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Facile fabrication of magnetically responsive PDMS fiber for camouflage
Journal of Colloid and Interface Science 483 (2016) 11–16
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Facile fabrication of magnetically responsive PDMS fiber for camouflage Shenglong Shang a, Qinghong Zhang a, Hongzhi Wang a,⇑, Yaogang Li b,⇑ a b
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China Engineering Research Centre of Advanced Glasses Manufacturing Technology, MOE, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China
g r a p h i c a l a b s t r a c t A new type of photonic crystal PDMS fiber exhibits tunable structural color upon exposure to external magnetic field.
a r t i c l e
i n f o
Article history: Received 13 June 2016 Revised 31 July 2016 Accepted 2 August 2016 Available online 9 August 2016 Keywords: PDMS fiber Magnetic field Responsive photonic crystal Fe3O4@C CNCs Chain-like structure Camouflage
a b s t r a c t A new type of photonic crystal PDMS fiber which exhibits tunable structural color upon exposure to external magnetic field is described in this article. The novel magnetic field responsive fiber was prepared from embedding ethylene glycol droplets (containing Fe3O4@C nanoparticles) into PDMS. In the presence of an external magnetic field, Fe3O4@C nanoparticles which dispersed in ethylene glycol droplets formed one dimensional chain-like structures along the magnetic field. As a result, the color of the fiber changes to yellow green. By contrast, when the magnetic field was removed, the color of the fiber will disappear and display its original color. Moreover, this novel PDMS fiber has good mechanical properties and could keep its color under a fixed magnetic field no matter it was stretched or squeezed. This study is expected to have some important applications such as none-powered and functionalized fibers for camouflage. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Camouflage has drawn more and more interests due to their special properties such as making animals or objects ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jcis.2016.08.005 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
invisible around the surrounding environment. Many researchers have done some excellent woks in this field, one of the most inconceivable works is the fabrication of invisibility cloaking [1–7], in which a device is used to render an object invisible to incident radiation. The device achieves invisibility often by bending the ray trajectory around the object. However, the problem is that the perfect invisibility is unachievable and the device cannot hide itself. Therefore, there also
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need some new strategies to achieve the purpose of camouflage. Many creatures in nature, such as chameleon, octopus and cuttlefish, are considered to be the great disguiser of the nature because of their capability to rapidly change their body’s color in response to the surrounding environment. The reason that these creatures achieve camouflage arises from the translocation of pigments or a rearrangement of reflective units within a large number of chromatophores [8]. Recent studies shown that chameleons shift their color through active tuning of a lattice of guanine nanocrystals within a superficial thick layer of dermal iridophores. By combining two superimposed populations of iridophores, it achieves efficient camouflage and dramatic display [9]. Inspired by the nature creatures, researchers have paid their attention to responsive photonic crystals in order to obtain chromatic transitions. Responsive photonic crystals (RPCs), which tuned their structural colors by external stimuli, have been widely researched in the past two decades. The RPCs display different colors under the stimulation of temperature [10–12], ions [13–15], pH [16], mechanical force [17,18], electrical field [19] and so on, which makes them hold great application in camouflage. However, few works have been done in this field and many challenges exist in developing responsive photonic crystals, including limited tunability of the band gap, a slow response to external stimuli, incomplete reversibility, and difficulty of integration into existing photonic devices [20]. In order to solve these problems, magnetically RPCs have been widely studied in the past decades [21–23]. Compared to the other RPCs, magnetically RPCs can rapidly responsive to magnetic field through its magnetic components incorporated into one dimensional chain-like photonic crystal structures. Because of its fast responsive to the external magnetic field, many applications have been developed, such as printing [24,25], humidity sensor [26,27], anti-counterfeiting materials [28]. However, the conventional film shape of RPCs makes them not suitable for the application of wearable electronics. To this end, some researchers have put their attention to the photonic-crystal-based structurally colored fibers (SCFs) [29–31]. These SCFs are fabricated through assembling colloidal crystals on the surface of the fibers. But the combination between colloidal crystals and the surface of the fiber is unstable, leading to the photonic structures which assembly on the surface of the fibers are easily broken. Herein, an efficient strategy for the preparation of magnetic field responsive (MFR) fiber is described. The MFR fiber was made of embedding EG droplets which contain Fe3O4@C nanoparticles into PDMS fiber in a microtubule. Then another PDMS was coated on the surface of the fiber after it was taken out from the microtubule. The color of the MFR fiber which arises from the assembly of Fe3O4@C nanoparticles under magnetic field can quickly switched between brown and yellow green. Moreover, the novel PDMS fiber also has good mechanical properties, the fiber will keep its color regardless of it was stretched or squeezed under a fixed magnetic field. In addition, the MFR fiber can be woven into fabrics for potential applications such as detection and camouflage. 2. Experimental 2.1. Chemicals and materials Ferrocene (Fe(C5H5)2, =98%), hydrogen peroxide (H2O2, =30%), acetone (C3H6O, =99%), ethylene glycol (C2H6O2 = 99%) were purchased from Shanghai Chemical Factory, China. Silicone liquid (10 cSt) and PDMS (Sylgard 184) were purchased from Dow Corning, PDMS (Sylgard 184) was supplied as a kit containing two separate components: the base material (part A) and the curing agent (part B). All the chemicals were of analytical reagent grade and used directly without any further treatment.
