Flexible MetalâOrganic Framework-Bacterial Cellulose

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Flexible Metal-Organic Framework-Bacterial Cellulose Nanocomposite for Iodine Capture Ai-Nhan Au-Duong, and Cheng-Kang Lee Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01360 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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Crystal Growth & Design

Flexible Metal-Organic Framework-Bacterial Cellulose Nanocomposite for Iodine Capture Ai-Nhan Au-Duong, Cheng-Kang Lee* Department of Chemical Engineering, National Taiwan University of Science and Technology 43 Keelung Rd. Sec. 4 Taipei, Taiwan 106 Email: [email protected] Keywords: Zeolitic imidazolate framework-8 (ZIF-8), functional bacterial cellulose, Iodine adsorption, BC based nanocomposite. ABSTRACT. Flexible nanocomposite pellicle consisted of bacterial cellulose (BC) and zeolitic imidazolate framework-8 (ZIF-8) nanoparticles was prepared by sequentially soaking 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 selfpolymerized dopamine, coating was applied in 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 photothermal feature to the ZIF-BC composite. ZIF-8 was well embedded inside BC pellicle with uniform shape and size 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

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(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 maintained 99% and 87% of its initial iodine uptake capacity at 2nd and 6th use, respectively. INTRODUCTION Metal organic frameworks (MOF) materials, a class of porous hybrid materials with defined cage structure allows selective capture and release of specific compounds, have been developed for potential applications in various areas

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. However, the powder-like MOFs crystals were found

difficulty in implementation for adsorption operation due to their inherent lightweight 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 process to be successfully operated. MOFs containing composite membranes based on rubbery polyurethane or poly(lactic acid) has been developed and retains all the advantages of MOFs particles but unable to work well in aqueous phase due to the hydrophobic nature of the polymeric membrane

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. Polyamide/MOF nanocomposite membrane has shown its easy

deployment in aqueous system, however, multi-steps 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 celluloseproducing 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 BC pellicle with very good physical and chemical stability. The nano-structured pellicle also provides a very high specific surface area that makes BC pellicle a very popular matrix for accommodating various


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nanoparticles (NP) 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 NP as catalytic membranes in fuel-cells 9, BC-iron oxide NP as magnetic membranes and BC-carbonaceous sphere for heavy metal removal

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,

have been reported. To the best of our

knowledge, the preparation and application of 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

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.

Zeolitic imidazolate framework-8 (ZIF-8), one of easily prepared and biocompatible MOFs, can seize iodine in its cage with binding energy approximately 3-fold 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 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, has a tendency to form complexes with the hemiacetal oxygen atom and hydroxyl groups of anhydroglucose unit of cellulose as reported elsewhere

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that might interfere the in situ ZIF-8 crystallization on the surface of cellulose


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nanofibers. Polydopamine (PDA) was recently reported that it 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

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. In addition, PDA coating has been reported to have a latent

photothermal feature that enable to immediately elevate local temperature upon irradiation with a near-infrared laser light

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. In this work, we demonstrate that well-shaped ZIF-8 nanocrystals

can be uniformly formed on the surface of BC nanofibers when PDA coating is applied in advance. ZIF-8 embedded in 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. EXPERIMENTAL Materials Zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and 2-methylimidazole (2-MI) were purchased from Sigma–Aldrich. Iodine (99.8% ACS) was obtained from Acros Organic. 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. 1 x 1 cm2 never-dried BC pellicle (10 pieces) was first immersed into 5 mL, 5 mg/mL dopamine hydrochloride solution with vigorously stirring at room temperature. After 2 h stirring, the pellicle soaked with dopamine solution was taken out and the


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excess amount of solution was removed by blotting with a paper towel. The blotted pellicle was dipped into 10 mL, 10 mM Tris-HCl (pH 8.5) buffer for 12 h to have dopamine being selfpolymerized 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 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 oC for 12 h before used to adsorb iodine. Iodine uptake. 1 x 1 cm2 of BC@Dopa-ZIF nanocomposite was placed in 1 mL iodine solution (1X PBS of pH 7 with 0.1 M KI) with a concentration 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 solid I2 for 1 cm2 of BC@Dopa-ZIF. The iodine vapor was generated by incubating the iodine containing bottle at 70oC. 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


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amount of iodine captured from iodine vapor was estimated by weight gain of the nanocomoposite. Regeneration of iodine loaded nanocompoiste. After iodine uptake, the BC@Dopa-ZIF-I2 was placed in a test tube containing 3 mL 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 last extraction batch as measured by 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 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@Dopa-ZIF was activated by heating at 150oC for 12h before carried out a next cycle of iodine uptake. Characterization. Scanning electronic microscope (SEM) (JEOL, Japan, JSM-6500F) was performed at an accelerating voltage of 15kV for observing the size and surface morphology of nanocomposite BC pellicle. Thermogravimetric analyzer (TGA, Model Diamond TG/DTA, Perkin Elmer) was employed to measure the decomposition temperature (in range of 50–700°C) of the samples under air atmosphere with heating rate of 10°C/min. The average size of ZIF-8 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 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 3.5o/min in step of 0.05o over the range of 10-60o. UV-Vis spectra were measured by using a JASCO V-530


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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 oC for 12 h in prior to measurement. 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 Fig. 1, particles of average size >500 nm appears on the surface of pristine BC pellicle at 2-MI/Zn ratio of 20 and nanofibers of BC can barely be observed. However, as the ratio increased to 40, particles with broader size distribution were unevenly formed on the surface of BC pellicle. When 2-MI/Zn ratio increased to 120, particles of smaller size were uniformly formed on the surface of nanofibers. Apparently, increase the 2-MI/Zn ratio would effectively reduce the size of particles formed in BC pellicle. The size reduction upon increase of 2-MI/Zn ratio for ZIF-8 crystalized in aqueous solution has also been observed by Kida et al.

