Electrospun polyacrylonitrile nanofibers loaded with silver nanoparticles by silver mirror reaction

Materials Science and Engineering C 51 (2015) 346–355

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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

Electrospun polyacrylonitrile nanofibers loaded with silver nanoparticles by silver mirror reaction Yongzheng Shi, Yajing Li, Jianfeng Zhang, Zhongzhen Yu ⁎, Dongzhi Yang ⁎ State Key Laboratory of Organic-Inorganic Composites, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e

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Article history: Received 6 May 2013 Received in revised form 4 March 2015 Accepted 9 March 2015 Available online 11 March 2015 Keywords: Electrospun Polyacrylonitrile Nanoparticles Antibacterial activity Biomaterials

a b s t r a c t The silver mirror reaction (SMR) method was selected in this paper to modify electrospun polyacrylonitrile (PAN) nanofibers, and these nanofibers loaded with silver nanoparticles showed excellent antibacterial properties. PAN nanofibers were first pretreated in AgNO3 aqueous solution before the SMR process so that the silver nanoparticles were distributed evenly on the outer surface of the nanofibers. The final PAN nanofibers were characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), transmission electron microscopy (TEM), TEM-selected area electron diffraction (SAED), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). SEM, TEM micrographs and SAED patterns confirmed homogeneous dispersion of the silver nanoparticles which were composed of monocrystals with diameters 20–30 nm. EDS and XRD results showed that these monocrystals tended to form face-centered cubic single silver. TGA test indicated that the nanoparticles loaded on the nanofibers reached above 50 wt.%. This material was also evaluated by the viable cell-counting method. The results indicated that PAN nanofibers loaded with silver nanoparticles exhibited excellent antimicrobial activities against gram-negative Escherichia coli (E. coli), gram-positive Staphylococcus aureus (S. aureus) and the fungus Monilia albicans. Thus, this material had many potential applications in biomedical fields. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Nowadays the emergence of new silver-based antimicrobial polymers represents a great challenge for both the academic world and industry [1]. There is a widespread concern about the preparation of nanofibers loaded with metal nanoparticles, especially silver nanoparticles, as a novel form of antimicrobial material [2,3]. With the introduction of silver nanoparticles, the material will be given some other properties owing to the fact that silver is non-toxic to human cells and effective against bacteria and viruses [4,5]. In that case, this material can be used in many biomedical applications. Electrospinning is a versatile and effective method that can produce fibers from materials of diverse origins, with diameters ranging from nanoscale to micrometer [6]. This technique has often been adopted for loading silver nanoparticles into porous polymer media [7]. Hong et al. [8] prepared polyvinyl alcohol (PVA) nanofibers containing silver nanoparticles by electrospinning PVA/silver nitrate (AgNO3) aqueous solution, followed by short heat treatment. Silver ions, known for their broad-spectrum antimicrobial activity, were reduced into silver nanoparticles in base gelatin solution containing 2.5 wt.% AgNO3. The antibacterial activity of this material was the greatest one against

⁎ Corresponding authors. E-mail address: [email protected] (D. Yang).

http://dx.doi.org/10.1016/j.msec.2015.03.010 0928-4931/© 2015 Elsevier B.V. All rights reserved.

Pseudomonas aeroginosa, followed by Staphylococcus aureus, Escherichia coli, and methicillin-resistant Staphylococcus aureus, respectively [9]. Polyacrylonitrile (PAN) is an important engineering polymer material that has been widely used in the application of filtration due to thermal stability, high mechanical properties and chemical resistance [10,11]. Therefore, PAN-Ag nanocomposites are expected to produce functional fibers with anti-electrostatic, fungicidal and ultravioletresisting effects [12,13]. In recent years, electrospun PAN nanofibers loaded silver nanoparticles have been prepared using different methods. For example, silver nanoparticles were prepared by electrospinning mixed solution of AgNO3 and PAN, the nanofibers were then reduced by using aqueous solution of N2H5OH [14,15]. NaBH4 was also used as chemical reduction to reduce Ag+ into Ag0 [16]. UV irradiation [17], atmospheric plasma treatment and high temperature were some other methods to prepare electrospun PAN nanofibers loaded silver nanoparticles [18,19]. In short, these common methods above have been reported in many papers. In this paper, a simple and effective method, silver mirror reaction (SMR), was developed to fabricate PAN nanofibers loaded silver nanoparticles. As we all know, SMR is an old and well known chemical reaction, which reduces Ag+ into Ag0 and generates silver attached to the tube wall to form a silver mirror. During the reaction, silver ions were urged to form homogeneous nucleation sites. In that case, the agglomeration during the electrospinning process could be avoided. Large amounts of silver loaded on the surface of PAN nanofibers could

