Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 5805−5811
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Biomimetic Optical Cellulose Nanocrystal Films with Controllable Iridescent Color and Environmental Stimuli-Responsive Chromism Yao-Dong He,† Ze-Lian Zhang,† Juan Xue,† Xiao-Hui Wang,† Fei Song,*,† Xiu-Li Wang,† Li-Li Zhu,‡ and Yu-Zhong Wang† †
Center for Degradable and Flame-Retardant Polymeric Materials (ERCPM-MoE), College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, 29 Wangjiang Road, Chengdu 610064, China ‡ The Affiliated Hospital of Guizhou Medical University, Guiyang 510000, China S Supporting Information *
ABSTRACT: As a wise and profound teacher, nature provides numerous creatures with rich colors to us. To biomimic structural colors in nature as well as color changes responsive to environmental stimuli, there is a long way to go for the development of free-standing photonic films from natural polymers. Herein, a highly flexible, controllably iridescent, and multistimuli-responsive cellulose nanocrystal (CNC) film is prepared by simply introducing a small molecule as both plasticizer and hygroscopic agent. The presence of the additive does not block the self-assembly of CNC in aqueous solution but results in the enhancement of its mechanical toughness, making it possible to obtain free-standing iridescent CNC films with tunable structural colors. In response to environmental humidity and mechanical compression, such films can change structural colors smoothly by modulating their chiral nematic structures. Notably, the chromism is reversible by alternately changing relative humidity between 16 and 98%, mimicking the longhorn beetle Tmesisternus isabellae. This chromic effect enables various applications of the biofilms in colorimetric sensors, anticounterfeiting technology, and decorative coatings. KEYWORDS: cellulose nanocrystal, chiral nematic structure, structural color, flexibility, environmental stimuli-responsive chromism
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INTRODUCTION In nature, there exist many photonic structures that generate brilliant colors, providing us inspirations for developing diverse artificial photonic materials with wide applications in optical devices, sensors, security indicators, and so forth.1−5 Different from colorant-based pigmentation, structural colors arise from the light interference in periodically layered or lattice structures, which are discovered in many animals and plants, such as insect cuticles, fish scales, bird feathers, fruits, and leaves.6−8 Inspired by these interesting phenomena, artificial structural colors have been created in the past decades with the assistance of photonic crystals, 9−11 Bragg stacks, 12 and chiral nematic liquid crystals.13−15 In contrast to the photonic materials with fixed structural colors, the counterparts with chromic effects, which refer to reversible color changes upon external stimuli, are more welcome regarding the demand on smart materials. Until now, impressive progress has already been made on colloidal photonic crystals that can mimic chameleons, tortoise beetles, hercules beetles, blue damselfish, and so on.16−19 Compared with that, however, we still have a long way to go for developing environmental stimuli-responsive chromic photonic materials from natural polymers. © 2018 American Chemical Society
Cellulose nanocrystal (CNC), generally prepared by acidcatalyzed hydrolysis of bulk cellulose, can well-disperse in water because of the introduction of sulfate ester groups on its surface as well as the resultant electrostatic repulsion,20 which enables the formation of chiral nematic liquid crystals. Even after evaporation of water to yield solid films, CNC can maintain the liquid crystal structure and present iridescent colors if its helical pitch is located in the visible wavelength region.21,22 To control its iridescence as a result of the tunable pitch, different approaches, including ultrasound treatment, desulfation, and introduction of salts or water-soluble polymers, have been widely reported.23−28 Nevertheless, mechanisms to make the resultant CNC materials sense environmental changes in terms of chromism are rarely illustrated. Zhang et al.29 prepared thick CNC films in Petri dishes by a conventional solution-cast method, which showed humidity-responsive chromism. Liu and co-workers30 employed a layer-by-layer approach to develop iridescent optical CNC coatings on silicon wafers that could Received: December 4, 2017 Accepted: January 23, 2018 Published: January 23, 2018 5805
DOI: 10.1021/acsami.7b18440 ACS Appl. Mater. Interfaces 2018, 10, 5805−5811
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Figure 1. (a) TEM image of the as-prepared CNC; (b) zeta potential of CNC suspension; (c) POM images of neat CNC and CNC/glycerol suspensions (inserted photos are the high-magnification images taken at the locations marked with red ellipses); (d) chiral nematic pitch as a function of glycerol percentage for CNC/glycerol suspensions.
