ZnO as an Efficient Nucleating Agent for Rapid, Room Temperature

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ZnO as an Efficient Nucleating Agent for Rapid, Room Temperature Synthesis and Patterning of Zn-Based Metal−Organic Frameworks Erika Zanchetta,† Luca Malfatti,‡ Raffaele Ricco,§ Mark J. Styles,§ Fabio Lisi,§,∥ Campbell J. Coghlan,⊥ Christian J. Doonan,⊥ Anita J. Hill,§ Giovanna Brusatin,*,† and Paolo Falcaro*,§ †

Industrial Engineering Department and INSTM, University of Padova, Via Marzolo 9, 35131 Padova, Italy Laboratorio di Scienza dei Materiali e Nanotecnologie, CR-INSTM, Universitá di Sassari, Palazzo Pou Salid, Piazza Duomo 6, 07041, Alghero (SS), Italy § CSIRO Manufacturing Flagship, Private Bag 10, Clayton South MDC, Victoria 3169, Australia ∥ School of Chemistry and Bio21 Institute, University of Melbourne, Parkville, 3010, Victoria, Australia ⊥ School of Chemistry and Physics, Centre for Advanced Nanomaterials, The University of Adelaide, Adelaide, SA 5005, Australia ‡

S Supporting Information *

ABSTRACT: The use of ZnO particles as efficient agents for seeding, growing, and precisely positioning metal−organic frameworks (MOFs) is described. Ceramic seeds have been successfully used for the preparation of Zn-based MOFs with a number of different carboxylic acids: terephthalic acid, 2aminoterephthalic acid, 1,3,5-benzenetricarboxylic acid, 2,6naphthalenedicarboxylic acid, and 4,4′-biphenyldicarboxylic acid. In situ synchrotron small-angle X-ray scattering and electron microscopy experiments were employed to determine the effect of the concentration of ZnO nanoparticles, temperature, and time on MOF growth. Under optimized conditions, MOF crystals were found to form in several minutes. This unprecedented capacity to seed MOF formation was used to control the growth of crystals in precise locations. Accordingly, we employed this seeding technique to position porous MOF crystals on paper strips (lateral flow), or within glass and PDMS microchannels (120 μm width and 100 μm height). These data demonstrate that ZnO nanoparticles are versatile seeding agents for the growth of porous crystals in a number of different microfluidic platforms.

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growth,15 conversion from ceramics, contact printing,16 electrochemical deposition,17 or nucleating agents,18 whereas “topdown” methods employ resist features patterned by X-ray19 and optical20,21 lithography to position preformed MOF crystals. In both cases, the ultimate aim is to develop rapid, simple, versatile, and scalable methods that facilitate the synthesis of MOFs in well-defined locations. To this end, an efficient and versatile technique that engenders the rapid growth of MOFs on specific substrate locations involves the use of homogeneous or heterogeneous nucleating agents to seed crystal growth.18 Homogeneous nucleation of MOFs is typically induced using pre-synthesized nanocrystals of identical elemental composition to the target frameworks metal nodes.22−29 A uniform seed layer of the corresponding MOF can be deposited on a support for secondary growth, allowing composite membranes or films to be obtained.22−26 Thus, in addition to the commonly used dip-coating,24,25,30 some novel seeding routes, such as thermal seeding27 and reactive seeding,28,29 have also been developed. In heterogeneous nucleation, the crystallization facilitator is a

INTRODUCTION Metal−organic frameworks (MOFs), or porous coordination polymers (PCPs), are a fascinating class of solid-state materials that are of significant interest due to their ultrahigh surface areas and pore volumes,1 tuneable structure metrics,2 and varied framework architectures.3,4 Due to their modular synthesis, these aforementioned features can be controlled through judicious choice of their organic and inorganic components.5 As a result, MOF research has concentrated on the design and synthesis of materials for application to gas storage6,7 and separation,8,9 catalysis,10 drug delivery, and sensing.11,12 A topic of increasing interest is to expand the potential technological applications of MOFs through the development of protocols for their integration into miniaturized platforms such as microfluidic or lab-on-a-chip devices. To fully realize their vast potential, new and efficient methods for precisely positioning and patterning MOF crystals are being explored.13 Strategies that have been employed for incorporating MOF crystals into miniaturized devices can be broadly categorized as either “bottom-up” or “top-down” approaches. In the “bottomup” approach, MOF growth is induced in predetermined locations by means of surface functionalization,14 UV-induced © XXXX American Chemical Society