2.2. Synthesis of Fe3O4@C nanoparticles Fe3O4@C colloidal nanoparticles were synthesized using a hightemperature hydrolysis reaction procedure that had been reported previously [22]. In a typical experiment, ferrocene (0.50 g) was dissolved in acetone (60 ml). After intense sonication for 15 min, hydrogen peroxide (2.0 ml) was slowly added to the above mixture solution and was vigorously stirred for 30 min by magnetic stirring. Then the precursor solution was transferred to a Teflonlined stainless autoclave with a total volume of 80 ml and was heated to and maintained at 180 °C. After 72 h, the autoclave was cooled naturally to room temperature. The products were washed by acetone three times and re-dispersed in ethylene glycol (EG) solution as a concentration of 10 mg/ml for the further use. 2.3. Fabrication of magnetic field responsive PDMS fibers The preparation of magnetic field responsive PDMS fiber is illustrated in the Supporting Information (Scheme S1). First, 1.0 ml above homogeneous sample solution was mixed with 4.0 g PDMS precursor (the base material (part A) diluted by silicone liquids with the ratio of 4:1) and 0.4 g PDMS curing agent Part-B respectively. After complete mixing, the mixture suspension was left at room temperature for 1 h in order to minimize bubble formation. Then the mixture suspension was injected into a Teflon tube by using a syringe and curing it at 60 °C for 1 h. Afterwards, the PDMS fiber was taken out and immersed into PDMS precursor (part A and part B with the ratio of 10:1), followed by curing it for another 2 h at 60 °C. 2.4. Characterization Digital photos were obtained by a digital camera (Nikon D7000, Japan). TEM images were obtained using a transmission electron microscope (JEOL-2100F, Japan). Scanning electron microscopy was recorded using desktop scanning electron microscopy (Phenom G2 pro, America). X-ray diffraction pattern was characterized by X-ray diffractometer (XRD, Rigaku D/max2550 V X-ray diffractometer using Cu Ka irradiation (k = 1.5406 Å). Refection spectra of the magnetic responsive PDMS fibers were obtained using a fiber-optic spectrometer (G2000-Pro-Ex, China). The incident light was aligned perpendicular to the film for all the optical measurements, and optical micrograph were obtained by an optical microscopy (XPF-550, China) mounted on a CCD camera (TCC, 3.3 ICE, China). 3. Results and discussion The fabrication of MFR fiber relies on some magnetic responsive colloidal nanocrystal clusters. Herein, Fe3O4@C superparamagnetic colloidal nanocrystal clusters (SCNCs) which were dispersed in EG solution were used to fabricating MFR fiber. The Fe3O4@C SCNCs suspension displayed brown color in the absence of an external magnetic field. However, when an external magnetic field was applied, the color of the suspension changes to yellow green1, as shown in Fig. 1a and b. According to previous studies [22], this is because one dimensional chain-like structures formed in the suspension under an external magnetic field. Fig. 1c shows the corresponding TEM image of Fe3O4@C CNCs, it can be observed that the Fe3O4@C CNCs have a core-shell structure and the diameter of these particles is about 155 nm. Fig. 1d shows the X-ray diffraction patterns of Fe3O4@C SCNCs. From Fig. 1d, Fe3O4 (JCPDS file 19-0629) and C peaks can be observed clearly. 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.
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Fig. 1. (a, b) Digital photos of the Fe3O4@C SCNCs in ethylene glycol: (a) no external magnetic field was applied; (b) 0.25T external magnetic field was applied. (c) The corresponding TEM image of Fe3O4@C particles and (g) X-ray diffraction patterns of Fe3O4@C SCNCs.
Fig. 2a shows the digital photo of the MFR fiber which displayed brown color in the absence of an external magnetic field. When an external magnetic field was applied, the color of the fiber changes to yellow green and shows obviously increased contrast against the background, as shown in Fig. 2b. The typical reflection spectra of the fibers are shown in Fig. 2c, it can be observed that the diffraction wavelength has a peak at 570 nm under an external magnetic field. The reason of the MFR fiber could display color under magnetic field is that EG suspensions formed droplets in
PDMS matrix. Fe3O4@C CNCs which were dispersed in EG droplets could form one dimensional chain-like structures and display color under an external magnetic field. Moreover, the fiber retains good flexibility of the PDMS matrix and can be weaved into a knot while still displaying magnetically induced structural color (Fig. S2). The fiber is also very stable and it could display color under magnetic field after several month. Fig. 3 shows the scanning electron microscopy (SEM) images of the magnetic responsive PDMS fiber. From the SEM images, as
Fig. 2. (a, b) The color changes of the magnetic responsive PDMS fiber which fabricated in macro-space withdrawal/within an magnetic field and (c) the representative reflectance spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Scanning electron microscopy (SEM) images of magnetic responsive PDMS fiber. The surface of the fiber without coating another layer of PDMS at low (a) and high (d) magnification, the surface of the fiber after coating another layer of PDMS at low (b) and high (e) magnification and cross-sectional SEM images of the magnetic responsive PDMS fiber at low (c) and high (f) magnification.
