Continuous MetalâOrganic Framework Biomineralization on Cellulose

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Continuous Metal-Organic Framework Biomineralization on Cellulose Nanocrystals: Extrusion of Functional Composite Filaments Joseph J. Richardson, Blaise L. Tardy, Junling Guo, Kang Liang, Orlando J. Rojas, and Hirotaka Ejima ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06713 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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ACS Sustainable Chemistry & Engineering

Continuous Metal-Organic Framework Biomineralization on Cellulose Nanocrystals: Extrusion of Functional Composite Filaments Joseph J. Richardson,a,† Blaise L. Tardy,b,† Junling Guo,c Kang Liang,d Orlando J. Rojas,b,e,* Hirotaka Ejimaa,* aDepartment

of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. bDepartment of Bioproducts and Biosystems, School of Chemical Engineering, Vuorimiehentie 1, Aalto University, FI-00076 Aalto, Finland cWyss Institute for Biologically Inspired Engineering, 3 Blackfan Cir, John A. Paulson School of Engineering and Applied Sciences, Harvard University, Boston, Massachusetts 02115, United States. dSchool of Chemical Engineering and Graduate School of Biomedical Engineering, Library Rd, Kensington, University of New South Wales, Sydney NSW 2052, Australia. eDepartment of Applied Physics, School of Science, Puumiehenkuja, Aalto University, FI-00076 Aalto, Finland. † These authors contributed equally

Corresponding Authors * [email protected] (Orlando Rojas) * [email protected] (Hirotaka Ejima)

Keywords MOF, CNC, Hybrid material, Multi-scale hierarchical structure, ZIF, Surface initiated nucleation, Nanocrystalline cellulose, Porous material

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ABSTRACT

Growing metal-organic frameworks (MOFs) around biomolecules has recently emerged as a promising method to combine natural and synthetic materials. In parallel, cellulose nanocrystals (CNCs) have found use for forming a wide range of renewable nano- and macroscopic materials due to their bio-derived nature, high surface area, and high strength. Herein, we demonstrate the continuous nucleation of MOFs from the surface of CNCs, thereby forming hybrid hydrogels, aerogels, and porous assemblies that can be pre- or post-loaded with functional cargo. By simply mixing CNCs with MOF precursors, the biomineralization is initiated and takes place continuously where the MOFs simultaneously coat and crosslink the CNCs across a wide range of CNC and MOF precursor concentrations. Additionally, CNCs can be extruded into the MOF precursors to yield CNC-MOF filaments that can be pre-loaded with functional enzymes or postloaded with small fluorophores. Overall, our approach enables the rapid structural control of functional composites promising for a range of applications.

Introduction The formation of materials at interfaces allows for the generation of novel composites that, depending on the reaction speed, can be used to engineer hybrid materials with unique geometries and physico-chemical properties.1 Controlled nucleation processes, such as crystal growth, can be done at interfaces to generate hybrid materials with complex geometries across all length scales.2-4 A particularly promising and well-studied crystalline material useful for generating microporous hybrid structures is metal-organic frameworks (MOFs), composed of metal ions or clusters coordinated by organic linkers,56

where the well-defined pore size of MOFs allow capture/sieving for various

environmental applications.7-9 MOFs can be conformally formed at many interfaces;10-11 however MOF formation is generally slow and requires high temperatures, which has presented a limitation for wide-spread applicability.12-13 Recently, it was demonstrated that zeolitic imidazolate frameworks, and specifically the water stable and biocompatible ZIF-8 and ZIF-L, can rapidly form at room temperature around polysaccharides,14 and cellulose-based materials like cell walls,15-16 plants17 and cellulose nanofibers,18


