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Robust self-standing chitin nanofiber/nanowhisker hydrogels with designed surface charges and ultra-low mass content via gas phase coagulation Liang Liu, Rong Wang, Juan Yu, Jie Jiang, Ke Zheng, Lijiang Hu, Zhiguo Wang, and Yimin Fan Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b01278 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 13, 2016

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Fig 1 Fig 1. Characterization of chi 165x145mm (150 x 150 DPI)

ACS Paragon Plus Environment

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Fig 2 Fig 2. The formation scheme of 146x93mm (150 x 150 DPI)


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Fig 3 Fig 3. The photographs of a DE 165x76mm (150 x 150 DPI)


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Fig 4 Fig 4. The frequency and stres 164x122mm (150 x 150 DPI)


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Fig 5 Fig 5. The equilibrium swellin 146x52mm (150 x 150 DPI)


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Fig 6 Fig 6. The photographs of the 165x120mm (150 x 150 DPI)


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Fig 7 Fig 7. Cross-section SEM image 164x89mm (150 x 150 DPI)


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Fig 8 Fig 8. Nitrogen adsorption/des 136x129mm (150 x 150 DPI)


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Robust self-standing chitin nanofiber/nanowhisker

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hydrogels with designed surface charges and ultra-

3

low mass content via gas phase coagulation

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Liang Liu†, Rong Wang†, Juan Yu†, Jie Jiang†, Ke Zheng†, Lijiang Hu‡, Zhiguo Wang**§, Yimin

5

Fan*†

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† Jiangsu Key Lab of Biomass-based Green Fuel & Chemicals, College of Chemical

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Engineering, Nanjing Forestry University, Nanjing 210037, China

8

‡ Zhejiang Heye Health Technology Co., LTD, Dipu Town, Anji, Zhejiang, 313300, China

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§ Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, College of Light

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Industry Science and Engineering, Nanjing Forestry University, Nanjing 210037, China

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KEYWORDS: chitin nanofiber/nanowhisker hydrogel; pH-dependent; dye adsorption; surface

12

charge; deacetylation; TEMPO-oxidation

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ABSTRACT

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Partially deacetylated α-chitin nanofibers/nanowhiskers mixtures (DEChNs) and TEMPO-

3

oxidized α-chitin nanowhiskers (TOChNs) that had positive and negative charges, respectively,

4

were transformed into hydrogels with mass concentrations of 0.2, 0.4, 0.6, 0.8 and 1.0% under

5

ammonium hydroxide or hydrochloric acid “gas phase coagulation”. To the best of our

6

knowledge, 0.2% is the lowest mass content reported for the successful preparation of physical

7

self-standing hydrogels based on chitin nanofibers/nanowhiskers. The even and uniform

8

coagulation under “gas phase” is one of the key aspects of preparing hydrogels with quite low

9

mass content. The storage modulus achieved the highest value of 8.35 and 3.73 KPa for DEChN

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and TOChN hydrogels, respectively, at the mass concentration of 1.0%, and these are known to

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be the highest values reported in the literature for hydrogels at the same mass concentration of

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chitin nanofibers/nanowhiskers. The equilibrium swelling ratio (ESR) of both DEChN and

13

TOChN hydrogels decreased with increasing mass content at neutral pH. As the pH increased

14

from 2 to 10, the swelling degree of DEChN hydrogels decreased from 268 to 130, whereas the

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swelling degree of TOChN hydrogels increased from 128 to 242. Additionally, due to the

16

electrostatic attraction between the hydrogels and dyes, DEChN hydrogels had significant

17

adsorption of Reactive Blue 19, while TOChN hydrogels had effective adsorption of Basic Green

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4. The different pH-dependent swelling behavior and adsorption affinity of the DEChN and

19

TOChN hydrogels were related to their designed opposite surface charges corresponding to the

20

surface amino groups on the DEChNs and carboxyl groups on the TOChNs.

21 22


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Biomacromolecules

INTRODUCTION

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Chitin, which is a copolymer of β (1-4) linked, 2-amino-2-deoxy-D-glucan and 2-acetamido-2-

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deoxy-D-glucan, is known to be a biodegradable natural polymer with low toxicity. However, its

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insolubility in general organic solvents due to its rigid crystalline structure greatly restricts its

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applications1-4. When chitin is processed into hydrogels, membranes, scaffolds or any other

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forms, the use of specific solvents, such as LiCl/dimethylacetamide (DMAc), ionic liquids,

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strong alkali solutions, calcium chloride dehydrate-saturated methanol and aqueous alkaline

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systems (NaOH/Urea), is essential. However, these dissolution treatments always require special

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conditions and lead to the destruction of the crystal structure of chitins5-10. In contrast to the

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dissolution of chitin, chitin nanofibers/nanowhiskers with high crystallinity can be easily and

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stably dispersed in aqueous medium with high transparency and viscosity under acid, alkali or

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neutral conditions depending on their characteristics, and they become potential candidates for

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the preparation of hydrogels, membranes and any other assembled form of chitin

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nanofibers/nanowhiskers11-15.

