Preparation of Dually, pH- and Thermo-Responsive Nanocapsules in

Dec 27, 2011 - pH- and thermo-sensitive nanocapsules were successfully synthesized via inverse miniemulsion copolymerization of N-isopropyl acrylamide...
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Preparation of Dually, pH- and Thermo-Responsive Nanocapsules in Inverse Miniemulsion Zhihai Cao,†,‡,* Katharina Landfester,§ and Ulrich Ziener†,* †

Institute of Organic Chemistry III − Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany ‡ College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Xuelin Street 16, 310036 Hangzhou, People's Republic of China § Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ABSTRACT: pH- and thermo-sensitive nanocapsules were successfully synthesized via inverse miniemulsion copolymerization of N-isopropyl acrylamide (NIPAM), N,N′-methylene bisacrylamide (MBA), and a functional monomer, 4-vinyl pyridine (4-VP). The size and size distribution of nanocapsules were measured by dynamic light scattering (DLS). The particle morphology was observed by transmission electron microscopy (TEM). The final morphology of particles was strongly influenced by the hydrophobicity of functional monomers. The use of a hydrophilic functional monomer, acrylic acid, led to the formation of solid particles, while the use of the more hydrophobic functional monomer, 4-VP, resulted in the formation of nanocapsules. The particle morphology, size, and size distribution were investigated in terms of the content of 4-VP, MBA, and the type and content of surfactant. The pH- and thermo-sensitivities were characterized by measuring the size variation with the change of temperature and pH. The organic−inorganic nanocapsules were prepared by coating a layer of silica particles on the surface of the sensitive nanocapsules.

1. INTRODUCTION Materials triggered by environmental stimuli such as temperature, pH, ionic strength, or light have attracted intensive attention in the past years. 1,2 Among them, poly(Nisopropylacrylamide) (PNIPAM) is one of the most widely investigated environmentally responsive polymer.3−5 PNIPAM undergoes a reversible “coil to globule” volume phase transition when the temperature passes its lower critical solution temperature (LCST, about 32 °C).4 The popularity of PNIPAM is in part due to the easy tuning of the LCST or the introduction of multiresponsiveness by simply conducting a copolymerization of NIPAM with functional monomers which can produce hydrophilic, hydrophobic, pH sensitive, or ionic strength sensitive segments.6−8 Capsules with a stimulusresponsive polymeric shell are of high interest in the areas of controlled drug delivery, biology, catalysis, and so on due to the capacity to control the loading or release of guest materials from the capsules.9,10 Environmentally responsive capsules have been successfully prepared by using hard or soft templates in different heterogeneous polymerization systems.5,11−13 Deng et al.12 and Horecha et al.13 prepared thermo-sensitive PNIPAM microcapsules via inverse emulsion polymerization by using an aqueous droplet template. Compared to emulsion polymerization, miniemulsion polymerization is a more frequently used technique to synthesize capsules by using soft templates (hydrophobic14−22 or hydrophilic liquid core5,23−26) because of its droplet nucleation mechanism. Recently, Lu et al. reported the synthesis of PNIPAM nanocapsules via interfacially confined polymerization by using an amphiphilic PEO-RAFT agent in inverse miniemulsion.26 More recently, we © 2011 American Chemical Society

prepared narrowly size-distributed cross-linked PNIPAM nanocapsules via inverse miniemulsion polymerization by using an aqueous solution of a cobalt salt as template.5 We found that the particle size distribution and morphology strongly depends on the weight content of the cobalt salt, the cross-linker, and the ratio between monomers and water. The application of environmentally responsive capsules/ particles can be further improved by the incorporation of a second sensitivity to a specific environmental stimulus. For example, pH- and thermo-sensitive copolymers or nanoparticles have been prepared by copolymerization of NIPAM and acrylic acid (AA)27,28 or 4-vinyl pyridine (4-VP).6,29 However, there is no report on the preparation of dually, pH- and thermosensitive nanocapsules in inverse miniemulsion. In the present work, the pH- and thermo-sensitive nanocapsules were successfully synthesized via the copolymerization of Nisopropyl acrylamide (NIPAM), N,N′-methylene bisacrylamide (MBA), and a functional monomer, 4-vinyl pyridine (4-VP) in inverse miniemulsion. Furthermore, the organic−inorganic hybrid nanocapsules were prepared by the adsorption of silica particles on the surface by a process of heterocoagulation.

