Reduction of Silver Ions Using an Alkaline Cellulose Dope

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Reduction of Silver Ions Using an Alkaline Cellulose Dope: Straightforward Access to Ag/ZnO Decorated Cellulose Nanocomposite Film with Enhanced Antibacterial Activities Feiya Fu, Jiayuan Gu, Jinfeng Cao, Rongsheng Shen, Hanxiang Liu, Yanyan Zhang, XiangDong Liu, and Jinping Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03059 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Sustainable Chemistry & Engineering


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Reduction of Silver Ions Using an Alkaline Cellulose Dope:

2

Straightforward

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Nanocomposite Film with Enhanced Antibacterial Activities

Access

to

Ag/ZnO

Decorated

Cellulose

4 5

Feiya Fu,†‡ Jiayuan Gu,† Jinfeng Cao,§ Rongsheng Shen,† Hanxiang Liu,† Yanyan

6

Zhang, † Xiangdong Liu, *†‡ and Jinping Zhou§

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†

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Sci-Tech University, Xiasha Higher Education Park 2 Avenue-5, Hangzhou 310018, P.

Department of Materials Engineering, College of Materials and Textile, Zhejiang

10

R. China.

11

‡

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Zhejiang Sci-Tech University, Ministry of Education, Xiasha Higher Education Park 2

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Avenue-5, Hangzhou 310018, P. R. China.

14

§

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of Education, Wuhan University, Bayi Road 299, Wuhan 430072, China.

Key Laboratory of Advanced Textile Materials and Manufacturing Technology,

Department of Chemistry and Key Laboratory of Biomedical Polymers of Ministry

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*Corresponding Author: Tel: +86-571-86843785, E-mail: [email protected]

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ACS Paragon Plus Environment ACS Paragon Plus Environment


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ACS Sustainable Chemistry & Engineering 1

ABSTRACT: This work proposed a straightforward and sustainable strategy to

2

synthesize Ag/ZnO decorated cellulose nanocomposite. Firstly, zincate was

3

introduced into an aqueous cellulose-NaOH/urea dope to enhance the solution

4

stability. Secondly, AgNO3 was directly added into the cellulose dope without gelation,

5

and then was reduced into Ag NPs by cellulose chain. Finally, zincate transformed

6

into ZnO via biomimetic mineralization along with the regeneration of the cellulose

7

dope, and an Ag/ZnO decorated cellulose nanocomposite film was synthesized. The

8

Ag NPs with a mean diameter of 16.5 nm were well dispersed in the cellulose matrix,

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and some were doped on the surface of the embedded ZnO crystal. With increase of

10

the Ag loading, the nanocomposite film showed more rapid sterilization for E. coli

11

than S. aureus, and complete eradication could be achieved within 3 h for both

12

bacteria. This simple and environmental friendly method hopefully provided new

13

routes for large-scale production of antibacterial cellulose-based nanohybrids in

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industry.

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KEYWORDS: Cellulose dope, Aqueous NaOH/urea/zincate solvent, In-situ

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reduction, Ag nanoparticles, Antibacterial activity

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INTRODUCTION

2

Cellulose is the most abundant polysaccharide material on earth.[1] It has been wildly

3

used in textile, packaging and healthcare fields, due to its low-cost, biocompatibility,

4

and good mechanical properties.[2,3] However, pure cellulose is lack of antibacterial

5

activity. Moreover, it can be served as a breeding ground for bacteria because of its

6

good absorbability for people's sweat and sebum. This will greatly threaten human

7

health. Thus, developing cellulose products with good antibacterial activity is

8

essential and currently of great interest.[4,5]

9

Over the past decades, incorporating inorganic metal NPs into cellulose materials

10

has become a common method to enhance its antimicrobial activities.[6] For instance,

11

noble metal (Ag, Au),[4,7] Cu[8] and ZnO NPs,[9] were identified as preferred

12

antibacterial additives. In particular, significant focus has been devoted to Ag due to

13

its strong antimicrobial activity against bacteria, fungi, protozoa, and viruses.[10, 11]

14

Currently, tremendous efforts have been devoted to fabricate antibacterial

15

cellulose-Ag nanocomposites. For example, Shao et al. developed a simple blending

16

synthesis for bacterial cellulose-Ag nanocomposites, which displayed excellent

17

antibacterial performance for S. aureus and C. albicans.[12] Borkowski et al. prepared

18

antibacterial SiC/Ag/cellulose nanocomposites using hydrazine as reductant.[13] Yan et

19

al. reported the in-situ synthesis of Ag NPs (10-30 nm) on nano- and micro- fibrillar

20

cellulose via a spray technique, and it showed a strong ability to kill E. coil.[14] For the

21

conventional processing methods, toxic reducing agents or high temperature are

22

usually required.[14] In addition, extra capping or dispersing agents are generally 3



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necessary to obtain uniform and stable dispersions of NPs. Therefore, constructing

2

antibacterial cellulose-Ag nanocomposites using cost-effective, environmentally safe

3

reagents is in high demand in various fields.

4

Recently, the cost-efficient synthesis of noble metal nanohybrids using cellulose

5

as a green reducing agent has become a promising strategy.[15,

16]

6

Thielemans et al. reported that Pd NPs could be prepared from (Pd(hfac)2) with

7

cellulose nanocrystals (CNs) as both the reducing agent and support material.[17]

8

Moores et al. proved that CNs could serve as a reductant and support for synthesizing

9

Ag NPs.[18] However, the low volume output of CNs and their complicated surface

10

functionalization procedure are unfavorable for their practical applications. Most

11

recently, Han et al. developed a novel strategy to prepare Ag NP doped cellulose

12

microgels.[19] Briefly, cellulose was first dissolved in a NaOH/urea/H2O solvent to

13

obtain the cellulose gel. Then, under alkali conditions, Ag+ could be reduced into Ag0

14

by the cellulose microgels without using any reductant or heating treatment. This

15

method was cost-efficient and sustainable, however, as far as we know, there was no