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Hydrogels are water-swollen materials that maintain three-dimensional networks with the

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ability to absorb and retain a significant amount of water. There are two major types of

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hydrogels, synthetic and natural, depending on the origins5, 16. Chitin-based hydrogels belong to

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the natural hydrogels category and are considered to be some of the most promising materials

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because of their nontoxicity, biocompatibility and biodegradability, but the poor mechanical

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properties of chitin-based hydrogels have long been a problem17. Compared with dissolved

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chitin, chitin nanofibers/nanowhiskers have shown unique advantages, such as large surface area,

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high aspect ratio and entanglement, and high crystallinity for maintaining possibly high

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mechanical strength9, 18. These special properties make it possible to prepare tough, self-standing


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hydrogels based on chitin nanofibers/nanowhiskers. Furthermore, the complicated dissolution

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process of chitin can be avoided. Our previous study demonstrated a successful preparation of

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chitosan beads reinforced with chitin nanofibers, and the results showed that the addition of

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chitin nanofibers simultaneously improved the mechanical properties and the porosity of the

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composite beads, which seemed to be a contradiction19. J. Araki et al. also reported on anionic

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and cationic nanocomposite hydrogels reinforced with cellulose and chitin nanowhiskers. They

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found that all gels displayed an increase in Young’s modulus and a decrease in the swelling ratio

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with an increase in whisker content, and the presence of electrolytes was somewhat negligible

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for the preparation of composite gels20. Few studies to date have focused on chitin

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nanofiber/nanowhisker-based hydrogels. Thus far, sheet-like hydrogels processed from chitin

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nanofibers have been reported. Kentaro Abe et al. reported a method of preparing hydrogels

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based on β-chitin nanofibers via NaOH treatment, in which the chitin nanofiber suspension was

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diluted to 0.1 wt% and then vacuum-filtered and dewatered using a polytetrafluoroethylene

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membrane filter (0.1 µm mesh). Finally, the wet sheets obtained at a fiber content of

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approximately 15 wt% were immersed in NaOH solutions and kept at 25°C for 12 h9. However,

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this method has limitations, such as the time-consuming process and lack of uniformity of the

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hydrogels. Recently, a chitin nanofiber-based hydrogel was prepared through the neutralizing of

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acetic acid present in the aqueous chitin nanofiber dispersion by dialysis or addition of NaOH. In

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their study, the original chitin with 86-87% acetylation, which has sparsely free amino groups for

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surface charging, was used for chitin nanofiber and hydrogel preparation, and it was difficult to

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control and evenly neutralize the acetic acid by NaOH during the process21. Other kinds of

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hydrogels based on chitin nanofibers/whiskers have rarely been reported.


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In our previous studies, we prepared DEChN/chitosan composite beads with the assistance of a

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NaOH/ethanol coagulation bath. We also tried to prepare chitin nanofiber/nanowhisker-based

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hydrogels with this coagulation bath. However, the good fluidity of the chitin

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nanofibers/nanowhiskers

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nanofibers/nanowhiskers dispersions difficult to immerse in the NaOH/ethanol coagulation bath.

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After a series of experiments, we found that a “gas phase coagulation bath” was a good choice.

dispersions

at

relatively

low

concentration

made

chitin

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In this study, partial deacetylation or TEMPO-mediated oxidation was applied to prepare two

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types of chitin nanofibers/nanowhiskers with either surface amino or carboxyl groups; thus, the

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chitin nanofibers/nanowhiskers were designed to have opposite surface charges11, 12. Thereafter,

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a hydrogel was formed under “gas phase coagulation”. In detail, the partially deacetylated chitin

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nanofiber/nanowhisker mixture (DEChN) dispersion (original pH of 3-4) was treated with an

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ammonium hydroxide bath. Alternatively, the TEMPO-oxidized chitin nanowhisker (TOChN)

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dispersion (original pH of 7-8) was treated with a hydrochloric acid bath. The following three

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key aspects of this study should be noted here: (1) The surface functionalized (charged) chitin

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nanofibers/nanowhiskers were designed to prepare hydrogels to bring potential functionalization

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to the prepared hydrogels. (2) The “gas phase coagulation” gelation method was proposed to

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ensure sufficient and even coagulation and the uniformity of hydrogels. (3) The lowest (0.2%)

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chitin nanofiber/nanowhisker mass content was successfully transformed into hydrogels, and the

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highest storage modulus was achieved, compared to previous reports in the literature for chitin

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nanofiber/nanowhisker-based hydrogels at the same mass content.

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The gelation behavior of chitin nanofibers/nanowhiskers under “gas phase coagulation” as well

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as the structure morphology and the mechanical properties of the hydrogels were characterized

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by rheometer, scanning electron microscopy (SEM) and Brunauer-Emmet-Teller analysis (BET).


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Furthermore, due to the large number of amino groups in the DEChNs and carboxyl groups in

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the TOChNs, it is not surprising that these chitin nanofiber/nanowhisker-based hydrogels exhibit

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unique characteristics at different pH values. Accordingly, the pH sensitivity and the adsorption

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performance of electronegative Reactive Blue 19 and electropositive Basic Green 4 of the

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DEChN and TOChN hydrogels were also studied in this article.

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EXPERIMENTAL SECTION

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Materials. Chitin with a degree of N-acetylation (DNAc) of 91% was purified from

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swimming crab (Portunus trituberculatus) collected from Nantong, a seaside city in Jiangsu

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Province. Clear steps are described in detail in our previous article19 and are briefly summarized

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as the following steps: Crab shell wastes were soaked in 1 M HCl for 12 h, followed by

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treatment with 1 M NaOH for 12 h. These two steps were repeated three times for a full reaction.

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Then, the obtained residual solid was decolorized by immersing it in 0.5% (w/w) NaClO2, and

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the pH was adjusted to 5 using acetic acid. This suspension was heated for 2 h at 70°C. The

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purified alpha-chitin solid residues were obtained by centrifugation and stored at 4°C for further

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use. All other chemicals were used without any other purification.

