One-Dimensional Assembly of Silica Nanospheres: Effects of Nonionic

Aug 30, 2012 - breviated as PEO-PPO-PEO) triblock copolymers with varying hydrophilic−lipophilic balance (HLB) values. Scanning electron microscopy ...
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One-Dimensional Assembly of Silica Nanospheres: Effects of Nonionic Block Copolymers Shujun Zhou, Takeshi Sakamoto,† Junzheng Wang, Ayae Sugawara-Narutaki, Atsushi Shimojima, and Tatsuya Okubo* Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: The effects of polymers on the one-dimensional assembly of silica nanospheres (SNSs) in the liquid phase are systematically investigated using nonionic poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (abbreviated as PEO-PPO-PEO) triblock copolymers with varying hydrophilic−lipophilic balance (HLB) values. Scanning electron microscopy is employed for morphological observations of the polymer-mediated assemblies of SNSs on the basis of which the optimal pH for 1D assembly (pH1D) is determined. To clarify the polymers’ effects on the 1D assembly of SNSs, the relationships between pH1D and polymers’ HLB values, the numbers of hydrophilic EO and hydrophobic PO units, and the relative ratio of NPO/NEO are examined. Zeta potential measurements are conducted to investigate the electrostatic repulsion among the SNSs in the presence of block copolymers. It is found that the relative hydrophilicity of the block copolymers greatly affects the balance of interactions in the 1D assembly of SNSs. Block copolymers with large HLB values promote the 1D assembly of SNSs under near-neutral pH conditions, whereas the block copolymers with small HLB values promote 1D assembly under basic pH conditions. Therefore, the 1D assembly of SNSs is achieved over an extensive pH range (7.5−9.5) through the employment of block copolymers of different hydrophilic and hydrophobic block lengths.

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INTRODUCTION Assembled nanostructures of nanoparticles (NPs) offer unique optical, electrical, and magnetic properties that have promising applications in optoelectronic devices,1 data storage,2 sensors,3 and so on. Diverse assembled nanostructures such as 1D chains, 2D sheets, 3D crystals, and complex superstructures have been produced.4 Among them, 1D assembled nanostructures are of special interest because they provide unusual functions such as the directional transfer of photons, electrons, and spins.5 Onedimensional assembled nanostructures can be fabricated via 1D assembly of 0D NPs. Linear templates such as biomolecules,6 polymers,7 inorganic nanotubes,8 and lithographic patterns9 are usually employed to direct the 1D assembly of NPs. In the absence of templates, 1D assembly readily proceeds for NPs with intrinsic anisotropic properties such as magnetic or electric dipole moments,10 crystallographical anisotropy,11 and surface chemical anisotropy.12 The precise control of the template-free 1D assembly of isotropic NPs in solution is still difficult owing to the lack of obvious anisotropic interactions. The addition of polymers to colloidal NP systems introduces a wealth of © 2012 American Chemical Society

interactions, such as steric repulsion, depletion attraction, and hydrophobic attraction.13 These offer various means to manipulate colloidal behavior. Although many kinds of nanostructures have been prepared by polymer-mediated assembly,14 the polymer-mediated 1D assembly of isotropic NPs requires further exploration. Kang et al. took advantage of the sphere-to-string morphological transition of poly(styrene-block-acrylic acid) copolymer micelles to assemble Au NP chains.15 The Au NPs are first encapsulated with a cross-linked poly(styreneblock-acrylic acid) polymer shell, and then the solution conditions are controlled to induce the sphere-to-string transition of the polymer micelles. This method requires that the Au NPs should be well-encapsulated and the excess polymers should be removed from the solution. Nikolic et al. produced strings of CdSe/CdS NPs via the self-assembly of Received: June 15, 2012 Revised: August 30, 2012 Published: August 30, 2012 13181