shown in Fig. 3a and d, it can be clearly observed that the surface of the fiber contains many interspaces after it was taken out from Teflon tube. These interspaces mainly result from the evaporation or destruction of the EG droplets on the surface of fiber and the remaining bubbles produced in the process of the fiber fabrication. However, after infiltration with another layer of PDMS, the fiber had a uniform and smooth surface, as shown in Fig. 3b and e. The MFR fiber had a core-shell structure after coated with another layer of PDMS and the core-shell structure of the MFR fiber can be
clearly seen in the cross-sectional SEM image. As shown in Fig. 3c, the core structure was composed of PDMS with EG suspension of Fe3O4@C, it’s the functional part which would display colors under an external magnetic field. While, the shell structure consists of pure PDMS, it makes the core structure more stable. Fig. 3f demonstrates that the Fe3O4@C SCNCs suspension was encapsulated in the inner of the fiber, the Fe3O4@C micro-spheres can be clearly observed after the solvent evaporation. Previous studies have reported that EG molecules would evaporate through the polymer
Fig. 4. Schematic illustration of the magnetic responsive PDMS fiber displays color under magnetic field. Under an external magnetic field, Fe3O4@C CNCs which were dispersed in EG droplets formed one dimensional chain-like structures and displayed color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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network [20], in this work, the out layer PDMS prevent the evaporation of the EG molecules, made the MFR fiber had a long-term stability. The reason of the MFR fiber tuned its color under magnetic field can be explained in Fig. 4. In the absence of an external magnetic field, Fe3O4@C CNCs are well dispersed in EG droplets which were encapsulated in the fiber due to Brownian motion. However, when an external magnetic field was applied, the application of a magnetic field induces a magnetic attractive force between two adjacent Fe3O4@C micro-spheres along the magnetic field, with the balance of the electrostatic repulsion of these two adjacent spheres, the Fe3O4@C micro-spheres formed one-dimensional chain-like structures along the direction of the magnetic field. Because of the periodicity of these chain-like photonic crystal structures, the brilliant diffraction color was observed.
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Fig. 5a and b shows the digital photos of MFR fiber which displays color before and after stretching under an external magnetic field. It can be observed that the fiber could remain its color even though it was stretched, as shown in Fig. S3. From the optical micrograph of the magnetic responsive PDMS fiber, Fig. 5c and d, we can observe that EG droplets remain ball-like structures when no external force was applied. However, when the fiber was stretched, the droplets change to ellipsoid. But the color of the MFR fiber does not change as the morphological change of the droplets. The reason that the MFR fiber could remain its color when it was stretched can be explained in Fig. S1. Fig. S1 shows the schematic illustration of the magnetic responsive PDMS fiber which displays color before and after stretching. It can be predicted that although the morphology of the droplets get changed when the fiber was stretched, however, under the same external magnetic
Fig. 5. Photographs of the magnetic responsive PDMS fiber displays color under magnetic field before (a) and after (b) stretching and optical micrograph of the magnetic responsive PDMS fiber (without coating thin out layer PDMS) before (c) and after (d) stretching. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. The magnetic field responsive fiber displays the same color as leaf under magnetic field. (a) No external magnetic field was applied; (b) 0.25T external magnetic field was applied. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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field, the distance between each adjacent Fe3O4@C spheres (d1, d2) would keep the same. According to Bragg’s law, mk = 2nd sinh (m is the diffraction order, k is the wavelength of incident light, n is the effective refractive index, d is the lattice spacing, and h is the glancing angle between the incident light and diffraction crystal plane) [32], where m and 2n are constant in the same system, the angle (h) we observed is 90° in this experiment, so the diffraction wavelength k is dependent on the distance between each adjacent Fe3O4@C spheres (d1, d2). While the diffraction wavelength k keep the same when the MFR fiber was stretched or not, we can concluded that the distance of each adjacent Fe3O4@C spheres would keep the same under the same magnetic field, that is to say d1 = d2. Owing the responsive of the MFR fiber to the stimuli of the external magnetic field, it may find applications for camouflage. Liking many creatures in nature (such as cuttlefish, chameleon) whose will change their body’s color according to the changes of the environment. Fig. 6 demonstrates that the MFR fiber displays the same color as leaf under the magnetic field. When the external magnetic field was removed, the fiber just diffract brown color, it can be seen clearly on the leaf. However, when an external magnetic field was applied, the color of the fiber changes to yellow green, which makes it like part of the leaf. Moreover, the MFR fiber can also be woven into the leaf (Fig. S4), which makes them hold great potential for varies applications, such as the material of smart clothes. 