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Chemistry of Materials
MOFs Encapsulated in Photocleavable Capsules for UV-light Triggered Catalysis †
Willem P. R. Deleu, Guadalupe Rivero,¶, § Roberto F. A. Teixeira,¶ Filip E. Du Prez*,¶, Dirk E. De † Vos*, †
Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis, KU Leuven – University of Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium. ¶
Department of Organic and Macromolecular Chemistry Ghent University,Krijgslaan 281 S4-bis, B-9000, Ghent, Belgium KEYWORDS
ABSTRACT: A smart catalyst that can be activated by UV-light was prepared by encapsulating Metal-Organic-Framework (MOF) particles in microcapsules with a photocleavable polyurea shell. Addition of palmitic acid as a modulator during the synthesis of the Fe-terephthalate MOF MIL-88B(Fe) yields MOF particles with a hydrophobic surface. These particles can be successfully encapsulated via interfacial polymerization, as proven by a combination of PXRD, SEM and EDX techniques. By the incorporation of photolabile groups in the polymer shell, UV light can be employed to trigger degradation of the capsules, releasing the catalytic load. Catalyst activation triggered by UV light was monitored by recording the rates for catalytic tetramethylbenzidine (TMB) oxidation with H2O2. The reactivity could be controlled by tuning irradiation time, UV intensity or MOF loading. The activity of the capsules was up to 90 times higher after irradiation.
INTRODUCTION Polymeric microcapsules have been extensively used in the last decade in a wide range of applications in which the con1 tent needs to be shielded from the environment. Important technological developments have been achieved by their 2–4 implementation in smart materials and textiles, drug de5–7 8,9 livery devices and biosensors, self-healing materials, etc. The preparation method and the shell composition can be selected in accordance to the core material and the final applications. Among the techniques reported in literature, 10 the layer-by-layer assembly and the use of microfluidic 11 devices afford microcapsules with a high level of control over size and precise loading but in a relatively complex and very limited-scale production. Interfacial polymerization is an established method for the encapsulation of substances in the core due to its high yield and the fast, straightforward 12 procedures starting from emulsions. Polymerization occurs at the interface and the shell grows around the organic droplets, leading to the microcapsule formation. The challenge of an encapsulation approach is not only the preparation procedure but involves the mechanism of content release from the polymeric microcapsules as well. Shell rupture can be achieved by mechanical means (e.g. pressure, shear stress) but it can also happen in response to an external trigger. The latter strategy appears to be more relevant when a controlled release is required. In this respect, the light-triggered release from microcapsules can be achieved by the incorporation of photolabile groups in the shell struc13 14 ture. Previously, some of us reported a simple method for the design of photocleavable microcapsules by the incorpora-
tion of 6-nitroveratroyloxycarbonyl (NVOC) units in the polyurethane solid shell. Exposure to long-wave UV light (365 nm) caused degradation of the shell and controlled release of the encapsulated liquid. An attractive target for encapsulation are catalytic compounds, as this opens the way to triggered catalysis. The catalyst used should resist all encapsulation procedures and remain inactive as long as the microcapsules are intact. At first sight, homogeneous catalysts dissolved in the liquid core seem best suited as they would immediately be dispersed in the surrounding of the capsule after breakage. However, a potential difficulty is that they could leak, even from pristine capsules, as the liquid core can slowly diffuse through the capsule walls over a certain time period, ranging from weeks 14 to months. For instance, as much as 10 % of encapsulated dyes has been observed to leak from liquid-immersed PU capsules over a period of 2 days. Such permeation problems could be avoided if particulate catalysts are chosen. For successful encapsulation, the particles should disperse in the organic phase during capsule formation and have no further effect on the emulsion stability or on the ensuing polymerization reaction. MOFs are a class of porous materials made from metal ions or clusters held together by organic linkers. By variation of the metal, organic linker or additives, it is possible to synthesize a huge range of different MOFs. Variations in the synthesis procedure can produce changes in crystal shape, cata15–17 lytically active sites and porosity as desired. This versatility makes them well suited for encapsulation as also their outer surface can be modified to avoid interferences with the capsule formation.