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Nanofibrillation of chitin. Partially deacetylated chitin nanofibers/nanowhiskers mixtures

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(DEChNs) and TEMPO-oxidized chitin nanowhiskers (TOChNs) were prepared by the methods

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described in previous papers12, 22. In short, DEChNs were prepared as follows: purified chitin

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was deacetylated in 35% (w/w) NaOH solution at 90°C for 4 h. Then, the partially deacetylated

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chitin, with a degree of deacetylation of 28%, was washed with deionized water and stored at

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4°C until used. Finally, the partially deacetylated chitin was dispersed in distilled water at a

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concentration of 0.4% (w/v), and drops of acetic acid were added to adjust the pH to 3 under

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constant stirring. The obtained suspension was homogenized and treated with ultrasonication to


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fabricate nanofibers/nanowhiskers. After centrifugation, aqueous DEChN dispersion stock at pH

2

3-4 was prepared.

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TOChNs were prepared as follows: chitin (1 g) was suspended in water (100 ml) containing

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TEMPO (0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol). The oxidation of chitin was

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started by adding NaClO (5 ml, 7 mmol). The pH of the slurry was maintained at 10 at room

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temperature by continuous addition of 0.5 M NaOH using a pH-Stat titration system. When no

7

further consumption of the alkali was observed, the oxidation was quenched by adding a small

8

amount of ethanol. After the pH was adjusted to 7 with 0.5 M HCl, the mixture was washed with

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deionized water by repeated centrifugation. The water-insoluble fraction with a carboxyl group

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content of 0.5 mmol/g was obtained and stored at 4°C. For the preparation of TOChNs, the

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oxidized chitin was suspended in deionized water at a concentration of 0.4% (w/v), and the

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obtained suspension was then homogenized and treated with ultrasonication. After

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centrifugation, the TOChN dispersion stock at pH 5-8 was prepared.

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Preparation of hydrogels. To prepare hydrogels, the prepared DEChN and TOChN

15

dispersion stocks were concentrated or diluted to prepare chitin nanofibers/nanowhiskers

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dispersions with concentrations of 0.2, 0.4, 0.6, 0.8 and 1.0 wt%, which were assigned as

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DEChN-0.2, DEChN-0.4, DEChN-0.6, DEChN-0.8 and DEChN-1.0 or TOChN-0.2, TOChN-

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0.4, TOChN-0.6, TOChN-0.8 and TOChN-1.0 accordingly. Subsequently, the prepared chitin

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nanofibers/nanowhiskers dispersions were poured into a plastic beaker and placed in a larger

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vessel containing the appropriate ammonium hydroxide solution (for DEChN hydrogels) or

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hydrochloric acid solution (for TOChN hydrogels). After being maintained at room temperature

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for 12 h, all the chitin nanofibers/nanowhiskers dispersions were transformed into the

23

corresponding hydrogels, the pH of DEChN hydrogel was around 11.0 and the pH of TOChN


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hydrogel was around 1.0. Thereafter, the hydrogels were washed with aqueous ethanol (50 wt%)

2

to remove most of the residues, followed by immersion in distilled water until the wash

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supernatant was neutral.

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Characterization of chitin nanofiber/nanowhisker dispersions

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FTIR measurements. FTIR spectra were obtained by a Nicolet Antaris FTIR-ATR analyzer

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at room temperature. The samples were prepared with freeze-dried nanofiber/nanowhisker

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dispersion. All spectra were recorded from 400 to 4000 cm-1.

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Dynamic light scattering. DLS measurements were performed using a Zetasizer NanoSeries

9

instrument (Malvern Instruments Ltd., UK) equipped with a He–Ne laser (633 nm) at 25°C and

10

an angle of 173°. The data were analyzed using the Zetasizer software provided with the

11

instrument to determine the average particle size volume of the chitin nanofibers/nanowhiskers.

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Before analyzing, the chitin nanofibers/nanowhiskers dispersion with mass content of 0.1% was

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centrifuged at 10000 rpm to remove impurities.

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TEM observations. The morphology and fiber width of the chitin nanofibers/nanowhiskers

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were observed by transmission electron microscopy. A drop of the suspension (0.05 wt%) was

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deposited on electron microscope grids coated with a carbon-reinforced formvar film and was

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allowed to dry, followed by observation using a JEOL JEM-1400 electron microscope at an

18

acceleration voltage of 80 kV.

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Fluid property analysis. The rheological properties of the chitin nanofibers/nanowhiskers

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dispersions were characterized by the steady shear method using an RS6000 (HAAKE,

21

Germany) rheometer equipped with a Kegel C35/1° TiL and a cone plate C35/1° TiL at 25°C.

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The degassed chitin nanofibers/nanowhiskers dispersion of 200 µL was heated or cooled to the

23

desired temperature directly in the rheometer during the measurement.


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Biomacromolecules

Characterization of hydrogels

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Determination of mechanical properties. The samples were subjected to a strain sweep and

3

a frequency sweep test on the RS6000 (HAAKE, German) rheometer equipped with a Platte P20

4

TiL and cone plate P20 TiL. The modulus was recorded to define the linear viscoelastic zone in

5

which the modulus was independent of the applied strain23.

