Noncovalent Functionalization of Carbon Nanotubes with Sodium

Since their discovery in 1991,1 carbon nanotubes (CNTs) have attracted wide interest ...... Jayaprakash Khanderi , Rudolf C. Hoffmann , Jörg Engstler...
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J. Phys. Chem. C 2007, 111, 1223-1229

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Noncovalent Functionalization of Carbon Nanotubes with Sodium Lignosulfonate and Subsequent Quantum Dot Decoration Yangqiao Liu, Lian Gao,* and Jing Sun State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai, 200050, China

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ReceiVed: September 15, 2006; In Final Form: NoVember 9, 2006

A novel noncovalent approach is developed for the functionalization of multiwalled carbon nanotubes (MWNTs) using a biopolymer obtained in the cellulose industryssodium lignosulfonate (SLS). Using a simple physical grinding technique, the SLS-functionalized MWNTs can be well-dispersed in water with a MWNTs-equivalent solubility as high as 1.5 mg/mL, and the high stability can be maintained for more than 3 months. The SLSfunctionalized MWNTs have been characterized by transmission electron microscopy (TEM), Raman spectra, and zeta-potential measurements. It is proposed that π-π stacking and the hydrophobic interactions are dominant mechanisms for the interaction between SLS and MWNTs, and the anionic polymeric nature of SLS imparts the high stability of MWNTs via the electrosteric repulsion. By taking advantage of the diverse anionic groups on the SLS-functionalized MWNTs as anchorage centers, SnO2 and CdS quantum dots with the size of 4-6 nm are controllably and uniformly decorated on the MWNTs surface using an in-situ formation method. These MWNTs/quantum dot hybrid materials are highly dispersible in water, thus opening possibilities for their prospective technological applications.

1. Introduction Since their discovery in 1991,1 carbon nanotubes (CNTs) have attracted wide interest for their unique structure and outstanding properties.2 However, exploring the chemistry of CNTs at the molecular level is greatly limited due to their inherent insolubility in water and organic solvents. Therefore, many attempts have been made to improve the solubility of CNTs in common solvents. There are two alternative routes to reach this goal, namely, covalent and noncovalent functionalization. Covalent functionalization generally involves the oxidative formation of carboxyl functionalities and consequently grafting organic moieties onto the tubes.3 However, covalent functionalization disrupts the one-dimensional electronic structure and the desired optical properties.4 In contrast, the noncovalent functionalization routes are considered more promising since the electronic structure and properties of the CNTs can be better preserved. Up to now, a wide range of chemicals has been reported to functionalize CNTs noncovalently, such as surfactants,5 polymers,6 oligosaccharides,7 and so forth. However, most of the noncovalent functionalization techniques involve a process of long-time sonication for uniformly wrapping the CNTs with solubilizers, which will inevitably lead to some damage to the graphitic structures of CNTs and/or, in some cases, make them shorter.8,9 Thus, it becomes increasingly important to explore novel dispersants that can functionalize CNTs by more simple and nondestructive ways. On the other hand, the carbon nanotubes/quantum dot hybrid nanostructures attract more and more interest since they are believed to be useful for building blocks for optoelectronic devices, solar energy conversion,10 and photocatalysis.11 To date, several types of quantum dots such as CdSe, TiO2, CdS, and SnO2, and so forth, have been bound to the surface of * Corresponding author. Tel: 0086-21-52412718. Fax: 0086-2152413122. E-mail: [email protected].