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Figure 1. Synthesis and use of MOFs encapsulated in photolabile capsules: (A) hydrophobic, palmitate-modified P-MIL-88B and photolabile oligomers are added to the organic phase of the emulsion to yield (B) photolabile microcapsules with MOF crystals inside. (C) The photolabile oligomers in the shell degrade when exposed to UV light allowing buffer to reach the MOF and release homogeneous iron species that can catalyze the oxidation of TMB. Moreover, the relatively low stability of some MOFs towards water and/or acids is an advantage compared to more stable heterogeneous catalysts. They are stable enough to survive the encapsulation procedure, but after capsule synthesis a simple treatment, e.g. exposure to a buffer solution can suffice to dissolve any free MOF particles, leaving only MOF particles that are successfully encapsulated. Moreover, when UV-light triggers the disintegration of the capsules, a buffer can leach the metals from the MOF particles, turning them effectively into homogeneous catalysts, ready for action. Here, we present the first type of a smart catalyst system developed by encapsulating MOF particles in microcapsules, as shown in the overview of Figure 1. The surface modification of catalytically active MOF particles with palmitate allows their dispersion in the oil droplets of an emulsion. Polymerization at the interface of these oil droplets then results in photolabile microcapsules with MOF particles in the core. For this purpose, polyurea was selected as shell material since it can be readily doped with photolabile 14 groups. This microcapsule design features a dormant catalyst that can be activated by means of UV-irradiation. Release
of the MOFs is monitored with a test reaction in order to investigate the effect of UV-irradiation on the system.
EXPERIMENTAL SECTION MOF synthesis: MIL-88B(Fe) synthesis was adapted from 18 ® literature. Synthesis was done in a 250 ml Schott DURAN pressure plus bottle by dissolving terephthalic acid (BDC) (1.66 g), palmitic acid (12.8 g) or acetic acid (4.7 g) as modulator, FeCl3.6H2O (2.7 g) and NaOH (0.32 g) in dimethylformamide (DMF) (50 ml). After 18 h at 110 °C or 80 °C for modulated and non-modulated materials respectively, the precipitate was removed by centrifugation and washed with DMF, water and ethanol. The samples were successively dried at 60 °C and 100 °C overnight. ®
UiO-66 (Zr) was synthesized in a 1 L Schott DURAN pressure plus bottle by dissolving BDC (7.5 g), acetic acid (27 g) as modulator, ZrCl4 (10.5 g) and HCl (4.5 ml, 37%) in DMF (465 ml). After 17 h at 120 °C the precipitate was removed by centrifugation and washed twice with DMF and then twice with ethanol. The samples were first dried at 60 °C followed by drying at 150 °C overnight. For the post-synthetic modifi-
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Chemistry of Materials
cation of the UiO-66 with oleic acid; dry UiO-66 (5 g) was dispersed in water (500 g) and oleic (5 g) acid was added. The mixture was vigorously shaken for several minutes until the UiO-66 and oleic acid formed a solid second phase and no more MOF was dispersed in the water. The water was then removed and the solid phase was washed twice with heptane to remove unbound oleic acid. Microcapsule synthesis and characterization: The synthesis of the NVOC-based diisocyanate oligomer was per14 formed as reported previously . All reactants and solvents were used as received. P-MIL-88B (0.5 – 1 g) was dispersed in a mixture of ethyl acetate (12.5 ml) and ethyl phenylacetate (12.5 ml). While P-MIL-88B can be dispersed in a range of organic solvents, this particular mixture was used to favor the complete solubilization of MDI-trimer Suprasec 2020 (10 g) (MDI = methylene diphenyl diisocyanate) and of the photolabile oligomer (0.248 g), which were also added to the organic phase. An 1% aqueous solution of sodium dodecyl sulfate (10 g) was placed in a double walled cylindrical glass reactor (500 mL, DuoLab Reactors Radleys) equipped with an external circulating heating bath (Julabo F12) and a fourbladed Teflon mechanical stirrer running at 450 rpm. The organic phase was then added to the reactor and homogenized for one minute. Then, a solution of 2,4,6triaminopyrimidine (TAP) (2.33 g) in water (100 ml) was added to the emulsion, at 20 ml/min addition rate. After 5 minutes at room temperature, the reactor was slowly heated up to 76 °C, during 15 minutes. The capsules were collected by vacuum filtration in a Buchner funnel, washed with water and dried in a vacuum oven for two hours at 40 °C. All filtrated liquids remained colorless, indicating that all MOF particles are retained in the capsule sample. The dried capsules were sieved (250 µm mesh) and only the fraction smaller than 250 µm was used for catalytic experiments. UV activation: All glasswork, cuvettes and stirring rods used for activation and reaction were soaked overnight in an EDTA solution (1.5 g EDTA, 200 ml H2O) before being washed with deionized water and acetone and dried. Encapsulated MOF (0.01 g) was dispersed in a buffer solution (2.76 ml, Na-acetate/acetic acid buffer, pH 4, 0.1 M, and 0.4 mM of Pluronic F127 dispersant to disperse the hydrophobic MOFs) in a clear 8ml glass vial with stirring rod. Samples were stirred at 250 rpm in a UV-reactor for defined times for activation. Covered samples of encapsulated MOF were wrapped in tinfoil to prevent exposure to UV-light. Two different UVreactors were used to activate the samples: UV-reactor 1 has eight Hitachi FL 8BL-B 8 W lamps installed with a constant inside temperature of 35 °C while UV-reactor 2 has two Philips hpl-n 440 W lamps installed with a constant inside temperature of 50 °C.
