Alginate Hydrogel: A Shapeable and Versatile Platform for in Situ

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Alginate Hydrogel: A Shapeable and Versatile Platform for In-Situ Preparation of MOF-Polymer Composites He Zhu, Qi Zhang, and Shiping Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04505 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 18, 2016

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ACS Applied Materials & Interfaces

Alginate Hydrogel: A Shapeable and Versatile Platform for In-Situ Preparation of MOF-Polymer Composites He Zhu,a Qi Zhang,*b and Shiping Zhu*a a

Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7. E-mail: [email protected] b

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, China, 310014. E-mail: [email protected]

KEYWORDS: metal-organic framework, alginate hydrogel, composite, in-situ growth, absorbent.

ABSTRACT: This work reports a novel in-situ growth approach to incorporate metal-organic framework (MOF) materials into alginate substrate, which overcomes the challenges of processing MOF particles into specially shaped structures for real industrial applications. The MOF-alginate composites are prepared through the post-treatment of metal ion cross-linked alginate hydrogel with MOF ligand solution. MOF particles are well distributed and embedded in, as well as on the surface of the composites. The macroscopic shape of the composite can be designed by controlling the shape of the corresponding hydrogel, thus MOF-alginate beads,

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fibers, and membranes are obtained. In addition, four different MOF-alginate composites, including HKUST-1-, ZIF-8-, MIL-100(Fe)-, and ZIF-67-alginate, were successfully prepared using different metal ion cross-linked alginate hydrogels. The formation mechanism is revealed and the composite is demonstrated to be an effective absorbent for water purification.

Introduction Metal-organic frameworks (MOFs) are an emerging class of porous crystalline materials, prepared from self-assembly of metal ions and organic bridging ligands. Their highly ordered periodic porous structures give superior surface areas up to 7,000 m2/g,1, 2 which is not attainable by traditional porous materials. In addition, high thermal stability, tunable pore property, and various chemical reactive sites have attracted great attention from many different areas, including gas storage,3, 4 gas separation,5, 6 chemical sensor,7, 8 catalysis,9, 10 proton conductor,11, 12

battery,13, 14 and drug delivery.15, 16 Numerous types of MOFs have been designed, synthesized, modified, and studied in their

particle forms.17-20 It has been well demonstrated that MOF particles possess excellent properties. However, in real commercial applications, particle forms limit their potential and specially ordered shapes and morphologies are often required. Thus, increasing efforts have been focused on processing MOF particles into spheres,21,

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chains,23 membranes,24-27 and structures with

higher dimension.28 Significant efforts have been made to deposit MOFs onto chemically stable and cost efficient shapeable substrates, such as synthetic polymers29,

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and ceramics.6,

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However, this deposition approach is severely constrained by physical and chemical requirements of the substrates. Surface modification is usually needed prior to the deposition in order to increase their compatibility.33, 34


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Integration of MOF particles into polymer substrate through blending has then been demonstrated as an alternative to prepare robust and flexible MOF-polymer composites.35-41 Incorporation of MOF particles into two-dimensional membranes, i.e. mixed-matrix membranes (MMMs), has been widely studied. Various polymer substrates have been reported for preparation of MMMs, such as polyimide,36,

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polybenzimidazole,42,

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polysulfone,44,

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poly(ionic liquid),46 and poly(vinylidene fluoride),47, 48 mainly for gas separation applications. Meanwhile, incorporation of MOF particles into three-dimensional polymer substrates has also been reported, either prepared via electrospun of polymer/MOF solution,40, 41 or through high internal phase emulsion as template.49 However, MOF-polymer composites prepared from simple blending often suffer from particle aggregation and poor particle-polymer matrix interaction, which can harm the composite performance.50 In-situ MOF growth, on the other hand, is a promising approach to overcome these drawbacks, however, it has been much less studied. Alginate, as one of the natural polymers, derived from brown seaweeds, is consisted of 1,4-linked β-D-mannuronic (M block) and α-L-guluronic acid residues (G block).51,