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sense different vapors. It should be noted, however, that these films are not freestanding because of the high fragility of CNC. To overcome the shortage, MacLachlan’s group22,31 introduced urea-formaldehyde and amino-formaldehyde resins into the CNC matrix, respectively, constructing mesoporous photonic CNC films sensitive to humidity and pressure. The presence of these thermoset resins did not disturb the self-assembly of CNC, however, along with the heat curing, volume shrinkage of such thermosets is difficult to avoid, even worse, resulting in apparent wrinkling of CNC films. Very recently, Zhou et al. reported the fabrication of flexible and humidity-responsive CNC films by introducing poly(ethylene glycol) as an additive.32 Inspired by the changed coloration of longhorn beetles Tmesisternus isabellae from golden at dry state to red at wet state,33 herein, a small molecule, glycerol, is used as a plasticizer as well as a hygroscopic agent for CNC to develop flexible and multiresponsive chiral nematic films. By controlling the length and diameter of CNC rods, a blue-iridescent CNC film is fabricated. Additionally, glycerol is introduced to tune the iridescent colors as well as the mechanical toughness of the CNC films. Depending on the addition amount of glycerol, controllably red-shifted iridescence and enhanced tensile strain are realized. In response to environmental humidity and pressure, in particular, the films can reversibly change structural colors, indicating potential applications in colorimetric sensors, anticounterfeiting technology, and decorative coatings.
EXPERIMENTAL SECTION
Materials. Cotton wool (product no. YZB/Chuan 0177-2013) was provided by Kangda Health Materials Co. (Sichuan, China). Sulfuric acid (CAS no. 7664-93-9) was purchased from Kelong Co. (Sichuan, China.) Glycerol (CAS no. 56-81-5) was obtained from Ruijin Co. (Tianjin, China). Deionized water was obtained from a Milli-Q Plus water purification system (Millipore, USA) and was used as a solvent in all experiments. Preparation of Cellulose Nanocrystal Films. CNC was prepared by treating dried cotton wool (34 g) with a sulfuric acid solution (64 w/w %, 300 mL) at 55 °C for 1 h, followed by adding cold deionized water (3 L) to quench the reaction. Such solution was left overnight, and the suspension was centrifuged at 9000 rpm for 5 min, washed with deionized water several times, and dialyzed against deionized water with dialysis membrane tubes (3500 molecular weight cut-off) until the complete removal of sulfuric acid at 25 °C. Finally, the as-prepared CNC dispersion was concentrated to the concentration of 8.5 wt % with a rotary evaporation apparatus. Prior to the film formation, different amounts of glycerol (0.2−1.0 g) were added in the aqueous CNC suspension (20 g) and mixed at 25 °C for 12 h, followed by casting such solution into polytetrafluoroethylene molds (5 cm in diameter) and drying at 45 °C to give freestanding CNC films. A pure CNC film in the absence of glycerol was also prepared as a reference. Characterization. FT-IR spectra were recorded on a Fourier transform infrared spectrometer (Nicolet 6700, USA) within a wavenumber range of 4000−700 cm−1. Molecular composition of samples (concentration: 0.5 mg/L) was determined in terms of element analysis by inductively coupled plasma−atomic emission spectrometry (Iris Advantage 1000, USA). Zeta potential measurement was conducted on the CNC dispersion sample with the concentration of 0.01 wt % by Zetasizer (Zetasizer Nano ZS90) at 5806
DOI: 10.1021/acsami.7b18440 ACS Appl. Mater. Interfaces 2018, 10, 5805−5811
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Figure 2. (a) Photographs of solid CNC films containing different amounts of glycerol; (b) UV−vis extinction spectra of neat CNC and CNC/ glycerol films; (c) maximum extinction wavelength and chiral nematic pitch of CNC and CNC/glycerol films as a function of glycerol percentage. room temperature to measure the zeta potential and understand the surface charge density. For each sample, six measurements were performed to give the average values. X-ray diffraction (XRD) patterns were collected by using an X-ray diffractometer (XRD-6100, Shimadzu) between 5° and 80° with a Ni-filtered Cu Kα radiation (1.5418 Å). Texture of samples was observed using polarized optical microscopy (POM, Eclipse 50i