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Multifunctional Cellulosic Scaffolds from Modified Cellulose Nanocrystals Eldho Abraham, David Ernst Weber, Sigal Sharon, Shaul Lapidot, and Oded Shoseyov ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13528 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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ACS Applied Materials & Interfaces

Multifunctional Cellulosic Scaffolds from Modified Cellulose Nanocrystals Eldho Abraham,† David E Weber,† Sigal Sharon,† Shaul Lapidot,‡ and Oded Shoseyov†* †

R.H. Smith Institute of Plant Sciences and Genetics and The Harvey M. Krueger Family Center for

Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Israel. ‡

Melodea Ltd, Faculty of Agriculture, Hebrew University of Jerusalem, Israel

Keywords: cellulose scaffold, freeze-drying, ultra-lightweight, oleophilic, lipophilic, βlactoglobulin, drug delivery

Abstract: A biobased cellulosic scaffold material was made through freeze-drying icetemplating of functionalized cellulosic nanomaterials. The resulting interconnected highly porous scaffold was primarily composed of highly esterified, strong network of ultrathin cellulosic layers. The prepared cellulosic scaffold material displayed multifunctional properties of hydrophobicity, oleophilicity and lipophilicity, which could selectively absorb milkfat, hydrophobic proteins, various organic solvents and oils. Diverse potential for the structural and medical applications, such as tissue engineering, drug delivery, oil and fat accumulation are proposed. Nanostructured porous materials are widely used due to their unusual properties, such as very low density, high specific surface area, static mechanical properties, fatigue resistance, and permeability. The use of traditional porous materials that are made of silica and carbon includes applications as adsorbents for various liquids, air purification, superconducting, catalysts support, ultralight composites, fire retardant insulation, lubricants, and many other areas.1 However, these inorganic porous materials (e.g., silica) are limited by their brittleness,2 while carbon-based materials (e.g., carbon nanotubes3 and graphene4) exhibit very high conductivity, toxicity and a relatively high cost of manufacturing, shortcomings which have limited their applications. This triggered a rising demand in functionalized polysaccharides and cellulosic nanomaterials for making foams and porous materials for structural and biomedical applications due to its environmental friendliness, biocompatibility and competent specific properties.5 For various biomedical application purposes, cellulosic scaffolds are intended to display specific chemical, mechanical, and topological 1 ACS Paragon Plus Environment


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characteristics of modified pendants and reactive surface hydroxyls, thus performing as an adequate substrate onto which cells are able to attach and communicate.6,7 Thus several recent studies have attempted to prepare organic foams from polysaccharides with applications in biomedical, biophysical and composite disciplines.7-12

Figure 1. Schematic of the scaffold fabrication and its multifunctional properties: (a) Schematic of the structure of acetylated cellulose nanocrystals (ACNC). (b) ACNC/ethanol stored syringe. (c) ACNC dispersed in water. (d) Dispersed ACNC particles and growing ice crystals during freezing. (e) Ice templating of ACNC crystals in between the channels of ice crystals. (f) Internal SEM of the scaffold. (g) Schematic of the scaffold structure with protruded ester pendants. (h) Scaffold condensed with accumulated milkfat. (i) Oil saturated scaffold. (j) Water drops and paraffin oil on the top of the ACNC scaffold. Recently, cellulosic nanofibers were used in advanced applications like biological template for electrical and magnetic materials, flexible magnetic aerogels,13 stiff magnetic nanopapers and lightweight substrates for supercapacitors.14 However, most of the porous cellulosic materials are made either through an excess of organic reagents, multistep processes and high catalyst concentrations or a combination of these measures. Moreover, to date, the methods designed for the functionalisation of cellulosic nanomaterials to perform them in a hydrophobic environment are inadequate and unsatisfactory since most of them are yielding a maximum degree of substitution (DS) 1. Recently our group developed highly acetate esterified cellulose nanocrystals (ACNC) through an environmental friendly single step 2 ACS Paragon Plus Environment

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esterification method with a DS value 2.18 (Figure S1a).15 This DS was considerably higher for modified CNC than ever reported in the literature (DS range 0.1-1).16,17 The very high DS in the crystal level of the cellulosic nanomaterial highlighted the uniqueness of the synthesised bionanomaterial (Figure S1b). In this work, we explore a stable scaffold biomaterial made from this cellulosic bioresources through aqueous fabrication method. The product is unusually ultra-lightweight and holds multi-functional mesoporous properties with a potential use in various biomedical and industrial applications.

