2330
J. Phys. Chem. C 2008, 112, 2330-2336
Hyperbranched Poly(amidoamine) as the Stabilizer and Reductant To Prepare Colloid Silver Nanoparticles in Situ and Their Antibacterial Activity Yongwen Zhang,† Huashong Peng,‡ Wei Huang,*,† Yongfeng Zhou,† Xuehong Zhang,‡ and Deyue Yan*,† School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites and School of Life Science & Biotechnology, Key Laboratory of Microbial Metabolism (Ministry of Education), Shanghai Jiao Tong UniVersity, 800 Dongchuan Road, Shanghai, 200240, People’s Republic of China ReceiVed: July 12, 2007; In Final Form: October 30, 2007
This study provided a facile and green method to prepare stable colloid silver nanoparticles in aqueous solution by utilizing the amine-terminated hyperbranched poly(amidoamine) (HPAMAM-NH2) as both stabilizer and reductant. The formation of silver nanoparticles was verified by FTIR, UV-vis, TEM, EDS, and XRD measurements. Monodispersed colloid silver nanoparticles with small particle sizes were obtained, and the average particle size could be effectively controlled from ca. 15 to 4 nm by simply adjusting the molar ratio of N/Ag in feed. The antibacterial activity of the HPAMAM-NH2/Ag nanocomposites was also investigated against Gram-positive and Gram-negative bacteria. They were able to efficiently inhibit the growth and multiplication of several bacteria, including Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Klebsiella mobilis, and the bacterial inhibition ratio reached up to 95% at a low silver concentration of 2.7 µg/mL.
Introduction In recent years, the polymer/metal (silver,1,2 platinum,3,4 palladium,5,6 gold,7,8 copper,9 cadmium sulfide,10 etc.) nanocomposites have received much attention due to their various potential applications in catalysts, electronics, chemical sensors, and so forth.11-14 Generally, the metal cations are first complexed with a certain polymer, and then reduced in situ by using various reducing agents to form the stable colloidal polymer/ metal nanocomposites. Here the polymers acted as stabilizers, templates, or protecting agents. For example, silver nanodecahedrons were fabricated under the assistance of linear poly(vinylpyrrolidone).13 Catalytic palladium nanoparticles were prepared in the film of linear poly(ethyleneimine) (PEI).14 Silver, platinum, and palladium nanoparticles were successfully encapsulated within poly(propyleneimine) dendrimers.15 All of these polymers possessed some strong electron-donating centers, such as amino or carboxylate groups, to facilitate the complex with the metal ions. In these works, the preparation of polymer/ metal nanoparticles often involved some tedious steps and extra reducing agents were required, including sodium borohydride (NaHB4), formaldehyde, sodium citrate, hydrazine, ascorbic acid, glucose, and solvents like N,N-dimethylformamide (DMF), ethylene glycol, etc. In addition, γ-ray or UV irradiation was also utilized as an alternative to reduce the metal cations.16-18 More recently, pure and alkylated linear PEIs have been reported as both reductants and stabilizers to prepare metallic nanoparticles. However, the stabilizing ability for metallic nanoparticles of pure linear PEI was not satisfactory due to its chain aggregation. This often resulted in the coagulation of metallic nanoparticles.19,20 As for alkylated linear PEI, its stabilizing * To whom correspondence should be addressed. E-mail: hw66@ sjtu.edu.cn (W.H.);
[email protected] (D.Y.). † State Key Laboratory of Metal Matrix Composites. ‡ Key Laboratory of Microbial Metabolism (Ministry of Education).
