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Journal of Macromolecular Science, Part A: Pure and Applied Chemistry Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lmsa20
Green and Facile Synthesis of Gold Nanoparticles Stabilized by Chitosan abc
Yuanpeng Wu
d
b
b
c
c
, Fang Zuo , Yuanhua Lin , Ying Zhou , Zhaohui Zheng & Xiaobin Ding
a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Chengdu 610500, PR China b
School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, PR China c
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China d
College of Chemistry and Environment Protection Engineering, Southwest University for Nationalities, Chengdu 610041, PR China Published online: 16 Apr 2014.
To cite this article: Yuanpeng Wu, Fang Zuo, Yuanhua Lin, Ying Zhou, Zhaohui Zheng & Xiaobin Ding (2014) Green and Facile Synthesis of Gold Nanoparticles Stabilized by Chitosan, Journal of Macromolecular Science, Part A: Pure and Applied Chemistry, 51:5, 441-446, DOI: 10.1080/10601325.2014.893142 To link to this article: http://dx.doi.org/10.1080/10601325.2014.893142
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Journal of Macromolecular Science, Part A: Pure and Applied Chemistry (2014) 51, 441–446 C Taylor & Francis Group, LLC Copyright ISSN: 1060-1325 print / 1520-5738 online DOI: 10.1080/10601325.2014.893142
Green and Facile Synthesis of Gold Nanoparticles Stabilized by Chitosan YUANPENG WU1,2,3, FANG ZUO4, YUANHUA LIN2∗, YING ZHOU2, ZHAOHUI ZHENG3, and XIAOBIN DING3∗ 1
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Chengdu 610500, PR China 2 School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, PR China 3 Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China 4 College of Chemistry and Environment Protection Engineering, Southwest University for Nationalities, Chengdu 610041, PR China
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Received May 2013, Accepted November 2013
A green and facile method has been developed to prepare gold nanoparticles (Au NPs) coated by chitosan in aqueous solutions. Herein, Au NPs capped with chitosan which acted as both the reducing and stabilizing agents were prepared through a two-step route at low temperature. HAuCl4 and chitosan initially reacted for 6 h at room temperature, and subsequently the mixture reacted for another 1 hour at 35◦ C. Ultraviolet visible spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction spectroscopy results confirmed the presence of Au NPs. Transmission electron microscopy indicated that the nanoparticles were dispersed individually. The analysis of thermogravimetric analysis and Fourier transform infrared spectroscopy showed that chitosan was capped on the surface of Au NPs. Keywords: Gold nanoparticles, chitosan, low reactive temperature, green route
1 Introduction Metallic nanostructures play a key role in nanoscience and nanotechnology. Nowadays, tremendous efforts have been made for developing new methods of preparing metal nanoparticles with different morphologies (1–3). In particular, the synthesis, assembly, and surface functionalization of gold nanoparticles (Au NPs) have attracted intensive attention owing to their unusually size-related optical properties, bioconjugation abilities, and easy functionalization (4). Therefore, the number of potential applications for Au NPs in biotechnology and medical research areas is growing drastically (5, 6). There is no doubt that the research of developing the synthetic methods of Au NPs is an important and enduring topic. ∗
Address correspondence to: Yuanhua Lin, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Chengdu 610500, PR China.Tel.: +86 28 85233426; Fax: +86 28 85233426; E-mail:
[email protected] ∗ Xiaobin Ding, Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China; E-mail:
[email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lmsa.
