Biomimetic Hybridization of Kevlar into Silk Fibroin: Nanofibrous

Biomimetic Hybridization of Kevlar into Silk Fibroin: Nanofibrous Strategy for Improved Mechanic Properties of Flexible Composites and Filtration Membranes Lili Lv,†,‡ Xiangsheng Han,†,‡ Lu Zong,† Mingjie Li,† Jun You,† Xiaochen Wu,*,† and Chaoxu Li*,†,‡ †

CAS Key Lab of Bio-based materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Songling Road 189, Qingdao 266101, PR China ‡ University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, PR China S Supporting Information *

ABSTRACT: Silk, one of the strongest natural biopolymers, was hybridized with Kevlar, one of the strongest synthetic polymers, through a biomimetic nanofibrous strategy. Regenerated silk materials have outstanding properties in transparency, biocompatibility, biodegradability and sustainability, and promising applications as diverse as in pharmaceutics, electronics, photonic devices and membranes. To compete with super mechanic properties of their natural counterpart, regenerated silk materials have been hybridized with inorganic fillers such as graphene and carbon nanotubes, but frequently lose essential mechanic flexibility. Inspired by the nanofibrous strategy of natural biomaterials (e.g., silk fibers, hemp and byssal threads of mussels) for fantastic mechanic properties, Kevlar was integrated in regenerated silk materials by combining nanometric fibrillation with proper hydrothermal treatments. The resultant hybrid films showed an ultimate stress and Young’s modulus two times as high as those of pure regenerated SF films. This is not only because of the reinforcing effect of Kevlar nanofibrils, but also because of the increasing content of silk β-sheets. When introducing Kevlar nanofibrils into the membranes of silk nanofibrils assembled by regenerated silk fibroin, the improved mechanic properties further enabled potential applications as pressure-driven nanofiltration membranes and flexible substrates of electronic devices. KEYWORDS: silk fibroin, Kevlar, nanofibrils, biomimetic hybrid, nanofiltration

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montmorillonite, apatite and aragonite) have been hybridized into regenerated silk fibroin to compete with its strongest natural analogues (e.g., natural silk fibers of spiders and Bombyx mori) as well as Kevlar, among the strongest synthetic polymers.10−13 Though exhibiting improved moduli and/or toughness, the resultant inorganic/organic composites may frequently sacrifice their flexibility due to the presence of high compositions of rigid inorganic fillers.4,14 As a matter of fact, many flexible biomaterials in nature adopt a soft nanofibrous hybridization strategy to achieve the highest performance of their polymeric building blocks. Beside the nanofibrils of silkworm silk, spiders secrete β-sheet-rich nanofibrils to produce web fibers with mechanic strength comparable to steel;15 cellulose fibrils construct hemp with high strength suitable for big product lines;16 marine mussels spin

variety of biomaterials in living systems rely on hierarchical hybridization of biomacromolecules with different natural inorganic/organic nanomaterials for incredible mechanic, optic and biological functions, such as collagen with apatite in bone, chitin with aragonite in nacre, and cellulose with lignin in wood, etc. These hybridization strategies have inspired the designing of various functional composites of biomacromolecules for biomimetic structural hierarchies and desired properties.1,2 In particular, fibers of silkworm silk (e.g., Bombyx mori) have fibrous organization (∼30 nm in diameter) of β-sheet-based crystals in the amorphous matrix of silk fibroin (SF) and sericin,3 and show outstanding mechanic properties in modulus (e.g., ∼15 GPa), strength (e.g., ∼0.5 GPa) and toughness (e.g., ∼60 KJ/kg).4,5 In order to develop silk-based functional materials (e.g., films, membranes, hydrogels, etc.) and serve diverse applications in biomedicine, wearable electronics, photonic devices, filtration membranes, etc.,6−9 synthetic/natural inorganic nanomaterials with high moduli (e.g., carbon nanotubes, graphene oxide, © 2017 American Chemical Society

