A Protective Film Produced by Whey Protein for Photonic ...yicaige.com/upload/news/1533280458.pdfA Protective Film Produced by Whey Protein for Photonic Crystals: Inspired by the Epidermis

J Bionic Eng 15 (2018) 713–721 Journal of Bionic Engineering DOI: https://doi.org/10.1007/s42235-018-0059-z http://www.springer.com/journal/42235

*Corresponding author: Yao Li E-mail: [email protected]

A Protective Film Produced by Whey Protein for Photonic Crystals: Inspired by the Epidermis Structure of Chameleon

Xiaoyi Chen1, Hongbo Xu1, Lei Pan1, Jiupeng Zhao1, Yao Li2*, Ying Song1

1. School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China 2. Center for Composite Materials and Structure, Harbin Institute of Technology, Harbin 150001, China

Abstract

Self-assembly technology of sub-micrometer-sized colloidal particles is the most promising approach for the preparation of large-area Photonic Crystals (PCs). However, PCs obtained by this method are facile to be destroyed by external factors such as friction, impact, and pollutants. The highly keratinized epidermis of chameleon skin acts as a protective role for the dermis with photon cells of the tunable band-gap structure. Inspired by the epidermis structure of chameleon, we use whey protein to develop a sort of protective film on the surface of artificially synthesized PCs. The film possesses positive mechanical properties that make the PCs friction and impact re-sistant. In addition, favorable resistance to water and CO2 could prevent PCs from being destroyed by pollutants. Consequently, PCs with protective film are well preserved when subjected to external factors (such as friction) and the optical properties of the PCs are successfully maintained, that may significantly promote the utilization of PCs in optical devices. Keywords: chameleon, photonic crystal, bioinspired protective film, whey protein Copyright © 2018, Jilin University.

1 Introduction

Artificially synthesized Photonic Crystals (PCs) are periodic elastic materials with diverse refractive indi-ces[1,2] that can control light propagation and effica-ciously inhibit the spontaneous emission of atoms. PCs are extensively employed in the fabrication of broad-band dielectric mirrors, Light-Emitting Diodes (LEDs), photonic crystal antennae, etc.[3–6]. In addition, re-searchers have discovered that the unique dispersion characteristics of PCs could produce various fantastic, interesting, and anomalous refraction effects[7–9], in-cluding negative refraction[10–12], self-collimation[13,14], and a super prism effect[15,16]. Consequently, PCs have considerable critical applications in various fields[17,18].

Several distinct sorts of PC structures from one-dimensional to three-dimensional (3D) have been acquired. To date, self-assembly technology of submicrometer-sized colloidal particles is the most prom-ising approach owing to its simplicity and low cost compared with top-down lithography techniques[19–21]. However, there exist two primary drawbacks which hinder the application of PCs. First, most self-assembly

PCs are formed into a conventional Face-Centered- Cubic (FCC) structure consisting of two phases (sub-microspheres and air). The weak van der Waals forces in the sub-microspheres spark off fragile PC ma-terials that are frequently destroyed by friction or colli-sions. Second, PC applications may be confined by pollutants such as dust particles and so on.

In order to overcome the fore-mentioned limita-tions, various previous attempts have focused on en-hancing the mechanical strength of PCs and protecting them from destruction. Several methods have been re-ported, such as thermal treatment to decrease pore di-mensions of the microspheres[22], coating microspheres lattices with a continuous silica layer to dominate the pore dimensions and increase the mechanical stability[23], fabricating elastic PCs using microspheres with a soft shell to diminish the air space within microspheres[24–26], and preparing hydrophobic PCs to safeguard them from fog or other pollutants[27,28].

We take inspiration from the epidermis structure of chameleons in nature, whose skin consists of epidermis and dermis. The orderly arrangement of the dermis cells forms the perfect natural PC structure. The main

Journal of Bionic Engineering (2018) Vol.15 No.4

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CEpidermis

Dermis

(a) (b) (c)

Fig. 1 (a) Chameleon; (b) chameleon skin structure, including the epidermis and dermis; (c) micrographs of PCs in the dermis tissue and a diagrammatic sketch of PC cells[29].

component of chameleon epidermis is keratin which possesses favorable toughness and rigidity that can prevent damage to the dermis when subjected to an ex-ternal force. Consequently, the highly keratinized outer epidermal layer plays a significant role in guarding the photon cells in the dermis tissue of chameleon. Fur-thermore, the epidermis presents favorable transmittance properties, and the light reflected by PCs in the dermal layer is almost unaffected by the epidermal layer[29,30]. Fig. 1 presents the representative structure of chameleon skin.