16

report for direct reduction of AgNO3 in a concentrated cellulose dope dissolved in

17

aqueous NaOH/urea system. That was probably because the reaction would occur

18

between AgNO3 and NaOH in the cellulose solution. The reaction could destroy the

19

“inclusion complex” consisted of NaOH and urea, leading to the gelation of the

20

cellulose dope.[20, 21]

For example,

21

In our previous work, it had been proved that zincate could form stronger

22

hydrogen bonds with cellulose chain, which was very beneficial for enhancing the 4


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stability of cellulose dope in aqueous NaOH solvent.[22,

2

multifunctional cellulose-ZnO nanohybrid could be obtained through a biomimetic

3

mineralization method.[24,

4

strategy to prepare Ag/ZnO decorated cellulose nanocomposite from a aqueous

5

cellulose-NaOH/urea/zincate dope by adding AgNO3. The added zincate in the

6

solvent could significantly delay the gelation of the mixture system. This made it

7

feasible for reduction of silver ions in a cellulose dope. Because of the good

8

dispersion of Ag NPs in the cellulose matrix, the as-prepared nanocomposite showed

9

good mechanical properties. In particular, this nanocomposite exhibited excellent

10

antibacterial activity and stability against S. aureus and E. coli. Owing to the

11

simplicity, low-cost and sustainability characteristics, the present pathway is suitable

12

for large-scale production of functional Ag/ZnO@cellulose nanohybrids.

25]

23]

Additionally,

In the present work, we proposed a straightforward

13

14

EXPERIMENTAL SECTION

15

Material. The cotton cellulose pulp was purchased from Hubei Chemical Fiber

16

Co. Ltd., (Xiangyang, China). Its viscosity average molecular weight (Mη) was 10.2 ×

17

104, which was measured at 25 °C and calculated from [η] according to the

18

Mark-Houwink equation.[26] NaOH, AgNO3 and other reagents were purchased from

19

Aladdin Industrial Inc.

20

Preparation of the Nanocomposite Film. The 7 wt% NaOH/ 12 wt% urea/ 0.8

21

wt% zincate aqueous solution was prepared according to the work and was precooled 5



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to -12 °C.[23] Then, a desired amount of cellulose (20 g) was added to the precooled

2

dissolving solvent (500 g) and stirred vigorously for 3 min to obtain a transparent

3

solution. Subsequently, 0.216 g of AgNO3 dissolved in a little water was added to the

4

cellulose solution to obtain the AgNO3-cellulose mixture solution system. After

5

storing the mixture solution system for a given time at 15 °C, the resultant mixtures

6

were spread onto a glass plate as a 0.5 mm thick layer and coagulated by glycol.

7

Finally, the coagulated films were washed by distilled water and dried at room

8

temperature to obtain the nanocomposite film. The films were coded AZ@RC-12 and

9

AZ@RC-24 when the storage time (or reaction time) of the mixture solution was 12

10

and 24 h, respectively. The nanocomposite film coagulated immediately from the

11

cellulose dope after adding AgNO3 was coded AZ@RC-0.

12

Antibacterial Performance. The antibacterial characteristics of the films were

13

tested against S. aureus and E. coli. The concentrations of the bacterium were

14

approximately 1.0×108 CFU/mL and were all CMCC. The disc diffusion method was

15

performed using Luria-Bertanimedium solid agar. The films and a filter paper with 10

16

mm in diameter were sterilized by UV irradiation and placed on cultured agar plates

17

coated with bacteria. The plates were placed at 37 °C for 24 h, and a bacterial

18

inhibition zone formed around the film. The width of the inhibition zone was

19

calculated according to previous work.[25] For each experiment, the samples were

20

illuminated with a white light source (28 W), emitting an average light intensity of

21

3750 lx. In control experiments, the samples were stored under dark conditions for the

22

same exposure times. 6


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Furthermore, a live/dead bacterial assay was performed to examine the viability

2

of the bacteria before and after the nanocomposite film treatment. A S. aureus bacteria

3

solution was mixed with 0.5 mL of a dye solution containing 1 µM SYTO 9 and 5 µM

4

propidium iodide (Invitrogen) for 20 min at room temperature. The bacteria were then

5

imaged using confocal microscopy (TCS SP5, Leica).

6

In the colony-forming count method, one piece of the nanocomposite film (1.0

7

cm×1.0 cm, 0.013 g) was immersed in 50 mL of the bacterial suspension. The

8

solution was then heated at 37 °C in a shaking incubator. The number of bacteria was

9

obtained using the surface spread plate method at 0, 1, 3, and 5 h intervals.

10

The laundering durability of the film was also tested as follows. The AZ@RC-24

11

film (1.0 cm×1.0 cm, 0.013 g) was washed by 2 wt% sodium dodecanesulfonate in a

12

beaker with a 300 rpm stirring rate. Then, the resultant films were rinsed with

13

deionized water (10 ml×4 times) and dried at room temperature. The antimicrobial

14

test of the laundered films was evaluated by the colony-forming count method

15

described above. The antibacterial ratio was calculated as follows:[27]

16

Antibacterial ratio =

17

where Nm and Nc are the numbers of the surviving bacteria for agar plates in the

18

presence or absence of the composite film, respectively. The residual quantility of the

19

NPs in the AZ@RC film could be calculated as following:

20

Residual quantity of NPs = 1 −

21

Where, w and w0 is the ZnO or Ag content in the film before and after the laundering

22

durability test, the w and w0 were calculated from the result detected from an

Nc − Nm × 100% Nc

(1)

w w0

(2)

7



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ICP-OES (Intrepid XSP Radial, Thermo, USA).