Received: May 5, 2017 Accepted: July 19, 2017 Published: July 19, 2017 8178

DOI: 10.1021/acsnano.7b03119 ACS Nano 2017, 11, 8178−8184

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Figure 1. (a) Three-step procedure followed to prepare aqueously stable ANF2h, i.e., homogenizing Kevlar fibers in DMSO saturated with KOH, aqueous exchange, and hydrothermal treating. (b) Synthesis mechanism of ANF2h. (c,d) TEM images of ANFs (c) and ANF2h (d). (e,f) FT-IR spectra (e) and Zeta-potential (f) of ANFs and ANF2h.

synthesized nanofibrils were hydrothermally treated under acidic conditions to introduce functional groups. By optimizing the hydrothermal treating condition and hybridizing compositions of ANFs in the final composites of SF, the fracture strength and elastic moduli could be twice as high as those of pure regenerated SF. Furthermore, the incorporation of ANFs could also improve the mechanic properties of SF nanofibrils. The final free-standing and porous membranes showed great potential as pressure-driven nanofiltration membranes and flexible electronic devices.

collagen nanofibrils into byssal threads for strong adhesion to rocks underwater.17 Following this inspiration, many of these biological nanofibrils were incorporated into silk fibroin for biomimetic structures and higher properties. For example, regenerated silk films were reinforced with assembled nanofibrils of marine chitin (∼3 nm) and showed an improved elastic modulus from ∼0.9 GPa to 2.7 GPa.18 An improved modulus of ∼2.2 GPa was obtained by combining amyloid fibrils of β-lactoglobulin with silk fibroins.19 Silk nanofibrils were also liquid-exfoliated from natural silk and then reinforced with regenerated silk fibroin as the fillers.20 However, broad applications of these biologic nanofibrils were hindered by their complicated production procedures, limited starting materials for production and inferior toughness to their inorganic counterparts. Some mass-synthesized polymers also have super mechanic properties comparable to inorganic nanomaterials. For instance, Kevlar fibers (i.e., poly(paraphenylene terephthalamide)) could achieve a modulus of 90 GPa and a tensile strength of 3.6 GPa due to its strong intermolecular bonding interactions, being capable of serving as starting materials for body armor.21 Thus, herein we proposed a biomimetic nanofibrous strategy to incorporate this synthetic polymer into SF for multiple properties. Commercial Kevlar fibers (Kevlar 49 yarns bought from DuPont) were first liquid-exfoliated into aramid nanofibrils (ANFs) with 20−30 nm in diameter via deprotonation of amide groups in the solvent of dimethyl sulfoxide (DMSO) saturated with KOH.22 In order to improve their surface hydrophilicity and compatibility for hybridization of SF, these

RESULTS AND DISCUSSION Owing to the strong interchain interactions (e.g., π−π stacking and hydrogen bonding) within their molecular backbones as well as high molecular orientation produced during their liquidcrystal spinning process, Kevlar fibers (normally with the diameter of 101−102 μm, Figure S1a and 1a) are recognized to be superior to steel and as one of the strongest synthetic materials. Their interchain interactions could be weakened or eliminated in the polar solvent of DMSO saturated with KOH (Figure 1a,b).22 Besides, interchain electrostatic repulsion could also show up due to possible deprotonation of −NH− groups, thus leading to gradual exfoliation of Kevlar fibers into homogeneous dispersions during a vigorously homogenizing process. The exfoliated product normally has one-dimensional fibrous morphologies, i.e., ANFs (Figure 1c and S1), whose diameter of 20−30 nm was comparable to those of biological nanofibrils of cellulose, silk and amyloidal proteins.21,23 The Xray diffraction (XRD) pattern of ANFs remains as the 8179

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Figure 2. (a) Three-step procedure followed to prepare SF/ANF2h films, i.e., SF regeneration, aqueously mixing SF with ANF2h, and solution casting. (b) 2D-WAXS analysis of poststretched SF/ANF2h film. (c) FT-IR spectrum of the poststretched SF film (1) and poststretched SF/ ANF2h film (2). (d,e) Cross-section SEM images of poststretched SF (d) and SF/ANF2h films (e).