We propose the structure of whey protein films similar to the epidermal layer of chameleon skin that may, therefore, prohibit the PCs from destruction in an analogous manner. In this study, a whey protein film is first successfully prepared as a protective layer for 3D self-assembly PCs, which would extremely broaden the application of PC materials. The introduced layer ex-hibits excellent impact and water resistance, maintaining the original optical properties of the PCs. Moreover, whey protein is a new type of biodegradable materials.

2 Materials and methods

2.1 Materials Whey protein powder (the protein content is more

than 93%) was purchased from New Zealand and its composition is given in Table S1. Transglutaminase (serving as a cross-linking agent) was purchased from Ajinomoto Company in Japan. Polystyrene, potassium peroxydisulfate, glycerol (serving as a plasticizer), L-cysteine (serving as a reductant), and CaCl2 were all analytically pure.

2.2 Preparation of mono-dispersed polystyrene (PS)

microspheres and PCs Mono-disperse PS microspheres were produced

through polymerization of styrene initiated by potassium persulfate. Specific steps were as follows: under the

protection of N2, 6 mL of polystyrene and 10 mL of potassium persulfate (1%) were mixed together at 72 ˚C. Mono-disperse PS microspheres was successfully pre-pared after being stirred for 24 h[31]. The diameters of PS microspheres in subsequent experiments were 226 nm, 270 nm, and 498 nm. The PS microspheres were as-sembled into PCs by vertical deposition method on glass substrates at 60 ˚C for 72 h[32].

2.3 Preparation of protective film

Whey protein was blended into deionized water (80 g/1000 mL) with glycerol (40 g/1000 mL) and L-cysteine (0.6 mmol·L−1), maintained for 15 min at 90 ˚C, and then cooled to 50 ˚C. Transglutaminase (35 U·g−1) was added to the solution at pH 7.0 (using 1.0 mol·L−1 NaOH or 1.0 mol·L−1 HCl to adjust). The resulting solution was maintained for 70 min at 50 ˚C, heated to 85 ̊ C for 3 min, and ultimately cooled to 25 ̊ C. The surfaces of PCs were coated with the film-forming solution of whey protein and dried in a drying oven. We defined the 226-nm PCs as PC-226, the 270-nm PCs as PC-270, the 226-nm PCs with the protective film as PCPF-226, and the 270-nm PCs with the protective film as PCPF-270.

2.4 Mechanical properties 2.4.1 Determination of tensile strength

A texture analyzer (Stable Micro Systems from England) was adopted to detect the maximum load of the films which were fastened with the screw. The parame-ters of the instrument settings were as follows: the probe depth was 50 mm, the back and forth speed was 15 mm·s−1[33]. After the probe was started, the film was pressed until it ruptured. The averages of the maximum load were calculated after the peak value being meas-ured and recording the continuous measurement results of three samples.

The film thicknesses were measured in eight dif-ferent positions on the same film using a spiral mi-crometer, and then the averages were calculated. The tensile strength was calculated according to the follow-ing formula:

t ,pb d

σ =×

(1)

where σt is the tensile strength (MPa), p the maximum

Chen et al.: A Protective Film Produced by Whey Protein for Photonic Crystals: Inspired by the Epidermis Structure of Chameleon

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load (N), b the specimen width (mm), and d the speci-men thickness (mm).

2.4.2 Elongation at break

The elongation at break of films cut into strips (2.0 cm × 5.0 cm) was measured by the texture analyzer. The parameter settings were as follows: the initial clip distance 50 mm, the pulling rate 5 mm·s−1, and the ef-fective tensile distance 150 mm[33]. The length of the film when broken was recorded. Then elongation was calculated according to the following formula:

1 0

0

( )100%,

L LE

L−

= × (2)

where E is the elongation at break of the film (%), L1 the length of the film at break (m), and L2 the length of the original film (m).