POL). Prior to the measurement, a sample drop was deposited onto a glass slide. Scanning electron microscopy (SEM) measurement was conducted on a scanning electron microscope (JSM-5900LV, JEOL Co. Japan). Samples were operated in high vacuum mode at 5 kV accelerating voltage and fractured to expose their cross-sectional areas. Transmission electron microscopy (TEM) analysis was performed by using a transmission electron microscope (ZEISS LIBRA 200FE) at a voltage of 75 kV. The samples were prepared by depositing a suspension drop onto a wax sheet, followed by immersing a copper grid in for a few minutes. Structural coloration of film samples was measured using UV−visible spectroscopy (Varian Cary 50 spectrophotometer), in terms of the extinction intensity within the wavelength range of 300−800 nm. For each sample, five scans were recorded to give an average spectrum. Mechanical properties were investigated using a universal testing machine (model 3366, Instron Corp, USA) equipped with a 100 N load cell. The sample size was of 4 mm in width and 25 mm in length, and the tensile speed was set at 5 mm/min. Humidity- and Pressure-Responsive Property. Humidityresponsive performance was evaluated by placing the sample films in the containers with different relative humidities (RH), which were built as reported previously,34 that is, saturated anhydrous CaCl2, K2CO3, NaCl solutions and distilled water for 16% RH, 43% RH, 75% RH, and 98% RH, respectively. After being incubated for 1 h, the films were taken out, and their UV−vis extinction spectra were recorded. For the investigation on the pressure-responsive property, different compressive stresses were applied with a small-scale powder press machine (HW-01, Jinwei) on samples under the load range of 0−10 MPa for 3 s, followed by recording the changes of their UV−vis extinction spectra.
CNC is of rod-like morphology with the average length and diameter of 222 and 7 nm, respectively (Figure 1a). Because of the presence of negatively charged sulfate surface groups, CNC can disperse in water homogeneously without precipitation. The zeta potential and sulfur content of the CNC prepared in this work are −15.8 mV (Figure 1b) and 0.0124 mol/g. According to a previous report,36 colloidal particles tend to flocculate in case of the zeta potential is lower than 25 mV. For CNC, however, its agglomeration starts once the zeta potential becomes lower than 15 mV.28 As a result, the aqueous CNC dispersion prepared herein is stable. Additionally, the surface charge density is thereafter calculated as 0.34 e/nm2, according to the following equation28 σ=
MSO3 × % S × Na SA:V /dcell
(1)
where MSO3 is the molecular weight of sulfate half-ester, % S is the sulfur content, Na is the Avogadro’s number, SA:V refers to the surface to volume ratio, and dcell represents the density of cellulose. As reported previously,37,38 the surface charge density of CNC is required to be located within the region of 0.16− 0.66 e/nm2, in case of self-assembly in water. Therefore, it is reasonable to assume that the CNC we prepare can be used to develop iridescent films. According to the equation proposed by Gray,39 the critical concentration, where CNC dispersion starts to exhibit a lyotropic chiral nematic behavior, is calculated as approximately 5.04 wt %. Consequently, the CNC concentration of the film-forming dispersion is set as 8.5 wt %. As shown in Figure 1c, the characteristic birefringent and “fingerprint” textures are observed for the CNC dispersion by POM, from which the chiral nematic pitch (P) is measured as twice the distance between the fingerprint lines in the inserted enlarged photos. Accordingly, the P value of the neat CNC dispersion is 840 nm. The introduction of glycerol shows no obvious effect on the appearance of birefringent and “finger-
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RESULTS AND DISCUSSION Herein, CNC is prepared by an acid hydrolysis approach reported before.35 As determined by TEM, the as-prepared 5807
DOI: 10.1021/acsami.7b18440 ACS Appl. Mater. Interfaces 2018, 10, 5805−5811
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Figure 3. (a) FT-IR spectra of neat CNC and CNC/glycerol films; (b) typical SEM images of neat CNC and CNC-G0.6 films showing regular layered structure; (c) proposed molecular mechanism for the regulation of red-shifted structural color of CNC in the presence of glycerol.