scope. Scanning Electron Microscopy (SEM) images of microcapsules and energy dispersive X-ray spectroscopy (EDX) measurements were recorded with a Quanta 200 FEG FEI scanning electron microscope equipped with the EDXsystem Genesis 4000. In both cases, a low acceleration voltage of 5 kV was used in order to prevent any damage to the capsules. FTIR spectra were obtained with an IFS 66v S Bruker spectrophotometer. TGA experiments were done on a TGA Q500 (TA Instruments) under oxygen flow, over the temperature range of 50 to 650 °C at 5 °C per minute. UV-Vis spectra of reaction solutions were collected on a Cary 5000 UV-Vis-NIR spectrophotometer ( 220nm - 700nm).
RESULTS AND DISCUSSION Synthesis of MOFs and their modification for encapsulation. The stability of a well-defined oil-in-water (o/w) emulsion is crucial for the interfacial microencapsulation to succeed. Successful dispersion of the materials in the core of a forming microcapsule requires all core materials to be apolar, i.e. the surface of the MOF particles must be rendered 19 hydrophobic. Cohen et al. created hydrophobic MOFs by attaching long alkyl chains to amine-functionalized MOFs in a post-synthetic modification (PSM). Unfortunately, this approach cannot be used for PU capsules, as residual amine groups would also react with the isocyanates used for shell formation and modification. Alternatively, MOF synthesis modulators, typically monocarboxylic acids, could provide MOFs with a hydrophobic surface. These modulators can compete with linkers to bind on the metal clusters in the MOF crystal. It is also possible to exchange linker molecules of the framework for other linkers or monocarboxylic acids 20 after synthesis . Carboxylic modulators with a low pKa can bind anywhere in the crystal, while those with a higher pKa 16 21 than the ligand tend to bind only to the outer surface. This approach of selective binding of modulators to the surface was used to create a hydrophobic surface layer on the MOF crystal by adding a long chain fatty acid as modulator during the synthesis or by exchanging the hydrophobic carboxylates onto the surface during a PSM. Meanwhile, the polarity of MOF particle’s interior remained unchanged. Modulated MIL-88B(Fe) materials will hereafter be denoted as P-MIL-88B for palmitic acid and A-MIL-88B for acetic acid as modulator respectively.
Catalytic reactions: The activated capsules in the buffered solution where transferred to quartz cuvettes and TMB (0.12 ml , 4.2 mM in DMSO) and H2O2 (0.12 ml , 4.2 mM in H2O) where added to. TMB oxidation with H2O2 was followed in real time with UV-Vis to determine the reaction kinetics. Characterization: PXRD patterns were measured on a STOE StadiP operating in high-throughput mode using CuKa radiation with a wavelength of 1.5418 Å. Measurements were carried out between 2ϴ values of 0 and 60°. Scanning electron micrographs of MOFs were recorded on Au-coated samples on carbon grids using a Philips XL30 FEG micro-
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Figure 3. Biphasic partitioning of MIL-88B/P-MIL-88B and the effect on encapsulation. Left: Biphasic system with 5 ml water, 5 ml n-heptane and 0.1 g MOF: (x) A-MIL-88B, (y) PMIL-88B. Samples were mixed and vigorously shaken for 1 minute. Pictures taken 30 s after shaking (a) and after 24 h (c). Right pictures: SEM pictures of capsules containing PMIL88B (b) and failed encapsulation of A-MIL88B, showing MOF particles on the capsule surface (d).