Three samples selected from each kind of film were measured and the averages were calculated.

2.5 Barrier properties 2.5.1 Water permeability

A cone bottle (the capacity is 50 mL) containing 5 g of anhydrous CaCl2 powders was applied in the ex-periment. The whey protein film was covered the bottle mouth and sealed with melted paraffin. The bottle was placed in a desiccator at 80% relative humidity and weighed every 24 h for 5 d. The water absorption of CaCl2 powders was detected.

2.5.2 CO2 permeability

The Erlenmeyer flask (the volume is 50 mL) con-tained 5 mL of KOH saturated solution was covered by whey protein film and sealed with melted paraffin. The permeability of CO2 was determined with the alkali absorption method and calculated according to the fol-lowing formula:

Q = Δm/d × s. (3)

Parameter Q is CO2 permeability rate of the film [mg·(cm2·d)−1], Δm is the weight of CO2 reacted with KOH (mg).

2.6 Optical performance 2.6.1 Test of transmissivity

The film absorbance was measured at the maxi-

mum absorption wavelength of 600 nm (LAMBDA950 from PerkinElmer). The relationship between the ab-sorbance and transmissivity was as:

A = −lgT, (4)

where A is absorbance of the samples, T is transmissivity of the samples.

2.6.2 Test of reflection spectra

The reflection spectra were measured using an Ocean Optics Maya 2000 Pro spectrometer coupled to a six-around-one reflection/back-scattering probe, where both the incident and reflective angles were fixed at 0˚.

3 Results and discussion

3.1 Flow diagram of forming process of the protec-tive film on PCs As shown in Fig. 2, the whey protein film was

homogeneously covered on the surface of the PCs through casting method. The network structure was produced as a result of the interaction of side-chain radicals of protein. Meanwhile, the protein molecules were completely stretched, forming a long-chain adhe-sive layer by macromolecular cross-linking[34]. With the increase of drying time, a complete protective film was firmly adsorbed on the PC surface.

3.2 Effect of the whey protein film on the optical

properties of PCs As observed from Figs. 3a and 3b that 498 nm PS

microspheres were arranged into a Face-Centered-Cubic (FCC) structure. Figs. 3c and 3d definitely illustrate that the ordered arrangements of PCs after removal of the protective film were maintained (the surfaces of PCs were coated with whey protein films which were re-moved after being treated by steam at 90 ˚C for 2 min). The FCC structure of PCs under the film is fully retained. Figs. 4a and 4b reveal the optical photographs of the PCPF-226 and PCPF-270 samples which presented perfect iridescent colors. The reflection spectra of PC-226, PCPF-226, PC-270, and PCPF-270 are exhib-ited in Figs. 5a and 5b. The reflection spectra were ap-proximately identical, which indicated that the intro-duction of the protective film possessed slightly influ-ence on the PC reflection properties. The photonic band gaps were examined at 548 nm and 639 nm, and the


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Fig. 2 Schematic diagram of forming process of the protective film on PCs. (a) Film-forming solution of whey protein was coated on the surfaces of PCs; (b) a network structure was produced by crosslinking action of whey protein; (c) whey protein was solidified to form a complete PC protective film.

Fig. 3 SEM images of (a,b) PCs assembled from PS microspheres with diameters of 498 nm and (c,d) PCs coated with protective films initially (then removing the films).

(a) (b) Fig. 4 Optical photographs of (a) PCPF-226 sample and (b) PCPF-270 sample.

results were consistent with the values acquired through theoretical calculations (according to the Bragg–Snell law, the photonic band gap of PC-226 is 540 nm and that

PC with protective film

Wavelength (nm)

(a)

PC with protective filmPC

60

80

100

40

20

0600 800

Wavelength (nm)400

PC80

0

40

600 800400

(b) Fig. 5 (a) Reflection spectra of PC-226 and PCPF-226; (b) re-flection spectra of PC-270 and PCPF-270. of PC-270 is 645 nm). The results demonstrate that the ordered arrangements of PCs under the whey protein films were not destroyed and presented superior optical properties. The protective film not only played a pro-tective role but also had no impact on the optical prop-erties of the original PCs. This is analogous to the cha-meleon epidermis in nature, which barely obstructs the ordered structure of photon cells in the dermis.