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this study are shown in Table 1. F127 was purchased from SigmaAldrich Corporation. P65 and P123 were obtained from BASF

polymer−NP conjugates, which comprise hydrophobic CdSe/ CdS NPs bound with hydrophilic poly(ethylene oxide) chains.16 The amphiphilic polymer−NP conjugates undergo surfactant-like self-assembly and give string-like nanostructures at proper polymer/NP ratios. The above methods generally require surface modifications of the NPs, whereas 1D NP assembly mediated by surface-adsorbed polymers suggests a comparatively convenient approach. Babayan et al. reported the formation of rodlike silica aggregates using surface-adsorbed thermoresponsive polymers.17 The temperature is raised above the lower critical solution temperature of the surface-adsorbed polymers to trigger hydrophobic interactions, which promote the NP assembly. At the same time, a relatively strong electrostatic repulsion exists to prevent random particle aggregation. In their report, 1D assembly is achieved through selecting the thermoresponsive polymers and the incubation temperatures. The incubation temperatures are relatively high, and control over the 1D structures is limited. We reported the 1D assembly of SNSs in the liquid phase using the F127 amphiphilic block copolymer.18 The SNSs assembled into 1D chainlike structures at pH 7.2−7.5 in the presence of F127 (2 wt % F127, 2 wt % SiO2) after incubation at 60 °C for several days. Instead of pH adjustments, tuning the ionic strength by the addition of salt also led to the formation of chainlike structures, suggesting that the 1D assembly of SNSs is sensitive to electrostatic forces. SNSs are typical isotropic NPs. Facile methods have been developed to synthesize stable colloidal suspensions of SNSs with sizes smaller than 20 nm using a tetraethoxysilane−water biphasic reaction catalyzed by basic amino acids, amines, or ammonia.19 SNSs with controlled sizes from ca. 15 to 200 nm were formed by a seed-regrowth method.20 These SNSs show great potential as ideal building blocks for the study of 1D assembly. F127 is a PEO-PPO-PEOtype block copolymer, which is known to adsorb on the surface of silica through hydrogen bonding between the ether oxygens in PEO and the silanol groups of silica.21 F127 may modify the interparticle interactions of SNSs through surface adsorption; however, details concerning its effects on the 1D assembly of SNSs are not clear. Understanding the polymers’ effects on the 1D assembly of isotropic NPs such as SNSs is important for extending the polymer-mediated 1D assembly approach to a wide range of polymer−NP systems. In this study, the effects of PEO-PPO-PEO block copolymers on the 1D assembly of SNSs were investigated. We take advantage of pH adjustment as an experimentally convenient means to finely tune interparticle electrostatic repulsion. Because the 1D assembly results from a delicate balance of interactions,22 pH1D is believed to reflect the conditions when such balance is achieved. PEO-PPO-PEO block copolymers are commercially available with different block lengths.23 To develop clues regarding the polymers’ effects on the 1D assembly, the relationships between pH1D and the polymers’ HLB value, the number of hydrophilic EO (NEO) and hydrophobic PO units (NPO), and their relative ratio (NPO/ NEO) are discussed. Considering the sensitivity of the 1D assembly to pH conditions, zeta potential measurements are conducted to examine how electrostatic repulsion among the SNSs may be affected by the block copolymers.

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Table 1. Properties of the PEO-PPO-PEO Block Copolymers Used in This Studya polymer F68 F127 P65 L64 P123

average Mw (g/mol) 8350 12 600 3400 2900 5750

average NEO

average NPO

152 200 39 26 39

28 65 29 30 69

EO content (wt%) 80 70 50 40 30

HLB

CMT (°C)

29 22 17 15 8

46.0 21.5 31.5 28.5 14.0

a

The average Mw values are provided by the manufacturers. The average NEO and NPO values are calculated from the average Mw and EO content. The HLB values are adopted from ref 23a. The critical micellization temperature (CMT) values for 2.5 wt % polymer solutions are adopted from ref 23b.