Figure 2. (a) Unmodulated MIL-88 forms long needle-like crystals of varying shape while P-MIL-88 (b) forms shorter crystals of a more uniform size. An additional advantage of using a modulator in the synthesis of P-MIL-88B is that a phase-pure sample was obtained with a more uniform crystal size than the reported 18 crystals produced by a non-modulated synthesis (2.40 µm ± 0.34 µm length, 1.21 µm ± 0.19 µm width, see Figure 2. These positive effects were also shown by Pham et al. in the synthe15 sis of MIL-88B-NH2 particles, modulated with acetic acid. Using palmitic acid instead of acetic acid has a drastic effect on the hydrophobicity of the MOF surface, as shown in Figure 3. Due to the long aliphatic chains, P-MIL-88B is easily dispersed in heptane while no stable suspension with water is formed, even after vigorously shaking or sonicating the sample. The opposite holds for A-MIL-88B. The modulator approach for making hydrophobic MOFs did not succeed in the case of the Zr-terephthalate MOF UiO-66 as the synthesized material contained no fatty acids after washing and drying. Instead, a post-synthetic approach was used to coat hydrophobic carboxylates on the surface, inspired by a method used to coat iron oxide particles with carboxylic 22 acids. For this, oleic acid was used instead of palmitic acid as it is liquid at room temperature, although palmitic acid can be used as well if heptane is added to dissolve it at room temperature. The UiO-66 coated with oleic acid in the PSM, will be hereafter denoted as O-UiO-66.
The presence of hydrophobic carboxylates on P-MIL-88B and O-UiO-66 was confirmed by Fourier Transform Infrared spectra (FTIR) as shown in Figure 4. Methylene (-CH2-) -1 stretching vibration bands at 2852 and 2921 cm and an -1 asymmetric methyl (-CH3-) stretching band at 2964 cm were detected in the spectrum of P-MIL-88B. The absence of -1 the C=O stretching vibration of the free acid at 1702 cm for P-MIL-88B indicates that no physisorbed palmitic acid remained in the sample. The presence of oleate on O-UiO-66 was confirmed in the same way but a new C=O stretching -1 vibration at 1702 cm appeared for O-UiO-66, showing that not all oleic acid molecules are bound to the MOF surface. The amount of palmitate present on P-MIL-88B was determined by thermogravimetric analysis (TGA). Figure S1 shows that a weight loss of 3.8 % occurs around 245 °C for PMIL-88B while this is not observed for MIL-88B. This value is in agreement with the 3.4 % excess mass of P-MIL-88B compared to MIL-88B when calculating the theoretical mass based on the iron oxide fraction obtained after TGA. In order to obtain a qualitative measure for the hydrophobicity, a glass microscope slide was coated with A-MIL-88B or P-MIL88B by applying a MOF/ethanol dispersion on the glass and letting it slowly evaporate. Contact angle measurements on the P-MIL-88 coating showed a contact angle of 140° ± 3.8° while the coating with A-MIL-88 absorbed all water drops that came into contact with the surface.
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Chemistry of Materials acetate/ethyl phenylacetate were tested as organic solvents, but only cyclohexane, toluene and the ethyl acetate/ethyl phenylacetate mixture yielded capsules with MOFs in the core. Encapsulation with the other solvents either gave samples in which the MOF ended up outside the microcapsules, or no microcapsules were formed at all. This shows that the choice of solvents for the core is crucial for successful encapsulation. When the photolabile linker (Figure 1) was added during synthesis (0.248 g per 25 ml organic solvent), its solubility was too low in cyclohexane or toluene and no capsules were formed at all. On the other hand, the mixture of ethyl acetate/ethyl phenylacetate yielded the desired photolabile microcapsules with MOFs in the core. Presence of MOF particles inside the capsules was evidenced by PXRD in Figure 5. Empty PU capsules are not crystalline and show only a very broad reflection between 15 and 30 °2θ related to the amorphous polymer. Capsules with P-MIL-88B show reflections between 7 and 12 ° 2θ as expected for the breathing MIL-88B. As washing the encapsulated P-MIL-88 with an EDTA solution did not change the PXRD pattern, it could be concluded that the MOFs are inside the capsules, and hence shielded from the EDTA solution.