4. Conclusions In conclusion, a novel magnetic field responsive (MFR) fiber was fabricated by a simplicity method in micro-tube. The novel fiber will change its color through changing its inner structure when an external magnetic field was applied. Specially, the fiber can achieve camouflage like some creatures in nature which could change their skin’s color under the external stimuli. Such novel fibers can also be used as the magnetic detection device, and have a great advantage because it doesn’t need any extra device to provide energy. Furthermore, because of these fibers have enough strength and flexibility, it holds great promise for the development of smart textiles device with novel functions. Acknowledgements We gratefully acknowledge the financial support by Natural Science Foundation of China (Nos. 51572046, 51503035), Science and Technology Commission of Shanghai Municipality (13JC1400200), The Shanghai Natural Science Foundation (15ZR1401200), the Program for Professor of Special Appointment
(Eastern Scholar) at Shanghai Institutions of Higher Learning, Program of Shanghai Academic Research Leader (16XD1400100), Innovative Research Team in University (IRT1221), the Program of Introducing Talents of Discipline to Universities (No. 111-2-04) and the Fundamental Research Funds for the Central Universities (2232014A3-06). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.08.005. References [1] J.B. Pendry, D. Schurig, D.R. Smith, Science 312 (2006) 1780. [2] D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, Science 314 (2006) 977. [3] W. Cai, U.K. Chettiar, A.V. Kildishev, V.M. Shalaev, Nat. Photon. 1 (2007) 224. [4] R. Liu, C. Ji, J.J. Mock, J.Y. Chin, T.J. Cui, D.R. Smith, Science 323 (2009) 366. [5] L.H. Gabrielli, J. Cardenas, C.B. Poitras, M. Lipson, Nat. Photon. 3 (2009) 461. [6] N. Landy, D.A. Smith, Nat. Mater. 12 (2013) 25. [7] H.S. Chen, B. Zheng, L. Shen, H.P. Wang, X.M. Zhang, N. Zheludev, B. Zhang, Nat. Commun. 4 (2013) 2652. [8] A.V. Singh, A. Rahman, N.V.G. Sudhir Kumar, A.S. Aditi, M. Galluzzi, S. Bovio, S. Barozzi, E. Montani, D. Parazzoli, Mater. Des. 36 (2012) 829. [9] J. Teyssier, S.V. Saenko, D. Marel, M.C. Millinkovitch, Nat. Commun. 6 (2015) 6368. [10] Y. Takeoka, M. Watanabe, Langmuir 19 (2003) 9104. [11] M. Kumoda, M. Watanabe, Y. Takeoka, Langmuir 22 (2006) 4403. [12] H.R. Ma, M.X. Zhu, W. Luo, W. Li, K. Fang, F.Z. Mou, J.G. Guan, J. Mater. Chem. C 3 (2015) 2848. [13] J.H. Holtz, S.A. Asher, Nature 389 (1997) 829. [14] H. Saito, Y. Takeoka, M. Watanabe, Chem. Commun. (2003) 2126. [15] S.A. Asher, A.C. Sharma, A.V. Goponenko, M.M. Ward, Anal. Chem. 75 (2003) 1676. [16] A.C. Sharma, T. Jana, R. Kesavamoorthy, L.J. Shi, M.A. Virji, D.N. Finegold, S.A. Asher, J. Am. Chem. Soc. 126 (2004) 2971. [17] H. Fudouzi, T. Sawada, Langmuir 22 (2006) 1365. [18] B. Viel, T. Ruhl, G.P. Hellmann, Chem. Mater. 19 (2007) 5673. [19] A.C. Arsenault, D.P. Puzzo, I. Manners, G.A. Ozin, Nat. Photon. 1 (2007) 468. [20] J.P. Ge, Y.D. Yin, Angew. Chem. Int. Ed. 50 (2011) 1492. [21] L. He, M.S. Wang, J.P. Ge, Y.D. Yin, Acc. Chem. Res. 45 (2012) 1431. [22] H. Wang, Y.B. Sun, Q.W. Chen, Y.F. Yu, K. Cheng, Dalton Trans. 39 (2010) 9565. [23] W. Luo, H.R. Ma, F.Z. Mou, M.X. Zhu, J.D. Yan, J.G. Guan, Adv. Mater. 26 (2014) 1058. [24] H. Kim, J.P. Ge, J. Kim, S. Choi, Hosuk Lee, Howon Lee, W. Park, Y.D. Yin, S. Kwon, Nat. Photon. 3 (2009) 534. [25] H.B. Hu, J. Tang, H. Zhong, Z. Xi, C.L. Chen, Q.W. Chen, Sci. Rep. 3 (2013) 1484. [26] R.Y. Xuan, Q.S. Wu, Y.D. Yin, J.P. Ge, J. Mater. Chem. 21 (2011) 3672. [27] H.B. Hu, Q.W. Chen, K. Cheng, J. Tang, J. Mater. Chem. 22 (2012) 1021. [28] H.B. Hu, Q.W. Chen, J. Tang, X.Y. Hu, X.H. Zhou, J. Mater. Chem. 22 (2012) 11048. [29] Z.F. Liu, Q.H. Zhang, H.Z. Wang, Y.G. Li, Chem. Commun. 47 (2011) 12801. [30] N. Zhou, A. Zhang, L. Shi, K.Q. Zhang, ACS Macro Lett. 2 (2013) 116. [31] Z.F. Liu, Q.H. Zhang, H.Z. Wang, Y.G. Li, Nanoscale 5 (2013) 6917. [32] F. Leal Calderon, T. Stora, O. Mondain Monval, P. Poulin, J. Bibette, Phys. Rev. Lett. 72 (1994) 2959.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Facile fabrication of magnetically responsive PDMS fiber for camouflage Shenglong Shang a, Qinghong Zhang a, Hongzhi Wang a,⇑, Yaogang Li b,⇑ a b
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China Engineering Research Centre of Advanced Glasses Manufacturing Technology, MOE, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China
g r a p h i c a l a b s t r a c t A new type of photonic crystal PDMS fiber exhibits tunable structural color upon exposure to external magnetic field.