However, many synthetic routes of Au NPs relied on organic solvents so far like toluene and/or toxic reducing agents such as sodium borohydride, which caused serious environmental issues and was also harmful to animals and human beings when the nanoparticles were applied in biomedical field. In order to reduce the utilization of toxic chemicals and eliminate biological risks in pharmaceutical and biomedical applications, green methods are beginning to be developed for preparing Au NPs (7). A green approach should utilize environmental friendly solvent, nontoxic reducing and stabilizing agents. So some small bioactive molecules, polysaccharides and phytochemicals were exploited to serve as both reducing and stabilizing agents for the green preparation of Au NPs, including cellulose (8), starch (9), dextran (10), and chitosan (7, 11). In these ecofriendly natural phytochemicals, chitosan is an attractive agent due to its well-known nontoxicity, biocompatibility, biodegradability, and antibacterial properties (12). Since Huang reported that chitosan can be used as reducing and stabilizing agents to prepare Au NPs (11, 13), many groups have incorporated chitosan into Au NPs preparation and applications. For example, Zhu and Radhakumary described the preparation of Au NPs with chitosan as a stabilizer and reducer of the gold salt through thermal method (14, 15). Li developed an efficient route for
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442 the preparation of Au NPs by direct microwave irradiation of HAuCl4 and chitosan mixed solution (16). Okitsu prepared Au NPs from aqueous solution of NaAuCl4 containing chitosan under sonochemical reduction (17). In another group, the chitosan capped Au and Ag NPs were prepared under different conditions and the catalytic activity of metal-chitosan nanoparticles was tested for the reduction of 4-nitrophenol (18, 19). Very recently, Boca investigated the cytotoxic effects of Au NPs protected by chitosan found that the nanoparticles were biocompatible with Chinese Hamster Ovary cells in vitro(20). Although Au NPs capped by chitosan have been successfully prepared by various strategies as mentioned above, some problems are still present in those routes, such as using high reaction temperature or usually needing the assistance of microwave or ultrasonic technology, which may cause degradation of polymers or expensive and complex processes. Hence, there is a current drive to develop simple, low-cost and biologically acceptable routes for the preparation of Au NPs. The present work is to design and implement a low temperature, green, and facile method to synthesize chitosan stabilized Au NPs. Unlike previous procedures, our process is achieved in one pot, but involves two steps: initially, the mixture of chitosan and HAuCl4 reacted for 6 h at room temperature, and then, the mixture were heated to 35◦ C and reacted for another 1 h. To the best of our knowledge, the reaction temperature of the present approach is the lowest and the synthetic process is simple and green.
2 Experimental 2.1 Materials Gold chloride (HAuCl4 ), was purchased from SigmaAldrich. Chitosan (Mw: 100 kDa) was obtained from Sigma-Aldrich. It was purified with the reprecipitation method according to reference (21). All other reagents were used as received from commercial sources. 2.2 Synthesis of Gold Nanoparticles All glassware used was thoroughly cleaned using detergent and aqua regia solution (HCl-HNO3 , 3:1). A two-step approach was used to prepare Au NPs. Firstly, 0.05 g chitosan was dissolved in 140 mL distilled water under ultrasonic assistance. Then, 6 mL of an aqueous solution of 4.85 mmol/L HAuCl4 was added under constant stirring and the color of the solution turned to yellow. The mixture reacted for 6 h at room temperature and its color changed to pink. Secondly, 2 mL of 1 wt% aqueous acetic acid was added to the above solution and the mixture was heated to 35◦ C. The solution reacted for another 1 h and the color of the solution turned to wine red. Then the resulting solutions were naturally cooled to room temperature. The Au NPs colloid was purified by centrifugation and washed several
Wu et al. times with 2 wt% acetic acid and distilled water to remove the remaining reactants. After that, it was re-suspended in distilled water for further characterization. 2.3 Instruments Ultraviolet-visible spectra (UV-Vis) were measured on a VARIAN CARY 100 Conc spectrophotometer using a 1 cm path length quartz cuvette. Transmission electron microscopy (TEM) was carried out on a JEM-100CX instrument operating an acceleration voltage of 80 kV. TEM specimens were prepared by aspirating a sample onto a copper EM grid. X-ray photoelectron spectroscopy (XPS) was carried out on an XSAM-800 electron and take-off angle of 20◦ was used with X-ray source. The source energy was Al Ka radiation and resolution was 0.9. X-ray diffraction (XRD) data were collected on a Shimadzu XD-D1 X-ray diffractometer employing Cu-Ka radiation at 30 kV and 30 mA. Fourier transform infrared spectra (FTIR) were measured on a Nicolet 200SXV-1 FTIR spectrometer. The transmittance was recorded in the range of 4000–450 cm−1. Thermogravimetric analysis (TGA) was performed on a TA instrument Q50, at a scan rate of 10◦ C/min. The temperature was from room temperature up to 800◦ C under nitrogen atmosphere.