CNTs.12-14 The preservation of the electronic properties of CNTs is also of vital importance for these applications since the main role of CNTs in these hybrids is to speed up the transfer of photogenerated electrons. For these purposes, it is imperative to develop effective methods to attach quantum dots onto the sidewalls of CNTs whose structures have been preserved intact. Aiming at the above issues, we developed a simple and facile noncovalent functionalization route for multiwalled carbon nanotubes (MWNTs) using a novel solubilizerssodium lignosulfonate (SLS), which has never been reported before. SLS is an amorphous aromatic biopolymer, which massively arises as a byproduct when pulping woods. It has been widely used as technical surfactants with dispersing, stabilizing, and adhesive abilities.15-17 We found that the interaction of SLS with MWNTs is so spontaneous and strong that the MWNTs can be made highly soluble in water by simple physical grinding with SLS using an agate mortar. Furthermore, these functionalized MWNTs are controllably decorated with SnO2 and CdS quantum dots using an in-situ formation method. The prepared MWNTs/ quantum dot hybrids are highly stable in aqueous solvents, which may facilitate further manipulation and applications. 2. Experimental Details 2.1. Materials. MWNTs prepared by the catalytic decomposition of CH4 were purchased from Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences. The CNTs have lengths ranging from several hundred nanometers to several micrometers and with outer diameters about 10-30 nm. They were washed in concentrated HCl solutions and have a purity >95% as claimed by the manufacturer. Sodium lignosulfonate (SLS) with an average molecular weight of 100 000 was purchased from East China Chemicals Company (Wuhan, China). Lignosulfonates have no regular structure; however, they are mainly composed of phenylpropane

10.1021/jp066018z CCC: $37.00 © 2007 American Chemical Society Published on Web 12/30/2006

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SCHEME 1: Chemical Structure of a Typical Lignosulfonate Segmenta

a

Ref 18.

groups.18

segments with sulfuric acid The chemical structure of a typical segment of lignosulfonate is shown in Scheme 1.18 Distilled water was used in all studies. 2.2. Functionalization of MWNTs and Characterization. The MWNTs were functionalized as follows. A 0.1-g portion of MWNTs was mixed with 1 g of sodium lignosulfonate (SLS) in an agate mortar and ground for 2 h by hand, with the addition of a small quantity of water to avoid agglomeration. The obtained brownish slurry was diluted using 100 mL of distilled water. Then, the mixture was separated by filtration and the excess SLS was removed by repeated rinsing with distilled water. The obtained SLS-functionalized MWNTs are highly dispersed in water; however, they would lose their excellent solubility completely when dried due to the rebundling of exfoliated MWNTs via π-π coupling of adsorbed SLS on the sidewall.19 Thus, they are directly dissolved and stored in water immediately after washing. TEM and high-resolution transmission electron microscopy (HRTEM) measurements of the MWNTs before and after functionalization were carried out on a JEOL JEM-2010 with an acceleration voltage of 200 kV. Surface properties of the samples were characterized by zeta-potential measurements (Zetaplus, Brookhaven Instruments Corp., Holtsville, NY). In order to ensure a fairly constant ionic strength, 10-3 mmol KCl was added to per milliliter MWNT suspensions. Solutions of HCl and NaOH were used to adjust the pH values. Raman spectra of the aqueous suspensions were recorded using a Renishaw MicroRaman with an excitation length of 632.8 nm. Inductively coupled plasma (ICP) spectroscopy (Vista Ax simultaneous ICP-OES, Varian, Australia) analysis was taken to measure the Na content in the solid sample of pure SLS and SLS-functionalized MWNTs, and the SLS content in the SLSfunctionalized MWNTs was accurately determined on the basis of their ratio. 2.3. Synthesis and Characterization of CNTs/Quantum Dot Nanohybrids. For the synthesis of MWNTs/SnO2 hybrids, a specific volume of SLS-functionalized MWNTs solutions was mixed with 30 mL of 0.01 M SnCl4 aqueous solution by stirring for 30 min. Then, 30 mL of 0.04 M hydrazine solution was added dropwise into the above mixture with vigorous stirring. After a 30-min reaction, the mixture was placed into an autoclave and maintained at 150 °C for 24 h. The obtained product was filtered and washed several times with water. In order to avoid agglomeration, no drying step was taken and the wet MWNTs/SnO2 hybrid was directly dispersed in aqueous solvents. For the synthesis of MWNTs/CdS hybrids, a specific volume of SLS-functionalized MWNTs solutions was mixed with 30

Figure 1. Photographs of aqueous suspensions of (a) pristine MWNTs, and (b) SLS-functionalized MWNTs after 3-months standing.