Figure 3. (a) Stress−strain curves of poststretched SF/ANF2h films with different ANF2h contents. Film thickness: 36 ± 6 μm. (b) Mechanic analysis of stress−strain curves in (a). (c) Mechanic comparison of SF/ANF2h films with other SF-based materials in the literature. References were detailed in Table S1. (d) Influence of hydrothermal treatment time of ANFxh on mechanic properties of poststretched SF/ANFxh films. ANFxh composition: 2 wt %.

nizing, ultrasonication and the addition of surfactants) and chemical treatments (e.g., nitric acid treatment) of nanomaterials all failed to prepare homogeneous aqueous dispersion of ANFs. Inspired by the aqueous treatment of carbon nanotubes,28 ANFs were hydrolyzed through an acid-assisted hydrothermal method. After hydrothermally treating in the mixture of HNO3 and H2SO4 (concentration of HNO3 8.75 wt %; concentration of H2SO4 37.5 wt %) at 120 °C for 2 h, the as-gained ANF2h not only maintained their nanofibrous morphologies, but also became well-dispersed in neutral water (Figure 1a,d). Their FT-IR spectrum revealed several emerging hydrolysis-related adsorptions (Figure 1e), e.g., the CO stretching and O−H bending vibration absorption peaks

characteristic peaks of Kevlar (Figure S1e), which could be assigned to (110), (200), and (004) reflections, respectively.22,24 It indicates that the crystalline polyparaphenylene terephthalamide structures were reserved in the nanoscale. It has been proved that ANFs could serve as a promising type of reinforcing nanofillers of composites.22 For instance, the composite of polyurethanes and ANFs had a record-high modulus of 5.28 GPa and ultimate strength of 98.02 MPa.25 However, when transferring from DMSO into aqueous conditions to hybridize silk fibroin, ANFs lost their colloidal stability due to their intrinsic hydrophobic feature.26,27 As shown in Figure S2, the conventional methods for physical dispersion (e.g., mechanically stirring, high-pressure homoge8180

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Figure 4. (a) Two-step procedure followed to prepare SF nanofibrils/ANF2h membranes, i.e., SF fibrillation in the presence of ANF2h and vacuum filtration of fibril mixture. (b) Stress−strain curves of SF nanofibrils/ANF2h membranes with different ANF2h contents. (c) Water flux of SF nanofibrils/ANF2h membranes as filtration membranes with different ANF2h compositions. Membrane thickness: 18 ± 3 μm. (d) Separation evaluation of SF nanofibrils/ANF2h membrane as nanofiltration membranes. Membrane thickness: 18 ± 3 μm, ANF2h composition: 20 wt %.

in carboxyl centered at 1685 and 1424 cm−1, the CO bonds in diketone at 1576 cm−1, and the C−O stretching vibrations at 1285 cm−1.27 Their Zeta potential analysis further confirmed that the possible formation of negatively charged oxygencontaining groups increased aqueous colloidal stability of ANF2h (Figure 1f). The negatively charged surfaces of ANF2h enabled their aqueous hybridization of silk fibroin (SF). SF molecules were normally regenerated through a sequential process of degumming, dissolving and dialysis.19,29 The regenerated SF has an isoelectric point at ∼4.3 in its aqueous solution (Figure S3a),30 and thus is negatively charged at > pH 5. At neutral pH values, the mixture of ANF2h and SF was able to maintain colloidal stability without showing aggregation (Figure 2a). When casting on a polystyrene plate, a homogeneous composite film was produced by air-drying, in sharp contrast to inhomogeneous films of SF and ANFs without hydrothermal treatment (Figure S3b). Poststretching is normally an essential procedure to produce functional materials of regenerated SF. For example, when incubating in ethanol aqueous solution (80 vol %) followed by poststretching, films of pure regenerated SF would undergo a combinational process of molecular orientation and conformational transition from random and α-coil to β-sheets. The aligned β-sheet nanocrystals may improve their mechanic properties from brittleness to toughness as physical crosslinkers.31 As a matter of fact, the composite film of SF and ANF2h also showed an improved content of aligned β-sheets after poststretching. Moreover, when exposing the film perpendicularly to incident X-ray, the azimuthally anisotropic X-ray scattering peaks (i.e., ∼14.1 and ∼6.7 nm−1) characteristic of β-strands and β-sheets stacking distances became more pronounced for the composite of SF and ANF2h (Figure 2b and S4).19 This indicates that the presence of ANF2h facilitated the formation of β-sheets during this poststretching process. The higher content of β-sheets in the composite of SF and ANFs was further confirmed by FT-IR spectra and SEM (Figure 2c−e). High shear is known to favor the formation of

