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Bioinspired, Ultrastrong, Highly Biocompatible, and Bioactive Natural Polymer/Graphene Oxide Nanocomposite Films Wen-Kun Zhu, Huai-Ping Cong, Hong-Bin Yao, Li-Bo Mao, Abdullah M. Asiri, Khalid A. Alamry, Hadi M. Marwani, and Shu-Hong Yu* With the fast development of science and technology, one of the major scientific challenges in the field of materials science is to create novel, multifunctional materials with advanced performance, and therefore, build strong foundations for diverse fantastic fields.[1] In the process of evolution, nature has point out intrinsic laws to produce lightweight, strong, and hierarchical materials with exceptional properties and functionalities.[2,3] Biological materials, such as tooth, bone, and nacre, are complex, hierarchical, and heterogeneous nanocomposites providing superior mechanical properties and biocompatibility.[4,5] Thus, design and fabrication of bioinspired natural/biological materials with controllable architecture, especially with outstanding mechanical properties, is a viable approach for developing advanced functionalities. Inspired by the unique multiscale and multilevel “brickand-mortar”(B&M) structure in nacre, a series of bio-inspired
Dr. W.-K. Zhu, Dr. H.-B. Yao, Dr. L.-B. Mao, Prof. S.-H. Yu Division of Nanomaterials and Chemistry Hefei National Laboratory for Physical Sciences at Microscale Collaborative Innovation Center of Suzhou Nano Science and Technology Department of Chemistry The National Synchrotron Radiation Laboratory University of Science and Technology of China Hefei, Anhui 230026, P. R. China E-mail: [email protected] Dr. W.-K. Zhu State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials Southwest University of Science and Technology Mianyang, Sichuan 621000, P. R. China Prof. H.-P. Cong School of Chemistry and Chemical Engineering Hefei University of Technology Hefei, Anhui 230039, P. R. China Prof. A. M. Asiri, Dr. K. A. Alamry, Dr. H. M. Marwani Center of Excellence for Advanced Materials Research Chemistry Department Faculty of Science King Abdulaziz University Jeddah 21589, Saudi Arabia DOI: 10.1002/smll.201500486
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highly intense and ultratough composites have been reported by adopting diverse self-assembly techniques such as layerby-layer (LBL) assembly, vacuum filtration-assisted assembly, freeze-drying assembly, interface-assisted self-assembly and Langmuir–Blodgett (LB) assembly.[6–10] In these cases, 2D inorganic assembly units including glass flake, alumina flake, graphene oxide (GO), layered double hydroxides (LDH), nanoclay and flattened double-walled carbon nanotube, etc., serve as “bricks”, while polymers act as “mortar”.[2,11] Notably, GO nanosheet is an ideal kind of 2D inorganic building blocks for assembly of nacre-like materials, owing to its unique physicochemical properties and easily-accessed advantage.[12–14] Among GO-based composites, nacre-like GO-polymer materials have attracted great interests due to the endowed excellent mechanical performances.[15–17] Till now, a large number of reports have been focused on the assembly of GO with the synthesized polymers, rather than natural polymers. However, in consideration of the growing exhaustion of fossil resources, such as petroleum and coal in polymer industries, and the resulted wastes from the nondegradable organic polymer materials, it is pressing to reduce the production of traditional petroleum-based material and develop the novel composites with natural polymers in chemistry and materials science.[18–21] As a renewable natural polymer, konjac glucomannan (KGM) exhibits excellent biocompatible and biodegradable properties.[22,23] Meanwhile, it possesses great advantages of gelling, film-forming, antibacterial action, and low calorific value, enabling its broad applications in chemical, biological, food, and medical industries.[24–28] Furthermore, KGM is a kind of promising components for the construction of strong material with GO through the hydrogen-bond interaction, etc., due to abundant of hydroxyl groups in its polymer chains. In the present study, we report the preparation of KGM– GO nanocomposite films with nearly B&M hierarchical structure via a simple solution casting method. Compared with pure KGM film, the Young’s modulus and tensile strength of KGM–GO nanocomposite film with 7.5% GO are increased by 92.6% and 151.6%, respectively. It is worth to note that the ultimate tensile strength of such KGM–GO nanocomposite film can reach up to 183.3 MPa, exceeding many other biopolymer/GO films. Furthermore, the as-prepared
