Double cross-linked chitosan composite films developed with oxidized

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Double cross-linked chitosan composite films developed with oxidized tannic acid and ferric ions exhibit high strength and excellent water resistance Jie Yang, Man Li, Yanfei Wang, Hao Wu, Tianyuan Zhen, Liu Xiong, and Qingjie Sun Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01420 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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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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Biomacromolecules

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Double cross-linked chitosan composite films developed with oxidized

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tannic acid and ferric ions exhibit high strength and excellent water

3

resistance

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Jie Yang, Man Li, Yanfei Wang, Hao Wu, Tianyuan Zhen, Liu Xiong, Qingjie Sun*

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College of Food Science and Engineering, Qingdao Agricultural University (Qingdao,

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Shandong Province, 266109, China)

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*Correspondence author (Tel: 86-532-88030448, e-mail: [email protected])

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College of Food Science and Engineering, Qingdao Agricultural University, 266109, 700

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Changcheng Road, Chengyang District, Qingdao, China.

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ACS Paragon Plus Environment

Biomacromolecules 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract: There is tremendous scientific interest in developing biodegradable films through facile

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and versatile strategies. Although extensive studies on the preparation of chitosan films have been

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conducted, the reported results commonly present low mechanical strength and weak water

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resistance. In the present study, high strength and significantly water resistance single-cross-linked

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chitosan-oxidized tannic acid (SC-CS/OTA) composite films and double cross-linked

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chitosan/oxidized tannic acid/FeIII (DC-CS/OTA/FeIII) composite films were created through a

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Schiff base reaction and metal coordination. As a result, the optimal tensile strength of SC-CS/OTA

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composite films and DC-CS/OTA/FeIII composite films was 35.92 and 209 MPa, respectively.

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Notably, when compared with other chitosan-based films, the tensile strength of DC-CS/OTA/FeIII

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composite films was approximately three times stronger. Moreover, the water vapor permeability

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(WVP) values of the films with FeIII (0.66±0.03×10-10 g/m.h.Pa) was lower than that of films without

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FeIII (1.33±0.01×10-10 g/m.h.Pa). More importantly, WVP values of the DC-CS/OTA/FeIII

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composite films were 3−4 orders of magnitude lower than those of chitosan films previously

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reported. The SC-CS/OTA composite films (96.69%) and DC-CS/OTA/FeIII composite films

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(99.06%) also presented high DPPH radical scavenging activity. Furthermore, SC-CS/OTA and

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DC-CS/OTA/FeIII hydrogels were also prepared. This work can be widely applied in the food,

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biomedical science, and wastewater treatment fields.

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Keywords: Schiff base reaction, high mechanical strength, water vapor permeability, self-healing

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1. Introduction

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The development of biocompatible and biodegradable films with excellent mechanical

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behavior and water resistance has been subject to increasing interest due to the belief that they show

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promise for broad application in various fields such as food, biomedical science, wastewater 2


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treatment, soft robotics, sensor development, tissue engineering, and artificial skin research.1-5

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Specifically, polysaccharide-based films are receiving considerable attention because of their

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abundance, low-cost, excellent biodegradability, biocompatibility, and bioactivity.6-7 However,

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polysaccharide-based films are mechanically weak and exhibit low water resistance because of their

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relatively fragile polymer networks and hydrophilic nature.8 Therefore, nanocomposite films,9

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double network films,10 and ion films11 with good mechanical properties have been developed,

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representing a breakthrough in the film field. However, most polysaccharide-based films have

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demonstrated great hydrophilicity, leading to poor water resistance.12-13 Therefore, the development

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of a film with both sufficient mechanical properties and good water resistance is a challenging task.

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As an important polysaccharide, chitosan is obtained from the deacetylation of chitin (the

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second most abundant polysaccharide in nature). Chitosan has been widely applied in the

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development of edible films14-15 and hydrogels16-17 due to its biodegradable, biocompatible, non-

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toxic, and strong film-forming properties. Chitosan films are usually prepared by solvent

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evaporation, chemical cross-linking, or physical interactions with other compounds such as

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proteins.18-20 However, chitosan films produced by physical methods often possess poor mechanical

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and permeability properties when compared with those developed through chemical reactions.

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Generally, covalently cross-linked chitosan films are formed in a reaction between chitosan and a

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compound with two or more functional groups, such as epichlorohydrin or glutaraldehyde.21

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Although there are lot of advantages to using chemical cross-linkers (especially for enhancing

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mechanical strength), their use is limited due to the residues of the cross-linkers which may induce

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toxicity, reduce biocompatibility, or confer other undesirable effects on these materials. 22

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The Maillard reaction is a spontaneous chemical modification process that can happen in food3



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grade substances through mild, safe, and solvent-less procedures which are widely applied in the

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food matrix, such as baking products and flavors.23 In general, the Schiff base reaction is the initial

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stage in the Maillard reaction due to covalent bonding between the carbonyl groups and the free

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amine groups.24 In recent years, Gullón et al.25 introduced glucose into the chitosan molecule to

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obtain a chitosan-glucose derivative through the Maillard reaction which could improve the

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solubility of chitosan at a neutral or basic pH. In another experiment, Bozic et al.26 used chitosan

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combined with oxidized tannic acid to prepare modified chitosan films which presented effective

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ABTS•+ cation radical scavenging activity and improved antimicrobial activity. However, the

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mechanical and physiochemical properties of modified chitosan films were not determined.

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Moreover, Santoc et al.27 reported that oxidized tannic acid could cross-link zein through a Maillard

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reaction to form films, and that the use of these films could be explored with respect to enhance the

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water resistance of zein films. However, to the best of our knowledge, the use of oxidized tannic

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acid to modify chitosan for increased toughness and improved water resistance in chitosan films by

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Schiff base reaction and metal coordination has not yet been reported.

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Tannic acid, a gallic ester of D-glucose, is recognized by its antioxidant capacity due to the

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multiple phenolic groups that can interact with biological macromolecules.28 Natural polyphenol

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and FeIII can form a supramolecular organic-metal network;29-32 this combination has attracted

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widespread interest due to the diverse properties it presents. Moreover, these two substances are

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generally recognized as safe (GRAS) by the U.S. Food and Drug Administration.33 Three galloyl

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groups from tannic acid can react with each FeIII ion to form a stable octahedral complex, allowing

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each tannic acid molecule to react with several FeIII centers to form a cross-linked network. Ejima

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et al.34-35 have reported on the development of tannic acid-FeIII versatile nanofilm by the one-step 4


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coating of coordination complexes. In addition, Mao et al.36 used poly(ε-caprolactone)

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nanocomposites filled with tannic acid-FeIII to enhance the mechanical and gas barrier properties of

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film.