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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 BC pellicle is first soaked in zinc ions solution for the preparation of BC@ZIF composite, a higher zinc ions concentration will be created near the cellulose surface due to the complexion interaction. Once 2-MI was added, the 2-MI/Zn ratio near cellulose surface will be much lower than that of bulk solution. In other words, the actual 2MI/Zn ratio on cellulose surface will be much lower than 20 and 40 for the cases of BC@ZIF


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composites prepared at bulk 2-MI/Zn ratio of 20 and 40, respectively. It has been observed that zinc hydroxide and basic zinc nitrate by-products or amorphous compounds will be formed along with ZIF-8 at 2-MI/Zn ratio lower than 40

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. Thus, the poor quality and quantity of ZIF-8

particles observed on the surface of pristine BC pellicle at 2-MI/Zn ratio of 20 and 40 can be ascribed to the significantly reduced 2-MI/Zn ratio near the cellulose surface. Increase the bulk 2-MI/Zn ratio to 120 can prevent the local 2-MI/Zn ratio near the cellulose surface from dropping too low so that small but well-formed ZIF-8 particles could be crystallized on BC pellicle (Fig. 1G). In order to insulate the complexion reaction between zinc ions and cellulose surface, oxidative self-polymerization of dopamine was employed to generate a PDA adlayer on the surface of cellulose nanofibers of BC pellicle (BC@Dopa). As shown in Fig. 1A and B, the smooth surface of nanofibers in pristine BC pellicle disappeared but with nanoparticles formation along nanofibers when self-polymerized dopamine coating was applied to 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 FE-SEM image. Evidently, the nanoparticles observed on the surface of nanofibers were resulted from the self-polymerization of dopamine. The PDA adlayer on BC nanofibers surface was expected to insulate the complexion interaction between zinc ions and cellulose so that in situ formation of ZIF-8 will not significantly be interfered. As shown in Fig. 1D and F, the surface density and uniformity of ZIF-8 formed on PDA coated pellicle are much higher than that on pristine BC pellicle. In addition to the insulation effect, the affinity interaction between ZIF-8 and PDA itself as reported by several researchers

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may also

contribute to the well-formed ZIF-8 on PDA coated pellicle. As 2-MI/Zn ratio increased to 120 (Fig. 1H), well-defined rhombic dodecahedron shaped ZIF-8 particles with uniform size of 127 ±


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14 nm were observed on 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, abundant of ZIF-8 particles were observed in the cross-section of PDA coated BC pellicle (Fig. 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 BC pellicle structure. Therefore, the ZIF-8 embedded PDA coated BC pellicle (BC@Dopa-ZIF) prepared at 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 BC pellicle, powder X-ray diffraction (XRD) analysis was employed. As shown in Fig. 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 ZIF8 nanoparticles in BC@Dopa-ZIF. This is in good agreement with what observed in the FE-SEM images (Fig. 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 by-product was generated during the preparation of BC@ZIF. As discussed earlier, the formation of by-product may be ascribed to the complexion 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 by-products 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 Fig. 2B, only ZIF-8 and BC@Dopa-ZIF shows a large increase in N2-sorption at a low relative pressure


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(80%) could be observed at ~300 oC for BC and BC@Dopa which was resulted from the decomposition of the cellulose structure. In contrast, only ~30% weight loss was observed for BC@Dopa-ZIF at 300 oC followed by a continuous weight loss until ~500 oC. Apparently, the slow weight loss observed in the range of 300 oC to 500 oC may attribute to the degradation of organic ligand, 2-MI of embedded ZIF-8. Based on 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 2-MI, 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


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strength of BC@Dopa-ZIF is shown in Fig. 2D that the tensile strength of BC@Dopa-ZIF (1.30 MPa) is approximately 50% higher than that of the BC (0.78 MPa) and BC@Dopa (0.91 MPa) at a slightly lowered strain (~4%). The enhanced strength can be ascribed to the presence of ZIF-8 in BC@Dopa as a role of physical cross-linker for the nanofibers of BC pellicle. BC@Dopa-ZIF for iodine uptake BC@Dopa-ZIF was explored for its adsorptive application and iodine was used as a model compound. For iodine vapor uptake, the dried BC@Dopa-ZIF pellicle was in contact with iodine vapor generated by heating iodine solid in a closed vessel at 70oC for 1 h. As shown in Fig. 3, the color of all the samples turned into black after iodine vapor adsorption. However, after overnight incubation at room temperature in a laminar flow hood, BC sample returned back to its original color. The initial black color of BC@ZIF sample faded away while BC@Dopa-ZIF sample remained the same color intensity. Evidently, iodine molecules once captured by the cage structure of ZIF-8 embedded in BC@Dopa-ZIF are not able to escape at room temperature under atmospheric pressure. Approximately, about 1.87 g of iodine could be captured from iodine vapor by 1 g of BC@Dopa-ZIF as measured by the weight gain. The loading capacity of ZIF-8 nanoparticles was 27% higher (2.56 g I2/g particles) than BC@Dopa-ZIF which is consistent with the larger specific surface area possessed by ZIF-8 nanoparticle as demonstrated in the BET results. In contrast, BC@ZIF shows a less effective iodine uptake (

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