β-sheets in SF materials,19 while the stretched SF/ANF2h exhibits more obvious β-sheet (1625−1640 cm−1, 1510−1530 cm−1) and β-turn (1665−1690 cm−1) peaks than pure SF.32,33 During the poststretching process, high-modulus ANF2h unambiguously produced higher shear and alignment to their adjacent SF molecules, due to excellent physical compatibility of ANF 2h and SF matrix. The fibrous cross-section morphologies (Figure 2d,e) were consistent with the modulus mismatch between ANF2h and the amorphous SF matrix, where stronger ANF2h tended to protrude and created rougher fractured surfaces. The introduction of ANF2h could greatly enhance the mechanical properties of regenerated SF materials (Figure 3 and S5). With the compositions of ANF2h as low as 2 wt %, the final composite (after poststretching) could achieve the ultimate stress, Young’s modulus and toughness of 210.43 MPa, 6.25 GPa and 833.96 MJ m−3 respectively (Figure 3a,b), which were two times as high as those of pure SF film (114.32 MPa, 3.39 GPa and 354.2 MJ m−3). Its Young’s modulus was also three times higher than that of pure ANFs films (i.e., 1.8 GPa in Figure S6). This mechanical property is also higher than or comparable to those of other SF composites with reinforcing nanofillers (e.g., carbon nanotube and other inorganic materials) (Figure 3c and Table S1). The reinforcing effect of ANFs also depends strongly on the period time of their hydrothermal treatment. With shorter time of hydrothermal treatment (e.g., 1 h), the ANF1h still lacked essential colloidal stability in water to prepare homogeneous composites with SF (Figure S7a), thus leading to much poor mechanic properties (Figure S7). With much longer hydrothermal treatment (e.g., >3 h), ANFs started to lose their nanofibrous morphologies and destroyed into irregular pieces (Figure S7b,c). Though having better transparency (Figure S7e), the final composite films gave degenerative mechanic properties (Figure 3d, S7f,g). The optimal hydrothermal treatment was proved to be 2 h, which could maintain nanofibrous morphologies of ANFs while endow them with proper aqueous stability and compatibility with SF matrix. 8181

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pores (nonzero rejection may be due to different charged states of metal ions and SF nanofibrils). We further evaluated the recyclability of SF nanofibrils/ANF2h membranes by rinsing with H2O or ethanol after filtration (Figure S14). The dyes (or proteins) left on the surface of membranes could be easily removed by immersing the membranes in ethanol (or H2O) for 1 h. The porous nanostructure and separation performance of these membranes could well maintain after reusing for 7 times, allowing their potential applications as filter membranes. Thanks to their mechanical properties, the hybrid membranes of SF nanofibrils and ANF2h could also serve as flexible substrates of electronic devices. By vacuum-filtering on the SF nanofibrils/ANF2h hybrid membrane, Au single-crystal microflakes (synthesized with the assistance of SF nanofibrils in Figure 5a) were patterned on this flexible membrane (Figure

