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Facile Fabrication of AgCl@Polypyrrole-Chitosan Core-Shell Nanoparticles and Polymeric Hollow Nanospheres Daming Cheng, Haibing Xia, and Hardy Sze On Chan* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117 543 Received June 30, 2004. In Final Form: September 17, 2004 A one-step sequential method for preparing AgCl@polypyrrole-chitosan core-shell nanoparticles and subsequently the formation of polypyrrole-chitosan hollow nanospheres is reported. The formation of the core and the shell is performed in one reaction medium almost simultaneously. Transmission electron microscopy (TEM) images show the presence of core-shell nanoparticles and hollow nanospheres. Ultraviolet-visible (UV-vis) studies reveal that AgCl was formed first followed by polypyrrole. X-ray diffration (XRD) and UV-vis studies show that AgCl was present in the core-shell nanoparticles and could be removed completely from the core.
Introduction The ability to obtain, control, manipulate, and modify structures at the nanometer scale is of great scientific and technological interest to scientists.1 Recently, there has been increasing interest in the fabrication of coreshell nanomaterials. These nanomaterials may have better performance compared with that of the individual core or shell material. For example, Klabunde et al.2 demonstrated that core-shell nanoparticles such as Fe2O3@MgO and Fe2O3@CaO had greatly enhanced efficiencies over pure MgO and CaO catalysts for SO2 adsorption, H2S removal, and chlorocarbon destruction. Generally, there are two major routines for synthesizing core-shell nanomaterials.3 The first one consists of the in situ synthesis of nanoparticles in the polymer matrix. The second one involves the polymerization of the monomers around the nanoparticles. Core-shell nanoparticles such as Au@polypyrrole,4 Fe2O3@polystyrene,5 and Au@HCMS6 have been reported using these approaches. A recent example of materials design via self-assembly is the fabrication of core-shell particles by the layer-by-layer adsorption of different materials.1,7 Fine control of the shell can be achieved using this method. We provide here a facile one-step pathway to the synthesis of AgCl@polypyrrole-chitosan (AgCl@PPy-CS) nanoparticles. This method does not need to prepare nanoparticles or polymer matrix in advance. The formation of the core and the shell is simple and can be performed sequentially in the same reaction medium. AgCl is chosen as the core, which is based on the consideration that silver halides are one type of important semiconductors. If certain conditions are controlled to avoid photo-decom* Corresponding author. Phone: (65) 6874 2833. Fax: (65) 6779 1691. E-mail:
[email protected]. (1) Caruso, F.; Spaova, M.; Salguesirno-Maceria, V. Adv. Mater. 2001, 13, 1090. (2) (a) Decker, S.; Klabunde, K. J. J. Am. Chem. Soc. 1996, 118, 12465. (b) Cames, C. L.; Kuabunde, K. J. Chem. Mater. 2002, 14, 1806. (3) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (4) Marinakos, S. M.; Novak, J. P.; Brosseau, L. C.; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (5) Wang, Y.; Teng, X.; Wang, J. S.; Yong, H. Nano Lett. 2003, 3, 789. (6) Kim, M.; Sohn, K.; Na, H. B.; Hyeon, T. Nano Lett. 2002, 2, 1383. (7) (a) Kaltenpoth, G.; Himmelhaus, M.; Slansky, L.; Caruso, F.; Grunze, M. Adv. Mater. 2003, 15, 1113. (b) Ji, T.; Lirtsman, V. G.; Avny, Y.; Davidov, D. Adv. Mater. 2001, 13, 1253.