2

Cell Cytotoxicity. The AZ@RC-24 film was cut into discs and sterilized under

3

UV light for 2 h. African green monkey SV40-transformed kidney fibroblast (COS-7)

4

cells were seeded at a density of 5×104 cells/well in a 24-well plate in DMEM

5

containing 10% FBS and incubated in a humidified atmosphere of 5% CO2 at 37 °C

6

for 24 h. Subsequently, the film was put into the 24-well plate at concentrations of 1.0,

7

5.0, 10, 50 and 100 mg mL-1, the hydrogel was removed after 48 h and the medium

8

was replaced with 200 µL of fresh medium. Then 20 µL of MTT solutions were added

9

for 4 h. After that, the medium was removed and 150 µL of DMSO was added and

10

mixed. The absorbance was measured at 570 nm using a microplate reader (BIORAD,

11

Model 550, USA). The sample without the nanocomposite was used as the control.

12

The relative cell viability was calculated as the following equation[28]:

13

Cell viability =

OD570(samples) × 100% OD570(control)

(3)

14

Where OD570(control) was obtained in the absence of samples and OD570(samples)

15

was obtained in the presence of film sample. Further, cells after incubation with the

16

AZ@RC film were rinsed twice with PBS solution. 200 µL PBS containing

17

calcein-AM (2 µМ) and ethidium homodimer-1 (4 µM) was added to each well for

18

staining cells at 37 °C under 5% CO2 for 30 min. The stained cells were observed by a

19

confocal microscopy (TCS SP5, Leica).

20

Characterization. The dynamic rheology measurement was carried out on an

21

ARES RFSIII rheometer (TA Instruments, USA). A parallel plate was used to measure

22

the shear storage modulus (G′) and loss modulus (G′′) as functions of temperature (T),

23

or time (t). The values of the strain amplitude were checked to ensure that all 8


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measurements were set as 10%, which was within a linear viscoelastic regime. For

2

each measurement, a fresh cellulose dope or mixture solution was prepared, and then

3

was poured into the parallel plate instrument.

4

XRD measurements were performed on an XRD diffractometer (D8-Advance,

5

Bruker, U.S.A.) with Cu-Kα radiation (λ=0.154 nm). The XRD data were collected in

6

the 2θ region from 6 to 80° at a scanning speed of 2°/min. The crystallinity index

7

(CI, %) of cellulose in the naohybrid films was calculated using the following

8

formula[29]:

9

CI =

I 020 − I am × 100% I 020

(4)

10

where I020 and Iam are the maximum intensity of the principal peak (020) lattice

11

diffraction (2θ = 21.7° for cellulose II) and amorphous cellulose diffraction (2θ = 16°

12

for cellulose II), respectively. X-ray photoelectronspectra (XPS) were recorded using

13

a VGE scalab 200 system with an aluminum anode (Al Kα = 1486.3 eV) operating at

14

510 W. The solid-state UV-vis diffuse reflectance spectra of the films were measured

15

on a PerkinElmer Hitachi U-4100H UV-Vis-NIR spectrometerin the range of 200-800

16

nm with a resolution of 1 nm.

17

SEM images were obtained using a field emission scanning electron microscope

18

(FE-SEM, Ultra 55, Zeiss, Germany). The wet films were frozen in liquid nitrogen,

19

immediately snapped and then, freeze-dried. All the samples were coated with gold

20

for the FESEM observations.TEM, high-resolution TEM (HRTEM) and selected area

21

electron diffraction (SAED) images were obtained using an FEI Tecnai F20 electron

22

microscopeat 200 kV. Ultrathin slices of the films were obtained by sectioning the 9



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films on an LKB-8800ultratome.

2

The thermal stability of the samples was analyzed on a Mettler-Toledo TGA

3

(TGA/SDTA851). Approximately 10 mg of each film sample was placed in a standard

4

aluminum pan and heated from ambient temperature to 800 °C at a heating rate of 10

5

°C/min under an air atmosphere. The samples were vacuum dried at 40 °C for 24 h

6

before the afore-mentioned measurements. The mechanical properties of the films

7

were evaluated in the dry state using a universal testing machine (KES-G1, Japan)

8

according to the ASTM Method D 882-88. The samples were maintained at 20 °C and

9

75% RH for 12 h before the testing was performed.

10 11

RESULTS AND DISCUSSION

12

Fabrication of Ag/ZnO NPs Decorated Cellulose Nanocomposite Films. To

13

investigate the influence of AgNO3 on the stability of cellulose dope in aqueous

14

NaOH/urea system, its G′ and G′′ as function of temperature and time were tested and

15

their crossover was set as the apparent gel point. As shown in Figure 1a, the gelation

16

temperature of the cellulose in aqueous NaOH/urea solvent (CNU) was 17.5 °C. With

17

introduction of 0.8 wt% zincate, the gelation temperature of the dissolved

18

cellulose-NaOH/urea/zincate (CNUZ) dope increased to 49.7 °C. Similarly, the

19

gelation time (Figure 1b) of the CNUZ system (124.6 min) was significantly longer

20

than that (18.5 min) of the CNU solution. After adding AgNO3, the gelation

21

temperature for CNUZ decreased to 46.1 °C, which was still larger than that of the

22

CNU dope (17.0 °C) (Figure 1a). In particular, as shown in Figure 1b, the gelation 10


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time of AgNO3-CNU mixture (73.8 min) was 12 times longer than that (6.2 min) of

2

the AgNO3-CNUZ mixture. Therefore, the addition of zincate was very helpful to

3

enhance the stability of mixed AgNO3-cellulose solution system, which could

4

guarantee the implement of the reduction process.

5

The entire procedure for preparing the composite film is illustrated in Figure 2a.

