Fabrication of 3D Photonic Crystals from Chitosan That Are

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Fabrication of 3D Photonic Crystals from Chitosan That Are Responsive to Organic Solvents Guanbo Huang,†,‡ Yibing Yin,†,‡ Zeng Pan,†,‡ Mingxi Chen,†,‡ Lei Zhang,†,‡ Yu Liu,*,‡,† Yongli Zhang,§ and Jianping Gao†,‡ †

Department of Chemistry, School of Science, Tianjin University, No.92, Weijin Road, Tianjin 300072, P. R. China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, P. R. China § Tianjin Huanhu Hospital, No.122, Qixiangtai Road, Tianjin 300060, P. R. China ‡

S Supporting Information *

ABSTRACT: Inspired by photonic nanostructures in nature, such as the hair-like chaetae on the body of sea mice, inverse opal photonic crystals films were fabricated with chitosan, a kind of biomacromolecule found in nature. First, monodispersed polystyrene (PS) colloidal crystal templates with different particle sizes were prepared. The inverse opal films (IOFs) were fabricated through in situ cross-linking of the PS templates. The IOFs contain periodically ordered interconnecting pores that endow the films with photonic stop bands and structural colors, which are visible to the naked eye. The IOFs exhibit rapid reversible changes in their structural colors and reflectance peaks in response to alcohols and phenols. Possible mechanisms for the shifts in the IOF’s reflectance peaks are proposed. The changes in the IOFs in response to alcohols and phenols provide a potential way to visually detect these organic solvents.

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INTRODUCTION During the last few decades, much effort has been devoted to the construction of photonic nanomaterials.1 Photonic crystals (PhCs) are materials with periodic optical nanostructures and spatially ordered lattices.2,3 The periodic arrangement of the dielectric material endows these materials with a property known as the photonic band gap (PBG). This property has given rise to many applications especially for PhCs with tunable PBGs. Some of these include low-loss resonators,4,5 optical fibers,6,7 switches/ logic gates,8−10 sensors,11,12 and display devices.13,14 For example, Burgess et al. developed a technique for patterning multiple chemical functionalities throughout the inner surfaces of ordered porous PhC structures.15 The same group also developed a selective colorimetric indicator for organic liquids.16 Fenzl et al. reported PhC-based sensors made from polystyrene (PS) for organic solvents.17 Many PBGs in the reported PhCs are direction-dependent stop bands instead of complete band gaps, but these vary with the type of material. To date, many photonic nanostructures from one-dimensional (1D) to three-dimensional (3D) have been developed.12,18,19 Three-dimensional photonic nanostructures were first investigated in the 1960s.20 Nature contains various 3D structures which have developed through billions of years of evolution.21 The inverse opal structure in butterflies is made from a biomacromolecule, chitosan, which also exists in many other organisms.22 Aphroditidae contain chaetae made of bundles of chitin.23 The chaetae forms a lateral belt of iridescence around the animal.24 Chitosan (partially deacetylated chitin) is a natural © XXXX American Chemical Society

polysaccharide as well as a biomass resource, and it has attracted a great deal of attention.25 Recently, chitosan has been used as a biomaterial due to its biocompatibility, nontoxicity, and edibility.26−29 Choi et al. reported that freeze-drying and templating technologies can be used to tune the pore sizes of chitosan materials with inverse opal structures, and thus, the materials can be utilized as drug carriers or scaffolds for cell cultures.30 In the recent years, many different responsive PhCs made of hydrogel have been reported. They exhibit reversible diffraction shifts in response to external stimuli such as temperature31 and chemical substances.32 However, many hydrogels contain the residual raw monomers or oligomers (such as acrylic amide) which are hazardous to the environment and organisms. This limits their application. So, inspired by the chitin photonic structures created by nature, we chose chitosan to build the responsive PhCs. Chitosan is a natural product that is easily obtained from biological resources and is a reproducible material. The cross-linked product, uncross-linked macromolecules, and the 2-amino-2-deoxy-β-D-glucopyranose “monomer” are safe to organisms. Products made of chitosan have good biocompatibility and are easily degraded by organisms.33 Methanol, ethanol, and 1-propanol are organic solvents and are important chemicals in the production of pharmaceuticals Received: July 29, 2014 Revised: October 16, 2014