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highlighting a promising route for both synthesizing MOFs and forming biomoleculeMOF constructs. Cellulose nanocrystals (CNCs) are some of the most promising high aspect ratio building blocks for engineering advanced materials,19-21 as they are the smallest amongst nanocellulose, instituting a clear opportunity to form high surface area materials via their controlled assembly.22 CNCs are high strength anisotropic, bio-based particles with characteristic dimensions ranging between 3 and 70 nm in width and ≈35 – 3000 nm in length, depending on the source. Moreover, CNCs are highly crystalline and possess sulfate half-esters that provide colloidal stability, although both surface charge and crystallinity can be altered during the isolation process.22 Importantly, cellulose nanocrystals are the product of carbon capture, have negligible toxicity, and are degradable via bio-chemical pathways23 while still being insoluble in the majority of conventional organic solvents.24 The continuous integration of CNCs with highly functional MOFs remains unexplored and the combination of environmental friendliness and outstanding physico-chemical characteristics of CNCs make them ideal building blocks for engineering novel composite materials for high-volume and low-cost applications.25 Herein, we utilize the ability of ZIFs to rapidly form around polysaccharides and negatively charged surfaces to gel CNCs into hybrid networks (Scheme 1a). In contrast with other approaches where pre-formed MOFs are mixed with CNCs,13 our process utilizes a biomineralization approach to grow the MOFs directly on the CNCs in one step, where gelation and biomineralization, occur simultaneously after addition of the MOF precursors (Scheme 1b). This study focuses specifically on sol-gel transitions mediated by the nucleation of MOFs and its influence on the rate of gelation. Additionally, compared to the use of cellulose nanofibers for mineralization,18 cellulose nanocrystals enable a wider range of concentrations to be used as colloidal crowding occurs at higher concentrations. Furthermore, being smaller than other nanocelluloses, they enable finer control over the porous architectures that they form. For biomineralization, the negative charge of CNCs, owing to the sulfate half-ester groups, is conducive for concentrating metal ions, which results in nucleation of the MOF around the CNCs at MOF precursor concentrations that normally would not result in MOF growth. We demonstrate the



formation of hydrogels by rheological testing using the common “tube-inversion method”27-28 and describe biomineralization-induced gelation as a function of total solid content or relative ratio of CNCs and MOFs precursors. Moreover, crystallization of the MOFs occurs after gelation of the mixtures as demonstrated by synchrotron small-angle x-ray scattering (SAXS)26 and UV-Vis spectroscopy. We discuss in-depth, the aggregation and nucleation processes, and their coupling in the biomineralization-induced gelling. We also studied thermal degradation and surface area characteristics by thermogravimetric assay (TGA) and nitrogen adsorption isotherm, respectively. We demonstrate the controlled extrusion of the resultant composites into wet-spun filaments (Scheme 1c), and finally introduce pre- and post-loading strategies for forming loaded, functional composites.

Scheme 1. Illustration of the gelation of CNC with MOFs. (a) The MOF precursors (2methylimidazole (2MI) and Zn2+ ions) interact with the cellulose sub-units of CNCs, which (b) results in ultra-fast hydrogel formation. (c) Extrusion of cellulose nanocrystals in water with noncrystallizing mixtures of MOF precursors, resulting in the ultra-fast gelation of the extruded fibers in CNC-MOF composites. Materials and Methods