Structure of ACNC crystal with protruded ester pendants is schematically represented in Figure 1a. The schematic of the fabrication of multifunctional cellulosic scaffold by the freeze-drying ice templated structure of water dispersed ACNC is depicted in Figure 1b-e. Injecting ACNC dispersed in ethanol (Figure 1b) to water with very low concentrations (< 1 wt.%) will result a stable highly viscous jelly precipitation (Figure 1c). This is due to the inter-particle electrostatic repulsion in an aqueous environment by the protruded pendants of ACNC. The precipitated, highly viscous jelly suspension was then frozen at −70 °C (Figure 1d). The sample was then freeze-dried at a −88 °C, under vacuum, resulting in a highly porous scaffold with continuous, interconnected strong network of ultrathin modified cellulosic layers (Figure 1f). The freeze dried ice templated ACNC scaffold shows multifunctional properties of lipophilic (milkfat accumulated by scaffold, Figure 1h), oleophilic (selectively absorbed oil from an oil/water mixture, Figure 1i) and hydrophobic characteristics. The affinity to oils and highly hydrophobic behaviour of the scaffold was demonstrated by dropping paraffin oil and water on the scaffold (Figure 1j). Figure 1c-f schematically represents the dispersion of esterified cellulose particles in water medium, ice crystal growth, the mechanism of the lamellar geometry formation by ice templated structure of ultrathin cellulosic layers in the freeze-drying chamber.

When

freezing the highly viscous stable jelly water suspension of cellulosic particles with suitable granulometry, under steady-state conditions, ice crystals gradually grew in the same direction as the temperature gradient, creating a lamellar microstructure oriented in a direction parallel to the movement of the freezing front (Figure 1d).18 The forming ice crystals eventually expelled the dispersed modified cellulosic particles from the growing freezing front and squeezed them into the space between ice crystals or dendrites (Figure 1e). During the freezing process, the concentration of cellulose particles gradually increased within the channels between growing ice crystals. Consequently, all neighbouring cellulosic crystals 3 ACS Paragon Plus Environment


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accumulated in this microspace, enabling assembly of cellulosic particles after reaching a critical concentration. In sea ice, pure hexagonal ice platelets, which display randomly oriented horizontal c crystallographic axes, and various impurities present in sea water (salt, soil, biological organisms), are expelled from the core ice crystal as it takes form, and become entrapped within channels between the ice crystals. This same phenomenon, enhanced by the interaction between the modified cellulose particles19 and the ice templating of nanomaterials, drove self-organization of the esterified nanocellulosic particles (Figure 1f) into a highly porous cellulosic scaffold with protruded ester pendants15 (Figure 1g). During the freeze-drying, ice-molecules that entrapped between the cellulosic particles were gradually sublimed, leaving the template of cellulose particles intact. The concentrated cellulose particles rearranged along the freezing direction into compact and well assembled cellulose microlayers, held together by van der Waals forces and strong intermolecular (C2OH and O-2′ ester) hydrogen bonds.20

Figure 2. (a) FTIR and (b) Solid-state 13C NMR of the scaffold. To elucidate the chemical structure and multifunctional properties of the scaffold, FTIR (Figure 2a) and solid

13

C NMR analysis (Figure 2b) were performed after

freeze-drying. The distinct intensities of the FTIR absorption peak of the methyl ester group (-COO-) at 1737 cm-1 provided clear-cut evidence for presence of ester pendants on the scaffold structure. The C-H bending (1365 cm-1) and stretching vibrations of acetyl esters at 1223 cm-1 peaks, were ascribed to the methyl esters of the ACNC molecules. The normalized solid CP-MAS

