A new method to position and functionalize metal-organic framework crystals

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With controlled nanometre-sized pores and surface areas of thousands of square metres per gram, metal-organic frameworks (MOFs) may have an integral role in future catalysis, filtration and sensing applications. In general, for MOF-based device
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  ARTICLE  NATURE COMMUNICATIONS | 2:237 | DOI: 10.1038/ncomms1234 | www.nature.com/naturecommunications  ©    2011   Macmillan Publishers Limited. All rights reserved. Received 15 Jul 2010 | Accepted 9 Feb 2011 | Published 15 Mar 2011 DOI: 10.1038/ncomms1234 With controlled nanometre-sized pores and surface areas of thousands of square metres per gram, metal-organic frameworks (MOFs) may have an integral role in future catalysis, filtration and sensing applications. In general, for MOF-based device fabrication, well-organized or patterned MOF growth is required, and thus conventional synthetic routes are not suitable. Moreover, to expand their applicability, the introduction of additional functionality into MOFs is desirable. Here, we explore the use of nanostructured poly-hydrate zinc phosphate ( α -hopeite) microparticles as nucleation seeds for MOFs that simultaneously address all these issues. Affording spatial control of nucleation and significantly accelerating MOF growth, these α -hopeite microparticles are found to act as nucleation agents both in solution and on solid surfaces. In addition, the introduction of functional nanoparticles (metallic, semiconducting, polymeric) into these nucleating seeds translates directly to the fabrication of functional MOFs suitable for molecular size-selective applications. 1  CSIRO, Division of Materials Science and Engineering, Private Bag 33, Clayton South MDC, Victoria 3169, Australia. 2  Associazione CIVEN, Via delle Industrie 5, Venezia 30175, Italy. 3  Institute of Biophysics and Nanosystems Research, Austrian Academy of Sciences, Schmiedlstraße 6, Graz 8042, Austria. 4  School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, Perth 6009, Western Australia . 5  School of Mathematical Sciences, Faculty of Science, Monash University, Clayton, Victoria 3800, Australia. 6  Centre for Micro-Photonics and CUDOS, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia. Correspondence and requests for materials should be addressed to P.F. (email: paolo.falcaro@csiro.au) or to D.B. (email: dario.buso@csiro.au). A new method to position and functionalize metal-organic framework crystals Paolo Falcaro 1 , Anita J. Hill 1 , Kate M. Nairn 1 , Jacek Jasieniak 1 , James I. Mardel 1 , Timothy J. Bastow 1 , Sheridan C. Mayo 1 , Michele Gimona 1 , Daniel Gomez 1 , Harold J. Whitfield 1 , Raffaele Riccò 2 , Alessandro Patelli 2 , Benedetta Marmiroli 3 , Heinz Amenitsch 3 , Tobias Colson 4 , Laura Villanova 5  & Dario Buso 1,6  ARTICLE   NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1234 NATURE COMMUNICATIONS | 2:237 | DOI: 10.1038/ncomms1234 | www.nature.com/naturecommunications  ©    2011   Macmillan Publishers Limited. All rights reserved. O rdered nanoporous systems with a large and accessible sur-face area are desirable in many applications, which require controlled molecular transport properties 1 . Metal-organic frameworks (MOFs) possess high surface areas 2 , adaptable surface chemistries, pore sizes 3  and structures 4  that make them leading candidates to separate 5 , capture 6 , store 7 , deliver 8 , transport 9 , sense 10  and catalyse 11  molecules. MOFs (or coordination polymers with permanent porosity  4 ) also have exciting potential as light-weight molecular selective sieves, because of their extremely high surface areas, low density, interconnected cavities and very narrow pore size distributions 12 . Some frameworks are also adaptive materials which respond to external stimuli (for example, light, electrical field, presence of particular chemical species), promising new advanced practical applications 6,10,13. To gain adoption in devices, a straightforward method to deposit MOFs onto chemically differing substrates must first be developed, preferably avoiding substrate functionalization protocols. In addi-tion to permitting controlled MOF growth at specific locations on planar substrates, this method must also be adaptable to more complex two-dimensional (2D) and three-dimensional (3D) sur-face morphologies. Achieving such facile deposition and growth requires a new level of control in the field of