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It is a well-known polymer that can form

hydrogels though ionotropic gelation under mild conditions.53 Different divalent and trivalent metal ions have been reported to be able to cross-link alginate and prepare hydrogels.54-56 This type of metal ion cross-linked hydrogels provides a great candidate as in-situ MOF growth template for preparation of MOF-polymer composite materials. In this work, we report a robust and straightforward in-situ growth method for preparation of MOF-alginate composites. We demonstrate that the metal ion cross-linked alginate hydrogels can be converted into MOF-alginate composites through a post-treatment of the hydrogels in MOF ligand solutions. The obtained MOF-alginate composites retain the shape of the


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corresponding hydrogels, which enables us to control the shape of the composite in the macroscopic level. MOF particles are well distributed in the alginate structure, as well as decorated on the surface. These composite materials can be easily used as convenient absorbents for removal of dye from water. This simple method for composite preparation greatly facilitates processing MOF materials into various shapes and morphologies often desired in real applications. It can combine the advantages of MOF materials in application and polymers in processing and fabrication.

Experimental Materials. Copper nitrate trihydrate (99-104%), iron (III) chloride hexahydrate ( ≥ 98%), trimesic acid (95%), zinc nitrate hexahydrate (98%), cobalt nitrate hexahydrate (≥98%), 2methylimidazole (99%), sodium formate (≥99%), alginic acid sodium salt, and Rhodamine B ( ≥95%) were purchased from Sigma-Aldrich and used without further purification. All water used was purified Type I water with a resistivity of 18.2 MΩ• cm (Barnstead NANOpure DIamond system, Thermo Scientific, Asheville, NC). Metal ion cross-linked hydrogels. All hydrogels were prepared with the same protocol. Here we use Cu2+ cross-linked hydrogel as an example. The bead-like hydrogel was prepared by adding 3 mL 10 mg/mL sodium alginate aqueous solution dropwisely into 15 mL 30 mg/mL copper nitrate trihydrate aqueous solution. The prepared hydrogel beads were left in the solution overnight and then washed with water (X2) and ethanol (X2) to remove the excess Cu2+. The fiber-like hydrogel was prepared by continuously injecting the sodium alginate solution into the Cu2+ solution using syringe with a needle on top. The membrane-like hydrogel was obtained by drop casting the sodium alginate solution on a glass slide, which was then carefully immersed


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into the Cu2+ solution. The other hydrogels were prepared under the same condition, except for in different metal aqueous solutions. In addition, the Co2+ and Zn2+ cross-linked hydrogels were soaked in their metal ion solution for 2 days to assure sufficient metal ion exchange and washed with water (X2) and methanol (X2). MOF-alginate composites. The prepared Cu2+ and Fe3+ hydrogels were transferred into 15 mL 30 mg/mL trimesic acid ethanol solution for MOF growth under 85 oC for 18 hours. The prepared Co2+ and Zn2+ hydrogels were transferred into 20 mL methanol solution containing 0.2592 g 2-methylimidazolate and 0.1072 g sodium formate for MOF growth under 85 oC for 24 hours. The obtained MOF-alginate composites were washed with ethanol (X2) and hot ethanol (X2), and then dried under 80 oC. Dye absorption of MIL-100-alginate composite. The MIL-100-alginate composite prepared from 0.5 mL sodium alginate solution was placed in 10 mL 1 mM Rhodamine B aqueous solution for 5 days before UV-Vis characterization. The Fe3+ cross-linked hydrogel prepared from 0.5 mL sodium alginate solution was also tested for dye absorption under the same condition for comparison. Characterization. MOF-alginate composites with a 5 nm platinum coating were characterized on JEOL JSM 7000 Scanning Electron Microscopy (SEM), the energy-dispersive X-ray spectroscopy characterization (EDX) was also done with the same equipment. Powder X-ray diffraction (PXRD) was carried out on Cu SMART6000 rotating anode diffractometer. UV-Vis absorption of dye contaminated water was conducted on DU 800 UV/Vis Spectrophotometer. Thermogravimetric analysis (TGA) was performed on TA Q5000 thermogravimetric analyzer with a ramp rate of 10 ̊C/min. Nitrogen isotherm was conducted on Quantachrome AutosorbIQ2-MP at 77K.