a r t i c l e
i n f o
Article history: Received 13 June 2016 Revised 31 July 2016 Accepted 2 August 2016 Available online 9 August 2016 Keywords: PDMS fiber Magnetic field Responsive photonic crystal Fe3O4@C CNCs Chain-like structure Camouflage
a b s t r a c t A new type of photonic crystal PDMS fiber which exhibits tunable structural color upon exposure to external magnetic field is described in this article. The novel magnetic field responsive fiber was prepared from embedding ethylene glycol droplets (containing Fe3O4@C nanoparticles) into PDMS. In the presence of an external magnetic field, Fe3O4@C nanoparticles which dispersed in ethylene glycol droplets formed one dimensional chain-like structures along the magnetic field. As a result, the color of the fiber changes to yellow green. By contrast, when the magnetic field was removed, the color of the fiber will disappear and display its original color. Moreover, this novel PDMS fiber has good mechanical properties and could keep its color under a fixed magnetic field no matter it was stretched or squeezed. This study is expected to have some important applications such as none-powered and functionalized fibers for camouflage. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Camouflage has drawn more and more interests due to their special properties such as making animals or objects ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jcis.2016.08.005 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
invisible around the surrounding environment. Many researchers have done some excellent woks in this field, one of the most inconceivable works is the fabrication of invisibility cloaking [1–7], in which a device is used to render an object invisible to incident radiation. The device achieves invisibility often by bending the ray trajectory around the object. However, the problem is that the perfect invisibility is unachievable and the device cannot hide itself. Therefore, there also
12
S. Shang et al. / Journal of Colloid and Interface Science 483 (2016) 11–16
need some new strategies to achieve the purpose of camouflage. Many creatures in nature, such as chameleon, octopus and cuttlefish, are considered to be the great disguiser of the nature because of their capability to rapidly change their body’s color in response to the surrounding environment. The reason that these creatures achieve camouflage arises from the translocation of pigments or a rearrangement of reflective units within a large number of chromatophores [8]. Recent studies shown that chameleons shift their color through active tuning of a lattice of guanine nanocrystals within a superficial thick layer of dermal iridophores. By combining two superimposed populations of iridophores, it achieves efficient camouflage and dramatic display [9]. Inspired by the nature creatures, researchers have paid their attention to responsive photonic crystals in order to obtain chromatic transitions. Responsive photonic crystals (RPCs), which tuned their structural colors by external stimuli, have been widely researched in the past two decades. The RPCs display different colors under the stimulation of temperature [10–12], ions [13–15], pH [16], mechanical force [17,18], electrical field [19] and so on, which makes them hold great application in camouflage. However, few works have been done in this field and many challenges exist in developing responsive photonic crystals, including limited tunability of the band gap, a slow response to external stimuli, incomplete reversibility, and difficulty of integration into existing photonic devices [20]. In order to solve these problems, magnetically RPCs have been widely studied in the past decades [21–23]. Compared to the other RPCs, magnetically RPCs can rapidly responsive to magnetic field through its magnetic components incorporated into one dimensional chain-like photonic crystal structures. Because of its fast responsive to the external magnetic field, many applications have been developed, such as printing [24,25], humidity sensor [26,27], anti-counterfeiting materials [28]. However, the conventional film shape of RPCs makes them not suitable for the application of wearable electronics. To this end, some researchers have put their attention to the photonic-crystal-based structurally colored fibers (SCFs) [29–31]. These SCFs are fabricated through assembling colloidal crystals on the surface of the fibers. But the combination between colloidal crystals and the surface of the fiber is unstable, leading to the photonic structures which assembly on the surface of the fibers are easily broken. Herein, an efficient strategy for the preparation of magnetic field responsive (MFR) fiber is described. The MFR fiber was made of embedding EG droplets which contain Fe3O4@C nanoparticles into PDMS fiber in a microtubule. Then another PDMS was coated on the surface of the fiber after it was taken out from the microtubule. The color of the MFR fiber which arises from the assembly of Fe3O4@C nanoparticles under magnetic field can quickly switched between brown and yellow green. Moreover, the novel PDMS fiber also has good mechanical properties, the fiber will keep its color regardless of it was stretched or squeezed under a fixed magnetic field. In addition, the MFR fiber can be woven into fabrics for potential applications such as detection and camouflage. 2. Experimental 2.1. Chemicals and materials Ferrocene (Fe(C5H5)2, =98%), hydrogen peroxide (H2O2, =30%), acetone (C3H6O, =99%), ethylene glycol (C2H6O2 = 99%) were purchased from Shanghai Chemical Factory, China. Silicone liquid (10 cSt) and PDMS (Sylgard 184) were purchased from Dow Corning, PDMS (Sylgard 184) was supplied as a kit containing two separate components: the base material (part A) and the curing agent (part B). All the chemicals were of analytical reagent grade and used directly without any further treatment.