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In this work, we describe a facile, green method using Schiff base-mediated covalent cross-

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link and metal coordination that can effectively develop single-cross-linked chitosan-oxidized

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tannic acid (SC-CS/OTA) composite films and double cross-linked chitosan/oxidized tannic

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acid/FeIII (DC-CS/OTA/FeIII) composite films to improve the mechanical properties and water

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resistance of chitosan films. First, SC-CS/OTA composite films of different tannic acid oxidation

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degrees were prepared. Then, the physical, mechanical, and antioxidant properties of the SC-

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CS/OTA and DC-CS/OTA/FeIII composite films were characterized. In order to expand application,

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SC-CS/OTA and DC-CS/OTA/FeIII hydrogels were also created, and the mechanical, rheological,

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and self-healing properties of the hydrogels were analyzed.

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2. Materials and methods

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2.1. Materials

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Chitosan (Mw=1092±50 kDa) with a degree of deacetylation ∼90% was purchased from the

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Shanghai Ryon Biological Technique Co., Ltd. (Shanghai, China). Tannic acid and laccase were

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obtained from the Sigma–Aldrich Corporation (USA). 2,2-diphenyl-1-picrylhydrazyl (DPPH, 96%)

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and anhydrous ferric chloride (FeCl3≥99.9%, FeIII) were purchased from the Beijing Solarbio

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Science & Technology Co., Ltd. (Beijing, China). Other reagents used were of analytical grade.

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2.2. Preparation of laccase oxidized tannic acid solution

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Tannic acid (0.85 g) was dissolved in a 50 mL, pH 6.5 phosphate buffer (100 mM). Then, the

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tannic acid was mixed with 0.0125 g (75 U), 0.025 g (153 U), and 0.05 g (206 U) of laccase and 5



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incubated under constant stirring for 24 h at 30 °C. Oxidized tannic acid was formed by oxidative

99

self-polymerization. The product was centrifuged (12,000 g, 1 h) and washed three times with

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distilled water. The sediment was frozen at −18 °C and then lyophilized for 48 h to obtain oxidized

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tannic acid powder.

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2.3. Determination of carbonyl content

103

The carbonyl content was determined according to the titrimetric method.37 Three kinds of

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laccase oxidized tannic acid samples (0.25 g) were suspended in 25 mL of distilled water. The

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solution was heated to 40 °C and adjusted to a pH of 3.2 with 0.1M HCl. Then, 4 mL of

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hydroxylamine reagent were added to the solution. The flask was stoppered and agitated in a 40 °C

107

water bath for 4 h. The excess hydroxylamine was determined by rapidly titrating the reaction

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mixture to a pH of 3.2 with 0.1M HCl. A blank determination with only a hydroxylamine reagent

109

was performed in the same manner. The hydroxylamine reagent was prepared by first dissolving 5

110

g hydroxylamine hydrochloride in 20 mL of 0.5M NaOH before the final volume was adjusted to

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100 mL with distilled water. The carbonyl content was calculated as follows:

112 113

Percentage of carbonyl content = [(Vb- Vs) × 0.1 × 0.028 × 100] / W

(1)

where Vb is the volume of HCl used for the blank (mL), Vs is the volume of HCl required for

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the sample (mL), 0.1 is the molarity of HCl, and W is the sample weight (g).

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2.4. The formation of SC-CS/OTA and DC-CS/OTA/FeIII films and hydrogels

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The SC-CS/OTA composite films and hydrogels were prepared by dissolving 3% chitosan

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powder in a 2% acetic acid solution. Then, the oxidized tannic acid dispersion of 4 mL (1.7%, w/v)

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was uniformly mixed into a 20 mL chitosan solution (pH=3). The pH of the solution was adjusted

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to 6.0 using 3 M NaOH and homogeneously mixed. The final volume of the solution was 25 mL. 6


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The SC-CS/OTA hydrogel was prepared by adding 5 mL of uniform chitosan solution to a small 10

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mL beaker, which was then incubated at a constant temperature (50 °C) and humidity (75%) in a

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chamber for 0, 12, 24, 48, and 72 h. The SC-CS/OTA films were prepared by adding a uniform

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CS/OTA solution to Petri dishes (Ф = 7 cm), and then reacted for 0–72 h at 50 °C. Finally, the films

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were dried overnight at 50 °C.

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DC-CS/OTA/FeIII films and hydrogels were prepared by adding 0.5 mL of different

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concentrations of FeIII into 3% chitosan solution. Other operations were consistent with the above

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procedures applied in developing the SC-CS/OTA films and hydrogels.

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For the DC-CS/FeIII/OTA hydrogel preparation, 20 mL of 3% chitosan solution was first added

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into 0.5 mL of 2.0% FeIII solution in a small 10 mL beaker and mixed well. Then, the solution was

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vacuum freeze dried to form a CS-FeIII aerogel. Finally, the CS-FeIII aerogel was soaked into

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different pH values (pH = 1 or 6) of oxidized tannic acid solution for 1, 2, and 4 h at 30 °C to obtain

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DC-CS/FeIII/OTA hydrogels.

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The preparation of chitosan-laccase-tannic acid simple mixing (CS-TA) films was conducted

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as follows. Tannic acid (0.85 g) was dissolved in a 50 mL phosphate buffer (100 mM). The solution

135

was mixed uniformly with 0.025g of laccase (153 U) and incubated for 0 h (30 °C). Subsequently,

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4 mL of the tannic acid-laccase solution was added to 20 mL of chitosan solution. The pH of the

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solution was adjusted to 6.0 using 3M NaOH and homogeneously mixed. The final volume of the

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solution was 25 mL. A CS-TA film was prepared by adding 25 mL of uniform chitosan-laccase-

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tannic acid solution to Petri dishes which were then placed in a constant temperature (50 °C) and

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humidity (75%) chamber for 12 h then dried overnight (50 °C).

141

The preparation of pure chitosan films was performed with procedures in accordance with the 7



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literature.38 Briefly, 25 mL of 2.4% chitosan solution was added into Petri dishes which were then

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placed in a constant temperature (50 °C) and humidity (75%) chamber for 48 h. Then, the Petri

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dishes were dried at 50 °C overnight.