Corporation. F68 and L64 were received from Adeka Corporation as gifts. Si wafers were cleaned with Semico Clean 56 (Furuuchi Chemical) in an ultrasonic bath for 15 min and then with water for 15 min at room temperature, followed by rinsing with ethanol and air drying. All chemicals were used without further purification. Deionized water was used as the solvent in all experiments. Synthesis of SNSs. SNSs with a diameter of ca. 15 nm were synthesized using the biphasic reaction method, as previously reported.19a,b First, a solution of L-lysine was prepared by dissolving 0.037 g of L-lysine in 34.8 g of deionized water in a 110 mL glass vial, and then 2.6 g of TEOS was added. The molar ratio of TEOS/Llysine/H2O is 1:0.02:154.4. The mixture was allowed to react at 60 °C for 24 h using a water bath. Magnetic stirring at 500 rpm was applied during the reaction using a 2-cm-long Teflon-coated stirring bar. Finally, a homogeneous, optically clear suspension of SNSs (2 wt % SiO2, pH ∼9.4) was obtained. Polymer-Mediated 1D Assembly of SNSs. Different PEO-PPOPEO block copolymers (Table 1) were used to mediate the 1D assembly of SNSs. The block copolymers consist of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) blocks arranged in a linear fashion of PEO-PPO-PEO (Figure 1). The polymer-mediated

Figure 1. Molecular structure of PEO-PPO-PEO block copolymer. 1D assembly was conducted as follows. First, each polymer was added to as-prepared SNS suspensions at a polymer/SiO2 weight ratio of 1:1. The polymers were completely dissolved at room temperature after 24−48 h, giving homogeneous polymer−SNS mixtures containing 2 wt % polymer and 2 wt % SiO2. Subsequently, the pH of the mixtures was adjusted using 0.01−1 M HCl or 0.1−1 M NaOH aqueous solutions. Finally, the polymer−SNS mixtures were statically incubated at 60 °C for 3−10 days. Characterization. The morphology of the products was observed using a Hitachi S-900 scanning electron microscope (SEM) at an accelerating voltage of 6 kV. All samples are diluted with deionized water (10 times) and deposited onto clean Si wafers by dip coating at a withdrawal speed of 10 mm/min. UV-ozone treatment of the samples was performed to remove organic species. To minimize the charging of electrons during SEM observation, the samples were sputtered with Pt in an Ar atmosphere using an ion sputtering system (Hitachi E-1030) prior to SEM analysis. Zeta potential measurements were performed on the Zetasizer Nano ZS90 instrument (Malvern Instruments Corp., U.K.) using a folded capillary zeta potential cell. Electrophoretic

EXPERIMENTAL SECTION

Materials. Tetraethoxysilane (TEOS) was purchased from Tokyo Chemical Industry Co., Ltd. L-Lysine was purchased from Wako Pure Chemical Industries. The PEO-PPO-PEO triblock copolymers used in 13182

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mobility is converted into zeta potential using the Smoluchowski approximation. To study the zeta potential of the SNSs in the presence of different block copolymers, samples were prepared by dissolving the polymers with a polymer/SiO2 weight ratio of 1:1 in as-prepared SNSs suspensions. Measurements were conducted at 60 °C to ensure that all polymers were examined under their micellization conditions. To study the zeta potential of the polymer-containing SNS suspensions under different pH conditions, the F127−SNS system was chosen because it allows examination over a relatively wide pH range without inducing immediate aggregation. Samples were prepared by dissolving F127 in as-prepared SNS suspensions (1:1 w/w F127/SiO2), and pH adjustments were performed for the resulting mixtures using 0.01−1 M HCl aqueous solutions. All measurements were conducted at room temperature (25 °C).

Figure 3. SEM images of the polymer-mediated 1D assemblies of SNSs after 7 days of incubation at 60 °C: (a) the P65−SNS suspension at pH 8.5, (b) the F127−SNS suspension at pH 7.5, and (c) the F68−SNS suspension at pH 7.6.