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Table 1 The different conditions of pretreatment process in AgNO3 solution and different conditions of SMR process. Pretreatment process Fig. 2

AgNO3 concentration (wt.%) 2 5 0.05 0.1 1 10

SMR process Fig. 3

(1) After pretreatment of PAN nanofibers in 2 wt.% AgNO3 solution (2) after pretreatment of PAN nanofibers in 10 wt.% AgNO3 solution (3) 5wt.%AgNO3 solution

improve the antibacterial efficiency of the material. It has been reported that SMR is still being used as an efficient method for preparing membranes coating silver nanoparticles [20]. Before SMR process, PAN nanofibers were pretreated in AgNO3 aqueous solution. In order to obtain homogeneous silver ions' nucleation sites, the morphology of PAN nanofibers before and after SMR process was characterized by SEM and TEM. XRD and EDS analysis were used to detect the crystal structure and chemical composition of silver nanoparticles. TGA test indicated that the amounts of loaded nanoparticles on the nanofibers. Finally, antibacterial evaluation was done to verify the excellent antibacterial properties of PAN nanofibers loaded silver nanoparticles. The results showed that this kind of material was a promising candidate as biomedical material.

2. Experimental 2.1. Materials
MW ¼ 100; 000 93 wt% was supplied by Polyacrylonitrile Courtaulds Co., Ltd (UK). N,N-dimethylformamide (DMF) was purchased

Concentration 7%

18 h Fig. 2c 18 h Fig. 2i

24 h Fig. 2d 24 h Fig. 2j

1h Fig. 3c 10 min Fig.3g

2h Fig. 3d 1h Fig.3h

30 h Fig. 2e 30 h Fig. 2k

48 h Fig. 2f 48 h Fig. 2l

from Tianjin Tiantai Fine Chemicals Co. (China). Silver nitrate (AgNO3), ammonium hydroxide (25 wt.%–28 wt.%) and formaldehyde (37 wt.%– 40 wt.%) were supplied by Beijing Chemical Reagent Company. All these reagents were used without further purification. 2.2. Preparation of PAN nanofibrous membranes by electrospinning PAN was first dissolved in DMF at different concentrations (7%, 10%, 14%, 17%, 22%) and stirred at room temperature for 12 h to obtain a transparent homogenous solution. The electrospinning process was performed at room temperature. The above solution was collected into a 5 mL syringe equipped with a single nozzle. The best electrospinning conditions were explored by changing different parameters. They were set as follows: the syringe was fixed on an electric pump set to maintain flow rate of the spin dope at 0.02 mm/s. A high-voltage power supply (BGG4-21, BMEI Co. Ltd., China) was employed to apply positive charge between the syringe tip and collector. The voltage used for electrospinning was 16 kV. An aluminum foil was used as collector, and the tip-to-collector distance was fixed at 20 cm. All the PAN nanofibrous membranes were folded on the surface of aluminum foil.

10%

17%

Pretreatment time 6h 12 h Fig. 2a Fig. 2b 6h 12 h Fig. 2g Fig. 2h 12 h Fig. 2m 12 h Fig. 2n 12 h Fig. 2o 12 h Fig. 2p SMR time 10 min 30 min Fig. 3a Fig. 3b 10 min 10 min Fig. 3e Fig. 3f

14%

22%

Fig. 1. SEM images of PAN electrospinning nanofibers under different polymer concentrations (a–e).