position, silver halide can be used in heterogeneous catalysis.8 Although silver halide nanoparticles have been extensively investigated, there are few examples describing the synthesis and properties of core-shell nanoparticles containing silver halide. Besides, our AgCl@PPyCS nanoparticles could also be used as templates to produce PPy-CS hollow nanospheres. Polymeric hollow spheres have recently been cited as novel types of carriers and nanoreactors with designed properties because they exhibit controllable permeability and surface functionality.9 This should enable many applications such as the controlled release of drugs10 or gene therapy.11 Experimental Section Preparation of AgCl@PPy-CS Core-Shell Nanoparticles. A 0.01 g portion of AgNO3 and 0.025 mL of pyrrole were first added to 20 mL of chitosan solution (Aldrich, Mw ) 300 000, 1 wt %, in 0.05 M HNO3). A 0.14 g portion of oxidant FeCl3 (Aldrich, in 5 mL of water) was introduced to the dispersion. The polymerization was conducted at 2 °C for 12 h. After polymerization, the reaction mixture was purified by dialysis using a membrane bag (Spectrum Medical Industries, Inc., Mw cutoff of 3500) in 0.05 M HNO3. The dispersion after dilution was used for transmission electron microscopy (TEM) and ultravioletvisible (UV-vis) characterizations. Finally, the AgCl@PPy-CS core-shell material was obtained by precipitating with acetone and drying in a vacuum at 40 °C for 24 h. Preparation of PPy-CS Hollow Nanospheres. A 0.005 g portion of AgCl@PPy-CS core-shell material was first dispersed in 20 mL of distilled water by ultrasonication. The pH of the dispersion was adjusted to 13 by 5 M NH4OH and stirred for 24 h. The formed silver ammonia ions were removed by dialysis with a membrane bag (Mw cutoff of 3500) in 0.1 M NH4OH. After dialysis, the pH of the dispersion was adjusted to 4 with 2 M HNO3. The PPy-CS hollow nanospheres were obtained by precipitating in acetone and drying in a vacuum at 40 °C for 24 h. Characterization. TEM images of the samples were recorded on a Philips CM10 electron microscope at an accelerating voltage of 100 kV. The diameters of the nanoparticles were determined from TEM images by counting 100 nanoparticles. X-ray diffration (8) Kakuta, N.; Goto, N.; Ohkita, H.; Mizushima, T. J. Phys. Chem. B 1999, 103, 5917. (9) Shchukin, D. G.; Suknorukov, G. B.; Mo¨hwald, H. Angew. Chem., Int. Ed. 2003, 42, 4472. (10) Thurmond, K. B.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1997, 119, 6656. (11) Kmiec, E. B. Am. Sci. 1999, 87, 240.
10.1021/la048377w CCC: $27.50 © 2004 American Chemical Society Published on Web 10/14/2004
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Scheme 1. Schematic Illustration of the Preparation of AgCl@PPy-CS Core-Shell Nanoparticles and PPy-CS Hollow Nanospheres
(XRD) patterns of the samples were recorded on a Siemens D5005 diffractometer equipped with a Cu KR (1.5405 Å) X-ray source. The powders were mounted by double-sided adhesive tape on plastic sample holders. A Shimadzu (UV1601PC) UV-vis spectrophotometer was used to measure the absorption of the dispersions containing core-shell nanoparticles or hollow nanospheres.
Results and Discussion The overall procedure of forming AgCl@PPy-CS coreshell nanoparticles and PPy-CS hollow nanospheres is illustrated in Scheme 1. AgNO3 and pyrrole were first introduced to a solution of chitosan before the introduction of oxidant FeCl3. The formation of AgCl core and PPy-CS shell took place in the same reaction medium, but the formation rates are different. AgCl nanoparticles formed instantly due to the rapid reaction rate between silver and chlorine ions. The polymerization of pyrrole initiated by Fe3+ began at the same time, but it was a relatively slow process.12 After polymerization, a shell of PPy-CS composite is formed on the surface of AgCl nanoparticles. The core could further be removed by treating the nanoparticles with NH4OH. The TEM images presented in Figure 1 illustrate the nanostructures that were obtained at various times in the preparation. Figure 1a shows an image of AgCl nanoparticles immediately after FeCl3 had been added to the reaction medium. At this stage, the polymerization of pyrrole has just started and no polymeric shell was observed. Stabilized by chitosan, AgCl nanoparticles disperse very well in aqueous media with diameters of 20 ( 2 nm. Figure 1b shows a representative image of coreshell nanoparticles after the polymerization of pyrrole for 12 h. AgCl nanoparticles surrounded by a spherical polymer shell (PPy-CS) are evident in the image. Welldefined particles are readily observed with light contrast shell and the dark contrast cores of AgCl. The mean diameter of core-shell nanoparticles is around 50 ( 5 nm (diameter of core, 20 ( 3 nm; thickness of shell, 15 ( 4 nm). The polymer appears to have nucleated on and grown outward from the AgCl particles. Most of the cores are spherical in shape and are just the same size as those nanoparticles shown in Figure 1a. The structure of a representative poly(aminosaccharide) segment of chitosan is shown in Scheme 1. Amine groups are known to behave normally with respect to protonation in aqueous solution. In our case, chitosan not only stabilizes AgCl nanoparticles but also prevents the aggregation of polypyrrole efficiently.13 AgCl@PPy-CS (12) Stejskal, J. J. Polym. Mater. 2001, 18, 225.