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All the obtained dried films were stored in a desiccator with 75% humidity for 72 h at 25 °C

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before further characterization.

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2.5. Measurement of film properties

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2.5.1. Film thickness

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A thickness meter (MP0, Fischer, Germany) was used to determine film thicknesses. The

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average values were obtained from ten random points for each sample.

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2.5.2. Light transmittance and opacity

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The light transmittance of each film was monitored by a UV-Vis spectrophotometer (1601 PC,

153

Tokyo, Japan) at 600 nm. A rectangular film was directly placed into the cuvette, and the opacity

154

was calculated with the following formula:39

155 156 157

𝐴

(2)

Opacity (A600/mm) = 𝑥

where A was the absorbance of films and 𝑥 was the film thickness (mm). 2.5.3. Film color measurements

158

Film color was measured with a CR-400 chromameter (Valencia, Spain). The chromaticity

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parameters (a*, b*) and lightness (L*) values of the films were recorded, and total color differences

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(ΔE) were calculated with the following formula:

161 162 163

ΔE = [(ΔL)2 + (Δa)2 + (Δb)2]1/2

(3)

2.5.4. Water vapor permeability Water vapor permeability (WVP) tests were conducted based on a modified ASTM (1995) 8


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method E96 by measuring gravimetrically.40 Each film was sealed on a conical bottle mouth (Ф=30

165

mm) filled with of dried CaCl2 (≈15 g), which was then placed in the desiccator (100% humidity).

166

The conical bottle was weighed every 12 h to calculate the WVP as follows:

167

WVP(g / m  h  Pa ) 

md AtP

(4)

168

where m, d, A, t, and P are the weight increment (g), thickness (m), area exposed (m2), time

169

for permeation (h) of the film, and the water vapor partial pressure difference across the film (Pa).41

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2.5.5. Water solubility and degree of swelling of the films

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The water solubility and degree of swelling of the films were determined by the gravimetric

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method.42 The rectangular films (1×4 cm2, m1) were dried (105 ºC, 24 h) to achieve the dry mass

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(m2). Then, the films were immersed in distilled water for 24 h (25 ºC) to obtain a constant mass

174

(m3). Finally, the films were dried for 24 h (105 ºC, m4). 𝑚2 ― 𝑚4 𝑚4

175

Water solubility (%) =

176

Deegree of swelling (%) =

177

Moisture content (%) =

178

(5)

𝑚3 ― 𝑚2 𝑚2

𝑚1 ― 𝑚2 𝑚1

(6) (7)

2.5.6. Determination of the mechanical properties

179

The mechanical properties of the films were evaluated with an INSTRON 5943 electronic

180

universal testing machine. Rectangular samples (10 mm×100 mm) were tested at 20 mm/min of the

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tensile rate. The strength and elongation at break as well as Young’s modulus were obtained based

182

on the stress-strain curves.

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2.5.7. Thermal properties

184

Thermogravimetric analysis of the films (≈ 5 mg) was carried out using a TGA/DSC 2

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(METTLER TOLEDO, Greifensee, Switzerland) from 30 to 600 °C with a heating rate of 10 °C/min. 9



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2.5.8. Fourier transform infrared (FTIR) spectroscopy analysis

187

The spectra of tannic acid, laccase-oxidized tannic acid, and both the SC-CS/OTA and DC-

188

CS/OTA/FeIII composite films were recorded using a Nicolet 6700 FTIR spectrometer (Thermo

189

Scientific, USA). A total of 32 scans at a resolution of 4 cm-1 were accumulated using rapid-scan

190

software in OMNIC 8.0 to obtain a single spectrum. The FTIR spectra in transmittance mode was

191

recorded in range of 400–4000 cm-1.

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2.5.9. Microstructural morphology

193

The cross-section morphology of all dried films was observed with a JEOL 7500F scanning

194

electron microscope. After lyophilization, the samples were sputtered with gold for observation.

195

The test was operated at 2 kV.

196

2.5.10. Antioxidant activity of the films

197

The antioxidant activity of the samples was tested in accordance with the method of Yu et al.43

198

Briefly, samples were mixed with 4.0 mL of 100 mM DPPH solution dissolved in methanol. The

199

final concentrations of the samples were 1.0, 2.0, 3.0, 4.0 and 5.0 mg/mL, respectively. Then, each

200

solution was incubated for 60 min in the dark at 25 °C. Finally, the absorbance rates of the resulting

201

solutions were recorded at 517 nm. DPPH radical scavenging activity was calculated with the

202

following equation:

203 204

DPPH radical scavenging activity (%) =

𝐴0 ― 𝐴1 𝐴0

× 100

(8)

where A0 is the absorbance of the control, and A1 is the absorbance of the supernatant in the

205

film sample tube.

206

2.6. Measurement of hydrogel properties

207

2.6.1. Mechanical properties of the hydrogels 10


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Compressive measurements of the SC-CS/OTA and DC-CS/OTA/FeIII hydrogels were

209

performed using a texture analyzer (TA-XTplus, Stable Micro Systems, Surrey, U.K.). Cylindrical

210

hydrogel sample (d = 1.5 cm, h = 1.5 cm) were compressed at 1 mm/s of a strain rate with a P 36R

211

probe. Cyclic tests under 50% of strain were conducted in subsequent trials which were performed

212

five times immediately after the initial loading.

213

2.6.2. Rheological test

214

The rheological measurements of the SC-CS/OTA and DC-CS/OTA/FeIII hydrogels were

215

analyzed with a strain-controlled rheometer (MCR102, Anton Paar, Graz, Austria). The hydrogels

216

were in the shape of a cylinder (Ф = 50 mm, h = 1 mm).

217

Frequency sweeps: To monitor the storage modulus (G') and loss modulus (G″), The

218

frequency sweeps were measured with an angular frequency range of 0.1−100 rad/s and a strain of

219

1% (within the linear viscoelastic region) at 25 °C.