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and pesticides. These three alcohols have similar structures (a terminal hydroxyl group) but different densities, polarities, and refractive indexes. An inexpensive and facile method to measure the concentrations of them in aqueous solutions would be beneficial. Phenolic compounds are major environmental pollutants.34 The detection of them is important in environment monitoring, waste management, and also clinical toxicology. Samples containing phenols are typically analyzed via chromatography or spectrophotometry. These methods have high sensitivities but can be complicated and expensive. Convenient, low-cost visual detection for these compounds at relatively low concentrations would be beneficial. Inspired by the structures of photonic materials in nature and the literature, inverse opal films (IOFs) were synthesized using PS templates and chitosan in this work. The morphologies of the IOFs were observed using scanning electron microscopy, and the response of the films methanol, ethanol, 1-propanol, phenol, 4chlorophenol, and 4-nitrophenol were investigated. The mechanism for the responses (i.e., the changes in the structural color) was also investigated.

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Figure 1. Preparation scheme for the IOFs: fabrication of the PS opal template (a), cross-linking of chitosan precursor (b), and removal of PS opal template (c).

EXPERIMENTAL SECTION

linking of the chitosan was conducted at 37 °C for 24 h. After that, the sample was immersed in toluene to completely remove the PS opal template (Figure 1c). A thin colored film of the chitosan PhC with an inverse-opal structure was obtained. Characterization and Responsivity Test of the IOFs. Optical photos of the PS opals and IOFs were taken with a NIKON CoolPix4300 digital camera. The PS opal templates and IOFs were coated with gold using a sputter coater (Desk-II; Denton Vacuum, U.S.A.) and then their microstructures were observed using scanning electron microscopy (SEM, S-4800, HITACHI, Japan) at 12 to 20 keV. To measure the response of the IOF to organic solvents, the IOF was immersed in a solvent until it reached a swelling equilibrium, at which point the reflectance spectrum was measured with a miniature fiber optic spectrometer (AvaSpec-2048-SPU). The stop band results were calculated from five parallel measurements. The physical sizes of the IOFs were measured using a ruler with vernier scale. Two points along one side of a specific IOF was marked, the distance between them was measured and after immersion in some solvent, the same distance was measured again. All the measurements were repeated five times, and the average results were used for the calculation of the equilibrium swelling degree (the ratio of the size at a given condition to the size at the original dry condition). Afterward, the IOF was rinsed three times with deionized water and then dried for subsequent tests. An Abbe refractometer (WAY-2S) was employed to measure the refractive indexes of all the tested liquids. All the indexes were measured at 22 °C using 589.3 nm (the sodium D line).