2. EXPERIMENTAL SECTION 2.1. Materials. The monomers, N-isopropyl acrylamide (NIPAM, Acros, 97.0%), acrylic acid (AA, Merck, 99.0%), and the cross-linker, N,N′-methylene bisacrylamide (MBA, Merck, 98.0%,) were used as Received: October 21, 2011 Revised: December 6, 2011 Published: December 27, 2011 1163

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Table 1. Formulations, Particle Sizes, and PDIs for All Experiments monomers (g)

a

run

surfactant/weight percent (wt%a)

initiator/mass (g)

NIPAM (g)

MBA (g)

AA or 4-VP/mass (g)

Z-average size (nm)

PDI

1 2 3 4 5 6 7 8 9 10

P(E/B)-PEO1/4 P(E/B)-PEO1/4 P(E/B)-PEO1/4 P(E/B)-PEO1/4 P(E/B)-PEO1/4 P(E/B)-PEO1/4 P(E/B)-PEO1/4 P(E/B)-PEO1/4 P(E/B)-PEO2/4 P(E/B)-PEO1/5

APS/0.021 AIBA/0.025 AIBA/0.025 AIBA/0.025 AIBA/0.025 AIBA/0.025 AIBA/0.025 AIBA/0.025 AIBA/0.025 AIBA/0.025

0.45 0.45 0.5 0.475 0.425 0.4 0.425 0.425 0.425 0.45

0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.025 0.1 0.1

AA/0.050 4-VP/0.050 4-VP/0.000 4-VP/0.025 4-VP/0.075 4-VP/0.100 4-VP/0.075 4-VP/0.075 4-VP/0.075 4-VP/0.050

205 276 338 307 346 276 238 204 387 276

0.050 0.088 0.023 0.017 0.065 0.103 0.111 0.117 0.063 0.062

Relative to the polar mixture.

received. 4-Vinyl pyridine (4-VP, Aldrich, 95%) was purified by distillation under reduced pressure, and stored in the refrigerator prior to use. The block copolymer surfactant, poly(ethylene-co-butylene)-bpoly(ethylene oxide) (P(E/B)-PEO), was synthesized according to the literature.30,31 The molecular weight of P(E/B)-PEO used in this paper was 6200 g·mol−1 (P(E/B)-PEO1) and 7000 g·mol−1 (P(E/B)PEO2), as determined by 1H NMR spectroscopy. The initiators, 2,2′azobis(2-methylpropionamide) dihydrochloride (AIBA, Acros, 98.0%) and ammonium persulfate (APS, Merck, 98.0%), the apolar solvent cyclohexane (CH, 99.5%, VWR Prolabo) and the metal salt cobalt(II) tetrafluoroborate hexahydrate (Co(BF4)2·6H2O, Aldrich, 99.0%) were used as received. Demineralized water with Milli-Q grade (resistivity is 18 MΩ.) was used. 2.2. Preparation of Inverse Miniemulsion and Polymerization. The surfactant P(E/B)-PEO was dissolved in 12.5 g CH under magnetic stirring. Co(BF4)2, AIBA, or APS, and the monomers were first dissolved in water to form homogeneous polar solutions. The polar solution was mixed with the surfactant solution. After 15 min pre-emulsification under strong magnetic stirring, the mixture was treated with 120 s ultrasound with a Branson 450W digital sonifier at 90% amplitude in an ice bath to prepare a miniemulsion. The initial miniemulsion was introduced to the reactor and purged with argon for 3 min under magnetic stirring. The argon-protected reaction mixture was placed in a preheated oil bath at 65 °C and stirred for 5 h. The respective amounts and detailed conditions are given in Table 1. 2.3. Preparation of Aqueous Dispersion of Nanocapsules. The original dispersion was redispersed in water according to the following protocol. About 0.5 g of the original dispersion was added to a solution of 45 mg SDS in 15 g water. Then the mixture was strongly agitated for 30 min, followed by 2 min ultrasonication with 70% amplitude in an ice bath. The aqueous dispersion was poured into an open glass bottle and stirred at room temperature for more than 2 days to remove CH. The aqueous dispersion was used to characterize the thermo- and pH- sensitivities of the nanocapsules in water. 2.4. Preparation of Aqueous Dispersion of Organic− inorganic Nanocapsules. The organic−inorganic nanocapsules were prepared according to the following protocol. About 0.25 g of the original dispersion was added to a diluted aqueous dispersion of silica sol. Then the mixture was strongly agitated for 5 min, followed by 2 min ultrasonication with 70% amplitude in an ice bath. The aqueous dispersion was poured into an open glass bottle and stirred at room temperature for more than 2 days to remove CH. 2.5. Characterization. 2.5.1. Dynamic Light Scattering. The size and size distribution (as PDI) were measured by DLS (NanoZetasizer, Malvern Instruments) at 20 °C under the scattering angle of 173° at a wavelength of 633 nm. Particle sizes and PDIs are given as the average of five measurements. The PDI is a measure of the particle size distribution, and is a dimensionless number that describes the heterogeneity of the sample; it can range from 0 (monodisperse) to 1 (polydisperse). For the measurement of thermo-sensitivity, the aqueous dispersion was diluted with water, and the particle size was measured every 2 °C in the range from 20 to 50 °C. Prior to the