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the cross-section of the composite film became rough with the well-defined layered structure. Furthermore, it was clearly shown that the tensile fracture surfaces of KGM–GO-7.5% film had typical ductile fracture characteristics (Figure 1d). On the other side, due to the enhanced surface roughness of KGM–GO nanocomposites with the increased GO content, the contact angles of KGM, KGM–GO-2.5%, and KGM–GO-7.5% films were 82°, 94°, and 117°, respectively, revealing the increased Figure 1. a) Schematic illustration of the fabrication procedure of KGM–GO nanocomposite hydrophobicity (Figure S2, Supporting films. Cross-sectional SEM images of b) KGM, c) KGM–GO-2.5%, and d) KGM–GO-7.5% Information).[29,30] nanocomposite films. In order to understand the chemistry of KGM and GO interactions, KGM–GO nanocomposite films show high biocompat- we characterized KGM and KGM–GO nanocomposite ibilities for boosting the growth of RAW264.7 cells and films using FT-IR spectra (Figure 2a). From the spechydroxyapatite (HAP) nanocrystals. These results indicate trum of pure KGM, it was investigated that the absorpthat the obtained KGM–GO nanocomposites have potential tion bands of 3315, 2886, 1722, 1639, and 1061 cm−1 were applications in food package, scaffold for tissue engineering, assigned to OH, C H, C O, C O and C6 OH groups, respectively.[31–34] While the absorption bands at 870 and and so on. The surface morphology of GO nanosheets was char- 819 cm−1 were characteristic vibrations of the mannose acterized by transmission electron microscopy (TEM) and unit in KGM.[25,38] Comparing the spectra of pure KGM atomic-force microscopy (AFM) (Figure S1, Supporting and KGM–GO nanocomposite films, the following changes Information). The TEM images showed that most GO sheets took place in the nanocomposite films. The vibration bands had obvious wrinkles. The typical tapping-mode AFM image of OH, C O and C O were weakened and down-shifted of GO sheets revealed that most GO sheets had heights of to 3309, 1715, and 1634 cm−1, respectively, indicating the ≈1.14 nm, which was characteristic of a fully exfoliated gra- strong intermolecular hydrogen bonds between GO and phene sheet. This favorable structure and oxygen-containing KGM.[34] Such confirmed interactions in the assembly films functional groups of GO facilitate its assembly with polymer would build solid foundations for the resulted enhanced into 2D film. Figure 1a illustrates the simple, efficient, and mechanical properties. The rheological properties of KGM and KGM–GO soluscalable preparation procedures of KGM–GO nanocomposite films by using GO as the nanoscale building block tions used for film casting were investigated to further dig and natural polymer KGM as the organic adhesion agent, out the interactions of KGM with GO (Figure 2b). It showed respectively. In the assembly process, KGM–GO solution that both of the storage modulus G′ and loss modulus G″ had was stirred and followed by sonication to allow full interac- the raised tendency with increasing GO content (Figure 2c). tions between KGM molecules and GO nanosheets. Dried This result was probably induced by the interactions between in a Petri dish at room temperature, the nanocomposite film GO and KGM, also beneficial for enhancing the mechanical was formed during such a low-speed evaporation-induced properties of the as-prepared nanocomposites.[35,36] Clearly, the color of KGM–GO mixtures was changed from colorassembly process. The representative cross-sectional SEM images of KGM less to dark brown with weight percentages of GO to KGM and KGM–GO nanocomposites with different GO concen- increased from 0 to 7.5% (inset picture in Figure 2c). With trations are shown in Figure 1b–d. As GO content increased, increasing GO contents, the slope of storage modulus against
Figure 2. a) FT-IR spectra of KGM and KGM–GO-7.5% nanocomposite films. Rheological properties of KGM and KGM–GO solutions: b) the strain scanning and c) the moduli dependent on GO concentration in the form of linear coordinates, inset showing the color changes of KGM–GO solutions from colorless to dark brown with the increased GO contents. small 2015, 11, No. 34, 4298–4302
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Figure 3. Mechanical properties of KGM and KGM–GO with different GO concentrations. a) Stress–strain curves of KGM–GO nanocomposite films. b) Effect of GO weight fraction on Young's modulus and ultimate strain of KGM–GO nanocomposite films. c) GO-based composite films with different kinds of inorganic building blocks (Black symbols: artificial polymer-GO composites; Blue symbols: natural polymer-GO composites; Brown symbol: pure GO. Red symbol: our KGM–GO-7.5% nanocomposite film; PCDO: 10, 12-pentacosadiyn-1-ol, PAH: poly(allylamine) hydrochloride, PVA: poly(vinyl alcohol), PMMA: poly(methyl methacrylate), PDA: polydopamine, PEI: polyetherimide, PAA: polyallylamine, CS: chitosan, SF: silk fibroin.