mL of 0.01 M Cd(CH3COO)2 aqueous solution. Then, 30 mL of 0.01 M Na2S solution was added slowly into the above mixture with vigorous stirring. Subsequently, the above mixtures were placed into an autoclave and maintained at 110 °C for 5 h. The product was filtered and washed several times with water and dispersed in water for storage. Morphology and composition of the MWNTs/SnO2 and MWNTs/CdS hybrid materials were characterized using TEM, HRTEM, electron diffraction spectra (EDS), and selected area electron diffraction (SAED) analysis. 3. Results and Discussion Our strategy for functionalizing MWNTs is found very effective in promoting the dispersion of MWNTs in water. Figure 1 compares the photographs of aqueous dispersions of pristine MWNTs and SLS-functionalized MWNTs after standing for 3 months. The pristine MWNTs completely precipitated to the bottom of the vials; the SLS-functionalized MWNTs still form a stable black suspension, with no significant phase separation or observable aggregations of nanotubes after standing for such a long time. The solubility of the SLS-functionalized MWNT in water was determined according to a method reported in the literature.20 A sufficient amount of SLS-functionalized MWNTs sample was dispersed in water by vigorous stirring for 2 h and allowed to stand overnight. A specific volume of the top homogeneous, clear supernatant was carefully transferred into a vial and the solid SLS-MWNTs were obtained by vacuum drying. The weight of the recovered SLS-functionalized MWNTs was accurately determined. Since ICP analysis shows that the Na contents in the SLS and SLS-functionalized MWNTs are 9.13 and 1.37%, respectively, which represents the presence of 15% SLS in the SLS-functionalized MWNTs sample, the MWNT-equivalent aqueous solubility can be estimated and determined to be 1.5 mg/mL. This solubility is higher than the value of 1.0 mg/mL reported for PAA-stabilized MWNTs obtained by prolonged sonication.21 The above results strongly indicate that the van der Waals interaction between MWNTs has been effectively overcome by simple grinding with SLS. The dispersion state of the pristine and SLS-functionalized MWNTs can also be observed by comparing their TEM images, as presented in Figure 2, parts a and b. It can be seen that the pristine MWNTs exist as bundles and are heavily entangled with each other to form 3D networks. In contrast, the functionalized nanotubes are well dispersed in water individually and no significant entanglement is observed (Figure 2c). It is worthy to note that the MWNTs exhibit no obvious length decrease or new damage after functionalization, which is different from the

Lignosulfonate Functionalization of CNTs

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Figure 2. TEM and HRTEM images of aqueous dispersions of (a, c) pristine MWNTs, and (b, d) SLS-functionalized MWNTs.

Figure 4. Zeta potential as a function of pH for (a) pristine MWNTs, and (b) SLS-modified MWNTs.

Figure 3. Raman spectra of (a) pristine MWNTs, and (b) SLS-modified MWNTs.

severe CNTs breakage after 4-min sonication in the presence of peptide dispersants.9 Thus, it can be found that the mild grinding approaches employed effectively preserve the structure of CNTs.20,22 Furthermore, comparing the HRTEM images shown in Figure 2, parts b and d, direct evidence for the presence of SLS on MWNTs surface can be found. The pristine MWNTs surfaces are very smooth and demonstrate clear edges. In contrast, the MWNTs after functionalization are fully covered

by a uniform amorphous polymer layer with a thickness of about 1-2 nm, which results from the adsorption of SLS. The thickness and uniformity of the wrapped SLS layer obtained are comparable to those of the coatings on MWNTs using peptides,9 amphiphilic polycations,6 and sodium dodecyl sulfate (SDS)23 as dispersants after prolonged ultrasonication. The above results indicate that the physical grinding of MWNTs with SLS is a nondestructive functionalization route with high efficiency. Raman spectroscopy has been used extensively to probe the structural and electronic properties of carbon nanotubes. Typical Raman spectra of MWNTs contain two domains: the tangential

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SCHEME 2: Mechanism for the Functionalization of MWNTs and Quantum Dot Decoration