Received: August 6, 2014 Revised: November 30, 2014

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dx.doi.org/10.1021/cm502882a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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techniques, this oxide-to-MOF transformation can be used to prepare patterned ZIF-8 coatings.47 In this work, we present the first example of using ZnO nanoparticles as seeds for the heterogeneous nucleation of highly crystalline MOFs. This strategy exhibits several advantages over other heterogeneous seeds, such as zinc phosphate microparticles48 and silica nanoparticles,38 including room temperature synthesis, short processing times (MOF crystals in less than 30 min), and applicability to a broad range of Zn-based MOFs (terephthalic acid, 2-aminoterephthalic acid, 1,3,5-benzenetricarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-biphenyldicarboxylic acid). Further, we demonstrate that this technique can be used for positioning MOF crystals on different supports, such as the channels of a microfluidic device and paper fibers at the interface of a lateral flow of seed and precursor solutions. We note that this is the first demonstration that MOF reactions can be triggered directly on a paper support using the seeding method. Furthermore, MOF crystals can be grown precisely within the microfluidic channels or at their edges, providing an effective strategy for a controlled MOF crystal growth.

different material to the targeted MOF (these include different MOFs with different topologies, as well as ceramic and polymeric particles) that remains embedded in the crystal.18 An advantage of this approach over the homogeneous seeding is the available technology for positioning seeds that includes ion-induced focusing masks, micromolding in capillaries (MIMIC), microcontact printing (μCP), dip-pen nanolithography (DPN), and a range of evaporation induced self-assembly techniques.31−34 Furthermore, it has been shown to significantly reduce MOF synthesis times, relative to traditional solvothermal methods.35−37 Several approaches to heterogeneous nucleation have been explored,13 including employing chemically modified particles,38,39 polymeric nanospheres,40 or pure ceramics as seeds.41−43 Here, we present an effective bottom-up methodology for the controlled growth of Zn-based MOFs. This protocol employs readily available and inexpensive zinc oxide particles as heterogeneous nucleation agents for the rapid growth of a range of different MOFs (such as MOF-5, IRMOF-3, IRMOF8, IRMOF-10); we changed different parameters such as the particle size and the substrate used for the controlled MOF growth (Figure 1). Previously, ZnO nanostructures have been

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EXPERIMENTAL SECTION

Synthesis of Au-Functionalized ZnO Particles. Au-modified ZnO microparticles were obtained as follows: 200 mg of a commercial ZnO powder (AnalaR BDH) was dispersed in 4 mL of ethanol (EtOH). Thereafter, 60 mg of tetrachloroauric acid (HAuCl4) and 40 μL of hydrogen peroxide H2O2 (30% v/v, Merck) were added to the dispersion. After stirring for 5 h, the dispersion was centrifuged and the microparticles were washed with EtOH, dried, and treated for 30 min at 650 °C. Finally, 200 mg of the as-prepared microparticles were dispersed in 250 μL of EtOH. Energy-dispersive X-ray analysis (EDX) characterization of the ZnO microparticles before and after functionalization with Au is proposed in Figure S1a,b (Supporting Information). Synthesis of Fe3O4 Particles Modified with ZnO. Fe3O4 microparticles were synthesized according to the synthesis of Hui and co-workers.49 In detail, 2.95 g of trisodium citrate dihydrate, 1.6 g of NaOH, and 170 mg of NaNO3 were dissolved in 190 mL of deionized water in a round-bottom flask, and the solution was heated to 100 °C. Afterward, 10 mL of a 0.1 M Fe(II) solution (27.8 g/L iron(II) sulfate heptahydrate) was quickly added in a single portion; the resulting mixture rapidly became black and was maintained at 100 °C for an hour with vigorous stirring. After cooling down to room temperature, the magnetic material was separated with an external magnet and washed thrice with water and EtOH, and finally dried in an oven at 110 °C overnight. Then, the composite Fe3O4/ZnO microparticles were obtained by a two-step sol−gel method. At first, 5 mL of NaOH (10 wt %) in water was added to 22.5 mL of water, 500 mg of Zn(NO3)2·6H2O (>99%, Aldrich), and 30 mg of Fe3O4 microparticles. Then, with a magnet, the responsive particles were collected while the supernatant was removed. After washing with acetone, the particles were added to a second solution containing 2 g of Zn(NO3)2·6H2O dissolved into 22.5 mL of water. The solution was left at 85 °C for 1 h until flocculation and precipitation of the particles occurred. After washing with acetone and centrifuging, the microparticles were evaporated for 12 h at 50 °C and treated at 650 °C for 30 min. Then, 30 mg of the annealed microparticles were dispersed in 150 μL of EtOH. Energy-dispersive X-ray analysis (EDX) characterization of the Fe3O4 particles before and after functionalization with ZnO is proposed in Figure S1c,d (Supporting Information). Synthesis of MOFs Using ZnO Microparticles. IRMOF-3 was obtained in 50 mL of N,N-diethylformamide (DEF) starting with 1.88 g (6.3 mmol) of Zn(NO3)2·6H2O (>99%, Aldrich) and 1.3 mmol of aminoterephthalic acid NH2TA. Once all the precursors had dissolved, the Au@ZnO or ZnO@Fe3O4 seeds were added to the as-prepared

Figure 1. Outline of the heterogeneous nucleation of MOFs by pure and functionalized ZnO microparticles (a, c, d) and nanoparticles (b, e, f, g) demonstrated in this work. ZnO nanoparticles can also be used to induce the growth of MOF crystals on paper and within microfluidic channels as shown in (f) and (g), respectively.

