Silk fibroin fibers supported with high density of gold nanoparticles: fabrication and application as catalyst
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- Xia, Y., Wan, J. & Gu, Q. Gold Bull (2011) 44: 171. doi:10.1007/s13404-011-0024-7
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Silk fibroin (SF) fibers were modified with sulfonated polyaniline and, via an in situ redox technique, a high density of gold (Au) nanoparticles were supported directly on the surface of the fiber. The morphology, formation, and application of the as-prepared product, Au/SPANI-modified SF composite fiber, were investigated. By controlling the concentration of HAuCl4, the density of Au nanoparticles on the composite fiber could be effectively adjusted. It is suggested that sulfonated polyaniline contributes to the generation of a high density of Au nanoparticles supported on the SF fibers. The composite fiber exhibited good activity when taking the reduction of p-nitrophenol as a model reaction.
KeywordsFibers Nanoparticles Scanning electron microscopy (SEM)
Nanostructured noble metals have been widely used as catalysts for a variety of organic reactions [1, 2, 3] due to their surface-to-volume ratio and high surface energy. To overcome the problem of particle aggregation and catalyst deactivation, the metal is generally immobilized on a solid support such as carbon, a metal oxide, or polymer microspheres . However, in recent years researchers have shown an increasing interest in the preparation and use of polymer fiber-supported noble metal catalysts [5, 6, 7].
Silk fibroin (SF), which can be obtained by removing the outer sericin of silk fiber, has been extensively studied in biomedical and chemical fields . Indeed, recent years theoretical and experimental studies have uncovered a number of novel phenomena [9, 10]. One of the most interesting finds is that a noble metal ion can be reduced by SF fiber to form zero-valent metal [11, 12, 13], which could lead to a new class of polymer fiber-supported catalysts. The tyrosine residue (Tyr) component in the SF has strong electron donating complex properties and can provide a redox-active site to construct nanostructured noble metal crystallites. However, even when the reaction is carried out at a high concentration of noble metal ions, only a low density of noble metal nanoparticles are generated on the surface of SF fiber due to the low content of tyrosine residue (5.17 mol%)  on the SF backbone. This restricts the catalytic activity of SF fiber-supported noble metal composites. Therefore, identifying a novel method to produce high density of noble metal nanoparticles on the SF fiber is of significant interest and importance to the field.
In our previous reports [14, 15], we showed that water-soluble sulfonated polyaniline (SPANI) could reduce noble metal ions at room temperature. Herein, we build on these data by demonstrating a convenient route to prepare a high density of Au nanoparticles on the SF fiber. This as-prepared composite fiber could have many other applications in a range of fields such as biosensors and textiles.
Fresh Bombyx mori cocoons were purchased from Maanshan Textile Corporation, Anhui Province, China. Aniline, (NH4)2S2O8, HSO3Cl, 1, 2-dichloroethane, and HAuCl4 were purchased from Beijing Chemical Co. in their reagent grades. Those reagents were used without further purification.
Preparation of SF fiber
Fresh B. mori cocoons were degummed twice with a 5-g/L anhydrous sodium carbonate solution at 100°C for 1 h, then were rinsed thoroughly in warm distilled water and dried in vacuum at 40°C.
The modification of SF fibers with SPANI
Generating a high density of Au nanoparticles on the surface of SF fibers
Typically, SPANI-modified SF fibers (0.001 g) were dispersed in 20 mL of HAuCl4 aqueous solution (10 mmol L−1) without stirring for 24 h at room temperature. Then, the product, Au/SPANI-modified SF composite fiber, was obtained after being filtered with distilled water and dried in vacuo at 40°C. Au/SPANI-modified SF composite fibers with different density of Au nanoparticles could be obtained by adjusting the concentration of HAuCl4 aqueous solution.
Analysis of the loading and content of Au nanoparticles on SF fibers
Catalytic activity of Au/SPANI-modified SF composite fibers
A reaction mixture of water (6 mL), p-nitrophenol aqueous solution (5 mL, 1.5 × 10−3 mol L−1), dried Au/SPANI-modified SF composite fiber (0.001 g) was put in a beaker and stirred. To this mixture, NaBH4 aqueous solution (2 mL, 1.5 mol L−1) was then added. The progress of the conversion of p-nitrophenol to p-aminophenol was monitored via UV–Vis spectroscopy by recording the time-dependent absorption spectra of the reaction at a regular time interval of 5 min at room temperature.
The catalytic activity of Au/SF composite fibers was also investigated according to the same procedure.
Instruments and measurements
Scanning electron microscopy (SEM) and optical microscope experiments were performed with JSM-5610 and XPT-7 microscopes, respectively. Fourier transform infrared spectra (FTIR) were recorded on a BrukerVECTOR220 spectrometer. X-ray diffraction (XRD) data were acquired with a Rigaku D/MAX-RC diffractometer using CuKα radiation in the 2θ range of 35–80° at 45 kV. UV–Vis spectra were recorded on a UV-240 spectrometer (Shimadzu, Japan).
Results and discussion
The modification of SF fibers with SPANI
Quantitative analysis of main elements in SF fibers before and after being modified by SPANI
The weight of main elements (wt.%)
The characterization of Au/SPANI-modified SF composite fibers
The formation of Au/SPANI-modified SF composite fibers
The effect of [HAuCl4] on the density of Au nanoparticles on the au/SPANI-modified SF composite fibers
The catalytic activity of Au/SPANI-modified SF composite fibers
For Au/SPANI-modified SF composite fiber, the Au nanoparticles are firmly integrated with the SF fiber even after repeated washing in water. This fact implies that the composite fibers could be used repeatedly. To support this point, we have not found obvious change of the rate constant, even after using the same sample on 16 occasions. Indeed, when compared with many other Au-based catalysts, the Au/SPANI-modified SF composite fiber catalyst described here has an obvious advantage of easy and convenient operation in the reaction system. Moreover, the tedious and time-consuming regeneration process which often includes precipitating, filtering, and redispersing can be simplified by washing only with excess water. These facts combine to make such composite fibers excellent materials for practical catalyst applications.
SF fibers supported with high density of Au nanoparticles have been easily obtained by modifying SF with sulfonated polyaniline (SPANI) and subsequently in situ reduction of AuCl4− on the fiber surface. In addition to the tyrosine residues on the SF backbone, SPANI also played an important role in controlling the formation and shape of Au nanoparticles on the surface of SF fibers. The as-prepared composite fibers exhibited good catalytic activity and may have other potential applications in a range of fields such as biosensors and textiles.
We thank Dr. Trevor Keel (World Gold Council) for help with formatting the manuscript, and are grateful for Anhui Provincial Natural Science Foundation (10040606Q08), China Postdoctoral Science Foundation (20110491393), SRTP Fund of Anhui University of Technology (2011013), and the financial support (2010004) from the Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University.
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