GO concentration of KGM–GO solutions was much steeper than that of loss modulus. For example, compared with pure KGM, G′ of the mixture with 7.5% GO content was significantly increased by 136.5%, while G″ was improved by 102.9%. Furthermore, it was found that the storage modulus G′ and loss modulus G″ of pristine GO were very low, indicating the small effect of GO contents on the modulus of the suspensions (Table S1 and Figure S3, Supporting Information). These analysis suggested the strong interactions of GO with the molecular chains of KGM and the restricted mobility of KGM molecules. Then, equations (1) and (2) were simulated to measure the interaction between KGM and GO. The contribution of GO on increasing the KGM’s modulus was also shown in Figure S4 (Supporting Information), which remaped Figure 2c logarithmically (Table S2, Supporting Information). It was shown that, the increase of KGM modulus followed a power function law in the concentration of GO. As for its storage modulus, the power function was Y1 = 2 + 0.014 X 1.6
(0 ≤ X ≤ 7.5)
(1)
And for its loss modulus Y2 = 1.8 + 0.005 X 2
(0 ≤ X ≤ 7.5)
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(2)
where Y1 and Y2 represent log (G′) and log (G″) respectively, and X% is the weight percentage of GO to KGM. From the power function, we can make the interaction rule between KGM and GO visualized. In pristine KGM solution, the storage modulus is larger than loss modulus, indicating that the reversible entanglements in KGM molecules is more than irreversible entanglements, and the interaction between KGM is relatively loose not so intensive as that in irreversible entanglements system.[37] However, incorporated with GO, the index of power function of loss modulus is larger than that of storage modulus. It indicates that the increase of irreversible entanglements exceeds the reversible entanglements in KGM–GO solution, and the interaction between KGM– GO molecules became much stronger. Therefore, GO indeed contributes to the strong interaction through hydrogen bond due to its abundant of oxygen-containing groups. The above-mentioned analysis quantified the dependence of rheological properties of composites on the GO concentration, and proved the controlled properties of KGM by the incorporated GO content. These strongly indicated the intensive interactions between KGM molecules and GO nanosheets. In order to prove the strong interactions between KGM and GO contributing to enhanced mechanical properties of their macroscopic assemblies, the tensile stress–strain curves were carried out on pure GO and KGM–GO nanocomposite
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Such a novel stability in aqueous solution was attributed to the formation of B&M hierarchical structures between KGM and GO nanosheets under their strong interactions.[39,40] Moreover, with the assembly of GO into KGM, the thermal stability was correspondingly improved, as revealed from TG curve of KGM–GO-7.5% composite film moved toward the high temperature zone (Figure S8, Supporting Information). Furthermore, the cytotoxicity of KGM and KGM–GO film was evaluated by culturing RAW264.7 cells on them. KGM or KGM–GO film was firstly casted onto plates, subsequently rinsed intensively with deionized (DI) water to remove residual dust. The morphology of cells was recorded by microscopy. As shown in Figure 4a–c, RAW264.7 cells with well-defined central nucleus were flatly spreading on KGM or KGM–GO film. KGM and KGM–GO film were noncytoFigure 4. The morphology images of RAW264.7 cells grown on control plates and films for toxicity, as confirmed by more than 90% 48 h. a) control sample film, b) KGM film, c) KGM–GO-7.5% nanocomposite films, respectively, of the cells alive and forming a confluent and subsequently stained with calcein (green) and propidium iodide (red) as a part of the monolayer on the film surface after 48 h of standard live-dead test (scale bars, 50 µm). d) Cell viability of RAW264.7 cells on control exposure through the confirmatory live– plate and film measured by cell counting assay after being cultured for 0, 24, 48, and 72 h, dead test (calcein and propidium iodide respectively. Error bars indicate the standard deviation (SD). staining) on the cells. Meanwhile, the cell number assay was conducted to examine films with different weight percentages of GO to KGM in the viability of RAW264.7 cells on KGM or KGM–GO film. Figure 3. Compared with pure KGM, Young’s modulus and As shown in Figure 4d, the proliferation of cells was not tensile stress of KGM–GO composite