Nature-Inspired One-Step Green Procedure for Enhancing the

Jul 4, 2016 - The final goal of this study was to develop antimicrobial food-contact materials based on a natural phenolic compound (tannic acid) and ...
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Nature inspired one step green procedure for enhancing the antibacterial and antioxidant behavior of chitin film: controlled interfacial assembly of tannic acid onto chitin film Yuntao Wang, Bin Li, and Jing Li J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b01859 • Publication Date (Web): 04 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016

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Journal of Agricultural and Food Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Journal of Agricultural and Food Chemistry

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Nature inspired one step green procedure for enhancing the

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antibacterial and antioxidant behavior of chitin film: controlled

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interfacial assembly of tannic acid onto chitin film

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Yuntao Wangac, Jing Liac, Bin Li abc*

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a

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430070, Hubei, China

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b

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of Technology, Wuhan 430068, China

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c

College of Food Science and Technology, Huazhong Agricultural University, Wuhan

Hubei Collaborative Innovation Centre for Industrial Fermentation, Hubei University

Key Laboratory of Environment Correlative Dietology (Huazhong Agricultural

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University), Ministry of Education, China

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Corresponding Author:

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*E-mail: [email protected]

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ABSTRACT

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The final goal of this study was to develop antimicrobial food-contact materials

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based on a natural phenolic compound (tannic acid) and chitin, which is the second

25

most abundant polysaccharide on earth, using interfacial assembly approach. Chitin

26

film has poor antibacterial and antioxidant ability, which limits its application in

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industrial fields such as active packaging. So in this study, a novel one step green

28

procedure was applied to introduce antibacterial and antioxidant properties to chitin

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film simultaneously by incorporation of tannic acid into the chitin film through

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interfacial assembly. The antibacterial and antioxidant behavior of chitin film has

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been greatly enhanced. Hydrogen bonds and hydrophobic interaction were found to be

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the main driving forces for the interfacial assembly.

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assembly of tannic acid onto chitin film demonstrated a good way for developing

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functional materials that can be potentially applied in industry.

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KEYWORDS:

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antioxidant, one step green procedure

chitin,

tannic

acid,

Therefore, controlled interfacial

interfacial

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assembly,

antibacterial,

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INTRODUCTION

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In recent years, developing polymer materials with antimicrobial and

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antioxidant function has fuelled huge interests of scientists due to their wide range of

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potential applications1-6. It is well known that antimicrobial agents may reduce or

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inhibit microbial contaminations for food, which is one of the major concerns for

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human health6. On the other hand, in industrial applications such as food packaging

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field, materials with antioxidant function can reduce the excessive levels of free

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radicals and extend the shelf life of the food products by preventing oxidation of food

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components7. So it is of great significance to fabricate polymer materials with both

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antibacterial and antioxidant function simultaneously.

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Combination of polymer with inorganic nanoparticles such as zinc oxide8,

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silver nanoparticles9,

10

, gold nanoparticles11 and copper nanoparticles12, graft of

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antimicrobial groups or antimicrobial substances on polymer13, 14, and incorporation

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of antibacterial agents15 such as 1,3,5-triazinederivatives to polymer16 are commonly

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used methods to improve the antibacterial performance of polymer materials.

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However, these approaches usually involve complicated procedures and/or toxic

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chemical agents to immobilize antimicrobials or toxic chemicals including heavy

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metals, which might raise food safety concerns. Therefore, it is in high demand to

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develop highly effective antibacterial materials with antioxidant function from natural

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and renewable resources via a green processing procedure17, 18.

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Tannic acid (TA) is a water-soluble polyphenol that is commonly found in

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many plants and is generally considered to be safe in the field of food science19. TA is

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also known for its antibacterial and antioxidant activities5. These properties suggested

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a potential utilization of TA in developing active packaging materials2, 20. Several

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studies demonstrated the potential of tannic acid as an antimicrobial agent against

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foodborne pathogens such as Staphylococcus aureus and Escherichia coli,

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representing Gram-positive and Gram-negative strains, respectively19, 21. However,

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research on antibacterial and antioxidant composite materials containing tannic acid

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have been rarely reported.