used for pseudomorphic replication where the ceramic is progressively consumed and replaced by the MOF.44 For example, zinc oxide nanorods45 and nanoparticles46 can be transformed into a zeolitic imidazolate framework (ZIF-8) when in contact with 2-methylimidazole. Additionally, by taking advantage of well-established ZnO deposition and patterning B



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growing media. The product was separated after 10, 30, and 60 min at 95 °C and washed with N,N-dimethylformamide (DMF) and toluene. Synthesis of Different MOFs Using ZnO Nanoparticles. The synthetic protocol for the MOFs obtained with different carboxylic ligands was as follow: 50 mL of N,N-diethylformamide (DEF) was added to a solid mixture of 1.88 g of Zn(NO3)2·6H2O (>99%, Aldrich) and either 1.3 mmol of dicarboxylic acid (terephthalic acid TA, 2aminoterephthalic acid NH2TA, 2,6-naphthalenedicarboxylic acid NDC, 4-4′-biphenyldicarboxylic acid BPDC) or 0.85 mmol of tricarboxylic acid (1,3,5-benzenetricarboxylic acid BTC). Once the precursors were dissolved, 20 μL of ZnO nanoparticle dispersion (50 nm mean size, 40 wt % in EtOH, Aldrich) was added to 5 mL of precursor solution just before heating at 95 °C. The as-prepared growing medium was left at 95 °C for 1 h. Finally, the synthesized MOFs were washed with N,N-dimethylformamide (DMF) and toluene. The potential effect of EtOH in the MOF growth16 was investigated; we found that no MOF crystals were observed in the time frame used for ZnO seeding (Figure S2, Supporting Information) for the range of ethanol volume (2−200 μL of EtOH). In Situ Measurements (SAXS and NMR). For the in situ characterization by small-angle X-ray scattering (SAXS), a bespoke reaction cell was constructed. The cell, measuring 2 cm × 2 cm, was obtained using two cover glass slides separated by a 2 mm gap and fastened together with silicon. Two openings were left at the top of the cell to allow for the entrance of the growing solution. Once filled with the solution, the cell was illuminated by a high-intensity X-ray beam at the SAXS beamline of the Australian Synchrotron, and the scattered radiation was registered by the detector (Pilatus 1M). An on-axis video camera allowed for parallax-free sample viewing and alignment at all times before and during exposure, enabling precise and rapid sample alignment. In order to monitor the growth of the MOFs at different temperatures, the cells filled with the ZnO seed and precursor solutions were immediately placed in thermal contact with a precision temperature controller (Linkam cell). Activation. The as-synthesized crystals were washed with fresh DMF, followed by dry EtOH (×5) and then allowed to soak in dry EtOH overnight. The sample was further washed with dry EtOH (×2) over a period of 6 h. The EtOH-exchanged material was then subject to supercritical activation using a Samdry-PVT-3D Critical-PointDryer; samples were solvent-exchanged with LCO2 over 1 h and held at the supercritical point for another 1 h. The N2 adsorption isotherm was measured using a Micromeritics 3Flex analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA) at 77 K. Brunauer−Emmett−Teller (BET) surface areas were calculated using experimental points at a relative pressure that follows the Rouquerol criteria.50 The pore size distribution was calculated by the Barret−Joyner−Halenda (BJH) method. In Situ NMR for Deprotonation Measurements. The 1H NMR ex situ experiments were conducted on a Bruker BioSpin UltraShield NMR spectrometer operating at 400 MHz (9.4 T magnet), using DMF-d7 as solvent and 1,4-dimethoxybenzene (DMB) as internal standard. The 1H NMR in situ experiments were conducted on a Bruker BioSpin AvanceIII NMR spectrometer operating at 500 MHz (11.7 T magnet). The ZnO effect was measured ex situ, to avoid any interference due to the ZnO suspension: 16.3 mg of DMB and 16.3 mg of terephthalic acid were dissolved in 4 g of solvent. Subsequently, 7.8 mg of ZnO nanopowder was added and aliquots of the suspension were spiked after 1, 5, 10, 20, 60, and 180 min, quickly filtered through glass wool directly into 7 in. NMR tubes, and analyzed. The triethylamine effect was estimated in situ, due to the impossibility to easily remove the base from the reaction mixture: the established procedure proposed by Yaghi et al.51 was followed, using 6.33 mg of terephthalic acid, 6.33 mg of DMB, 500 μL of DMF-d7, and 10.6 μL of triethylamine. In all cases, the decrease of the −COOH integral at 13.5−14.0 ppm from terephthalic acid was monitored against the −CH3 signal at 3.5−4.0 ppm from the internal standard and used to calculate the residual amount of free terephthalic acid vs time. Calculation of the Crystal Size. The crystal sizes for the data given below were obtained analyzing the SEM images with ImageJ software (Wayne Rasband, National Institutes of Health, USA, http://