G-band within 1550-1605 cm-1 and the disorder-induced D-band at ∼1350 cm-1.24 This D-band is due to defect sites in the hexagonal framework of carbon nanotubes walls, which results from the disorder induced by sp3 hybridization. Thus, the intensity ratio of the D-band to the G-band (ID/IG) is widely used as a measure of sidewall covalent derivatization or defect introduction.24 Figure 3 reveals Raman spectra of pristine MWNTs and SLS-functionalized MWNTs. Contrary to the generally observed ID/IG increase for the covalent and ordinary noncovalent functionalization of CNTs by ultrasonication,6 the ID/IG ratio in this study decreases from 1.80 to 1.20 after the SLS functionalization. The origin of this ID/IG decrease is still

unclear; however, it can be at least regarded as evidence that SLS modifies MWNTs via noncovalent bonds instead of chemical functionalization and no sidewall defects are introduced in the process of grinding. We suspect that the decreased ID/IG ratio is an indication of the enhanced resonance processes of Raman scattering due to the exfoliation of the nanotubes. Wise et al. also reported a similar ID/IG decrease when dispersing SWNTs in polyimide.25 Another significant change of the Raman curves after functionalization with SLS is that the Gand D-bands of MWNTs shift to higher wavenumbers from 1579 and 1326 cm-1 to 1591 and 1335 cm-1, respectively. This systematic upshift of the Raman peaks is indicative of the strong

Figure 5. (a, b) TEM images; (c) HRTEM images; and (d) EDS spectrum of the MWNTs/SnO2 hybrid materials prepared from SLS-functionalized MWNTs, inset of Figure 5d shows their SAED patterns; and (e) TEM images of MWNTs/SnO2 hybrid materials prepared from pristine MWNTs.

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Figure 6. (a, b) TEM images; (c) HRTEM images; and (d) EDS spectrum of the MWNTs/CdS hybrid materials prepared from SLS-functionalized MWNTs, inset of Figure 6d shows their SAED patterns; and (e) TEM images of MWNTs/CdS hybrid materials prepared from pristine MWNTs.

attachment of SLS onto MWNTs. The strong attraction increases the energy necessary for vibrations to occur, thus leading to the upshift of the Raman peaks.6 A similar phenomenon have been reported for the MWNTs wrapped by alginic acid8 and poly(acrylic acid).21 Zeta-potential measurements are taken to give insight to the alteration of the surface charge of MWNTs after SLS functionalization, and the results are shown in Figure 4. It is evident that the isoelectric point of the pristine MWNTs is about 3.7, which is consistent with our previous results.26 In contrast, the zeta potential for the SLS-functionalized MWNTs becomes much more negative in the whole pH range. The zeta potential is decreased to about -45 mV between pH 5 and 10, after the SLS modification. The SLS contains various anionic groups, including sulfonate (SO3-; pKa ) 1.5), carboxylate (COO-; pKa ) 5.1), and phenolic hydroxyl (PhOH; pKa ) 10.5) groups, which can increase the negative charges on the MWNTs once adsorbed. Thus, the increment in negative charges by SLS functionalization can be regarded as additional evidence for their adsorption on MWNTs. It is worth noting that in our experiments, the SLS and MWNTs were mixed at about pH 7, where both the MWNTs and the SLS are negatively charged (Figure 4); however, the zeta-potential decrease is still very significant in spite of the electrostatic repulsion. This zeta-potential decrease further confirms that the adsorption of SLS on MWNTs is spontaneous and strong, which is consistent with the TEM and Raman results.

The above results indicate that SLS acts as an excellent solubilizer for MWNTs. Even after repeated washing and filtration, the SLS-functionalized MWNTs also present good solubility. This is quite different from that observed during the solubilization of CNTs by peptides27 and surfactants,28 for which the removal of free dispersants would cause a significant decrease of the solubility. These indicate that the SLS adsorbs onto MWNTs more strongly and irreversibly than the ordinary dispersants. The dispersing abilities of organic molecules for CNTs are closely related to their chemical structures. Molecules containing aromatic groups are capable of forming more specific and more directional π-π stacking interactions with the graphitic surface of nanotubes,29 a phenomenon which has been demonstrated by the superior dispersing abilities of SDBS compared to those of SDS. In this study, the SLS used is an amphiphilic polyelectrolyte with a molecular weight as high as 100000. The backbone of SLS is essentially hydrophobic which consists of many aromatic rings linked with C3 ether and carbon bonds.30 Thus, besides the hydrophobic attraction of the alkyl chains with the CNTs, the great number of aromatic rings on the SLS bone are also expected to provide many active attaching sites onto CNTs sidewalls by the π-π stacking mechanism. The combined hydrophobic and π-π stacking interactions facilitate the strong binding of SLS on MWNTs surfaces, even by simple physical grinding. The SLS layer provides steric repulsion to help the MWNTs overcome the van der Waals force at contact. From another aspect, the SLS also contains various