Materials. Styrene (98.0%), sodium dodecyl benzenesulfonate (SDBS) (97.0%), and ammonium persulfate (APS) (99.0%) were purchased from Alfa Aesar. Chitosan (85% deacetylated, average molecular weight 100 kDa) was purchased from Zhejiang Golden-shell Biochemical Co. Glutaraldehyde (25% aqueous solution), absolute acetic acid (99.0%), methanol (99.5%), ethanol (≥99.7%), 1-propanol (99.0%), phenol (99.0%), 4-nitrophenol (analytical grade), 4chlorophenol (analytical grade), toluene (≥99.5%), acetone (99%), ethyl acetate (EA, ≥99.5%), chloroform (TCM, 99%), dichloromethane (DCM, 99%), sodium chloride (99.5%), and the other chemicals were all purchased from Tianjin Chemical Reagent Co. All chemicals were used as received, except for styrene, which was redistilled before use to remove the polymerization inhibitor. The glass slides (76.2 × 25.4 × 1 mm3) were cleaned in an alcohol ultrasonic bath for 15 min, followed by rinsing in deionized water. Preparation of PS Colloids, PS Opal Templates, and Chitosan IOFs. The monodispersed PS colloids and PS opal templates were prepared according to our previous method.35 Briefly, 100 mL of distilled water was poured into a three-neck flask, and then the desired amount of emulsifier (SDBS) and initiator (APS) were added with stirring under a N2 gas atmosphere. The flask was heated to 75 °C in a water bath and 25 mL of purified styrene was slowly added. The reaction was terminated after about 9 h to give a monodispersed PS colloid. To remove the large aggregated blocks of PS and the impurities (unreacted styrene monomers, APS and SDBS), the emulsion was filtered through a gauze cloth and then placed in a dialysis bag (14 000 molecular weight cutoff) and dialyzed with ultrapure water. The prepared PS colloid was then used to fabricate the PS colloidal crystal templates by a vertical deposition method as shown in Figure 1a. The PS colloid was placed into clean containers, and then clean glass slides were inserted vertically into the colloid. The containers were kept in a vacuum oven for 2−3 days at atmospheric pressure, during which time the temperature and humidity were controlled at 35 °C and 40% RH to prevent the templates from breaking. After the solvent completely evaporated, PS opal templates were obtained. IOFs were prepared by using a capillary-attraction-induced method (Figure 1b). First, the precursor was made by dissolving 0.2 g of chitosan in 20 mL of 1% (v/v) aqueous acetic acid mixed with 10 mL of 1% (m/ m) glutaraldehyde solution. A glass slide partially covered with the PS opal template was tilted about 15° to the horizontal and the precursor was then placed on the upper portion of the uncoated area on the glass slide next to the upper edge of the PS opal template. The precursor solution slowly flowed downward along the glass slide and infiltrated the colloidal crystals until the template became transparent. The glass slide with the filled PS opal template was then put into an oven and the cross-

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RESULTS AND DISCUSSION Choice of Materials. PS colloids were used to prepare templates for the fabrication of PhCs with inverse opal structures. Styrene, the monomer of PS, can easily be polymerized, and the size of the PS colloid particles can be controlled with the aid of a surfactant. The PS colloid was synthesized through an emulsion polymerization reaction, and the colloid possessed good monodipersity and was suitable for the fabrication templates with opal structures (Figure S1a in the Supporting Information). Moreover, the PS particles can be easily removed from the final product with an appropriate solvent such as toluene. Chitosan was chosen as the matrix to build the IOFs. As mentioned previously, chitosan has advantages which are important for its applications in areas like bioengineering and biomedicine. In addition, there are large numbers of highly active hydroxyl groups and amino groups on the macromolecule chains

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Figure 2. SEM images of IOFs fabricated from PS opal templates with different diameters: 201 nm (a), 236 nm (b), 250 nm (c), 280 nm (d), and 319 nm (e) (bars are 100 nm).

colors (Figure 3a,b, S3a,b). There is a linear relationship between the wavelength of IOF reflection peaks (y, nm) and the diameters of the air spheres (x, nm): y = 1.6437x + 31.228, R2 = 0.9532 (Figure S4). The stop bands of the IOFs exhibited good reproducibility with standard deviations of a few nanometers and no variation in structural color. However, the correlation coefficient (0.9532) is lower than those for the PS opals and the chitosan-filled PS opals (0.9982 and 0.9942 shown in Figure S2 and S3c, respectively), which is due to defects in the IOFs. Responsivity of IOFs to Alcohol−Water Mixtures. When the dry IOFs were immersed in pure water, they all showed distinct changes in their structural colors (Figure 4); all of the colors red-shifted. The color changes are caused by shifts in the stop bands. The precise mechanism for the stop band shifts was