3 Results and Discussion Chitosan has been utilized as a reducing agent to prepare Au NPs at different temperatures. However, due to the low reducing ability of chitosan, these preparing processes usually carried out at an elevated temperature (11, 22) which may cause degradation of chitosan chains and other undesirable problems (23). In the present work, chitosan was used as both reducing agent and stabilizing agent to prepare Au NPs capped by chitosan under mild condition. The approach proceeded through a two-stage process and the synthetic procedure could be easily monitored by naked eyes from the change of the color of the reactive solution. The generation of nanoparticles, as evident from the methodology itself, did not involve complex synthetic procedures or any costly, toxic chemicals. The major advantages of the present approach are that the reaction temperature is lowest and the procedure is facile and eco-friendly. The low reaction temperature could be interpreted as follows. The solubility of chitosan is low in pure water. It is usually dissolved in acidic water to reduce Au3+. However, the reduction of Au3+ is not easier in acidic condition than that in alkaline or neutral condition (24), so elevated reaction temperature is needed. In our approach, nucleation process of Au NPs was processed under neutral condition (without acetic acid) and the reduction of Au3+ can be realized at low reaction temperature. In the growth process, acetic acid was added and the mixture was heated to 35◦ C and reacted for an hour at that temperature, so the whole
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Fig. 1. UV-Vis spectrum Au nanoparticles dispersed in aqueous solution. The inset shows the picture the Au nanoparticles.
temperature of the reaction was lower than the conventional route. Additionally, the as-synthesized Au/chitosan NPs were highly stable in solution and could be stored under refrigerator for several months (more than 5 months). The stability of nanoparticles may be attributed to the binding of chitosan chains to the surface of Au through amino group (7). So, the nanoparticles can be easily stored and the surface of nanoparticles can be conveniently modified.
Fig. 3. XPS spectrum of Au nanoparticles (a) and Au (4f) regions of XPS (b).
Fig. 2. TEM image of Au nanoparticles. The bar in the picture is 100 nm.
The optical property of the nanoparticles was characterized by UV-Vis spectrum. Figure 1 shows the UV-Vis absorption spectrum of Au NPs. The absorption maximum is observed at 534 nm, which is the characteristic surface plasmon resonance (SPR) band of Au NPs (25). The characteristic SPR band of the reacted solution indicates that the zero-valent Au NPs are successfully formed. Figure 1 inset shows photos of the as-prepared nanoaparticles dispersed in distilled water. The color of the solution as shown in the inset is wine red which is coincident with the Au NPs in other reports (26). The dispersion and nanoparticle size of the as-prepared Au NPs in aqueous solutions can be determined by TEM images. Figure 2 shows the TEM image of Au NPs. It is clearly seen from the TEM image that the metal nanoparticles have formed through the above route. The Au NPs disperse singlely and have no aggregation in solution. The good dispersion may be attributed to completely protection
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Fig. 4. XRD patterns of Au nanoparticles.
of Au NPs by chitosan. The mean diameter of Au NPs as shown in Figure 2 is ca. 34.5 nm. XPS spectra are widely applied to confirming the existence of a particular element in the surface of a material and further to distinguishing the different forms of the same element. XPS spectra as shown in Figure 3 are used to determine the surface compositions of elements in the asprepared nanoparticles. Figure 3a shows that the nanoparticles contain Au, C, N, and O, indicating that the surface is
Fig. 5. FTIR spectra of Chitosan and Au/chitosan.