1228 J. Phys. Chem. C, Vol. 111, No. 3, 2007 kinds of anionic groups. On adsorption, these groups extrude outside in order to minimize electrostatic repulsion. These ionized groups of SLS cause higher negative charge, which is proved by the zeta-potential measurements (Figure 4). The repulsive forces between the same charged SLS layers render the MWNTs electrostatically repulsive and highly hydrophilic, which makes them highly stable in aqueous solvents. As a conclusion, the SLS have a strong interaction with the MWNTs and efficiently stabilize the MWNTs through an electrosteric mechanism, as commonly suggested for other polyelectrolyte stabilizers, such as poly(acrylic acid),21 and polycations,6 and so forth. The conformation of SLS adsorbed onto MWNTs is schematically presented in Scheme 2. A similar conformation of polyvinylpyrrolidone (PVP) wrapped onto CNTs has been reported by others, in which the hydrophobic alkyl backbone is in contact with the nanotube surface and pyrrolidone groups are exposed to water.31 Similar to the CNTs functionalized by other solubilizers containing various functional groups, the present SLS-functionalized MWNTs may find applications in nanocomposites, selfassembly, layer-by-layer deposition, and so forth.32 In our study, we further examine the application of the SLS-functionalized MWNTs as supports for decorating SnO2 and CdS quantum dots. Scheme 2 also depicts the possible mechanism for the insitu decoration of quantum dots. Figure 5 shows the TEM images of the SnO2-decorated MWNTs, which clearly indicate that the MWNTs are uniformly decorated with 4-6-nm SnO2 nanoparticles, and scarcely unbound particles can be found. Since the attachment of the SnO2 nanoparticles involves interaction of Sn4+ and anionic groups of SLS, the decoration also serves to confirm our scheme that the anionic groups extrude outside. It is also noted that the interaction between nanotubes and quantum dots is quite strong, because SnO2 nanoparticles could not be ripped off the nanotubes surfaces through washing.33 The HRTEM micrograph (Figure 5c) shows the good crystallinity of the attached SnO2 nanoparticles. EDS measurements (Figure 5d) exhibit peaks corresponding to Sn, O, and C, confirming the composition of SnO2. The C signal is attributed to the supporting MWNTs and the SLS polymers. The inset of Figure 5d presents the corresponding SAED pattern for the MWNTs/SnO2 hybrid. Several diffused diffraction rings appear in accordance with the (321), (301), (211), (200), (101), and (110) indices of tetragonal SnO2 (JCPDS No. 77-0452).34 In contrast, for the MWNTs/SnO2 hybrids prepared from pristine MWNTs (Figure 5e), the CNTs are still bare and almost no nanoparticles have been decorated on the sidewall. These results suggest that the SLS coating plays an important role in decorating the CNTs with nanoparticles. Similar to the SnO2-decorated MWNTs, the CdS quantum dots also decorate on SLS-functionalized MWNTs surface quite uniformly, as shown in Figure 6a-c. The CdS quantum dots are not spherical, but have a rod shape with diameters around 5 nm and lengths about 7-10 nm. EDS measurements (Figure 6d) exhibit peaks corresponding to Cd, S, O, and C, confirming the composition of CdS. The C and O signals are attributed to the supporting MWNTs and the SLS polymers. The corresponding SAED patterns (inset of Figure 6d) taken from the nanoparticles on the wall of MWNTs exhibit four rings corresponding to the (111), (220), (311), and (331) planes of cubic CdS (JCPDS 42-1411). Combining the results from EDS and SAED, we can confirm that the nanoparticles on the MWNTs are CdS. Similar to the case of the MWNTs/SnO2 hybrids, the CdS nanoparticles also cannot decorate on the pristine CNTs surface, as seen from Figure 6e.