MOF synthesis.Further to spatial control of MOF growth, full exploitation of their properties in next generation sensing, separation, catalysis and delivery devices may require the introduction of extrinsic function-ality (for example, magnetism, luminescence and so on). Ideally, the functionality would reside within the core of the MOF so that the favourable selective barrier properties provided by the framework could impart molecule-specificity to the devices. Incorporation of functional species in MOFs has to date been demonstrated through post-impregnation mostly of metal nanospecies by chemical vapour deposition 14  and one-pot synthesis (adding either the functional species 15  or its precursors 16  directly into the MOF growing medium). Both of these approaches cause the doping species to grow indis-criminately inside the MOF cavities and on the MOF outer surface. 󰀀e resulting lack of spatial control of the functional components within the MOF crystals compromises the molecular selectivity of the final composite.Here, we present a novel and straightforward method (Fig. 1) to simultaneously achieve MOF positioning and functionalization. 󰀀e method is based on our discovery that a class of nanostructured α -hopeite microparticles shows an exceptional ability to nucleate MOFs. 󰀀ese microparticles are used as seeds for growing MOFs in solution (Fig. 1a), on any flat surface (Fig. 1b) and on complex 2D/3D surface morphologies (Fig. 1c). In addition, the introduction of active species (either metal, semiconductor or polymer nanoparticles) directly into the framework via the α -hopeite microparticles, permits functional-ization solely within the framework core, and not on its outer surface (Fig. 1d). Unlike the previously reported studies on functionalization of MOFs, the unprecedented spatial control that is offered through our method enables various functionalities to be easily included within each MOF crystal, without compromising the size-selective properties of the framework. A single crack or pinhole could make the putative sieving ineffective with respect to subsequent selective chemical reactivity; however, the MOF composites described here are free of such defects. 󰀀e method allows for the MOF shell to behave as a molecular sieve for the encapsulated functional species (Fig. 1e). We focus on the archetypal MOF-5 (refs 3, 17) to bench-mark the use of the α -hopeite microparticles as nucleating seeds and as carriers for embedding controlled functionality. Given the pecu-liar resemblance of these inorganic microparticles to the desert rose mineral, we have named them as desert rose microparticles (DRMs). 󰀀is report demonstrates that using DRMs to nucleate MOFs not only promotes faster MOF formation in solution but also enables controlled MOF formation on any surface chemistry and morphol-ogy. In addition, DRMs offer the unprecedented means to effectively encapsulate a range of functional nanospecies of diverse chemical nature (for example, metal, polymer, semiconductor) within MOFs. 󰀀e successful coupling of the sieving effect of the MOF matrix and the preserved functionality of the embedded species opens a new route for the development of a future generation of micro-reactors for selective sensing and catalysis. Results Nucleating properties of DRMs in solution . We have optimized a one-pot synthesis to grow MOF-5 through a surfactant assisted (Pluronic F-127) method. Once added to a traditional MOF-5 precursor solution, this particular surfactant coordinates the Zn 2 + ions 18,19  and contemporaneously provides an abundant source of phosphate (see Supplementary Fig. S1). 󰀀ese factors enable the rapid and iso-directional formation of inorganic poly-hydrate zinc phosphate nanoflaked microparticles (DRMs, see Fig. 2a) within the framework precursor solution. 󰀀e chemical and morphological characterization of the DRMs is presented in Supplementary Figures S2, S3 and Supplementary Table S1. 󰀀e nanostructured microparticles form within the first minute of reaction inside the cloudy surfactant suspension that constitutes the MOF-5 precursor solution, and in the following 3 h the DRMs grow up to tens of microns. As the DRM formation precedes the growth of MOF-5, DRMs can effectively act as heterogeneous nucleation seeds for the framework crystals (Fig. 2b–d and Supplementary Fig. S4).󰀀e micron dimensions of both the DRMs and the MOF-5 crys-tals ensure that optical microscopy is a convenient tool to study DRM α -HopeitemicroparticleMetal, polymer orsemiconductornanoparticles Figure 1 | Possibilities offered by desert rose microparticles.  ( a ) The ceramic desert rose microparticles (DRMs, α -hopeite microparticles) enable the preparation of MOF-5 in a one-pot synthesis, reducing the crystal formation time. ( b ) DRMs can be used to seed MOF-5 growth on any substrate, without the need for chemical functionalization of the surface. ( c ) The ceramic microparticles have been combined with a lithographed surface affording spatial control of MOF growth. ( d ) The α -hopeite microparticles can be synthesized in the presence of functional nanoparticles, including metal (Pt and Pd), polymer (PTFE) and semiconductor (CdSe-CdS-ZnS quantum dots) nanoparticles. The functionalized DRMs can be subsequently used to grow MOF-5. One advantage of this synthesis route is the ability to embed nanoparticles solely in the core of the MOF crystals. ( e ) The method enables the preparation of new molecular sieves in which, depending on their size, only selected molecules can diffuse through the framework (blue molecules) accessing the functional core; diffusion is not allowed for bigger molecules (yellow molecules). With this synthesis route new devices such as selective catalysts and luminescent sensors can be prepared.  ARTICLE    NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1234 NATURE COMMUNICATIONS | 2:237 | DOI: 10.1038/ncomms1234 | www.nature.com/naturecommunications  ©    2011   Macmillan Publishers Limited. All rights reserved. the nucleation and growth processes within our surfactant assisted growth solution. During the first 3 h of reaction, the only micron-sized species to form within the surfactant suspension are the α -hopeite DRMs of Figure 2a. Aer 3 h, heterogeneous nucleation of MOF-5 takes place on the nanometre-sized flakes of the DRMs to form cubic crystallites (Supplementary Figs S5–S8). Prolong-ing this growth time allows single crystal (Fig. 2b) or multifaceted (Fig. 2c) MOF-5 crystallites to develop, with sizes as large as 100 µ m. We observed that the controlled MOF-5 growth through hetero-geneous crystallization reduces the MOF-5 synthesis time by 70% when compared with conventional solvothermal methods.Statistical analysis of individual MOF-5 crystals indicates that on average 96% contain one α -hopeite DRM when synthesized in dimethyl formamide (DMF; Fig. 2b). By adopting an optimized two-step thermal process, up to 98% of these DRMs generate a crystal of MOF-5 (Fig. 2c). X-ray microtomography analysis (Fig. 2d) confirms that the embedded microparticles sit at the centre of each cubic crystal. 󰀀e microparticle spheres have a density similar to the surrounding coordination polymer and possess a lower density shell. 󰀀e resulting crystals appear in the SEM images of Figure 2e,f.Scanning electron microscopy of MOF-5 synthesized using the surfactant-directed method reveals that the spherical shape of the α -hopeite DRMs is produced by the arrangement of multiple 150–200 nm thick interpenetrating plates (Fig. 2g). Furthermore, it can be understood from Figure 2h,i that the microparticles that nucle-ate and promote growth of the MOF-5 crystals are progressively embedded deeper within the framework during the growth stage. Spatial control of MOF growth . During a single-pot synthesis, α -hopeite DRMs can be isolated before MOF-5 nucleates, and conse-quently re-dispersed into a variety of solvents as exogenous nuclea-tion seeds. When incubated in a fresh N,N- diethyl formamide (DEF)-based MOF-5 growth medium, a significant improvement in crystal quality can be achieved (Supplementary Figs S9, S10). 󰀀e use of the nucleating agents in this case more than doubles the crystal growth rate compared with control samples.󰀀e seeded nucleation mechanism for the growth of MOF-5 can be adapted directly to solid substrates. 󰀀is feature is demonstrated for α -hopeite DRMs deposited directly onto an alumina or silicon substrate (Fig. 3a). As shown in Figure 3b–d, immersion of this sub-strate into a fresh DEF-based MOF-5 precursor solution results in crystallite growth of the framework exclusively from the micropar-ticles (a continuous layer of crystals grown on the α -hopeite seeds is shown in Supplementary Fig. S11). Optical imaging through the focal plane of the frameworks reveals that several α -hopeite DRMs can be detected within the crystals. 