6

Firstly, cellulose was dissolved into a aqueous NaOH/urea/zincate solvent to form a

7

homogeneous solution (Figure 2b). Then, AgNO3 was directly added to the cellulose

8

solution for its reduction. As is known, cellulose was composed of β-(1,4)-linked

9

D-glucose units and contained three hydroxyl groups. The hydroxyl groups formed

10

complex inter- and intra-molecular hydrogen bonds in its aggregation state, exhibiting

11

a weak reduction capacity.[19, 30] However, when the cellulose pulp was dissolved, the

12

hydrogen-bonding network would be destroyed and plenty of reducing ends was

13

exposed, showing better reduction capacity.[30, 31] Therefore, as given in Figure 2c, the

14

color of the reaction mixture was light yellow at the beginning and gradually changed

15

to dark brown with increase of reduction time. This could be regarded as a signal of

16

the reducing process of Ag+ to Ag0.[32] After coagulation, the dissolved cellulose was

17

regenerated along with the attached Ag NPs. Meanwhile, the zincate transformed into

18

ZnO by the biomimetic mineralization of cellulose.[33] The films showed a bright and

19

even brown color, which indicated the homogeneous dispersion of the NPs in it

20

(Figure 2d). In addition, the films could be facilely folded in half, exhibiting good

21

flexibility. As determined by ICP-OES analysis, the Ag element and ZnO content in

22

the AZ@RC films was in the range of 0.14-0.38 wt% and 11.2-12.2 wt%, respectively 11



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(Table 1), when the reduction time varied from 0 to 24 h.

2

Structure and Morphology of the Films. The XRD patterns of the AZ@RC

3

film are shown in Figure 3a. All three AZ@RC films exhibited typical diffraction

4

peaks at 2θ = 12, 20, and 22°, which were ascribed to the (1 1 0), (110), and (020)

5

planes of cellulose II, respectively.[34] In addition, the three films showed diffraction

6

peaks at 2θ = 31.9, 34.5, 36.4, 47.6, 56.9, 62.9, 68.2°, and 69.3°, which corresponded

7

to the (100), (002), (101), (102), (110), (103), (112) and (201) planes of wurtzite ZnO

8

(ICDD card no. 36-1451), respectively.[33] This indicated the addition of AgNO3 did

9

not hinder the biomineralization of ZnO caused by cellulose. Compared to the

10

AZ@RC-0 film, two new peaks at 2θ = 37.9 and 77.0° emerged in the AZ@RC-12

11

film, which were characteristic of the (111) and (200) peaks, respectively, of cubic

12

silver (JCPDS 04-783).[35] With the increase of the reduction time, the peak intensity

13

of cubic silver in the AZ@RC-24 films slightly increased but was still not obvious.

14

This might be caused by the low Ag loading content in the AZ@RC films. However,

15

the corresponding peaks could be clearly observed in the composite film prepared by

16

adding double quality of AgNO3 in the cellulose solution. The results confirm the

17

successful reduction of Ag+ to Ag0 by the cellulose dope and the creation of Ag/ZnO

18

decorated cellulose nanohybrid using a facial strategy.

−

19

The UV-vis spectra of the AZ@RC nanohybrid films are shown in Figure 3b. The

20

AZ@RC-0 film only exhibited a strong absorption in the UV region from 300 to 350

21

nm, which was assigned to the absorption of the ZnO crystals.[24] With the increase in

22

the reduction time, a surface plasmon resonance (SPR) band at approximately 400 nm 12


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was observed for the AZ@RC-12 and AZ@RC-24 films.[19] In addition, the SPR band

2

in the AZ@RC-24 film underwent a slight red shift and was broader than that of the

3

AZ@RC-12 film. This implied that larger Ag NPs and a wider size distribution

4

probably occurred as the reduction time increased. The results matched well with the

5

XRD result and further proved the successful preparation of the Ag/ZnO NPs

6

decorated cellulose composite films.

7

To know more about the chemical composition of the AZ@RC nanocomposite

8

films, XPS tests were also performed. As shown in Figure 4a, AZ@RC-0 film

9

displayed two binding energy peaks at 367.5 eV and 373.5 eV, which should be

10

assigned to of Ag 3d5/2 and Ag 3d3/2, respectively.[36] However, in the spectrum of

11

AZ@RC-12 film (Figure 4b), the corresponding binding energies were located at

12

368.0 eV and 373.9 eV, respectively, showing an obvious negative shift. With the

13

reduction time increasing, the binding energy of Ag 3d hardly changed in the

14

AZ@RC-24 film (Figure 4c). Generally, the Ag 3d5/2 peaks for the oxidation states of

15

Ag (I) and Ag (0) were always in the ranges of 367.5-367.8 eV and 368.0-368.4 eV,

16

respectively.[36] Therefore, a minimal amount of Ag+ should mainly exist as Ag2O in

17

the AZ@RC-0 film. As the reduction time reached 12-24 h, Ag+ was successfully

18

reduced into Ag0 in the AZ@RC-12 and AZ@RC-24 films. In the case of the Zn 2p

19

spectra (Figure 4d-4f), the peaks at 1022.6, 1022.5 and 1022.0 eV corresponded to the

20

Zn 2p3/2 orbit, and the peaks at 1045.7, 1045.6and 1045.1 eV were attributed to Zn

21

2p1/2 orbit.[37] In addition, the observed spin-orbit splitting between Zn 2p3/2 and

22

2p1/2 was approximately 23 eV, which indicated a normal state Zn2+ of ZnO in the 13



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AZ@RC films. Additionally, the binding energy of Zn 2p shifted to a lower binding

2

energy with prolong of the reduction time. This possibly indicated some Ag NPs were

3

deposited on the surface of ZnO and that electron transfer occurred from Ag to the

4

conduction band in ZnO.[37]

5

Figure 5 shows FE-SEM images of the AZ@RC nanocomposite film. Because of

6

the liquid-liquid demixing during the coagulation process, both the surface and

7

cross-section of the cellulose matrix in AZ@RC film exhibited a porous structure. As

8

expected, flower-like ZnO crystals were found on the surface and inside of the

9

nanocomposite. In addition, since the pore structure of the cellulose scaffold could

10

control the growth of the crystal, the ZnO crystal inside the films were smaller

11

(223.9-284.5nm) than those on the surface (380.7-429.2nm).[38] However, as

12

illustrated in Figure 5a, apart from the obvious ZnO particles, only little NPs with a

13

mean size of 18.6 nm were observed in the AZ@RC-0 film, and the mostly should be

14

the Ag2O crystals, as proven by the XPS test. For the AZ@RC-12 and AZ@RC-24

15

films, some spherical Ag NPs with the mean sizes of 20.2 and 26.3 nm emerged and

16

uniformly dispersed in the cellulose matrix, respectively.