220

Self-healing properties: For rheological demonstration of the self-healing abilities of

221

hydrogels with different concentrations of FeIII, each hydrogel was immediately transferred into a

222

cylindrical cast and measured. In a non-linear time sweep experiment, the initial G' and G″ of the

223

hydrogels were recorded at 1% of strain and an angular frequency of 5 rad/s for 100 s. Then, a

224

sudden strain of 100% (angular frequency of 5 rad/s) was applied for 100 s to destroy the hydrogel.44

225

2.7. Statistical analysis

226

Triplicate samples of all quantitative results were obtained. The results were reported as

227

average values and standard deviations. Statistical analysis was performed with Duncan’s multiple

228

range tests using the SPSS V. 17 statistical software package (SPSS Inc., Chicago, IL, USA).

229

3. Results and discussion 11



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Laccase oxidization

Tannic acid

Oxidized tannic acid NH2 NH2

Schiff base reaction

Chitosan

First crosslinking SC-CS/OTA composite film FeIII

Metal coordination

Second crosslinking DC-CS/OTA/FeIII composite film

Scheme 1. Illustration of formation mechanism of SC-CS/OTA composite film and DCCS/OTA/FeIII composite film.

12


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The formation mechanisms of the SC-CS/OTA and DC-CS/OTA/FeIII composite films are

231

briefly demonstrated in Scheme 1. The SC-CS/OTA composite film was formed by a Schiff base

232

reaction between the chitosan and oxidized tannic acid. Then, the FeIII enabled coupling with the

233

phenolic hydroxyl in the oxidized tannic acid by metal coordination to construct the second cross-

234

linking of the DC-CS/OTA/FeIII composite films.

235

3.1. Oxidation of tannic acid Tannic acid Oxidized tannic acid

A

3+

Without Fe

B

0.4% 0.6% 0.8% 1.0% 2.0% 4.0% 6.0%

1536 1325

1204

1634

4000

3500

3000

2500

2000

1500

1000

1636 1540

500

-1

Wavenumber (cm )

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

C

0h 12 h 24 h 48 h 72 h

40 35

D 250

25 20 15

0.4% 0.6% 0.8% 1.0% 2.0% 4.0% 6.0%

200 Stress (MPa)

30 Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

150 100

10

50

5 0

0 0

50

100

150

200

250

0

300

20

40

60

80

100

120

Strain (%)

Strain (%)

Figure 1. FTIR spectra of tannic acid and oxidized tannic acid (A), and DC-CS/OTA/FeⅠⅠⅠ composite film with different concentration FeIII (B). Tensile stress-strain curves of SC-CS/OTA composite films with different Schiff base reaction time (C) and DC-CS/OTA/FeⅠⅠⅠ composite films (D) with different concentration of FeIII. 236

The FTIR spectra of the tannic acid and laccase oxidized tannic acid are shown in Figure 1A.

237

For tannic acid, peaks at 3200–3700 cm-1 and 1325 cm-1 were attributed to the vibration of the

238

phenolic hydroxyl groups. The peaks at 1204 cm-1 and 1536 cm-1 were associated with the aromatic 13



239

ring C=O and C=C stretching vibrations, respectively. The peak at 1000–1150 cm-1 corresponded

240

mainly with the ethers (C-O-C).

241

The laccase can oxidize phenols and convert them into reactive o-quinones.45 For the oxidized

242

tannic acid, the absorption peak at 1634 cm−1 was attributed to carbonyl stretching, which could be

243

due to the formation of a quinone in the intermediate step. Also, it is worth noting that the hydroxyl

244

groups of tannic acid at that point have been drastically transformed.46

245

The content of the carbonyl groups of the oxidized tannic acid is presented in Table 1. The

246

carbonyl content of the oxidized tannic acid significantly increased as the additions of laccase

247

increased. The carbonyl contents for three kinds of samples (75, 153, and 206 U) were 0.112 ± 0.004,

248

0.246 ± 0.005, and 0.358 ± 0.015%, respectively. The solution of laccase oxidized tannic acid

249

changed from light yellow to shades of brown in accordance with the increasing degree of tannic

250

acid oxidation (Figure S1). Table 1. Carbonyl contents of laccase oxidized tannic acid. Sample

Carbonyl content (%)

75 U 153 U 206 U

0.112±0.004c 0.246±0.005b 0.358±0.015a

Values are means ± standard deviation of three replications. Mean values in the same column with different letters are significantly different (p < 0.05). Sample 75, 153, and 206 U: the oxidized tannic acid with different laccase activity. 251

3.2. Effect of tannic acid oxidation degree on WVP of SC-CS/OTA films

252

The CS-TA films and SC-CS/OTA composite films were prepared with casting methods. The

253

thickness of the CS-TA films was 142±7 μm, while the thicknesses of the SC-CS/OTA composite

254

films ranged from 138 ± 6 to 126 ± 1 μm (Table S1). Moreover, the thicknesses of the SC-CS/OTA

255

composite films with high degrees of tannic acid oxidation were lower than those with low degrees

256

of tannic acid oxidation. 14


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The WVP value is an important parameter for films because one of their main functions is to

258

hinder the transfer of moisture between the coated materials and the surrounding atmosphere (Aider,

259

2010).47 The WVP values of three SC-CS/OTA composite films were lower than that of the CS-TA

260

film (Table S1). The WVP of films with high degrees of tannic acid oxidation were lower than those

261

with low degrees of tannic acid oxidation. A possible explanation for this result could be that

262

oxidized tannic acid containing more carbonyl groups could crosslink with more chitosan molecules,

263

which could make the network structure of films more compact. Compared to the CS-TA film (80.40

264

± 1.70 × 10-10 g / m.h.Pa), the WVP values of the SC-CS/OTA composite films (0.94 ± 0.04 × 10-10

265

g / m.h.Pa) decreased remarkably by two orders of magnitude. Aljawish et al. 48 also found that the

266

WVP values of ferulic acid grafted chitosan film were lower than those of pure chitosan film. The

267

WVP values of the SC-CS/OTA composite films were more than 3 orders of magnitude lower than

268

those of tannic acid crosslinked chitosan films (3× 10-7 g / m.h.Pa) previously reported by Rivero et

269

al.49 In their work, only a small amount of oxidized tannic acid was incorporated into chitosan

270

molecules without keeping a certain period of reactions, which resulted to insufficient crosslinking.

271

We chose the oxidized tannic acid with a moderate oxidation degree (153 U) to conduct the

272

following investigation. Table 2. Physical properties of SC-CS/OTA composite films with different Schiff base reaction time.