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structures was also carried out (Figure S1). The morphology of the chainlike structures is not obviously different from that observed by SEM, suggesting that the substrates have a negligible influence on the chainlike structures. The mode of assembly can be systematically controlled by pH, as we previously reported.18 The SNSs assemble into chainlike structures at pH1D after incubation at 60 °C. A timedependent investigation of the F127−SNS system at pH 7.5 (pH1D with F127) shows that chainlike structures are already observed after incubation at 60 °C for 1 day and they become longer with increased incubation time (Figure S2). The SNSs remain isolated above pH1D, and they form heavily branched structures or even irregular aggregates below pH1D after incubation. The stability of the assembled structures was tested using sonication. The chainlike structures (F127−SNS system, pH 7.5) are still robust after 30 min of sonication (Figure S3), revealing that they possess mechanical strength due to Si−O− Si bonds formed between the neighboring SNSs. The reversibility of the assembly process was investigated through readjusting the pH of the suspension of silica chainlike structures. Because the isoelectric point of colloidal silica is pH 2 to 3,24 raising the pH of the suspension leads to stronger electrostatic repulsion between the SNSs and promotes their dispersion. The suspension of chainlike structures of the F127− SNS system (pH 7.5) was readjusted to pH 9.2, followed by incubation at 60 °C. The chainlike structures exhibited no obvious morphological difference after 3 days of incubation (Figure S4), suggesting that the 1D assembly process is not reversed through pH adjustment. Such structural stability offers ease of handling and further processing of the chainlike structures. Different block copolymers mediate the 1D assembly of the SNSs under different optimal pH conditions, thus some interplay of polymers and pH conditions may contribute to the 1D assembly. In general, 1D assembly is the result of wellbalanced interparticle attractions and repulsions.22 On one hand, the addition of block copolymers to the SNS suspensions induces non-DLVO interactions.13 On the other hand, pH adjustments tune the electrostatic repulsion among the SNSs. In a sense, pH1D can be viewed as an index of the conditions affording a balance of interactions. It is known that the EO unit is hydrophilic at temperatures lower than 80 °C and the PO unit is hydrophobic at temperatures higher than 20 °C.25 Therefore, the block copolymers are amphiphilic under the current experimental temperature conditions (25−60 °C). The self-assembly principle of amphiphiles suggests that the hydrophilic−hydrophobic ratio is important for balancing the interfacial energy of the hydrophilic−hydrophobic interface and the repulsive energy of the hydrophilic groups because this

RESULTS AND DISCUSSION One-Dimensional Assembly of SNSs with Different Block Copolymers. After incubation at 60 °C in the presence of block copolymers, the SNSs typically form three kinds of structures (dispersed particles, 1D chainlike structures, and particle aggregates) within pH 7.0−9.5. We defined the 1D chainlike structures as structures consisting of at least five continuously connected SNSs before branching occurs. The chainlike structures form through the 1D assembly of SNSs in the liquid phase, which has been confirmed by turbidity and cryo-TEM investigations.18 The adhesion of SNSs is strong enough (as discussed later) that the chainlike structures can be transferred to substrates. Well-formed chainlike structures are generally observed by SEM after incubation for several days. Figure 2 shows representative SEM images of the polymer-

Figure 2. SEM images of the polymer-mediated assemblies of SNSs. P123−SNS suspensions after 7 days of incubation at 60 °C: (a) pH 9.9, (b) pH 9.4, and (c) pH 8.5. L64−SNS suspensions after 5 days of incubation at 60 °C: (d) pH 9.5, (e) pH 9.0, and (f) pH 8.5.

