Green Synthesis of Silk Fibroin-Silver Nanoparticle Composites with Effective Antibacterial and Biofilm-Disrupting Properties Xiang Fei,† Minghui Jia,‡ Xin Du,§ Yuhong Yang,∥ Ren Zhang,∥ Zhengzhong Shao,† Xia Zhao,*,‡ and Xin Chen*,† †

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai, 200433, People’s Republic of China ‡ Department of Otorhinolaryngology−Head and Neck Surgery and §Department of Laboratory Medicine, Huashan Hospital, Fudan University, Shanghai, 200040, People’s Republic of China ∥ Research Centre for Analysis and Measurement, Fudan University, Shanghai 200433, People’s Republic of China S Supporting Information *

ABSTRACT: Natural polymer Bombyx mori silk fibroin is used as a biotemplate to produce silver nanoparticles in situ under light (both incandescent light and sunlight) at room temperature. Silk fibroin provides multiple functions in the whole reaction system, serving as the reducing agent of silver, and the dispersing and stabilizing agent of the resulted silver nanoparticles. As the reaction needs not any other chemicals and only uses light as power source, the synthetic route of silver nanoparticles reported here is rather environment-friendly and energy-saving. The silk fibroin-silver nanoparticle composite prepared by this method can be stably stored in a usual environment (room temperature, exposure to light, and so forth) for at least one month. Such a silk fibroin-silver nanoparticle composite shows an effective antibacterial activity against the methicillin-resistant Staphylcoccus aureus (S. aureus) and subsequently inhibits the biofilm formation caused by the same bacterium. Moreover, a maturely formed biofilm created by methicillin-resistant S. aureus can be destroyed by the silk fibroin-silver nanoparticle composite, which meets the demand of clinical application. Therefore, the silk fibroin-silver nanoparticle composite prepared by this clean and facile method is expected to be an effective and economical antimicrobial material in biomedical fields.

INTRODUCTION There has been increasing interest in the development of clean synthetic procedures for nanoproducts targeted for biomedical applications.1 Utilization of nontoxic chemicals, environmentally benign solvents, and renewable materials are some of the key issues that merit important consideration in a green synthetic strategy. In nature, biological systems have developed biomineralization approaches in the combination of the biology and nanotechnology, offering unparalleled opportunities for excellent physicochemical properties in natural biomaterials2 and providing inspiration for material scientists to design advanced nanomaterial.3−5 As an important component in biomineralization, various biological systems, such as viruses,6 proteins,7 and peptides7−11 broaden the possible application fields of the biomineralization in electronics and nanobiotechnology. They often act as a biotemplate during the © 2013 American Chemical Society

preparation of novel biomaterials and nanodevices in the form of inorganic−organic hybrid composites.7,10,12−14 Silk fibroin derived from Bombyx mori silkworm silk is a commonly used biomacromolecule with unique sequencespecific self-assembly behavior and substrate recognition properties.15 Silk fibroin has been used as a biomedical material for long a time because of its good biocompatibility, controllable biodegradability, and easy fabrication into different forms, such as fibers, films, gels, and three-dimensional scaffolds.16,17 As other proteins, silk fibroin is also a good candidate for a soluble template for biomineralization. In our previous works, we have proved that silk fibroin plays an Received: September 22, 2013 Revised: October 20, 2013 Published: October 30, 2013 4483 | Biomacromolecules 2013, 14, 4483−4488