nanoparticles disperse very well in acidic aqueous media. FeCl3 also plays two roles during the formation of AgCl@PPy-CS. One is that chlorine ions provide building blocks to form AgCl nanoparticles, and the other is that ferric ions act as the oxidant for the polymerization of pyrrole. The diameter of the AgCl core depends on the concentration of AgNO3 and the amount of chitosan in the reaction medium. The thickness of the shell, on the other hand, depends on the concentration of pyrrole, the amount of chitosan, and the polymerization time. Figure 2a shows the core-shell nanoparticles with enlarged cores (30 ( 2 nm) compared with Figure 1b. Figure 2b shows coreshell nanoparticles with even bigger cores (60 ( 5 nm) and thinner shells (10 ( 2 nm) compared with Figure 1b. The reaction temperature should be low enough (2 °C) to get uniform nanoparticles. At a higher polymerization temperature (15 °C), multiple AgCl nanoparticles inside one polymer shell are observed (Figure 2c). The UV-vis absorption spectra of the formation of AgCl and AgCl@PPy-CS nanoparticles at different stages are presented in Figure 3 (spectra a and b, respectively). Spectrum a was taken immediately after FeCl3 had been added to the reaction medium. An absorption peak appears at 256 nm which is related to the surface plasmon resonance of AgCl nanoparticles.14 This means AgCl particles are nanosized.15 No additional peaks related to polypyrrole were observed at this stage. Spectrum b was taken when the polymerization was completed. In addition to the surface plasmon band related to AgCl, a broad absorbance at 446 nm corresponds to the π-π* transition of the polypyrrole chain that was observed.16 UV-vis spectra not only confirm the formation of AgCl and polypyrrole but also prove that AgCl was formed first followed by polypyrrole. Even after long periods of sunlight exposure, no additional optical absorption was developed in the 300-500 nm range, where silver metal has been reported to absorb.17 This is because the polymer shell may act as an electron-hole scavenger to offer an efficient pathway for their recombination, thus avoiding electronsilver ion formation.18 (13) Khor, E.; Whey, J. L. H. Carbohydr. Polym. 1995, 26, 183. (14) Husein, M.; Rodil, E.; Vera, J. Langmuir 2003, 19, 8467. (15) Mulvaney, P. In Semiconductor Nanoclusterss Physical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier: Amsterdam, The Netherlands, 1997; pp 99-123. (16) Selvan, S. T.; Spatz, J. P.; Kolk, H. A.; Mo¨ller, M. Adv. Mater. 1998, 10, 132. (17) Zhang, H.; Mostafavi, M. J. Phys. Chem. B 1997, 101, 8443. (18) Calandra, P.; Longo, A.; Marciano, V.; Liveri, V. T. J. Phys. Chem. B 2003, 107, 6724.
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Figure 1. TEM images of (a) AgCl nanoparticles, (b) AgCl@PPy-CS nanoparticles, and (c) PPy-CS hollow nanospheres. Reaction conditions: 20 mL of chitosan (1 wt %, in 0.05 M HNO3), 0.01 g of AgNO3, 0.025 mL of pyrrole, and 0.14 g of FeCl3. The reaction temperature is 2 °C.