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Materials: The cellulose nanocrystals, CNCs (CAS No. 7789-20-0) were obtained from the USDA’s Forest Products Laboratory (FPL, Madison, WI) and acquired through the Process Development Center in the University of Maine, U.S.A.. They were obtained as an 11 % slurry (w/w), with a sulfur content in the dry CNC solids is 0.95 wt % or 297 mM/kg of CNC, and used as received. A thorough characterization of the CNC suspension was performed in previous studies including for the specific batch used herein.29-30 Millipore (Synergy UV) milli-Q water was used to dilute the CNC suspension. Acetone, ethanol, zinc acetate (ZA), 2-methylimidazole (2MI), o-dianisidine, horse radish peroxidase, hydrogen peroxide (30% v:v), and (4′,6diamidino-2-phenylindole) (DAPI) were obtained from Sigma Aldrich. Photographic images were taken using a digital camera at a resolution of 4128  3096. Preparation of CNC-MOFs Gels: For 1:1:1 gels, 150 µL of CNC solution (11%, 5.5%, 2.2%, or 0.55% starting concentration) was added to a 1.5 mL tube. Next, 150 µL of stock 2MI solution at 800 mM, 160 mM, or 32 mM was added to the CNC solution and mixed vigorously. Finally, 150 µL of stock ZA solution at 200 mM, 40 mM, or 8 mM, was added and immediately mixed. For 3:1:1 gels, 300 µL CNC, 100 µL 2MI, then 100 µL ZA were mixed. Preparation of CNC-MOF Extruded Gels: Identical stock concentrations as above were used and first 10 mL of 2MI and 10 mL of ZA were mixed, followed by the slow addition of the stock CNC solution (1 mL) from a pipette tip. Note that the extrusion size could be changed by cutting the tip off the end of the pipette tip to change the extrusion diameter. Note that the extrusion process was verified independently on 4 continents/countries (Asia/Japan, Australia/Australia, Europe/Finland, and North America/USA). Preparation of CNC-MOF aerogels: The gels were dialyzed against acetone in a 50 MWCO regenerated cellulose (spectrum labs). The gel-containing membranes then underwent critical point drying in a Leica EM CPD300. 75 cycles of liquid CO2 exchange with acetone occurred at 10 °C. Thereafter, the chamber was heated to 40 °C and the supercritical CO2 was vented off slowly. Thereafter the N2 adsorption isotherms of the obtained aerogels were directly recorded using Micrometrics ASAP 2020 (Tristar II). Thermogravimetric analysis was conducted using a TGA 8000™ Thermogravimetric Analyzer (PerkinElmer) with a ramp temperature of 10 °C per min. Preparation of Enzyme-Loaded CNC-MOF Fibers: 800 mM 2MI were mixed with 40 mM ZA to a total volume of 20 mL. Afterwards, 1.35 mL of CNC suspension in DI water and 150 uL



of 20 mg mL-1 of a stock solution of HRP in sodium phosphate buffered at pH 7 (50 mM) were mixed and the 1.5 mL mixture as well as 1.5 mL of a suspension of CNC were slowly extruded through the MOF precursors solution. The resulting intertwined filaments were left to sit for an hour or 16 h and were assessed for activity afterward. Prior to the assay, the supernatant was replaced by immersion of the vial into 1 L of a diluted solution of sodium phosphate adjusted to pH 7, this process was repeated up to 6 times for the enzyme containing vial and 3 times for the control experiments, without enzymes. Afterwards, the volume suspension was adjusted to 20 mL and 5 mL of sodium phosphate (pH 7, 50 mM) were added as well as 160 µL of a solution of 5 mg mL-1 o-dianisidine dihydrochloride. Quickly afterwards, 5 µL of hydrogen peroxide from a stock solution of 30% v:v was added. The suspensions were rewashed to assess the recyclability as described above. Post-loading with 4 ′ ,6-diamidino-2-phenylindole: 5 µL of 1 mg mL-1 of 4′,6-diamidino-2phenylindole was added to 1 mL of water containing extruded filaments. After 30 min, the free dye was washed away by removing the liquid and washing 3 times with 1 mL water. The filaments were then imaged under the UV excitation/emission filter on an inverted Olympus IX71 microscope (Olympus). UV-Vis Spectroscopy: Transmission spectra were obtained with an UV-Vis spectrophotometer (Nanodrop OneC, Thermo Fisher Scientific). All experiments were carried at room temperature (24 °C). Samples were blanked against the deionized water used for preparing all solutions. Nonspecific transmittance was recorded from 190–850 nm in 0.5 nm increments. Scanning Electron Microscopy: The morphology of the biomineralized CNCs were examined using a field emission scanning electron microscope (FE-SEM) (Zeiss SigmaVP, Germany) operating at 1.6 kV and a working distance of 4 mm. A small piece of the sample was fixed on a carbon tape and then sputtered with Pd-Au alloy (4 nm). Samples were prepared by flash drying a small amount of the gel onto silicon chips at 80 °C. Small-angle X-ray scattering: Synchrotron small-angle X-ray scattering (SAXS) data were collected at the SAXS/WAXS beamline at the Australian Synchrotron, and the diffraction patterns were collected using a Pilatus 1M detector and analyzed using Scatterbrain.