717

110.00

90.00

70.00

50.00

Elon

gatio

n at

bre

ak (%

)

(a)

4.0 5.0 6.0 7.0 8.0 9.0Concentration of glycerol (g/100 mL)


0.80

0.40

1.60

(b)

Tens

ile st

reng

th (M

Pa)


61.00

56.00

(c)

51.00

46.00

Wat

er p

erm

eabi

lity

(mg·

cm−2

·d)


50.00

48.00

46.00

(d)

44.00

CO

2pe

rmea

bilit

y (m

g·cm

−2·d

)

Fig. 6 Effects of glycerol concentration on the film properties including (a) elongation at break, (b) tensile strength, (c) CO2 permeability, and (d) water permeability.

3.3 Effects of plasticizer (glycerol) on the protective

film properties For the protective film, the evaluating indicator

generally involved the mechanical properties that ap-peared as tensile strength and elongation at break. Film rigidity was indicated by the tensile strength and film toughness was indicated by the elongation at break. Favorable mechanical properties of the protective film could make PCs friction and impact resistant. Water and CO2 permeability were measured to assess the barrier properties of the film. Excellent barrier properties of the film could restrict the PC layer from being destroyed by pollutants, fog, and other factors.

The chameleon epidermis consisted predominantly of keratin molecules that contain numerous sequences of alpha helices and beta folding formed through hydrogen bonds among side-chain radicals of proteins[30]. Con-sequently, chameleon epidermis presented preferable toughness properties. In this study, the small molecule (i.e., glycerol) was added into the film-forming solution so that hydrogen bonds would form in the process of

crosslinking and solidifying of whey proteins to upgrade the elongation at break of the film.

Fig. 6a shows the relationship between the elonga-tion at break and glycerol concentration. The elongation at break markedly amplified owing to the engendering of massive hydrogen bonds through the intervention of glycerol and hydrophilic amino acid residues in the whey protein side-chains. Meanwhile, the rigid interac-tion among polypeptide chains was weakened, which resulted in a significant reduce of tensile strength[35], as revealed in Fig. 6b. When the glycerol concentration was over 6 g/100 mL, the decrease of elongation at break was ascribed to the lessened densification of the film.

Figs. 6c and 6d illustrate that the water and CO2 permeability, respectively; both of which increased with increasing glycerol concentration. The primary factor was the compactness of the film dwindled dramati-cally[36,37]. The results of the statistical analysis are shown in Table S2 (supplementary information). Fig. S1 (supplementary information) demonstrates that the transmissivity of 600 nm was above 90%. By adding


718

Elon

gatio

n at

bre

ak (%

)

Tens

ile st

reng

th (M

Pa)

Wat

er p

erm

eabi

lity

(mg·

cm−2

·d)

CO

2pe

rmea

bilit

y (m

g·cm

−2·d

)

Fig. 7 Effects of L-cysteine concentration on the film properties including (a) elongation at break, (b) tensile strength, (c) water perme-ability, and (d) CO2 permeability.

glycerol as a plasticizer, elongation at break was sig-nificantly upgraded, nevertheless, the barrier properties of the whey protein film for water and CO2 diminished. These results indicate that the variation trends of me-chanical properties and permeability with increasing glycerol concentration were consistent with Fang’s study[38].

3.4 Effects of reductant (L-cysteine) on the proper-

ties of protective film The keratinized chameleon skin epidermis pos-

sesses excellent barrier properties to water and gas ow-ing to the exposure of hydrophobic groups of the pro-teins[39]. Based on using glycerol to modify whey protein, disulfide bonds were deoxidized by L-cysteine for im-proving the mechanical strength and reducing the per-meability of water and CO2. As seen from Fig. 7a and Fig. 7b that the elongation at break decreased with in-creasing L-cysteine concentration, whereas the tensile strength of the film augmented. The permeability of both water and CO2 lessened (Figs. 7c and 7d). L-cysteine breaks the disulfide bonds in the molecules of whey