These are all essential to maximize the reinforcing effect of ANFs for stronger SF composites. Recently biological nanofibrils attracted great interests to prepare nanofiltration membranes with super biocompatibility and biodegradation, SF nanofibrils (assembled by SF) received particular attention for applications due to their natural abundancy and facile preparation.34 However, SF nanofibrils produced through physical exfoliation from natural fibers were normally highly branched and had uneven diameters,35 thus showing the difficulty in controlling uniform nanopores for nanofiltration. On the other hand, SF nanofibrils assembled “bottom-up” from aqueous SF solutions,19 though having welldefined nanofibrous morphologies (Figure S8), were normally brittle and resulted in the difficulty in preparing freestanding and tough nanofiltration membranes. For instance, a typical membrane from pure SF nanofibrils had an ultimate stress at ∼13 MPa and strain at break of ∼1.35%.19 With nanofibrous morphologies as well as high toughness,36 ANFs would be a promising type of reinforcing filler to produce nanofiltration membranes of SF nanofibrils for practical applications. By coincubated ANF2h with the SF solution (7 vol % ethanol at pH 9.5) for 2 days, SF nanofibrils grew in the presence of ANF2h (Figure 4a), and homogeneous hybrid membranes were then produced by vacuum-filtering the resultant solution (Figure S9a−i). The final hybrid membrane had improved mechanic properties by gradually increasing the composition of ANF2h up to 20 wt % (Figure 4b and S9j). The ultimate stress of 52.4 MPa and toughness of 143.2 MJ m−3 were achieved, which were much higher than the membranes produced by pure SF nanofibrils (2.7 MPa, 1.9 MJ m−3). In sharp contrast, heterogeneous morphologies and relatively low mechanical properties were obtained in the membranes prepared by directly mixing SF nanofibrils and ANF2h (Figure S10). The improved mechanic properties of SF nanofibrils/ANF2h hybrid membranes may be an indicator of strong interfacial interactions between SF nanofibrils and ANF2h, which prevent the crack growth and pull-out of nanofibrils during tensile tests (Figure S11). The free-standing hybrid membranes (thickness of 4−25 μm) with nanoporous structure and good mechanical properties promised a potential application in pressure-driven filtration. Their water flux could be tuned not only by the ANF2h composition, but also by their thicknesses. With the composition of ANF2h of 50 wt %, the hybrid membrane (thickness 18 ± 3 μm) had the water flux much higher than the membrane produced from either pure SF nanofibrils or pure ANF2h (Figure 4c). This water flux matched well with the pore size distribution of these membranes (Figure S12). Notably, the membrane of pure ANF2h had relatively low pore sizes because of possible structural collapse during the drying process, which may be resulted from π−π interactions and hydrogen bonding among ANF2h. Moreover, the separation performance of the SF nanofibrils/ANF2h membrane could also be adjusted through changing the membrane thickness (Figure S13a), e.g., the water flux of 300 L h−1 m−2 bar−1 with the thickness of ∼4 μm. As shown in Figure 4d and S13b, the pore size was proved to be a crucial factor for separation performance, the cutoff size was evaluated to be 2−3 nm for SF nanofibrils/ANF2h membrane with ANF2h composition of 20 wt % and thickness 18 ± 3 μm. The dyes (e.g., Rhodamine B and Congo red), proteins (e.g., bovine serum albumin and lysozyme), and Au nanoparticles (the size of ∼6 nm) all have the rejection above 96%, while metal ions (e.g., Cr3+, Co2+) passed through the nanometric

Figure 5. (a) Pathway and SEM image of Au microflakes synthesized in the presence of SF nanofibrils. The inset gives optic microscopic image of Au microflakes. (b) Patterning Au microflakes on SF nanofibrils/ANF2h membrane as flexible electronic circuit. The inset shows few layers of Au microflakes depositing on the membrane. (c) Conductivity evaluation of flexible electronic circuit.

5b, S15a and S15b), which could be twisted, bended and wrapped without showing any clear conductivity decay (Figure 5 and S15). Additionally, due to the outstanding biocompatibility and biodegradability of SF, the substrate would crack into pieces after incubating in the solution of protease K (0.1 mg/ mL) for 10 days (Figure S15d).

CONCLUSIONS Through Kevlar fibrillation in combination with proper hydrothermal treatments, Kevlar was proved to be a promising reinforcement agent with good mechanical performance and excellent compatibility with silk fibroin (SF). The biomimetic nanofibrous strategy enabled successful hybridization of regenerated SF films with Kevlar nanofibrils with the ultimate stress and Young’s modulus 2-fold. Moreover, Kevlar nanofibrils could also hybridize and reinforce the membrane of silk nanofibrils. The reinforced mechanic properties promise potential applications as pressure-driven filtration membranes and flexible substrates of electronic devices. 8182