17

To further investigate the microstructure of the nanocomposite, TEM

18

observations combined with HRTEM and SAED were performed for the AZ@RC-24

19

film. Figure 6a shows the uneven dispersion of the ZnO particles with large diameters

20

of 98.3-760.8 nm in the cellulose matrix. The size of some ZnO crystals in the TEM

21

images was relatively larger than that in the SEM images. This might be caused by the

22

sectioning procedure during the preparation of the ultrathin slices for the TEM test, 14


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which damaged the structure of the ZnO crystal. In contrast, as illustrated in Figure 6a

2

and 6b, spherical Ag NPs were well dispersed in the cellulose matrix and little

3

aggregation occurred. The statistical histogram (inset in Figure 6b) shows that the

4

sizes of the Ag NPs were mainly in the range of 2.6 to 32.9 nm with a mean size of

5

16.5 nm, which was larger than that (7.3 nm) in AZ@RC-12 film (Figure S1). In

6

addition, it was noted that some Ag NPs were doped on the surface of the ZnO

7

particles, which could lead to a metal-semiconductor oxide heterostructure. This

8

phenomenon matched well with the XPS results. The HRTEM analysis shows that the

9

ZnO NPs in the film had lattice spacings of 1.93 A° and 2.47 A ° (Figure 6c), which

10

corresponded to the (102) and (101) planes of wurtzite ZnO.[39] In addition, the lattice

11

fringes with an interplanar spacing of 0.236 nm were observed and ascribed to the

12

(111) planes cubic Ag (Figure 6d).[39] Moreover, the SAED pattern of ZnO and Ag

13

crystal exhibited spotty diffraction rings in Figure 6c and diffraction spots in Figure

14

6d, which confirmed the synthesized NPs mediated by cellulose were well

15

crystallized.[40]

16

Physical Properties of the Films. The stress-strain curves of the AZ@RC

17

nanocomposite films are shown in Figure 7. The tensile strength (σb), elongation at

18

break (εb) and Young’s modulus (E) of the AZ@RC-0 film were 45.1 MPa, 4.3% and

19

2.0 GPa, respectively. As the Ag+ (Ag2O) was reduced to Ag NPs, the mechanical

20

properties of the AZ@RC films were significantly improved. In particular, the σb, εb

21

and E values of the AZ@RC-24 film reached 55.2 MPa, 4.7% and 3.6 GPa,

22

respectively. The values were comparable to those of the CN@cellulose 15



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


nanocomposite (σb = 54.2 MPa, εb = 4.8% and E = 3.7 GPa).[41] As summered in

2

Table 1, the crystallinity index of cellulose II for the different AZ@RC had little

3

change (23-25%). Thus, high strength of the nanocomposite should be attributed to

4

the uniform distribution of Ag NPs in the cellulose matrix and their electrostatic

5

interactions formed at the interfaces.

6

The thermal decomposition of the composite films was investigated by TGA

7

under an air atmosphere. As shown in Figure 7b, for all the AZ@RC films, an initial

8

weight loss at a temperature below 150°C was observed due to the evaporation of

9

adsorbed moisture.[42] Also, the first weight loss step (337-340 °C), which was

10

attributed to the onset of cellulose decomposition, almost unchanged. The values were

11

very similar to that of the pure RC film (332 °C)[22] and ZnO-RC nanocomposite (336

12

°C).[33] However, with loading of more Ag NPs, the complete degradation temperature

13

markedly decreased from 564 to 486 °C, which was obviously lower than that of the

14

RC film (690 °C)[22] and ZnO/RC composite film (620 °C).[33] This phenomenon

15

should be attributed to the strong catalytic capacity of the Ag NPs, which accelerated

16

the breakdown of carbon skeleton.[43] The residual weight for AZ@RC-0, AZ@RC-12

17

and AZ@RC-24 were 12.5, 12.1 and 11.8 wt%, respectively. The values were

18

basically similar with the sum of the NPs determined by ICP-MS, and its gradual

19

decreasing trend should be caused by the decreased loading content of ZnO in films.

20

Antibacterial Activity and Cytotoxicity. Figure 8a-8h shows the inhibition zone

21

test for the films against E. coli and S. aureus in light. The filter paper did not indicate

22

any detectable inhibition zones. In contrast, all the AZ@RC films indicated significant 16


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inhibition zones. As summarized in Table 1, the width of inhibition zones (Winh)

2

around AZ@RC-10 against S. aureus (3.0 mm) was larger than that for E. coli (2.7

3

mm). With the increase of the Ag content, the Winh values gradually increased for both

4

bacteria. Especially, the Winh values grew faster for E. coli compared to S. aureus. To

5

better observe the antibacterial effect, the S. aureus bacteria before and after contact

6

with the AZ@RC-24 film were investigated by SEM. As shown in Figure 8i, the S.

7

aureus showed smooth bodies before contact with the film. In contrast, cellular

8

deformation and surface collapse were found after exposure to the film for 2 h (Figure

9

8j). In addition, as proved by the corresponding fluorescence images, the S. aureus

10

changed from green to red after the settlement, which further indicated disruption in

11

the bacterial cell wall and its membrane.

12

The antibacterial properties of the AZ@RC nanocomposite films were also

13

quantitatively evaluated using the colony-forming count method. As shown in Figure

14

9a and 9b, the number of viable bacteria for filter paper was almost constant.

15

However, within 1 h of exposure, a 5.7-log, 7-log and 7.1-log reduction in the viable

16

bacteria of S. aureus were observed for the AZ@RC-0, AZ@RC-12 and

17

AZ@RC-24films (Figure 9a), respectively. After 3 h, nearly 100% reduction was

18

reached for both the AZ@RC-12 and AZ@RC-24 films. After 5 h of exposure, the S.