ability was greatly improved. However, the self-reduction was completed over a long period of time.21,22 As a new sort of polymers with quasispherical branched architecture and special solution/melting properties, hyperbranched polymers have received more and more attention, and have been widely applied as rheological additives, drug carriers, self-assembly precursors, etc.23 However, compared with the linear polymers, only a limited number of works have reported the application of hyperbranched polymers in preparing colloidal polymer/metal nanocomposites. For instance, Tiller and coworkers adopted the amphiphilically modified hyperbranched PEI to stabilize silver nanoparticles.24 They further used the network film based on the hyperbranched PEI containing double bonds to load silver nanoparticles.25 Here Li[HBEt3] or ascorbic acid was used as the reducing agent. Lu et al. also reported hyperbranched poly(amine ester) as the template to prepare stable gold nanoparticles, and NaHB4 was applied as a reductant.26 In these pioneering works, the hyperbranched polymers did show some fascinating advantages in preparing polymer/ metal nanocomposites, such as the quasispherical branched structure with many inner cavities and almost nonexistence of chain entanglements. Nevertheless, without exception, extra reducing agents were needed in these systems in order to achieve efficient and complete reduction. Herein, we designed and synthesized a new kind of stabilizer, i.e., an amine-terminated hyperbranched poly(amidoamine) (HPAMAM-NH2), to produce colloid silver nanoparticles. More importantly, this hyperbranched polymer was found to serve as a highly effective self-reducing agent. The advantages of this method are as follows: (1) No extra reducing agent was needed. (2) The process was conducted at room temperature, under normal pressure, and in aqueous solution, so it is a green route. (3) The obtained silver nanoparticles have several excellent properties, including long-term dispersion stability, small particle sizes, and a narrow size distribution controlled by the amount
10.1021/jp075436g CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008
Preparation of Colloid Silver Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2331
SCHEME 1: Schematic Plot of Preparing HPAMAM-NH2/Ag Nanocomposites
of HPAMAM-NH2 in the feed. In addition, it is generally recognized that the materials containing silver nanoparticles exhibit some effective antibacterial properties.24,25,27 Hence the antibacterial properties of the as-prepared HPAMAM-NH2/Ag nanocomposites were also studied. As expected, they showed highly antibacterial activity in aqueous Luria-Bertani (LB) medium against several Gram-positive and Gram-negative bacteria.
Materials. N,N′-Methylene bisacrylamide (MBA, 96%) and 1-(2-aminoethyl)piperazine (AEPZ, 99%) were purchased from Acros and used as received. Silver nitrate (AgNO3, >99%) and sodium sulfide (Na2S, 99%) were purchased from Sinopharm Chemical Reagent Co. The hyperbranched polyester Boltorn H20 was purchased from Perstorp AB Co. Hyperbranched polyglycerol (HPG, Mn ) 5000, Mw/Mn ) 1.8, degree of branching (DB) ) 0.5) was synthesized according to the literature.28 Hyperbranched PEI (Mn ) 10 000, Mw ) 25 000) was purchased from Aldrich. Ultrapure water (resistivity >18 MΩ‚cm) processed by the Milli-Q plus system (Millipore Co.)
was used to prepare all the aqueous solutions. Other solvents were used as received without further purification. The inoculants used here were Gram-positive and Gram-negative prokaryotes of clinical interest, i.e., Staphylococcus aureus (S. aureus, ATCC 6538), Bacillus subtilis (B. subtilis, ATCC 21332), Escherichia coli (E. coli, ATCC 8739), and Klebsiella mobilis (K. mobilis, ATCC 13048). All of them were purchased from Fisher. Synthesis of HPAMAM-NH2. HPAMAM-NH2 was synthesized according to the method described in our previous paper;29 DB ) 0.42, Mn ) 2.6 × 104, and Mw/Mn ) 2.3. The total amine value was 0.092 mol/g, which was measured by the titration method described in Supporting Information S1. Preparation of Colloid Silver Nanoparticles. The preparation was performed based on the N/Ag molar ratio in feed calculated by the total amine value of HPAMAM-NH2 and the concentration of AgNO3 aqueous solution. A typical procedure is as follows: 5 mL of AgNO3 aqueous solution (0.1 mol/L) was dropped into 10 mL of aqueous solution of HPAMAMNH2 (16.3 g/L) under vigorous stirring within 5 min. Then it was kept stirring at room temperature for 24 h in the dark. The
Figure 1. FTIR spectra of (a) neat HPAMAM-NH2 and (b) HPAMAMNH2/Ag nanocomposites. N/Ag ) 15.
Figure 2. UV-vis absorption spectra of colloid silver nanoparticles reduced and stabilized by HPAMAM-NH2 at different time points. N/Ag ) 15.
Experimental Section
2332 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Zhang et al.
Figure 3. (a) TEM image of HPMAM-NH2/Ag nanocomposites (inset: SAED pattern of the silver nanoparticles). (b) High-resolution TEM image of a single silver nanoparticle. N/Ag ) 15.
