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A Fast and Scalable Approach for Synthesis of Hierarchical Porous Zeolitic Imidazolate Frameworks and One-Pot Encapsulation of Target Molecules Hani Nasser Abdelhamid,*,† Zhehao Huang,† Ahmed M. El-Zohry,‡ Haoquan Zheng,*,† and Xiaodong Zou*,† †

Inorganic and Structural Chemistry and Berzelii Centre EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden ‡ Department of Chemistry, Ångström Laboratory, Box 523, SE-751 20 Uppsala, Sweden S Supporting Information *

ABSTRACT: A trimethylamine (TEA)-assisted synthesis approach that combines the preparation of hierarchical porous zeolitic imidazolate framework ZIF-8 nanoparticles and one-pot encapsulation of target molecules is presented. Two dye molecules, rhodamine B (RhB) and methylene blue (MB), and one protein (bovine serum albumin, BSA) were tested as the target molecules. The addition of TEA into the solution of zinc nitrate promoted the formation of ZnO nanocrystals, which rapidly transformed to ZIF-8 nanoparticles after the addition of the linker 2-methylimidazole (Hmim). Hierarchical porous dye@ ZIF-8 nanoparticles with high crystallinity, large BET surface areas (1300−2500 m2/g), and large pore volumes (0.5−1.0 cm3/g) could be synthesized. The synthesis procedure was fast (down to 2 min) and scalable. The Hmim/Zn ratio could be greatly reduced (down to 2:1) compared to previously reported ones. The surface areas, and the mesopore size, structure, and density could be modified by changing the TEA or dye concentrations, or by postsynthetic treatment using reflux in methanol. This synthesis and one-pot encapsulation approach is simple and can be readily scaled up. The photophysical properties such as lifetime and photostability of the dyes could be tuned via encapsulation. The lifetimes of the encapsulated dyes were increased by 3−27-fold for RhB@ZIF-8 and by 20-fold for MB@ZIF-8, compared to those of the corresponding free dyes. The synthesis approach is general, which was successfully applied for encapsulation of protein BSA. It could also be extended for the synthesis of hierarchical porous cobalt-based ZIF (dye@ZIF-67).

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INTRODUCTION Metal−organic frameworks (MOFs) are a new class of porous materials with large surface areas, tunable pore sizes, and functionalities.1−6 Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs consisting of tetrahedrally coordinated metal ions and imidazolate linkers.7 Among different ZIFs, ZIF-8, built from zinc ions and 2-methylimidazole (Hmim), has gained considerable attention due to the high surface area, and high thermal and chemical stability.7 Microporous ZIF-8 can be prepared using various methods including solvothermal method,7 chemical vapor deposition (CVD),8 and others.9 ZIF-8 has shown potential applications for carbon dioxide capture,10 hydrogen storage,11 small molecule separation,11 catalysis,12 drug delivery13 and as a cathode material for lithium−sulfur battery (S@ZIF-8).14 In order to overcome the size and diffusion limits of the micropores, it is interesting to introduce mesopores into ZIF crystals to promote mass transfer kinetics. Hierarchical porous ZIF-8 with interconnected microand mesopores can be synthesized using sodium dodecyl sulfate,15 block co-oligomer micelles,16 drug,13 or cetyltrimethylammonium bromide with histidine as cotemplates.17 © XXXX American Chemical Society

Organic dyes have been widely used for applications such as photothermal treatment,18 imaging,19 textile industry (representing 60% of world dye consumption),18 sensing,19 photoredox catalysis,20 and as solid-state dyes (SSD)21 and semiconductors.22 They have long excitation wavelengths and large absorption coefficients. Organic dyes offer high reproducibility for labeling and show well-described fluorescence resonance energy transfer (FRET) with monoexponential emission decay.23 On the other hand, organic dyes often have short lifetimes, low thermal and photochemical stability, and easily undergo photobleaching (fading).23 Therefore, it is highly demanding to improve the photophysical properties of the dyes; one such approach is to encapsulate organic dyes into an MOF.24 It has been reported that dye encapsulation into MOFs could fine-tune the emission color and efficiently enhance the fluorescence quantum yield of the dye.25 The improved performance of the dyes encapsulated in the MOFs offered new applications.26 Received: May 10, 2017