were significantly influenced by different kinds of films within culture time. enhanced with increasing GO contents (Figure 3a,b). When The number of RAW264.7 cells on KGM or KGM–GO the GO content was high to 7.5%, Young’s modulus and ten- film was almost the same as that on control plates after 24, sile strength of the composite were increased by 92.6% and 48, and 72 h of culture (Figure 4d). These results indicated 151.6%, achieving 16.8 GPa and 183.3 MPa, respectively. that such nontoxic, high-strength KGM–GO film could Compared with the reported composite films as plotted in be used as ideal substrate for cell adhesion and growth, Figure 3c, KGM–GO-7.5% film presented in our work exhib- promising it a potential scaffold for tissue engineering and ited wonderful mechanical properties. Its tensile stress was biomedical device. Moreover, the biomineralization perseveral times higher than previous GO-based nanocomposite formances of KGM and KGM–GO-7.5% nanocomposite films and papers,[1,8,9,12–15,17,19–21] for example, 2.28 times films were also proved. SEM and XRD results showed higher than synthesized polymers-GO films (PVA–GO film, that more HAP nanocrystals were grown on KGM– 80.2 MPa),[8] and 1.37 times higher than natural polymers- GO-7.5% film, indicating the improved bioactivity of KGM GO films (CS–GO film, 133.1 MPa), etc.[19] Nevertheless, through the composite with GO nanosheets (Figure S9, when the GO content was further improved to 10%, the Supporting Information). tensile strength of the composite film was greatly decreased In summary, we have demonstrated an easy, safe, and (Figure S5a, Supporting Information). The high viscosity of simple assembly method to fabricate KGM–GO comKGM damaged the uniform dispersion of GO at a high ratio posite films under the strong hydrogen-bonding interactions and resulted in the aggregation, which was responsible for between GO nanosheets and KGM molecules. The resulted such an attenuation of mechanical property, as revealed from KGM–GO-7.5% composite film exhibits significantly the appearance of characteristic diffraction peak of GO at enhanced mechanical properties, higher than any other GO11° in the XRD pattern of KGM–GO-10% film (Figure S6, based films reported so far. Significantly, the composite films Supporting Information). also possess excellent biocompatibility and bioactivity toward Fantastically, the prepared KGM–GO-7.5% composite growth of RAW264.7 cells and HAP. The strong, biocompatfilms showed the strong water-resistant performance, as ible, and bioactive natural polymer/GO composite films prerevealed from the nonredispersion of the KGM–GO-2.5% sented in our work show the promise for wide applications and KGM–GO-7.5% composite films after soaking in water in diverse fields of food package, tissue engineering, and biofor 15 d by shaking (Figure S7, Supporting Information). logical scaffold. small 2015, 11, No. 34, 4298–4302
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements W.-K.Z., H.-B.Y. contributed equally to this work. This work is supported by the Ministry of Science and Technology of China (Grant Nos. 2014CB931800 and 2013CB933900), the National Natural Science Foundation of China (Grant Nos. 21431006, 91227103, 21061160492, and J1030412), Scientific Research Grant of Hefei Science Center of CAS (Grant No. 2015SRG-HSC038), Hainan Province Science and Technology Department (Grant No. CXY20130046), Sichuan Province Science and Technology Pillar Program(2014GZ0185), and the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (12zxnp08).
[1] Y. Q. Li, T. Yu, T. Y. Yang, L. X. Zheng, K. Liao, Adv. Mater. 2012, 24, 3426. [2] Q. Cheng, L. Jiang, Z. Tang, Acc. Chem. Res. 2014, 47, 1256. [3] L. J. Bonderer, A. R. Studart, L. J. Gauckler, Science 2008, 319, 1069. [4] L. Li, C. Ortiz, Nat. Mater. 2014, 13, 501. [5] E. Munch, M. E. Launey, D. H. Alsem, E. Saiz, A. P. Tomsia, R. O. Ritchie, Science 2008, 322, 1516. [6] S. Deville, E. Saiz, R. K. Nalla, A. P. Tomsia, Science 2006, 311, 515. [7] Y. Gao, L.-Q. Liu, S.-Z. Zu, K. Peng, D. Zhou, B.-H. Han, Z. Zhang, ACS Nano 2011, 5, 2134. [8] K. W. Putz, O. C. Compton, M. J. Palmeri, S. T. Nguyen, L. C. Brinson, Adv. Funct. Mater. 2010, 20, 3322. [9] A. Satti, P. Larpent, Y. Gun'ko, Carbon 2010, 48, 3376. [10] A. Tao, P. Sinsermsuksakul, P. Yang, Nat. Nanotechnol. 2007, 2, 435. [11] H. B. Yao, H. Y. Fang, X. H. Wang, S. H. Yu, Chem. Soc. Rev. 2011, 40, 3764. [12] D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H. Dommett, G. Evmenenko, S. T. Nguyen, R. S. Ruoff, Nature 2007, 448, 457.