clinical laboratory of Huashan Hospital affiliated to Fudan University, which has the ability to form biofilm. A single isolated colony of the target bacteria was inoculated in 4 mL of Muller−Hinton broth (MHB) and was cultured to a midlog phase at 37 °C with shaking (220 r/min). Then, the bacteria liquid in midlog phase was resuspended in the physiological saline to an optical density (OD) of 0.5 at 600 nm, which corresponds to the concentration of 1.5 × 108 colony forming units per milliliters (CFU/mL). Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) Determination. The antimicrobial activity of RSF-AgNPs composite was evaluated by determination of MIC and MBC against Sa006 in vitro. MIC and MBC testing were performed according to the Clinical and Laboratory Standards Institute (CLSI, 2011) guidelines for macro-dilution in broth.38 The bacterial suspension was resuspended in MHB to 2 × 105 CFU/mL and dispensed into nine tubes (1 mL per tube), then equal volume of the 2-fold serially diluted concentrations of RSF-AgNPs composite solution were added into the tubes. The final concentration of AgNPs in these tubes was from 0 to 153.6 mg/L, using pure MHB medium as a control. MIC was defined as the lowest concentration that yielded no visible growth of bacteria after 24 h incubation at 37 °C. MBC was determined by plating 100 μL suspension collected from the MIC test tubes without visible bacteria growth onto the Muller− Hinton (MH) agar plates and then incubated at 37 °C for another 24 h. The lowest concentration for monoclonal number in the plate was five or less and defined as the MBC.38 All the tests were performed at least in triplicate. Antibacterial Activity of RSF-AgNPs Composite against Biofilm. In order to test the antibacterial activity of RSF-AgNPs composite against the maturely formed biofilm, the bacteria suspension was diluted in tryptic soy broth supplemented with 0.5% glucose (TSBg) to approximately 1 × 105 CFU/mL. Then, 1 mL of such bacteria suspension was transferred to a cell culture dish (NEST Biotechnology Co., Ltd.) to incubate for 24 h at 37 °C without shaking. After the biofilms were formed in the dishes, the previous TSBg media were changed by the fresh TSBg containing different amount of RSF-AgNPs composite and continued to incubate for another 10 h. The concentration of AgNPs was set as MIC, MBC, 2 × MBC, and 5 × MBC, respectively. After 10 h incubation, the dishes were sequentially rinsed with physiological saline three times to remove the planktonic bacteria. The viability and structure of the biofilms were detected by means of confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Characterizations. Transmission electron microscopy (TEM) was performed with a Hitachi H-600 transmission electron microscope (Japan) at 75 kV. High-resolution transmission electron microscopy (HRTEM), selective area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX) were performed on a JEM2010F transmission electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. UV−vis spectra were measured by a Hitachi U-2910 spectrophotometer. For SEM detection, the biofilm samples were placed in 2.5 wt % glutaraldehyde solution for 24 h, and then dehydrated in a series of ethanol aqueous solutions with the increased concentration until the specimens were placed in absolute ethanol. Afterward, the specimens were critical-point dried in carbon dioxide, mounted on the scanning electron microscope stubs, sputter coated with gold palladium, and then examined with a Hitachi SU8010 scanning electron microscope (Japan) at an accelerating voltage of 20 kV. For CLSM detection, the biofilm samples were immersed in 1 mL of physiological saline in which contained 1.5 μL aliquots of component A (Syto 9) and component B (propidium iodide) from the BacLight LIVE/DEAD kit (Invitrogen, Molecular Probes) to stain in darkness at room temperature for 15 min, Then, they were analyzed with a Leica TCS SP5 confocal scanning laser microscope (Leica Microsystems, Germany) at certain wavelengths (for Syto 9 at 515− 530 nm, and for propidium iodide, >600 nm).

important role in the biomineralization process for inducing and regulating the morphologies and lattice structures of different inorganic nanostructures.18−22 Nowadays, the development of antibiotic-resistant bacteria has created a myriad of new challenges within the healthcare field. In terms of public health, the methicillin-resistant Staphylcoccus aureus (MRSA) accounts for a considerable part of the reported cases of S. aureus infections all over the world. People estimate 20 000 deaths occurred in the United States annually because of MRSA infections.23 On the other hand, planktonic bacteria in the nosocomial environment or during the manufacturing processes of medical supplies can form colonies on their surfaces to create biofilms, leading to the wide-spreading of infectious diseases by contacting those contaminated surfaces. Bacterial biofilms consist of bacteria and self-secreted extracellular polymeric substances (EPS),24 which are extremely resistant to conventional antibiotics because of acquired resistance, limited diffusion, and inactivation of antibiotics in EPS.23,25 Silver nanoparticles (AgNPs) are well-known for their antibacterial property to a broad spectrum of bacteria and have been used increasingly in detergents, plastics, food storage containers, antiseptic sprays, catheters, bandages, and textiles, and so forth.26 Nevertheless, practical applications of AgNPs are often hampered by their easy oxidization weakness, which may cause the loss of the antibacterial activity.27 To overcome this problem, different organic27−29 and inorganic templates30−36 have been employed to stabilize AgNPs through the formation of nanocomposites. However, those methods always need complex and tedious procedures, and face the problems of high cost and poor biocompatibility. In this article, we introduce an easy and environmentally friendly route to in situ preparation of AgNPs using silk fibroin as a biotemplate at room temperature under light exposure. Then, we report the antibacterial and biofilm-disrupting properties of the resulted silk fibroin-AgNPs composite.