2.2. Synthesis of Fe3O4@C nanoparticles Fe3O4@C colloidal nanoparticles were synthesized using a hightemperature hydrolysis reaction procedure that had been reported previously [22]. In a typical experiment, ferrocene (0.50 g) was dissolved in acetone (60 ml). After intense sonication for 15 min, hydrogen peroxide (2.0 ml) was slowly added to the above mixture solution and was vigorously stirred for 30 min by magnetic stirring. Then the precursor solution was transferred to a Teflonlined stainless autoclave with a total volume of 80 ml and was heated to and maintained at 180 °C. After 72 h, the autoclave was cooled naturally to room temperature. The products were washed by acetone three times and re-dispersed in ethylene glycol (EG) solution as a concentration of 10 mg/ml for the further use. 2.3. Fabrication of magnetic field responsive PDMS fibers The preparation of magnetic field responsive PDMS fiber is illustrated in the Supporting Information (Scheme S1). First, 1.0 ml above homogeneous sample solution was mixed with 4.0 g PDMS precursor (the base material (part A) diluted by silicone liquids with the ratio of 4:1) and 0.4 g PDMS curing agent Part-B respectively. After complete mixing, the mixture suspension was left at room temperature for 1 h in order to minimize bubble formation. Then the mixture suspension was injected into a Teflon tube by using a syringe and curing it at 60 °C for 1 h. Afterwards, the PDMS fiber was taken out and immersed into PDMS precursor (part A and part B with the ratio of 10:1), followed by curing it for another 2 h at 60 °C. 2.4. Characterization Digital photos were obtained by a digital camera (Nikon D7000, Japan). TEM images were obtained using a transmission electron microscope (JEOL-2100F, Japan). Scanning electron microscopy was recorded using desktop scanning electron microscopy (Phenom G2 pro, America). X-ray diffraction pattern was characterized by X-ray diffractometer (XRD, Rigaku D/max2550 V X-ray diffractometer using Cu Ka irradiation (k = 1.5406 Å). Refection spectra of the magnetic responsive PDMS fibers were obtained using a fiber-optic spectrometer (G2000-Pro-Ex, China). The incident light was aligned perpendicular to the film for all the optical measurements, and optical micrograph were obtained by an optical microscopy (XPF-550, China) mounted on a CCD camera (TCC, 3.3 ICE, China). 3. Results and discussion The fabrication of MFR fiber relies on some magnetic responsive colloidal nanocrystal clusters. Herein, Fe3O4@C superparamagnetic colloidal nanocrystal clusters (SCNCs) which were dispersed in EG solution were used to fabricating MFR fiber. The Fe3O4@C SCNCs suspension displayed brown color in the absence of an external magnetic field. However, when an external magnetic field was applied, the color of the suspension changes to yellow green1, as shown in Fig. 1a and b. According to previous studies [22], this is because one dimensional chain-like structures formed in the suspension under an external magnetic field. Fig. 1c shows the corresponding TEM image of Fe3O4@C CNCs, it can be observed that the Fe3O4@C CNCs have a core-shell structure and the diameter of these particles is about 155 nm. Fig. 1d shows the X-ray diffraction patterns of Fe3O4@C SCNCs. From Fig. 1d, Fe3O4 (JCPDS file 19-0629) and C peaks can be observed clearly. 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.
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Fig. 1. (a, b) Digital photos of the Fe3O4@C SCNCs in ethylene glycol: (a) no external magnetic field was applied; (b) 0.25T external magnetic field was applied. (c) The corresponding TEM image of Fe3O4@C particles and (g) X-ray diffraction patterns of Fe3O4@C SCNCs.
Fig. 2a shows the digital photo of the MFR fiber which displayed brown color in the absence of an external magnetic field. When an external magnetic field was applied, the color of the fiber changes to yellow green and shows obviously increased contrast against the background, as shown in Fig. 2b. The typical reflection spectra of the fibers are shown in Fig. 2c, it can be observed that the diffraction wavelength has a peak at 570 nm under an external magnetic field. The reason of the MFR fiber could display color under magnetic field is that EG suspensions formed droplets in
PDMS matrix. Fe3O4@C CNCs which were dispersed in EG droplets could form one dimensional chain-like structures and display color under an external magnetic field. Moreover, the fiber retains good flexibility of the PDMS matrix and can be weaved into a knot while still displaying magnetically induced structural color (Fig. S2). The fiber is also very stable and it could display color under magnetic field after several month. Fig. 3 shows the scanning electron microscopy (SEM) images of the magnetic responsive PDMS fiber. From the SEM images, as
Fig. 2. (a, b) The color changes of the magnetic responsive PDMS fiber which fabricated in macro-space withdrawal/within an magnetic field and (c) the representative reflectance spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Scanning electron microscopy (SEM) images of magnetic responsive PDMS fiber. The surface of the fiber without coating another layer of PDMS at low (a) and high (d) magnification, the surface of the fiber after coating another layer of PDMS at low (b) and high (e) magnification and cross-sectional SEM images of the magnetic responsive PDMS fiber at low (c) and high (f) magnification.