mediated assemblies of SNSs after 5−7 days of incubation at 60 °C. When P123 is used, particles that are mostly dispersed are observed at pH 9.9 (Figure 2a). The SNSs assemble into 1D chainlike structures at pH 9.4 (Figure 2b) and aggregate with each other at pH 8.5 (Figure 2c). On the basis of these observations, pH1D in the P123−SNS system is determined to be ca. 9.4. When L64 is employed, the SNSs are mostly dispersed at pH 9.5 (Figure 2d). They assemble into chainlike structures at pH 9.0 (Figure 2e) and form particle aggregates at pH 8.5 (Figure 2f). Hence, pH1D is ca. 9.0 for the L64−SNS system. Similarly, pH1D values for the P65−SNS, F127−SNS, and F68−SNS systems are determined to be ca. 8.5, 7.5, and 7.6, respectively (Figure 3). TEM observation of the chainlike 13183

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The morphologies systematically distribute into three regionsI, II, and IIIthat correspond to dispersed particles, 1D chainlike structures, and particle aggregates, respectively. The diagram shows that pH1D roughly shifts from a basic to a near-neutral pH range with increasing HLB values of the polymer. Colloidal silica has an isoelectric point at pH 2 to 3.24 The surface charge density of SNSs increases with increasing pH (above 3.0), thus SNSs should experience stronger electrostatic repulsion under basic pH conditions than under near-neutral pH conditions. The shift of pH1D in parallel with the HLB values of the polymers implies that the balance of interactions in the 1D assembly is affected by the polymers’ relative hydrophilicity. Strong electrostatic repulsion is needed to achieve a balance of interactions in the 1D assembly when relatively hydrophobic polymers (P123, L64, and P65, HLB < 20) are used, whereas weak electrostatic repulsion is favorable with the relatively hydrophilic polymers (F127 and F68, HLB > 20). This diagram can also be considered to be a stability diagram because the suspensions in the three regions display corresponding macroscopic changes (Figure 4b). Suspensions in region I are optically clear after incubation, which indicates good colloidal stability. Those in region II show turbidity, suggesting a certain degree of aggregation because of the formation of 1D assemblies. The suspensions in region III become opaque gels or undergo precipitation, which usually indicates heavy aggregation and poor colloidal stability. Onedimensional assembly will not occur if the SNS suspensions are too stable, whereas the SNSs form randomly aggregated structures if they are completely destabilized. The partial loss of colloidal stability under the proper pH conditions is a favorable circumstance for 1D assembly.27 To develop clues to the effects of EO and PO units on 1D assembly, the relationships between pH1D and NEO (Figure 5a) as well as between pH1D and NPO (Figure 5b) are examined. pH1D does not clearly correlate to NEO or NPO as it correlates to HLB, which is reasonable because HLB is determined by NEO and NPO at the same time. Still, pH1D changes with varying NEO (or NPO) at fixed NPO (or NEO) values. For example, both F127 and P123 have similar numbers of PO units (65−69), whereas F127 has approximately 4 times more EO units than P123 (Table 1). As shown in Figure 5a, pH1D shifts from ∼7.5 to ∼9.4 with decreasing NEO. The F68−SNS and P65−SNS (or L64−SNS) systems also indicate such a correlation between the pH1D shift and NEO at fixed NPO. However, both P123 and P65 have an NEO of ca. 39, whereas P123 (NPO, ∼69) has more PO units than P65 (NPO, ∼29). The P123−SNS and P65−SNS systems show that pH1D shifts from ∼8.5 to ∼9.4 with

balance determines the structures of the amphiphile assemblies.16 When amphiphilic block copolymers are used to mediate the 1D assembly of SNSs, the difference between the ratios and lengths of the hydrophilic and hydrophobic blocks should influence the balance of interactions in the 1D assembly. Dependence of pH1D on the Polymers’ HLB Value, NEO, NPO, and NPO/NEO. The relative ratio of hydrophilic and hydrophobic blocks in amphiphilic polymers determines their HLB value.26 For PEO-PPO-PEO block copolymers, HLB is expressed as23a HLB = −36.0