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Chitin is the second most abundant polysaccharide on earth which is usually

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derived from the shells of crabs or shrimps. But it is less developed in the past

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because of the difficultly in dissolving chitin22. The discovery of new solvent has

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brought opportunities for the development of chitin-derived new materials recently23,

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24

. Our previous research has developed several functional materials from chitin23, 25,

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26

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improved functionalities including better chemical stability and excellent mechanical

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properties24, 25, indicating their great potential applications in different fields. So it is

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of great significance to functionalize chitin films to broaden its applications.

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Moreover, regenerated chitin films from the above solvent have porous structure,

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which is conducive to interfacial modification24,

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molybdate nanoparticles in the porous chitin matix to enhance the antibacterial

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activity of the chitin matix9. But the possible toxic effect of silver nanoparticles to

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human health may limit their food related applications, suggesting a need of “green”

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alternative active agents such as tannic acid.

, and found out that regenerated chitin film prepared from NaOH/urea exhibited

25

. Tang immobilized silver

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Plant polyphenol is tightly bound with polysaccharides such as cellulose and

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pectin in the cell walls of plant, which defense plants against various kinds of

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pathogens27. Inspired by this phenomenon, we developed one step environmental

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friendly approach for preparing chitin based composite films with antibacterial and

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antioxidant activity via controllable interfacial assembly of tannic acid onto chitin

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film. The film was also examined for its structure and physicochemical properties. In

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addition, the mechanism for the interfacial assembling tannic acid into the chitin film

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was investigated. The information obtained from the present study could be applied to

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fabricate other functional chitin-derived materials.

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MATERIALS AND METHODS Materials Chitin was purchased from Golden-Shell Biochemical Co. Ltd (Zhejiang, China).

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All other chemical regents were purchased from Aladdin Chemical Reagent Co., Ltd.

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(Shanghai, China).

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Preparation of Chitin/tannic acid Films

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Chitin films (RC) was prepared according to our previous method and the

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prepared films were stored in 20% (v/v) alcohol under 4 °C for further use25. Tannic

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acid solutions of different concentration were obtained by dissolving 20 g tannic acid

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in 1L distilled water first and then proper dilution to desired concentration. For

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investigation of tannic acid assembly under different pH, the pH of initial tannic acid

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solution was adjusted to desired value with sodium hydroxide and hydrochloric acid.

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A piece of chitin film (1.5 cm×1.5 cm) was added into a stoppered flask containing 20

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mL tannic acid solution and the flask was then shaken for 3 h at a speed of 150 rpm

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under 30 °C. Then the film was taken, the absorbance of tannic acid was determined

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by UV spectrophotometer at wavelength of 270nm and the concentration was

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calculated according to the standard curve of tannic acid. The adsorption capacity

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(AC) was calculated according to the following formula: AC = (madd -mleft)/mc, where

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madd means the initial mass of tannic acid, mleft means the mass of tannic acid left after

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adsorption in the solution and mc means the dry mass of chitin film. All the

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experiments were conducted three times.

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Characterization

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Fourier transform infrared (FT-IR) spectra of RC and RC-TA were recorded with

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a FT-IR spectrometer (FT-IR 615, Japan). The optical transmittance of chitin films

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with thickness of 80 µm was determined on a UV-visible spectroscopy (UV-1750)

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from 300 to 800 nm. The mechanical properties of chitin films were assessed with a

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tensile tester (CMT 6503, Shenzhen SANS Test machine Co. Ltd., China) based on

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previous research28. The surface morphology of chitin films was characterized with

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scanning electron microscopy (SEM) (Hitachi S4800, Japan).The water vapor

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transmission rate (WVTR) of chitin films was determined based on the method in

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previous research28.

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In Vitro Antibacterial Activity Assay

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The antibacterial activity of the chitin films was studied with the inhibition zone

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method5. Gram-negative Escherichia coli (E. coli cicc 10032) and Gram-positive

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Staphylococcus aureus (S.aureus cicc 10201) obtained from department of food

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microbiology, Huazhong Agricultural University was chosen as the representative

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bacterial. Initially they were stored in glycerol (30 wt%) under -70 °C, then they were

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activated in nutrient broth medium at 37 °C for 24 h before usage and the

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concentration is about 5.0–10.0 × 1010 CFU/mL. Then the bacterial was diluted with

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physiological saline to obtain bacterial suspension with concentration around 5.0–10.0

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× 105 CFU/mL. Round disks with a diameter of 6 mm were obtained from chitin films.