19

aureus were completely eradicated. Similarly, the number of viable E. coli also

20

showed a remarkable decrease with the increase of exposure time, and the bacteria

21

were also eliminated within 5 h for all three films. Notably, AZ@RC-0 film showed a

22

more rapid sterilization for S. aureus than E. coli while the situation was the reverse 17



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Page 18 of 41


for the AZ@RC-24 film. This phenomenon was basically consistent with the

2

inhibition zone test. In the literature, recyclability concerns have been raised

3

regarding developing antibacterial reagents.[6] For example, to achieve slow release of

4

the silver ion, a core-shell GO-Ag NPs/BC microfibers was fabricated using a

5

micro-fluidic

6

microfibers decreased to 70-80% for either E. coli or S.aureus after 6 washing cycles

7

of reuse. However, as illustrated in Figure 9c, the antibacterial ratio of the

8

AZ@RC-24 films was nearly invariable (99.6%) for both S. aureus and E. coli after

9

70 laundering cycles. Additionally, the durability of the inorganic NPs in the film was

10

also investigated in Figure 9d. The residual quantity of ZnO showed a slow decline

11

with the increase of the washing cycles, and 91.2% of the ZnO content was

12

maintained after 70 washing cycles. In contrast, the corresponding value of the Ag

13

NPs quickly dropped and decreased to 63.3% after identical washing cycles. This

14

should result from the significantly smaller diameter of Ag NPs compared to that of

15

the ZnO particles, which made it easier to escape from the cellulose matrix. However,

16

the durability of the Ag NPs in the AZ@RC film was still superior to that of the

17

cotton/Ag NPs modified by a chitosan derivative binder (37.6% remained after 30

18

laundering cycles).[27] Here, as demonstrated in the SEM and TEM images, the Ag

19

NPs anchored on the surface and inner part of the cellulose matrix, and the porous

20

cellulose assisted with regulating the fall off rate of the Ag NPs.

wet-spinning

device.[44]

However,

antimicrobial

efficiency

of

21

It was known that ZnO was a photocatalyst material. Thus, a contrastive study on

22

the antibacterial activity of AZ@RC film was conducted under light and darkness. As 18


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shown in Figure S2, the Winh values for both the AZ@RC-12 and AZ@RC-24 film

2

against E. coli were larger when exposed to a white light source (28 W) than that

3

under darkness. However, the quantitative assessment proved that the E. coli could

4

also be eliminated within 5 h in the presence of AZ@RC-24 in dark. Besides, to study

5

the effect of Ag particle size on bactericidal activity, ZnO in the AZ@RC film was

6

removed in dilute H2SO4 solution and the obtained Ag-cellulose (Ag@RC) film was

7

used for testing. As illustrated in Figure S3, with the mean sizes of imbedded Ag NPs

8

varied from 7.3 to 16.5 nm, the Winh values of the obtained Ag@RC-12 film

9

containing 0.24 wt% Ag NPs was comparable with that of Ag@RC-24 film

10

containing 0.38 wt% Ag NPs. The result was consistent with the previous

11

observation[45] that smaller Ag NPs showed a better antibacterial activity because of

12

more efficient Ag-bacteria interaction.

13

There could be three plausible ways (Figure 9e) for the antibacterial activity of

14

AZ@RC film in light[46, 47]: (1) oxidative stress caused by reactive oxygen species

15

(ROS) that were synthesized through reactions between photocatalytically generated

16

electrons/holes and H2O/O2, (2) the presence Ag reduced the chance of electron-hole

17

pair recombination, and promoted the generations of ROS, and (3) release of Zn2+,

18

and Ag+ ions from the films that were cytotoxins to microbes. As for the AZ@RC-0

19

film, the antimicrobial activity should be generated mainly through route 1. S. aureus

20

was composed of peptidoglycan, and it was more susceptible to ROS.[46,

21

Therefore, the AZ@RC-0 film displayed better antibacterial activity against S. aureus

22

than E. coli. With the increase of Ag content, more silver ions were released and route 19


47]


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


2 occurred. As a result, the antimicrobial activity gradually increased from AZ@RC-0

2

to AZ@RC-24. Further, due to the fact that S. aureus cell surface was less negatively

3

charged than that of E. coli, the AZ@RC-24 film with more Ag NPs showed faster

4

elimination rate for E. coli than S. aureus.[48] However, in darkness, only route 3 could

5

take place and thus antibacterial activity of the nanocomposite decreased.

6

The cytotoxicity of the AZ@RC film was evaluated using COS-7 cells as an in

7

vitro model. As shown in Figure 10a of the MTT results, the cell viability values

8

decreased with increasing concentrations. The phenomenon was consistent with that

9

of silver-reinforced cellulose hybrids synthesized using a microcrystalline cellulose

10

solution.[31] Especially, when the concentration of AZ@RC-24 film was 100 mg mL-1,

11

the cell viability value was only 25.0 %. Low cell viability value indicated that

12

AZ@RC film with high concentration had a strong cytotoxicity and bad

13

biocompatibility.[30] However, the value (25.0 %) was still higher than that (13.1%) of

14

cellulose/Ag/AgCl hybrids with a concentration of 100 mg mL-1.[31] The relatively

15

low cytotoxicity should attribute to the slow leakage of Ag NPs and little releasing of

16

Ag+ as illustrated in Figure 9d. In particular, it was noted that the AZ@RC-24 film

17

showed good cellular compatibility (cell viability, 92.2-96.8%) at a concentration of

18

1-5 mg mL-1, and this value was 2.8-17.8 times higher than the value (0.26 mg mL-1)

19

that could completely eradicated bacteria within 3-5 h. Besides, staining was

20

employed to examine the growth situation of cells after MTT assay. As illustrated in

21

Figure 10b, little red (dead) cells were found in the image and some of the live (green)

22

cells exhibited a spindle shape. This result visually illustrated the good cell 20


Page 20 of 41

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compatibility of the AZ@RC-24 film at lower concentration.

