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Dyeing and Functionalization of Wearable Silk Fibroin / Cellulose Composite by Nano Colloidal Array Dan Yan, Li-li Qiu, Kenneth J. Shea, Zi-hui Meng, and Min Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11576 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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Dyeing and Functionalization of Wearable Silk Fibroin / Cellulose Composite by Nano Colloidal Array Dan Yan, † Lili Qiu,*, †Kenneth J. Shea, ‡ Zihui Meng,*, †Min Xue†

†School

of Chemistry and Chemical Engineering, Beijing Institute of Technology,

Beijing, 102488, China

‡Department

of Chemistry, University of California, Irvine, 92697-2025, USA

KEYWORDS: silk fibroin, photonic crystal, wearable device, humidity detection, organic solvent

ABSTRACT: A wearable silk fibroin (SF) / cellulose composite is reported. It is structural dyed and functionalized by embedding three dimensional (3D) or two dimensional (2D)

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poly (methyl methacrylate) (PMMA) and polystyrene (PS) nano colloidal arrays to form opal and inverse opal silk methylcellulose photonic crystal films (SMPCF). The brilliant color of SMPCF is utilized for naked eye detection of humidity and trace amount (0.02%) of H2O content in organic solvents. Five types of VOCs (volatile organic compounds) gases were detected. By alternately exposed to organic solvents of methanol, acetonitrile, acetone, ethanol, isopropanol, n-butanol, carbon tetrachloride and toluene, 3D inverse opal SMPCFs displayed an excellent sensing performance with instantaneously color changes from green to red. The organic solvents sensitive SMPCF is wearable by integrated into a rubber glove. This composite has the potential to be used in wearable real-time sensing materials.

INTRODUCTION

Silk is a natural animal fiber, and its main component is silk fibroin (SF), which accounts for 70%-80% of the total fiber mass. Compared with traditional synthetic materials, SF shows excellent biocompatibility, degradation performance, wide sources, and remarkable mechanical properties, 1,2 which makes it wearable and flexible. Various

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methods have been used to convert SF solutions into different functional materials, such as 3D photonic crystals, 3,4 1D and 2D diffraction gratings, 5 wave guide, 6 and laser optics, 7,8 which further diversify the potential biological and photonic applications of SF biophotonic devices. During our experiments, it is found that the pure SF film is very brittle, and it is difficult to support the photonic crystal array and the inverse opal structure to obtain a complete film. The application of pure SF as a wearable flexible material is limited due to its brittleness and low tensile strength. Therefore, it is often necessary to composite with other materials to improve its properties and broaden the application range.

Cellulose is an ideal matrix for photonic crystal (PhC) sensors. 9,10 It has inherent biodegradability and biocompatibility. 11-13 Natural cellulose is renewable, non-toxic, environmental friendly and more resistant than other polymers. Combining SF with cellulose not only achieves good biocompatibility, but also improves its mechanical strength. Furthermore, silk fibroin has outstanding advantages for optical applications due to its transparency, low surface roughness, nano-scale processability and

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mechanical durability. 14-17 Therefore, it has the potential to be modified for desired optical properties.

Conventional chemical dyeing techniques have many defects such as toxicity, carcinogenicity and fading. 18,19 In recent years, artificial structural coloring materials have attracted wide attentions because they do not require chemical dyes, have bright colors, good color retention, and meet the requirements of sustainable development. 2022

The PhC is a typical artificial structure colored material whose structural colors were

generated by light tuned by nanoscale periodic structures 23 which could be subdivided into opal (O-) and inverse opal (I-) structures. 24,25 These periodic structures can suppress the propagation of the iso-optical frequency and exhibit a bright diffractive color that covers the entire visible spectrum according to the Bragg diffraction law. 26,27 The properties of PhCs depend highly on their spatial structure and the change of the effective refractive index (ERI). 28 It has been used in a variety of stimulating response sensing devices. 29 Combined with its cost efficiency and simple synthetic method,

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PhCs have become more popular in optical switching devices, 30,31 optical sensors 32,33 and display devices. 34,35

Herein, we proposed a wearable and flexible material by combining SF with cellulose, functionalizing and dyeing it by embedding a 3D or 2D PMMA or PS colloidal array in the silk methylcellulose (SMC) (Scheme 1). The application of this material was further explored. It is used to respond to humidity and VOCs. The brilliant color of SMPCF is also utilized for the naked eye detection of different organic solvents and trace amount of H2O in organic solvents. A wearable optical sensor is constructed by fabrication of a piece of SMPCF into a rubber glove. Due to its outstanding biocompatibility, 36,37 salient mechanical strength and wide application range, the SMPCF has a promising potential to be used as a wearable optical sensor in vivo.