Preparation of Regenerated Silk Fibroin (RSF) Solution. The raw B. mori cocoon silk consists of fibroin fibers that are bound together by sericin, a hydrophilic gumlike coating protein. The degumming (removing the sericin) and dissolving process of B. mori silk followed the established procedures.37 In brief, the silk fibers were degummed twice with 0.5 wt % NaHCO3 aqueous solution at 100 °C for 30 min, washed with distilled water, and allowed to air-dry at room temperature. The degummed B. mori silk fibers were dissolved in 9.3 mol/L LiBr aqueous solution at 60 °C. The silk fibroin-LiBr solution was dialyzed against deionized water for 72 h at room temperature with a semipermeable membrane (MEMBRA-CEL, 12 000−14 000 MWCO) to remove the salt. The dialyzed silk fibroin solution was centrifuged at 6000 r/min for about 5 min and the supernatant was collected and then stored at 4 °C for further use. The final concentration of RSF solution was about 4 wt % and was diluted to 1 wt % by adding deionized water. Preparation of RSF-AgNPs Composite Solution. Five to eighty milligrams of AgNO3 powders were added into 5 mL of 1 wt % RSF solution to form a transparent RSF-AgNO3 mixture solution. The final AgNO3 concentration in the mixture was 1−16 mg/mL. Then, the RSF-AgNO 3 solution was exposed under the light with an incandescent bulb (40 W, from Philips) and incubated at room temperature for 24 h to produce RSF-AgNPs composite solution. Afterward, RSF-AgNPs composite solution was stored at 4 °C for further use. Preparation of Bacterial Suspension. The bacterial strain used in this study was Sa006, a clinical isolate of MRSA obtained from the 4484 | Biomacromolecules 2013, 14, 4483−4488



nature of Try residues (Figure 1a). In the current study, we set the RSF concentration to 1 wt % in the reaction system, so we need to determine the suitable concentration of AgNO3 to be reduced. Fluorescence analysis was chosen as an effective method to do this because the loss of fluorescence from Tyr residues reflected the oxidation degree of the phenolic groups in them. The more Ag+ ions were reduced, the more Tyr residues were oxidized, and the less fluorescence intensity was observed. Figure 1b indicates that the fluorescence intensity does not change when the AgNO3 concentration is larger than 8 mg/mL. This means almost all of the Tyr residues in silk fibroin were used (oxidized) at [AgNO3] = 8 mg/mL, so we decided to set [AgNO3] = 4 mg/mL in our reaction system to make sure that all the Ag+ ions can be reduced to Ag. The initial RSF-AgNO3 solution was colorless, but after the exposure to the light, the color gradually turned to yellow (see inset in Figure 3b). Such a change was attributed to the sizeand shape-dependent surface plasmon resonance (SPR) of AgNPs44 formed in the solution in the visible region and was characterized by UV−vis spectroscopy (Figure 2a). The main SPR band was at 440 nm, but a small shoulder can be found at 347 nm, suggesting the possible existence of different size and morphology of AgNPs.45 TEM images (Figure 2b,c) show the particle size distribution of the AgNPs. The average size of AgNPs was 12.0 ± 2.1 nm, which was obtained by measuring 100 nanoparticles. HRTEM image of an individual nanoparticle indicates the d-spacing of the crystallographic plane is 0.23 nm, which agrees well with the distance of (111) lattice plane of Ag (Figure 2d).46 The electron diffraction patterns (inset in Figure 2d) prove the single crystal nature of the synthesized AgNPs. The local elemental composition of the product was confirmed as Ag element by EDX microanalysis at a single nanocrystal level (Supporting Information Figure S1). The explanation about how polypeptides like silk fibroin reduce Ag+ to Ag and subsequently form AgNPs can be found in the literature.47,48 Figure 3a is the time-resolved UV−vis spectra recorded from beginning of the reaction to 32 h. The main absorption at 440

Figure 1. (a) Possible mechanism of the reduction of Ag+ ion by Tyr residues in silk fibroin chain. (b) Fluorescence spectra of RSF/AgNO3 reaction systems with different AgNO3 concentrations (from 0 to 16 mg/mL) after 24 h at room temperature ([RSF] = 1 wt %).