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2.3. Pretreatment of PAN nanofibrous membranes in AgNO3 aqueous solution The PAN nanofibrous membranes were first cut into 2.5 cm × 2.5 cm pieces and then soaked in AgNO3 aqueous solution (2 wt.%, 5 wt.%) for pretreatment for 18 h and 24 h, respectively. The whole pretreatment process was performed in a dark container at room temperature to avoid exposure under visible light and UV irradiation. After pretreatment, the PAN nanofibrous membranes were picked up and freezedried for 24 h. The different conditions of the pretreatment process and SMR process have been listed in Table 1.

was stirred just until the brown precipitate dissolved. The PAN nanofibrous membranes after pretreatment were then immersed in the above solution. Formaldehyde (1 wt.%) was finally added into the beakers dropwise. It could be seen that the silver film began to form on the walls of the beakers within about 1 min. The beakers were continuously shaken until they had a silver mirror coating on the wall. After that, the beakers were kept for a few minutes until the reaction was completed. The samples were washed with distilled water three times before analysis and test.

2.4. Silver nanoparticles loaded by SMR process

2.5. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS)

10 milliliters of 2 wt.% and 5 wt.% AgNO3 aqueous solution were placed in two 100 mL beakers, respectively. Concentrated ammonia (2.5 wt.%) was added dropwise into the beakers while the solution

The morphology of PAN nanofibrous membranes sputter-coated with gold was observed by a scanning electron microscope (Hitachi S4700, Hitachi Company, Japan) with an accelerating voltage of 20 kV

(a) Pretreatment of PAN nanofibers in AgNO3 solution at different time (fixed concentration of AgNO3 solution)

2wt%AgNO3 6h

2wt%AgNO3 12h

2wt%AgNO3 18h**

2wt%AgNO3 30h

2wt%AgNO3 48h

5wt%AgNO3 6h

5wt%AgNO3 18h

5wt%AgNO3 24h

5wt%AgNO3 30h

2wt%AgNO3 24h

5wt%AgNO3 12h

5wt%AgNO3 48h

(b) Pretreatment of PAN nanofibers in AgNO 3 solution at different concentration (fixed 12h in AgNO3 solution)

m

n

0.05wt%AgNO3 12h

0.1wt%AgNO3 12h

o

p

1wt%AgNO3 12h

10wt%AgNO3 12h

Fig. 2. SEM images of pretreating PAN nanofibers in AgNO3 solution: at different time (a); at different concentration (b).

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(1) SMR at different time after pretreatment 18h of PAN nanofibers in 2wt% AgNO3 solution

SMR 10min

SMR 30min

SMR 1h

SMR 2h

(2) SMR after pretreatment 18h of PAN nanofibers in 10wt% AgNO3 solution (different magnification)

e

f

SMR 10min

SMR10min

(3) SMR after pretreatment 18h of PAN nanofibers in 5wt% AgNO3 solution

g

h

SMR10min

SMR 1h

Fig. 3. SEM images of PAN nanofibers after SMR process: (1) at different time after pretreatment 18 h in 2 wt.% AgNO3 solution; (2) after pretreatment 18 h in 10 wt.% AgNO3 solution; (3) SMR after pretreatment 18 h of PAN nanofibers in 5 wt.% AgNO3 solution.

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so as to compare the different morphologies of nanofibers loaded with silver nanoparticles before and after pretreatment. Energy dispersive spectrometer (EDS) was used in conjunction with SEM for elemental analysis of the surface component.

Company, Tokyo, Japan) under area detector operating at a voltage of 40 kV and a current of 50 mA using Cu Kα radiation (λ = 0.154 nm). The scanning rate was 1°/min in the 2θ range from 10° to 90°. 2.8. Thermogravimetric analysis (TGA)

2.6. Transmission electron microscopy (TEM) and TEM-selected area electron diffraction (SAED) After pretreatment in AgNO3 aqueous solution, a few PAN fibers were loaded on the copper screen carefully, and dried in a dark vacuum condition, and these fibers were characterized by transmission electron microscopy (TEM, JEM3010 JEOL) and TEM-selected area electron diffraction (SAED). The SAED patterns confirmed that these silver particles were composed of monocrystals. 2.7. X-ray diffraction (XRD) The XRD patterns of the samples were recorded on a wide-angle X-ray diffraction analyzer (WAXD, D/Max 2500 VB2 +/PC, Rigaku

