Article Cite This: Cryst. Growth Des. 2018, 18, 356−363
pubs.acs.org/crystal
Flexible Metal−Organic Framework-Bacterial Cellulose Nanocomposite for Iodine Capture Ai-Nhan Au-Duong and Cheng-Kang Lee* Department of Chemical Engineering, National Taiwan University of Science and Technology, 43Keelung Rd. Sec. 4, Taipei, Taiwan 106 S Supporting Information *
ABSTRACT: A flexible nanocomposite pellicle consisting of bacterial cellulose (BC) and zeolitic imidazolate framework-8 (ZIF-8) nanoparticles was prepared by sequentially soaking the BC pellicle in zinc ion and 2-methylimidazole (2-MI) solutions. The quality and quantity of ZIF-8 in situ crystallized in the pellicle was significantly improved when polydopamine, oxidatively self-polymerized dopamine, coating was applied prior on the surface of BC nanofibers. The polydopamine coating not only insulates the cellulose structure from competitive complex interaction with zinc ions employed for ZIF-8 formation but also can provide a photothermal feature to the ZIF-BC composite. ZIF-8 was well embedded inside the BC pellicle with a uniform shape and size of about 127 nm. Approximately, 70% (w/w) of the as-prepared nanocomposite (BC@ Dopa-ZIF) is ZIF-8, which demonstrated a high iodine uptake capacity from vapor (1.87 ± 0.18 g I2/g) and aqueous I2/KI solution (1.31 ± 0.02 g I2/g). The iodine captured by BC@Dopa-ZIF could be effectively released by irradiation of laser light of 808 nm due to the photothermal effect. The photothermal regeneration made BC@Dopa-ZIF maintain 99% and 87% of its initial iodine uptake capacity at the second and sixth use, respectively.
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INTRODUCTION Metal−organic frameworks (MOF) materials, a class of porous hybrid materials with a defined cage structure that allows selective capture and release of specific compounds, have been developed for potential applications in various areas.1−3 However, the powder-like MOFs crystals were found to be difficult in implementation for adsorption operation due to their inherent light weight and submicron size. Therefore, confining MOFs inside a solid support without changing their inherent selectivity and uptake capacity may be a practical alternative for MOFs based adsorption processes to be successfully operated. MOFs containing composite membranes based on rubbery polyurethane or poly(lactic acid) have been developed and retain all the advantages of MOFs particles but are unable to work well in aqueous phase due to the hydrophobic nature of the polymeric membrane. 4,5 A polyamide/MOF nanocomposite membrane has shown its easy deployment in an aqueous system; however, a multistep preparation was required to obtain the desired layer-by-layer structure.6 Bacterial cellulose (BC) pellicle is generally formed at the air−liquid interface by cellulose-producing bacteria such as Acetobacter xylinus. It has a highly hydrated structure with >95% water content and mainly consists of randomly webbed cellulose nanofibers of 20−30 nm in diameter. BC nanofiber possesses a highly crystalline structure that provides the BC pellicle with very good physical and chemical stability. The nanostructured pellicle also provides a very high specific surface © 2017 American Chemical Society
area that makes the BC pellicle a very popular matrix for accommodating various nanoparticles (NPs) for the production of functional composites. Specifically, BC-hydroxyapatite as a scaffold for cartilage regeneration,7 BC-AgNp for antimicrobial wound dressing,8 BC-Ag, Pt, Pd NPs as catalytic membranes in fuel cells,9 BC-iron oxide NP as magnetic membranes,10 and a BC-carbonaceous sphere for heavy metal removal11 have been reported. To the best of our knowledge, the preparation and application of a MOFs-BC composite (BC@MOFs) have never been reported yet. In this work, we report a straightforward in situ growth method for the preparation of a flexible BC@MOFs pellicle composite. Several MOFs have demonstrated their potential for practical applications in removal of