RESULTS AND DISCUSSION In Situ Prepared RSF-AgNPs Composite Solution. B. mori silk fibroin contains 18 amino acid residues, among which the tyrosine (Tyr) has strong electron donating property. On the basis of the previous studies on the synthesis of noble metal (Ag and Au) nanoparticles with either single Tyr molecule or polypeptides and proteins containing Try residues in both our laboratory39 and other research groups,40−43 we believe that silk fibroin is also capable of reducing Ag+ to Ag by the redox-active

Figure 2. UV−vis spectra (a) and TEM (b−d) images of the synthesized RSF-AgNPs composites ([RSF] = 1 wt %, [AgNO3] = 4 mg/mL). 4485 | Biomacromolecules 2013, 14, 4483−4488



a rapid growth period from 0 to 20 h and then levels off. Therefore, we set the reaction time as 24 h in the present study to make sure the formation of AgNPs is complete. We should point out that AgNPs can be also formed if the RSF-AgNO3 solution was exposed under sunlight.49 However, it is not controllable to use sunlight, so we chose an incandescent lamp for this fundamental research. For real applications, it would be no problem to use sunlight and would probably need a long time to allow the reaction to be completed. Individual AgNPs are easily oxidized and/or aggregated in air that may affect their practical uses,50,51 so the stability of AgNPs has been considered as a critical key for the real application. In our case, RSF-AgNPs composite formed a stable yellow colloidal solution and almost had no change for at least one month. Figure 3c shows the UV−vis spectra as well as the appearance of RSF-AgNPs composite solutions, which scarcely change after 30 days storage either in the dark or in the light. During the AgNPs formation process, silk fibroin chains connect to the surface of nanoparticles.47,48 Silk fibroin is a protein, a natural nonionic surfactant, so it surely enhances the stability of AgNPs in the aqueous dispersions.52 Therefore, silk fibroin also acts as a dispersing and stabilizing agent of the synthesized AgNPs like soy protein39 and bovine serum albumin.53 Antibacterial Property of RSF-AgNPs Composite. In this article, we evaluated the antibacterial activity of our RSFAgNPs composite with a clinical isolated MRSA. We cultured the bacteria in MHB liquid media containing different amount of RSF-AgNPs composite for 24 h, and the result was shown in Figure 4a. It can be found that the bacteria suspension became turbid if there was no RSF-AgNPs composite in the mixture, suggesting the bacteria proliferated rapidly in MHB medium. With the increase in concentration of RSF-AgNPs composite in MHB medium, the bacteria suspension was still turbid until the AgNPs concentration in the mixture reached 19.2 mg/L. As the MIC is defined as the lowest concentration of an antimicrobial material that can inhibit bacteria growth, so the MIC of AgNPs in our RSF-AgNPs composite solution is 19.2 mg/L. Subsequently, we plated the bacteria suspensions in which the AgNPs concentration was larger than MIC onto the MH agar plates and incubated at 37 °C for another 24 h. The result showed that when the AgNPs concentration increased to 76.8 mg/L, the monoclonal number of the bacteria growing on the MH agar plate was less than 5 (Figure 4b). Therefore, the MBC of AgNPs in our RSF-AgNPs composite was identified as 76.8 mg/L. As our RSF-AgNPs composite exhibited quite effective antimicrobial activity to MRSA, as shown above, in the next step we tested its effect on the inhibition of the biofilm formation. The result showed that the biofilms were formed if the concentration of RSF-AgNPs was no more than a quarter of MIC (Supporting Information Figure S2a−c). When the concentration increased to half of MIC, although the bacteria still grew, the biofilm was failed to form (Supporting Information Figure S2d). Of course, it was understandable that MRSA bacteria were killed or their growth was inhibited when the concentration of RSF-AgNPs reached MIC (Supporting Information Figure S2e). In comparison with the inhibition effect on the biofilm formation by the bacteria, the ability to destroy of RSF-AgNPs composite toward the maturely formed biofilm created by MRSA is more promising in real clinical application. Here, we used SEM and CLSM to examine the structures of the biofilms

Figure 3. (a) Time-resolved UV−vis spectra of RSF-AgNO3 reaction solution. (b) The change of the absorption at 440 nm at different reaction time, and the inset is the corresponding digital photos of the reaction system. (c) Comparison of UV−vis spectra of RSF-AgNPs composite solutions: black line, fresh synthesized; red line, stored in the dark for 30 day; green line, stored in the light for 30 days (the inset is the corresponding digital photos).