󰀀e existence of embed-ded microparticles and the truncated shape of the otherwise cubic crystals suggest a heterogeneous nucleation mechanism 20 .In a subsequent experiment, the DRM seeding effect was tested using a support with a complex 3D geometry (Fig. 4a–d). A 100- µ m-thick film of commercial SU-8 resist was deposited on a silicon substrate and patterned using deep X-ray lithography, to obtain an array of high aspect ratio vertical wells (30–50 µ m diameter). Under optimized conditions, an average of one DRM could be deposited per well (Fig. 4e,f). Immersion of the DRM infiltrated surface into a MOF-5 growth solution caused frame-work crystals to nucleate inside the wells (Fig. 4g). 󰀀e crystallite sizes were found to be constrained until they had outgrown the well height, whereupon they grew freely in all directions (Fig. 4h). Further evolution of the system resulted in the merging of MOF crystals to form an interpenetrated crystalline structure. 󰀀is structure could be detached from the silicon wafer to form a free-standing support, with the channels in the SU-8 resin occupied by MOFs. Details of the procedure and examples of MOF growth on different 3D geometries are presented in Supplementary Figures S12 and S13. Figure 2 | Desert rose microparticles and their nucleating effect.  ( a – c ) Optical microscope images of the desert rose microparticles (DRMs) and MOF-5 crystals formed using DRMs as seeds. ( a ) DRMs formed in the MOF growing medium containing Pluronic F-127 after 3 h reaction at 95 °C (scale bar, 100 µ m). The picture shows a group of DRMs that grew within the suspended cloud of Pluronic F-127; the cloud is also visible in the image. The average size of the DRMs is in the tens of microns range. The picture was taken of a drop of solution extracted from the mother batch and dropped directly on a glass microscope slide. ( b ) MOF-5 crystals formed around the DRMs. The image was taken of a drop of solution extracted after 8 h reaction at 95 °C (scale bar, 200 µ m, inset scale bar, 50 µ m). ( c ) Two-step heating synthesis resulting in multicrystalline growth on each DRM (scale bar, 200 µ m). ( d ) X-ray microtomography performed on two cubic crystals of MOF-5 nucleated around two DRMs. The small images on the top-left are the measured tomography of the crystals’ exterior surfaces. The main image is a cross-section of the cubic crystals. The image was obtained slicing the 3D tomographs along a section plane, which intersects both of the crystal centres and is perpendicular to the observer’s view. The exposed section demonstrates that the DRMs are sitting in the geometric centre of the cubic crystals. ( e – i ) SEM images of the MOF-5 crystals grown with the assistance of the DRMs. ( e ) Overview of cubic MOF-5 crystals resulting from the one-pot synthesis (scale bar, 500 µ m). ( f  ) MOF-5 crystals grown using the two-step synthesis (scale bar, 50 µ m). ( g ) Close-up of a DRM particle. This image reveals the faceted features of the DRMs surface (scale bar, 5 µ m). The inset of the figure shows the electron diffraction pattern of the DRMs, indexed as α -hopeite. ( h ) DRMs partially embedded in the MOF structure (one-pot synthesis). Scale bar, 5 µ m. ( i ) DRMs almost completely embedded in the MOF structure (two-step synthesis). Scale bar, 10 µ m.  ARTICLE   NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1234 NATURE COMMUNICATIONS | 2:237 | DOI: 10.1038/ncomms1234 | www.nature.com/naturecommunications  ©    2011   Macmillan Publishers Limited. All rights reserved. Fabrication of functional framework nanocomposites . By adopt-ing a straightforward combinatorial method, the DRMs can also be doped with functional nanoparticles. We optimized a series of protocols to effectively incorporate metal (Pt and Pd), semiconduc-tor (luminescent CdSe-CdS-ZnS quantum dots, QDs) and polymer (polytetrafluoroethylene, PTFE) nanoparticles with the DRMs. In a typical synthesis, nanoparticles of a specific functionality are intro-duced into the DRM growth solution. During this step, we observed that the nanoparticles spontaneously decorate the DRMs. In a first experiment, the reaction was allowed to proceed (one-pot approach) and the functionalized DRMs nucleated MOF-5 crystals directly in the same reaction batch. In a second experiment, the functional-ized DRMs were isolated and inoculated into a DEF-based MOF growth medium as external seeds and produced high quality frame-work composites. In both the tests, the observed kinetics of MOF-5 growth showed no appreciable change compared with the synthesis without nanoparticles. In each case, the seeded MOF-5 synthesis yields framework composites containing the extrinsic functional-ity srcinating from the nanoparticles decorating the DRMs (see