Figure 4. XRD spectrum of resulting silver nanoparticles. N/Ag ) 15.
molar ratio of N/Ag in the mixture was 15. In this process, the light yellow solution gradually changed into the deep brown colloid, indicating that silver cations (Ag+) were reduced into elemental silver (Ag0). By adjusting the molar ratio of N/Ag (N/Ag ) 2, 10, 20, and 30, respectively), a series of colloid silver nanoparticles were obtained. The method based on Na2S was conducted to confirm no remaining Ag+ in the resulting colloidal solution.24 X-ray photoelectron spectroscopy (XPS) measurements further confirmed that the reduction of Ag+ to Ag0 was complete. The data are shown in Supporting Information S2. Characterization. 1H and 13C NMR spectra were recorded by a Varian Mercury Plus 400M Hz spectrometer. The molecular weights were measured on a Perkin-Elmer Series 200 system against polystyrene calibration with DMF as the eluent. For Fourier transform infrared (FTIR) measurement, the colloidal silver solution was poured into acetone and the resulting precipitates were dried for characterization. FTIR spectra were performed on a PE PARAGON 1000 spectrometer. UV-vis absorption spectra were recorded on a Perkin-Elmer Lamdba 20/2.0 UV-vis spectrometer. Transmission electron microscopy (TEM; a JEOL JEM-2100F electron microscope operated at 200 kV) was used to measure the particle size and morphology of the silver nanoparticles in the colloids. The selected-area electron diffraction (SAED) pattern was recorded using the same TEM
operated at 200 kV. The colloidal silver solutions were centrifuged, and the resulting powders were dried for X-ray diffraction (XRD) measurement. The XRD patterns were taken in the 2θ range of 20-80° at a scanning rate of 4°/min using Cu KR radiation. XPS measurements were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg KR radiation (hν ) 1253.6 eV). The whole spectra and the narrow spectra of all the elements with high resolution were both recorded by using RBD 147 interface (RBD Enterprises, USA) through AugerScan 3.21 software, and the data were processed by the same software. Antibacterial Test. An antibacterial test was performed by the LB broth method.30,31 The inhibition ratio based on the bacterial optical density (OD) was presented as the antibacterial efficiency. The OD values were obtained based on the duplicated tests. Typically, 10 mL of sterilized LB broth was added into a sterile flask. The cultured microorganism (S. aureus, B. subtilis, or K. mobilis) in the stationary phase was inoculated into the flask containing the LB broth solution. The HPAMAM-NH2/ Ag colloid in a certain silver concentration was dropped into the flask. The mixture was incubated overnight at 28 °C in a shaking incubator. The same procedure was done with stationary phase cultured E. coli and incubated with shaking at 37 °C. After incubation for 12 h, the OD values of the bacterial broth medium at 600 nm (OD600) were measured by a UV-vis spectrophotometer. The culture with pure LB broth served as the control. The inhibition ratios for the HPAMAM-NH2/Ag colloids were calculated as follows:
inhibition ratio (%) ) 100 - 100[(At - A0)/(Ac - A0)] where A0 corresponded to the OD values for bacterial broth medium before incubation; At and Ac were OD values for HPAMAM-NH2/Ag colloid sample and control sample after incubation for 12 h, respectively. Results and Discussion The schematic plot for preparation of the HPAMAM-NH2/ Ag nanocomposites is demonstrated in Scheme 1. The hyperbranched polymer HPAMAM-NH2 was first synthesized via the Michael addition polymerization from MBA and AEPZ. When AgNO3 solution was added to the aqueous solution of HPAMAMNH2, Ag+ was complexed with HPAMAM-NH2 and then
Preparation of Colloid Silver Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2333
Figure 5. TEM images of silver nanoparticles reduced and protected by HPAMAM-NH2. (a) N/Ag ) 2; (b) N/Ag ) 10; (c) N/Ag ) 20; (d) N/Ag ) 30. Inset: Corresponding size distribution histogram from the TEM. The solid line is a Gaussian fit to the data.