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under the applied pressure of 1 bar by using CrCl3, CoCl2, bovine serum albumin, lysozyme, Au nanoparticles (∼6 nm),23 Rhodamine B, and Congo red as the model filtering objects. The concentrations of filtering objects before and after the filtration were measured by a UV−vis spectrophotometer. Au Microflakes Patterning on the Membrane. Au microflakes were synthesized as the following: 40 mL of SF nanofibrils dispersion (0.1 wt %) was adjusted to pH 2. Chloroauric acid was then added to achieve the final concentration of 5 mM. Au microflakes were obtained by incubating the mixture at 80 °C for 24 h. After washing for 3 times and dispersing in H2O, Au microflakes were loaded on the membrane through vacuum filtration. Characterization. Morphologies and structure of the samples were characterized by transmission electron microscopy (TEM) (Hitachi H7650, Japan). A drop of the solution was casted onto a carbon support film on a copper grid, and the excess solution was removed after 30 s by blotting using a filter paper. Emission scanning electron microscopy (FESEM) (JEOL 7401, Japan) was used at acceleration voltage of 10 kV. Atomic force microscope (AFM) in tapping mode was performed on Agilent 5400 at a scan rate of 1 Hz equipped with silicon nitride cantilevers (Bruker). Wide angle X-ray Scattering (WAXS) measurements were carried out using a Rigaku microfocused source based on Cu Kα radiation (λ = 0.154 nm), the sample-to-detector distance gives the scattering vector windows of 1.9−20 nm−1. The recorded scattered intensity distributions, collected on a Triton 200 mm gas-filled multiwire detector, were azimuthally integrated yielding scattered intensity as functions of the scattering vector. Zeta potential testing was carried by NanoBrook 90 plus Pals (Brookhaven, American). Fourier transform infrared (FTIR) analyses were performed on a Nicolet 6700 FT-IR spectrometer (American). X-ray diffraction (XRD) measurements were taken on a X-ray diffractometer (Bruker D8 ADVANCE) using Cu Kα (λ = 1.5406 Å) radiation. UV−vis spectrophotometric analyses were performed using a DU800 UV−vis spectrophotometer. An electromechanical universal testing machine (CMT 6503, MTS systems China Co. Ltd.) was used to evaluate mechanical properties of the composites with the gauge length of 15 mm and the tensile speed of 2 mm/min, and each sample was tested for at least 3 parallel samples. The rectangular samples (5 × 40 mm2) were equilibrated at ∼25 °C and relative humidity of ∼50% for 24 h for further measurements.