imagej.nih.gov/ij), version 1.48. A minimum of 30 measures for each image was used to calculate the average and the standard deviation. Lateral Flow Reaction on a Paper Strip. First, two solutions were prepared: one containing the MOF precursors (1.88 g of Zn(NO3)2·6H2O and 1.3 mmol of aminoterephthalic acid NH2TA in 50 mL of DMF), and the other one containing the ZnO nanoparticle dispersion (ZnO dispersion: DMF = 0.5:1 volume ratio). The opposite ends of a filter paper strip were immersed at 95 °C for 3 h into the asprepared solutions, allowing the species to ascend the paper by capillary absorption from both sides. The reaction was conducted in a closed vessel. MOF Growth in Microchannels and Microfluidic Circuits. The step-by-step protocol used for MOFs growth into microfluidic channels was as follows. First, the master was patterned on silicon substrates through a photolithographic process by using SU8-2100 negative photoresist (MicroChem, Newton, MA) to obtain a final thickness of 100 μm, according to the manufacturer’s data sheet.52 Second, the polydimethylsiloxane (PDMS, SYLGARD 184 kit; Dow Corning, Midland, MI) mold was obtained by casting a 10:1 base/ curing agent ratio solution on the SU-8 structures and curing for 2 h at 70 °C. Thereafter, the PDMS mold was released from the silicon master by cutting and peeling off, and punched with a 21G stainless steel needle (Small Part Inc., Logansport, IN) to obtain inlet/outlet holes.53,54 The substrate for MOFs growth and patterning was obtained by spin-coating the ZnO nanoparticle dispersion (50 nm mean size, 40 wt % in EtOH, Aldrich) on cleaned silicon substrates at 1000 rpm for 30 s. The ZnO nanoparticle coating was then treated at 650 °C for 3 h in order to sinter the ZnO film. The PDMS mold was pressed onto the ZnO NPs substrate and gently fastened to it with a clamp. Finally, the MOF growing medium (0.188 g of Zn(NO3)2· 6H2O, 0.024 g of 2-aminoterephthalic acid, and 20 μL of ZnO NPs in 5 mL of DMF) was injected at room temperature with a syringe through holes in the channels’ edge. The solution injection was repeated up to 4 times to maximize the MOF coverage inside the microchannels, before peeling-off the PDMS mold from the substrate. MOF Growth in Microchannels and Sequestration of Polycyclic Aromatic Hydrocarbon. A glass microchannel (300 μm × 120 mm) was obtained by cutting the end of a glass capillary (300 μm diameter, from Charles Supper Inc.). The inside of the microchannel was filled with a dispersion of ZnO nanoparticles (50 nm mean size, 40 wt % in EtOH, Aldrich) by capillary action. The solvent was then evaporated at room temperature under vacuum (60 min), leaving the inside walls of the microchannel coated with the ZnO NPs. A subsequent thermal treatment (150 °C for 30 min) was performed to improve the adhesion of NPs onto the glass. Then, a solution of MOF precursors (0.188 g of Zn(NO3)2·6H2O and 0.028 g of 2,6-naphthalenedicarboxylic acid in 5 mL of DMF) was injected into the microchannel at a controlled flux of 1 μL min−1. Once the microchannel was completely filled with the precursor solution, the sample was thermally treated (90 °C for 30 min) to induce the growth of IRMOF-8. This process (injection and thermal treatment) was repeated 4 times to maximize the MOF coverage inside the microchannel. Finally, the IRMOF-8 crystals were washed by injecting 50 μL of EtOH inside the microchannel (flux of 20 μL min−1). The sequestration experiment was conducted by injecting a 350 nM solution of 1,2-benzanthracene (>99%, Aldrich) in tetrahydrofuran (THF) into the IRMOF-8 coated microchannel at a controlled flux of 5 μL min−1. The solution from the channel was collected, diluted in THF, and characterized via fluorescence spectroscopy to evaluate the benzanthracene concentration. The fluorescence emission spectra were acquired in the 350−600 nm spectral range, with a 347 nm excitation wavelength, using a FluoroLog spectrofluorimeter (Horiba Scientific, Edison, NJ).

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RESULTS AND DISCUSSION ZnO Microparticle Induced MOF Formation. We initially investigated the heterogeneous nucleation and growth of IRMOF-3 on ZnO micropowder in DMF at 95 °C using