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DOI: 10.1021/acs.inorgchem.7b01191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) PXRD patterns of the products formed during the synthesis at room temperature (RT) using water as the solvent over the time (2−30 min). (b) TEM image of rice-shaped ZnO particles formed after the addition of TEA into the Zn(NO3)2 solution. Each rice-shaped ZnO particle is an aggregate of ZnO nanoparticles. (c) TEM image of the final RhB@ZIF-8 nanocrystals after 30 min reaction time. Mesopores can be observed in the ZIF-8 crystals. The molar ratio of Hmim/Zn/RhB was 10:1:0.01. respectively, and the product was collected. The reaction at the shortest time (2 min) was quenched in an ice bath to prevent further crystal growth. It was observed that a white precipitate was formed immediately after the addition of TEA into the Zn(NO3)2·6H2O solution. The white precipitates were collected before and after adding the RhB solution for further characterization. The influence of the TEA concentration was tested by changing the amount of TEA added in the synthesis from 0.70 mmol (0.1 mL) to 7.0 mmol (1.0 mL) and 14 mmol (2.0 mL), respectively, while keeping the other synthesis parameters the same. The reaction time was 30 min. To study the influence of the Hmim/Zn ratio, the synthesis was carried out using different volumes of the Hmim solution, 0.46 (1.4 mmol), 1.6 (4.8 mmol), 2.3 (6.9 mmol), 3.9 (11.2 mmol), 5.5 (16.5 mmol), 11.0 mL (33.0 mmol), respectively, while keeping the Zn2+ content the same (0.67 mmol). The corresponding molar ratios of Hmim/Zn were ∼2, 7, 10, 17, 25, and 50, respectively. The synthesis procedure was the same as described above. The same synthesis procedure was also carried out using various amounts of the dye solutions, 1.0 (2 μmol), 5.0 (10 μmol), 10 (20 μmol), and 15 mL (30 μmol) of the RhB solution and 1.0 mL of the MB solution (20 μmol), respectively, were used instead of 4.0 mL of the RhB solution. ZIF-8 was also synthesized without the addition of any dye using 2.0 mL (14 mmol) of TEA following the same procedure as described above. For comparison, ZIF-8 was also synthesized according to the reported protocol (Hmim/Zn ratio of ∼70).33 Synthesis Scale-Up of RhB@ZIF-8. A large scale synthesis of RhB@ZIF-8 was performed. Ten milliliters of TEA (70 mmol) was added to 40 mL of the Zn(NO3)2·6H2O solution (34 mmol). Then, 20 mL of the RhB solution (40 mmol) was introduced, followed by adding 115 mL of the Hmim solution (345 mmol). The reaction mixture was stirred at room temperature (RT) for 2 h. The product was collected using centrifugation (13 000 rpm, 30 min), washed with water and ethanol. The yield was ∼79% (based on the Zn salt). Postsynthetic Treatment. A postsynthetic treatment of RhB@ ZIF-8 (RhB, 20 μmol) using reflux was carried out. Typically, ca. 0.15 g of the RhB@ZIF-8 nanoparticles was refluxed in methanol (30 mL) for 2h. The material was separated using centrifugation (13000 rpm, 30 min) and washed with water and ethanol (2 × 40 mL). Synthesis of BSA@ZIF-8. A stock solution of BSA (1 mg/mL) was prepared by dispersing 50 mg BSA in 50 mL deionized water. The synthesis procedure was the same as that of RhB@ZIF-8, by using 0.10 mL (0.70 mmol) of TEA and substituting RhB by BSA. Different volumes of the BSA stock solution (1, 2, 3, and 4 mL, respectively) were applied. Synthesis of dye@ZIF-67. The synthesis procedure of dye@ZIF67 was the same as that of RhB@ZIF-8, by substituting Zn(NO3)2· 6H2O with the Co(NO3)2·6H2O solution. A total of 0.67 mmol of Co(NO3)2·6H2O, 0.7 mmol of TEA (0.10 mL), 8 μmol of the RhB (4 mL) or MB (0.40 mL) solution, and 6.9 mmol of Hmim were used. The reaction mixture was completed to 24 mL using deionized water. Characterization. The materials were characterized using powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared

The encapsulation of organic dyes into MOFs can be achieved using multistep processes,24,25,28,29 one-pot synthesis,30,31 and a perturbation-assisted nanofusion synthesis.32 Recently, our group reported a one-pot synthesis approach to encapsulate target molecules such as anticancer drugs and dyes in ZIF-8 by controlling the pH (pH = 8) of the reactant solution using sodium hydroxide (NaOH).13 The synthesis was simple, and high loadings of the target molecules could be achieved (14−20 wt %). However, the method requires a slow and controlled addition of NaOH solution and is difficult to scale up. Thus, a simpler, faster, and scalable synthesis approach for dye encapsulation and preparation of hierarchical porous ZIF-8 is required. Herein, we present a simple one-pot synthesis approach to encapsulate organic dyes into ZIF-8 (dye@ZIF-8) nanoparticles using an organic base triethylamine (TEA). Synthesis parameters including the reaction time, the base and dye concentrations, and the Hmim/Zn ratio, were investigated. Photophysical properties of the encapsulated dyes were evaluated by fluorescence spectroscopy and time-correlated single photon counting. The one-pot synthesis approach has been extended to encapsulate proteins (bovine serum albumin, BSA) in ZIF-8 and dye molecules into Co-based ZIF-67 (dye@ ZIF-67).

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EXPERIMENTAL SECTION

Materials. 2-Methylimidazole (Hmim), triethylamine (TEA, density 0.70 g/mL, 7.0 M), Zn(NO3)2·6H2O and Co(NO3)2·6H2O, rhodamine B (RhB, Figure S1) and BSA were purchased from SigmaAldrich (Germany). Methylene blue (MB, Figure S1) was purchased from Fluka (India). Deionized water was used for the preparation of all solutions. A Zn(NO3)2·6H2O solution (0.84 M) was prepared by dissolving 25 g of Zn(NO3)2·6H2O in 100 mL of deionized water. Two dye solutions were prepared by dissolving 0.10 g of rhodamine B in 100 mL of water (2.0 mM) and 0.0064 g of methylene blue in 1 mL of water (20 mM), respectively. A solution of Hmim (3.0 M) was prepared by dissolving 62.5 g of Hmim in 250 mL of water. Synthesis of Dye@ZIF-8. All synthesis experiments were carried out in a fixed final volume of 24 mL. All products were collected using centrifugation (13000 rmp, 30 min), washed using water and ethanol (2 × 40 mL), and dried overnight in an oven at 85 °C. A typical onepot synthesis of dye@ZIF-8 is presented below. In a glass scintillation vial, 0.10 mL (0.70 mmol) of TEA was added to 0.8 mL of the Zn(NO3)2·6H2O solution (0.67 mmol). Then, 4.0 mL of the RhB solution (8 μmol) was added, and followed by addition of 2.3 mL of the Hmim solution (6.9 mmol). The final molar ratio of Zn/Hmim/ TEA was 1:10:1. The reaction volume was completed to 24 mL using deionized water. The reaction mixture was stirred for 30 min before the product was collected. In order to follow the crystallization process, the reaction was stopped after 2, 5, 10, 20, and 30 min stirring, B

DOI: 10.1021/acs.inorgchem.7b01191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (FTIR), isotherm nitrogen adsorption−desorption, fluorescence spectroscopy, and time correlated single photon counting (TCSPC).34 The amount of encapsulated dyes was determined using 1H NMR. Details of the characterization techniques are described in the Supporting Information.