shown in Fig. 3a and d, it can be clearly observed that the surface of the fiber contains many interspaces after it was taken out from Teflon tube. These interspaces mainly result from the evaporation or destruction of the EG droplets on the surface of fiber and the remaining bubbles produced in the process of the fiber fabrication. However, after infiltration with another layer of PDMS, the fiber had a uniform and smooth surface, as shown in Fig. 3b and e. The MFR fiber had a core-shell structure after coated with another layer of PDMS and the core-shell structure of the MFR fiber can be
clearly seen in the cross-sectional SEM image. As shown in Fig. 3c, the core structure was composed of PDMS with EG suspension of Fe3O4@C, it’s the functional part which would display colors under an external magnetic field. While, the shell structure consists of pure PDMS, it makes the core structure more stable. Fig. 3f demonstrates that the Fe3O4@C SCNCs suspension was encapsulated in the inner of the fiber, the Fe3O4@C micro-spheres can be clearly observed after the solvent evaporation. Previous studies have reported that EG molecules would evaporate through the polymer
Fig. 4. Schematic illustration of the magnetic responsive PDMS fiber displays color under magnetic field. Under an external magnetic field, Fe3O4@C CNCs which were dispersed in EG droplets formed one dimensional chain-like structures and displayed color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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network [20], in this work, the out layer PDMS prevent the evaporation of the EG molecules, made the MFR fiber had a long-term stability. The reason of the MFR fiber tuned its color under magnetic field can be explained in Fig. 4. In the absence of an external magnetic field, Fe3O4@C CNCs are well dispersed in EG droplets which were encapsulated in the fiber due to Brownian motion. However, when an external magnetic field was applied, the application of a magnetic field induces a magnetic attractive force between two adjacent Fe3O4@C micro-spheres along the magnetic field, with the balance of the electrostatic repulsion of these two adjacent spheres, the Fe3O4@C micro-spheres formed one-dimensional chain-like structures along the direction of the magnetic field. Because of the periodicity of these chain-like photonic crystal structures, the brilliant diffraction color was observed.
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Fig. 5a and b shows the digital photos of MFR fiber which displays color before and after stretching under an external magnetic field. It can be observed that the fiber could remain its color even though it was stretched, as shown in Fig. S3. From the optical micrograph of the magnetic responsive PDMS fiber, Fig. 5c and d, we can observe that EG droplets remain ball-like structures when no external force was applied. However, when the fiber was stretched, the droplets change to ellipsoid. But the color of the MFR fiber does not change as the morphological change of the droplets. The reason that the MFR fiber could remain its color when it was stretched can be explained in Fig. S1. Fig. S1 shows the schematic illustration of the magnetic responsive PDMS fiber which displays color before and after stretching. It can be predicted that although the morphology of the droplets get changed when the fiber was stretched, however, under the same external magnetic
Fig. 5. Photographs of the magnetic responsive PDMS fiber displays color under magnetic field before (a) and after (b) stretching and optical micrograph of the magnetic responsive PDMS fiber (without coating thin out layer PDMS) before (c) and after (d) stretching. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. The magnetic field responsive fiber displays the same color as leaf under magnetic field. (a) No external magnetic field was applied; (b) 0.25T external magnetic field was applied. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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field, the distance between each adjacent Fe3O4@C spheres (d1, d2) would keep the same. According to Bragg’s law, mk = 2nd sinh (m is the diffraction order, k is the wavelength of incident light, n is the effective refractive index, d is the lattice spacing, and h is the glancing angle between the incident light and diffraction crystal plane) [32], where m and 2n are constant in the same system, the angle (h) we observed is 90° in this experiment, so the diffraction wavelength k is dependent on the distance between each adjacent Fe3O4@C spheres (d1, d2). While the diffraction wavelength k keep the same when the MFR fiber was stretched or not, we can concluded that the distance of each adjacent Fe3O4@C spheres would keep the same under the same magnetic field, that is to say d1 = d2. Owing