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Results and Discussion MOF-alginate composites were obtained by a post-treatment of the corresponding metal ion cross-linked hydrogels with MOF ligand precursor solutions. Hong Kong University of Science and Technology-1(HKUST-1)-alginate was chosen as a model system to demonstrate the feasibility of this method. Specifically, 3 mL 10 mg/mL sodium alginate aqueous solution was dropwisely added into 15 mL 30 mg/mL copper nitrate trihydrate aqueous solution. Spherical hydrogel beads were formed instantaneously, and left in the Cu2+ solution overnight for complete cross-linking. The prepared hydrogel beads were then washed with water twice and rewashed with ethanol twice to remove the excess Cu2+. The hydrogel beads were then added into 15 mL 30 mg/mL trimesic acid (TMA) ethanol solution and kept under 85 oC for 18 hours for the preparation of HKUST-1-alginate composite, as shown in Figure 1A. The formed Cu2+ crosslinked hydrogel was transparent with a light blue color (Figure 1B & 1C), while turned opaque with a sky blue color after incubation in the TMA ethanol solution (Figure 1D). The formation of HKUST-1 crystals was confirmed by powder X-ray diffraction (PXRD) (Figure S1).


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Figure 1. (A) Schematic of the preparation of MOF-alginate composite. Photographs of (B) alginate hydrogels cross-linked by Cu2+ right after the addition of sodium alginate aqueous solution into Cu2+ aqueous solution, (C) alginate hydrogels cross-linked by Cu2+ after washed with water and ethanol, and (D) HKUST-1-alginate hydrogels. The ultrafast cross-linking between metal ion and alginate assisted in control over the macroscopic shape of the hydrogel. The shape was then inherited by the MOF-alginate composite after treated with the ligand solution (Supporting Information Figure S2). A spherical bead composite was achieved by adding the alginate aqueous solution dropwisely into the metal ion aqueous solution, as demonstrated above (Supporting Information Figure S2A). A fiber-like composite was obtained by continuously injecting the alginate aqueous solution in syringe into metal ion aqueous solution (Supporting Information Figure S2B). A membrane shape composite was realized by drop casting the alginate aqueous solution on top of a glass slide, which was then


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carefully immersed into metal ion aqueous solution (Supporting Information Figure S2C). These macroscopic-shape-designable MOF-alginate composites have high potential in paving the way of processing MOF particles for real industrial applications. The morphology was investigated by scanning electron microscopy (SEM) with the fiber-like Cu2+ cross-linked hydrogel and its corresponding HKUST-1-alginate composite. The fibers were obtained by directly drying the prepared samples under 80 oC. Wrinkles were seen on surface of the dried Cu2+ cross-linked hydrogel due to shrinkage upon drying (Figure 2E). A mesoporous structure was evident both on the surface and inside the dried hydrogel (Figure 2E & 2F). The HKUST-1-alginate composite showed a surface (Figure 2A & 2D) and internal (Figure 2B & 2C) porous structure similar to the hydrogel. The composite also contained a large amount of octahedral HKUST-1 particles well distributed and embedded in (Figure 2B & 2C), as well as on top of the surface (Figure 2D). HKUST-1 loading of the fiber-like composite was 23.8 ± 1.3 wt.% determined by thermogravimetric analysis (TGA) (Figure S3 & Table S1), suggesting about 58% copper inside the hydrogel converted into HKUST-1 particles. The theoretical maximum of HKUST-1 loading is 36.6 wt.% if all of Cu2+ converted into MOF particles.


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Figure 2. SEM images of the dried Cu2+ cross-linked hydrogel and the corresponding HKUST-1alginate composite. (A, B, C) Cross-section of the HKUST-1-alginate composite with different magnification. (D) Surface of the HKUST-1-alginate composite. (E) Surface of the Cu2+ crosslinked hydrogel. (F) Cross-section of the Cu2+ cross-linked hydrogel. We then hypothesized that various MOF-alginate composites can be prepared with this protocol under appropriate MOF synthetic conditions, because alginate is cross-linkable by different metal ions. Herein, we prepared four different MOF-alginate composites, including HKUST-1-alginate, zeolitic imidazolate framework-8(ZIF-8)-alginate, Material Institute de Lavoisier-100(Fe)(MIL-100(Fe))-alginate, and zeolitic imidazolate framework-67(ZIF-67)alginate composite, based on corresponding Cu2+, Zn2+, Fe3+, and Co2+ cross-linked hydrogels, respectively (Figure 3). The formation of different MOFs within the alginate matrix was confirmed by PXRD (Figure S1). All composites had similar morphologies. MOF particles could be seen on the surface, as well as inside the alginate matrix, as revealed by SEM characterization (Figure 3 & Figure S4-S6). This proves the robustness and versatility of this method in


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preparation of different MOF based alginate composites, which could be potentially used in different applications.