Results and Discussion


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In the biomineralization-induced gelation experiments, the CNC suspension was first mixed with different concentrations of 2-methylimidazole (2MI) (1:1 by volume). The CNC suspension dispersed completely, even upon mixing with 800 mM of 2MI (final 2MI concentration of 400 mM), suggesting limited interaction between CNCs and 2MI. Upon addition of an equivalent of zinc acetate (ZA) (i.e., 1:1:1 CNC:2MI:ZA volume ratio), biomineralization of the CNCs initiated and the suspension gelled following a kinetics that depended on the precursors’ ratio, as seen by tube-inversion experiments (Figures 1 and S1-S4). The concentrations were varied from 0 to 267 mM for 2MI, 0 to 66.7 mM for ZA, and 0.183 to 3.76 % for CNC. Slow (> 5 min) aggregation and gelation occurred upon addition of concentrated ZA (>13.3 mM) to CNC suspension (3.67%) in the absence of 2MI (Figure S1, bottom right panel). This is in accordance with previous metal-CNC cross-linking studies.31 However, when ZA was added to 2MI-CNC mixtures, gelling occurred rapidly (< 5 s) across a wide range of concentrations and ratios, without inhomogeneous clumping (Figures 1 and S2–4). This was also confirmed when observing the gels under SEM (Figure S5). Specifically, rapid gelling occurred at most ratios and concentrations, with the single biggest factor being the concentration of the CNC suspension (Figures 1 and S1). For example, no gelation was seen at the lowest concentration of CNC (0.183%, data not shown), while gelation occurred in all but the lowest MOF precursor concentrations at an intermediate CNC concentration (1.83%). These observations highlight that the mechanism of gelation involves simultaneous nucleation and biomineralization, specifically around and between individual CNCs. CNCs are known to concentrate metal ions and act as nucleating agents for metal nanoparticles due to the sulfur content of the CNCs,32 and this can locally increase the concentration of MOF precursors and allow for MOF nucleation.33 Another important factor governing the speed of gelation was the molar ratio of 2MI to ZA, where a higher concentration of 2MI compared to ZA allowed the quickest gelation and avoided metal-CNC cross-linking. An excess of 2MI is beneficial for driving the formation of ZIF-8 in aqueous conditions.34 At the highest concentration of CNCs (3.67 %), the concentration of sulfate charges are still only 1.99 mM, which is considerably lower than the concentration of 2MI. After the MOF seed forms, it is likely aids in the crosslinking of the gels as only a limited number of sulfate



groups per individual CNC need to be interacting with the MOF to allow for gelation due to the size and aspect ratio of CNCs. This continuous gelation process is substantially different from MOF-based composites utilizing pre-formed MOF particles or pre-gelling the MOF-support using metallic ions.13,18

Figure 1. Concentration-dependent gelation of CNC as a function of the concentrations of the MOF precursors (ZA, 2MI). “Slow gravitational flow” describes viscous fluids exhibiting total inversion after several minutes while fast gravitational flow represents immediate inversion (i.e. liquid-like behavior). Additional information on the kinetics of gel formation and lower CNC concentration effects are provided in Figure S1. UV-Vis spectroscopy showed increases in turbidity related to the gelling process (measured at 280 nm herein), and the higher MOF precursor concentrations showed clear peaks corresponding to Zn-2MI interaction (~240–250 nm) (Figure 2). Of note is that a peak corresponding to Zn-CNC interactions (~220–230 nm) was also apparent in many samples, which explains how the MOFs interact and cross-link the CNC (Figure 2 and Figures S6–10). An excess of 2MI was identified as important for MOF formation as seen by UV-Vis (Figures S6–8), which matches the gelation results and further suggests that gelation is due to the formation of MOFs. Similarly, having a high concentration of MOF precursors was more critical than having a high CNC concentration, as seen by the blue peak-shift and decreased turbidity in UV-Vis at decreasing MOF precursor