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[13] S. Park, D. A. Dikin, S. T. Nguyen, R. S. Ruoff, J. Phys. Chem. C 2009, 113, 15801. [14] Q. F. Cheng, M. X. Wu, M. Z. Li, L. Jiang, Z. Y. Tang, Angew. Chem. Int. Ed. 2013, 125, 3838. [15] Y. Tian, Y. Cao, Y. Wang, W. Yang, J. Feng, Adv. Mater. 2013, 25, 2980. [16] H. P. Cong, J. F. Chen, S. H. Yu, Chem. Soc. Rev. 2014, 43, 7295. [17] K. Hu, L. S. Tolentino, D. D. Kulkarni, C. Ye, S. Kumar, V. V. Tsukruk, Angew. Chem. Int. Ed. 2013, 52, 13784. [18] H. Bao, Y. Pan, Y. Ping, N. G. Sahoo, T. Wu, L. Li, J. Li, L. H. Gan, Small 2011, 7, 1569. [19] Y. Pan, T. Wu, H. Bao, L. Li, Carbohydr. Polym. 2011, 83, 1908. [20] C. Wan, M. Frydrych, B. Chen, Soft Matter 2011, 7, 6159. [21] R. Li, C. Liu, J. Ma, Carbohydr. Polym. 2011, 84, 631. [22] B. Li, B. Xie, J. F. Kennedy, Carbohydr. Polym. 2006, 64, 510. [23] H. Yan, B. Cai, Y. Cheng, G. Guo, D. Li, X. Yao, X. Ni, G. O. Phillips, Y. Fang, F. Jiang, Food Hydrocolloids 2012, 26, 383. [24] L. Cheng, A. Abd Karim, C. Seow, Food Chem. 2008, 107, 411. [25] H. Nie, X. Shen, Z. Zhou, Q. Jiang, Y. Chen, A. Xie, Y. Wang, C. C. Han, Carbohydr. Polym. 2011, 85, 681. [26] J. Li, J. Ji, J. Xia, B. Li, Carbohydr. Polym. 2012, 87, 757. [27] O. Tatirat, S. Charoenrein, W. L. Kerr, Carbohydr. Polym. 2012, 87, 1545. [28] J. Liu, K. Zhu, T. Ye, S. Wan, Y. Wang, D. Wang, B. Li, C. Wang, Food Res. Int. 2013, 51, 437. [29] A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Adv. Mater. 1999, 11, 1365. [30] K. K. Lau, J. Bico, K. B. Teo, M. Chhowalla, G. A. Amaratunga, W. I. Milne, G. H. McKinley, K. K. Gleason, Nano Lett. 2003, 3, 1701. [31] B. Li, Z. Xu, B. Xie, Eur. Food Res. Technol. 2006, 223, 132. [32] H. Yu, Y. Huang, H. Ying, C. Xiao, Carbohydr. Polym. 2007, 69, 29. [33] J. Chen, C. Liu, Y. Chen, Y. Chen, P. R. Chang, Carbohydr. Polym. 2008, 74, 946. [34] X. Ye, J. Kennedy, B. Li, B. Xie, Carbohydr. Polym. 2006, 64, 532. [35] C. Valle’s, R. J. Young, D. J. Lomax, IanA. Kinloch, J. Mater. Sci. 2014, 49, 6311. [36] P. Manivel, S. Kanagaraj, A. Balamurugan, N. Ponpandian, D. Mangalaraj, C. Viswanathan, J. Appl. Polym. Sci. 2014, 131, 1. [37] Z. Gao, R. X. Su, R. L. Huang, W. Qi, Z. M. He, Nanoscale Res. Lett. 2014, 9, 1. [38] W. Lin, Q. Li, T. Zhu, J. Ind. Eng. Chem. 2012, 18, 934. [39] M. El Achaby, Y. Essamlali, N. El Miri, A. Snik, K. Abdelouahdi, A. Fihri, M. Zahouily, A. Solhy, J. Appl. Polym. Sci. 2014, 131, 41042. [40] D. Han, L. Yan, W. Chen, W. Li, Carbohydr. Polym. 2011, 83, 653.
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Received: February 16, 2015 Revised: May 14, 2015 Published online: June 19, 2015
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Nanocomposites
Bioinspired, Ultrastrong, Highly Biocompatible, and Bioactive Natural Polymer/Graphene Oxide Nanocomposite Films Wen-Kun Zhu, Huai-Ping Cong, Hong-Bin Yao, Li-Bo Mao, Abdullah M. Asiri, Khalid A. Alamry, Hadi M. Marwani, and Shu-Hong Yu* With the fast development of science and technology, one of the major scientific challenges in the field of materials science is to create novel, multifunctional materials with advanced performance, and therefore, build strong foundations for diverse fantastic fields.[1] In the process of evolution, nature has point out intrinsic laws to produce lightweight, strong, and hierarchical materials with exceptional properties and functionalities.[2,3] Biological materials, such as tooth, bone, and nacre, are complex, hierarchical, and heterogeneous nanocomposites providing superior mechanical properties and biocompatibility.[4,5] Thus, design and fabrication of bioinspired natural/biological materials with controllable architecture, especially with outstanding mechanical properties, is a viable approach for developing advanced functionalities. Inspired by the unique multiscale and multilevel “brickand-mortar”(B&M) structure in nacre, a series of bio-inspired
Dr. W.-K. Zhu, Dr. H.-B. Yao, Dr. L.-B. Mao, Prof. S.-H. Yu Division of Nanomaterials and Chemistry Hefei National Laboratory for Physical Sciences at Microscale Collaborative Innovation Center of Suzhou Nano Science and Technology Department of Chemistry The National Synchrotron Radiation Laboratory University of Science and Technology of China Hefei, Anhui 230026, P. R. China E-mail: [email protected] Dr. W.-K. Zhu State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials Southwest University of Science and Technology Mianyang, Sichuan 621000, P. R. China Prof. H.-P. Cong School of Chemistry and Chemical Engineering Hefei University of Technology Hefei, Anhui 230039, P. R. China Prof. A. M. Asiri, Dr. K. A. Alamry, Dr. H. M. Marwani Center of Excellence for Advanced Materials Research Chemistry Department Faculty of Science King Abdulaziz University Jeddah 21589, Saudi Arabia DOI: 10.1002/smll.201500486