of chitosan so it can be easily cross-linked using a cross-linker like glutaraldehyde. Further, many of the functional groups, especially the alkaline amino groups, remain unreacted after the cross-linking reaction and the macromolecule is hydrophilic due to its polar groups. However, once the polymer is crosslinked, it does not dissolve in water. Morphology and Stop Bands of the IOFs. Figure 2 shows the SEM photos of the IOFs prepared from the chitosan-filled PS templates. The IOFs have 3D ordered face center cubic structures which are derived from the structure of the PS opals. The average center-to-center distance between the air spheres in the IOFs are smaller than those in the original PS opals. This shrinkage is from contraction due to water loss in the IOFs and is responsible for the blue-shifts that occur in the IOF structural C

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74% of the total volume, whereas the walls occupy 26%. So na can be approximated by eq 3: na =

26%·n w2 + 74%·n p2

(3)

where nw and np are the refractive indexes of the IOF material and the material filling the pores, respectively.38 The refractive index of water and chitosan are 1.333 and 1.513, respectively.39 In dry IOFs, the pores are filled with air so the dry IOFs have a refractive index of 1.155 because the refractive index of air is about 1. When the IOF is immersed in water, the value of na changes to 1.382 (1.197 times 1.155). The λmax of the IOF in water is 560 nm, which is 1.244 times that of the dry IOF (450 nm). There is a slight difference between the change in the refractive index and the change of λmax predicted by eq 2. This is due to the swelling behavior of the IOF in water. Although the IOFs made of chitosan do not dissolve in water after cross-linking, they can still swell in water. Calculated with the size change of the dry IOF and IOF immersed in water, the equilibrium swelling degree of the IOF is 1.04 in water. So the results are in agreement with eq 2. As shown in Figure 5, when the concentration of ethanol in the mixtures increased (from pure water to pure ethanol), the

Figure 3. Photos (a) and reflectance spectra (b) of IOFs made from PS opal templates with different diameters (from left to right: 201, 236, 250, 280, and 319 nm).

Figure 4. Color changes of IOFs made from different PS templates in response to water.

then investigated using IOFs made from the 250 nm PS colloid template. When the IOFs were immersed in ethanol−water mixtures of different proportions, they exhibited different structural colors as shown in Figure 5. The maximum reflection wavelength of the IOFs (λmax) can be estimated from the following modified Bragg diffraction equation: λmax = 1.633

d ·S · na 2 − sin 2 θ m

Figure 5. Photos (a) and reflectance peaks (b) of IOF in different ethanol−water mixtures (all percentages are the concentration of ethanol (v%)).

(1)

where d is the diameter of the PS colloidal particles, m is the Bragg order of diffraction, na is the average refractive index of the material, θ is the angle measured between the normal and the plane of the colloidal crystal, and S is the equilibrium swelling degree of the IOF.36,37 According to eq 1, λmax is dependent on five factors, but two of them, θ (0) and m (1), are fixed. For each specific IOF, d is a constant and depends only on the particle size of the PS colloid template. So, S and na are the factors that may affect λmax, and the relationship simplifies to λmax ∝ nadS

reflectance peak blue-shifted a total of about 100 nm and the structural colors changed from bright green to bluish violet. The refractive indexes of water and ethanol are 1.333 and 1.362, respectively. When more ethanol was present, the average refractive index increased, and this causes a red shift in the IOF structural color. However, because chitosan is a hydrophilic polymer, it does not dissolve in ethanol. Thus, ethanol makes the IOF to shrink; in other words, the value for S decreases and causes a blue shift in the structural color. Because the value of S changed more than the value of na, the overall result was a blue shift.