Methods section and Supplementary Figs S14–S19 for details). An elemental analysis of the sectioned crystals confirms signals of plati-num, palladium, cadmium or fluoro species srcinating exclusively from the microparticles (Fig. 5a–f). No trace of the functional nano-particles was recorded in the framework matrix or the framework external surface.To demonstrate perfect encapsulation of the functional species within the MOF framework, we tested the effectiveness of the framework as a size-sieving shell. As a proof of concept, we show that the luminescent QDs/DRM/MOF-5 composites can be used to discriminate thiols of different size, thus acting as selective molecular sieve sensors.QDs that are dispersed in DEF undergo rapid quenching of the steady-state photoluminescence when thiols are injected (Supple-mentary Fig. S20). 󰀀e quenching mechanism is associated with the Figure 3 | Desert rose microparticle assisted growth of MOF-5 on a flat surface.  ( a ) Schematic representation of DRMs used for the nucleation of MOF-5 on a silicon surface. DRMs extracted from the growing medium are first dried on a substrate, which is subsequently immersed into a fresh DEF-based MOF-5 precursor solution. MOF-5 crystals are observed to form on the deposited bed of DRMs in the form of truncated cubes because of the flat surface of the support. ( b – d ) Optical microscope images of a single truncated MOF-5 crystal grown on a bed of DRMs (scale bar, 100 µ m). The three images were taken on the same crystal at different microscope focal planes. Next to each image, a 3D schematic illustrates the focal plane position related to the corresponding image. Figure 4 | Desert rose microparticles used for positioning MOF-5 on patterned supports.  ( a – d ) Schematics of the procedure used to spatially drive MOF-5 formation using DRMs in pre-defined positions on a lithographed substrate. X-ray lithography was used to produce a membrane with an even distribution of wells ( a ) on a commercial resist (SU-8) layer previously deposited on a silicon substrate. ( b ) Subsequently, drops of a DRM suspension in DMF were deposited by drop-casting on the patterned substrate. ( c ) The substrate was vacuum dried to fill the lithographed wells with DRMs. ( d ) The seeded substrate was then inserted into a standard MOF-5 growing medium to induce MOF formation within the membrane holes. ( e – h ) SEM images showing seeding and MOF crystal growth on the lithographed substrate. ( e ) The SU-8 membrane obtained after the lithographic procedure. The wells have a diameter of 40 µ m and a depth of 100 µ m (scale bar, 100 µ m). ( f  ) DRMs located in a hole of the substrate after the drop-casting and drying processes (scale bar, 10 µ m). ( g ) MOF-5 crystals growing within each one of the lithographed holes. The micrograph was taken after 5 h reaction time at 95 °C in the MOF-5 growing medium (scale bar, 50 µ m). The insets show crystals emerging from the holes during growth (scale bar, 20 µ m). ( h ) Substrate after 10 h reaction at 95 °C. MOF-5 crystals have grown out of the holes and appear to be interconnected with their neighbours, which is a prerequisite for the controlled formation of a continuous layer of MOF. Scale bar, 200 µ m.  ARTICLE    NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1234 NATURE COMMUNICATIONS | 2:237 | DOI: 10.1038/ncomms1234 | www.nature.com/naturecommunications  ©    2011   Macmillan Publishers Limited. All rights reserved. rapid and efficient photo-generated hole capture by surface-bound thiolates. 󰀀e exact yield of luminescence quenching is directly related to the local concentration of thiol species 21 . 󰀀e change in the luminescence yield of QDs that are incorporated within a MOF can therefore be used as a direct indicator of the concentration of thiol species that have diffused through the porous framework. One of the quenching thiols is a custom non-commercial copolymer, and details of its synthesis and characterization can be found in the Methods section and in Supplementary Figures S21, S22 and in Supplementary Table S2.Luminescent MOF-5 crystals were fabricated using DRMs doped with CdSe-CdS-ZnS core-shell QDs emitting at 576 nm (Supple-mentary Fig. S23). 󰀀e resulting QDs/DRM/MOF-5 composites were loaded into DEF-filled cuvettes, and then solutions of thiols of different molecular size were injected. In situ  emission spectra of the samples were recorded at 10 min intervals for times between 20 and 50 h. Equivalent experiments conducted on QDs dispersed directly in DEF show rapid and efficient luminescence quenching, regardless of the thiol molecular dimension (Fig. 6a–c). However, for the QDs/DRM/MOF-5 composites significant differences between the lumi-nescence quenching results were observed depending on the size of the thiol (Fig. 6d–f). 