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MOFs were not affected by the particles functionalization (i.e., Fe3O4 and Au). Therefore, this strategy can also be used for the fabrication of MOF-based composites (also called framework composites).18 The versatility of the proposed synthetic method was verified by preparing Zn-based MOFs with different organic linkers (terephthalic acid, 2-aminoterephthalic acid, 1,3,5-benzenetricarboxylic acid, 2,6-naphthalenedicarboxylic acid and 4,4′biphenyldicarboxylic acid). Because of their density of the ZnO particles (5.81 g cm−1) and size, the microparticles will adversely affect the surface area of the resulting MOFs composite. Therefore, we selected smaller ZnO nanoparticles for the remaining studies (see Figure S4, available in the Supporting Information, for ZnO nanoparticles characterization). Analysis of MOF Crystal Growth Rates. To assess the rate of MOF crystal growth, we employed optical microscopy and in situ SAXS measurements. Optical microscope images (Figure S5 of the Supporting Information) of solutions containing the MOF precursors and ZnO nanoparticles at 95 °C show the formation of MOF crystals only a few minutes after the addition of the nanoparticles. The morphology of MOF crystals afforded were elucidated by SEM (Figure 4). This result demonstrates that ZnO particles are effective and versatile precursors for the heterogeneous nucleation of different Zn-based MOFs. Without seeds, the spontaneous crystallization occurred after 15 h with BTC to form MOF-4, 16 h with TA to form MOF-5 or NDC to form IRMOF-8, 19 h with BPDC to form IRMOF-10, and 21 h with NH2TA to form IRMOF-3. To gain an understanding of the kinetics of seed-induced crystal growth, we performed in situ SAXS measurements on solutions of varied synthetic conditions (e.g., different ligands, solvents, temperatures, and seed concentration) (Figure 5). Individual 5 mL solutions were prepared containing either terephthalic acid, 2-aminoterephthalic acid, or 1,3,5-benzenetricarboxylic acid as the organic component, and to each was added a 20 μL solution of ZnO. These solutions were then heated to 95 °C, and their ensuing crystal growth was examined by SAXS. Close inspection of the SAXS data indicated that, for each case, MOF crystal growth occurred within minutes subsequent to the injection of ZnO particles, thus confirming the optical microscope images in Figure S5 (also see Figures S6 and S7, Supporting Information, for SAXS data). The resulting 2D scattering patterns have been radially integrated and plotted as functions of the reaction time. Because of the high sensitivity of the technique, the in situ study was used to assess MOF crystal formation. The time evolution of the patterns observed for MOF-5, IRMOF-3, and MOF-4 is reported in Figure 5b−d. Fluctuations in the intensity of the diffraction peaks were observed due to MOF crystals passing in and out of the volume illuminated by the Xray beam in the process of settling to the bottom of the reaction cell. The experiment is schematically represented in a multimedia file in the Supporting Information. We then turned our efforts to understanding the effect of temperature during the synthesis. Remarkably, SAXS measurements revealed that, by using ZnO nucleating agents, MOFs could be readily synthesized at room temperature. This synthetic protocol gave rise to MOF crystals of analogous morphology to their solvothermally synthesized counterparts. Furthermore, we observed that the size of the crystals decreased with decreasing synthesis temperature (Supporting Informa-

optical microscopy (available in the Supporting Information as Figure S3). Figure 2 shows the SEM images of IRMOF-3 crystals 10, 30, and 60 min after the addition of ZnO microparticles. Close

Figure 2. (a−c) SEM images of the MOFs obtained at 95 °C 10, 30, and 60 min after the addition of ZnO microparticles into the MOF precursor solution. As the framework’s growth proceeds, the microparticles are increasingly embedded inside the crystals. (d−f) Schemes summarizing the heterogeneous nucleation and growth of MOFs on ZnO microparticles.

inspection of these images reveals that crystals with sharp cubic facets are obtained in just 10 min subsequent to seeding the precursor solution. Additionally, the SEM images clearly show that, over the time course studied, the microparticles become embedded within the MOF (Figure 2c). This process is concomitant with crystal growth. The average size of crystals grows from 5.8 ± 1.8 μm in 10 min to 7.9 ± 1.6 μm after 30 min reaction time, and 9.0 ± 2.7 μm after 60 min. Although the morphology of the MOF crystals grown on the ZnO seeds resembles a single crystal, a previous study has shown that nanoparticles can induce the formation of two or more crystalline domains that are slightly misaligned.55 To further demonstrate the nucleating capacity of ZnO particles, we prepared gold-functionalized ZnO microparticles through a metal reduction method and ZnO-modified iron oxide particles obtained with the sol−gel approach, since Au and Fe atoms facilitated the detection of particles within the MOF framework by EDX (Figure S1). The reaction for the MOF growth around the modified ZnO microparticles was kept at 95 °C for 60 min, and the resulting samples were characterized by EDX (Figure 3). The co-location of Zn, Au, and Fe in the elemental maps from SEM confirms the presence of ZnO-functionalized microparticles within the crystals. Both the morphology and the kinetics of synthesized

Figure 3. SEM and energy-dispersive X-ray maps of MOF crystals grown on Au-modified ZnO (a−c) and Fe3O4 micropowders modified with ZnO (d−f). The Fe3O4-functionalized MOFs (also called magnetic framework composites56) are responsive to a magnetic field (g). Both the scale bars (a, d) are 5 μm. Complete information is available in the Supporting Information (Figure S1). D



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Figure 4. SEM images of the MOFs prepared with different ligands for 1 h at 95 °C. In the insets: the corresponding ligand structure. From left to right: MOF-5, IRMOF-3, MOF-4, IRMOF-8, and IRMOF-10.