In order to understand the rapid kinetics of the ZIF-8 formation even at the low Hmim/Zn ratios, we followed the evolution of the pH value and the temperature of the reaction mixture during the synthesis with a Hmim/Zn molar ratio of 2 and 10 (Figure S4). The zinc nitrate solution was slightly acidic (pH = 5.8−6.7). After the addition of TEA (0.7 mmol, Zn/ TEA = 1:1), the pH value increased rapidly to ca. 10.3 and remained above 10.0 throughout the synthesis (30 min, Figure S4a). Further addition of the dye and Hmim did not make noticeable changes of the pH. The temperature increased slightly (∼2 °C) throughout the synthesis (Figure S4b). TEM and SEM images show the RhB@ZIF-8 particles have a wide size distribution from 50 to 200 nm (Figure 2 and Figure S5). ZnO nanoparticles were found in the samples with low Hmin/ Zn ratios, even though no ZnO peak was observed in the PXRD patterns. The amount of ZnO decreased with the increase of the Hmim concentration. The RhB@ZIF-8 crystals synthesized at different Hmim/Zn ratios show a broad size distribution (50−200 nm, Figure S5). In addition, mesopores of ca. 20 nm were observed in RhB@ZIF-8 crystals (Figure 2). Because ZnO does not dissolve at such a high pH (pH > 10), the rapid formation of ZIF-8 was likely via solid-to-solid transformation of the in situ formed ZnO. The TEA assisted in situ formation of ZnO nanocrystals not only shortened the synthesis time of ZIF-8, but also promoted the formation of ZIF-8 at low Hmim/Zn ratios. Solid-to-solid transformation of ZnO to ZIF-8 was previously reported using ZnO as the metal source.37 ZnO nanoneedles were observed during the synthesis of ZIF-8 when Zn carbonate basic was used as the Zn source and methanol as the solvent.38 However, this method required a higher Hmim/Zn ratio (>23) and longer synthesis time (>48 h) compared to the present TEA-assisted ZIF-8 formation (2 min). Basic conditions were required for the formation of ZnO.39 TEA plays several important roles in the synthesis of hierarchical porous ZIF-8. It promotes the formation of ZnO nanoparticles which facilitate the formation of ZIF-8 and offers deprotonation of the imidazole and dye molecules.37 The increase of the TEA amounts affected neither the crystallinity (Figure S6) nor the yields of RhB@ZIF-8, which were 79, 62, and 68% for 0.7, 7.0, and 14.0 mmol TEA, respectively. The BET surface areas obtained from the nitrogen adsorption isotherms (Figure 3a) were also similar for RhB@

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RESULTS AND DISCUSSION Triethylamine (TEA) Assisted Synthesis of RhB@ZIF-8 via Transformation of in Situ Formed ZnO Nanocrystals. PXRD patterns show that the white precipitate formed after the addition of TEA in the Zn(NO3)2·6H2O solution was zinc oxide (ZnO, ICSD #191642, Figure 1a). The ZnO crystals remained after the addition of RhB (Figure S2). TEM images show that the ZnO particles have rice-shaped morphology built of ZnO nanocrystal aggregates of ∼20 nm in size (Figure 1b, Figure S3). The PXRD patterns after the addition of RhB and Hmim show a rapid transformation of ZnO to pure crystalline RhB@ZIF-8 at RT, in less than 2 min after the addition of Hmim (Figure 1a). The crystallinity of ZIF-8 was preserved during the synthesis from 2 to 30 min. TEM images show wellshaped ZIF-8 nanocrystals with mesopores of 20−60 nm in sizes, and no ZnO crystals were observed (Figure 1c). TEA was previously used in the RT synthesis of ZIF-8, but it was directly added into the solution of Hmim (to deprotonate the linker) and no ZnO formation was reported.26 PXRD patterns show that RhB@ZIF-8 crystals could be synthesized with a wide range of Hmim/Zn molar ratios, from 2 to 50 (Figure 2a). No impurity ZnO peaks are observed in

Figure 2. TEM images of RhB@ZIF-8 using Hmim/Zn molar ratios of (a) 2; (b) 7; (c) 10; and (d) 24.

