (Fig. 5). Thepresence of [H2TMPyP][ p-tosyl]4 in the pores of rho-ZMOF-1was demonstrated by the dark red color of the thin film andUV-vis spectra of the fully washed thin film (red line inFig. 6), where the spectrum shows the characteristic fiveabsorption bands associated with the free-base porphyrin(λmax = 434, 522, 556, 593 and 648 nm).

Accordingly, this result reveals that porphyrin was notmetallated by the In3+ cations present in the assembly condi-tions. The IJ[H2TMPyP]ij p-tosyl]4)-functionalized thin filmwas subjected to intensive washing with acetonitrile over-night, which did not lead to the release of the porphyrin from

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Fig. 4 Confocal laser fluorescence micrographs demonstrating thepost-synthetic encapsulation of dyes such as acridine yellow (A), methyleneblue (B), and Nile red (C), and the in situ encapsulation of the ijH2TMPyP]ijp-tosyl]4 porphyrin moiety (D) in the oriented rho-ZMOF-1 thin films.

Fig. 5 PXRD patterns of the as-synthesized rho-ZMOF-1 thin film (red)and after the in situ encapsulation of porphyrin (black).

Fig. 6 Reflectance UV-vis spectra of rho-ZMOF-1 (black), porphyrin-impregnated rho-ZMOF-1 (red), and porphyrin-impregnated rho-ZMOF-1after metallation with CoIJII).

Fig. 7 XP spectra of porphyrin-impregnated rho-ZMOF-1 after meta-llation with CoIJII).

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the framework. These findings prove the encapsulation of thefree-base porphyrin (not just surface interactions) and sup-port the absence of any porphyrin leaching, mainly due tothe relatively smaller size of the window openings to theα-cages, that is, 8MR.6a The presence of In3+-free porphyrinsallows for potential metallation of the free-base porphyrin,which can allow for its utilization and exploration as a plat-form for metalloporphyrin-based catalysis. Indeed, we havesuccessfully metallated the encapsulated free-base [H2TMPyP][p-tosyl]4 porphyrin moiety, by a post-encapsulation approach,through immersion of the ([H2TMPyP][p-tosyl]4)-functionalizedrho-ZMOF-1 thin film to various solutions of transition metalions. Metallation of the thin film by Co2+ cations, for example,

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was accomplished via its incubation in a 0.1 M methanol solu-tion of the corresponding metal nitrate at room temperaturefor up to 24 h. The thin film was subsequently washed withacetonitrile and methanol several times and air-dried at 40 °C.The expected metallation was confirmed by UV-vis studies, asindicated by the red-shifted Soret bands and collapse of theQ-band multiplets upon metallation (blue line in Fig. 6).6a

Metallation was also confirmed by X-ray photon spectros-copy (XPS), which provides the rho-ZMOF-1 thin film elemen-tal composition, confirming all of the elements such as In, C,O, N, and Co2+ (2p peaks) (Fig. 7). This observation demon-strates complete molecular access to the porphyrin moietyand into the mesopore system of the rho-ZMOF-1 thin film.

The adsorption kinetics of VOCs with different molecularweights at the corresponding vapor pressure and at 298 Kwas also studied on the highly oriented rho-ZMOF-1 thin

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Fig. 8 QCM profiles of specific mass uptake of VOCs at saturatedvapor pressure and room temperature for rho-ZMOF-1.

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films grown on the QCM gold substrate. The oriented thinfilms were grown directly on the QCM gold substrate, whichallowed us to study the adsorption kinetics of some VOCssuch as alcohols, acetone, and dichloromethane at theircorresponding vapor pressure and at 298 K. Due to the ultra-sensitivity of the QCM device, it was possible to sense themass changes in the nanogram range.10 As shown in Fig. 8,we observed no clear decrease in the kinetics of the adsorp-tion rate with increasing molecular weight of the alcoholsdue to the large size of the rho-ZMOF-1 8MR window. Indeed,the fastest and highest uptake was found for methanol, ace-tone, and ethanol, which is due to their small size and higherdipole moment (2.91, 1.70 and 1.69 D for acetone, methanoland ethanol, respectively). In the case of 1-propanol, higherand faster uptake was observed over 2-propanol and 1-butanol.However, an interesting very slow uptake was detected fordichloromethane at 298 K and within the same measurementtime. The observed slow uptake of dichloromethane is possiblydue to its relatively lower dipole moment (1.14 D).

Conclusions

In summary, highly oriented rho-ZMOF-1 thin films werefabricated by solvothermal growth of two COOH-terminatedself-assembled monolayers (SAM). It was shown for the firsttime that rho-ZMOF-1 thin films could be grown in a highlyoriented fashion on COOH-terminated SAMs. Moreover, theencapsulation of different molecules such as acridine yellow,methylene blue, Nile red, and porphyrin within the internalcavities of the rho-ZMOF-1 thin films was achieved, and thesewere characterized using different techniques such as XRD,UV-vis, and confocal fluorescence microscopy. The thin filmsof rho-ZMOF-1 grown on the QCM substrates showed slow

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guest uptake for dichloromethane over other solvents. Thisfinding may help indicate the desired feature to maximizethe forces driving the particular kinetic separation of mole-cules such as dichloromethane. This study suggests thatmesoporous ZMOFs are promising materials for applications,especially when serving as hosts for the encapsulation ofnumerous functional moieties within a stable pore systemfeaturing a well-defined molecular framework.

Acknowledgements

Research reported in this publication was supported by theKing Abdullah University of Science and Technology (KAUST).

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