6

Dye adsorption studies. Adsorption experiments were performed as follows according to the

7

literature24, 25. First, 5 g of DEChNs and TOChNs with mass concentrations ranging from 0.2

8

wt% to 1.0 wt% were transformed into hydrogels under gas phase coagulation. The hydrogels

9

were washed with deionized water until the washing supernatant was neutral. Then, 50 ml

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solutions of Reactive Blue 19 (90 mg/L) were treated with DEChN hydrogels, and 50 ml

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solutions of Basic Green 4 (30 mg/L) were treated with TOChN hydrogels in 50 ml glass

12

beakers. The pH was carefully adjusted to 1.5 and 8.0 using HCl and NaOH, respectively. The

13

solutions were stirred for 24 h using a KS260 (IKA, Germany) shaker at room temperature.

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Finally, the solution was filtered, and the absorbance of dye in the solution was determined with

15

a UV-vis spectrophotometer.

16

Evaluation of swelling properties and pH sensitivity. The gravimetric method was

17

employed to measure the swelling ratios of hydrogels in distilled water at room temperature. The

18

pH of solutions was adjusted to 7 using HCl or NaOH. The equilibrium swelling ratio (ESR) was

19

calculated as

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ESR=Ws/Wd

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where Ws was the weight of the swollen hydrogel and Wd was the weight of the dried

22

hydrogel.


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The swelling degree (SD) at each pH value at room temperature was determined by following

2

the same method as used for measuring the equilibrium swelling ratio. In detail, after hydrogels

3

were prepared, the swelling solutions were adjusted to the desired pH value by addition of HCl

4

or NaOH. After immersion for 12 h to reach the swelling equilibrium, the hydrogels were

5

removed from the solution, and the excess moisture was gently removed with filter paper and

6

weighed. The SD was calculated as

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SD= Wt/Wo

8

where Wt was the weight of the hydrogel after reaching the swelling equilibrium and Wo was

9 10 11 12

the weight of the dried hydrogel26, 27. Characterization of aerogels After solvent exchange (by ethanol and tert-butyl alcohol), hydrogels were frozen at -78°C and vacuum-dried for 2 days at -50°C.

13

Scanning electron microscopy (SEM). The SEM images were obtained using a JEOL-JSM

14

7600F (JEOL, Tokyo, Japan) scanning electron microscope. Samples were coated with gold

15

before examination.

16

BET analysis. The BET analysis was conducted using a TriStar 3020 (Micromeritics, USA)

17

analyzer. Before the adsorption measurements, the samples were de-gassed under vacuum for at

18

least 8 h at 353 K.

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RESULTS AND DISCUSSION

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Chitin nanofibers/nanowhiskers with designed surface charges. First, two kinds of chitin

21

nanofibers/nanowhiskers with designed surface charges were prepared for hydrogel formation, as

22

shown in Fig 1. Partial deacetylation and TEMPO oxidization was applied to introduce different


10

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active groups, amino groups and carboxyl groups, to bring corresponding positive and negative

2

surface charges11, 12, respectively, as shown in Fig 1.b and Fig 1.g.

3 4

Fig 1. Characterization of chitin nanofibers/nanowhiskers with designed surface charges:

5

Photographs of DEChN dispersions (a) and TOChN dispersions (f) with concentrations from

6

0.2% to 1.0%; Reaction mechanism of partial deacetylation (b) and TEMPO-mediated oxidation

7

(g); Viscosity of DEChN dispersions (c) and TOChN dispersions (h) with concentration from

8

0.2% to 1.0%; TEM images of DEChN (d) and TOChN (i); FTIR spectra of DEChN (e) and

9

TOChN (j).


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When the partially deacetylated chitin was disintegrated under weak acid aqueous conditions,

2

the surface cationic (positive charged) amino groups ensured that the nanofibrillated DEChNs

3

remained stably dispersed in water due to the electrostatic repulsion with a zeta-potential of +48

4

mV (Fig 1.a). Based on the same basic theory, the surface anionic (negative charged) carboxyl

5

groups of TEMPO-oxidized chitin ensured the TOChNs were stably dispersed in water under

6

weak alkali conditions with a zeta-potential of -46 mV (Fig 1.f). The photographs in Fig 1.a and

7

Fig 1.f show transparent DEChN and TOChN chitin nanofiber/nanowhisker dispersions with

8

mass concentrations from 0.2% to 1.0%, and the corresponding viscosities are shown in Fig 1.c

9

and Fig 1.h. As the concentration increased, the viscosity of the DEChN dispersion or TOChN

10

dispersion increased correspondingly as a result of increasing crosslinking opportunities between

11

chitin nanofibers/nanowhiskers. Furthermore, at the same concentrations, DEChN dispersions

12

possessed a higher viscosity than TOChN dispersions within the shear ranges. This difference in

13

viscosity was caused by the different morphologies of DEChNs and TOChNs. The TEM images

14

of the chitin nanofibers/nanowhiskers shown in Fig 1.d and Fig 1.i clearly demonstrate the rod-

15

like morphology of the DEChNs compared to the TOChNs. The average size (by volume) of the

16

DEChNs and TOChNs was found to be 726.55±27.75 nm and 580.15±7.95 nm, respectively,

17

determined by DLS measurements. The average width of the DEChNs and TOChNs was

18

measured to be 8.8±1.25 nm and 11.5±2.1 nm by TEM analysis. The DEChNs showed a thinner

19

width and higher aspect ratio than the TOChNs, which is in accordance with the literature and is

20

one of the most important factors that influences the viscosity28. Fig 1.e and Fig 1.j exhibit the