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Scheme 1. Schematic illustration of the fabrication protocol to SMPCF.

RESULTS AND DISCUSSION

Characterization of 2D and 3D O-SMPCF and I-SMPCF. The SEM images of the PMMA arrays, 3D O-SMPCF and 3D I-SMPCF prepared with 255 nm PMMA microspheres were displayed in Figure 1. As shown in Figure 1a, the colloidal particles with 255 nm diameter were self-assembled into 3D closely packed face-centred cubic (FCC) structure. These periodic structures were kept well after been embedded inside the silk

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methylcellulose matrix (Figure 1b). In Figure 1c, a well-ordered and air-containing porous structure was obtained after etched by toluene. The diameter of these pores was 235 nm. After the removal of PMMA array, the diameter of the pores was about 20 nm smaller due to the shrinkage of SMC. Hexagonally close-packed 2D array, opal and inverse opal SMPCFs can be observed in Figure 1e and f. As shown in Figure 1d and e, the 2D close-packed single layer formed by 600 nm PS particles was successfully prepared. This 2D array was not destroyed during the polymerization processes of SMC matrix. After removing the colloidal template, the well-ordered monolayer periodic porous structure was formed as shown in Figure 1f. The diameter of the pores is about 240 nm. It was about 360 nm smaller than that of the corresponding template PS spheres. The colors of 3D SMPCFs were recorded by a digital camera at a fixed angle. The results were showed in Figure 1g and h. The color of the SMPCF changed significantly before and after the colloidal removal. The flexibility of the film can be observed from the Figures. The addition of cellulose effectively improved the mechanical strength and flexibility of silk fibroin, as a result, its wearability has a prominent improvement. It could be bent and twined more than 150 times without any

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cracks observed on the surface. During bending or twining, the structure color of the SMPCFs changed due to its angular dependence.

Figure 1. SEM images of a) 3D PMMA arrays b) 3D O-SMPCF c) 3D I-SMPCF d) 2D PS arrays e) 2D O-SMPCF f) the 2D I-SMPCF, photographs of the g) 3D O-SMPCF h)

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3D I-SMPCF, i) diffraction wavelength responses of the 3D opal and inverse opal SMPCFs (The insets are the structural colors observed for 3D O-SMPCF and 3D ISMPCF under normal incident light, respectively).

The maximum diffraction wavelength (λmax) of the O-SMPCF and I-SMPCF follows the Bragg’s law equation (1), 38

1

max  1.633  d / m  D / D0   neff2  sin 2   2

(1)

where d is the planar spacing; m is the order of Bragg diffraction, here m=1; neff is the effective refractive index (ERI) of the PhC at given conditions; D/D0=1 is the swelling ratio of the PhC; θ is the angle of the incident light and the normal of the crystal surface. As for neff, it could be calculated by equation (2).

1

neff   n12 f  n22 1  f   2

(2)

We assumed a close-packed array with a volume fraction, f, of 0.74. For the OSMPCF, neff is the ERI of PMMA particles (n1, 1.48) and SMC (n2, 1.71); while for the ISMPCF, neff is the ERI of air (n1’, 1.00) and SMC (n2’, 1.71). The λmax of SMPCFs could

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be observed in the visible spectral region and SMPCFs present different structural colors. After removing the PhC array, the λmax was blue-shifted, and the structural color changed from red to blue (Figure 1i). The λmax of the O-SMPCF is 647 nm and the λmax of the I-SMPCF is 469 nm. The theoretical values of O-SMPCF and I-SMPCF calculated by equation (1) were 642 nm and 470 nm. The band diagrams calculated by plane-wave expansion (by Rsoft Photonics CAD Suite) of 3D O-SMPCF and 3D I-SMPCF were shown in Figure 2. The λmax of O-SMPCF and I-SMPCF calculated by plane-wave expansion were 641nm and 461nm. The theoretical calculations are consistent with the experimental results. The band gaps are 17nm and 52nm, respectively. After removing the PhC template, the band gap of I-SMPCF is increased by 35 nm.