Films

Thickness (μm)

Opacity (A600/mm)

WVP (10-10g /m.h.Pa)

Solubility (%)

Swelling degree (%)

Moisture content (%)

0h 12 h 24 h 48 h 72 h

141±1a 132±6ab 128±1b 122±6bc 117±4c

0.638±0.017e 2.288±0.082d 2.438±0.018c 2.656±0.051b 5.043±0.081a

10.81±0.28a 6.66±0.17b 4.25±0.18c 3.27±0.06d 1.33±0.01e

19.72±0.42a 17.18±0.81b 12.20±0.53c 11.14±0.35cd 10.56±0.43d

46.79±1.09a 28.53±1.24b 19.29±0.51c 19.15±0.23c 19.07±0.56c

17.42±0.87a 16.41±0.65a 17.26±0.71a 16.58±0.64a 15.59±0.64a

Values are given as mean ± standard deviation. Means in each column with different letters are significantly different (p < 0.05). The activity of laccase was 153 U. 273

3.3. Effect of the Schiff base reaction time on the physical properties of SC-CS/OTA composite 15



274

films

275

The effect of the Schiff base reaction time on the physical properties of the SC-CS/OTA

276

composite films is shown in Table 2. The thicknesses of the films with different Schiff base reaction

277

times varied between 117 ± 4 and 141 ± 1 μm. The color of the films was a slightly yellow without

278

experiencing the Schiff base reaction, but it turned brown after the Schiff base reaction. With the

279

increasing Schiff base reaction time, the brown color deepened. The prolonged reaction time led to

280

a decrease from ≈ 80% to ≈ 30% in light transmittance at 600 nm and strong ultraviolet absorption

281

(Figure S2), which suggested that the SC-CS/OTA composite films could protect packaging

282

products against ultraviolet damage.

283

The L*, a*, and b* values of chitosan film and the SC-CS/OTA composite films prepared at

284

different Schiff base reaction times were presented (Table S2). The L* value decreased with the

285

increasing reaction time, suggesting that the SC-CS/OTA composite films had dark colors.

286

Moreover, the a* and b* values of the SC-CS/OTA composite films were larger than those of the

287

chitosan film, indicating that the SC-CS/OTA composite films changed in color toward red or

288

yellow shades. An increase in △E was found for the SC-CS/OTA composite films with the

289

increasing Schiff base reaction time, indicating that the SC-CS/OTA composite films were more

290

colored. These results were similar to those of the tea polyphenol grafted chitosan composite film

291

reported by Wang et al.50

292

The WVP of the SC-CS/OTA composite films (6.66 ± 0.17–1.33 ± 0.01 × 10-10 g / m.h.Pa)

293

after Schiff base reaction was lower than that of the SC-CS/OTA composite films not subject to the

294

Schiff base reaction (10.81 ± 0.28 × 10-10 g / m.h.Pa) (Table 2). With an extension of the Schiff base

295

reaction time, the WVP value of the SC-CS/OTA composite films significantly reduced. The 16


Page 16 of 37

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Biomacromolecules

296

decrease in WVP by the lengthened reaction time could probably be attributed to the higher degree

297

of crosslinking and the more compact network structure of SC-CS/OTA composite film. Along

298

similar lines, Wang et al.51 demonstrated the colloidal complexation of zein and the hydrophobic

299

region of tannic acid through hydrophobic interaction.

300

Water solubility and the degree of swelling are important factors in determining the application

301

of bio-based films. As shown in Table 2, the moisture content (15.61–17.42%) of the films was not

302

affected by the Schiff base reaction time. The water solubility of the SC-CS/OTA composite films

303

decreased from 19.72 ± 0.42% to 10.56 ± 0.43%, and the swelling degree of the film decreased from

304

46.79 ± 1.09% to 19.07 ± 0.56% with the increasing reaction time. The excellent water resistance

305

of SC-CS/OTA composite films could be due to the better crosslink between OTA and CS resulted

306

in extremely compact structure that hinders water penetration into the films via capillary force. It is

307

thus important to note that the water solubility and swelling degree values of the SC-CS/OTA

308

composite films were lower than those of chitosan-polyphenol film (19.93% and 353.92%,

309

respectively).52 Furthermore, the swelling degree of SC-CS/OTA composite films was much lower

310

than that of tannin acid cross-linked gelatin films (>600%). 53

311

The films with high mechanical properties designed for packaged foods and other products can

312

protect their physical integrity.54 Figure 1C and Table S3 show the typical stress−strain curves,

313

tensile strength, Young’s modulus, and elongation at break values for the SC-CS/OTA composite

314

films with different Schiff base reaction times. The values of tensile strength, Young’s modulus,

315

and elongation at break of oxidized tannic acid-chitosan films not subject to a Schiff base reaction

316

were 8.58 ± 0.57 MPa, 837.12 ± 4.07 MPa, and 275 ± 20 %, respectively. Among the different Schiff

317

base reaction times of the films that were tested, the SC-CS/OTA composite film prepared at 72 h 17



Page 18 of 37

318

exhibited the highest tensile strength (35.92 ± 1.32 MPa) and the lowest elongation at break (131 ±

319

4%). The tensile strength of SC-CS/OTA composite film increased by 32% that of the chitosan-

320

based films that had been previously reported (Table 4). In the literature, genipin cross-linked

321

chitosan films were shown to present strong tensile strength and low elongation at break.55 More

322

recently, Aljawish et al.48 found that the tensile strength of chitosan films increased after ferulic acid

323

grafting. Table 3. Physical properties of SC-CS/OTA composite films with different FeIII. FeIII concentration (%)

Opacity (A600/mm)

WVP (10-10g /m.h.Pa)

Solubility (%)

Swelling degree (%)

Moisture content (%)

0.4 0.6 0.8 1.0 2.0 4.0 6.0

14.366±0.235g 16.662±0.370f 22.231±0.609e 24.852±1.064d 27.817±1.296c 30.542±1.050b 34.873±1.093a

2.24±0.10b 1.90±0.07c 1.42±0.04d 0.97±0.04e 0.66±0.03f 1.82±0.06c 2.93±0.11a

10.77±0.47c 9.06±0.34d 8.30±0.29d 8.16±0.20de 6.82±0.45e 14.55±0.50b 29.21±1.25a

25.31±0.69g 28.03±0.66f 30.81±0.58e 35.06±0.23d 38.73±0.53c 40.82±1.44b 43.19±0.58a