radioactive iodine isotopes that are liberated during nuclear fuel treatment and nuclear accidents due to their correct pore size and strong iodine bonding interaction.1,12−20 A very stable pillared and double-walled zinc(II) MOF has shown its substantially higher iodine loading kinetic than that of activated carbon particle, and zeolite 13X in cyclohexane.21 Novel 3D MOF (JLU-Liu14) has also exhibited its ability in controlled uptake and release of iodine in alcohol solution.22 The zeolitic imidazolate framework-8 (ZIF-8), one of the easily prepared and biocompatible MOFs, can seize iodine in its cage with binding energy approximately 3-fold Received: September 24, 2017 Revised: November 26, 2017 Published: December 7, 2017 356
DOI: 10.1021/acs.cgd.7b01360 Cryst. Growth Des. 2018, 18, 356−363
Crystal Growth & Design
Article
Figure 1. FE-SEM images of ZIF-BC composites prepared with and without polydopamine (PDA) coating. Without PDA coating: (A) BC, (C) BC@ZIF (2-MI/Zn = 20), (E) BC@ZIF (2-MI/Zn = 40), and (G) BC@ZIF (2-MI/Zn = 120). With PDA coating: (B) BC@Dopa, (D) BC@ Dopa-ZIF (2-MI/Zn = 20), (F) BC@Dopa-ZIF (2-MI/Zn = 40), and (H) BC@Dopa-ZIF (2-MI/Zn = 120). by blotting with a paper towel. The blotted pellicle was dipped into 10 mL of 10 mM Tris-HCl (pH 8.5) buffer for 12 h to have dopamine being self-polymerized on cellulose nanofibers. After polydopamine (PDA) self-polymerization treatment, the pellicle was rinsed thoroughly with deionized water to remove the unreacted dopamine and loosely adhered PDA particles. In Situ Formation of ZIF-8 Crystals in BC Pellicle. BC pellicles or PDA-coated BC pellicles were immersed into zinc nitrate hexahydrate solution (250 mM, 0.25 mL/cm2 pellicle) for 1 min under a vigorous vortex. 2-Methylimidazole (1.25 M, 10 mL) was then immediately poured into the mixture. Alternatively, different molar ratios of 2-MI/ Zn solutions were prepared by changing the concentration of 2-MI solution. ZIF-8 crystals were grown at room temperature for 12 h under rotary shaking of 150 rpm. The ZIF-8 embedded pellicle was then washed 3 times with DI water and lyophilized. The lyophilized nanocomposite was designated as BC@ZIF or BC@Dopa-ZIF and activated at 150 °C for 12 h before used to adsorb iodine. Iodine Uptake. The 1 × 1 cm2 of BC@Dopa-ZIF nanocomposite was placed in 1 mL of iodine solution (1X PBS of pH 7 with 0.1 M KI) with a concentration that varied from 1−10.4 mg/mL at room temperature for 24 h. At the end of iodine uptake, absorbance at 350 nm of the supernatant solution was measured to estimate the amount of iodine captured based on a standard iodine calibration curve prepared in pH 7, 1X PBS containing 0.1 M KI. For the uptake of iodine vapor, BC@Dopa-ZIF was hung in a tightly capped glass bottle containing 0.02 g of solid I2 for 1 cm2 of BC@Dopa-ZIF. The iodine vapor was generated by incubating the iodine containing bottle at 70 °C. After completion of iodine adsorption, BC@Dopa-ZIF-I2 was then air-dried at room temperature for 24 h to remove the iodine molecules not confined in the cage of ZIF-8. The amount of iodine captured from iodine vapor was estimated by weight gain of the nanocomopposite. Regeneration of Iodine Loaded Nanocompoiste. After iodine uptake, the BC@Dopa-ZIF-I2 was placed in a test tube containing 3 mL of ethanol for extraction regeneration. Usually, 6−8 repeated batch extractions with 25 min for each batch were carried out, and no I2 could be detected in the supernatant of the last extraction batch as measured by a UV−vis spectrophotometer. Six cycles of I2 adsorption and regeneration were carried out to evaluate the operational stability of the nanocomposite. For photothermal regeneration, near-infrared laser light irradiation (1500 mW, 808 nm) was spotted uniformly over the whole piece of BC@Dopa-ZIF-I2 for 1 min and repeated for 3 times. After regeneration, BC@Dopa-ZIF was washed 3 times with deionized water and lyophilized overnight. Lyophilized BC@DopaZIF was activated by heating at 150 °C for 12 h before carrying out the next cycle of iodine uptake. Characterization. Scanning electronic microscopy (SEM) (JEOL, Japan, JSM-6500F) was performed at an accelerating voltage of 15 kV for observing the size and surface morphology of the nanocomposite