The thermal behavior of neat PAN nanofibers and nanofibers loaded silver nanoparticles were examined by Q100 TA Instruments (TA Instruments, New Castle, U.S.). Measurements were conducted over a temperature range of 25–800 °C, at a heating rate of 20 °C/min under nitrogen purge. 2.9. Antibacterial evaluation The antibacterial activity of PAN nanofibers loaded with silver nanoparticles was tested against bacteria and fungus using the viable cellcounting method. E. coli (gram-negative), S. aureus (gram-positive), and M. albicans (fungus) were chosen to detect antibacterial properties of this material. The bacteria and fungus were propagated on agar and

(a) Pretreatment of PAN nanofibers in 2wt% AgNO3 solution at different time

18h

24h

18h

24h

(b) Pretreatment of PAN nanofibers in 5wt% AgNO3 solution at different time

18h

24h

Fig. 4. TEM images and SAED patterns: pretreatment of PAN nanofibers in 2 wt.% AgNO3 solution at different times (a); pretreatment of PAN nanofibers in 5 wt.% AgNO3 solution at different times (b).

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broth, and diluted to 1 × 106 colony forming units (CFU)/mL for testing. PAN nanofibrous membranes loaded with silver nanoparticles were cut into pieces (15 cm × 15 cm). The bacteria suspension (5 mL) containing approximately 1 × 106 CFU/mL of the test organism was pipetted onto the samples, completely covering the nanofibrous membranes. For comparison, the neat PAN nanofibrous membranes were also tested. The samples were kept for 24 h at room temperature. Then tryptone was used to elute the bacteria from the samples. Gradient dilution was done and the concentration of the dilute solution was 10−1, 10−2 and 10−3. 0.1 mL of the dilute solution was mixed with agar and broth for bacteria incubation. After 24 h (for E. coli and S. aureus) and 48 h (for M. albicans) incubation at room temperature, bacterial colonies were counted. The results were photographed for further evaluation.

351

conditions is characterized in Fig. 1. It could be seen that when the concentration of PAN/DMF solution was 7%, there was a large number of spherical bead structure. With the concentration increased, the beaded structure was significantly reduced. When the concentration of PAN/ DMF solution reached 17%, smooth nanofibers with uniform diameter distribution were obviously observed. Except for the concentration, the effects of other electrospinning parameters on PAN nanofibers were also explored, but there the display of fiber morphology is not necessary in this paper because electrospinning of PAN nanofibers has been reported in many articles before and the aim of this study is modifying the PAN fibers after electrospinning. The best electrospinning conditions were set just as follows: electric field voltage of 16 kV, syringe speed of 0.02 mm/s, and a tip-to-collector distance of 20 cm, PAN solution concentration of 17%.

3. Results and discussion The nanoparticles attached to the cell membrane and also penetrated inside the bacteria. The size and shape of the nanoparticle implies that it has a large surface area for contact with the bacteria and cell. And it has an obvious advantage over bigger particles, and the nanoparticles with a diameter smaller than 10 nm can interact with cell and bacteria to produce electronic effects, which could enhance the reactivity of the nanoparticles. In addition, the antibacterial effect of the spherical nanoparticles was better than that of rod shaped ones. Below, the crystallization process of silver on the nanofibers will be investigated in detail. 3.1. Surface morphology of electrospun PAN nanofibers In order to obtain PAN nanofibers which had a smooth surface and homogeneous distribution, the surface morphology of electrospun PAN nanofibers under different electrospinning concentration

3.2. Surface morphology of electrospun PAN nanofibers after pretreatment In an attempt to examine the adsorption effect and crystallization ability of silver ions, PAN nanofibers were pretreated in AgNO3 aqueous solution at different times from 6 h to 48 h. The effect of pretreatment at different concentrations of AgNO3 aqueous solution from 0.05 wt.% to 10 wt.% was also demonstrated. As shown in Fig. 2(a), it could obviously be seen that when PAN nanofibers were pretreated in 2 wt.% AgNO3 aqueous solution at 18 h, lots of silver nanoparticles were formed and evenly distributed on the surface of the nanofibers. With the pretreatment time increased, the number of silver nanoparticles no longer continued to increase and the morphology of the nanofibers appeared curved. However, Fig. 2(b) shows that with the concentration of the AgNO3 solution increased, the number of silver nanoparticles increased but the morphology distributed unevenly.