the PXRD patterns. The yields were similar for all the experiments (71 ± 4%, calculated based on the Zn salt). Previous reports showed that the Hmim/Zn ratio was important in the formation of ZIF-8.33 It was difficult to synthesize ZIF-8 with Hmim/Zn ratio 79% (based on Zn salt). Influence of the RhB Concentration. PXRD shows that high crystalline and pure RhB@ZIF-8 could be synthesized with a wide range of RhB concentrations, from an RhB/Zn ratio of 0.01 (2 μmol RhB) to 0.15 (30 μmol RhB) (Figure S11). The BET surface areas, and micro- and mesopore volumes at different RhB concentrations obtained from the nitrogen adsorption isotherms (Figure 3b) are given in Table 1. For samples obtained with low RhB amounts (2−10 μmol), the mesopore volumes were small (0.04−0.08 cm3/g). The BET surface areas and micropore volumes were similar, within the range of 1344−1384 m2/g and 0.47−0.49 cm3/g, respectively. When the RhB amount was increased to 20 μmol, both the micropore and mesopore volumes increased significantly, to 0.51 and 0.24 cm3/g, respectively. The surface area also increased slightly (1475 m2/g). Further increase of the RhB amount led to a significantly increased BET surface area (2545 m2/g) and micropore volume (0.83 cm3/g), but a decreased mesopore volume (0.17 cm3/g). SEM images show that the RhB@ZIF-8 crystals have a wide particle size distribution, 50− 200 nm (Figure S12). TEM shows that mesopores are present in all RhB@ZIF-8 samples (Figure 5). At low RhB concentrations, the mesopores are fewer and larger (10−20 nm). At a high RhB concentration (30 μmol RhB), the mesopores are smaller (3 times at 0.7 mmol TEA to by 27 times at 14 mmol TEA. The lifetime of the MB@ ZIF-8 (0.7 mmol TEA) was 1.00 ns, which is 20 times longer than that of the free solid MB (0.05 ns). The increase of lifetimes for dye@ZIF-8 is due to the reduced aggregation and deprotonation of the dye molecules via encapsulation into ZIF8 framework. Apparently, high TEA concentrations were needed to break down the aggregation of the dyes in solid forms and enhance deprotonation.30 The dye@ZIF-8 materials have large potentials for applications as solid state dyes, laser dyes, and sensors that require long lifetimes.46 The photostabilities of the free dyes and dye@ZIF-8 are given in Figure 7c−d. The photostability of dye@ZIF-8 was lower than that of the free dye. The decrease of the dye photostability is due to the increase of the dye rigidity, which is supported by FTIR (Figure S22). The enhanced rigidity of the dye molecules in ZIF-8 increased the bond energy and led to the increase of photochemical alteration upon continuous irradiation. These rigid dyes (RhB@ZIF-8 and MB@ZIF-8) are ideal for FRET.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01191. Detailed sample characterization; chemical structures of the dyes; PXRD patterns and TEM image of the reaction intermediate ZnO; PXRD patterns and SEM/TEM images of RhB@ZIF-8 and dye@ZIF-67 obtained under different conditions; pH and temperature changes during the synthesis; N2 adsorption−desorption and pore size distribution; 1H NMR spectra; FTIR spectra; lifetime (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.N.A.). *E-mail: [email protected] (H.Z.). *E-mail: [email protected] (X.Z.).

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CONCLUSIONS A novel approach for synthesis of hierarchical porous ZIFs at RT using TEA as the base and water as the solvent is reported. The addition of TEA promoted in situ formation of ZnO nanocrystals that via solid-to-solid transformation not only accelerated the formation of ZIF-8 (2 min) but also significantly reduced the amount of linker (Hmim/Zn can be reduced to 2). Dye molecules and biomolecules can be encapsulated into ZIF-8 nanocrystals in one pot during the synthesis. Hierarchical porous ZIF-8 nanoparticles with microand mesopores can be synthesized using the synthesis approach, with high crystallinity, large BET surface areas (>1300−2500 m2/g), and pore volumes (>0.5−1.0 cm3/g). The pore structure of dye@ZIF-8 can be tuned by changing the TEA or dye concentration, as well as by postsynthesis treatment using reflux. The TEA concentration affects mainly the mesopore volumes and the density of the mesopores, but not the surface areas and micropore volumes. At very low dye concentrations (2−10 μmol in 24 mL), the surface areas and pore volumes are similar for all the RhB@ZIF-8 materials. When the dye concentrations were increased (20−30 μmol), the surface areas and micropore volumes increased significantly, while the mesopore volumes decreased. The size of the mesopores decreased and the mesopore density increased with the increase of the dye concentration. The hierarchical structure could be tuned by postsynthetic treatment using reflux, leading

ORCID

Xiaodong Zou: 0000-0001-6748-6656 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Swedish Research Council (VR) and the Swedish Governmental Agency for Innovation Systems (VINNOVA). The EM facility was supported by the Knut and Alice Wallenberg Foundation.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01191 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b01191 Inorg. Chem. XXXX, XXX, XXX−XXX