󰀀e smallest thiol (ethanethiol), which can read-ily diffuse through the MOF-5 cage, quenched 90% of the emission in about 3 h; the medium-sized thiol (2-amino-6-mercaptopurine ribo-side), the size of which is comparable with the dimension of MOF-5 cavities, showed a remarkably slower quenching effect (about 30% emission drop in 21 h); the biggest thiol molecule (thiol terminated copolymer n -isopropyl acrylamide/acrylic acid/ t  -butyl acrylamide) did not provoke any detectable emission quenching, even aer 51 h following injection. 󰀀e vastly different fluorescence response of the thiols can be attributed to the sieving effect of the MOF-5 crystal, which effectively discriminates the quenching molecules by their size. Additional details can be found in the Methods section and in Supplementary Figures S24–S27. Discussion 󰀀e role played by a surfactant (Pluronic F127) in the classical sol- vothermal synthesis of MOF-5 has been investigated for the first time. 󰀀is surfactant has been used to create a novel ceramic micro-particle in which nanosized flakes are arranged to form a spheri-cal microscopic superstructure. 󰀀e unexpected windfall of these α -hopeite microparticles, here named as DRMs, is to offer an ideal combination of chemistry and morphology for MOF-5 growth. As an immediate consequence, DRMs enhance the growth rate of the framework by a factor of three compared with conventional solvo-thermal synthesis.󰀀e versatility of the DRMs can be extended through their isola-tion and then use as heterogeneous seeds to trigger MOF nucleation on either flat surfaces or 3D objects. A number of recent studies have explored a variety of protocols and patterning technologies 22–26  to grow MOFs in selected areas 27 , all of which have been limited to planar surfaces. To extend the 2D lithography methods for MOF growth to more complex 3D surfaces, we showed that DRMs can be used to fabricate free-standing MOF-5 forms using lithographically patterned substrates. We showed that positioning the DRMs allows for spatially controlled growth of MOF-5 and offers new routes to fabricate 2D or 3D architectures of MOF-5. With this method, there is no need for chemical functionalization of the surface because positioning the DRMs is enough to localize the framework growth. Here, we propose new routes for the simultaneous fast growth and controlled positioning of frameworks; both features are fundamen-tal prerequisites for framework device fabrication.An additional degree of freedom offered by the DRMs is the possibility to use them as carriers for functional species within the    C  o  u  n   t  s   C  o  u  n   t  s   C  o  u  n   t  s 3,0004,0004,0006,0008,000 MOF-5 DRM Pd MOF-5DRM funct. 2,0002,0001,0001,0008006004002000000 0 06422 4 6 82 4 6 8 100 5 10 15 200 5 10 15 20 5 10 15 20Energy (keV)Energy (keV)Energy (keV)Energy (keV)Energy (keV) ZnZnZnZnZnZn    C  p  s   C  p  s 105ZnZnSiSi DRM PTFE DRM QDs DRM PtPPPdPdCOZnZnZnZnZnZnZnPtPt PtPtPOSiKZnZnSiSiCdPOOF F Figure 5 | Desert rose microparticles functionalized with functional nanoparticles.  ( a ) SEM of a MOF-5 crystal sectioned along its median plane to expose the functionalized embedded desert rose microparticle (DRM). This MOF-5 crystal was grown around a DRM functionalized with Pd. Similar images have been obtained for each reported system involving functionalized DRMs (DRM funct . with funct. = Pd, Pt, QDs or PTFE nanoparticles). Scale bar, 5 µ m. ( b ) Energy dispersive X-ray analysis (EDX) of MOF-5 crystal sections. Only signals from MOF-5 constituents (C, Zn, O) are detected. The Si signal is related to the silicon wafer used as support for MOF-5 crystal analysis. ( c – f  ) EDX of functionalized DRMs. ( c ) Pd functionalized DRM revealing L α  and L β  emission from Pd atoms. ( d ) Pt functionalized DRM showing L α  and L β  emissions from Pt atoms. ( e ) EDX of DRMs functionalized with CdSe-CdS-ZnS core-shell quantum dots revealing L α  emission of Cd only from the microsphere section. ( f  ) Presence of a weak signal of F on the microsphere embedding PTFE nanoparticles. The experiment demonstrates the accuracy offered by the DRM approach in the control of the spatial distribution of functional species within a MOF framework.
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