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concentrations (Figure S9). Without 2MI, the ZA-dependent gelation was more sensitive to reduction of the ZA concentration than to decreases in CNC concentration (Figure S10), again highlighting the importance of 2MI. SAXS analysis demonstrated that two different phases of Zn(2MI)2 were present, depending on the ratio of the precursors. The ZIF-8 isoform was only present at the highest concentration of MOF precursors, while ZIF-L was present at a more moderate ratio of 4:1 (2MI:ZA) (Figure 2a). This result is also in agreement with studies on pure MOF synthesis, where high 2MI:ZA ratios tends to produce ZIF-8 and low 2MI:ZA ratios tend to produce ZIF-L specifically.35 Additionally, the crystal phase took time to appear, which supports the idea that there is competition between 2MI and CNC for ZA. These two points matches previous reports on the biomineralization of ZIFs around celluloserich polysaccharides both in solution and in plants.14,

17

These results highlight that

crystalline MOFs are forming throughout the gel, and thermogravimetric analysis demonstrated similar decomposition to ZIF-8 and ZIF-L depending on the precursor concentrations, with the lowest precursor concentrations yielding a decomposition similar to CNCs (Figure S11).18,

35

Moreover, nitrogen adsorption experiments highlighted an

increase in surface area with increases in MOF precursor concentrations and/or decreases in CNC concentration, with results comparable to other MOF-cellulose constructs reported in the literature.18 Importantly, no further activation of the MOF-CNC aerogels was undertaken to highlight the contribution of the mesopores from CNC aerogels to act as support for the microporous MOFs, thus yielding multi-scaled porous architectures (Figure S12). The comparable results with previous literature,18 in terms of surface area, therefore represent a minimum value for the composites presented herein. Specifically, surface areas of 299.1, 348.5 and 509.9 m2 g-1 were measured for the aerogels formed from the gels formed at a final concentration of CNC of 3.67 % and 2MI-ZA precursor concentrations of 53.3-13.3, 267-13.3 or 267-67.7 mM, respectively. These results highlight that the concentration of the MOF precursors has a substantial impact on surface area. Surface areas of 436.9 and 557 m2 g-1 were measured for the aerogels obtained from gels formed at a CNC concentration of 1.67 % with precursors mixtures at concentrations of 53.3-13.3 or 267-67.7 mM, respectively. Therefore, for a lower concentration CNC, the impact of the MOFs on the surface area is reduced. Therefore, the CNC-ZIF gel pore



structure is likely multi-scaled, with macro- and mesopores from the gelled CNC framework and micropores (< 2 nm) arising from the MOFS grown onto the CNCs. Because the ZA interacts with both 2MI and CNC there could be an equilibrium where the initially gelled suspensions might destabilize as the MOF seeds grew and scavenged the ZA cross-linkers of the gel. Interestingly, when the samples were left to sit overnight, previously gelled samples became runny liquids again, likely due to competition between forming continuous networks (i.e., gels) and forming discrete particles (i.e., MOF particles) (Figure S2). Discrete particles could be seen under the microscope in the samples that de-gelled confirming this hypothesis (Figure S13). De-gelling occurred for the samples at and below 0.73% CNC, and for all the samples at the lowest ZA concentration (2.7 mM), highlighting that a sufficient quantity of CNCs was crucial for cross-linking the gel, and that both 2MI and the CNC surfaces were competing to interact with Zn. Specifically, this destabilization happened at lower CNC concentrations in samples with the lowest ZA concentration. Interestingly, it also occurred in some of the samples at the highest concentration of MOF precursors, which is likely due to the fact that MOFs can form in water at this high concentration if given enough time. Therefore, it is possible that the ZA-induced aggregation gelation could be disrupted upon interaction of the metal with 2MI for the “intermediately stable” gels shown in Figure 1.


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Figure 2. MOF formation and crystallization as well as aggregation development in the mixed systems as analyzed by a) SAXS and b) UV-Vis. In a) the initial state of the mixture is elucidated immediately after mixing and after 4h. A, 3.67% CNC / 267 mM 2MI / 66.7 mM ZA, B, 3.67% CNC / 53 mM 2MI / 66.7 mM ZA, C, 1.83% CNC / 53 mM 2MI / 66.7 mM ZA, D, 0.73% CNC / 53 mM 2MI / 66.7 mM ZA, and E, 0.18% CNC / 53 mM 2MI / 66.7 mM ZA. SAXS patterns obtained from control ZIF-8 and ZIF-L synthesized conventionally in the absence of CNCs are also shown. The spectra in b) correspond to the highest stock or mixture concentrations reported in this manuscript. Extrusion can be used to shape nanocelluloses into gels, and we therefore examined the conditions necessary for successful extrusion of the CNCs as this significantly widen the scope of application of such gels.36-38 We first mixed the MOF precursors at the same ratios as the tube-inversion experiments, where the precursors are at a ratio that requires a