2

Apart from possessing desirable features as packaging film in the antibacterial

3

field, the AZ@RC nanohybrids had the versatility to form materials with other device

4

geometries. As shown in Figure S4, through a wet-spinning process, we could obtain

5

flexile solid filaments, which might have potential applications in textile industry and

6

catalysis field. Besides, a porous structure AZ@RC aerogel with low density (0.18

7

g/cm3) could be prepared by freeze-drying. The entire synthesized procedure just

8

proceeded at ambient temperature without employing any heating and reducing,

9

capping, or dispersing agents, which endowed the AZ@RC nanohybrids with

10

sustainability characteristics and made it promising for industrial application.

11 12

CONCLUSIONS

13

In summary, Ag/ZnO NPs decorated cellulose nanocomposite films were successfully

14

prepared by directly mixing a cellulose-NaOH/urea/zincate solution with AgNO3. The

15

zincate contributed to the stability of the mixture solution, which facilitated the

16

reduction of AgNO3 in the cellulose dope. With the reduction time from 0 to 24h, the

17

Ag content in the nanocomposite varied from 0.14% to 0.29%, and mean diameter of

18

the Ag NPs was 16.5 nm. Further, ZnO crystal could be achieved from zincate during

19

the coagulation process. Owing to the good interactions between the NPs and

20

cellulose matrix, the σb, εb and E values of the nanocomposite reached 55.2 MPa, 4.7%

21

and 3.6 GPa, respectively. The Ag/ZnO NPs decorated cellulose nanocomposite

22

showed good cell compatibility at lower concentration which was sufficient to 21



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


eliminate both E. coli and S. aureus bacteria within 3h. This mild and straightforward

2

synthetic approach opened a new window for preparing high value-added

3

cellulose-based nanohybrid materials.

4 5

ASSOCIATED CONTENT

6

Supporting Information

7

The Supporting Information is available free of charge on the ACS Publications

8

website

9

TEM images of AZ@RC-12 films, inhibition zone tests under light and

10

darkness, the effect of Ag NPs sizes on the antimicrobial activity, and

11

pictures of the AZ@RC filaments and aerogel. (PDF)

12 13

AUTHOR INFORMATION

14

Corresponding Author

15

*Tel: +86-571-86843785, E-mail: [email protected]

16

Notes

17

The authors declare no competing financial interest.

18 19

ACKNOWLEDGEMENTS

20

This work was financially supported by Public Welfare Technology Application

21

Research Project of Zhejiang Province (2017C33154), the Science Foundation of

22

Zhejiang Sci-Tech University (ZSTU) under Grant (15012080-Y), Zhejiang Top

23

Priority Discipline of Textile Science and Engineering (2014YBZX03), the Young 22


Page 22 of 41

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Researchers Foundation of Key Laboratory of Advanced Textile Materials and

2

Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University

3

(2015QN03 and 2016QN02), the Natural Science Foundation of China (51573167),

4

the Scientific Research Foundation for the Returned Overseas Chinese Scholars, and

5

the State Education Ministry (1101603-C).

6 7

8

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Y.,

photocatalyst

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(38) Hu, W.; Chen, S.; Zhou, B.; Wang, H., Facile synthesis of ZnO nanoparticles

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based on bacterial cellulose. Mat. Sci. Eng. R 2010, 170 , 88-92.

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(39) Ding, J.; Zhu, J.; Yao, P.; Li, J.; Bi, H.; Wang, X., Synthesis of ZnO-Ag hybrids

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and their gas-sensing performance toward ethanol. Ind. Eng. Chem. Res. 2015, 54 ,

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150811050629003.

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(40) Ko, Y. C.; Fang, H. Y.; Chen, D. H., Fabrication of Ag/ZnO/reduced graphene

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oxide nanocomposite for SERS detection and multiway killing of bacteria. J. Alloy.

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Compd. 2016, 695, 1145-1153.

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(41) Huang, H. D.; Liu, C. Y.; Zhang, L. Q.; Zhong, G. J.; Li, Z. M., Simultaneous

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nanocomposite films by interfacial hydrogen bonding. ACS Sustainable Chem. Eng.

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2015, 3, 317-324.

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(42) Bai, Q.; Xiong, Q.; Li, C.; Shen, Y.; Uyama, H., Hierarchical porous carbons

and

toughening

of

carbon

nanotube/cellulose

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conductive

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high-performance supercapacitor electrodes. ACS Sustainable Chem. Eng. 2017, DOI:

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10.1021/acssuschemeng.7b02488.

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(43) Maleki, A.; Movahed, H.; Ravaghi, P., Magnetic cellulose/Ag as a novel

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eco-friendly

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nicotinonitriles. Carbohyd. Polym. 2017, 156, 259-267.

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Rapid fabrication of composite hydrogel microfibers for weavable and sustainable

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antibacterial applications. ACS Sustainable Chem. Eng. 2016, 4, 6534-6542.

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(45) Zhang, H.; Chen G., Potent antibacterial activities of Ag/TiO2 nanocomposite

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2905-2910.

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(46) Ghosh, S.; Goudar, V. S.; Padmalekha, K. G.; Bhat, S. V.; Indi, S. S.; Vasan, H. N.,

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ZnO/Ag nanohybrid: synthesis, characterization, synergistic antibacterial activity and

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its mechanism. Rsc. Adv. 2012, 2 , 930-940.

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(47) Li, Z.; Zhang, F.; Meng, A.; Xie, C.; Xing, J., ZnO/Ag micro/nanospheres with

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continuous synthesis method. Rsc. Adv. 2014, 5, 612-620.

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GFP-expressing antibiotic resistant E. coli. Colloid. Surface. B 2014, 115, 359.