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The original chitin film was used as the negative control. One hundred micro-liters of

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the above diluted bacterial suspension was pipette onto sterilized nutrient agar plate

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and coated uniformly. Then the obtained disks after sterilization with an ultraviolet

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radiation lamp for 30 min were closely placed on the surface of nutrient agar plate to

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contact the bacteria. After incubation at 37 °C for 12 h, the diameter of the inhibition

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zones were determined by a micrometer. All the experiments were conducted for

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three times.

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Antioxidant Capacity

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The antioxidant capacity of the films was measured based on the reported

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method29 with slight modification.

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DPPH• free radical scavenging

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A piece of round chitin or chitin/tannic acid film (6 mm diameter) was added to

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5 mL of DPPH• radical solution (0.2 g/L) and the mixture was shaken for 3 h in the

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dark. Then the absorbance of DPPH• radical solution was determined with a

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UV-visible spectroscopy (UV-1750) at 517 nm and the antioxidant capacity was

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calculated based on the following equation:

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DPPH• scavenging ratio=[(ABSC-ABSS)]/ ABSC·100%

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where ABSC means the initial absorbance of the DPPH• radical solution and ABSS

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means the absorbance of the DPPH• radical solution after shaking with the sample for

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3h.

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ABTS•﹢free radical scavenging

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ABTS•﹢free radical scavenging ratio was determined with the method similar

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to that of the above DPPH• radical except that the absorbance was determined at 734

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nm.

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RESULTS AND DISCUSSION

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Fabrication and Characterization of Chitin-Tannic Acid Films

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Chitin film was fabricated via a green way in previous works with the advent of 24 ,25

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new solvent

. Additionally, it was found that the obtained chitin film had

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homogeneous structure, high tensile strength, good thermal stability, and perfect gas

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barrier properties 24 ,25. So functionalization of chitin film to broaden its application is

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of great significance. In this study, chitin film was modified through interfacial

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assembly of tannic acid to broaden its application.

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Optical images of the chitin-tannic acid films

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Optical images of the chitin-tannic acid films were shown in Figure S1.In this

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experiment, chitin film was immersed in tannic acid solution with different

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concentration and shaken for 3 h under 30 ℃. As shown in Figure S1, the transparent

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chitin film gradually become much darker with increasing the initial concentration of

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tannic acid solution, indicating that more tannic acid was assembled onto the chitin

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film.

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Transmittance of chitin-tannic acid films

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Transmittance affects the application of polymer films. So the transmittance

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values were determined by UV−visible spectroscopy to study the assembly of tannic

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acid on the light barrier properties of the chitin films. It can be seen from Figure1a

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that the transmittance gradually decreased with increasing content of tannic acid in the

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chitin films. Moreover, all the RC-TA films especially RC-TA20 showed a low

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transmittance value in the wavelength range of 200-400 nm. These results

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demonstrated that the assembly of TA enhanced the UV/Visible light barrier

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properties of the chitin films, resulting in good barrier for radiation in the UV range.

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For packaging application such as in the field of food, a film with high UV barrier

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property can prevent the oxidation of lipid, which can then prolong the shelf life of

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food30. It has also been reported that gelatin films containing polyphenol possessed

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good barrier properties for ultraviolet and visible light in the range of 200-800 nm 31,

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Mechanical property of chitin-tannic acid films

.

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Mechanical property is another important character of polymer films. In the

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present study the chitin films were evaluated for their mechanical properties using a

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tensile tester. Because the chitin films after the drying process were very brittle, in

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this experiment, the chitin films were first plasticized by immersion in 6

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wt% glycerol for 12 h before air drying for test. Figure1b showed the stress-strain

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curves of RC and RC-TA films. The tensile strength (TS) of chitin film was 9.67 MPa,

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while for the tannic acid incorporated chitin film, it obviously increased with

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increasing tannic acid content. The TS of RC-TA20 was 13.24 MPa, which increased

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by 36.9 % compared with that of RC. Therefore, assembly of tannic acid led to

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strengthen of chitin film. The result is consistent with those for the gelatin-polyphenol

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films30,

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mechanical strength of the gelatin films through stronger hydrophobic interaction and

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additional hydrogen bonds.

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tannic acid increased the tensile strength (TS) of chitin films while did not change the

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elongation at break (EAB) of chitin films significantly.