MATERIALS AND METHODS Materials. Bombyx mori cocoon was collected from Shandong, China. Kevlar 49 yarns (density: 1.44 g/cm3, elongation: 2.4%, modulus: 112 GPa, tenacity: 2.08 N/tex, Mw: 40 000 g/mol) were obtained from DuPont. Proteinase K was bought from Solarbio Science & Technology Co., Ltd., Beijing. Bovine serum albumin (BSA) was bought from Sangon Biotech Co., Ltd., Shanghai. Lysozyme was purchased from Sigma-Aldrich. Other reagents were obtained from Sinopharm Chemical Reagent Co. Ltd., China. Ultrapure water (resistance: 18.2 MΩ cm−1) was used to prepare all the solutions. Regeneration of Silk Fibroin. Silk fibroin was regenerated following a sequential procedure of degumming, dissolving, and dialysis as described in the literature.29 In brief, Bombyx mori cocoon was degummed twice in the boiling solution of 0.5 wt % NaHCO3 for 30 min. After thoroughly washing in deionized water, degummed silk fibroin was dissolved in the aqueous solution of LiBr (9.3 M) at 60 °C for 1 h under continuously stirring. The obtained solution of silk fibroin (10 wt %) was then dialyzed with Millipore water at 4 °C, followed by centrifugation at 8000 rpm to remove any insoluble residue. The dialyzed solution was further concentrated up to ∼5 wt % through reverse dialysis against polyethylene glycol aqueous solution (10 wt %, 20 kDa) at 4 °C. Liquid Exfoliation and Hydrothermal Treatment of Kevlar into ANFs. In a typical experiment,22 bulk Kevlar yarns (1.0 g) were homogenized vigorously in DMSO (500 mL) at 25 °C for 7 days in the presence of KOH (1.5 g), followed by filtrating and washing with deionized water. The obtained solid (10 mg) was then dispersed in a mixture (25 mL) of HNO3 (concentration: 8.75 wt %) and H2SO4 (concentration: 37.5 wt %), and heated in an autoclave at 120 °C for different periods of time (1−4 h), the products were denoted as ANFxh according to the hydrothermal reaction time (x = 0, 1, 2, 3, 4). After cooling down to room temperature, the product was filtrated and thoroughly washed with deionized water to neutral pH, whose viscosity-average molecular weight of ∼18 300 g/mol was determined by Ubbelohde viscometer in 100.3 wt % (0.3 wt % excess SO3) sulfuric acid as the solvent. Hybridization of SF with ANFs. The solution of ANF2h (5 mg/ mL) was ultrasonicated for 5 min and then mixed with the solution of SF (50 mg/mL). After casting on a polystyrene weighting plate (diameter of 5 cm), the composite films were air-dried in a desiccator. Their thicknesses were controlled by the amount of solution mixture for casting and measured by using SEM. The composition of ANF2h was tuned within 1−6 wt %. Poststretching treatment was performed as the following:22 The films were submerged in ethanol solution (80 vol %) for 2 h and then in H2O for 30 min, before mounting on a tensile testing machine and stretching to an optimized strain of 100% at a drawing speed of 20 mm/min. These films were dried at the final strain at ∼25 °C and relative humidity of ∼50% for 2 h. The toughness (MJ/m3), a parameter that demonstrates the work required to fracture the sample per unit volume, was calculated from the area below the tensile stress−stretch curve until fracture. The Young’s modulus (GPa) was calculated from the slope of the initial linear position of the stress−stretch curves. Hybridization of SF Nanofibrils with ANFs. Ethanol was added into the aqueous solution of SF to achieve the silk concentration of 0.2 wt % and ethanol composition of 7 vol %. After adding different amount of ANF2h (5, 10, 20, 50 wt %), the mixture was incubated at 25 °C and pH 9.5 for 2 days to assembly SF nanofibrils. The final composite membranes were fabricated by vacuum-filtrating the mixture with filtering membranes (pore size: 0.22 μm, diameter: 4.4 cm). Their thicknesses were controlled by the amount of mixture for filtration. Separation Evaluation of Filtering Membranes. Water flux was evaluated by vacuum-filtering water through the filtering membrane (inner diameter: 44 mm) under 1 bar and calculated by J = V/(A·t·p), where V is the volume of water (unit: L), A is the effective film filtration area (unit: m2), t is the filtration time (unit: h) and p is the applied pressure (unit: bar). The rejection ratio was evaluated

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03119. Optical images, Zeta-potential, WAXS profiles, SEM images, TEM images, AFM images, XRD partterns, pore size distributions, stress−strain curves and mechanical properties of SF, SF nanofibrils, ANFs, SF/ANFs, and SF nanofibrils/ANFs; Photographs, recyclability and water flux of SF nanofibrils/ANF2h membrane; Characterizations of Au microflakes patterned SF nanofibrils/ ANF2h membrane (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jun You: 0000-0001-8918-0930 Chaoxu Li: 0000-0003-0276-834X Notes