print” textures but on the P values, depending on the addition amount. For specific CNC/glycerol dispersions, which are named in accordance with the amount of glycerol, their P values as a function of the glycerol percentage are illustrated in Figure 1d, presenting a positive correlation. To prepare iridescent CNC films, water is evaporated by a conventional solution-cast method. The photos of such CNC/ glycerol films taken at the vertical direction are shown in Figure 2a, where different iridescent colors can be seen, that is, the color of such films changes from blue to red gradually with the increased amount of glycerol. The different “colorations” on the films are attributed to the fact that the light with certain wavelength is extinct when passing through the films. A further quantitative analysis conducted by UV−vis extinction spectroscopy indicates that a gradual red shift of the maximum extinction wavelength occurs from 347 to 610 nm with the incorporation of glycerol (Figure 2b). Generally, the iridescent colors are regarded as a result of Bragg reflection. Therefore, the P values of the solid CNC/glycerol films can be calculated by the following equation:40 λ = nav P sin θ
the plasticizer, the band is obviously narrowed and shifts to low wavenumber. Additionally, from the cross-sectional image of the representative blend film, CNC-G0.6, the parallel layered structure is still well-maintained, compared with the neat CNC film (Figure 3b). Nevertheless, the height of an individual layer of the blend film is much greater than that of the neat one. Figure S1 presents the XRD patterns of neat CNC and CNC/ glycerol blends, where the characteristic crystalline peaks belonging to the (101), (101)̅ , (002), and (040) lattices of cellulose are observed for all samples. This suggests that the crystal structure of cellulose is not destroyed after the addition of glycerol. On the basis of the FTIR and XRD results, it is reasonable to believe that glycerol molecules diffuse into the CNC scaffold, located in the amorphous region as well as in the mesopores among the rods, and form hydrogen bonding with CNC at the hydroxyl groups of the glucose ring. As a result, the presence of glycerol occupies some free volumes of CNC, resulting in the increase in the periodic arrays (in terms of P) as well as the red shift of structural colors; the proposed mechanism is presented in Figure 3c. As well-known, mechanical properties are of great importance to practical applications. For the neat CNC film, however, it is impossible to perform the tensile test because of the high intrinsic brittleness. In contrast, the addition of glycerol induces the obviously increased tensile toughness of the CNC films. Particularly, the elongation at break exceeds 2% for CNC-G0.6 (Figure 4a), which is larger than those in many previous reports.22,32,41,42 The detailed tensile strength and elongation at break as a function of glycerol amount are provided in Figure S2. As shown in the inserted photo, moreover, the CNC-G0.8 film can be easily bent, presenting good flexibility that can be hardly realized for the neat CNC film. Owing to the high hygroscopicity of glycerol, the resulting films are readily able to capture water in a humid environment. Accordingly, their responses to RH are investigated in detail.
(2)
where nav is the average refractive index of the CNC-G film, P is the pitch, and θ the incident angle of light. The effect of the glycerol amount of the calculated P value is shown in Figure 2c. Concretely, P of such films increases from 227 to 402 nm with the increased glycerol amount. The results clearly explain why different iridescent colors are presented on the CNC films, suggesting that the iridescent property can be controlled by simply changing the addition amount of glycerol. To clarify the mechanism for the controllable iridescence, Fourier transform infrared spectroscopy (FTIR) measurement is performed on the neat CNC and CNC/glycerol films. As shown in Figure 3a, the absorption band of neat CNC within 3300−3500 cm−1 attributing to O−H stretching suggests the existence of hydrogen bonding. Once introducing glycerol as 5808
DOI: 10.1021/acsami.7b18440 ACS Appl. Mater. Interfaces 2018, 10, 5805−5811
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Figure 4. (a) Tensile property of neat CNC and CNC/glycerol films (inserted digital photograph is to show the bending flexibility of the CNC-G0.8 film); (b) humidity-responsive effect and (c) corresponding cross-sectional structure of the CNC-G0.8 film at different relative humidities; (d) reversible humidity response over 10 consecutive cycles between the relative humidities of 16 and 98%; (e) proposed mechanism for the reversible humidity-responsive structural color change of CNC films; (f) pressure-responsive effect and (g) corresponding chromism mechanism of the CNCG0.8 film.