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SEM images of chitin-tannic acid films

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, where the interactions between protein and polyphenol enhanced the

So these results demonstrated that the incorporation of

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SEM images were taken to research the effect that the assembly of tannic acid

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exerted on the microstructure of chitin films. It can be seen from Figure 2 that the

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tannic acid incorporated chitin films exhibited much more compact structures than the

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chitin film. This effect was more pronounced as more tannic acid was incorporated

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into the film, which was probably caused by the distribution of tannic acid in the

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polymeric network. But on the whole, there was no obvious effect on the pore

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structure of the chitin films.

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FTIR of chitin-tannic acid films

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The successful assembly of tannic acid onto chitin film was further confirmed

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from FTIR measurements. As shown in Figure 3a. The band located at 3500–3100

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cm-1 was assigned to the O–H stretching and N-H stretching in chitin molecules23.

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This band became much broader compared to that of chitin film due to the

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incorporation of tannic acid. The remarkable bands near 2900 cm-1 could be from C-H

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stretch vibration33, which shifted to higher wavenumbers because of the composite

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film formation. For RC-TA5, the increasing intensity of the peak at 1716 cm-1 can be

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explained by the vibration of carbonyl ester group in TA5. The peak at 1448 cm-1 is

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attributed to the in plane bend of C–O–H in hydroxyl group of TA34. The band at

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1203 cm-1 and 1323 cm-1 were ascribed to C–O stretch of polyols and C–O stretch of

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the acid group in TA, respectively. The peak at 758 cm-1 is associated with the out

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plane bend of C–H in phenyl group5, 34.

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Water Vapor Transmission Rate (WVTR) of chitin-tannic acid films

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WVTR affects the application performance of polymer films. For food

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packaging application, to prevent moisture transfer between food and the surrounding

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environment is a primary function of the packaging film 35, and a lower WVTR is

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needed for the film. The WVTR of chitin films incorporated with different amount of

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TA were determined at 30 ℃ under different humidities. As shown in Figure3b, the

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WVTR decreased from 8.66 g/h/m2 to 3.66 g/h/m2 under RH = 31% and from 35.29

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g/h/m2 to 25.03 g/h/m2 under RH = 75% with increasing content of TA from 0 (RC) to

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50.4% (RC-TA20). The 57.7% reduction in WVTR under RH = 31% indicated that

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incorporation of TA into chitin films improved hydrophobicity or reduced water

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vapor permeability of the resulting films. Presumably, TA assembled into chitin film

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cross-linked with chitin through hydrogen bond or hydrophobic interaction. The

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larger amount of TA might be associated with an increased density of the RC-TA film

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and a decreased free volume. Also, hydrophilic bonding between chitin film and water

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might be reduced because assembly of TA may limit the availability of hydrogen

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groups of chitin, which further lead to a decrease in the affinity of chitin film towards

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water. Similar phenomenon was reported that hydrogen bond and hydrophobic

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interaction between polyphenols and protein lead to a less hydrophilic poly-phenol

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protein interface36.

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Mechanism for the Interfacial Assembly of Tannic Acid onto

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Chitin Film

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In order to investigate the mechanism for the interfacial assembly of tannic acid

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onto chitin film, assembly of the tannic acid was conducted under various conditions

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such as different pH, temperature and ionic strength. The effect of pH (Figure S2a)

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and ionic strength (Figure S2b) on the adsorption capacity of chitin film towards

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tannic acid indicated electrostatic attraction did not dominate the driving force for the

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assembly of tannic acid. The influence of temperature on the adsorption capacity of

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chitin film towards tannic acid was then investigated. As shown in Figure4a, the

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adsorption capacity decreased from 1013 mg/g to 157 mg/g when the temperature

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increased from 30 ℃ to 70 ℃. Presumably, Brownian movement of tannic acid in the

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solution increased with increase in temperature and the hydrogen bonding between

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tannic acid and regenerated chitin were weaker compared with that under low

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temperature, which further led to decrease in the adsorption capacity.