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highly intense and ultratough composites have been reported by adopting diverse self-assembly techniques such as layerby-layer (LBL) assembly, vacuum filtration-assisted assembly, freeze-drying assembly, interface-assisted self-assembly and Langmuir–Blodgett (LB) assembly.[6–10] In these cases, 2D inorganic assembly units including glass flake, alumina flake, graphene oxide (GO), layered double hydroxides (LDH), nanoclay and flattened double-walled carbon nanotube, etc., serve as “bricks”, while polymers act as “mortar”.[2,11] Notably, GO nanosheet is an ideal kind of 2D inorganic building blocks for assembly of nacre-like materials, owing to its unique physicochemical properties and easily-accessed advantage.[12–14] Among GO-based composites, nacre-like GO-polymer materials have attracted great interests due to the endowed excellent mechanical performances.[15–17] Till now, a large number of reports have been focused on the assembly of GO with the synthesized polymers, rather than natural polymers. However, in consideration of the growing exhaustion of fossil resources, such as petroleum and coal in polymer industries, and the resulted wastes from the nondegradable organic polymer materials, it is pressing to reduce the production of traditional petroleum-based material and develop the novel composites with natural polymers in chemistry and materials science.[18–21] As a renewable natural polymer, konjac glucomannan (KGM) exhibits excellent biocompatible and biodegradable properties.[22,23] Meanwhile, it possesses great advantages of gelling, film-forming, antibacterial action, and low calorific value, enabling its broad applications in chemical, biological, food, and medical industries.[24–28] Furthermore, KGM is a kind of promising components for the construction of strong material with GO through the hydrogen-bond interaction, etc., due to abundant of hydroxyl groups in its polymer chains. In the present study, we report the preparation of KGM– GO nanocomposite films with nearly B&M hierarchical structure via a simple solution casting method. Compared with pure KGM film, the Young’s modulus and tensile strength of KGM–GO nanocomposite film with 7.5% GO are increased by 92.6% and 151.6%, respectively. It is worth to note that the ultimate tensile strength of such KGM–GO nanocomposite film can reach up to 183.3 MPa, exceeding many other biopolymer/GO films. Furthermore, the as-prepared
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the cross-section of the composite film became rough with the well-defined layered structure. Furthermore, it was clearly shown that the tensile fracture surfaces of KGM–GO-7.5% film had typical ductile fracture characteristics (Figure 1d). On the other side, due to the enhanced surface roughness of KGM–GO nanocomposites with the increased GO content, the contact angles of KGM, KGM–GO-2.5%, and KGM–GO-7.5% films were 82°, 94°, and 117°, respectively, revealing the increased Figure 1. a) Schematic illustration of the fabrication procedure of KGM–GO nanocomposite hydrophobicity (Figure S2, Supporting films. Cross-sectional SEM images of b) KGM, c) KGM–GO-2.5%, and d) KGM–GO-7.5% Information).[29,30] nanocomposite films. In order to understand the chemistry of KGM and GO interactions, KGM–GO nanocomposite films show high biocompat- we characterized KGM and KGM–GO nanocomposite ibilities for boosting the growth of RAW264.7 cells and films using FT-IR spectra (Figure 2a). From the spechydroxyapatite (HAP) nanocrystals. These results indicate trum of pure KGM, it was investigated that the absorpthat the obtained KGM–GO nanocomposites have potential tion bands of 3315, 2886, 1722, 1639, and 1061 cm−1 were applications in food package, scaffold for tissue engineering, assigned to OH, C H, C O, C O and C6 OH groups, respectively.[31–34] While the absorption bands at 870 and and so on. The surface morphology of GO nanosheets was char- 819 cm−1 were characteristic vibrations of the mannose acterized by transmission electron microscopy (TEM) and unit in KGM.[25,38] Comparing the spectra of pure KGM atomic-force microscopy (AFM) (Figure S1, Supporting and KGM–GO nanocomposite films, the following changes Information). The TEM images showed that most GO sheets took place in the nanocomposite films. The vibration bands had obvious wrinkles. The typical tapping-mode AFM image of OH, C O and C O were weakened and down-shifted of GO sheets revealed that most GO sheets had heights of to 3309, 1715, and 1634 cm−1, respectively, indicating the ≈1.14 nm, which was characteristic of a fully exfoliated gra- strong intermolecular hydrogen bonds between GO and phene sheet. This favorable structure and oxygen-containing KGM.[34] Such confirmed interactions in the assembly films functional groups of GO facilitate its assembly with polymer would build solid foundations for the resulted enhanced into 2D film. Figure 1a illustrates the simple, efficient, and mechanical properties. The rheological properties of KGM and KGM–GO soluscalable preparation procedures of KGM–GO nanocomposite films by using GO as the nanoscale building block tions used for film casting were investigated to further dig and natural polymer KGM as the organic adhesion agent, out the interactions of KGM with GO (Figure 2b). It showed respectively. In the assembly process, KGM–GO solution that both of the storage modulus G′ and loss modulus G″ had was stirred and followed by sonication to allow full interac- the raised tendency with increasing GO content (Figure 2c). tions between KGM molecules and GO nanosheets. Dried This result was probably induced by the interactions between in a Petri dish at room temperature, the nanocomposite film GO and KGM, also beneficial for enhancing the mechanical was formed during such a low-speed evaporation-induced properties of the as-prepared nanocomposites.