protein so the molecular weight of polypeptide chain is reduced and internal hydrophobic groups are exposed, making the whey protein facile to create more hydro-phobic interactions. L-cysteine also contains sulfhydryl groups (–SH), which lead to the redistribution of disul-fide bonds. Consequently, considerable intermolecular disulfide bonds and excessively hydrophobic interac-tions were reshaped by sulfhydryl (–SH) and hydro-phobic groups during the subsequent formation process of the film. The network structure was considerably enhanced, which intensified the tensile strength. Meanwhile, L-cysteine also exposed the hydrophobic groups of internal molecules, which boosted the inter-molecular hydrophobic interactions and reduced the film permeability. However, elongation at break dwindled significantly due to extensive cross-linking of the in-termolecular disulfide bonds, causing film rigidity en-hancement[40]. Taking the above factors into account, an L-cysteine concentration (0.6 mmol·L−1) was deter-mined. In this study, the results are summarized in Table S3. The influences of L-cysteine concentration on transmissivity were petty (Fig. S2).


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Fig. 8 Effects of cross-linking time on the film properties including (a) elongation at break, (b) tensile strength, (c) water permeability, and (d) CO2 permeability.

3.5 Effects of cross-linking agent (transglutaminase) on the film properties The epidermis structure of chameleon exhibits im-

pressive mechanical properties owing to the in-tramolecular and intermolecular cross-linking in kera-tin[30,39]. Inspired from this, the properties of whey pro-tein film were reinforced by transglutaminase cross- linking. Both the elongation at break and the tensile strength of the film increased dramatically after cross-linking (Figs. 8a and 8b). Figs. 8c and 8d illustrate that the permeability of water and CO2 diminished. These results indicate that the mechanical properties of the film and barrier properties for water and CO2 were upgraded through the transglutaminase cross-linking. The longer was the reaction time and the more extensive was the cross-linking reaction, the more closely spaced was the network structure of the whey protein. Trans-glutaminase was used to promote the amino interaction and carbonyl groups, which accomplish the cross- linking of intramolecular and intermolecular groups, improving the mechanical properties and resistance to water and CO2 of the films. The elongation at break amplified rapidly at first due to the intramolecular

cross-linking which played a leading role in the initial reaction. Meanwhile, the reaction of intermolecular was weakened, resulting in the decline of tensile strength (as shown in Tables S3 and S4 of supplementary informa-tion). However, the intermolecular cross-linking started to augment with the extension of the reaction time, eventually achieving a balance between intramolecular and intermolecular cross-linking[41,42]. The statistical analysis results can be seen in Table S4 (supplementary information). The film transmissivity exhibited minor change at the maximum absorption wavelength of 600 nm (Fig. S3 in supplementary information). Hard-ness scratch testing of the protective film is shown as a video in electronic supplementary information. The PCs without a protective film were easily destroyed in the 5B hardness; however, the PCs with the protective film were still intact when suffered from a hardness of 5H.

4 Conclusion

A PC protective film inspired by the epidermis structure of chameleon was prepared by using the whey protein material. The acquired film demonstrated ex-cellent barrier properties (for water and CO2) and me-


720

chanical properties that the tensile strength topped 2.2 MPa and the elongation at break exceeded 200%. With the favorable deformability of protein materials, the protective film could be applied to diverse substrates on which PCs were assembled such as glass, polyimide, and polydimethylsiloxane etc. Designing varieties of novel functional PC devices has become a topic research in the field of optics owing to their high stability and rapid propagation velocity. The protective film pos-sesses preferable mechanical properties and excellent barrier properties to pollutants, which will promote the application of PCs in optical devices. Furthermore, flourishing biodegradable materials are crucial for the future of polymer materials because they may partially or completely replace finite petrochemical resources and realize sustainable and environmentally friendly devel-opment.

Acknowledgment

We appreciate National Natural Science Founda-tion of China (Nos. 51572058 and 51502057), National Key Research & Development Program (Nos. 2016YFB0303903 and 2016YFE0201600), the Interna-tional Science & Technology Cooperation Program of China (Nos. 2013DFR10630 and 2015DFE52770), and Foundation of Equipment Development Department (No. 6220914010901).

* All supplementary materials are available at http://www.springer.com/journal/42235.

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