NPO + 33.2 NPO + NEO

where NEO and NPO are the numbers of EO and PO units, respectively. The HLB value of the PEO-PPO-PEO block copolymers ranges from 0 to 30 and is a measure of the polymers’ relative hydrophilicity. In general, PEO-PPO-PEO block copolymers with HLB values larger than 20 are relatively hydrophilic and those with HLB values smaller than 20 are relatively hydrophobic. A morphology diagram is established on the basis of SEM observations (Figure 4a). Three typical morphologies (dis-

Figure 4. (a) Morphology diagram of the polymer−SNS suspensions after incubation at 60 °C for 5−7 days. (b) Representative photographs of the macroscopic appearance of the polymer−SNS suspensions in regions I, II, and III. The photographs correspond to the L64−SNS suspensions at (I) pH 9.5, (II) pH 9.0, and (III) pH 7.5 after 5 days of incubation at 60 °C.

persed particles, 1D chainlike structures, and particle aggregates) are charted as a function of pH and HLB values.

Figure 5. Relationships between pH1D and (a) NEO, (b) NPO, and (c) NPO/NEO of the block copolymers: (○) P123, (▽) L64, (△) P65, (●) F127, and (■) F68. 13184

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increasing NPO values (Figure 5b). Finally, pH1D is plotted as a function of NPO/NEO (Figure 5c). The pH1D systematically shifts from ∼7.5 to ∼9.4 with increasing NPO/NEO. Therefore, decreasing the number of hydrophilic EO units accompanied by an increase in the number of hydrophobic PO units leads to a shift of pH1D to higher values. PEO-PPO-PEO block copolymers adsorb onto hydrophilic silica surfaces through hydrogen bonding interactions.21 Because the PEO blocks have an affinity for both silica and water, they may preferentially locate near the silica surface and in the outer regions of the adsorbed layer.21a Those at the outer regions of the adsorbed polymer layers give rise to steric repulsion.28 Usually PEO-PPO-PEO block copolymers with long PEO blocks and large HLB values are effective steric stabilizers for particles in aqueous solutions.29 However, the adsorbed PEO-PPO-PEO layers are also found to strengthen the net attraction between surfaces.28 Hydrophobic attraction may be involved because the hydrophobic blocks tend to fuse with each other at small distances.30 On the basis of these findings, we conclude that the adsorbed PEO-PPO-PEO block copolymers affect the interparticle interactions of SNSs by providing steric repulsion and hydrophobic attraction. The shift of pH1D with the polymers’ HLB value, NEO, and NPO suggests that electrostatic repulsion, steric repulsion, and hydrophobic attraction play important roles in balancing interactions in the 1D assembly. Block copolymers with large HLB values and relatively long PEO blocks (e.g., F127 and F68) may provide strong steric repulsion between the SNSs. When these polymers are used, the SNSs are well stabilized by both steric and electrostatic repulsions. The repulsive barriers need to be weakened for the SNSs to assemble in a 1D manner. Decreasing pH reduces the electrostatic repulsion among the SNSs and facilitates their mutual approach. Hydrophobic attraction may come into play when the SNSs approach over small interparticle distances. However, block copolymers with small HLB values and relatively short PEO and long PPO blocks (e.g., P123) may provide weak steric repulsion but cause strong hydrophobic attraction among the SNSs. A relatively strong repulsive barrier is preferred to avoid random particle aggregation; therefore, a balance of interactions in the 1D assembly is achieved at higher pH, where the electrostatic repulsion among the SNSs is stronger. Electrostatic Repulsion of the Polymer-Containing SNS Suspensions. The adsorbed block copolymers may modify the level of electrostatic repulsion among the SNSs. The magnitude of electrostatic repulsion can be estimated from the zeta potential. The zeta potential and colloidal stability of the SNS suspensions at pH 9.4 in the absence and presence of block copolymers are summarized in Table 2. The zeta potential of the P123-containing SNSs could not be determined because they begin to assemble at the measurement temperature (60 °C) and well-dispersed samples are required for zeta potential measurements. The SNSs have a zeta potential of ca. −62 mV at pH 9.4 in the absence of block copolymers. Such a low negative zeta potential suggests that the SNSs retain a high level of negative surface charges (deprotonated silanol groups) and relatively strong electrostatic repulsion exists among them. The absolute zeta potential of SNSs considerably decreases in the presence of polymers. Because the PEO-PPO-PEO block copolymers are nonionic, such a decrease in the measured zeta potential can be attributed to an outward shifting of the slipping plane by the adsorbed polymers.31 A decrease in the absolute zeta potential