18.41±0.69c 18.47±0.18c 17.03±0.32c 20.11±0.58b 16.98±0.31c 23.26±1.08a 24.36±1.04a

Values are given as mean ± standard deviation. Means in each column with different letters are significantly different (p < 0.05). 324

3.4. Effect of different concentrations of FeIII on DC-CS/OTA/FeIII composite film properties

325

FeIII ions were introduced into the CS/OTA homogeneous solutions to promote the occurrence

326

of metal coordination between FeIII and oxidized tannic acid. The resulting solution evolved into a

327

black color, which indicated the formation of a metal−tannic acid coordinate bond. The solution

328

was cross-linked into films or hydrogels when the pH value was adjusted to 6. Furthermore, the

329

addition of FeIII also decreased the light transmittance of the DC-CS/OTA/Fe III composite films

330

(Table 3). The opacity values of the films increased from 14.366 ± 0.235 to 34.873 ± 1.093

331

A600/mm with the increase of concentration of FeIII from 0.4% to 6.0%. Table 3 also shows the

332

WVP values of films with different concentrations of FeIII. The WVP values of films with 2.0% Fe

333

III

were the lower than those of other films. Moreover, the WVP values of the DC-CS/OTA/FeIII 18


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Biomacromolecules

334

composite films were lower than those of the SC-CS/OTA films (1.33 ± 0.01 × 10-10 g / m.h.Pa).

335

The WVP of DC chitosan composite film was 0.66 ± 0.03 × 10-10 g / m.h.Pa, which was lower by

336

4~5 orders of magnitude than that of other chitosan-based films in the literature presented in Table

337

4. Table 4. Comparison of water solubility, swelling, WVP, and tensile strength of DC-CS/OTA/FeIII composite film. Film

Solubility (%)

CS-TA film SC-CS/OTA film DC-CS/OTA/FeIII composite film polyphenol-chitosan film Chitosan-ethyl ferulate film Chitosan -genipi film Chitosan/gelatin composite film Chitosan/silver nanoparticle bionanocomposite film Chitosan-Tween 80 edible film Chitosan film

—— 10.56 6.82 19.93 —— 20 26.95 28.7 19.6 24.99

Swelling degree (%)

WVP (g /m.h.Pa)

Tensile strength (MPa)

Reference

—— 19.07 38.73 353.92 —— —— 103

80.40×10-10 1.33×10-10 0.66×10-10 —— 1.30×10-6 —— 3.45×10-7

—— 35.92 209 —— 58.7 82.2 32.04

This work This work This work Ref.55 Ref.48 Ref.57 Ref.59

270

3.28×10-7

55.44

Ref.63

113 213

2.39×10-6 3.92×10-7

107 27.14

Ref.61 Ref.59

338

Packaged foods will be susceptible to discoloration, oxidative deterioration, off-flavoring, and

339

nutrient loss once they are exposed to visible and UV light.52 The light transmittance of DC-

340

CS/OTA/Fe III composite films with different concentrations of Fe III is shown in Figure S3. The

341

DC-CS/OTA/Fe

342

transmittance, which indicated that the DC films exhibited high barrier abilities against UV light.

343

Thus, these films could potentially be applied in food system manufacturing to retard lipid oxidation

344

induced by UV light.

III

composite films, in the range of 300–360 nm, exhibited very low UV light

345

Table S4 shows the color parameters of the DC-CS/OTA composite films with different

346

concentrations of FeIII. The introduction of FeIII onto composite films caused a significant decrease

347

in L*. In addition, the a* and b* values of the composite films decreased with the increasing FeIII 19



348

concentration, indicating that DC composite films became darker and evolve toward shades of green

349

and blue in color. However, the total color differences (ΔE) of the DC composite films containing

350

different concentrations of FeIII were not significant.

351

The solubility, swelling degree, and moisture content of DC-CS/OTA/FeIII chitosan films with

352

different concentrations of FeIII are shown in Table 3. The water resistance of films is vital when

353

they are applied in food packaging.56 When the concentration of FeIII was 2.0%, the solubility and

354

swelling degree of the DC composite films was the lowest among the film samples. In fact, the

355

solubility and swelling degree of the composite films (6.82 ± 0.45% and 13.73 ± 0.53%, respectively)

356

were lower than those of chitosan/silver nanoparticle bio-nanocomposite films (19.6% and 113%,

357

respectively).57 Moreover, the moisture content of DC composite films was in the range of 15–25%.

358

The mechanical strength of the DC composite films is well adjustable. The concentration of

359

FeIII in the films can affect their mechanical strength. The tensile stress–strain curves of composite

360

films with various FeIII concentrations are shown in Figure 1D. As the concentration of FeIII

361

increased from 0.4% to 2.0%, the tensile stress of the films improved gradually from 42 to 209 MPa,

362

whereas the strain decreased from 86% to 65%. However, as the concentration of FeIII increased

363

from 2.0% to 6.0%, the tensile stress of the films decreased gradually from 209 to 11.86 MPa,

364

whereas the strain increased from 65% to 119%. Importantly, as compared to the SC-CS/OTA

365

composite films, the mechanical strength of the DC-CS/OTA/FeIII composite films was much

366

stronger, indicating that noncovalent interaction also occurs between FeIII and chitosan chains or

367

tannic acid.58 Moreover, the tensile strength of DC chitosan composite film was 209 MPa, which

368

was approximately three times stronger than the reported strength of other chitosan-based films,

369

such as chitosan/silver nanoparticle bio-nanocomposite films listed in Table 4.59 20


Page 20 of 37

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Biomacromolecules

370

To determine if the Schiff base reaction changed the thermal stability of SC-CS/OTA

371

composite films, TGA profiles of the SC films prepared at different reaction times were studied

372

(Figure S4A and B). The thermal degradation of the films could be divided into two stages. The first

373

stage exhibited a minor mass loss of approximately 20% below 100 °C, which was ascribed to the

374

vaporization of absorbed water. In the second stage, the mass of films showed a sharp decrease in

375

the temperature range of 200–300 °C. Typically, after the Schiff base reaction, the maximum loss

376

rate of the SC composite films appeared at a lower temperature (265 °C) than that of the film not

377

subject to a Schiff base reaction (275 °C). These results indicate that the SC films exhibited worse

378

thermal stability compared to the films not subject to a Schiff base reaction. Moreover, the TGA

379

profiles of the DC-CS/OTA films were also studied (Figure S4C and D). The DC chitosan films

380

also exhibited two steps in weight loss. The first (occurring around 50–100 °C) was ascribed to the

381

removal of water from the films. The second (occurring around 250–300 °C) was mostly due to the

382

degradation of chitosan and oxidized tannic acids in the films. The degraded temperature of the

383

composite films had no change with the adding of FeIII.