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research whether there was hydrophobic interaction between tannic acid and chitin

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film, tannic acid was absorbed by chitin film in the presence of various concentration

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of sodium dodecyl sulfate (SDS), which can destroy the hydrophobic interaction

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between macromolecules37. It was found from Figure4b that the adsorption capacity

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decreased significantly from 1019 mg/g to 749 mg/g in the presence of 6.25 mM SDS,

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indicating that there was strong hydrophobic interaction between chitin and tannic

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acid. It has been reported that chitin possessed amphiphilic structure because of the

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particular sugar ring structure38, which made nano chitin good emulsifier for

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stabilization of oil in water emulsion23. While there were abundant hydrophobic

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groups such as phenyl group in tannic acid5, so it was understandable that there was

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strong hydrophobic interaction between chitin film and tannic acid.

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demonstrated that hydrogen bond and hydrophobic interaction were the main driving

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force for the interfacial assembly of tannic acid onto chitin film.

All these results

277 278

Antibacterial and Antioxidant Activities of the Chitin Films

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S.aureus (Gram-positive bacteria) and E.coli (Gram-negative bacteria) are two

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representatives of the most common pathogens that frequently result in food

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poisoning and skin infections3, so it is important to investigate the antibacterial effects

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against these microorganisms. The antibacterial activity of RC and RC-TA against the

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E. coli and S.aureus was investigated with the inhibition zone method. Obviously, it

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can be seen from Figure 5 and Figure S3 that chitin films hardly displayed any

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bacterial inhibition zones. However, an obvious bacterial inhibition zones can be seen

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for RC-TA and the diameter of the inhibition zone become larger for RC-TA

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containing larger content of TA. But on the whole, the inhibition zone for S.aureus

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(11-19 mm) is larger than that for E. coli (7.5 -10.8 mm) under the same condition,

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demonstrating a stronger effect for S. aureus than E. coli. The mechanism behind the

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antibacterial activity of tannic acid might be related to its ability to inactivate enzymes

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and the surface proteins on the bacterial cells because tannic acid has strong

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interaction with proteins through hydrogen binding and hydrophobic interactions21,39 ,

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and the difference in the cell wall structures of Gram-positive and Gram-negative

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bacteria made S. aureus more vulnerable to tannic acid attack8,

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demonstrated that the incorporation of tannic acid significantly enhanced the

296

antibacterial performance of chitin film.

40

.These results

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The antioxidant capacity of the chitin-tannic acid composite films was

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determined by scavenging the DPPH• and ABTS•﹢ radicals. Because the absorbed

299

amount of tannic acid is so large for RC when the initial concentration of tannic acid

300

is above 0.5 g/L that it was difficult to determine the antioxidant capacity of the

301

composite films. So RC-TA0.05, RC-TA0.1, RC-TA0.25 and RC-TA0.5 were chosen

302

for the antioxidant assay. It can be seen from Figure 6 that the antioxidant capacity of

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the composite films significantly increased with increasing content of tannic acid in

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the films. The original chitin film scavenged 1.8% of the DPPH• radicals and 7.0% of

305

the ABTS•﹢radicals, while the chitin-tannic acid based film (RC-TA0.5) scavenged

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90.2% of the DPPH• radicals and 90.5% of ABTS•﹢. These results indicated that

307

chitin films with controllable antioxidant capacity can be obtained, which are

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promising to be used in the field of active packaging.

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The strong hydrogen bond and hydrophobic interaction between chitin and tannic

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acid made it possible that large amount (1086 mg/g) of tannic acid can be assembled

311

easily onto chitin film under pH 3 and 30 ℃, resulting in highly effective antibacterial

312

and antioxidant films.

313

acid composite films obtained directly from chitin because chitin cannot be dissolved

314

in common solvents and it is underutilized in the past. Tannic acid was tightly bonded

315

with chitin film due to the strong interaction between regenerated chitin and tannic

316

acid. The method was superior in the low cost of raw materials and the simple process

317

to obtain functional materials without the usage of toxic substances. It is expected that

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this method can be extended to fabricate polymer films/hydrogels with superior

319

antimicrobial and antioxidant properties for advanced industrial applications.

As far as we know, there has been no report of chitin/tannic

320 321 322 323 324 325

ABBREVIATIONS USED:

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RC (Regenerated Chitin film), TA (Tannic acid), RC-TA (Regenerated Chitin/ tannic

327

acid film), AC (adsorption capacity), TS (tensile strength), RH (Relative Humidity),

328

WVTR (Water Vapor Transmission Rate), SDS (sodium dodecyl sulfate), E. coli

329

(Escherichia coli), S.aureus (Staphylococcus

aureus)

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Funding:

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This work was financially supported by the National Natural Science Foundation of

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China (No. 31371841) and the Fundamental Research Funds for the Central

333

Universities (Program No. 2015BQ041).