reduced in situ to form colloid silver nanoparticles. Since the amino groups in some alkylated linear PEI21,22 or alkylamines32-34 were able to reduce metal ions, we guess that the amino groups in HPAMAM-NH2 may play a similar role. To prove the hypothesis, hyperbranched polymers without amino groups, such as hyperbranched polyglycerol (HPG) and hyperbranched polyester H20, were used for the same preparation. However, no silver particles were produced under the same dark conditions (see Supporting Information S3). Evidently, amino groups (including 1°, 2°, and 3° amines) in HPAMAM-NH2 provide a suitable environment to reduce Ag+ into Ag0. In this process, a one-electron transfer between the various amines and Ag+ is suggested (Scheme 1).35-38 The nitrogen atom in the amino groups of HPAMAM-NH2 loses one electron to form the oxidized HPAMAM-NH2, and Ag+ acquires the electron to be reduced into Ag0. In fact, except for the reduction ability, HPAMAM-NH2 also shows strong interactions with the reduced silver nanoparticles. Figure 1 displays the FTIR spectra of HPAMAM-NH2 and the HPAMAM-NH2/Ag nanocomposites. As can be seen, the absorption peaks of amide I, II, and III at 1660, 1538, and 1356 cm-1 shifted to 1654, 1542, and 1354 cm-1, respectively, after
HPAMAM-NH2 reacted with AgNO3. Besides, some shifts of the N-H stretching vibrational bands were also found at 32863052 cm-1. These changes indicate that coordination existed between the silver nanoparticles and HPAMAM-NH2 through its inner and surface amine groups, as represented in Scheme 1. The nonbonding electrons of the amines, the carbonyl, and nitrogen in the amide moiety may be donated to the metallic silver and form a stable polymer/Ag complex. The formation of stable HPAMAM-NH2/Ag nanocomposites was further monitored by the UV-vis absorption band of silver nanoparticles. As shown in Figure 2a, only one characteristic peak at 286 nm was observed for the pure HPAMAM-NH2. However, a new strong absorption band appeared at 447 nm after AgNO3 solution was added to the aqueous solution of HPAMAM-NH2 and stirred for 24 h (see Figure 2b). This can be attributed to the surface plasmon absorption of silver colloids. Due to the effect of the positive charges in HPAMAM-NH2, the absorption spectrum of the protected silver nanoparticles assigned to the surface plasmon resonance showed a strong red shift.39 As for the HPAMAM-NH2/Ag colloid, its absorption was almost unchanged even after storing for 1 month (see Figure
2334 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Zhang et al.
Figure 6. Antibacterial evaluation of HPAMAM-NH2/Ag nanocomposites against various bacteria after incubation for 12 h. (a) N/Ag ) 10; (b) N/Ag ) 20; (c) N/Ag ) 30.
2c). In fact, the HPAMAM-NH2/Ag colloid was very stable without any precipitation for more than 6 months. A representative TEM micrograph of the colloidal HPAMAMNH2/Ag nanocomposites is shown in Figure 3a. The silver nanoparticles are spherical and of uniform particle size, and the average particle size is about 8.2 nm. The SAED pattern in the inset of Figure 3a shows the (111), (200), (220), and (311) planes of the silver nanoparticles.40 The high-resolution image of a single silver nanoparticle in Figure 3b reveals that the wellcrystallized silver nanoparticles were really produced. The XRD pattern of the silver nanoparticles (Figure 4) further reveals that they are crystalline in nature, and the diffraction peaks match those of the cubic silver phase in the Joint Committee for Power Diffraction Studies database (PDF 4-783).41 According to the Scherrer formula, the crystalline size of silver nanoparticles is ca. 8.4 nm, which agrees well with the TEM results. We found that the morphologies of the colloid silver nanoparticles were affected greatly by the polymer concentration in the feed. A series of HPAMAM-NH2/Ag colloidal solutions at the different N/Ag ratios from 2 to 30 were obtained. The TEM images and the particle size distribution histograms of the resulting colloid silver nanoparticles are