Figure 3. (i) HKUST-1-alginate composite. (ii) ZIF-8-alginate composite. (iii) MIL-100(Fe)alginate composite. (iv) ZIF-67-alginate composite. (A) MOF structure. (B) Photographs of the fiber-like metal-ion cross-linked hydrogel. (C) Photographs of the corresponding MOF-alginate composite. (D, E) SEM images of the MOF-alginate composite. All fibers were prepared with 1 mL sodium alginate solution. The mechanism involved in the formation of MOF-alginate composites was proposed based on the energy-dispersive X-ray spectroscopy characterization (EDX). EDX elemental maps of the cross-section of the HKUST-1- and ZIF-67-alginate composites showed distinct element distributions (Figure 4). Carbon, oxygen, and copper were uniformly distributed within the selected area. There were no sodium and nitrogen found in the case of HKUST-1-alginate


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composite (Figure 4A). However, in the case of ZIF-67-alginate composite, only carbon was uniform. Oxygen and sodium, nitrogen and cobalt were consistent with each other, respectively. The distributions of oxygen and sodium were exclusive from those of nitrogen and cobalt (Figure 4B). This difference between copper- and cobalt-based MOF-alginate composite provided some insight into the formation mechanism. Alginate is a copolymer composed of G blocks, GM alternative blocks, and M blocks, among which only G blocks are believed to participate the gel formation, which forms an “egg-box” structure (Figure 4F).52 Hence, the ionic bindings between free GM and M blocks with metal ions provide good sources for MOF growth. However, the degree of affinity between divalent metal ion and alginate decreases as Pb > Cu > Cd> Ba > Sr > Ca > Co, Ni, Zn > Mn.53, 57 Therefore, all sodium ions were replaced by copper but were not by cobalt in preparation of the hydrogels. Thus, no sodium was found in the HKUST-1-alginate, while abundant in the ZIF-67alginate. This indicated that cobalt cross-linked only with G blocks, but did not bind with GM and M blocks as copper did, due to its low degree of affinity. During the formation of HKUST-1 particles, the copper ions bound with GM and M blocks could easily react with TMA ligand and formed the MOF particles (Figure 4D). In the case of ZIF-67 as comparison, only the crosslinked cobalt could serve as metal source for the MOF growth, thus the cross-linking points were broken during the formation of ZIF-67 particles (Figure 4E). However, due to the insolubility of alginate in alcohol, the structure of ZIF-67-alginate was preserved.


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Figure 4. Cross-section back-scattered electron images of (A) HKUST-1- and (B) ZIF-67alginate composites and their corresponding EDX elemental maps (Scale bar 60 µm). (C) Chemical composition of sodium alginate, HKUST-1, and ZIF-67. Schematic of the formation of (D) HKUST-1-alginate composite, (E) ZIF-67-alginate composite, and (F) “egg-box” model of metal ion cross-linked alginate. In order to support this hypothesis, HKUST-1 and ZIF-67 based alginate composites were immersed in water, respectively. The former remained its structure, while the latter was completely disassembled within an hour, due to the breaking of cross-linking points. The ZIF-8alginate composite was disassembled in water as well, because the degree of affinity of zinc