(2)

If a dry IOF is immersed in water, the value of na will change. At the same time, if an IOF swells or shrinks in water, the value of S will change. In an inverse opal, the pores take up approximately D

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reproducibility (standard deviations of a few nanometers and no variation in structural color). The λmax of IOF in mixed alcohol solutions was also studied in methanol solution with different amount of ethanol. The methanol concentration was set as 10% and ethanol was 10− 200% of the methanol concentration. The addition of ethanol resulted in a blue shift of the IOF structural color. When the ethanol/methanol ratio was increased from 0 to 20%, the blue shift was small and when the ethanol was increased from 20 to 200%, a decline of λmax occurred (Figure 7).

The responsivities of the IOFs to methanol−water and 1propanol-water mixtures were also studied. These results along with those for ethanol−water mixtures are shown in Figure 6a.

Figure 7. Maximum reflection wavelength of IOF in 10% methanol solution with different ratio of ethanol to methanol.

Responsivity to Aqueous Phenol Solutions. The IOFs prepared with 250 nm PS colloid templates were also tested in solutions of phenols. When the IOFs were immersed in aqueous solutions of three phenolic compounds (phenol, 4-chlorophenol, and 4-nitrophenol), the structural colors changed within a few seconds. The results are shown in Figure S5. For all three phenols, higher concentrations of phenol resulted in larger red shifts of the maximum reflection wavelengths. Accordingly, the structural colors of IOFs changed from green to yellow orange. Meanwhile, the values of λmax of the IOFs in different phenols solutions exhibited good reproducibility, the standard deviations were only a few nanometers, and the structural colors showed no obvious variation. The refractive indexes of the phenol-filled pores in the IOFs are approximately the same as those for the IOFs immersed in water. This is because the concentration of phenol in the pores is quite small and therefore not enough to significantly change the refractive index of the material. (The refractive indexes of all the phenols solutions at the experimental conditions is 1.333, which is the same as that for pure water). So for the phenols, the S value was the decisive factor responsible for the change of structural color. Meanwhile, the IOF responds to 4-nitrophenol at the lowest concentration, as shown in Figure S5. The pKa values of phenol, 4-chlorophenol, and 4-nitrophenol are 9.94, 9.38, and 7.15, respectively. So all these phenols dissociate to some extent and provide protons in water. IOFs made of chitosan are basic due to large amounts of amino groups. When immersed in phenol solutions, the amino groups in the IOFs capture the protons from the phenols molecules and become positively charged as shown in Figure 8. The positively charged groups then repel each other and cause the system to expand and as a result, S increases. Among the three phenols, 4-nitrophenol has the lowest pKa, so the IOF responds to 4-nitrophenol at the lowest concentration. Responsivities of the IOFs to pH, Other Organic Solvents, and Ionic Strength. To further verify the swelling mechanism of the chitosan IOFs in response to phenols, they

Figure 6. Maximum reflection wavelength vs concentration of alcohol (a), calculated average refractive indexes of the IOF immersed in different mixtures (b), equilibrium swelling degree of the IOF in different mixtures (c).

For all the alcohols, the IOFs inhibited similar blue shifts in their structural colors as the concentrations of alcohol increased. The average refractive indexes of the IOF immersed in the different mixtures were calculated using eq 3, and the results are shown in Figure 6b. The refractive indexes of water, methanol, ethanol, and 1-propanol are 1.333, 1.329, 1.362, and 1.420, respectively, the average refractive index of IOF changed when the alcohol switched. However, the polarities of water, methanol, ethanol, and 1-propanol are 10.2, 5.1, 4.3, and 4.0, respectively. Thus, the hydrophilic IOF shrank the least in methanol and the most in 1propanol, that is, the S value of the IOF in methanol was the highest and that in 1-propanol was the lowest (Figure 6c). The contributions of the different alcohols to na and S are in opposite directions and so the observed color changes (i.e., the changes in λmax) are almost the same. Moreover, the values of λmax of the chitosan IOFs in different alcohol solutions exhibited good E

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Figure 8. IOF swelling mechanism in response to aqueous phenol solutions.

bactericide, no bacterial colonies were visible after 6 months of storage. The IOFs also showed no changes in their physical properties.