higher over charge-transfer complexes on other organic adsorbents.1 Moreover, ZIFs can also work as efficient and economical adsorbents for iodine removal in aqueous solution which has a negative impact on iodine capture by other hydrophilic adsorbents.23 By taking advantage of the high iodine uptake capacity of ZIF-8, the ZIF-BC nanocomposite (BC@ZIF) was employed for iodine uptake from vapor and aqueous solution. However, zinc ions, one of the main ingredients for ZIF-8 preparation, have a tendency to form complexes with the hemiacetal oxygen atom and hydroxyl groups of the anhydroglucose unit of cellulose as reported elsewhere24 that might interfere with the in situ ZIF-8 crystallization on the surface of cellulose nanofibers. It was recently reported that polydopamine (PDA) can be easily coated on various substrate surfaces via oxidative self-polymerization of dopamine under mild alkaline pH.25 PDA coating on the surface of BC nanofibers has also been achieved for the preparation of AgNP decorated BC pellicle.26 In addition, a PDA coating has been reported to have a latent photothermal feature that enables one to immediately elevate the local temperature upon irradiation with a nearinfrared laser light.27,28 In this work, we demonstrate that wellshaped ZIF-8 nanocrystals can be uniformly formed on the surface of BC nanofibers when a PDA coating is applied in advance. ZIF-8 embedded in a PDA-coated BC pellicle (BC@ Dopa-ZIF) retains its high iodine uptake capacity from iodine vapor and I2/KI aqueous solution. The captured iodine can be released not only by ethanol extraction but also more effectively by near-infrared (NIR) laser light irradiation due to the photothermal effect. The regenerated BC@Dopa-ZIF is also studied for its reusability for iodine capture.
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EXPERIMENTAL SECTION
Materials. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 2methylimidazole (2-MI) were purchased from Sigma−Aldrich. Iodine (99.8% ACS) was obtained from Acros Organic. The bacterial cellulose pellicle was obtained from Harvest Belle Biotech Co. (New Taipei City, Taiwan) and further purified by thoroughly washing with deionized water. All other chemicals were reagent grade. Preparation and Characterization of Iodine Loaded ZIF-8. Polydopamine Coating on BC Pellicle. The 1 × 1 cm2 never-dried BC pellicle (10 pieces) was first immersed into 5 mL of 5 mg/mL dopamine hydrochloride solution with vigorous stirring at room temperature. After 2 h stirring, the pellicle soaked with dopamine solution was taken out and the excess amount of solution was removed 357
DOI: 10.1021/acs.cgd.7b01360 Cryst. Growth Des. 2018, 18, 356−363
Crystal Growth & Design
Article
Figure 2. XRD patterns (A), BET analysis (B), TGA (C), and typical tensile stress−strain curves (D) of pristine BC, BC@Dopa, BC@ZIF(2-MI/Zn = 120), and BC@Dopa-ZIF(2-MI/Zn = 120). BC pellicle. A thermogravimetric analyzer (TGA, Model Diamond TG/DTA, PerkinElmer) was employed to measure the decomposition temperature (in range of 50−700 °C) of the samples under an air atmosphere with a heating rate of 10 °C/min. The average size of ZIF8 was determined from SEM images by using ImageJ software. Powder X-ray diffraction (XRD) was applied to the samples loaded onto a quartz plate in a Bruker D2 Phaser, X-ray Diffractometer using Cu Kα radiation (30 kV, 10 mA, and λ = 1.5406 nm) about 2θ with a scanning speed of 3.5°/min in a step of 0.05° over the range of 10− 60°. UV−vis spectra were measured by using a JASCO V-530 spectrophotometer for iodine concentration estimation. Brunauer− Emmett−Teller (BET) measurements were executed using nitrogen sorption at 77 K up to 1 bar by Belsorp-max (Japan). Samples were activated by heating under vacuum at 150 °C for 12 h prior to measurement.