29



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

Page 30 of 41


1

Table 1 Results of ICP-OES, X-ray diffraction, tensile testing, and antibacterial activity test with AZ@RC nanocomposite filmsa. Sample

WZ (wt%)

WA (wt%)

CI

dZ-SEM(nm)

dA-SEM (nm)

(%)

Surface

Inside

Surface

σb (MPa)

εb (%)

E (GPa)

Winh (mm) E. coli

S. aureus

AZ@RC-0

12.1

0.14

25

406.2

238.1

20.2

45.1

4.3

2.0

2.7

3.0.

AZ@RC-12

11.6

0.24

25

390.7

223.9

21.2

51.6

4.5

2.6

3.2

3.3

AZ@RC-24

11.2

0.38

23

429.2

250.5

26.3

55.2

4.7

3.6

3.5

3.5

2

a

3

diameter of ZnO and Ag from the SEM image; CI, crystallinity index of cellulose; σb, tensile strength; εb, elongation at break; E, Young’s

4

modulus; Winh, width of the inhibition zone.

WZ and WA, content of Ag element and ZnO content in the films determined by the ICP-OES analysis; dZ-SEM and dA-SEM, the average

30

ACS Paragon Plus Environment

ACS Paragon Plus Environment

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1

2 3

Figure 1. (a) Temperature dependence of G′ (solid symbol)) and G′′ (open symbol)

4

for 4.5wt% cellulose dope in NaOH/urea system (CNU) and in NaOH/urea/zincate

5

system (CNUZ) with or without adding AgNO3. (b) Time dependence of G′ and G′′

6

for the different cellulose solution (4.0 wt%) system at 25°C; The data were shifted

7

along the vertical axis by 10a to avoid overlap. 31



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

1

2 3

Figure 2. (a) Preparation of the AZ@RC nanocomposite films: (I) dissolving of

4

cellulose in aqueous NaOH/urea/zincate solvent, (II) reduction of Ag+ in cellulose

5

dope, (III) film casting and coagulation, (IV) AZ@RC nanocomposite film. (b)

6

Photographs of dissolved cellulose dope, (c) the mixed AgNO3-cellulose dope system

7

after different storage time, and (d) the resultant nanocomposite films.

8

32


Page 32 of 41

♦Ag

∇

∇

AZ@RC-D

(103) (220) (112) (201)

♦

(110)

∇

∇ ZnO

(200) (102)

Cellulose

∇

♦ ∇

∇

♦

∇ ∇

(311)

(a)

♦

AZ@RC-24 AZ@RC-12 AZ@RC-0

10

20

30

40

50

60

70

80

2θ θ (degree)

1

1.6

(b)


ZnO Ag

1.4

Absorbance (a.u.)

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


(100) (002) (101) (111)

Page 33 of 41

1.2

1.0

0.8 300

400

500

600

700

800

Wavelength (nm)

2 3

Figure 3. (a) XRD patterns and (b) solid-state UV-vis diffuse reflectance spectra of

4

the AZ@RC nanocomposite films.

5

33



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

1 2

Figure 4. XPS spectra of the AZ@RC nanocomposite films: (a-c) Ag 3d region, and

3

(d-f) Zn 2p region.

4

34


Page 34 of 41

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1 2

Figure 5. SEM images of the (a1-c1) surfaces and (a2-c2) cross-sections of the

3

nanocomposite films: (a1, a2) AZ@RC-0, (b1, b2) AZ@RC-12, and (c1, c2)

4

AZ@RC-24, Inset is the high-resolution SEM images.

5

35



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

1 2

Figure 6. (a, b) TEM and (c, d) HRTEM images of the AZ@RC-24 nanocomposite

3

films, inset in (b) is the particle size distribution of the Ag NPs and (c, d) SAED

4

patterns of the films.

5

36


Page 36 of 41

Page 37 of 41

60

(a) 50

Stress,MPa

40 30 20 AZ@RC-24 AZ@RC-12 AZ@RC-0

10 0 0

1

2

3

4

5

Strain (%) 1

100

(b) DTG (%/min)

486

80

Weight (%)

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


337 500 564

60


340 100

200

300

400

500

600

700

800

Temperature (°C)

40 AZ@RC-0 AZ@RC-12 AZ@RC-24

20

0 100

200

300

400

500

600

700

800

Temperature (°C) 2 3

Figure 7. (a) Stress-strain and (b) TG curves of the AZ@RC nanocomposite films;

4

Inset shows the DTG curves of the films under an air atmosphere.

5

37



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

1 2

Figure 8. Inhibition zone test of the AZ@RC nanocomposite films against (a-d) S.

3

aureus and (e-h) E. coli: (a, e) filter paper, (b,f) AZ@RC-0, (c, g) AZ@RC-12 and (d,

4

h) AZ@RC-24. Representative (i, j) SEM and (k, l) fluorescence images for the

5

live/dead bacterial staining assay of S. aureus (i, k) before and (j, l) after contact with

6

the AZ@RC-24 film.

7 8

38


Page 38 of 41

Page 39 of 41

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1 2

Figure 9. Viable cell numbers for (a) S. aureus and (b) E. coli in the AZ@RC films

3

with different contact times. Effect of washing cycles on (c) antibacterial ratio and (d)

4

residual quantity of NPs for the AZ@RC-24 film. (e) Schematic representation of

5

possible antibacterial mechanism of the AZ@RC film.

39



1

(a) 100

Cell viability (%)

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

80 60 40 20 0 1.0

5.0

10.0

50.0

100.0

Concentration (mg/ml)

2

3 4

Figure 10.Viabilities of COS-7 cells (MTT assay) incubated with the AZ@RC-24

5

film, and the fluorescence images of live (green) and dead (red) COS-7 cells after the

6

MTT assay. The control group was subjected to the same treatments without adding

7

the film.

40


Page 40 of 41

Page 41 of 41

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1

For Table of Contents Use Only

2

3 4

The straightforward reduction of silver ions in a cellulose dope enables the sustainable

5

fabrication of Ag/ZnO decorated cellulose nanocomposite film with enhanced

6

antibacterial activities.

41


Reduction of Silver Ions Using an Alkaline Cellulose Dope

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