Element CK NK OK AgL

Element CK NK OK AgL

Wt% 07.13 01.23 01.84 89.80

At% 36.43 05.40 07.06 51.10

Wt% 58.09 19.04 06.78 16.09

At% 71.45 20.08 06.26 02.20

Element CK NK AgL

Wt% 06.55 01.24 92.21

At% 36.61 05.97 57.42

Fig. 5. SEM image and EDS analysis of PAN nanofibers: pretreatment in 2 wt.% AgNO3 solution (a); SMR after pretreatment in 2 wt.% AgNO3 solution (b); SMR after pretreatment in 10 wt.% AgNO3 solution (c).

Y. Shi et al. / Materials Science and Engineering C 51 (2015) 346–355

It has been generally accepted that there is coordination between cyano nitrogen of PAN and silver ions [21]. Cyano nitrogen of PAN can donate its lone-pair electrons from occupied 2p orbitals to empty s orbitals of silver ions to form σ-bond. The back-donation of electron density from occupied d orbitals of silver ions into the empty *-2p antibonding orbitals of cyano nitrogen leads to the formation of π-bonds. This kind of coordination makes PAN an ideal carrier of silver ions. When the pretreatment in the AgNO3 solution continued for a long time or the concentration of the AgNO3 aqueous solution was too high, silver ions which were on the surface of the nanofibers were easy to agglomerate, therefore the morphology of PAN nanofibers after pretreatment showed uneven distribution.

110

100

d, e c

90

Weight(%)

352

b a, PAN

80

b,2%AgNO3 18h, SMR 1h c,2%AgNO3 18h, SMR 10min

70

d, 5%AgNO3 18h, SMR 1h

60

e,5%AgNO3 18h, SMR10min

50

3.3. Surface morphology of silver nanoparticles loaded by SMR process

a,

40

To increase the amounts of silver nanoparticles loaded on the surface of PAN naofibers, the nanofibers after pretreatment were further subjected to the SMR process. Transformation from silver ions to silver nanoparticles in solution could be observed visibly as a color change before and after the SMR process. Fig. 3(1) shows SEM images of PAN nanofibers loaded with silver nanoparticles via SMR at different times. With the reaction time increased, the morphology of silver appeared sheet shape and silver distribution was uneven. When SMR continued for 10 min, silver nanoparticles on the surface of nanofibers showed excellent morphology and distributed evenly. This might be attributed to agglomerating of silver which was on the walls of the beakers after SMR for a long period of time. SMR after pretreatment of PAN nanofibers in 10 wt.% and 5 wt.% AgNO3 solution is also presented in Fig. 3(2) and (3). Large amounts of silver nanoparticles were evenly distributed on the surface of the nanofibers. It showed that the high concentration of AgNO3 solution pretreatment preferred to form large numbers of silver nanoparticles. The nanofibers after pretreatment tended to have more silver nanoparticles loaded as the same effect of nuclei.

0

200

400

600

800

1000

Temperature ( ) Fig. 7. Thermogravimetric curves of silver nanoparticles loaded on the surface of PAN nanofibers via SMR at different times.

were observed to be uniformly formed on and within the entire nanofiber matrix, and no significant aggregation of silver was observed. The silver nanoparticles across the nanofibers were all spherical, and their average size was about 20 nm to 30 nm. This suggested that AgNO3 solution could promote the growth of silver nanoparticles by covering and encapsulating. Therefore, Ag+ ions acted as a nucleater to the aggregation of silver nanoparticles. In addition, with pretreatment time increased, the number of silver nanoparticles increased and appeared uniformly distributed. When the pretreatment time reached 24 h, the nanoparticles were uniformly distributed. This suggested that adequate pretreatment time is preferred to form uniform nanoparticles. The SAED patterns are also shown in Fig. 4. Spotted patterns indicate the monocrystallinity of the nanoparticles.