[35,36] Clearly, the color of KGM–GO mixtures was changed from colorassembly process. The representative cross-sectional SEM images of KGM less to dark brown with weight percentages of GO to KGM and KGM–GO nanocomposites with different GO concen- increased from 0 to 7.5% (inset picture in Figure 2c). With trations are shown in Figure 1b–d. As GO content increased, increasing GO contents, the slope of storage modulus against
Figure 2. a) FT-IR spectra of KGM and KGM–GO-7.5% nanocomposite films. Rheological properties of KGM and KGM–GO solutions: b) the strain scanning and c) the moduli dependent on GO concentration in the form of linear coordinates, inset showing the color changes of KGM–GO solutions from colorless to dark brown with the increased GO contents. small 2015, 11, No. 34, 4298–4302
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Figure 3. Mechanical properties of KGM and KGM–GO with different GO concentrations. a) Stress–strain curves of KGM–GO nanocomposite films. b) Effect of GO weight fraction on Young's modulus and ultimate strain of KGM–GO nanocomposite films. c) GO-based composite films with different kinds of inorganic building blocks (Black symbols: artificial polymer-GO composites; Blue symbols: natural polymer-GO composites; Brown symbol: pure GO. Red symbol: our KGM–GO-7.5% nanocomposite film; PCDO: 10, 12-pentacosadiyn-1-ol, PAH: poly(allylamine) hydrochloride, PVA: poly(vinyl alcohol), PMMA: poly(methyl methacrylate), PDA: polydopamine, PEI: polyetherimide, PAA: polyallylamine, CS: chitosan, SF: silk fibroin.
GO concentration of KGM–GO solutions was much steeper than that of loss modulus. For example, compared with pure KGM, G′ of the mixture with 7.5% GO content was significantly increased by 136.5%, while G″ was improved by 102.9%. Furthermore, it was found that the storage modulus G′ and loss modulus G″ of pristine GO were very low, indicating the small effect of GO contents on the modulus of the suspensions (Table S1 and Figure S3, Supporting Information). These analysis suggested the strong interactions of GO with the molecular chains of KGM and the restricted mobility of KGM molecules. Then, equations (1) and (2) were simulated to measure the interaction between KGM and GO. The contribution of GO on increasing the KGM’s modulus was also shown in Figure S4 (Supporting Information), which remaped Figure 2c logarithmically (Table S2, Supporting Information). It was shown that, the increase of KGM modulus followed a power function law in the concentration of GO. As for its storage modulus, the power function was Y1 = 2 + 0.014 X 1.6
(0 ≤ X ≤ 7.5)
(1)
And for its loss modulus Y2 = 1.8 + 0.005 X 2
(0 ≤ X ≤ 7.5)
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(2)
where Y1 and Y2 represent log (G′) and log (G″) respectively, and X% is the weight percentage of GO to KGM. From the power function, we can make the interaction rule between KGM and GO visualized. In pristine KGM solution, the storage modulus is larger than loss modulus, indicating that the reversible entanglements in KGM molecules is more than irreversible entanglements, and the interaction between KGM is relatively loose not so intensive as that in irreversible entanglements system.[37] However, incorporated with GO, the index of power function of loss modulus is larger than that of storage modulus. It indicates that the increase of irreversible entanglements exceeds the reversible entanglements in KGM–GO solution, and the interaction between KGM– GO molecules became much stronger. Therefore, GO indeed contributes to the strong interaction through hydrogen bond due to its abundant of oxygen-containing groups. The above-mentioned analysis quantified the dependence of rheological properties of composites on the GO concentration, and proved the controlled properties of KGM by the incorporated GO content. These strongly indicated the intensive interactions between KGM molecules and GO nanosheets. In order to prove the strong interactions between KGM and GO contributing to enhanced mechanical properties of their macroscopic assemblies, the tensile stress–strain curves were carried out on pure GO and KGM–GO nanocomposite