“green” and “red”. Within 10-run alternations, the reversibility keeps well in terms of the nearly unchanged maximum extinction wavelength. Accordingly, the proposed mechanism is provided in Figure 4e. More importantly, the trigger signal to induce the chromism is not just limited as humidity. Giving the film a pressure can also cause a similar consequence. In particular, a monotonic dependence of the wavelength shift on the applied pressure is found, that is, the higher pressure, the greater blue shift of the maximum extinction wavelength (Figure 4f). As a clearer illustration, an “S”-shaped stamp is employed to press on the CNC-G0.8 film, leaving a blue mark after uplifting (Figure 4g), which can be attributed to the shortened P distance. Interestingly, the blue coloration is able to turn red once placing the film in high-RH condition; this phenomenon can be well-explained by the close maximum extinction wavelengths of such film before and after the successive press-moisture absorption treatment. Owing to the poor elasticity, however, the “S” mark remains still in spite of the color recovery.
Taking CNC-G0.8 as the representative, it is stored at different RH levels. As shown in Figure 4b, the extinction wavelength of the CNC-G0.8 film shifts from 586 to 704 nm when the RH increases from 16 to 98%, indicating the red-shifted structural coloration. To understand whether the water molecules have such power to trigger the color variation, SEM images of the film are recorded after the treatment at different RH levels. From Figure 4c, we can see that the film thickness increases with the RH level, that is, from 91 ± 2 μm in the case of 16% RH to 155 ± 5 μm in the case of 98% RH. As a result, it can be reasonably deduced that the increased film thickness accounts for the increase of P value as well as the red shift of structural color. In addition, all other films except the neat CNC one have humidity-responsive chromism performance; the detailed maximum extinction wavelengths are presented in Figure S3. This indicates that the presence of glycerol contributes to realize the humidity-responsive chromism function of CNC films. Besides that, the chromism is reversible in case of the changed environmental humidity. As shown in Figure 4d, when the CNC-G0.8 film is placed at 16% RH and 98% RH alternately, its structural color changes reversibly between 5809
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(3) Galisteo-López, J. F.; Ibisate, M.; Sapienza, R.; Froufe-Pérez, L. S.; Blanco, Á .; López, C. Self-assembled photonic structures. Adv. Mater. 2011, 23, 30−69. (4) Qiao, W.; Huang, W.; Liu, Y.; Li, X.; Chen, L.-S.; Tang, J.-X. Toward Scalable Flexible Nanomanufacturing for Photonic Structures and Devices. Adv. Mater. 2016, 28, 10353−10380. (5) Yu, K.; Fan, T.; Shuai, L.; Zhang, D. Biomimetic optical materials: Integration of nature’s design for manipulation of light. Prog. Mater. Sci. 2013, 58, 825−873. (6) Kinoshita, S.; Yoshioka, S. Structural colors in nature: the role of regularity and irregularity in the structure. ChemPhysChem 2005, 6, 1442−1459. (7) Parker, A. R.; Townley, H. E. Biomimetics of photonic nanostructures. Nat. Nanotechnol. 2007, 2, 347−353. (8) Vignolini, S.; Rudall, P. J.; Rowland, A. V.; Reed, A.; Moyroud, E.; Faden, R. B.; Baumberg, J. J.; Glover, B. J.; Steiner, U. Pointillist structural color in Pollia fruit. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 15712−15715. (9) Yablonovitch, E. Inhibited spontaneous emission in solid-state physics and electronics. Phys. Rev. Lett. 1987, 58, 2059−2062. (10) John, S. Strong localization of photons in certain disordered dielectric superlattices. Phys. Rev. Lett. 1987, 58, 2486−2489. (11) Míguez, H.; Yang, S. M.; Tétreault, N.; Ozin, G. A. Oriented Free-Standing Three-Dimensional Silicon Inverted Colloidal Photonic Crystal Microfibers. Adv. Mater. 