The authors declare no competing financial interest. 8183

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ACKNOWLEDGMENTS Chinese “1000 youth Talent Program”, National Natural Science Foundation of China (No. 21474125), Shandong “Taishan Youth Scholoar Program”, Shandong Provincial Natural Science Foundation (No. JQ201609 and ZR2016EEB25) and Shandong Collaborative Innovation Center for marine biomass fiber materials and textiles are kindly acknowledged for financial support. The authors also thank Dr. Antoni Sánchez-Ferrer from Swiss Federal Institute of Technology (ETH Zürich) for his kindly help on WAXS. REFERENCES (1) Li, C.; Born, A. K.; Schweizer, T.; Zenobi Wong, M.; Cerruti, M.; Mezzenga, R. Amyloid-Hydroxyapatite Bone Biomimetic Composites. Adv. Mater. 2014, 26, 3207−3212. (2) Wegst, U. G.; Bai, H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O. Bioinspired Structural Materials. Nat. Mater. 2015, 14, 23−36. (3) Xu, G.; Gong, L.; Yang, Z.; Liu, X. What Makes Spider Silk Fibers so Strong? From Molecular-Crystallite Network to Hierarchical Network Structures. Soft Matter 2014, 10, 2116−2123. (4) Koh, L. D.; Cheng, Y.; Teng, C. P.; Khin, Y. W.; Loh, X. J.; Tee, S. Y.; Low, M.; Ye, E.; Yu, H. D.; Zhang, Y. W.; Han, M. Y. Structures, Mechanical Properties and Applications of Silk Fibroin Materials. Prog. Polym. Sci. 2015, 46, 86−110. (5) Shao, Z.; Vollrath, F. Materials: Surprising Strength of Silkworm Silk. Nature 2002, 418, 741−741. (6) Zhu, B.; Wang, H.; Leow, W. R.; Cai, Y.; Loh, X. J.; Han, M. Y.; Chen, X. Silk Fibroin for Flexible Electronic Devices. Adv. Mater. 2016, 28, 4250−4265. (7) Kundu, B.; Kurland, N. E.; Bano, S.; Patra, C.; Engel, F. B.; Yadavalli, V. K.; Kundu, S. C. Silk Proteins for Biomedical Applications: Bioengineering Perspectives. Prog. Polym. Sci. 2013, 39, 251−267. (8) Vepari, C.; Kaplan, D. L. Silk as a Biomaterial. Prog. Polym. Sci. 2007, 32, 991−1007. (9) Tansil, N. C.; Koh, L. D.; Han, M. Y. Functional Silk: Colored and Luminescent. Adv. Mater. 2012, 24, 1388−1397. (10) Hu, K. S.; Gupta, M. K.; Kulkarni, D. D.; Tsukruk, V. V. UltraRobust Graphene Oxide-Silk Fibroin Nanocomposite Membranes. Adv. Mater. 2013, 25, 2301−2307. (11) Kharlampieva, E.; Kozlovskaya, V.; Gunawidjaja, R.; Shevchenko, V. V.; Vaia, R.; Naik, R. R.; Kaplan, D. L.; Tsukruk, V. V. Flexible Silk-Inorganic Nanocomposites: from Transparent to Highly Reflective. Adv. Funct. Mater. 2010, 20, 840−846. (12) Römer, L.; Scheibel, T. The Elaborate Structure of Spider Silk. Prion 2008, 2, 154−161. (13) Chae, H. G.; Kumar, S. Rigid-Rod Polymeric Fibers. J. Appl. Polym. Sci. 2006, 100, 791−802. (14) Yin, Y.; Hu, K.; Grant, A. M.; Zhang, Y. H.; Tsukruk, V. V. Biopolymeric Nanocomposites with Enhanced Interphases. Langmuir 2015, 31, 10859−10870. (15) Nova, A.; Keten, S.; Pugno, N. M.; Redaelli, A.; Buehler, M. J. Molecular and Nanostructural Mechanisms of Deformation, Strength and Toughness of Spider Silk Fibrils. Nano Lett. 2010, 10, 2626−2634. (16) Elsakhawy, M.; Hassan, M. L. Physical and Mechanical Properties of Microcrystalline Cellulose Prepared from Agricultural Residues. Carbohydr. Polym. 2007, 67, 1−10. (17) Coyne, K. J.; Qin, X.-X.; Waite, J. H. Extensible Collagen in Mussel Byssus: A Natural Block Copolymer. Science 1997, 277, 1830− 1832. (18) Jin, J.; Hassanzadeh, P.; Perotto, G.; Sun, W.; Brenckle, M. A.; Kaplan, D.; Omenetto, F. G.; Rolandi, M. A Biomimetic Composite from Solution Self-Assembly of Chitin Nanofibers in a Silk Fibroin Matrix. Adv. Mater. 2013, 25, 4482−4487. (19) Ling, S.; Li, C.; Adamcik, J.; Shao, Z.; Chen, X.; Mezzenga, R. Modulating Materials by Orthogonally Oriented β-Strands: Compo8184

DOI: 10.1021/acsnano.7b03119 ACS Nano 2017, 11, 8178−8184