Figure 4. FTIR spectra of synthesized MOFs and modulators. -CH2- stretching vibrations in P-MIL-88B and O-UiO-66 confirm the presence of respectively palmitate and oleate on the MOF while these are absent for A-MIL-88B and UiO-66. P-MIL-88B shows no free -COOH groups, indicating all palmitic acid is bound to the MOF surface while O-UiO-66 still has free -COOH groups. 23
MIL-88B is known to be a ‘breathing material’ and powder X-ray diffraction patterns (PXRD) change depending on whether the structure is open or closed. Patterns of the PMIL-88B and A-MIL88 materials initially showed a crystalline structure in between those of the closed and open conformation of MIL-88B (based on the CIF file from CCDC). Dispersing the samples in ethanol and measuring them in a wet state opens the conformation as shown by PXRD on Figure S2. This confirms the topology of P-MIL-88B and AMIL88 to be that of MIL-88B(Fe). Although both modulated MOFs adopt the open conformation, the time needed to open the structure in ethanol significantly increases from a few minutes for A-MIL-88B to nearly 24 h for P-MIL-88B. In other words, the palmitate species on the surface form a barrier that remains permeable, yet slows down the swelling process. This also indicates that few or no palmitate moieties are located inside the pores, since bulky groups in the crystal’s interior structure make it easier to open the structure or can even cause permanent porosity for N2, as for the tetra18 methylated variant of MIL-88B. Encapsulation of MOFs in microcapsules. As a first step towards encapsulating MOFs, the microcapsules were produced without the photolabile linker, starting from proce1424 dures for PU-based capsules, published by some of us. The choice of organic solvent for the core is crucial as this is the phase of the emulsion that interacts most with the MOF particles during encapsulation. Cyclohexane, toluene, chlorobenzene, mineral oil, dioxane and a 1:1 mixture of ethyl
Figure 5. PXRD of MOFs encapsulated in PU microcapsules with different core solvents: (a) empty capsules with toluene as core, (b) P-MIL-88 in capsules with toluene as core, (c) PMIL-88 in capsules with cyclohexane as core, (d) P-MIL-88 in photolabile capsules with ethyl acetate/phenyl ethylacetate as core, (e) P-MIL-88B, (f) O-UiO-66 in capsules with ethylacetate/phenyl ethylacetate as core, (g) O-UiO-66. Scanning electron microscopy (SEM) pictures show that no P-MIL-88 can be observed outside the capsules (Figure 3). OUiO-66 can also be successfully encapsulated. The PXRD pattern of microcapsules with O-UiO-66 clearly shows the original reflections of UiO-66 (Figure 5), while no O-UiO-66 crystals are observed outside the capsules on SEM pictures (Figure S3). The iron content of the P-MIL-88 containing capsules was determined by elemental analysis and TGA in order to calculate the theoretical amount of MOF entrapped in the capsules. Figure 6 shows the relation between the amount of MOF used for encapsulation (mass ratio of MOF/organic core) and the actual MOF load in the eventual capsules (g MOF/ g capsule). Depending on the solvent composition used, the MOF loading somewhat varies for a same mass ratio of MOF/ organic core, but the main parameter to control the MOF loading in the capsules clearly is the starting
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amount of MOF used for encapsulation. At MOF/ organic core mass ratios higher than 0.06, it becomes hard to keep the particles suspended in the organic phase and sedimentation of MOF particles occurs, making encapsulation impossible for such ratios.