Liu et al. The above-mentioned results indicate that SLS-functionalized MWNTs are good supports for preparing MWNTs/quantum dot hybrids, which may have unique properties different from those of bulk materials. From another aspect, the MWNTs/SnO2 and MWNTs/CdS composites are highly stable in aqueous solvents and no precipitation was observed for over 6 months. In the control experiments, the SnO2 and CdS prepared without MWNTs completely precipitate after several days. It is believed that anchorage of quantum dots on the SLS-functionalized MWNTs impedes them from coagulation and increases their stability in liquid solvents. This opens possibilities for their prospective technological applications in many fields. 4. Conclusions In conclusion, we have developed a straightforward and facile route for the functionalization of MWNTs. The advantage of this route includes the high functionalization efficiency, the simple grinding technique, the retention of the electronic structure of the MWNTs, and the low cost of the SLS dispersant. This route is expected to be suitable for industrial scale-up preparation of functionalized MWNTs. The large variety of modification methods for lignosulfonates reported thus far,35 such as methylolation, phenolation, desmethylation, and fractionation provides a future prospective for improving the dispersing abilities of SLS. Furthermore, the functionalized MWNTs provide excellent supports for the controllable and uniform decoration of semiconductor nanoparticles such as SnO2 and CdS. The semiconductor nanoparticle-decorated MWNTs hybrids demonstrate long-time stability (>6 months) at ambient temperatures. These features make them useful for fabricating photovoltaic cells, light-emitting diodes, biosensors, and photocatalysts. Also, the versatility of the method can be extended to decorate other nanocrystals onto CNTs. Acknowledgment. The project was supported by the National Natural Science Foundation of China (Nos. 50372077, 50572114, and 50602049) and Shanghai Nanotechnology Promotion Center (Grant No. 0652nm022). References and Notes (1) Iijima, S. Nature 1991, 354, 56-58. (2) Harris, P. Carbon nanotubes and related structures: New materials for the twenty-first century; Cambridge University Press: Cambridge, 2001. (3) Hong, C. Y.; You, Y. Z.; Pan, C. Y. Chem. Mater. 2005, 17, 22472254. (4) Barone, P. W.; Baik, S.; Heller, D. A.; Strano, M. S. Nat. Mater. 2005, 4, 86-92. (5) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253-1256. (6) Sinani, V. A.; Gheith, M. K.; Yaroslavov, A. A.; Rakhnyanskaya, A. A.; Sun, K.; Mamedov, A. A.; Wicksted, J. P.; Kotov, N. A. J. Am. Chem. Soc. 2005, 127, 3463-3472. (7) Chambers, G.; Caroll, C.; Farrell, G. F.; Dalton, A. B.; Mcnamara, M.; in het Panhuis, M.; Byrne, H. J. Nano Lett. 2003, 3, 843-846. (8) Liu, Y.; Liang, P.; Zhang, H. Y.; Guo, D. S. Small 2006, 2, 874878. (9) Zorbas, V.; Ortiz-Zcevedo, A.; Dalton, A. B.; Yoshida, M. M.; Dieckmann, G. R.; Draper, R. K.; Baughman, R. H.; Yacaman, M. J.; Musselman, I. H. J. Am. Chem. Soc. 2004, 126, 7222-7227. (10) Robel, I.; Bunker, B. A.; Kamat, P. V. AdV. Mater. 2005, 17, 24582463. (11) Jiang, L. Q.; Gao, L. Mater. Chem. Phys. 2005, 91, 313-316. (12) Banerjee, S.; Wong, S. S. Nano Lett. 2002, 2, 195-200. (13) Cai, Z. X.; Yan, X. P. Nanotechnology 2006, 17, 4212-4216. (14) Chen, Y.; Zhu, C.; Wang, T. Nanotechnology 2006, 17, 30123017. (15) Ratinac, K. R.; Standard, O. C.; Bryant, P. J. J. Colloid Interface Sci. 2004, 273, 442-454.

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