measurement, the sample was equilibrated for 5 min at each temperature. For the measurement of the pH sensitivity, the aqueous solution was diluted by the pH-adjusted aqueous solution. 2.5.2. Transmission Electron Microscopy (TEM). TEM measurements were performed on a Philips EM 400 Microscope. 1.5 μL of the original dispersion was diluted with 3 mL of CH, and then 2.0 μL of the diluted sample was placed on a 400-mesh carbon-coated copper grid and dried at room temperature overnight.

3. RESULTS AND DISCUSSION 3.1. Preparation of pH- and Thermo-Responsive Nanocapsules via Inverse Miniemulsion. The synthesis of thermo-sensitive nanocapsules via inverse miniemulsion polymerization has been reported in our previous contribution.5 In the present report, we tried to step further to prepare dually, pH- and thermo-sensitive nanocapsules via copolymerization of N-isopropyl acrylamide (NIPAM), N,N′-methylene bisacrylamide (MBA), and a functional monomer which can form pH-sensitive polymers in inverse miniemulsion. The aqueous solution of Co(BF4)2 was used as soft template to form a capsule morphology, and the ratio between monomers and water was fixed at 0.6:1 on the basis of the previously optimized conditions.5 Acrylic acid (AA) and 4-vinyl pyridine (4-VP) are two widely used monomers to introduce pH sensitivity to copolymers or nanohydrogels.8,27−29 As a hydrophilic monomer, AA could be easily mixed with the other polar reagents used in this work. The Z-average size and PDI of the resulting particles were 205 nm and 0.05, respectively. However, the TEM image in Figure 1a revealed

Figure 1. TEM images of particles or nanocapsules synthesized in inverse miniemulsion with different functional monomers (a, acrylic acid; b, 4-vinyl pyridine; the scale bars are 600 nm for both images; see Table 1, runs 1−2).

the formation of solid particles instead of nanocapsules due to the introduction of AA. We assume that the unsuccessful formation of a capsule morphology is caused by the increase of 1164