Figure 6. Amount of MOF encapsulated as a function of the original MOF/organic core ratio used for encapsulation. Different solvents were used as core: cyclohexane (♦), 50/50 v/v mixture ethyl acetate/ethyl phenylacetate (▲), toluene (■). MOF release by UV irradiation and catalytic test reaction. The morphology of PU microcapsules was not affected by the incorporation of the photolabile units in the shell, as seen in Figure S3. Comparison of the microcapsules with photolabile units before and after UV exposure shows no visible changes in the external aspect, but important changes in the elemental composition of the surface were detected with energy dispersive X-ray spectroscopy (EDX). Although an accurate quantitative comparison is not possible, there is obviously a significant increase in the iron content on the surface of the sample after UV exposure (Figure S4). This shows that UV light does not break the capsules into pieces but rather degrades the shell, creating cracks and holes. Buffer from the reaction solution can then enter the leaky capsules and dissolve the MOFs inside, releasing dissolved iron compounds. Release of the active iron species after UV exposure was quantified by comparing the catalytic activity of irradiated samples with that of samples kept in the dark. As a test reaction, the catalyzed oxidation of tetramethylbenzidine (TMB) with H2O2 in an acetate buffer was used because this reaction results in formation of a blue chargetransfer complex (CTC), with absorption maxima at 370 nm 25 and 655 nm . The decrease of TMB concentration was monitored at 310 nm (Figure 7A). However, for H2O2 /TMB ratios higher than 0.5, the blue CTC can undergo another oxidation to the fully oxidized TMB (diimine derivate) with an absorp26 tion maximum at 450 nm . MIL-88B is an active catalyst for the reaction and behaved similarly as NH2-MIL-88B, for which this catalytic oxidation has been reported in literature 27 . As UiO-66 showed no activity in this reaction, only PU microcapsules with P-MIL-88B were further considered for the study of the triggered catalysis.
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The initial rate of CTC formation is dependent on the rate • of OH radical formation by iron species from H2O2 and on the TMB concentration in solution. As the concentrations of H2O2 and TMB are the same for all experiments, the initial rate of TMB oxidation depends only on the concentration of available iron released by UV irradiation. The initial rate of CTC formation (hereafter called v’) was measured in situ with UV-Vis spectroscopy by following the absorption at 370 nm. This v’ value was used to investigate the effect of irradiation time, UV-lamp intensity and MOF loading in the capsule. Sample A, containing 3.4 wt% of P-MIL-88B, was activated in a first type of UV-reactor with a 64 W illumination intensity for varying periods of time. A second ‘dark’ sample was covered with tinfoil and was exposed to identical experimental conditions in order to evaluate the influence of the UV-irradiation while keeping the effect of stirring and the higher temperature in the reactor (35 to 50 °C). Figure 7 B shows v’ values for the irradiated samples (blue line) and the dark samples (red line) as a function of the time in the UVreactor. It is clear that the UV-exposure time has a strongly positive influence on the reactivity of the samples. The dark samples also showed a slightly increased activity with longer times in the reactor, but the effects are not significant compared to the activity of UV exposed samples. This minimal activity may be attributed to capsules being broken after the synthesis or crushed during stirring. A blank sample without capsules was also measured; it showed no absorption at 370 nm after 2 h. If the UV irradiation would shatter the capsules into fragments, MOF release would be instantaneous and catalytic activity would instantly enhance. Instead, a gradual increase of activity is observed with longer irradiation time, pointing to a gradual degradation of the shell and release of active species. In order to investigate whether the oxidation is a heterogeneously or homogeneously catalyzed process, a sample of A was activated for 1 h in the first UV-reactor and centrifuged before reaction to remove all solid capsules and MOF particles. The sample was still reactive after centrifugation (v’ = -1 0.121 M h ), indicating that the catalyst is a homogeneous iron species. Thus, after capsule degradation by UVirradiation, the MOFs are degraded by the acetate buffer to soluble iron acetate species that can oxidize TMB with H2O2.