Table 2. Zeta Potential and Colloidal Stability of the Polymer−SNS and Polymer-Free SNS Suspensions at pH 9.4a samples

zeta potential (mV) at 60 °C

colloidal stability after incubation at 60 °C

F68−SNS F127−SNS P65−SNS L64−SNS P123−SNS SNSs

−20.2 ± 1.1 −7.3 ± 0.4 −16.9 ± 1.0 −15.3 ± 1.0 N.A. −62.0 ± 2.4

good good good good poor good

a

The polymer−SNS suspensions contain 2 wt % polymer and 2 wt % SiO2. The polymer-free SNS suspension contains 2 wt % SiO2.

indicates weakened electrostatic repulsion, thus the polymers afford electrostatic shielding effects.32 The absolute zeta potentials of the L64-, P65-, F127-, and F68-containing SNS suspensions decrease to well below 30 mV, a level at which poor colloidal stability is usually predicted.33 However, these suspensions remain dispersed even after incubation at 60 °C because the adsorbed polymers provide a certain degree of steric stabilization.28,34 The zeta potential of polymer-containing SNS suspensions under varying pH conditions is studied in the F127−SNS system (Figure 6). This system is chosen because it allows

Figure 6. (a) Zeta potential of the F127−SNS (●) and polymer-free SNS (□) suspensions in the pH range of 7.5−9.0 at 25 °C. Photographs of the (b) F127−SNS and (c) polymer-free SNS suspensions in the pH range of 7.5−9.0 after incubation at 60 °C for 3 days.

measurements over a relatively wide pH range (7.5−9.0) without undergoing immediate aggregation. The absolute zeta potential of the SNS suspensions both with and without F127 decreases with decreasing pH (Figure 6a), reflecting weakening electrostatic repulsion as the pH approaches the isoelectric point of the SNSs. The stability of these same SNS suspensions (with and without F127) is examined by observing their macroscopic changes after incubation at 60 °C for 3 days. In the presence of F127, the SNS suspension remains optically clear at pH 9.0 whereas it becomes turbid at pH 7.5 after incubation (Figure 6b). In the absence of polymers, the SNS suspensions at pH 7.5 and 9.0 both remain optically clear after incubation (Figure 6c), suggesting good colloidal stability. These observations agree with the majority of reports suggesting that silica suspensions are stable at high pH (above 6.0) and low salt concentrations.24 The colloidal stability of SNSs under such conditions is attributed to the existence of strong electrostatic repulsion. Therefore, F127 13185