384

3.5 FTIR analysis of DC-CS/OTA/FeIII composite films

385

For SC-CS/OTA composite film, the peak of carbonyl stretching at 1634 cm−1 was shifted to

386

1636 cm−1, which indicated the formation of the OTA-CS amide bond during Schiff reactions

387

(Figure 1B).60 The formation of the network of DC chitosan composite films is apparently associated

388

with the formation of metal-coordination bonds between ferric ions and -OH groups of tannic acid,

389

which is confirmed by the FTIR spectra. Compared with SC composite film, the peak of 3000–3500

390

cm-1 in the DC composite films actually appeared as two small peaks with the increasing of

391

concentration of FeIII, lending support that there was a formation of metal coordination bonds. This 21



392

Page 22 of 37

was in line with the results reported by Fan et al.61 and Laxmi et al.62

B

A

10 μm

10 μm

Figure 2. SEM image of SC-CS/OTA composite film (A) and DC-CS/OTA/FeⅠⅠⅠ composite film (B). 393

3.6. Microstructural morphology analysis

394

The cross-section morphologies of the SC-CS/OTA and DC-CS/OTA/FeIII composite films are

395

shown in Figure 2A and B. Pure chitosan film and the CS-TA films not subject to a Schiff base

396

reaction all exhibited a horizontal fringe-like appearance with a thickness of 140 μm (Figure S5). In

397

contrast, the morphologies of the SC-CS/OTA composite films were substantially rough, and the

398

horizontal fringe decreased with a thickness of 120 μm. The rough morphologies of the SC

399

composite films were probably due to the oxidized tannic acid and increase of the viscosity of the

400

CS/OTA system. This phenomenon was in accordance with the results of the study conducted by

401

Aljawish et al.48 The cross-section structure of the DC-CS/OTA/FeIII composite films was much

402

smoother with a thinner thickness of 110 μm than that of the SC-CS/OTA composite films. Because

403

of the tighter inner structure, the DC-CS/OTA/FeIII composite films had a higher tensile strength

404

and excellent water resistance.

405

3.7. Antioxidant activity of the SC-CS/OTA and DC-CS/OTA/FeIII composite films

22


Page 23 of 37

100

Scavenging activity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

80 Chitosan film Tannic acid SC-CS/OTA composite film DC-CS/OTA/Fe composite film

60 40 20 0

1

2

3

4

5

Sample concentration (mg/ml)

Figure 3. Dose-dependent DPPH radical scavenging activity of chitosan film, tannic acid, SCCS/OTA composite film, and DC-CS/OTA/FeIII composite film. 406

The DPPH radical scavenging method is used to evaluate the antioxidant ability of chitosan film.63-64

407

Figure 3 shows the DPPH radical scavenging activity of pure chitosan film, tannic acid, SC-

408

CS/OTA composite films, and DC-CS/OTA/FeIII composite films. First, a much stronger DPPH

409

radical scavenging activity for both SC-CS/OTA and DC-CS/OTA/FeIII films was displayed in

410

comparison to that of pure chitosan film. By increasing the concentration of film, the antioxidant

411

activity of the SC-CS/OTA and DC-CS/OTA/FeIII composite films increased. The radical

412

scavenging activity percentages of the SC-CS/OTA and DC-CS/OTA/FeIII composite films were

413

96.69% and 99.06%, respectively, when the film concentration was 5 mg/mL; these percentages

414

were higher than those for pure chitosan film (16.53%) and tannic acid (93.64%). Schreiber et al.

415

(2013)53 found that chitosan films grafted with gallic acid remarkably enhanced the radical

416

scavenging activity. With an increase in the grafting ratio of gallic acid, the chitosan-gallic acid

417

films presented higher antioxidant activity; this finding was reported by Wu et al.65

418

3.8. Characteristics of SC-CS/OTA and DC-CS/OTA/FeIII hydrogels

419

Both films and hydrogels are widely recognized as smart and relatively low-cost materials. In 23


Biomacromolecules

420

practice, hydrogels are the films containing water. In order to expand the application of SC-CS/OTA

421

and DC-CS/OTA/FeIII films, characteristics of SC-CS/OTA and DC-CS/OTA/FeIII hydrogels were

422

also explored. The initial pH value of the CS/OTA solution was ∼4 (Figure S6), which did not form

423

a hydrogel even after a Schiff base reaction. However, after adjusting the pH to 6 and initiating a

424

Schiff base reaction, the CS/OTA solution changed to a hydrogel. Because the Schiff base reaction

425

occurred between the chitosan and oxidized tannic acid at a pH of 6, this enhanced the cross-link

426

points.

427

For the DC-CS/OTA/FeIII hydrogel, the concentration of FeIII enormously affected its cross-

428

link density, which further had an influence on its mechanical performance levels. All the hydrogels

429

exhibited a gel-like character because their G' was greater than their G", and their tanδ < 1 (Figure

430

4A, B and C). Furthermore, the G' and G" of the DC-CS/OTA/FeIII hydrogel were higher than that

431

of the SC-CS/OTA hydrogel. One possible reason was that increasing the concentration of FeIII led

432

to a high cross-link density. However, the moduli G' and G" decreased with the concentration of

433

FeIII at more than 8.0%. A possible explanation for this phenomenon can be assigned to the high

434

concentration of FeIII, which resulted in less compact coordinate complexes.66 3+

Without Fe 3+ With 0.8% Fe 3+ With 1.0% Fe 3+ With 2.0% Fe 3+ With 4.0% Fe 3+ With 6.0% Fe 3+ With 8.0% Fe 3+ With 10.0% Fe 3+ With 15.0% Fe

A 10000 1000


B 1000 G'' (Pa)

3+

G' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

100

100

10

10

0

20

40

60

80

100

0

20

40

60

80

Angular Frequency (rad/s)


24


100

Page 25 of 37

3+


0.30 0.25 0.20

D 0.025

3+


0.020 Stress (MPa)

C 0.35 Tan

0.015 0.010

0.15

0.005 0.10

0.000

0.05 0

20

40

60

80

100

0

20

40

60

F

E

Before

Compress

80

100

Strain (%)