3.4. TEM analysis after pretreating PAN nanofibrous membranes in AgNO3 solution

3.5. EDS analysis of silver nanoparticles loaded by SMR

The silver nanoparticles embedded in PAN nanofibers that were pretreated in AgNO3 solution at different times were confirmed by TEM images. From Fig. 4(a) and (b), individual silver nanoparticles

Elementary analysis of PAN nanofibers loaded with silver nanoparticles was carried out by using SEM-EDS, as shown in Fig. 5. The results showed that carbon, nitrogen, oxygen and silver were the principal

Fig. 6. XRD patterns of (A): PAN (a); SMR for 1 h (b) or 10 min (c) after pretreatment in 2 wt.% AgNO3 for 24 h; SMR for 10 min after pretreatment in 2 wt.% AgNO3 (d) or 10 wt.% AgNO3 (e) for 18 h; (B) and (C) the standard XRD spectra of single silver.

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elements of PAN nanofibrous membranes. EDS analysis thus provided direct evidence that silver particles existed on the PAN nanofiber furface. As seen in Fig. 5, the silver content of PAN nanofibers after pretreatment reached 16.09 wt.%, whereas loaded silver content reached 89.80 wt.% (SMR after pretreatment of PAN nanofibers in 2 wt.% AgNO3 solution for 10 min) and 92.21 wt.% (SMR after pretreatment of PAN nanofibers in 10 wt.% AgNO3 solution for 10 min). The loaded silver content seemed to be very high after SMR due to the strong anchoring mechanism of the ion–dipole interaction and the subsequent reduction of the silver particles on the large surface area of the PAN

a

d

3.6. XRD spectra of silver nanoparticles loaded by SMR process To prove the presence of silver nanoparticles loaded on the surface of PAN nanofibers and investigate their crystal structure, X-ray analysis

c

E. coli

g

E. coli

f

e

S. aureus neat PAN nanofibers

S. aureus

S. aureus

i

h

Monilia albican neat PAN nanofibers

j

nanofibrous membranes. The weight fraction of oxygen in AgO was approximately 12.9 wt.%. The content of oxygen shown in Fig. 5 was so little (6.78 wt.% and 1.84 wt.%, respectively), and combined with XRD results, we could conclude that the element of silver existed in PAN polymer matrix in the form of single silver instead of silver oxide.

b

E. coli neat PAN nanofibers

Monilia albican

k

353

Monilia albican

l

control suspension of bacteria, (j) E. coli , (k) S. aureus, (l) Monilia albican. Fig. 8. Antibacterial test plates of E. coli (b, c), S. aureus (e, f) and M. albicans (h, i) after treatment with PAN nanofibrous membranes loaded with silver nanoparticles; E. coli (a), S. aureus (d) and M. albicans (g) after treatment with neat PAN nanofibers; control suspension of bacteria, (j) E. coli, (k) S. aureus, (l) M. albicans. (b, e, h): PAN nanofibrous membranes SMR 10 min after pretreatment 18 h with 2 wt.% AgNO3 solution. (c, f, i): PAN nanofibrous membranes SMR 10 min after pretreatment 18 h with 5 wt.% AgNO3 solution.