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Figure 2. Band diagram calculated by plane-wave expansion of a) 3D O-SMPCF b) 3D I-SMPCF.

Response of 3D I-SMPCF to humidity. Due to the sensitivity of the material to humidity, it can optically indicate changes in humidity after been dyed. Humidity was detected using the devices shown in Figure 3a. In Figure 3b, we could see that the λmax of the 3D I-SMPCF was blue-shifted with the increase of humidity, and the intensity of the λmax gradually decreased. The structural color gradually faded and eventually became colorless. As the humidity increased, the interaction between the water molecules and the polar protein chains of the silk fibroin leaded to a conformational change of the silk fibroin from random coiling to the β-sheet structure, 46 as a result, the 3D I-SMPCF shrank, the particle spacing reduced and the λmax blue-shifted. This structural change is irreversible.

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Figure 3. a) Device diagram of humidity detection b) λmax responses of 3D I-SMPCF to humidity.

Response of 3D I-SMPCF to trace amount of water. Trace amount of water is usually the most concerned impurities in organic solvents. Many reactants or catalysts of organic reactions are sensitive to water, such as Friedel-Crafts reaction, 39 Grignard reaction 40 and so on. The presence of water deactivates the reactants and poisons the catalyst, which seriously affects the reaction. Therefore, the control of water content in organic solvents is of crucial significance in numerious applications such as drug synthesis, food processing, biopharmaceuticals and environmental monitoring. 41,42 A series of detection methods are developed from the classical Karl-Fischer titration to gas chromatography and small molecule fluorescent probe methods. 43 There are still

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certain deficiencies such as the use of reagents with high toxicity, cumbersome operation and time-consuming, expensive reagents, etc. Since the 3D I-SMPCF is sensitive to H2O, its periodic structure can be by the H2O and the structural color can change correspondingly. Therefore, the H2O content in the organic solvent could be detected by the 3D I-SMPCF. As shown in Figure 4, the 3D I-SMPCFs were immersed in acetone with 1%, 0.2% and 0.02% volume of H2O, after removed from the solution, the structural colors changed. With the increase of water content, the interaction between silk fibroin and water leaded to the change of protein configuration, the microstructure of the 3D I-SMPCF was destroyed to different degrees, and the structural color appeared faded accordingly. By this method, the trace water in the organic solvent can be quantitatively detected by the naked eye, which detection limit is as low as 0.02%. And the method is non-toxic, simple, convenient and low-cost.

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Figure 4. The structural colors of the 3D I-SMPCF before and after immersed into the acetone which water contents were a) 1% b) 0.2% and c) 0.02% respectively (The response time is 1 min).

Detection of VOC vapor by 3D SMPCF. Volatile organic solid or liquid under normal temperature and pressure is called VOCs. When the concentration of VOCs in indoor air is too high, it is easy to cause acute poisoning, such as headache, dizziness and other symptoms. People may suffer from liver poisoning or even coma soon, and some may be life-threatening. Living in a room contaminated with VOCs for a long time can cause chronic poisoning, liver and nervous system damage, and cause leukemia. 44, 45

As shown in the Figure 5, the SMPCF can be used to detect VOC gas. When contacting with different content of VOC gas, the position and intensity of the λmax will change to different degrees, thereby indicating the VOC gas content in the air. Figure 5a shows the response of acetone gas detected by 3D O-SMPCF. It can detect acetone gas with a volume ratio as low as 0.084%. As the volume ratio increased, the

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concentration of acetone adsorbed on the PMMA sphere increased, and its ERI increased, as a result, λmax red-shifted and the intensity of the λmax decreased due to the influence of the surface gas.