The crystalline structure of the AgCl@PPy-CS nanoparticles was analyzed using XRD (Figure 4). The XRD spectrum of AgCl@PPy-CS shows several sharp diffraction peaks together with a broad peak. The sharp peaks at 2θ ) 27.9, 32.3, 46.4, 55.0, and 57.6° are assigned to the (111), (200), (220), (311), and (222) planes of the facecentered cubic (fcc) structure of AgCl crystal.19 The positions of the peaks are in excellent agreement with JPCDS data. The broad peak centered at 2θ ) 25.5° corresponds to the amorphous structure of the polypyrrole-chitosan composite.20 No diffraction peak characteristic of Ag2O or Ag is observed, which confirms that the component of the core of the nanoparticle is AgCl. (19) Rogez, J.; Garnier, A.; Knauth, P. J. Phys. Chem. Solids. 2002, 63, 9. (20) Liu, J.; Wan, M. J. Mater. Chem. 2001, 11, 404.
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Figure 2. TEM images of AgCl@PPy-CS nanoparticles prepared under different reaction conditions: 20 mL of chitosan (1 wt %, in 0.05 M HNO3), x g of AgNO3, and y mL of pyrrole, where (a) x ) 0.02 and y ) 0.025, (b) x ) 0.05 and y ) 0.01, and (c) x ) 0.01 and y ) 0.025. The reaction temperature for parts a and b is 2 °C, while that for part c is 15 °C.
On the basis of the core-shell structure of AgCl@PPyCS, we anticipate that AgCl could serve as a template for forming nanosized polymer hollow spheres. To achieve this, a dispersion containing AgCl@PPy-CS nanoparticles was treated with NH4OH to remove AgCl:
AgCl(s) + NH4OH f Ag[(NH3)2]+ + ClSince the polymer shell was permeable for ions and small organic molecules,10a,21 silver ammonia ions could be easily removed by dialysis. PPy-CS hollow nanospheres are stable in acidic media but insoluble in basic media. This is due to the presence of chitosan in the shell.13 This (21) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, M.; Budde, A.; Mo¨hwald, H. Colloids Surf., A 1998, 137, 253.
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Figure 3. UV-vis absorption spectra of (a) AgCl nanoparticles, (b) AgCl@PPy-CS core-shell nanoparticles, and (c) PPy-CS hollow nanospheres. The reaction conditions are the same as those of the nanoparticles prepared in Figure 1.
property may be used to control the permeability of the PPy-CS hollow nanospheres. Figure 1c is a typical TEM image of PPy-CS hollow nanospheres. The image shows strong contrast between the dark ring and the pale center of the spherical particles, which indicate the formation of hollow structures. The diameter of the hollow nanospheres is ∼50 ( 5 nm, while the diameter of the hole is ∼20 ( 3 nm. The UV-vis absorption spectrum of PPy-CS hollow nanospheres is shown in Figure 3 (spectrum c). The peak at 252 nm that is related to the surface plasmon resonance of AgCl nanoparticles has disappeared. The broad band centered at 460 nm confirms the presence of polypyrrole.16 The XRD pattern of the polymer hollow nanospheres is shown in Figure 4. Spectrum b shows a broad band centered at 2θ ) 25.5°, which is consistent with the amorphous structure of PPy-CS.20 Compared with the XRD pattern of AgCl@PPy-CS, the sharp peaks that correspond to AgCl
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Figure 4. XRD patterns of (a) AgCl@PPy-CS nanoparticles and (b) PPy-CS hollow nanospheres. The reaction conditions are the same as those of the nanoparticles prepared in Figure 1.
crystals are absent. This means that AgCl has been completely removed form the nanoparticles. Conclusions We have reported a one-step sequential method for preparing AgCl@PPy-CS core-shell nanoparticles with a typical diameter of 50 ( 5 nm. The size of the core and thickness of shell could be controlled by adjusting the reaction parameters. AgCl@PPy-CS nanoparticles may find applications in photography or the development of new catalysts. The AgCl core can further be removed to form PPy-CS hollow nanospheres. These polymeric hollow nanospheres may have applications in drug delivery, cell transplantation, and removal of waste. We are currently exploring the possibility of introducing certain molecules into the core of the hollow nanospheres by adsorbing them on AgCl nanoparticles. LA048377W