2002, 14, 1805−1808. (12) Choi, S. Y.; Mamak, M.; von Freymann, G.; Chopra, N.; Ozin, G. A. Mesoporous Bragg Stack Color Tunable Sensors. Nano Lett. 2006, 6, 2456−2461. (13) Shopsowitz, K. E.; Qi, H.; Hamad, W. Y.; Maclachlan, M. J. Free-standing mesoporous silica films with tunable chiral nematic structures. Nature 2010, 468, 422−425. (14) Khan, M. K.; Giese, M.; Yu, M.; Kelly, J. A.; Hamad, W. Y.; Maclachlan, M. J. Flexible mesoporous photonic resins with tunable chiral nematic structures. Angew. Chem., Int. Ed. 2013, 52, 8921−8924. (15) Shopsowitz, K. E.; Hamad, W. Y.; MacLachlan, M. J. Flexible and Iridescent Chiral Nematic Mesoporous Organosilica Films. J. Am. Chem. Soc. 2012, 134, 867−870. (16) Lee, G. H.; Choi, T. M.; Kim, B.; Han, S. H.; Lee, J. M.; Kim, S.H. Chameleon-Inspired Mechanochromic Photonic Films Composed of Non-close-Packed Colloidal Arrays. ACS Nano 2017, 11, 11350− 11357. (17) Vigneron, J. P.; Pasteels, J. M.; Windsor, D. M.; Vértesy, Z.; Rassart, M.; Seldrum, T.; Dumont, J.; Deparis, O.; Lousse, V.; Biró, L. P.; Ertz, D.; Welch, V. Switchable reflector in the Panamanian tortoise beetle Charidotella egregia(Chrysomelidae: Cassidinae). Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2007, 76, 031907. (18) Rassart, M.; Colomer, J.-F.; Tabarrant, T.; Vigneron, J. P. Diffractive hygrochromic effect in the cuticle of the hercules beetle Dynastes hercules. New J. Phys. 2008, 10, 033014. (19) Kasukawa, H.; Oshima, N.; Fujii, R. Mechanism of light reflection in blue damselfish motile iridophore. Zool. Sci. 1987, 4, 243− 257. (20) Rånby, B. G. Fibrous macromolecular systems. Cellulose and muscle. The colloidal properties of cellulose micelles. Discuss. Faraday Soc. 1951, 11, 158−164. (21) Fernandes, S. N.; Almeida, P. L.; Monge, N.; Aguirre, L. E.; Reis, D.; de Oliveira, C. L. P.; Neto, A. M. F.; Pieranski, P.; Godinho, M. H. Mind the Microgap in Iridescent Cellulose Nanocrystal Films. Adv. Mater. 2017, 29, 1603560. (22) Giese, M.; Khan, M. K.; Hamad, W. Y.; MacLachlan, M. J. Imprinting of Photonic Patterns with Thermosetting Amino-Formaldehyde-Cellulose Composites. ACS Macro Lett. 2013, 2, 818−821. (23) Dong, X. M.; Gray, D. G. Induced Circular Dichroism of Isotropic and Magnetically-Oriented Chiral Nematic Suspensions of Cellulose Crystallites. Langmuir 1997, 13, 3029−3034. (24) Beck, S.; Bouchard, J.; Berry, R. Controlling the reflection wavelength of iridescent solid films of nanocrystalline cellulose. Biomacromolecules 2011, 12, 167−172.
CONCLUSIONS In summary, highly flexible and iridescent CNC films with humidity- and pressure-responsive chromism effects are successfully prepared by introducing glycerol as the plasticizer as well as the hygroscopic agent. The iridescent color of such films is easily modulated from blue to red by controlling the addition amount of the small molecule. The presence of glycerol endows high flexibility to the CNC films, which can be bent and stretched with an elongation at break higher than 2%. When being exposed to different relative humidities, the films demonstrate reversible color changes. Furthermore, the films can quantificationally sense the compression pressure by presenting iridescent color changes. On the basis of the above properties and functions, the CNC films are expected to show applications in colorimetric sensors, anticounterfeiting, and decorative coatings. For instance, the films can be designed in the form of security signs, labels, and optical components, which can perceive external environmental changes by chromism. This behavior, conversely, can help to distinguish the original and the fake. Besides that, the film can be also used as a functional coating displaying optional colors that we want, which is desirable for decoration. Nevertheless, it should be mentioned that, as well-known, the surface migration of small molecules from polymeric films is generally difficult to avoid, especially in high humidity-environments, which might result in the loss of structural colors and relevant properties. Consequently, functional additives with higher stability in CNC will be explored in our future work for the long-term service of such environmental stimuli-responsive chromic photonic biobased materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18440. XRD patterns, mechanical properties, and humiditydependent maximum extinction wavelength of neat CNC and CNC/glycerol films (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone/Fax: 86-28-85410755. ORCID
Fei Song: 0000-0001-5229-4379 Xiu-Li Wang: 0000-0002-2732-9477 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (51773133 and 51421061) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT1026).
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