21

FTIR spectra of the DEChNs and TOChNs, respectively, compared with the original chitin

22

sample. The band at 1030 cm-1 is due to the C-O stretching vibration of the chitin skeleton and

23

can be used as an internal standard, while the bands at 1740 cm-1 and 1560 cm-1 correspond to


12

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Biomacromolecules

1

carboxyl and amide II groups, respectively. Therefore, the absorption ratios of A1740/A1030 and

2

A1560/A1030 could represent the carboxylate content of TOChNs and the degree of DNAc of

3

DEChNs, respectively. From the spectra in Fig 1.e, the ratio between the absorption at 1560 and

4

1030 cm-1 decreased, which corresponded to the decrease in the degree of N-acetylation and

5

indicated the successful deacetylation of the DEChNs. In the spectra in Fig 1.j, an absorption

6

band appeared at 1740 cm-1 in the TOChN FT-IR spectra with no obvious change in the ratio of

7

A1560/A1030, which verified the existence of free carboxyl groups after TEMPO-mediated

8

oxidation without the influence of the DNAc11, 19, 36.

9

Gel formation by “gas phase coagulation”. Until now, sheet-like chitin nanofiber hydrogels

10

could be easily prepared through vacuum filtration using a membrane filter9, 17, 18. However, this

11

method had disadvantages such as high energy consumption and low efficiency, and more

12

importantly, it was difficult to control homogenous coagulation and formation of the hydrogel.

13

Therefore, finding a more convenient method of producing chitin nanofiber/nanowhisker-based

14

hydrogels has scientific and practical significance. In our previous studies, we prepared

15

DEChN/chitosan composite beads with the assistance of a NaOH/ethanol coagulation bath19. In

16

the process of fabricating physical DEChN/chitosan composite hydrogels, an alkaline

17

coagulation bath was necessary for neutralization of amino groups that lead to the disappearance

18

of ionic repulsions between polymer chains. The hydrogel was formed by the physical

19

crosslinking of chitin nanofibers/nanowhiskers and polymer chains29. This theory was also

20

suitable for the pure DEChN- and TOChN-based hydrogels. However, the good fluidity of chitin

21

nanofibers/nanowhiskers

22

nanofibers/nanowhiskers dispersions difficult to immerse in traditional coagulation baths, such

23

as sodium hydroxide solution. Recently, dialysis or dialysis combined with the addition of NaOH

dispersions

at

relatively

low


concentrations

made

chitin

13

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

Page 22 of 39

1

solution was used for preparing chitin nanofiber hydrogels. Hydrogels with a mass content of

2

0.4% were prepared successfully. However, this method was complex, and mechanical

3

properties of the hydrogels were limited due to the influence of dialysis21. After a series of

4

experiments, we found that a “gas phase coagulation bath” was a good choice, as shown in Fig 2.

5 6

Fig 2. The formation scheme of chitin nanofiber/nanowhisker-based hydrogels.

7

Fig 2 displays the detailed steps taken in preparing chitin nanofiber/nanowhisker-based

8

hydrogels. First, chitin nanofibers/nanowhiskers were dispersed evenly in aqueous solution due

9

to the electrostatic repulsion resulting from the cationic amino groups in the DEChNs under

10

weak acid conditions or the anionic carboxyl groups in the TOChNs under weak alkali

11

conditions11, 12. When the chitin nanofibers/nanowhiskers dispersions were treated with a gas

12

phase coagulation bath, the stability of the dispersion was broken. In the case of DEChN-based

13

hydrogels, the diffusion of volatile ammonia neutralized the acetic acid in the dispersion and the

14

surface amino cations of the DEChNs. Then, ionic repulsion between the DEChNs disappeared,


14

Page 23 of 39

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Biomacromolecules

1

and the hydrogel was formed by the physical crosslinking of the DEChNs. The formation of the

2

TOChN hydrogel was similar to that of the DEChN hydrogel; in this case, ammonia was

3

replaced by hydrochloric acid. The diffusion of volatile hydrochloric acid neutralized the alkali

4

in the dispersion and the surface carboxyl anions of the TOChNs, resulting in physical

5

crosslinking between the TOChNs and formation of the TOChN hydrogel. The greatest

6

advantage of “gas phase coagulation” was avoiding direct contact between the coagulation bath

7

solution and the chitin nanofibers/nanowhiskers dispersion. In addition, this method was easy to

8

use, and the choice of the coagulation bath was diverse; any volatile acidic or alkaline liquid

9

could be applied. To the best of our knowledge, this gas phase coagulation was proposed and

10

applied for the first time in this report.

11

After being treated with the gas phase coagulation bath for 12 h, transparent chitin

12

nanofibers/nanowhiskers dispersions turned into hydrogels, as shown in Fig 3. This phenomenon

13

also indicated that the sol-gel transition occurred in the chitin nanofibers/nanowhiskers

14

dispersions.

15

Fig 3.a and Fig 3.e show the photographs of self-standing and tough DEChN and TOChN

16

hydrogels, respectively, with a chitin nanofiber/nanowhisker concentration of 1.0%. When the

17

chitin was transformed to a hydrogel, almost no volume shrinkage was observed. The thickness

18

of the prepared hydrogels was approximately 5 mm, and both the DEChN and TOChN hydrogels

19

could easily bear the weight of 200 g without obvious deformation, as shown in Fig 3.c and Fig

20

3.g.