13

C NMR spectra provided structure of the

basic components of the ACNC scaffold, expressed by DS and crystalline behavior. The scaffold featured two prominent signals in addition to basic cellulose structure, one at 170 ppm and the other at 20.6 ppm, which were assigned to the resonance of the

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carbonyl carbon and the terminal methyl carbons of the acetyl ester groups, respectively. Extrapolated normalized CP-MAS

13

C NMR spectra reveals that the DS

of the ester functionalisation of the scaffold was 2.18. The element detection analysis (CHNS) of CNC and scaffold were supported this relatively high DS (Figure S2). Crystalline and disordered cellulose components were detected in the solid-state

13

C

NMR spectra of the scaffold as downfield and upfield signals of C4 or C6 carbons respectively.21 FTIR and solid CP-MAS

13

C NMR analysis confirmed that the

fabricated scaffold backbone possesses highly modified cellulose molecules, which render

them

multifunctional

properties

of

lipophilicity,

oleophilicity

and

hydrophobicity. TEM analysis of the ACNC crystals (Figure S3a,b) confirmed that they displayed individual nanodimension in size. The cellulose I structure of the CNC material was preserved during the highly modified iodine catalysed esterification which was clear from the XRD analysis of ACNC (Figure S4). TEM and XRD findings confirmed the fact that the acetyl ester substitution by iodine-catalysed reaction preserved the core crystalline cellulose structure. The ice templating of this crystalline ACNC through freeze-drying in water medium forms a continuous interconnected porous cellulosic layered structure with a spongy foam and white in appearance (Figure 3a). Density and compactness of the scaffolds depends on the starting concentration of ACNC in water. SEM images of the fabricated scaffolds with different ACNC/water starting concentrations were also analysed (Figure S3c-f). They possessed densities between 3.3 kg m−3 and 7.5 kg m−3. Scaffolds were physically resistant to deformation, as shown by placing a 300 g weight on the top of 1 cm2 scaffold sample having density of 7.5 kg m−3 (Figure 3c). Density of the ACNC scaffold was lower than those reported for other cellulosic porous materials 8,10 due to the crystalline behavior of the starting nanomaterial and the effective pre-functionalization before the fabrication of the scaffold material. SEM micrographs revealed significantly distinct orientations of the assembled ACNC particles in various parts of the ice templated structure. Since the bottom and side walls were in direct contact with the freezing container, unidirectional formation of ice crystals began from the sides of the container, where ACNC concentrations were highest, resulting in stable and strong thin nanofilaments on the sides of the container (Figure 3d,e). The texture of the core structure was somewhat different from that of the outer and middle parts (Figure 3f). The middle part of the scaffolds displayed a porous structure (Figure 3h), composed of interconnected thin sheets with a pore diameter of 0.1-10 µm and a cell wall thickness of 40–80 nm (Figure 3i). Freezing starts from the peripheral portion and gradually 5 ACS Paragon Plus Environment


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slink into the middle part of the container. These thin sheets are resulted from the slower freezing of the middle part of the ACNC particles, as compared to the outer portions of the container (Figure 1d). Thus the core of the scaffolds displayed a regular two-dimensional assembly (2D) of interconnected thin sheets, the middle part of the scaffold was mostly composed of a random two and three dimensional (3D) assembly of thin networks and nanofilaments, while the outer portion of the structure featured strong, thick nanofilaments. The bottom and outer sides of the scaffolds contained strong thin sheet-like structures, which may protect them from failure upon application of internal or external forces (Figure 3d). The inner interconnected thin sheets and random networks of cellulosic nanofilaments (Figure 3fi) results highly porous structure of the scaffold material. Moreover it gives flexibility to the material and the capacity to hold absorbed solvents and oils. Thermal stability of the fabricated scaffolds expressed a notable enhancement (15.4%) from the unmodified CNC due to the ester functionalisation of the crystals together with lower thermal conductivity of the porous structure (Figure S5).19 This enhancement of the scaffolds could be advantageous for using as a scaffold for tissue engineering7, biological template for electrical and magnetic materials and other high temperature structural applications.