Figure 6. MOF-4 nucleation time as a function of (a) synthesis temperature and (b) ZnO nanoparticle concentration added to the starting MOF precursor solution. These results were obtained with in situ SAXS measurements. The nucleation of MOF-4 starts within a few minutes after the addition of the seeds at 95 °C and remains below half an hour at room temperature, while maintaining the crystal morphology (SEM images in the inset refer to MOF-4 obtained at 30, 50, and 95 °C). Increasing the quantity of NPs decreases MOFs nucleation time: the trend shows a plateau for the nanoparticle concentration above 80 μL/5 cm3 precursors solution.

Supporting Information). These results confirm that the addition of ZnO particles as nucleating agents within the MOF precursor solution has the dual benefit of decreasing the nucleation time and reducing the temperature required for MOFs synthesis. To investigate if the primary role of ZnO nanoparticles was to facilitate ligand deprotonation, an in situ NMR study was performed. Previous work has shown that addition of a base (e.g., triethylamine) can significantly increase the kinetics of MOF formation.51 Our result shows that the extent of the ligand deprotonation by the ZnO nanoparticles is only about 1/ 5 of that observed for triethylamine (Figure S10, Supporting Information). This result suggests that the induced rapid formation of MOF crystals by ZnO is not simply driven by an increase in the pH of the bulk reaction solution.

Figure 5. (a) Scheme of the setup used for small-angle X-ray scattering (SAXS) in situ measurements: the X-ray beam is passed through a cell filled with the MOF precursor solutions, and the scattered radiation is registered by the detector. The cell is attached to a heating microscope stage (Linkam cell). (b−d) MOFs nucleation and growth at 95 °C as a function of time when nucleating agents (ZnO NPs) are added to the precursor solution. SAXS-based diffraction patterns of MOF-5, IRMOF-3, and MOF-457 as a function of time show that nucleation starts within minutes after addition of the seeds.

tion, Figure S8). An example of these data is shown in Figure 6a. We note that attempts to grow MOF crystals at room temperature without the presence of ZnO seeds resulted in no crystal detection even after days of reaction (Figure S9, E



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heterogeneously seeded MOF closely approaches that of the MOF synthesized via traditional solvothermal methods. Further validation of this assumption was obtained with energy-dispersive X-ray spectroscopy. These data show that the X-ray emission peaks attributable to zinc (Kα: 8.637 and Lα: 1.012 keV) and oxygen (Kα: 0.523 keV) proportionally decreased with the synthesis temperature (Figure S16 of the Supporting Information). The difference in Zn and O content can be related to ZnO nanoparticles’ contribution to the mass of the crystals. As expected from our data at 95 °C, the content of Zn in the MOF closely matches MOF prepared using the traditional solvothermal method. Accordingly, the specific surface area of the MOF grown from ZnO seeds is a function of the synthesis temperature. ZnO Nanoparticles for Controlled MOF Growth. To explore the potential of this synthetic protocol to control MOF nucleation, a paper support was used. To date, the growth of MOFs (HKUST-1 and Zn2(adc)2(dabco)2) on a paper support has been achieved by employing inkjet printing by Terfort’s group.60 However, in this present work, we sought to ascertain if MOF formation could be controlled on a paper support using the heterogeneous nucleation protocol. Accordingly, we immersed one end of a strip of filter paper in MOF precursor solution and the other end in a dispersion of ZnO nanoparticles in DMF to induce crystal formation at the area where the respective solutions mix (Figure 8). A lateral flow of the MOF precursors as well as the nanoparticles was induced by capillary forces, and the nucleation of MOFs could be observed at the interface between MOF precursors and ZnO nanoparticles. To experimentally confirm these observations, we performed EDX mapping on the filter paper washed with THF. The region with the MOF crystals presents a higher concentration of Zn; this can be explained considering the increased amount of Zn as part of the Zn-based MOF (Figure 8c,d, and Figure S17 in the Supporting Information). The region with the crystals grown on the paper fibers was further investigated at a higher magnification (Figure 8e−h); while the Zn signal overlaps the shape of the MOFs cubic crystals, the oxygen and carbon signals come evenly from the MOF crystals and the fibers (Figure S18, Supporting Information). Thus, we conclude that MOF nucleation and growth were the direct result of contact of the ZnO seeds and MOF precursors at the solvent front. Indeed, this is the first time that the growth of MOFs on paper was achieved by simply controlling the position of the ZnO nanoparticles. We further investigated the potential of using ZnO seeds to precisely control the position of MOF crystals within microchannels. A 300 μm microchannel in glass was, therefore, coated with a ZnO film using a syringe pump for a dip-coating deposition. The microchannel was thermally treated and then injected with the seeded MOF solution to induce the crystal growth. All of the IRMOFs tested above were successfully grown in the microchannels. A specific example is shown in Figure S19c (Supporting Information), which shows an SEM image of truncated crystals on the cross section of the channel. A similar procedure was used with a microcontact printing technique.13 In this case, a PDMS mold with micrometric channels was pressed onto a ZnO-coated silicon substrate. The seeded MOF solution was introduced by multiple injections through holes at the channel ends at room temperature (see Figure 9a−e). The introduction of a solution already containing the nanoparticles was necessary in order to induce an effective MOFs nucleation. After removing the solvent through vacuum