Recovery

0.25

Oxidized tannic acid pH=1 1 h Oxidized tannic acid pH=1 2 h Oxidized tannic acid pH=1 4 h Oxidized tannic acid pH=6 1 h Oxidized tannic acid pH=6 2 h Oxidized tannic acid pH=6 4 h

0.20

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

0.15 0.10 0.05 0.00

0

20

40

60

80

Strain (%)

Figure 4. Storage G′ (A), loss G″ (B), tanδ (C) moduli function as time of SC-CS/OTA composite hydrogel with angular frequency sweep, and the compress stress−strain curves (D) of hydrogel with different concentration FeIII, (E) Snapshots showing compression process of hydrogel with 4% FeIII. (F) The compress stress−strain curves of DC-CS/FeIII/OTA hydrogel soaking in different pH of oxidized tannic acid for 1, 2, and 4 h. 435

Additionally, the compressive test for the SC-CS/OTA and DC-CS/OTA/FeIII hydrogels was

436

studied. Figure 4D presents the stress–strain curves for the hydrogels. The stress for the SC-CS/OTA

437

hydrogel was relatively lower (0.003 MPa) than that of the DC-CS/OTA/FeIII hydrogels. Moreover,

438

the stress of the DC composite hydrogel with a concentration of FeIII of 4.0% was the highest (0.021

439

MPa), with a strain of approximately 63%. The reason for this may be due to the interactions of the

440

metal-coordination bonds between the FeIII ions and the -OH groups of oxidized tannic acid, which

441

enhanced the mechanical performances of the DC composite hydrogels. As shown in Figure 4E, 25


120


442

upon compression to 50%, the DC-CS/OTA/FeIII hydrogel can almost recover to its original state.

443

Meanwhile, it was found that the addition order of oxidized tannic acid was essential to the

444

compressive mechanical strength of the hydrogels. As shown in Figure 4F, the DC-CS/OTA/FeIII

445

hydrogel had the highest strength of 0.2 MPa when the pH value of oxidized tannic acid was 6 and

446

soaked for 2 h. Upon being immersed into the pH 6 oxidized tannic acid solution, the outermost

447

layer of the chitosan made preferential contact with the oxidized tannic acid molecules, resulting in

448

a nonuniform enhancement of the network strength. Once a dense DC-CS/OTA/FeIII layer is formed,

449

it is difficult for oxidized tannic acid molecules to diffuse into the inner network of the chitosan-

450

FeIII hydrogel. This means that the DC-CS/OTA/FeIII hydrogel has a higher tensile strength than the

451

as-prepared DC-CS/OTA/Fe III hydrogel.

452

The cyclic compressive tests of the DC-CS/OTA/FeIII hydrogels were also evaluated. A distinct

453

hysteresis occurred in the first loading and unloading curves, (Figure S7), and the area of the

454

hysteresis reflected the DC-CS/OTA/FeIII hydrogels’ ability to dissipate energy.67 The DC-

455

CS/OTA/FeIII hydrogels exhibited high energy dissipation during the compressive condition, which

456

mainly depended on the metal coordination interactions between OTA and FeIII. The DC-

457

CS/OTA/FeIII hydrogels could recover about 80% of their shape after the first loading and almost

458

100% during the rest of the four cycles, which suggested that the DC-CS/OTA/FeIII hydrogels had

459

excellent recovery properties.

460

The self-healing behaviors of the DC-CS/OTA/FeIII hydrogels were also assessed through

461

rheology measurements. The hydrogel could recover to its original modulus rapidly when the strain

462

was reverted from 100% back to 1% (Figure S8 A, B, and C), which indicated that the sol−gel

463

transition of the DC-CS/OTA/FeIII hydrogels is reversible. Even in the case of an oscillatory strain 26


Page 26 of 37

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Biomacromolecules

464

with a large amplitude (100%), the G' value was also higher than the G", indicating that the network

465

of the DC-CS/OTA/FeIII hydrogel had maintained its integrity. The DC-CS/OTA/FeIII hydrogel

466

exhibited recovery of both the G' and G" without delay when the strain was returned to 1%. Even

467

further, this process was recyclable. Thus, hydrogels with 2%, 4%, and 6% FeIII presented rapid

468

self-healing abilities.

469

4. Conclusions

470

In summary, SC-CS/OTA and DC-CS/OTA/FeIII composite films and hydrogels were prepared

471

and demonstrated improved mechanical properties and excellent water resistance. First, the carbonyl

472

group of oxidized tannic acid interacted with the amino group of chitosan chains through Schiff

473

base reactions. Second, oxidized tannic acid was cross-linked by FeIII via metal coordination bonds,

474

thus leading to the formation of double cross-linking films and hydrogels. The DC-CS/OTA/FeIII

475

composite films demonstrated high levels of mechanical strength and good water retention abilities

476

in addition to significant free radical scavenging abilities. Compared to traditional packaging

477

materials, the films we developed are biodegradable and biocompatible with super mechanical and

478

barrier properties. Simultaneously, they also exhibit excellent antioxidant properties. Moreover, the

479

DC-CS/OTA/FeIII hydrogels demonstrated mechanical tenability and rapid self-healing ability. The

480

developed DC composite films and hydrogels are therefore a promising biomaterial for widespread

481

application.

482

Supporting Information

483

WVP value of SC-CS/OTA composite films with different degree of oxidation. Color value,

484

mechanical properties, light transmittance, TGA of SC-CS/OTA composite films and DC-

485

CS/OTA/FeIII composite films. Photograph of oxidized tannic acid and chitosan-oxidized tannic 27



486

acid solution. SEM image of control films. Mechanical properties of SC-CS/OTA/FeIII composite

487

hydrogel.

488

Acknowledgements

489

This work was supported by the National Key R&D Program of China (Project No.

490

2018YFD0400701) and the Special Funds for Taishan Scholars Project of Shandong Province (No.

491

ts201712058).

492

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Biomacromolecules

Table of Contents

Laccase oxidization

Tannic acid

Oxidized tannic acid NH2 NH2

Chitosan

Schiff base reaction

First crosslinking SC-CS/OTA composite film FeIII

Metal coordination

Second crosslinking DC-CS/OTA/FeIII composite film

37


Double cross-linked chitosan composite films developed with oxidized

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