substrate, such as CNCs, to nucleate MOF growth in a reasonable timescale, and extruded 11% and 5.5% CNC suspensions into shapeable gels. The concentration of ZA played the most important role, because at low ZA concentrations the CNCs dispersed into the solution rather than gelling, while intermediate concentrations of ZA were ideal for extrusion (20 mM) (Figure 3). Filaments produced from the various extrusion conditions are shown in Figure 3b,c and corresponding SEM of a dried filament in Figure 3b. SEM images of the surface of the corresponding filament highlight an absence of distinct MOF crystals upon washing thoroughly with ethanol (Figure S14). For all filaments, upon prolonged exposure to the precursor solutions (16h), large clusters formed and the filaments became rigid. In the process, they became more opaque and more difficult to isolate individually (Figure S15). A thick layer of nucleated MOFs can be observed by SEM, and below the MOFs the biomineralized CNC filament (Figure S15). Upon close examination of the CNCs surface it can be clearly seen that the nucleation of MOFs occurs from the CNCs surface, embedding CNCs in the process (Figure S16a). Furthermore, some degelling as described in Figure S13 was observed from CNC suspensions immersed into precursor mixtures at concentrations of 400 mM 2MI and 100 mM ZA wherein the degelled fragments correspond to CNC embedded within individual MOF particles (Figure S16b), further confirming the CNC-initiated mineralization of MOF particles.


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Figure 3. Evaluation of biomineralization-based extrusion of CNCs using ZIF-8. a) Precursors concentration dependence on extrudability. b) Extruded wet filaments for CNC extruded from a concentration of CNC of 5.5 % and final precursor concentrations of 2MI/ZA of 400/100 mM. The corresponding SEM of a filament washed with ethanol is shown below. c) Extruded wet filaments and corresponding extrusion conditions showing that higher concentration of precursor or CNC lead to a decrease in deformability. A wide range of functionalities are available via biomineralization and multi-scaled porous architectures.39 Herein, we illustrate two functionalities (1) dye uptake and (2) cobiomineralization of enzymes, utilizing post- and pre-loading strategies, respectively. The permeability and post-loading of small-molecules in the extruded composites was examined using a fluorophore (4′,6-diamidino-2-phenylindole, DAPI), which only adsorbed onto the surface of the extruded filaments, likely due to intercalation into the MOF pores as demonstrated previously (Figures S17).15 This highlights the gating effect of the MOFs as it prevented a small water-soluble molecule from interpenetrating into the hydrated gel filaments. It also suggests that the biomineralization could be denser on the outer section of the filament, thus leading to a core-shell type structure for the extruded filaments as described earlier for prolonged biomineralization processes (Figure S15). Pre-loading using a functional enzyme, namely horseradish peroxidase (HRP), was also explored by mixing the enzyme with the CNCs prior to extrusion into the MOF precursors. HRPs is an ideal model enzyme in that context as it has been shown to rapidly biomineralize MOFs with a near 100% encapsulation efficiency, while retaining its activity and even gaining thermal resistance due to the surrounding MOF architecture.40 Due to the presence of HRP, the extruded filament had a slight brown tint after extrusion (after 1h of biomineralization) and extensive washing of the supernatant (Figure 4), and the protein primarily resided at the edge of the filament in the MOF layer as determined by confocal microscopy (Figure S18).40 The activity of the HRP was assessed using o-dianisidine (ODD), which turns bright orange upon addition of hydrogen peroxide in the presence of HRP. The orange color developed from the gelled surface within a few seconds of adding hydrogen peroxide, confirming the presence of active enzymes in the extruded filaments. The reacted