We also assessed the dependence of the quantity of ZnO nanoparticles added to the starting solution on the rate of crystal growth. Using a fixed synthesis temperature of 30 °C, the nucleation of MOF-4 was monitored by in situ SAXS measurements, while the volume of the ZnO nanoparticles dispersion was varied between 5 and 140 μL into 5 mL of the precursor solution. Figure 6b shows that, as the quantity of nanoparticles increases, the crystal nucleation time decreases and a plateau (of around 5 min) can be detected for NP quantities exceeding 80 μL. Moreover, increasing the quantity of ZnO nanoparticles in solution resulted in a concomitant increase in the number of MOF crystals of decreased size (Figures S11−S13 of the Supporting Information). The low deprotonation efficiency of the ZnO nanoparticles, together with the direct correlation between the quantity of nanoparticles and the number of MOF crystal formed, supports a heterogeneous nucleation mechanism.58,59 We assessed the surface area of the MOFs synthesized via heterogeneous nucleation by performing 77 K N2 isotherms. Figure 7 shows isotherms collected on MOFs synthesized in

Figure 7. BET measurements of ZnO seeded IRMOF-3 obtained at different synthesis temperatures (from 25 to 95 °C) compared to IRMOF-3 synthesized using the classical solvothermal methods, 95 °C*). The BET measurement on the pristine ZnO nanoparticles is presented in Figure S15 (Supporting Information). The solid and hollow circles represent adsorption and desorption points, respectively. Lower synthesis temperatures correspond to lower value of MOFs surface area: this evidence can be explained considering that smaller crystals are obtained at lower temperature. Therefore, the ZnO nanoseeds occupy a major volume with respect to the bigger frameworks obtained at higher synthesis temperatures. This hypothesis is further confirmed by EDX measurements (Figure S16 of the Supporting Information).

the temperature range of 25−95 °C (BET plots as Figure S14, Supporting Information). The isotherms are best described as Type 1 in shape, essentially analogous to their solvothermally synthesized counterpart. BET analysis of the collected isotherms shows that the surface area increases with reaction temperature. This trend can be attributed to the nanoparticles/ MOF ratio. Given that these are gravimetric measurements, a higher ratio of nanoparticles/MOF will reduce the measurable surface area. Thus, as the crystal size increases with synthesis temperature (Figure S8 of the Supporting Information), the nanoparticles/MOF volume ratio decreases, resulting in higher surface areas. We note that, at 90 °C, the surface area of the F



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evaporation, the mold was released, leaving a well-anchored MOF pattern on the porous ZnO substrate surface (observation conducted by SEM after washing the substrate). We observed that the separate use of ZnO coating and ZnO particles provides a low density of crystals. We note that vacuum application drives the precursor solution to the channel edges, where the nucleation of MOFs starts (Figure 9g−i). As a consequence, for one injection of precursor solution, crystals were detected only at the outline of the channels, providing evidence that the crystal formation can be spatially controlled within the microfluidic channels due to the seeding process. Using multiple injections (up to 4), a fully covered channel can be obtained (Figure 9l−n). In the absence of ZnO nanoparticles, MOF crystals were not formed further, validating the need for NPs to nucleate MOFs at room temperature (Figure S20, Supporting Information). These experiments clearly show that this heterogeneous seeding protocol offers a facile method for producing Zn-MOF patterns using low-cost equipment, such as custom-built microfluidic devices. To demonstrate that the functional properties of MOFs grown in controlled locations were retained, and to highlight the potential application of such devices, we performed a molecular sequestration test. MOFs prepared with aromatic linkers have proven to be effective for polycyclic aromatic hydrocarbon (PAH) sequestration due to the π−π interactions between the aromatic ring of the framework and the polluting aromatic agent.61,62 A glass microchannel (300 μm × 12 cm) was decorated with IRMOF-8 using the ZnO NPs, as proposed above (Figure S19); then, 1,2-benzanthracene was used as a model to test the PAH sequestration capability.61 IRMOF-8 was selected due to the high affinity of the organic linker toward benzanthracene.63 Accordingly, a 350 nM benzanthracene solution was prepared; the fluorescence spectra of the diluted 1,2benzanthracene solution was confirmed to be stable over time, as previously reported,61 and corresponded to emissions reported in the literature,64 and the resulting solution was pushed through the MOF lined microchannel at a controlled flux of 5 μm min−1 (Figure 10a). Fluorescence emission spectra of the solution before and after sequestration by MOFs were collected to evaluate the benzanthracene concentration (Figure 10b). This benzanthracene concentration range was selected because the fluorescence intensity is proportional to the