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Such a novel stability in aqueous solution was attributed to the formation of B&M hierarchical structures between KGM and GO nanosheets under their strong interactions.[39,40] Moreover, with the assembly of GO into KGM, the thermal stability was correspondingly improved, as revealed from TG curve of KGM–GO-7.5% composite film moved toward the high temperature zone (Figure S8, Supporting Information). Furthermore, the cytotoxicity of KGM and KGM–GO film was evaluated by culturing RAW264.7 cells on them. KGM or KGM–GO film was firstly casted onto plates, subsequently rinsed intensively with deionized (DI) water to remove residual dust. The morphology of cells was recorded by microscopy. As shown in Figure 4a–c, RAW264.7 cells with well-defined central nucleus were flatly spreading on KGM or KGM–GO film. KGM and KGM–GO film were noncytoFigure 4. The morphology images of RAW264.7 cells grown on control plates and films for toxicity, as confirmed by more than 90% 48 h. a) control sample film, b) KGM film, c) KGM–GO-7.5% nanocomposite films, respectively, of the cells alive and forming a confluent and subsequently stained with calcein (green) and propidium iodide (red) as a part of the monolayer on the film surface after 48 h of standard live-dead test (scale bars, 50 µm). d) Cell viability of RAW264.7 cells on control exposure through the confirmatory live– plate and film measured by cell counting assay after being cultured for 0, 24, 48, and 72 h, dead test (calcein and propidium iodide respectively. Error bars indicate the standard deviation (SD). staining) on the cells. Meanwhile, the cell number assay was conducted to examine films with different weight percentages of GO to KGM in the viability of RAW264.7 cells on KGM or KGM–GO film. Figure 3. Compared with pure KGM, Young’s modulus and As shown in Figure 4d, the proliferation of cells was not tensile stress of KGM–GO composite were significantly influenced by different kinds of films within culture time. enhanced with increasing GO contents (Figure 3a,b). When The number of RAW264.7 cells on KGM or KGM–GO the GO content was high to 7.5%, Young’s modulus and ten- film was almost the same as that on control plates after 24, sile strength of the composite were increased by 92.6% and 48, and 72 h of culture (Figure 4d). These results indicated 151.6%, achieving 16.8 GPa and 183.3 MPa, respectively. that such nontoxic, high-strength KGM–GO film could Compared with the reported composite films as plotted in be used as ideal substrate for cell adhesion and growth, Figure 3c, KGM–GO-7.5% film presented in our work exhib- promising it a potential scaffold for tissue engineering and ited wonderful mechanical properties. Its tensile stress was biomedical device. Moreover, the biomineralization perseveral times higher than previous GO-based nanocomposite formances of KGM and KGM–GO-7.5% nanocomposite films and papers,[1,8,9,12–15,17,19–21] for example, 2.28 times films were also proved. SEM and XRD results showed higher than synthesized polymers-GO films (PVA–GO film, that more HAP nanocrystals were grown on KGM– 80.2 MPa),[8] and 1.37 times higher than natural polymers- GO-7.5% film, indicating the improved bioactivity of KGM GO films (CS–GO film, 133.1 MPa), etc.[19] Nevertheless, through the composite with GO nanosheets (Figure S9, when the GO content was further improved to 10%, the Supporting Information). tensile strength of the composite film was greatly decreased In summary, we have demonstrated an easy, safe, and (Figure S5a, Supporting Information). The high viscosity of simple assembly method to fabricate KGM–GO comKGM damaged the uniform dispersion of GO at a high ratio posite films under the strong hydrogen-bonding interactions and resulted in the aggregation, which was responsible for between GO nanosheets and KGM molecules. The resulted such an attenuation of mechanical property, as revealed from KGM–GO-7.5% composite film exhibits significantly the appearance of characteristic diffraction peak of GO at enhanced mechanical properties, higher than any other GO11° in the XRD pattern of KGM–GO-10% film (Figure S6, based films reported so far. Significantly, the composite films Supporting Information). also possess excellent biocompatibility and bioactivity toward Fantastically, the prepared KGM–GO-7.5% composite growth of RAW264.7 cells and HAP. The strong, biocompatfilms showed the strong water-resistant performance, as ible, and bioactive natural polymer/GO composite films prerevealed from the nonredispersion of the KGM–GO-2.5% sented in our work show the promise for wide applications and KGM–GO-7.5% composite films after soaking in water in diverse fields of food package, tissue engineering, and biofor 15 d by shaking (Figure S7, Supporting Information). logical scaffold. small 2015, 11, No. 34, 4298–4302
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements W.-K.Z., H.-B.Y. contributed equally to this work. This work is supported by the Ministry of Science and Technology of China (Grant Nos. 2014CB931800 and 2013CB933900), the National Natural Science Foundation of China (Grant Nos. 21431006, 91227103, 21061160492, and J1030412), Scientific Research Grant of Hefei Science Center of CAS (Grant No. 2015SRG-HSC038), Hainan Province Science and Technology Department (Grant No. CXY20130046), Sichuan Province Science and Technology Pillar Program(2014GZ0185), and the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (12zxnp08).
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Received: February 16, 2015 Revised: May 14, 2015 Published online: June 19, 2015
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