Figure 3. (a) ACNC scaffold made in falcon tube. (b) Scaffold made in a freezing chamber. (c) Scaffold with a density of 7.5 kg m−3 can sustain a 300 g weight. SEM of the: (d,e) outer top, (f,g) inner core, and (h,i) middle part. (j) Compressive stress−strain of the scaffolds. (k) Ultra-lightweight scaffold on the top of a dandelion without bending the flower’s seed heads. 6 ACS Paragon Plus Environment

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ACNC scaffolds are comparatively strong and its mechanical performance is competent with reported porous cellulosic materials.4,8,22 The ultra-lightweight of the material was confirmed with two different scaffolds (50+50 =100 ml) holding 0.3440 g weight (3.44 kg m-3) (Supporting information S1 movie). Figure 3j shows the compressive stress-strain analysis of the scaffolds which is typical of open-cell foam: linear elastic behavior at low strain (0-50%), followed by a cell collapse-related stress reduction at intermediate strains (50%-80%), and finally a plastic stiffening effect at high strain.23 ACNC scaffolds are flexible and the structure of the thin cellulosic layers could be preserved perfectly in the lower and intermediate compression strain area (Supporting information S2 movie). At 80% strain, the 0.2 wt. % scaffold possesses a compressive stress of 7 kPa while 0.4, 0.6, 0.8 and 1 wt. % scaffolds holds 10, 19, 27 and 41 kPa values respectively. Highly porous cellulosic scaffolds were able to absorb and dissipate the applied compressive stress which is clear from the strain region below 80%. In the plastic stiffening region, the final compressive strength shows a marked enhancement since these inner layers have been compressed in a way that the interconnected solid cellulosic layers came into contact closely with each other which increases the compressive resistance. ACNC scaffolds with highest density (1 wt. %) possess a final compressive strength of 240 kPa at 98% compressive strain. A large piece of the scaffold (35 cm3) placed on a dandelion caused no bending of the seed heads (Figure 3k), thanks to its ultra-lightweight and interconnected properties. To identify potential applications, the lipophilic, oleophilic and hydrophobic properties of the ACNC scaffolds were assessed. The selective absorptive capacities of fats and lipophilic proteins by the scaffold were assessed using cow’s milk with different fat contents (Figure S6). The dry content of normal homogenized cow’s milk is 12.5 wt.%, and constitutes fat (3.9 %), protein (3.7%) and lactose (4.8%). The dry weight of the scaffold treated with cow’s milk was 4.7-, 8.3- and 12.5-times greater than its original weight for 1, 2 and 3 wt.% fat milk, respectively. While cow’s milk contains more than 20 proteins, among which, β-lactoglobulin is among the most common food allergens, affecting mostly, but not exclusively, the infants. β-Lactoglobulin is a hydrophobic lipocalin protein, soluble in dilute salt solution as befits a globulin, with 162 amino acid residues (Mr ~18,400) that fold up into an 8-stranded, antiparallel β-barrel with a 3-turn α-helix on the outer surface and a ninth β-strand flanking the first strand.24 It binds a variety of ligands and it appears that it holds at least 3 independent binding sites namely retinol binding site, fatty acid binding site and polar aromatic binding site. Previous studies reveal that there is an internal cavity which can



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readily accommodate retinol in a manner similar to the related lipocalin, retinol-binding protein.24

Figure 4. (a) FTIR of the milk-treated (3% fat) scaffold. (b) DTG of ACNC scaffold and milk-treated (3% fat) scaffold. (c) Schematic of the β-lactoglobulin accumulated ACNC scaffold. (d) Absorption of paraffin oil with time by the scaffold from an oil/water mixture with final oil-saturated scaffold at 12s. Accumulation of milkfat and hydrophobic whey protein, β-lactoglobulin, by the scaffold were evident from FTIR (Figure 4a and Figure S7) and DTG analysis of the milk treated and untreated scaffold (Figure 4b). The FTIR analysis of the milk-treated scaffolds exhibited intense peaks of milkfat at 3267 cm-1 and 1741 cm-1 for the hydroxyl and carbonyl density of the fatty acids present in the milkfat, respectively. Absorption of whey protein βlactoglobulin was apparent from the intense peaks at 2920 cm-1 (C-H stretching) and 1637 cm1