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the hydrophilicity of the copolymers due to the incorporation of AA segments, which may prevent sufficiently expressed phase separation of the copolymers. In addition, because of the higher hydrophilicity of the copolymers containing AA units than plain poly(NIPAM), it could be expected that the interfacial tension between the aqueous core and the copolymers would decrease, while the interfacial tension between the copolymers and the apolar phase would increase. For the systems in which the formation of nanocapsules follows the mechanism of phase separation of polymers in the droplets, this change is definitely not favorable for the accumulation of copolymers at the interface to form nanocapsules on the basis of thermodynamic consideration.16 In order to confirm the above-mentioned analyses and prepare pH- and thermo-sensitive nano-objects with a capsule morphology, 4-VP, a relatively more hydrophobic monomer than NIPAM and AA, was used to replace AA. It is worth mentioning that although 4-VP is a hydrophobic monomer, it could completely dissolve in the aqueous solution of Co(BF4)2 and NIPAM, probably due to the formation of a complex between 4-VP and Co(BF4)2. As expected, most of the droplets evolved to form nanocapsules with a thick shell, see Figure 1b. The Z-average size and PDI of the nanocapsules were 276 nm and 0.088, respectively. It should be noted that the type of initiators used in run 1 and run 2 were APS and AIBA, respectively. However, we excluded the possibility that the differences in morphology of those runs were caused by using different initiators, because thermo-sensitive nanocapsules could be also synthesized by using APS as initiator.5 3.2. Variables Influencing the Particle Morphology, Size, And Size Distribution. 3.2.1. Content of 4-VP. The size and size distribution of nanocapsules were strongly dependent on the weight content of 4-VP (Table 1). The particle size decreased with the increase of the 4-VP content except for 15 wt % 4-VP with respect to NIPAM (run 5, Table 1). According to the PDIs in Table 1, the increase of the weight content of 4-VP led to a broadening of the particle size distribution, especially in the product with 20 wt % of 4-VP (PDI = 0.103, see Figure 2d). This could be ascribed to the enhancement of the molecular diffusion between the droplets due to the relatively higher hydrophobicity of 4-VP than NIPAM. This analysis was supported by the appearance of some large particles in Figure 2d. 3.2.2. Content of MBA. It has been reported that the incorporation of some cross-links in the shell could promote the formation of the capsule morphology. In addition, the size and size distribution depend strongly on the cross-linker amount.5 As shown in Figure 2c and Figure 3, the capsule morphology can be well observed in the product with 10 wt % or more of MBA to NIPAM, but became obscure at 5 wt % of MBA confirming the improved stabilization of the shell with increased cross-linking. The particle size distribution became broader in the products with low cross-linking density as indicated by the results of DLS and TEM measurements, consistent with our previous results.5 3.2.3. Surfactant Type and Content. The size and size distribution could be tuned by the type and content of surfactant. Compared to P(E/B)-PEO1, the size of the nanocapsules increased about 40 nm, but the size distribution remained almost the same by using P(E/B)-PEO2 as surfactant (see Table 1). This could be ascribed to the relatively higher hydrophilicity of P(E/B)-PEO2 than that of P(E/B)-PEO1, which may lead to a relatively lower efficiency to provide

Figure 2. TEM images of nanocapsules synthesized in inverse miniemulsion with different contents of 4-VP (a, 0 wt % of 4-VP to NIPAM; b, 5 wt %; c, 15 wt %; d, 20 wt %; the scale bars are 600 nm for all images; see Table 1, runs 3−6).

Figure 3. TEM images of nanocapsules synthesized in inverse miniemulsion with different MBA contents (a, 10 wt % of MBA to NIPAM; b, 5 wt %; the scale bars are 600 nm for all images; see Table 1, runs 7−8).

droplet stability as reported in reference 32. According to Figure 4, the particle morphology was not obviously influenced

Figure 4. TEM image of nanocapsules synthesized in inverse miniemulsion by using P(E/B)-PEO2 as surfactant (the scale bar is 600 nm; see Table 1, run 9).

by the change of surfactant and well-defined nanocapsules dominated in the final product. Compared to run 2, the size of nanocapsules remains constant with the increase of the content of P(E/B)-PEO1 1165

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Table 2. Sizes at 20 and 50 °C, and Swelling Ratios of Nanocapsules Containing Different 4-VP Contents

from 4 wt % to 5 wt % (run 10), but the size distribution of nanocapsules is narrowed, indicated by the decrease of PDI from 0.088 (run 2) to 0.062 (run 10). The narrow size distribution of nanocapsules of run 10 was also confirmed by TEM (see Figure 5).

run

4-VP content (wt % to NIPAM)

size at 20 °C (nm)

PDI at 20 °C

size at 50 °C (nm)