displayed in Figure 5. Some particle aggregates were observed clearly when the N/Ag ratio was 2 (Figure 5a). The average silver nanoparticle size decreased from 14.3 to 4.5 nm, and their size distribution narrowed when the N/Ag ratio was increased from 10 to 30. This indicates that a large amount of cationic amine groups are more effective to bind the silver particles and prevent them from growing further at the high N/Ag ratios. In fact, at the N/Ag ratio of 2, some dark silver powders were observed to precipitate from the colloidal solution after it was kept for 2 weeks. However, at the N/Ag ratios of 10, 20, and 30, the colloidal solutions were all stable for at least 6 months. Therefore, the silver nanoparticle sizes, the particle size distribution, and the
dispersing stability in water can be controlled easily by simply adjusting the N/Ag ratio in the feed. Evidently, the hyperbranched polymer reported here exhibited the efficiently stabilizing and self-reducing ability to produce stable silver nanoparticles. Now a question naturally occurs: why can it play such a role? As mentioned above, the linear PEI possessed the self-reducing ability to some extent. In addition, we also found that the hyperbranched PEI could reduce Ag+ by itself. However, its self-reducing efficiency was rather low (see Supporting Information S4). Thus, extra reducing agents were preferably adopted to produce the silver nanoparticles by using the hyperbranched PEI or its derivatives as templates.24,25 It is noteworthy that the theoretical content of amino groups (1°, 2°, and 3° amines) in HPAMAM-NH2 is lower than those in the linear PEI or hyperbranched PEI, but HPAMAM-NH2 shows much higher reducing efficiency and better stabilizing ability. Thus, it seems that the content of amino groups is not the dominating factor to decide the stabilizing and self-reducing ability of polymer templates. Based on the FTIR results in Figure 1, we suggest that the carbonyl groups (and even piperazine rings) may have some synergistic effects with the amino groups in the reducing and stabilizing process. These groups can “grab” silver ions from the solution due to the complex interactions between them, and greatly decrease the distance between the silver ions and the amino groups to promote the reduction process of silver ions without any extra reductant. The mechanism is under our further investigation. The antibacterial effects of the HPAMAM-NH2/Ag nanocomposites were evaluated by using the LB broth method. The antibacterial abilities of the above stable colloidal HPAMAMNH2/Ag nanocomposites with N/Ag ratios of 10, 20, and 30 were tested at different silver concentrations. The antibacterial ability of pure HPAMAM-NH2 was also tested as a contrast. We found that an almost transparent aqueous solution was obtained and no precipitate or coagulation was observed after
Preparation of Colloid Silver Nanoparticles adding a certain amount of colloidal HPAMAM-NH2/Ag nanocomposites into the bacterial broth medium and cultivating for 12 h (see Supporting Information S5). Such a homogeneous state could be kept for 2 months or longer. Obviously, the growth of the bacteria had been inhibited effectively by the HPAMAM-NH2/Ag nanocomposites. The inhibition ratios based on the OD600 data (see Supporting Information S6) were further investigated. When the N/Ag ratio was 10 as shown in Figure 6a, the inhibition activity against E. coli, S. aureus, B. subtilis, and K. mobilis was enhanced with increasing silver concentration. The bacterial inhibition ratio of the HPAMAM-NH2/Ag nanocomposites reached up to 95% at the silver concentration of 2.7 µg/mL. Besides, pure HPAMAMNH2 also showed some limited antibacterial ability with the inhibitation ratio less than 10%. Similar results were obtained when the N/Ag ratio was 20 or 30 (Figure 6b,c). Evidently, the silver nanoparticles are mainly responsible for the highly antibacterial activity of the colloidal HPAMAM-NH2/Ag nanocomposites. The antibacterial efficiency is comparable to or even exceeds that of