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Chemistry of Materials
Figure 7. Triggered catalysis with encapsulated P-MIL88B. (Left) Absorption spectra of reaction mixture containing sample A (vide infra) after different reaction times (60 min activated in UV reactor 1). (Right) initial rates v’[ m / h ] for sample A after different activation times in UV reactor 1. As shown before in Figure 7, longer irradiation times result in a higher MOF release and higher activities, but this is not the only method of accomplishing higher activity as shown in Table 1. Firstly, the amount of MOF inside the capsule can be increased, thus more catalyst is released. When comparing sample B against sample A with 6.5 wt% and 3.4 wt% of P-MIL-88 respectively, a 1.5-1.7 times higher activity is recorded, whether the samples are irradiated under the 64 W intensity of the first reactor, or under the more intense power of a second reactor (2 x 440 W Hg lamps). Secondly, a higher UV intensity can increase capsule degradation, res ulting in a faster activation of the sample. Table 1 shows that irradiating the samples with higher intensity Hg lamps in UV-reactor 2 results in more than fourfold higher v’ values for both samples A and B, while the v’ value of the dark samples remains almost the same. The 4-fold increase of v’ does not match with the 14-fold increase of power when comparing UV-reactor 1 and 2 (64 W compared to 880 W); this is ascribed to the fact that the mercury lamps in UVreactor 2 have a broader emission spectrum than the halogen lamps in UV-reactor 1 and emit less UV-light for the same amount of power.
Table 1. Initial rate values (v’) [ m / h ] for samples with different MOF loadings irradiated in different UV reactors for one h.
CONCLUSION A UV-responsive catalyst system has been designed and synthesized by encapsulating hydrophobic MOF particles in photolabile microcapsules via interfacial polymerization. Phase-pure MOFs with uniform crystal size were obtained by modulating the MIL-88B(Fe) synthesis with palmitic acid. Moreover, the presence of palmitate on the MOF surface creates a hydrophobic layer of alkyl chains around the crystal, allowing it to disperse in the emulsified organic droplets during encapsulation. This was one of several critical issues that have been tackled, others being solvent incompatibility and low encapsulation yields. Successful encapsulation of MOFs was verified with PXRD, SEM, EDX and TGA and capsules with different MOF loadings were prepared. Photolabile oligomers in the shell resulted in capsule degradation under UV-irradiation. This allowed the buffer solution to dissolve the MOFs inside and release catalytically active iron species, as demonstrated by the detection of the product derived from the catalytic oxidation of TMB with H2O2. The activity of the system could be precisely tuned by varying MOF load in the capsules, the irradiation time or the UVlight intensity. This high control over the catalytic activity is an essential feature for potential applications of reaction systems in which triggered catalysis is desired.
ASSOCIATED CONTENT Sample A
Sample B
3.4 wt%
6.5 wt%
MOF a) content
UV
dark
UV
dark
UV-reactor 1
0.049 ± 0.005
0.014 ± 0.001
0.071 ± 0.021
0.021 ± 0.003
UV-reactor 2
0.212 ± 0.019
0.017 ± 0.001
0.349 ± 0.025
0.020 ± 0.003
a)
MOF content is the wt% of MOF in the total mass of the capsule.
Supporting Information. TGA data of P-MIL-88 and AMIL-88; additional PXRD patterns of the free MOFs; additional SEM pictures of the encapsulated MOFs; EDX data; contact angle measurement; photographs of the UV-reactors. “This material is available free of charge via the Internet at http://pubs.acs.org.”
AUTHOR INFORMATION Corresponding Author †
E-mail:
[email protected], ¶
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Microcapsules of Stimuli-Responsive Polymers. J. Control. Release 2011, 149, 209–224.
Present Addresses §
Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Facultad de Ingeniería - Universidad Nacional de Mar del Plata, Juan B. Justo 4302, B7608FDQ, Mar del Plata, Argentina
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Author Contributions The manuscript was written through contributions of all authors.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT D.D.V and F.D.P. acknowledge the FWO Vlaanderen(grant G.0256.14 G.0486.12 G.0959.13) for support in the frame of a research project and the Belgian Science Policy Office Interuniversity Attraction Poles (IAP) programme in the frame of IAP 7/05 for financial support. D.D.V also thanks IWT Vlaanderen for support in the SBO program MOFShape.
ABBREVIATIONS MOF, metal organic framework; PXRP, powder x-ray diffraction; SEM, scanning electron microscopy; EDX, energy dispersive x-ray spectroscopy; TMB, tetramethylbenzidine; NVOC, 6-nitroverantroyloxycarbonyl; BDC, 1,4benzenedicarbocylic acid; DMF, dimethylformamide; MDI, methylene diphenyl diisocyanate; TAP, triaminopyrimidine; EDTA, ethylenediaminetetraacetic acid; PSM, post synthetic modification; FTIR, fourier transform infrared spectroscopy; TGA, thermogravimetric analysis; CTC, charge transfer complex.
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