increasing the 2-MI/Zn ratio would effectively reduce the size of particles formed in the BC pellicle. The size reduction upon increase of 2-MI/Zn ratio for ZIF-8 crystallized in aqueous solution has also been observed by Kida et al.29 and explained based on the fact that a higher nucleation density is generated at higher 2-MI/Zn ratio in the early stage of ZIF-8 crystallization, leading to a decrease in the average crystal size. Zinc ion is known to have a tendency to complex with the hemiacetal oxygen or hydroxyl group of an anhydrofructose unit of cellulose.30 When the BC pellicle is first soaked in zinc ions solution for the preparation of the BC@ZIF composite, a higher zinc ions concentration will be created near the cellulose surface due to the complexation interaction. Once 2-MI was added, the 2-MI/Zn ratio near the cellulose surface will be much lower than that of bulk solution. In other words, the actual 2-MI/Zn ratio on the cellulose surface will be much lower than 20 and 40 for the cases of BC@ZIF composites prepared at a bulk 2-MI/Zn ratio of 20 and 40, respectively. It has been observed that zinc hydroxide and basic zinc nitrate byproducts or amorphous compounds will be formed along with ZIF-8 at a 2-MI/Zn ratio lower than 40.29 Thus, the poor quality and quantity of ZIF-8 particles observed on the surface of a pristine BC pellicle at a 2-MI/Zn ratio of 20 and 40 can be ascribed to the significantly reduced 2-MI/Zn ratio near the cellulose surface. Increasing the bulk 2-MI/Zn ratio to 120 can prevent the local 2-MI/Zn ratio near the cellulose surface from
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RESULT AND DISCUSSION In Situ Crystallization of ZIF-8 in BC Pellicle. The pristine BC pellicle was first tried to accommodate ZIF-8 in situ crystallized for the preparation of a flexible BC@ZIF nanocomposite using different molar ratios of 2-MI/Zn. As shown in Figure 1, particles of average size > 500 nm appear on the surface of the pristine BC pellicle at a 2-MI/Zn ratio of 20 and nanofibers of BC can barely be observed. However, as the ratio increased to 40, particles with a broader size distribution were unevenly formed on the surface of the BC pellicle. When the 2MI/Zn ratio was increased to 120, particles of smaller size were uniformly formed on the surface of nanofibers. Apparently, 358
DOI: 10.1021/acs.cgd.7b01360 Cryst. Growth Des. 2018, 18, 356−363
Crystal Growth & Design
Article
Table 1. Porous Structure Parameters of As-Prepared Nanocomposites Determined by BET Analysis Vm (cm3 (STP) g−1) as,BET (m2 g−1) As,Lang (m2 g−1) total pore volume (cm3 g−1) mean pore diameter (nm)
BC
BC@Dopa
BC@ZIF
BC@Dopa-ZIF
ZIF-8
27.533 119.84
24.075 104.79
333.9
500.1
0.6745 22.513
0.6916 26.399
19.989 87.002 136.32 0.146 6.7143
1453.3 0.8306 2.6848
2176.7 1.2165 2.4604
dropping too low so that small, but well-formed, ZIF-8 particles could be crystallized on the BC pellicle (Figure 1G). In order to insulate the complexation reaction between zinc ions and the cellulose surface, oxidative self-polymerization of dopamine was employed to generate a PDA adlayer on the surface of cellulose nanofibers of the BC pellicle (BC@Dopa). As shown in Figure 1A,B, the smooth surface of nanofibers in the pristine BC pellicle disappeared but with nanoparticles formation along nanofibers when self-polymerized dopamine coating was applied to the BC pellicle. The morphology and size of the pristine nanofibers (56 ± 17 nm) were not much affected after PDA coating (57 ± 12 nm) as observed in the FESEM image. Evidently, the nanoparticles observed on the surface of nanofibers resulted from the self-polymerization of dopamine. The PDA adlayer on the BC nanofibers surface was expected to insulate the complexation interaction between zinc ions and cellulose so that in situ formation of ZIF-8 will not significantly be interfered. As shown in Figure 1D,F, the surface density and uniformity of ZIF-8 formed on