15

Biomacromolecules

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Page 24 of 39

1 2

Fig 3. The photographs of a DEChN hydrogel (a) and a TOChN hydrogel (e) and the

3

corresponding hydrogel bearing a 200 g weight (c) and (g), respectively; the adsorption of RB 19

4

(90 mg/L) into DEChN hydrogels (b, d) (contact time: 12 h, agitation speed: 150 rpm, pH 1.5)

5

and the adsorption of BG 4 (30 mg/L) into TOChN hydrogels (f, h) (contact time: 12 h, agitation

6

speed: 150 rpm, pH 8.0).

7

Additionally, an adsorption experiment was conducted for the removal of Reactive Blue 19

8

and Basic Green 4. Reactive Blue dyes, which are extensively used in textile industries, are water

9

soluble and anionic in nature, while Basic Green 4 is a cationic dye that is generally used for

10

dying cotton, wool, silk, and leather, among other things.25, 30. In general, the control of the

11

sorption performance of an adsorbent depends on three physical-chemical factors: first, the

12

nature of the adsorbent, such as its physical structure (porosity, particle size), its chemical

13

structure (ionic charge) and functional groups (variety, density); second, the chemistry and

14

accessibility of the adsorbate; and, finally, the solution condition, referring to its pH, ionic

15

strength, temperature and adsorbate concentration25. Among them, the pH of the dye solution


16

Page 25 of 39

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Biomacromolecules

1

played an important role in the whole adsorption process and particularly in the adsorption

2

capacity, influencing not only the surface of the adsorbent, the degree of ionization of the

3

material present in the solution and the dissociation of functional groups on the active sites of the

4

adsorbent but also the solution dye chemistry25. In this study, the adsorption experiment for RB

5

19 into DEChNs was conducted at pH 1.5. The DEChNs had positively charged amino groups at

6

low pH. Therefore, the electrostatic interaction between negatively charged –SO3- groups in the

7

Reactive Blue 19 molecule and positively charged –NH3+ groups in the DEChNs enhanced the

8

adsorption of RB 19 into the DEChN hydrogels. Meanwhile, the adsorption experiment for BG 4

9

into the TOChN hydrogel was conducted at pH 8. The TOChNs had negatively charged carboxyl

10

groups at high pH. The electrostatic interaction between positively charged BG 4 and negatively

11

charged TOChNs strengthened the dye adsorption. In this study, the influence of the adsorbent

12

mass of DEChN hydrogels on the sorption capacity of RB 19 was investigated with the initial

13

dye concentration of 90 mg/l at pH 1.5 with agitation (150 rpm) for 12 h. The results are reported

14

in Fig 3.b and Fig 3.d, showing that the percentage of dye removal increased with the adsorbent

15

dose and reached equilibrium value after 0.02 g (nanofiber/nanowhisker mass content) of

16

sorbent. The maximum RB 19 adsorption amount per gram of DEChN hydrogel reached 225

17

mg/g. Meanwhile, the adsorption of Basic Green 4 into TOChN hydrogels was studied with the

18

initial concentration of 30 mg/l at pH 8.0 with agitation (150 rpm) for 12 h. As shown in Fig 3.g

19

and Fig 3.h, almost all the dye was adsorbed after a dose of 0.03 g (nanowhisker mass content)

20

of sorbent. The maximum Basic Green 4 adsorption amount per gram of TOChN hydrogel

21

reached 50 mg/g. Both hydrogels showed effective adsorption of dyes and performed with

22

different affinities on the dye type due to their designed different surface charges.


17

Biomacromolecules

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Page 26 of 39

1

Mechanically strong and pH-sensitive hydrogels. The frequency sweep and stress sweep

2

measurement for the viscoelastic properties of the hydrogels was performed at room temperature.

3

The results are shown in Fig 4. The frequency sweep measurements of DEChN and TOChN

4

hydrogels are shown in Fig 4.a and Fig 4.b. The storage modulus (G’) and loss modulus (G”)

5

correspond to the elasticity and viscosity of materials, respectively31. The storage modulus (G’)

6

was a much larger value than the loss modulus (G”) for all hydrogels, which indicated a typical

7

hydrogel nature. The position and length of the linear viscoelastic region (LVR) indicated the

8

ability of the sample to resist flow as well as its ability over a range of stress32. LVRs for all the

9

samples are displayed in Fig 4.c and Fig 4.d. The storage modulus of DEChN and TOChN

10

hydrogels reached 120, 210, 400, 820, 8350 Pa and 150, 470, 1100, 3300, 3730 Pa, respectively,

11

with the nanofiber/nanowhisker mass concentration of 0.2, 0.4, 0.6, 0.8 and 1.0%. The large

12

increase in the storage modulus with the increasing nanofiber/nanowhisker mass concentration

13

suggested that the formation of a stronger hydrogel was the result of the increasing crosslinking

14

points between chitin nanofibers/nanowhiskers at higher content. Meanwhile, it was observed

15

that DEChN hydrogels had better mechanical properties than TOChN hydrogels at the same

16

concentrations. This finding indicated that longer chitin nanofibers/nanowhiskers with a higher

17

aspect ratio had a positive correlation with the elasticity of the hydrogel. To the best of our

18

knowledge, 0.2% is the lowest concentration at which physical chitin nanofiber/nanowhisker

19

hydrogels have been prepared, and the values for the storage modulus of DEChN hydrogels (8.35

20

KPa at 1.0%) and TOChN hydrogels (3.73 KPa at 1.0%) are the highest reported for chitin

21

nanofiber/nanowhisker-based hydrogels at the same concentration. Considering the absence of

22

any crosslinking agent in the preparation of these hydrogels, the good mechanical properties of

23

DEChN and TOChN hydrogels made them good candidates for exploring biomaterials.