the responsive of the MFR fiber to the stimuli of the external magnetic field, it may find applications for camouflage. Liking many creatures in nature (such as cuttlefish, chameleon) whose will change their body’s color according to the changes of the environment. Fig. 6 demonstrates that the MFR fiber displays the same color as leaf under the magnetic field. When the external magnetic field was removed, the fiber just diffract brown color, it can be seen clearly on the leaf. However, when an external magnetic field was applied, the color of the fiber changes to yellow green, which makes it like part of the leaf. Moreover, the MFR fiber can also be woven into the leaf (Fig. S4), which makes them hold great potential for varies applications, such as the material of smart clothes. 4. Conclusions In conclusion, a novel magnetic field responsive (MFR) fiber was fabricated by a simplicity method in micro-tube. The novel fiber will change its color through changing its inner structure when an external magnetic field was applied. Specially, the fiber can achieve camouflage like some creatures in nature which could change their skin’s color under the external stimuli. Such novel fibers can also be used as the magnetic detection device, and have a great advantage because it doesn’t need any extra device to provide energy. Furthermore, because of these fibers have enough strength and flexibility, it holds great promise for the development of smart textiles device with novel functions. Acknowledgements We gratefully acknowledge the financial support by Natural Science Foundation of China (Nos. 51572046, 51503035), Science and Technology Commission of Shanghai Municipality (13JC1400200), The Shanghai Natural Science Foundation (15ZR1401200), the Program for Professor of Special Appointment
(Eastern Scholar) at Shanghai Institutions of Higher Learning, Program of Shanghai Academic Research Leader (16XD1400100), Innovative Research Team in University (IRT1221), the Program of Introducing Talents of Discipline to Universities (No. 111-2-04) and the Fundamental Research Funds for the Central Universities (2232014A3-06). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.08.005. References [1] J.B. Pendry, D. Schurig, D.R. Smith, Science 312 (2006) 1780. [2] D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, Science 314 (2006) 977. [3] W. Cai, U.K. Chettiar, A.V. Kildishev, V.M. Shalaev, Nat. Photon. 1 (2007) 224. [4] R. Liu, C. Ji, J.J. Mock, J.Y. Chin, T.J. Cui, D.R. Smith, Science 323 (2009) 366. [5] L.H. Gabrielli, J. Cardenas, C.B. Poitras, M. Lipson, Nat. Photon. 3 (2009) 461. [6] N. Landy, D.A. Smith, Nat. Mater. 12 (2013) 25. [7] H.S. Chen, B. Zheng, L. Shen, H.P. Wang, X.M. Zhang, N. Zheludev, B. Zhang, Nat. Commun. 4 (2013) 2652. [8] A.V. Singh, A. Rahman, N.V.G. Sudhir Kumar, A.S. Aditi, M. Galluzzi, S. Bovio, S. Barozzi, E. Montani, D. Parazzoli, Mater. Des. 36 (2012) 829. [9] J. Teyssier, S.V. Saenko, D. Marel, M.C. Millinkovitch, Nat. Commun. 6 (2015) 6368. [10] Y. Takeoka, M. Watanabe, Langmuir 19 (2003) 9104. [11] M. Kumoda, M. Watanabe, Y. Takeoka, Langmuir 22 (2006) 4403. [12] H.R. Ma, M.X. Zhu, W. Luo, W. Li, K. Fang, F.Z. Mou, J.G. Guan, J. Mater. Chem. C 3 (2015) 2848. [13] J.H. Holtz, S.A. Asher, Nature 389 (1997) 829. [14] H. Saito, Y. Takeoka, M. Watanabe, Chem. Commun. (2003) 2126. [15] S.A. Asher, A.C. Sharma, A.V. Goponenko, M.M. Ward, Anal. Chem. 75 (2003) 1676. [16] A.C. Sharma, T. Jana, R. Kesavamoorthy, L.J. Shi, M.A. Virji, D.N. Finegold, S.A. Asher, J. Am. Chem. Soc. 126 (2004) 2971. [17] H. Fudouzi, T. Sawada, Langmuir 22 (2006) 1365. [18] B. Viel, T. Ruhl, G.P. Hellmann, Chem. Mater. 19 (2007) 5673. [19] A.C. Arsenault, D.P. Puzzo, I. Manners, G.A. Ozin, Nat. Photon. 1 (2007) 468. [20] J.P. Ge, Y.D. Yin, Angew. Chem. Int. Ed. 50 (2011) 1492. [21] L. He, M.S. Wang, J.P. Ge, Y.D. Yin, Acc. Chem. Res. 45 (2012) 1431. [22] H. Wang, Y.B. Sun, Q.W. Chen, Y.F. Yu, K. Cheng, Dalton Trans. 39 (2010) 9565. [23] W. Luo, H.R. Ma, F.Z. Mou, M.X. Zhu, J.D. Yan, J.G. Guan, Adv. Mater. 26 (2014) 1058. [24] H. Kim, J.P. Ge, J. Kim, S. Choi, Hosuk Lee, Howon Lee, W. Park, Y.D. Yin, S. Kwon, Nat. Photon. 3 (2009) 534. [25] H.B. Hu, J. Tang, H. Zhong, Z. Xi, C.L. Chen, Q.W. Chen, Sci. Rep. 3 (2013) 1484. [26] R.Y. Xuan, Q.S. Wu, Y.D. Yin, J.P. Ge, J. Mater. Chem. 21 (2011) 3672. [27] H.B. Hu, Q.W. Chen, K. Cheng, J. Tang, J. Mater. Chem. 22 (2012) 1021. [28] H.B. Hu, Q.W. Chen, J. Tang, X.Y. Hu, X.H. Zhou, J. Mater. Chem. 22 (2012) 11048. [29] Z.F. Liu, Q.H. Zhang, H.Z. Wang, Y.G. Li, Chem. Commun. 47 (2011) 12801. [30] N. Zhou, A. Zhang, L. Shi, K.Q. Zhang, ACS Macro Lett. 2 (2013) 116. [31] Z.F. Liu, Q.H. Zhang, H.Z. Wang, Y.G. Li, Nanoscale 5 (2013) 6917. [32] F. Leal Calderon, T. Stora, O. Mondain Monval, P. Poulin, J. Bibette, Phys. Rev. Lett. 72 (1994) 2959.