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was used. The crystalline characteristics of the silver nanoparticles on PAN nanofibrous membranes are shown in Fig. 6(A). In the XRD pattern of PAN nanofibers containing silver nanoparticles, five distinct peaks were observed. These peaks appeared at 2θ values of 38.1, 44.3, 64.4, 77.5, 81.5, which corresponded to the reflections of the (111), (200), (220), (311), (222) crystalline planes of silver, respectively. These values are close to those in the International Center for Diffraction Data (ICDD) card (card no. 4-783) which is shown in Fig. 6(B) and (C). In that case, silver nanoparticles tended to form face-centered cubic single silver after SMR. It could be seen that when SMR continued for 10 min, silver nanoparticles tended to have higher peaks compared with the others. The results were in good agreement with the SEM results. Moreover, the XRD patterns showed that pretreatment in different concentrations of AgNO3 solution at different times almost had no obvious effect on the crystal form of single silver. 3.7. TGA analysis of silver nanoparticles loaded by SMR process Fig. 7 shows TGA thermograms of both the neat PAN nanofibers and PAN nanofibrous membranes via SMR under nitrogen atmosphere. The profile of the neat PAN sample shows that there were two stages of mass loss for the thermal degradation of PAN. It could be seen that PAN nanofibrous membranes lost weight starting at 325 °C with cyclization and decomposition reaction of PAN and the char yield was about 45% up to 800 °C. For samples loaded with large amounts of silver nanoparticles, only one stage in the loss of weight was observed. For the four measured samples, the total loaded amount of silver could reach above 90 wt.% of the composites, and it depended on the different SMR reaction conditions and parameters, even if a sample with SMR of only 10 min and lowest pretreatment AgNO3 concentration of 2 wt.%. For Fig. 7(b) and (c), the char yields were close to 90% and 95%, respectively, and we can deduce that the possible composition of (PAN/nano-silver) should be 18/82 and 8/92 by wt.%. For Fig. 7(d) and (e), the char yield was close to 98%, this composition ratio should be 5/95. Such high loading of silver looks unbelievable because it did not cover the whole fiber surface, but we can't ignore the fact that the density difference between silver crystal and PAN polymer fiber is very surprising. Focusing on the point of application, if the fiber membranes loaded with silver were used in the antibacterial material field, the antibacterial effects of silver are related to the particle size, silver particles with size about 5 20 nm are better. Combination of Fig. 3 discussion, the AgNO3 solution concentration and SMR time were two important parameters for control of particle size and loading capacity of silver. The optimizing experiment parameters were as follow: PAN fibers were first pretreated 18 h in 2 wt.% AgNO3 solution before SMR process of 10 min. If smaller particle size and lower loading capacity were expected, reducing the SMR time was more effective. 3.8. Antibacterial activity Antibacterial properties of PAN nanofibers loaded with silver nanoparticles were tested on E. coli (gram-negative), S. aureus (grampositive) and M. albicans (fungi) microorganisms. For comparison, results for neat PAN nanofibers are also shown in Fig. 8. As seen in Fig. 8, neat PAN nanofibers showed no significant antibacterial activity. Conversely, PAN nanofibers loaded with silver nanoparticles exhibited complete inhibition indicating that the nanofibrous membranes were endowed with excellent antibacterial properties due to the introduction of silver nanoparticles. Silver nanoparticles have strong antibacterial properties since they attach to the cell walls and disturb cell-wall permeability and cellular respiration [22,23]. Nano-sized fibrous membranes provided relatively larger surface areas for contact with bacteria. Photographs of agar plates with the control bacteria suspension are also shown in Fig. 8. The absence of colony-forming units on the plates exposed to PAN nanofibers loaded with silver nanoparticles suggested a complete kill.

4. Conclusion Micro or nano-sized fibrous structures are unique as compared to normal fibrous structures by their high surface to volume ratio, fiber interconnectivity, micro scale interstitial space and also high porosity. Antibacterial PAN nanofibrous membranes loaded with silver nanoparticles were successfully prepared by electrospun nanofibers and silver mirror reaction. The optimizing experiment process was as follow: PAN fibers were first pretreated for 18 h in 2 wt.% AgNO3 solution before SMR process of 10 min. The TEM-selected area electron diffraction (SAED) patterns and XRD results proved the monocrystallinity of silver nanoparticles and they tended to form facecentered cubic single silver. The PAN nanofibrous membranes loaded with silver nanoparticles exhibited excellent antibacterial activity against E. coli (gram-negative), S. aureus (gram-positive) and M. albicans (fungi) microorganisms. These antibacterial nanofibrous membranes have potential applications including in antibacterial clothes, medical protection devices, etc.

Acknowledgments The authors would like to thank the National Natural Science Foundation of China (51273015) for its financial support.

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