reagents were then washed away and the assay was repeated to assess recyclability. A lower reactivity was observed, but still a bright orange color could be obtained using the same assay (Figure 4). A lower reactivity was observed, with the development of the color occurring over several seconds rather than immediately. A slightly dimmer color could be obtained using the same assay when observed after 1 g (Figure 4). The lower reactivity is potentially due to hydrogen peroxide inactivation of HRP.41-42 This catalysis also demonstrated the porosity of the MOFs, as the analyte was still able to access the HRP. Lastly, as demonstrated in numerous prior studies, biomineralization imbues heat resistance to various enzymes including HRP,40, 43-45 which synergizes outstandingly well with the high heat resistance of CNCs (> 230 °C).46-47 Collectively, these loading experiments highlight the versatility of the extruded CNC-MOF composites and demonstrate their functionality in ways that could be useful for water remediation, filtering, sensing and heterogeneous catalysis depending on the cargo.


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Figure 4. Formation of functional CNC-MOF filaments containing enzymes (HRP) by cobiomineralization of CNC and HRP. a) Scheme of the extrusion process using enzymes preloaded into the CNC suspension. b) Control fibers without enzyme appear white, and do not lead to any catalysis of the oxidation of the analyte (ODD), even in the presence of H2O2 for several hours. c) The HRP-loaded filaments have a brown tint due to the presence of HRP, and after the addition of H2O2, the ODD is converted and becomes red. Both images were taken after 1h for the assay assessing the activity of HRP. In summary, we explored the fundamental conditions necessary to gel CNC-MOF composites and demonstrate that under specific conditions, stable extruded constructs can be formed. Bio-sourced, high-aspect ratio and nano-sized CNCs act as ideal building blocks for such purpose. A biomineralization approach of ZA and 2MI was used to grow the MOFs around CNCs, and small molecules and enzymes could be pre- and postloaded, respectively. As the biomineralization of MOFs has been achieved for a variety of MOF types,40, 44-45 it is foreseeable that CNC-MOF composites other than ZIFs might be engineered. Moreover, as nanocellulosic building blocks are becoming central to circular economy ideals, their use as a framework for multi-scaled porous materials may significantly expand in the future. Extruded CNC-MOF composites loaded with cargo represent a unique multi-scaled material with promise in fields as diverse as energy and biomedicine. For example, the high surface area of both fibers and MOFs and the robustness of macroscopic constructs means that CNC-MOF hybrids could fit a unique niche for energy/environmental applications including catalysis, carbon-capture, or metal/toxin sequestration.48 Additionally, the high biocompatibility of cellulose, in combination with the extensive studies on drug delivery using MOFs, means that small molecule-loaded extruded CNC-MOF fibers could be used for wound healing or tissue scaffolds amongst other biomedical applications. A particularly appealing aspect of the proposed architectures is their use for filtration and water remediation purpose as nanocelluloses are biodegradable, non-toxic, and bio-sourced. Acknowledgements



Part of this research was undertaken on the Small/wide angle X-ray scattering (SAXS) beamline at the Australian Synchrotron, part of ANSTO. J.J.R. acknowledges Japan Society for the Promotion of Science (JSPS) funding project (ID PE17019). K.L. acknowledges Australia NHMRC Career Development Fellowship (APP1163786) and Scientia Fellowship program at UNSW. H.E. acknowledges Leading Initiative for Excellent Young Researchers (LEADER, MEXT) and Grants-in-Aid for Scientific Research (JP18K14000 and JP18K18802, JSPS). The European Research Commission is thanked for support via ERC Advanced Grant 78848 “BioElCell”. Order of first authorship was decided alphabetically by surname. Conflicts of interest There are no conflicts of interest to declare. Supporting Information Electronic supporting information can be found online from the publisher. Extended Figure 1, tube inversion assays, SEM of the formed composites, extended UV-Vis data pertaining to Figure 2 discussion, degelling and MOF particles formation, biomineralized filaments surface and cross sections, dye adsorption evaluation, TGA assays, nitrogen adsorption isotherms and confocal microscopy.

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TOC:

Cellulose nanocrystals can nucleate the biomineralization of metal organic frameworks, allowing for gelation and extrusion of the composites.


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Continuous MetalâOrganic Framework Biomineralization on Cellulose

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Continuous MetalâOrganic Framework Biomineralization on Cellulose