18

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Biomacromolecules

1

Furthermore, the feature of having amino groups on DEChN hydrogels and carboxyl groups on

2

TOChN hydrogels could greatly extend their applications.

3 4

Fig 4. The frequency and stress sweep of DEChN- (a, b) and TOChN (c, d)-based hydrogels.

5

Moisture content is an important factor in hydrogel characteristics23. Fig 5.a displays the

6

equilibrium swelling ratio of DEChN and TOChN hydrogels at room temperature in distilled

7

water under neutral pH. All of the hydrogels contained significant amounts of water, as indicated

8

by their equilibrium swelling ratio (ESR). With the increase in nanofiber/nanowhisker mass

9

content, the ESR of DEChN and TOChN hydrogels decreased from 540 to 93 and 420 to 112,

10

respectively. Therefore, chitin nanofiber/nanowhisker-based hydrogels with different ESRs could


19

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

Page 28 of 39

1

be obtained for utilization in diverse applications by changing the chitin nanofiber/nanowhisker

2

concentration 23.

3 4

Fig 5. The equilibrium swelling ratio (ESR) of DEChN and TOChN hydrogels related to the

5

chitin nanofiber/nanowhisker concentration in distilled water (a) and the swelling degree of

6

DEChN (b) and TOChN (c) hydrogels as a function of pH at room temperature

7

The amino groups on DEChNs and carboxyl groups on TOChNs could imbue the hydrogels

8

with pH-sensitive properties. The pH sensitivities of these two types of hydrogels at the same

9

concentration (0.6%) were compared, and the results are shown in Fig 5.b and Fig 5.c. With an

10

increase in pH, the swelling degree of DEChN hydrogels decreased from 268 at pH 2 to 130 at

11

pH 10. Meanwhile, the swelling degree of TOChN hydrogels increased from 128 to 242 as the

12

pH increased from 2 to 10. Both hydrogels were pH dependent. This phenomenon can be

13

understood easily and logically. The -NH2 and -NH3+ groups on DEChN interchanged depending

14

on the pH of the medium. Amino groups were in their –NH2 form, and strong hydrogen bonding

15

was formed between the -OH and -NH2 groups in the alkaline medium, which impaired the

16

diffusion of water into its structure. As a result, the swelling degree was reduced. When the pH

17

decreased, large amounts of protons were adopted to form -NH3+ groups, leading to a significant


20

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Biomacromolecules

1

decrease in the number of hydrogen bonds. Due to the increase in the number of positively

2

charged -NH3+ groups, the electronic repulsion became dominant, which facilitated the diffusion

3

of water molecules into the network to swell the hydrogel. In contrast, in the case of the TOChN

4

hydrogels, the -COO- and -COOH functional groups interchanged. When the medium was acid,

5

carboxylic groups were in their -COOH form. Strong hydrogen bonding formed between the -

6

OH and -COOH groups, which restricted the diffusion of water. As a result, the swelling degree

7

was reduced. When the pH was higher, most of the -COOH groups were dissociated to form -

8

COO- groups, leading to a significant decrease in the number of hydrogen bonds. Due to the

9

increase in the number of negatively charged -COO- groups, the electrostatic repulsion became

10

dominant, which facilitated the diffusion of water molecules into the network to swell the

11

hydrogel26.

12

Nano-structured hydrogels. To observe the structure and morphology of hydrogels, the

13

DEChN and TOChN hydrogels were first freeze-dried to form aerogels, as shown in Fig 6. The

14

aerogels shown in Fig 6 were formed from DEChN and TOChN hydrogels with a

15

nanofiber/nanowhisker mass concentration of 1.0%. The aerogel was light enough to self-stand

16

on a thin leaf, and both the DEChN and TOChN aerogels were tough enough to bear a 200 g

17

weight, as shown in Fig 6a, 6b and Fig 6d, 6e.


21

Biomacromolecules

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Page 30 of 39

1 2

Fig 6. The photographs of the DEChN aerogel (a) and TOChN aerogel (d) at

3

nanofiber/nanowhisker mass concentration of 1.0% and the corresponding aerogel bearing a 200

4

g weight (b) and (e), respectively; SEM images of DEChN (c) and TOChN (f) aerogels.

5

The corresponding cross-section SEM images of DEChN- and TOChN-based aerogels at a

6

mass concentration of 1.0% are shown in Fig 6c and 6f. The images indicate that all the aerogels

7

were supported by fiber-like textures. These fiber structures had diameters of approximately 10

8

nm, and aggregation could be observed due to the aggregation and incomplete nanofibrillation of

9

chitin33. For more detailed information, the SEM images of DEChN- and TOChN-based aerogels

10

with different nanofiber/nanowhisker mass content were also imaged, and they are shown in Fig

11

7. The similar fibrillar network and fiber-like textured nanostructures were observed for all of the


22

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Biomacromolecules

1

aerogel samples, which were clearly different from the structures of the aerogel prepared from

2

the dissolved and regenerated chitin solutions34. Rich pore structures that had a size below 1 µm

3

could be observed in all the hydrogels, which was consistent with findings from a previous

4

article21.