were immersed in solutions of different pH values, and the results are shown in Figure S6a. As the pH decreased from 7 to 6, the structural color changed from green to yellow and the maximum reflection wavelength red-shifted about 40 nm. The amount of acid does not significantly change the refractive index of the solution but influences the pH value of the solution which could cause change in the degree of swelling, so the red shift must be caused by changes in the degree of swelling. When the pH value was lower than 6, the IOF showed no more red shift because the concentration of acid exceeded the responsive range. The crosslinked chitosan IOF cannot swell without limit in acidic solution. In solutions with pH value higher than 7, the chitosan IOF did not exhibit any stop band shift because chitosan do not swell or shrink in alkaline solutions. The IOFs were then immersed in acetone, ethyl acetate, chloroform, and dichloromethane to test their responsivity to these organic solvents, and the results are as shown in Figure S6b. As in the alcohols, the IOFs shrank in the organic solvents because they all have weak polarities. The differences in the maximum reflection wavelengths are mainly from the changes in refractive indexes (Table S3). The responsivity of the chitosan IOFs to ionic strength was also tested using aqueous solutions with different sodium chloride concentrations, and the results are shown in Figure S6c. As the concentration of NaCl in water increased from 0 to 3 mol·L−1, the structural color stayed green, and no obvious changes were observed with the naked eye. When measured with a spectrometer, the maximum reflection wavelength was found to red-shift about 10 nm. The degree of swelling remained fairly constant because IOFs made of chitosan are nonionic in solutions with neutral pH value, which differ from hydrogels containing ionic components.40 Thus, the ionic strength did not affect the intermolecular forces or the swelling degree. The small shift in wavelength can mainly be attributed to the change of the refractive index of the salt solution as shown in Table S4. Recyclability, Stability, and Durability of the IOFs. The IOFs have good physical stabilities and are chemically inert due to the cross-linking. The IOFs that were soaked in alcohol or phenol solutions were easily recovered from those solutions. After the IOFs were removed from the solution, they were soaked in deionized water, dried, and then reused. Figure S7 shows that IOFs could be recovered and reused over 20 times at room temperature. In addition, even after a month, their reflectance spectra did not change much. The IOFs were stable in both the organic solvents and in the inorganic salt solutions. They were not damaged or dissolved by those liquids, which is good for their utilization in a variety of media. The IOFs were stored by first drying them in an oven and then placing them in a desiccator. Even without the use of a

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CONCLUSIONS IOFs with pore sizes of 201−319 nm were synthesized by a colloidal crystal templating method using the biomacromolecule, chitosan, and glutaraldehyde as the cross-linking reagent. The IOFs have stop bands in the visible-light region and exhibit brilliant structural colors. They undergo fast and reversible changes in their structural colors and in the corresponding reflectance peaks in response to alcohols and other organic solvents. These responses are mainly attributed to changes in the degree of swelling and the refractive index of the IOFs. A possible mechanism for the responsivity to phenolic compounds was proposed and proved through pH responsivities tests. The basic chitosan IOF contains amino groups which can capture protons from acidic aqueous solutions, and the amino groups become positively charged. They repel each other, and as a result, the IOF swells. The stop bands of the chitosan IOFs in different liquids exhibited good reproducibility (standard deviations of a few nanometers and no variation in structural color). The IOFs were stable in organic solvents which is advantageous for their applications in a variety of areas. The chitosan IOF could be used to detect chemicals using either their optical signals or a direct visual assessment. They would be useful for evaluating the amount of organics in water solutions, and they may also find potential bioapplications such as the rapid and convenient analysis of acidic target substances over a small pH range. Moreover, they could be applied to areas like environment management as a visual detector for some pollutants.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of the PS templates, stop band of the PS templates, and chitosan-filled templates. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-22-27403475. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of the National Science Foundation of China (21074089). F

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dx.doi.org/10.1021/bm501374t | Biomacromolecules XXXX, XXX, XXX−XXX