Figure 8. (a, b) Schematics of the experiment conducted on paper. The paper ends are immersed into two different vials: one containing MOF precursor solution (green solution) and the other filled with a dispersion of ZnO nanoparticles in DMF (0.5 mL of Aldrich ZnO dispersion in 1 mL of DMF, teal solution). Because of capillary absorption, the species ascend the paper from both sides: MOF nucleation starts along the interface between MOF precursors and ZnO nanoparticles. (c, d) SEM image and EDX measurements of the interface between MOF precursors (left) and ZnO nanoparticles (right), respectively. The MOFs grow on the fiber support as shown in the left side of the strip. Morphological and elemental analysis confirms the identity of the MOF crystals only along the interface (Zn: green; O: red; C: white). (e−h) The region with the MOFs is presented at 4 different magnifications.

Figure 9. (a−e) Schematic illustrations of the MOFs bottom-up positioning procedure using microfluidic channels as precursor solution confiners. The growth medium added with ZnO NPs is multiple injected inside the channels pressed on the substrate through holes at the channel ends. (f) Example of a pattern obtained. (g−i) SEM images at increasing magnification of MOFs pattern following the outline of the mold. This pattern was obtained with a single injection, followed by solvent vacuum evaporation. (l−n) SEM images at increasing magnification of MOFs pattern obtained after 4-times injection of the growth medium. G



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naphthalenedicarboxylic acid organic link of IRMOF-8 and PAH)63 and the enhanced diffusion in microchannels.65

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CONCLUSION A new protocol for the synthesis and positioning of Zn-based MOFs is presented here. The approach consists of using ZnO nanoparticles as seeds to induce heterogeneous nucleation of the frameworks. The versatility of this approach was demonstrated by using different linkers (terephthalic acid, 2aminoterephthalic acid, 1,3,5-benzenetricarboxylic acid, 2,6naphthalenedicarboxylic acid, 4,4′-biphenyldicarboxylic acid) to rapidly form a range of different MOFs. In situ SAXS measurements confirmed that the introduction of nucleating agents into MOF-precursor solutions offers several different advantages, including faster nucleation times (from 2 to 60 min) and moderate synthesis temperatures (from 25 to 95 °C) than achieved previously with any heterogeneous nucleation approach.18 However, the surface area of the composites shows a dependence on the synthesis temperature and ranges from 1300 to 2000 m2 g−1 for IRMOF-3. Nevertheless, the protocol offers the versatility to control the MOF crystal size as well as the nucleation time depending on the concentration of nanoparticles. We posit that this methodology provides an effective protocol for miniaturized device fabrication based on MOFs. This was demonstrated by controlling the growth on paper substrates, within glass and PDMS microchannels. Remarkably, the MOF growth has been shown in two different configurations obtaining MOF decorating the borders of the microfluidic channels or homogeneously growing on their surface. The practical use of such material was explored with 1,2-benzanthracene sequestration in a microfluidic device. In conclusion, the short processing time, low temperature synthetic conditions, low cost of equipment, as well as the easy control over the positioning of several Zn-MOFs make the presented procedure a viable industrial protocol for MOF synthesis and bottom-up patterning for MOF-based device applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 10. (a) Schematic illustration of the PAH sequestration experiment: the 1,2-benzanthracene solution is pushed through a microchannel decorated with IRMOF-8; the MOF crystals were grown from ZnO NPs as heterogeneous nucleation seeds. (b) Emission spectra of the PAHs solution at the μchannel entrance (start) and at the μchannel exit (end) collected in the 350−600 nm range using a 347 nm excitation wavelength. (c) Fluorescence emission at 409 nm of the PAHs solution before and after IRMOF-8 sequestration in the μchannel. A calibration curve correlating the fluorescence intensity and the molarity of the benzanthracene was used to calculate the PAHs concentration in solution after MOFs sequestration (180 nM).

EDX, SEM, optical images, in situ NMR, SAXS measurements, and fluorescent calibrations are reported in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.B.). *E-mail: [email protected] (P.F.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

concentration of the PAH (calibration curve proposed in the Supporting Information, Figure S21). The plot of the fluorescence emission at 409 nm versus concentration of 1,2-benzanthracene is reported in Figure 10c. The results showed that the MOF-filled channel sequestrated approximately 50% of the benzanthracene from the starting solution, confirming that the MOF-based microdevice could efficiently sequestrate 1,2-benzanthracene from a solution much faster than the previous reported experiments.61 This can be explained by the strong π−π interaction between the 2,6-

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this research was undertaken on the SAXS beamline at the Australian Synchrotron, Victoria, Australia, with the support of Dr. Nigel Kirby and Dr. Adrian Hawley. P.F. acknowledges the Australian Research Council (ARC, DECRA Grant H



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DE120102451), the AMTCP scheme, and the OCE science team for the Julius Award.

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