5 6

Fig 7. Cross-section SEM images of DEChN and TOChN aerogels at different

7

nanofiber/nanowhisker mass concentrations; the number in the following abbreviation represents

8

the chitin nanofiber/nanowhisker mass concentration (%): DEChN-0.2 (a), DEChN-0.4 (b),

9

DEChN-0.6 (c), DEChN-0.8 (d), TOChN-0.2 (e), TOChN-0.4 (f), TOChN-0.6 (g), and TOChN-

10

0.8 (h).

11

The BET analysis was applied to further characterize the pore structure of aerogels, as shown

12

in Fig 8.a and Fig 8.c. The results showed that both DEChN and TOChN hydrogels exhibited

13

type III nitrogen adsorption-desorption isotherms that had prominent adsorption at high relative

14

pressures (P/Po) and indicated the macropore adsorption. The BET surface areas for DEChN-0.4,


23

Biomacromolecules

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Page 32 of 39

1

DEChN-0.6, DEChN-1.0 and TOChN-0.4, TOChN-0.6, TOChN-1.0 were 115.40, 114.47,

2

141.96 m2g-1, and 166.05, 189.17, 172.74 m2g-1, respectively. As the concentration increased, a

3

slight increase in the specific surface area could be observed in the hydrogels. Compared to

4

DEChN aerogels, TOChN aerogels exhibited higher surface area at the same concentration. The

5

pore size distribution (Fig 8.b and Fig 8.d) calculated by the BJH method showed that DEChN

6

aerogels had a size distribution from 3 nm to 13 nm and that TOChN aerogels had a size

7

distribution from 15 nm to 25 nm, which was similar to a cellulose nanofiber aerogel35.

8 9 10

Fig 8. Nitrogen adsorption/desorption isotherms of DEChN hydrogels (a) and TOChN hydrogels (c) and pore size distribution of DEChN hydrogels (b) and TOChN hydrogels (d).


24

Page 33 of 39

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1

Biomacromolecules

CONCLUSIONS

2

Partially deacetylated α-chitin nanofibers/nanowhiskers mixtures (DEChNs) and TEMPO-

3

oxidized α-chitin nanowhiskers (TOChNs) that had positive and negative charges, respectively,

4

were transformed into hydrogels with chitin nanofiber/nanowhisker content of 0.2, 0.4, 0.6, 0.8

5

and 1.0% under “gas phase coagulation”. The prepared hydrogels had the following

6

characteristics: (1) The self-standing physical hydrogels could be formed with an ultra-low mass

7

content of 0.2% of chitin nanofibers/nanowhiskers. In addition, the hydrogels were tough and

8

achieved the highest storage modulus, 8.35 KPa for DEChN and 3.73 KPa for TOChN, at a mass

9

content of 1.0%, which is known to be the highest reported value compared to previously

10

reported values for chitin nanofiber/nanowhisker-based hydrogels at the same mass

11

concentration. (2) The chitin nanofibers/nanowhiskers with different surface functional groups

12

and opposite surface charges were designed to form hydrogels with unique surface electrical

13

properties. Several experiments verified the effects of surface charges on the hydrogels: I.

14

different dye adsorption affinities: DEChN hydrogels had a significant effect on the adsorption

15

of Reactive Blue 19, and TOChN hydrogels had effective adsorption of Basic Green 4 due to the

16

electrostatic attraction between the hydrogels and dyes. II. pH-dependent swelling behaviors: as

17

pH increased, the swelling degree of DEChN hydrogels decreased (268 at pH 2, 130 at pH 10).

18

Conversely, the swelling degree of TOChN hydrogels increased from 128 at pH 2 to 242 at pH

19

10 as a function of amino groups on DEChN and carboxyl groups on TOChN. The “gas phase

20

coagulation”

21

nanofiber/nanowhisker-based hydrogels with the above unique characteristics. Additionally, the

22

absence of any crosslinker in the preparation of the hydrogels and the feature of having opposite

23

charges on the DEChN and TOChN hydrogels, along with the simple fabrication process,

was

one

of

the

key

aspects

of

uniformly


forming

these

chitin

25

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

Page 34 of 39

1

excellent mechanical strength and special surface properties of DEChN and TOChN hydrogels,

2

make the prepared hydrogels good candidates for a wide range of applications.

3

AUTHOR INFORMATION

4

Corresponding Author

5

*E-mail: [email protected].

6

**E-mail: [email protected].

7

Notes

8

The authors declare no competing financial interest.

9

ACKNOWLEDGEMENTS

10

This research was supported by the National Forestry Public Welfare Industry Research

11

Project (201304609), the National Natural Science Foundation of China (31100426), the Natural

12

Science Foundation of the Jiangsu Higher Education Institutions of China (12KJA220001),

13

Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology (201503), the

14

Specialized Research Fund for the Doctoral Program of the Higher Education of China

15

(20133204110008) and the Priority Academic Program Development of Jiangsu Higher

16

Education Institutions (PAPD).

17

REFERENCES

18

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21

Critical Reviews in Food Science & Nutrition 2003, 43, (1), 61-87.

22

(3) Rinaudo, M., Chitin and Chitosan—General Properties and Applications. Cheminform 2007,


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For Table of Contents Use Only

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Title: Robust self-standing chitin nanofiber/nanowhisker hydrogels with designed surface

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charges and ultra-low mass content via gas phase coagulation

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Liang Liu, Rong Wang, Juan Yu, Lijiang Hu, Zhiguo Wang, Yimin Fan

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Robust Self-Standing Chitin Nanofiber ... - ACS Publications

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