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towards alginate is the same as cobalt. However, MIL-100-alginate composite remained its structure in water. Their EDX analyses showed similar results: MIL-100-alginate did not contain any sodium, while ZIF-8-alginate contained a lot (Figure S7). It should be noted that the EDX maps of ZIF-8-alginate showed a homogeneous distribution of all elements, which is different from those of ZIF-67-alginate, due to the smaller crystal size of ZIF-8. This experiment confirmed that the growth of ZIF-67 particles was only associated with the cross-linked cobalt ions. The cross-linking points were broken in order to support the ZIF-67 growth. In contrast, the copper ions loosely bound with GM and M blocks could serve as metal source for HKUST-1 growth, resulting in the formation of HKUST-1-alginate composite. This composite maintained its structure when immersed in aqueous solution, which was due to retention of the cross-linking points. Interestingly, we also found that these two types of composites possessed totally different surface areas. HKUST-1- and MIL-100-alginate, which remained cross-linked, showed low surface area, while ZIF-8- and ZIF-67-alginate showed high surface areas (ZIF-8: 563 m2 g-1, ZIF-67: 317 m2 g-1) (Table S2 and Figure S8). Our hypothesis is that, due to the retention of cross-linking points, the growth of HKUST-1 and MIL-100 crystal was confined within limited spaces, leading to the interpenetration of alginate chains into MOF pores, and thus reducing the accessible surface area. This could impose some limitations to potential applications, which require high surface areas. In the case of ZIF-8 and ZIF-67, the crystals could squeeze in between alginate chains due to the loss of cross-linking points, resulting in less or none interpenetration of alginate chains.


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Figure 5. (A) Photographs of 1 mM Rhodamine B aqueous solution, 1 mM Rhodamine B aqueous solution after 5 days soaking with Fe3+ cross-linked hydrogel, and 1 mM Rhodamine B aqueous solution after 5 days soaking with MIL-100(Fe)-alginate composite (from left to right). (B) UV-Vis spectrums of the aforementioned aqueous solutions. Finally, the prepared MOF-alginate composite was employed as absorbent for the removal of dye from contaminated water, which can be handled easily due to the confinement of MOFs within the composite. The purpose was to demonstrate potential uses of such composites. MIL100(Fe)-alginate composite was prepared from 0.5 mL sodium alginate solution and it was immersed into 10 mL 1 mM Rhodamine B aqueous solution. After 5 days of soaking, the solution became colorless while the MIL-100(Fe)-alginate composite became crimson (Figure 5). On the contrast, the corresponding Fe3+ cross-linked hydrogel showed little absorption (Figure 5), suggesting that the dye adsorption behavior was solely associated with MIL-100(Fe).

Conclusion In conclusion, we have demonstrated a novel method to incorporate MOF particles into alginate substrates. The metal ion cross-linked alginate hydrogels provided excellent templates


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for in-situ MOF growth, and the shapes of hydrogels were readily inherited by their corresponding MOF-alginate composites. MOF particles were well distributed and embedded inside, as well as decorated on surfaces of the composites. Four different types of MOF-alginate composites based on different metal ions were obtained, highlighting the robustness and versatility of this processing method. The mechanistic study revealed that the alginate hydrogels prepared with high affinity metal ions (Cu2+ for example) could be converted into MOF-alginate composites through coordination between ligand molecules and metal ions loosely bound with alginate GM and M blocks, while the G blocks remained cross-linked. However, a large amount of sodium ions bound with GM and M blocks remained in the alginate hydrogels prepared with low affinity metal ions (Co2+ for example). The cross-linked metal ions had to serve as metal source for the MOF growth, leading to disassembly of the obtained composites upon exposure to an aqueous solution because of loss of cross-linking points. Finally, it was demonstrated that the prepared composites were effective as an absorbent in treating dye contaminated water. This simple method greatly facilitates preparation of MOF materials into various shapes and morphologies, which really combines application property of MOF material with processability of polymers.

ASSOCIATED CONTENT Supporting Information. Photographs of HKUST-1-alginate composites with different shapes and TGA results are included. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author * S. Zhu. Email. [email protected] * Q. Zhang. Email. [email protected] ACKNOWLEDGMENT We sincerely thank the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Canada Research Chair (CRC) program of the Federal Government for supporting this research work. REFERENCES 1.

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Table of Contents Graphic and Synopsis Here A novel in-situ growth approach to incorporate metal-organic framework (MOF) materials into alginate substrate is reported here. The MOF-alginate composites are prepared through the posttreatment of various shaped metal ion cross-linked alginate hydrogels with MOF ligand solution. MOF particles are well distributed and embedded inside, as well as on the surface of the composites.


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Alginate Hydrogel: A Shapeable and Versatile Platform for in Situ

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