recently reported silver-containing materials, such as silver nanoparticles inside a carbon matrix, silver-doped titania materials, and silver nanoparticles prepared in the presence of some surfactant templates,42-44 which usually achieved excellent antibacterial effect at a relatively higher silver concentration of ca. 5 µg/mL or more. The effective antibacterial activity of the HPAMAM-NH2/Ag nanocomposites can be ascribed to the small and monodispersed silver nanoparticles. Such silver nanoparticles may have good penetration ability and a large surface-to-volume ratio to interact with the bacteria, thereby enhancing the extent of the bacterial elimination. It is generally recognized that silver nanoparticles have a high affinity to react with the sulfur- and phosphorus-containing compounds (such as DNA) in the bacterial cells. Therefore, they can attach to the surfaces of the bacterial cells and even penetrate them, leading to bacterial death.30,45,46 The bactericidal effect of our silver nanoparticles can also be ascribed to the release of silver from HPAMAM-NH2/Ag nanocomposites into surrounding bacterial cells driven by the strong interactions between silver and cells. Besides, the cationic HPAMAM-NH2 macromolecules are able to capture the surrounding negatively charged bacteria through the strong ionic interaction,47-49 which can effectively decrease the distance between the silver nanoparticles and bacteria, and facilitate the release of the active silver into the bacteria. Thus there is a synergistic antibacterial effect of HPAMAM-NH2 templates and the encapsulated silver nanoparticles. Conclusions In conclusion, a series of stable colloid silver nanoparticles were prepared by utilizing HPAMAM-NH2 as both the reductant and stabilizer through a facile and green process; the nanoparticles were able to effectively inhibit the growth and multiplication of several bacteria. The advantages of our method come from the structure and property benefits of the used hyperbranched polymer HPAMAM-NH2. (1) It has a unique quasispherical nanocage architecture with many amido moieties and amino groups (1°, 2°, and 3° amines), leading to the effective self-reducing ability and templating capacity to produce small colloid silver nanoparticles. (2) HPAMAM-NH2 is polar and possesses a highly branched structure, so it has excellent solubility in water and can effectively avoid chain entanglements in comparison with the linear polymers. These features make it a nice template to obtain monodispersed silver nanoparticles with long-term dispersion stability and ensure a green prepara-
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2335 tion process. In addition, HPAMAM-NH2 contains many terminal amine groups. They can be further modified to produce various hyperbranched polymer analogues with self-reduction/ stabilization abilities and designable functions, which can be used as novel templates to fabricate more functional metallic nanoparticles. Acknowledgment. The authors gratefully acknowledge the financial support provided by the National Basic Research Program (Nos. 2007CB808000, 2005CB623803), the National Natural Science Foundation of China (Nos. 50633010, 50503012, 30500014), and the Basic Research Foundation of Shanghai Science and Technique Committee (No. 07DJ14004). Supporting Information Available: Determination of the total amine values, detection method for completion of the reaction, comparative experiments of HPG and H20, comparative reducing efficiency of hyperbranched PEI, photographs of the bacterial medium containing HPAMAM-NH2/Ag nanocomposites after cultivation for 12 h, and OD600 data. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Balogh, L.; Swanson, D. R.; Tomalia, D. A.; Hagnauer, G. L.; McManus, A. T. Nano Lett. 2001, 1, 18. (2) Xu, H.; Xu, J.; Zhu, Z.; Liu, H.; Liu, S. Macromolecules 2006, 39, 8451. (3) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364. (4) Oh, S.-K.; Kim, Y.-G.; Ye, H.; Crooks, R. M. Langmuir 2003, 19, 10420. (5) Scott, R. W. J.; Ye, H.; Henriquez, R. R.; Crooks, R. M. Chem. Mater. 