Figure 4. Antibacterial activities of RSF-AgNPs composites. (a) MIC test shows the MHB liquid medium turbidity assays of bacteria suspensions after incubation with RSF-AgNPs composite for 24 h at 37 °C. (b) MBC test shows the colonies of the bacteria suspensions taken from the MIC test tubes and incubated for another 24 h on the MH agar plate.

nm was plotted as a function of time in Figure 3b and the corresponding optical photos were shown as an inset. It shows 4486 | Biomacromolecules 2013, 14, 4483−4488



Figure 5. SEM (small magnification, a1−d1; large magnification, a2−d2) and CSLM images (a3−d3, with the same magnification of a1−d1) of maturely formed MASA biofilm after contact with RSF-AgNPs composite with different concentration at 37 °C for 24 h (a1−a3, MIC; b1−b3, MBC; c1−c3, 2 × MBC; d1−d3, 5 × MBC). In the CSLM image, the live MASA emit green light, while the dead MASA emit red light.

and the corresponding bacterial activities after these biofilms were contacted with a different concentration of RSF-AgNPs composite. The MRSA created a thick and dense biofilm under the experimental condition (Supporting Information Figure S3). In the meantime, the proportion of the live MRSA in the biofilm was very high. When the biofilm contacted the RSFAgNPs composite with the concentration of MIC (Figure 5a1− a3), the structure of the biofilm and the proportion of the live bacteria in the biofilm seemed to have little difference from the control. With the increase of AgNP’s concentration to MBC, the structure of the biofilm became loose, showing an islandlike distribution (Figure 5b1−b3). Although there were still quite a lot of biofilm structures remaining, the number of dead bacteria significantly increased, indicating the strong bactericidal ability of RSF-AgNPs composite at MBC. When the concentration of RSF-AgNPs composite further increased to 2 × MBC (Figure5c1-c3), the biofilm started to break, so most of the bacteria were washed away by the physiological saline before the SEM and CLSM observation. Meanwhile, it can be seen that the bacteria that remained in the cracked biofilm were almost dead. If we continuously increased the concentration of RSF-AgNPs composite to 5 × MBC, the biofilm was totally destroyed (Figure 5d1−d3). Therefore, it clearly indicated that

our RSF-AgNPs composite had an obvious effect to destroy the structure of a maturely formed biofilm and kill the corresponding bacteria.


In this article, silk fibroin was used in situ to synthesize AgNPs by adding AgNO3 powders into RSF solution and exposing under 40 W incandescent light at room temperature. The reaction was found to happen under the sunlight as well. In the whole synthetic process, silk fibroin acted as the reducing, dispersing, and stabilizing agent, so no other chemical reagent was needed. Tyr residues in the silk fibroin backbone were thought to be responsible for the reduction of Ag+ into AgNPs in situ. The prepared RSF-AgNPs composite exhibited an effective antibacterial activity against MRSA and showed strong ability to destroy the maturely formed biofilm created by the same bacterium. Therefore, it is a rather green route to produce AgNPs compared to the reported method, and the product has the great potential as an antibacterial and biofilm-disrupting agent in real clinic applications. 4487 | Biomacromolecules 2013, 14, 4483−4488



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S Supporting Information *

EDX spectrum of the synthesized RSF-AgNPs composite, photos of biofilm formation phenomenon at different RSFAgNPs composite concentration, and SEM and CSLM images of the maturely formed MASA biofilm. This material is available free of charge via the Internet at


Corresponding Authors

*E-mail: (X.C.) [email protected]. Fax: +86 21 5163 0300. Tel: +86 21 6564 2866. *E-mail: (X.Z.) [email protected]. Fax: +86 21 6564 7072. Tel: +86 21 5288 7073. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. X.F. and M.J. contributed equally. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National High Technology Research and Development Program of China (863 Program) (No. 2012AA030309) and the National Natural Science Foundation of China (No. 21034003, 21274028, and 81271094). We thank Dr. Jinrong Yao and Dr. Lei Huang for their valuable suggestions and discussions.


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Green synthesis of silk fibroin-silver nanoparticle composites with effective antibacterial and biofilm-disrupting properties.

Natural polymer Bombyx mori silk fibroin is used as a biotemplate to produce silver nanoparticles in situ under light (both incandescent light and sun...
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