The 3D I-SMPCF was used to detect methanol, formaldehyde, acetonitrile and ethanol. The results were shown in Figure 5d-o. The λmax of methanol, formaldehyde and acetonitrile were red-shifted first and blue-shifted as the volume ratio increased to 0.998%, 0.400% and 2.022%, respectively (Figure 5e, h and k). In the initial stage, the gas entered the air hole of the 3D I-SMPCF, so its ERI was increased due to the mixing of VOC gas and air, as a result, the λmax red-shifts. As the volume ratio of the gas increased, the concentration of gas adsorbed on the surface of the 3D I-SMPCF increased accordingly. The interaction of the silk fibroin network with the VOC gas caused the 3D I-SMPCF to shrink, resulting in a decrease of lattice spacing. When it accounts for the main factor, the λmax blue-shifted. Since the interaction between ethanol and silk fibroin network was weak, the change of lattice spacing was not obvious. Therefore, its main factor was the change of the ERI after the air and the gas

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mixed, which caused the red shift of the λmax. From the above, acetonitrile has the strongest interaction with the silk protein network, and the ethanol has the weakest interaction. As the gas concentration on the surface of the SMPCF increased, the energy dissipation increased when the light passed through, resulting in the decrease of the intensity of the λmax.

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Figure 5. The responses of the 3D SMPCF to VOC gas a) λmax responses of 3D OSMPCF to acetone gas b) the relationship between the wavelength of λmax and volume ratio c) the relationship between the intensity of λmax and volume ratio of the acetone gas d)-f) λmax responses of 3D I-SMPCF to methanol gas g)-i) λmax responses of 3D ISMPCF to formaldehyde gas j)-l) λmax responses of 3D I-SMPCF to acetonitrile gas m)o) λmax responses of 3D I-SMPCF to ethanol gas.

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Visual detection of organic solvents. As the 3D I-SMPCF was immersed into different organic solvents, the organic solvent permeated into the internal structure of 3D ISMPCF and filled the air holes of the inverse opal. According to the Bragg diffraction, the band gap of the 3D I-SMPCF changed correspondingly, therefore causing the changes of structural color. Along with this principle and the good stability of 3D ISMPCF in the organic solvents, it was selected for naked eye detection of different organic solvents. To inspect the feasibility, the prepared 3D I-SMPCFs were immersed respectively into methanol, acetonitrile, acetone, ethanol, isopropanol, n-butanol, carbon tetrachloride and toluene, the ERIs of which were 1.3172, 1.3416, 1.3588, 1.3624, 1.3752, 1.3993, 1.4486, 1.4961, respectively. The structural colors of 3D I-SMPCFs were single and the specific surface area of inverse opal structures were high, therefore, the adsorption mass transfer rate of the 3D I-SMPCFs was quick. As shown in insets of Figure 6, the I-SMPCFs immersed in different organic solvents showed different structural colors and underwent a distinctly red-shift with the increase of ERI. Figure 7 shows the comparison of peak positions corresponding to different organic

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solvents. As the structural color of the I-SMPCFs changed from green to red, its corresponding reflection peak position was red shifted from 595nm to 646nm. The color changes can be finely tuned through dynamic modulation of ERI.

The λmax of 3D I-SMPCF in different organic solvents were also shown in Figure 6. As the increasing of the ERIs, the λmax red-shifted gradually. Due to the influence of the liquid on the surface of the film, the intensity of the λmax decreased. As the ERI of the organic solvent was close to the ERI of the film, the band gap of 3D I-SMPCF became narrowed, the intensity of the λmax decreased and the structural color became inconspicuous. The intensity of the λmax in the organic solvent was simultaneously related to the surface of the liquid on the 3D I-SMPCF and its ERI.

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Figure 6. The λmax responses of 3D I-SMPCF to a) methanol b) acetonitrile c) acetone d) ethanol e) isopropanol f) n-butanol g) carbon tetrachloride and h) toluene (B represents the 3D I-SMPCF before organic solvent infiltration). The insets are the structural colors for organic solvents above under normal incident light, respectively.

Figure 7. The comparison of peak positions corresponding to different organic solvents.

Wearable performance of the SMPCF. The SMPCF can be integrated into rubber gloves and fabrics as a wearable sensor due to its outstanding biocompatibility, flexibility, nontoxicity and portability. Compared with other rigid sensing materials, this material has obvious wearable advantages. SMPCF endows rubber gloves and fabrics

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sensing functions such as humidity monitoring, identification of organic solvents and detection of trace water, without deteriorating wearing comfort and their original functions. This design allows above tests to be performed directly during the experimental procedure without additional testing steps, which simplifies the operation and enables real-time monitoring. As shown in Figure 8, the 3D I-SMPCF was integrated into rubber gloves and fabric. The color changes occurred when the material was exposed to the organic solvent, indicating the types of organic solvent (Figure 7b and c). Due to the outstanding flexibility, wear resistance and mechanical strength of the SMPCF, it could be used repeatedly. Because of its excellent wearability, the SMPCF is of great significance for the application of wearable optical sensing device.