2003, 15, 3873. (6) Yeung, L. K.; Crooks, R. M. Nano Lett. 2001, 1, 14. (7) Kim, Y.-G.; Oh, S.-K.; Crooks, R. M. Chem. Mater. 2004, 16, 167. (8) Yamamoto, M.; Kashiwagi, Y.; Sakata, T.; Mori, H.; Nakamoto, M. Chem. Mater. 2005, 17, 5391. (9) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (10) Lemon, B. I.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 12886. (11) Tabuani, D.; Monticelli, O.; Chincarini, A.; Bianchini, C.; Vizza, F.; Moneti, S.; Russo, S. Macromolecules 2003, 36, 4294. (12) Trakhtenberg, L. I.; Gerasimov, G. N.; Aleksandrova, L. N.; Potapov, V. K. Radiat. Phys. Chem. 2002, 65, 479. (13) Gao, Y.; Jiang, P.; Song, L.; Wang, J. X.; Liu, L. F.; Liu, D. F.; Xiang, Y. J.; Zhang, Z. X.; Zhao, X. W.; Dou, X. Y.; Luo, S. D.; Zhou, W. Y.; Xie, S. S. J. Cryst. Growth 2006, 289, 376. (14) Kidambi, S.; Bruening, M. L. Chem. Mater. 2005, 17, 301. (15) Esumi, K.; Isono, R.; Yoshimura, T. Langmuir 2004, 20, 237. (16) Chou, K. S.; Lu, Y. C.; Lee, H. H. Mater. Chem. Phys. 2005, 94, 429. (17) Shchukin, D. G.; Radtchenko, I. L.; Sukhorukov, G. B. ChemPhysChem 2003, 4, 1101. (18) Khanna, P. K.; Subbarao, V. V. V. S. Mater. Lett. 2003, 57, 2242. (19) Sun, X. P.; Dong, S. J.; Wang, E. K. Polymer 2004, 45, 2181. (20) Sun, X.; Dong, S.; Wang, E. Langmuir 2005, 21, 4710. (21) Kuo, P.-L.; Chen, C.-C.; Jao, M.-W. J. Phys. Chem. B 2005, 109, 9445. (22) Chen, C.-C.; Hsu, C.-H.; Kuo, P.-L. Langmuir 2007, 23, 6801. (23) Gao, C.; Yan, D. Y. Prog. Polym. Sci. 2004, 29, 183. (24) Aymonier, C.; Schlotterbeck, U.; Antonietti, L.; Zacharias, P.; Thomann, R.; Tiller, J. C.; Mecking, S. Chem. Commun. 2002, 3018. (25) Ho, B. C. H.; Tobis, J.; Sprich, C.; Thomann, R.; Tiller, J. C. AdV. Mater. 2004, 16, 957. (26) Bao, C. Y.; Jin, M.; Lu, R.; Zhang, T. R.; Zhao, Y. Y. Mater. Chem. Phys. 2003, 82, 812. (27) Ghosh, S.; Banthia, A. K. J. Mater. Sci. 2007, 42, 118. (28) Tokar, R.; Kubisa, P.; Penczek, S. Macromolecules 1994, 27, 320. (29) Zhang, Y. W.; Huang, W.; Zhou, Y. F.; Yan, D. Y. Chem. Commun. 2007, 2587. (30) Melaiye, A.; Sun, Z.; Hindi, K.; Milsted, A.; Ely, D.; Reneker, D. H.; Tessier, C. A.; Youngs, W. J. J. Am. Chem. Soc. 2005, 127, 2285. (31) Yu, H. J.; Xu, X, Y.; Chen, X. S.; Lu, T. C.; Zhang, P. B.; Jing, X. B. J. Appl. Polym. Sci. 2007, 103, 125. (32) Selvakannan, P. R.; Kumar, P. S.; More, A. S.; Shingte, R. D.; Wadgaonkar, P. P.; Sastry, M. Langmuir 2004, 20, 295.
2336 J. Phys. Chem. C, Vol. 112, No. 7, 2008 (33) Selvan, S. T. Chem. Commun. 1998, 351. (34) Dai, J.; Bruening, M. L. Nano Lett. 2002, 2, 497. (35) Lazareva, N. F.; Vakul’skaya, T. I.; Albanov, A. I.; Pestunovich, V. A. Appl. Organomet. Chem. 2006, 20, 696. (36) Zhou, Y.; Itoh, H.; Uemura, T.; Naka, K.; Chujo, Y. Langmuir 2002, 18, 277. (37) Wang, X. Q.; Naka, K.; Itoh, H.; Park, S.; Chujo, Y. Chem. Commun. 2002, 1300. (38) Keki, S.; Torok, J.; Deak, G.; Daroczi, L.; Zsuga, M. J. Colloid Interface Sci. 2000, 229, 550. (39) Maillard, M.; Huang, P.; Brus, L. Nano Lett. 2003, 3, 1611. (40) Swami, A.; Selvakannan, P. R.; Pasricha, R.; Sastry, M. J. Phys. Chem. B 2004, 108, 19269. (41) Jin, R. H.; Yuan, J. J. J. Mater. Chem. 2005, 15, 4513. (42) Morones, J. R.; Elechiguerra, J. L.; Camacho, A.; Holt, K.; Kouri, J. B.; Ramirez, J. T.; Yacaman, M. J. Nanotechnology 2005, 16, 2346.
Zhang et al. (43) Thiel, J.; Pakstis, L.; Buzby, S.; Raffi, M. N. C.; Pochan, D. J.; Shah, S. I. Small 2007, 3, 799. (44) Sondi, I.; Salopek-Sondi, B. J. Colloid Interface Sci. 2004, 275, 177. (45) Feng, Q. L.; Wu, J.; Chen, G. Q.; Cui, F. Z.; Kim, T. N.; Kim, J. O. J. Biomed. Mater. Res. 2000, 52, 662. (46) Matsumura, Y.; Yoshikata, K.; Kunisaki, S.; Tsuchido, T. Appl. EnViron. Microbiol. 2003, 69, 4278. (47) Ye, W. J.; Leung, M. F.; Xin, J.; Kwong, T. L.; Lee, D. K. L.; Li, P. Polymer 2005, 46, 10538. (48) Lenoir, S.; Pagnoulle, C.; Detrembleur, C.; Galleni, M.; Jerome, R. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1214. (49) Sambhy, V.; MacBride, M. M.; Peterson, B. R.; Sen, A. J. Am. Chem. Soc. 2006, 128, 9798.