334

Notes:

335

The authors declare no competing financial interest.

336

Supporting Information description:

337

The supporting Information is available free of charge via the Internet at

338

http://pubs.acs.org.

339

Figure S1. Pictures of chitin-tannic acid films(RC-TA)after immersing chitin film in

340

tannic acid (TA) a(0g/L), b(1g/L),c(5g/L), d(10g/L),e(15g/L),f(20g/L) for 3h.

341

Figure S2. (a) Effect of pH on adsorption of tannic acid onto chitin film, (b) effect of

342

ionic strength on adsorption of tannic acid onto chitin film (pH=3), (c) effect of

343

concentration on adsorption of tannic acid onto chitin film.

344

Figure S3.Pictures that show inhibition zones of chitin/TA and chitin against E. coli

345

and S. aureus.

346 347

AUTHOR INFORMATION:

348

Corresponding Author:

349

*Telephone/Fax: +86-027-87282111

350

E-mail: [email protected]

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Figures Figure captions Figure1. (a)Transmittance spectra of RC and RC-TA films in the wavelength from 300 to 800 nm, (b) stress−strain curves of the RC and RC-TA films. Figure2. SEM images of the chitin film after freeze-drying (a) RC, (b) RC-TA5, (c) RC-TA10, and (d) RC-TA20. Figure3. (a)FT-IR spectra of the RC and RC-TA films, (b) water vapor transmission rate (WVTR) of films under different relative humidity. Different capital letters (A, B, C, D, E) for RH=31% and lowercase letters (a, b, c) for RH=75% indicate significant differences (P<0.05). Figure4. (a) Effect of temperature on adsorption of tannic acid onto chitin film, (b) effect of SDS on adsorption of tannic acid onto chitin film. Different lowercase letters in each graph(a, b, c, d, e) indicate significant differences (P<0.05). Figure5. Antimicrobial activities of RC and RC-TA against E. coli and S. aureus Different capital letters (A, B, C, D) for E. coli and lowercase letters (a, b, c, d, e, f) for S. aureus indicate significant differences (P<0.05). Figure6. Radical (DPPH• and ABTS• ﹢ )scavenging activities of RC and RC-TA Different capital letters (A, B, C, D, E) for DPPH• and lowercase letters (a, b, c, d) for ABTS•﹢indicate significant differences (P<0.05).

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Transmittance(%)

a

RC RC-TA5 RC-TA10 RC-TA20

60 50 40 30 20 10 0

b

14

Stress(Mpa)

300

10

400 500 600 Wavelength (nm)

700

800

12

8 RC RC-TA5 RC-TA10 RC-TA20

6 4 2 0 0

4

8 12 Strain(%)

16

20

Figure1

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Figure2

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a

RC

RC-TA5

TA

4000

3500

480

b

40

3000 2500 2000 1500 -1 Wavanumbers(cm )

a

35

c

WVTR(g/h/m2)

c

c

25 20 15 10

A

5

482

500

RH=31% RH=75%

b

30

481

1000

RC

B

C

D

E

RC-TA5 RC-TA10 RC-TA15 RC-TA20

Figure3

483 484 485 486 487 488 489 490

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Adsorption capacity (mg/g)

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a

a b

1000

c

800 600 400

d e

200 0

30

40

50 T(℃)

60

70

b

b

b

b

Adsorption capacity (mg/g)

491

b

a

1000 800 600 400 200 0

492

50 0 6.25 12.5 25 Concentration of SDS (mmol/L)

493

Figure4

494 495 496 497 498 499 500 501 502 503 504

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505 f

20 E.coli S.aureus

e

16

d

14

10

D

D

D C

B

8 6

c

b

12

A

a

A 20

15

R C -T

-T A

A 5

A 10

R C

R C -T

C -T R

C -T

RC

A 1

4 R

Inhibition Zone (mm)

18

Figure5

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E DPPH ABTS

80

d

D

60

c C

40 20

B A

a

ab

b

507

Figure6

R

506

C RC /T A 0. 05 R C /T A 0. 1 R C/ TA 0. 25 RC /T A0 .5

Scavenging rate rate (%)

100

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Graphic abstract

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