PDI at 50 °C

α

3 4 2 5 6

0 5 10 15 20

362 318 251 386 225

0.058 0.080 0.086 0.092 0.157

301 275 227 356 213

0.058 0.091 0.060 0.122 0.105

0.57 0.65 0.74 0.79 0.85

shall be noted that the capsule morphology allows the shell to expand not only to the exterior, which delivers the increased radius, but also to the interior, which cannot be determined by DLS. In addition, the pronounced change of size with temperature takes mainly place in the range of 28 to 36 °C. In addition to thermo-sensitivity, poly(NIPAM-co-4-VP) could also respond to the variation of solution pH due to the introduction of 4-VP units in the copolymers.29 The pHsensitivity of nanocapsules with different 4-VP contents was investigated by measuring the particle size in the solutions with pH values below (pH 3.1) and above (pH 10.1) the pKa of 4VP (5.3934). The sizes of nanocapsules without 4-VP units were 360 nm at both pH 3.1 and 10.1, not showing any pHdependence. With the incorporation of 4-VP units into the shell copolymers, the size started to display pH dependence. More or less, the sizes of nanocapsules at pH 3.1 were larger than those at pH 10.1, regardless of the 4-VP contents. This could be ascribed to the protonation of the nitrogen atom in the pyridine ring at pHs below the pKa of 4-VP to repel the neighboring charged pyridine ring, finally leading to the expansion of the nanocapsules. As the solution pH was above the pKa of 4-VP, the nanocapsules shrunk due to the deprotonation of the nitrogen atom. It should be pointed out that the dependence of size on the solution pH was not significant in the nanocapsules with 20 wt % of MBA because of the high cross-linking density. This was strongly supported by the fact that a significant size dependence (about 360 nm) on the solution pH was observed in nanocapsules containing 10 wt % of MBA and 15 wt % of 4VP (run 7). In addition, with the decrease of cross-links, the nanocapsules showed a more obvious thermosensitivity than that of the sample with 20 wt % of MBA (run 7, see Figure 7).

Figure 5. TEM image of nanocapsules synthesized in inverse miniemulsion by using 5 wt % of P(E/B)-PEO1 as surfactant (the scale bar is 600 nm; see Table 1, run 10).

3.3. pH- and Thermo- Sensitivity of Nanocapsules. Poly(NIPAM) undergoes a volume phase transition near its LCST caused by the deswelling (heating) or swelling (cooling) of polymer chains. As a result, the particle size will change with the temperature variation, and therefore, the thermo-sensitivity of the synthesized nanocapsules can be characterized by investigating the relationship between size and temperature. Regardless of the 4-VP content, the size of nanocapsules decreased with the increase of the temperature (see Figure 6).

Figure 6. Dependence of size of nanocapsules in water on temperature and content of 4-VP (solid symbols: heating step; open symbols: cooling step).

The capacity of the nanocapsules to respond to the change of temperature can also be indicated by the swelling ratio α which is defined as the ratio between the particle volume in the collapsed (Vcollapsed) and the swollen state (Vswollen).33 The results listed in Table 2 show that the increase of the 4-VP content in the copolymers lead to an increase of the swelling ratios, consistent with literature reports.6 This could be attributed to the increase of the hydrophobicity of copolymer chains with the increase of 4-VP content, leading to the deswelling of copolymer chains. Although a small hysteresis was observed, the nanocapsules show clearly reversible thermosensitivity. The swelling ratios of the nanocapsules are rather high compared to corresponding microgel systems.33 But it

Figure 7. Dependence of size of nanocapsules in water on temperature and content of MBA (see Table 1, runs 5 and 7).

3.4. Preparation of Organic−Inorganic Nanocapsules. The organic−inorganic hybrid nanocapsules were prepared by coating negatively charged silica particles (average size: ca. 22 nm) on the surface of the original nanocapsules via a process of heterocoagulation. The idea of using silica particles as model is 1166

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Table 3. Sizes of Nanocapsules Containing Different 4-VP Contents at pH 3.1 and 10.1a run

4-VP content (wt % to NIPAM)

MBA content (wt % to NIPAM)

size at pH 3.1 (nm)

PDI at pH 3.1

size at pH 10.1 (nm)

PDI at pH 10.1

3 4 2 5 6 7

0 5 10 15 20 15

20 20 20 20 20 10

360 319 257 396 227 577

0.053 0.089 0.071 0.111 0.139 0.491

360 311 245 382 217 217

0.080 0.090 0.076 0.111 0.128 0.135

a

Measured at 20 °C.