(amide I region), due to its denatured conformations, thanks to the high temperature (60 oC)

drying process of the accumulated scaffold.25 Thermogravimetric (TGA) and its differential thermal (DTG) analysis of the milk-treated scaffolds showed two major weight reductions (195 oC and 309 oC) and a minor weight loss at 274 oC. The first and second decompositions (195 oC and 274 oC) were due to lactose and proteins degradations, while the second major degradation at 309 oC corresponded to the degradation of milkfat.26 The milk-treated scaffold 8 ACS Paragon Plus Environment

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retained 31.2% of its weight at 400 oC (Figure S8), due to the presence of milkfat components, while its untreated counterparts retained only 9.1% (Figure S5). The small endothermic peak at 274 oC at DTG of the scaffold can be ascribed to the decomposition of the β-lactoglobulin and its derivatives. On the outer surface of the β-lactoglobulin, a solventaccessible hydrophobic cleft runs between the 3-turn α-helix that is packed against the outer surface. This cleft can accommodate fatty acids like palmitate, stearate and big aromatic ligands and the lining of the cleft pocket is very hydrophobic. There is a peptide carbonyl binding-site is present in this cleft cavity which possibly makes suitable interaction with the carbonyl carbon of the ACNC molecules (Figure 4c). The high density (DS=2.18) acetate ester pendants on the surface of the scaffold material gives them strong hydrophobic characteristics. The water contact angle on the surface of the scaffold was 140o, demonstrating its highly hydrophobic behaviour (Figure S9a). The instantaneous absorption of paraffin oil (coloured with a violet dye) by the scaffold from an oil/water mixture was demonstrated (time in seconds) together with the final oil saturated scaffold (Figure 4d). The carbonyl ester decorated cellulose surfaces were able to accumulate oils and organic solvents which render them oleophilicity. The oleophilic properties of the scaffold were further investigated by evaluating its absorption capacity of other organic solvents and oils (Figure S9b). Cm values with different solvents and oils, confirming its ability to act as an effective absorbent of organic solvents and oils. Absorption capacities of the ACNC scaffolds were significantly higher than those reported for other porous and biobased absorbents. For instance, 5−25 times more liquid was absorbed by ACNC scaffold than by silane-treated nanocellulosic sponges, TiO2-coated nanofibril cellulosic aerogels,27 silicatreated polyurethane sponges28 and superoleophilic paper.29 In summary, a multifunctional cellulosic scaffold with lipophilic, oleophilic and hydrophobic characteristics was fabricated by water freeze-drying fabrication of modified CNC. The resulting interconnected highly porous scaffold was primarily composed of highly esterified, strong network of ultrathin cellulosic layers. The prepared scaffold material displayed multifunctional properties of hydrophobicity, oleophilicity and lipophilicity. Accordingly, the functionalisation method adopted and the aqueous precipitation-freeze drying fabrication of the multifunctional scaffold will provide new opportunities for the design of novel, advanced, functional biomaterials with diverse potential applications in structural and medical applications such as, scaffold for tissue engineering30, drug delivery, oil absorption and to accumulation of hydrophobic proteins, fats, and lipids.5 9 ACS Paragon Plus Environment


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ASSOCIATED CONTENT Supporting Information Experimental details, elemental analysis, TEM of ACNC, SEM of the ACNC scaffolds, TGA and DTG of CNC and scaffolds, XRD of CNC and ACNC, accumulation of oils, milkfat and proteins by the scaffold, FTIR of milkfat-treated scaffold behaviour are provided. Two movies attached to show the ultra-lightweight and flexibility properties of the scaffold material.

*

AUTHOR INFORMATION Corresponding Author Email: [email protected]

Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the financial support of Melodea Ltd, MINERVA Center Bio-Hybrid Complex Systems, PBC fellowship, and Israel National Nanotechnology Initiative (INNI).

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Multi-functional cellulosic scaffold 885x449mm (150 x 150 DPI)

ACS Paragon Plus Environment

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