Wu et al. composed of Au and chitosans (N is one main component in chitosan). To further confirm the different forms of these elements in the nanoparticles, detailed Au 4f was measured. Figure 3b shows XPS Au 4f spectrum of the nanoparticles. The broad peaks locate at 83.9 eV and 87.7 eV corresponding to Au 4f7/2 and Au 4f5/2 , respectively. These results are mostly consistent with the emission of 4f photoelectrons from Au0 (27) thereby suggesting the successful formation of Au0 atoms in the nanoparticles. In order to investigate the crystallinity, the as-prepared nanoparticles were characterized by powder X-ray diffraction (XRD). Figure 4 is the XRD of the nanoparticles and it indicates that the as-prepared nanoparticle is the resulting material. The diffraction peak positioning at 2θ = 38.35, 44.39, 64.68, 77.61, and 81.72 can be attributed to Au (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes, respectively (28). These data in XRD patterns are coincident with that determined in other literatures (16, 29) and again prove the formation of Au NPs. FTIR spectra are a useful tool to identify the presence of specific chemicals containing certain functional groups. In order to further verify the successful formation of the chitosan capped Au NPs, FTIR spectra as shown in Figure 5 were collected. Figure 5(a and b) are the FTIR spectra of the pristine chitosan and Au/chitosan nanoparticles samples, respectively. The strong broad absorption band at 3300–3500 cm−1 correspond to the characteristic
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of residual or physically adsorbed water on the nanoparticles surfaces. In the second step, a distinct weight loss of ca. 48.5 wt% happens from 190◦ C to 340◦ C, which may be attributed to the thermal degradation of the chitosan polymers (33). The amount of chitosan coated on the surface of Au NPs could be calculated on the basis of weight loss of the second step in the TGA data and is found to be ca. 54.2 wt% of the mass of the Au/chitosan nanoparticles. In addition, in order to investigate the reactive effective of the Au3+ in the present method. After Au NPs were separated, the reactive solution was investigated by UV-Vis and the result indicated that there was no Au3+. This means that the reactive effective of Au3+ was probably 100%. And based on these results and the data of TGA, we can calculate that there are about 13.5% chitosan is absorbed on the Au NP after reaction and subsequent treatment relative to the amount of chitosan used in the starting of reaction. Fig. 6. TGA spectrum of Au/chitosan nanoparticles.
4 Conclusions stretching vibration of N–H, although the broad band may be due to overlapping between the N–H and the O–H stretching vibrations (19). The wavenumbers of 1652.4, 1596.5, 1419.7, 1080.5, and 899.5cm−1 in Figure 5(a) are tightly related to the vibration of N-H bending, C-N stretching, and N-H rocking bands. However, in Figure 5(b), these peaks shift to 1638.6, 1565.0, 1407.3, 1072.6, and 895.1 cm−1. The gradual blue shifting of these peaks can be attributed to the effective interaction between the amine of the chitosan back bone and Au NPs. The attachment of Au to nitrogen atoms in the chitosan molecules reduced the vibration intensity of the N-H bond due to the increase in molecular weight with Au binding, so the blue shift phenomenon occurred. The present results are consistent with those of similar condition in literature (30) and confirm that chitosan molecules immobilized on the surface of Au NPs through nitrogen atom. In addition, another obvious change that can be observed in the FTIR spectra happens at the wavenumbers of 2926.5 and 2865.3cm−1. These peaks may be assigned to C-H and O-H stretchings (31), and these transmittances are significantly decreased after capped on Au NPs. Since Au is unlikely to be combined with carbon atom, the results may therefore suggest that oxygen atoms in the hydroxyls could also be interacted with Au. However, it can be obviously observed that the effect between oxygen atoms and Au is weaker than that between nitrogen atoms and Au (31). It was known that the content of organic chemicals in the organic/inorganic hybrid materials can be tested through TGA measurements (32). The content of chitosan polymers in the as-prepared nanoparticles was studied by TGA as shown in Figure 6. According to the TGA data, there are two main weight loss steps in the whole heating process. In the first step, a weight loss of ca. 10.5 wt% from room temperature to 125◦ C is observed, which may be due to the loss
In conclusion, we have demonstrated a novel, low temperature, green, and convenient route for the fabrication of chitosans coated Au NPs. The nanoparticles were prepared through a two steps process: chitosan and HAuCl4 first were reacted at ambient temperature, and then the mixture was continuously reacted at 35◦ C. UV-Vis, TEM, XPS, and XRD results confirmed the successful formation of Au NPs capped with chitosan. FTIR indicated that chitosan was coated on the surface of Au NPs through amine group. TGA showed that the amount of chitosan in the mass of the Au/chitosan nanoparticles was ca. 54.2 wt%. Our next tasks are to try to investigate the formation mechanism of Au NPs in our approach.
Funding This work was financially supported by the Foundation of Science and Technology Department of Sichuan Province (No. 2012FZ0131), the Open Fund (No. PLN1112, 1201) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), the Science Foundation of Southwest Petroleum University (No. 2012XJZ011) and the National Natural Science Foundation of China (No. 51304166, 50903011).
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