the PDA-coated pellicle are much higher than those on the pristine BC pellicle. In addition to the insulation effect, the affinity interaction between ZIF-8 and PDA itself as reported by several researchers31 may also contribute to the well-formed ZIF-8 on the PDA-coated pellicle. As the 2-MI/Zn ratio increased to 120 (Figure 1H), well-defined rhombic dodecahedron shaped ZIF-8 particles with a uniform size of 127 ± 14 nm were observed on the PDA-coated BC pellicle. Apparently, the smaller size of ZIF-8 obtained is due to the increased 2-MI/Zn ratio which is consistent with that reported by Kida et al.31 In addition to the surface, an abundance of ZIF-8 particles were observed in the cross section of the PDA-coated BC pellicle (Figure S1). Evidently, the PDA-coated surface of BC nanofibers is very favorable for the in situ formation of ZIF-8 which can be well embedded inside of the BC pellicle structure. Therefore, the ZIF-8 embedded PDA-coated BC pellicle (BC@ Dopa-ZIF) prepared at a bulk 2-MI/Zn ratio of 120 was employed for further characterization and iodine uptake investigation. To further confirm the successful preparation of ZIF-8 embedded in the BC pellicle, powder X-ray diffraction (XRD) analysis was employed. As shown in Figure 2A, all characteristic peaks assigned to BC disappeared in the samples of BC@Dopa and BC@Dopa-ZIF. This demonstrates that PDA coating can well shield BC nanofibers in BC@Dopa. On the other hand, the XRD pattern of BC@Dopa-ZIF is consistent with that of ZIF-8 which reveals the existence of crystalline ZIF-8 nanoparticles in BC@Dopa-ZIF. This is in good agreement with what is observed in the FE-SEM images (Figure S1) that uniform ZIF-8 nanoparticles were embedded in BC@Dopa. In contrast, the BC@ZIF shows a strong unknown peak beside the feature peaks of crystalline BC and ZIF-8. In other words, a byproduct was generated during the preparation of BC@ZIF. As discussed earlier, the formation of byproduct may be ascribed to the complexation interaction of zinc ions toward
cellulose that reduces the 2-MI/Zn ratio on the surface of cellulose nanofibers, leading to the formation of zinc hydroxide and basic zinc nitrate byproducts or amorphous compounds.31 Brunauer−Emmett−Teller (BET) analysis using nitrogen sorption was also employed to determine the surface area of the as-prepared nanocomposites. As illustrated in Figure 2B, only ZIF-8 and BC@Dopa-ZIF show a large increase in N2 sorption at a low relative pressure (80%) could be observed at ∼300 °C for BC and BC@ Dopa which resulted from the decomposition of the cellulose structure. In contrast, only ∼30% weight loss was observed for BC@Dopa-ZIF at 300 °C, followed by a continuous weight loss until ∼500 °C. Apparently, the slow weight loss observed in the range of 300−500 °C may be attributed to the degradation of the organic ligand, 2-MI of embedded ZIF-8. On the basis of the observed weight loss, the amount of 2-MI of ZIF-8 embedded in BC@Dopa-ZIF was estimated to be ∼45% (w/ w). By including the amount of zinc ions coordinated with 2MI, the amount of ZIF-8 embedded in BC@Dopa-ZIF should be higher than 45%. It was determined to be about 70% by measuring the weight gain after ZIF-8 was in situ formed in BC@Dopa as shown in Table 2. The mechanical strength of BC@Dopa-ZIF is shown in Figure 2D; the tensile strength of Table 2. Mechanical Properties and Area Density of Pristine BC, BC@Dopa, and BC@Dopa-ZIF
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sample
elongation at break (%)
tensile strength (MPa)
area density (g/m2)
BC BC@Dopa BC@Dopa-ZIF
5.00 ± 1.43 6.27 ± 1.53 5.68 ± 1.70
0.78 ± 0.18 0.91 ± 0.24 1.65 ± 0.29
14.15 ± 2.67 18.5 ± 0.68 69.44 ± 8.83
DOI: 10.1021/acs.cgd.7b01360 Cryst. Growth Des. 2018, 18, 356−363
Crystal Growth & Design
Article
the larger specific surface area possessed by a ZIF-8 nanoparticle as demonstrated in the BET results. In contrast, BC@ZIF shows a less effective iodine uptake (