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Figure 8. Splicing diagram of the rubber gloves and fabric (top panel), the color responses of the stitched gloves to a) air b) methanol c) acetonitrile.

CONCLUSIONS

In summary, a wearable flexible material by composite SF with cellulose was proposed and was functionally dyed by embedding 3D or 2D PMMA or PS nano colloidal array in a SMPCF to obtain the optical performance. The SMPCFs have high stability, flexibility, and the structural color and reflection peak can change accordingly under different humidity conditions. It also can be used for naked eye detection of organic solvents and

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trace amount of water content in organic solvents (The detection limit is as low as 0.02%). The SMPCF has responses to VOC gas, so it has the potential to be used for monitoring VOCs. It has excellent wearability and color indicating performance after integrated with rubber gloves. Due to these obviously wearability, optical signals and efficient construction process, the SMPCFs have high promising to be used as a biowearable flexible optical sensing device.

ASSOCIATED CONTENT

Supporting Information

Experimental materials, preparation of colloidal particles and array, silk fibroin solution, dyeing SF and photographs of 3D I-SMPCF in different organic solvents (PDF)

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected] (L. L. Qiu).

* E-mail: [email protected] (Z. H. Meng).

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Present Addresses School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 102488, China

Funding Sources This work was financially supported by the National Natural Science Foundation of China (U1530141, 21874009 and 21804009).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (U1530141, 21874009 and 21804009).

ABBREVIATIONS

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SF, silk fibroin; 3D, three dimensional; 2D, two dimensional; SMPCF, silk methylcellulose photonic crystal film; PhC, photonic crystal; ERI, effective refractive index; FCC, face-centred cubic.

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(3) Kim, S.; Mitropoulos, A. N.; Spitzberg, J. D.; Tao, H.; Kaplan, D. L.; Omenetto, F. G. Silk Inverse Opals. Nat. Photonics. 2012, 6, 818-823.

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SYNOPSIS

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Scheme 1. Schematic illustration of the fabrication protocol to SMPCF.

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Figure 1. SEM images of a) 3D PMMA arrays b) 3D O-SMPCF c) 3D I-SMPCF d) 2D PS arrays e) 2D O-SMPCF f) the 2D I-SMPCF, photographs of the g) 3D O-SMPCF h) 3D I-SMPCF, i) diffraction wavelength responses of the 3D opal and inverse opal SMPCFs (The insets are the structural colors observed for 3D O-SMPCF and 3D I-SMPCF under normal incident light, respectively).

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Figure 2. Band diagram calculated by plane-wave expansion of a) 3D O-SMPCF b) 3D I-SMPCF.

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Figure 3. a) Device diagram of humidity detection b) λmax responses of 3D I-SMPCF to humidity.

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Figure 4. The structural colors of the 3D I-SMPCF before and after immersed into the acetone which water contents were a) 1% b) 0.2% and c) 0.02% respectively (The response time is 1 min).

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Figure 5. The responses of the 3D SMPCF to VOC gas a) λmax responses of 3D O-SMPCF to acetone gas b) the relationship between the wavelength of λmax and volume ratio c) the relationship between the intensity of λmax and volume ratio of the acetone gas d)-f) λmax responses of 3D I-SMPCF to methanol gas g)-i) λmax responses of 3D I-SMPCF to formaldehyde gas j)-l) λmax responses of 3D I-SMPCF to acetonitrile gas m)-o) λmax responses of 3D I-SMPCF to ethanol gas.

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Figure 6. The λmax responses of 3D I-SMPCF to a) methanol b) acetonitrile c) acetone d) ethanol e) isopropanol f) n-butanol g) carbon tetrachloride and h) toluene (B represents the 3D I-SMPCF before organic solvent infiltration). The insets are the structural colors for organic solvents above under normal incident light, respectively.

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Figure 7. The comparison of peak positions corresponding to different organic solvents.

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Figure 8. Splicing diagram of the rubber gloves and fabric (top panel), the color responses of the stitched gloves to a) air b) methanol c) acetonitrile.

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