that various inorganic particles can be possibly covered with a layer of silica shell by a sol−gel reaction.35,36 Therefore, the results concerning silica particles can be possibly generalized for other inorganic functional particles like magnetic particles, noble metal particles, and so on. The adsorption of silica particles on the surface of the nanocapsules was expected to rely on the interaction between the nitrogen atom (basic) of 4-VP and the surface hydroxyl group (acidic) of silica particles, which would strongly depend on the density of pyridine units on the shell. In addition to the interaction between nitrogen atoms and hydroxyl groups, the positive charges introduced by AIBA might also promote the adsorption of silica particles. Therefore, the ability of nanocapsules to adsorb silica particles was investigated with respect to the content of 4-VP ranging from 0 to 20 wt % of 4VP to NIPAM with no pH control and a mass ratio between the nanocapsules and silica particles of about 1. The control experiment without 4-VP units in the nanocapsules shows that only a part of the silica particles is adsorbed on the nanocapsules, whereas a large amount of free silica particles could be found in the TEM image (Figure 8a). Obviously, there is only a weak interaction between the nanocapsules and the silica particles. With the introduction of 4-VP units in the shell copolymers, most of the silica particles were adsorbed by the nanocapsules, and only a very small amount of free silica particles could be found in the TEM images (Figure 8b−e). Figure 8c reveals that a dense layer of silica particles is formed on the surface of the capsules, and the core−shell morphology of the capsules can be clearly distinguished. It should be pointed out that the organic−inorganic hybrid nanocapsules tend to form aggregates. We attribute the formation of aggregates partially to the relatively weaker colloidal protection ability of silica particles, compared to conventional surfactants like SDS. Further improvement of the colloidal stability of the dispersion is under investigation.

Figure 8. TEM images of organic−inorganic hybrid nanocapsules prepared via a process of heterocoagulation by using nanocapsules with different 4-VP contents (a, 0 wt % of 4-VP to NIPAM; b1 and b2, 5 wt %; c, 10 wt %; d, 20 wt %; the scale bars are 100 nm for b2, and 400 nm for the rest).

size distributions were relatively broadened with the increase of the 4-VP content. The introduction and increase of the MBA amount promoted the formation of intact nanocapsules and narrowed the size distribution. The size and size distribution could be tuned by the type and content of surfactant. The prepared nanocapsules showed obvious pH- and thermosensitivities. With the increase of 4-VP content in the copolymers, the capacity of the nanocapsules to respond to temperature was reduced, due to the increase of the hydrophobicity of the copolymers. The nanocapsules with 10 wt % of MBA to NIPAM showed an obvious response to solution pH and temperature. The organic−inorganic hybrid nanocapsules were successfully obtained by coating a layer of silica particles via heterocoagulation. The presence of 4-VP units in the shell copolymers was crucial for the adsorption of silica particles on the surface. We believe that the pH- and thermo-sensitivities, capsule morphology, and the adsorption of inorganic particles on the surface ensure wide applications of the particles synthesized by our presented technique.

4. CONCLUSIONS The dually pH- and thermo-responsive nanocapsules have been successfully prepared via free radical copolymerization of Nisopropyl acrylamide (NIPAM), 4-vinyl pyridine (4-VP), and N,N′-methylene bisacrylamide (MBA) in inverse miniemulsion. In the system using acrylic acid (AA) as functional monomer, most of the droplets formed solid particles after polymerization due to the hydrophilicity of AA, which probably prevented phase separation of the copolymers in the droplets. The welldefined nanocapsules with a dense shell dominated in the products with 4-VP as functional monomer. Generally, the particle morphology was insensitive to the variation of the 4-VP content, but the sizes of the nanocapsules decreased, and the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.C.), ulrich.ziener@uni-ulm. de (U.Z.).

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ACKNOWLEDGMENTS We greatly thank G. Weber for the synthesis of P(E/B)-PEOs. The financial supports from National Natural Scientific Foundation of China (NNSFC) project (51003023), the Hangzhou Normal University high-level talents start-up fund 1167

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(32) Kobitskaya, E.; Ekinci, D.; Manzke, A.; Plettl, A.; Ziemann, P.; Ziener, U.; Landfester, K. Macromolecules 2010, 43, 3294−3305. (33) Karg, M.; Pastoriza-Santos, I.; Rodriguez-González, B.; von Klitzing, R.; Wellert, S.; Hellweg, T. Langmuir 2008, 24, 6300−6306. (34) Pinkrah, V. T.; Snowden, M. J.; Mitchell, J. C.; Seidel, J.; Chowdhry, B. Z.; Fern, G. R. Langmuir 2003, 19, 585−590. (35) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Nano Lett. 2002, 2, 785−788. (36) Zhao, W.; Gu, J.; Zhang, L.; Chen, H.; Shi, J. J. Am. Chem. Soc. 2005, 127, 8916−8917.

(2011QDL04), and Deutsche Forschungsgemeinschaft (DFG) within the Cooperative Research Center SFB 569 are gratefully acknowledged.

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dx.doi.org/10.1021/la2041357 | Langmuir 2012, 28, 1163−1168