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renders the SNSs more susceptible to destabilization with decreasing pH. On the basis of the zeta potential measurements, we expect that the electrostatic shielding effects of F127 result in a partial loss of colloidal stability for the SNSs. The other block copolymers are also confirmed to have electrostatic shielding effects (Table 2), which can be attributed to the adsorbed polymers.31 The quantity of adsorbed polymers is determined from CHN chemical analysis of the precipitates separated from the polymer−SNS mixtures by centrifugation (Table S1). The measured amounts of block copolymers are generally large compared to those in previous reports (0.06−0.24 mg/m2),21 which may be due to the high polymer concentration of our samples. The polymer showing the highest retention in the SNS precipitates is P123, followed by F127, L64, P65, and F68. The P123−SNS suspension at pH 9.4 assembles and displays increased turbidity during the incubation period prior to centrifugation. Except for P123, we found that the retained amounts of polymers roughly correspond to their electrostatic shielding effects on the SNSs (F127 > L64 > P65 > F68). In other words, the electrostatic shielding effects are strengthened by increasing polymer adsorption. However, there is no clear relationship between the degree of shielding and pH1D, suggesting that other effects afforded by the polymers, such as steric repulsion and hydrophobic attraction, likely play important roles. Our results show that PEO-PPO-PEO block copolymers reduce the colloidal stability of SNSs through electrostatic shielding and afford steric repulsion and hydrophobic attraction during the 1D assembly of SNSs. However, this does not fully explain why the SNSs assemble in a 1D manner. To clarify whether the polymers contribute to the formation of 1D structures, polymer-free SNS suspensions under various pH conditions are incubated at 60 °C. The SNSs at pH 9.4 remain dispersed and form close-packed structures upon drying.19 At pH values close to the isoelectric point (pH 2 to 3), the SNSs form particle aggregates; however, no 1D chainlike structures are observed (Figure S5). The polymers should induce effects that drive particle connections in an anisotropic manner. The 1D assembly of spherical NPs with surface-attached polymers has been well studied both experimentally and theoretically.16,35,36 These studies agree with a scenario in which the particles initially form dimers when assembly begins and subsequent rearrangement of the surface-attached polymers causes steric crowding at the neck of the dimer. The steric crowding is believed to hinder lateral aggregation and favor the subsequent attachment of particles at the end of the dimer.16,35,36 An illustration of this scenario is given in Scheme 1. In the current system, the PEO-PPO-PEO block copolymers attach to the surfaces of the SNSs through hydrogen bonding interactions. Rearrangements and steric crowding of the block copolymers may occur to assist the 1D assembly of SNSs.

Scheme 1. Proposed Model of the 1D Assembly of SNSs Promoted by Rearranged Surface-Attached Polymers

the polymers tend to promote the 1D assembly of SNSs at higher pH, indicating that steric repulsion and hydrophobic attraction play important roles in balancing the interactions. The SNSs become more sensitive to destabilization with decreasing pH because the polymers afford electrostatic shielding effects on the SNSs. Our study demonstrates that the polymer-mediated 1D assembly approach is useful with different PEO-PPO-PEO block copolymers and provides insight into the polymers’ role in the 1D assembly of SNSs. Further investigations are in progress using non-(PEO-PPOPEO) block copolymers and SNSs. These chainlike structures have the potential to be used as hard templates for hollow anisotropic nanostructures with high surface area.

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ASSOCIATED CONTENT

* Supporting Information S

TEM image of the chainlike structures. SEM images of the chainlike structures after different incubation times, before and after sonication, and after pH readjustment. CHN chemical analysis of the adsorbed amounts of polymers on SNSs. SEM images of the polymer-free SNS suspensions. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS The effects of nonionic PEO-PPO-PEO block copolymers on the 1D assembly of SNSs are investigated. The polymers promote the assembly of the SNSs into 1D structures and influence the colloidal stability of the SNS suspensions. pH1D roughly shifts from a basic pH range (8.5−9.5) to a nearneutral pH (∼7.5) in conjunction with increasing HLB values of the polymers, suggesting that the relative hydrophilicity of the polymers influences the balance of interactions required for 1D assembly. Moreover, decreasing NEO and increasing NPO of

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +81-35841-7348. Fax: +81-3-5800-3806. Present Address

† Department of Chemistry and Biotechnology, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

Notes

The authors declare no competing financial interest. 13186

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (B) (23350098) from the Japan Society for the Promotion of Science. We thank Professor Yukio Yamaguchi (The University of Tokyo) for providing equipment for zeta potential measurements and Adeka Corporation for freely offering the block copolymers. S.Z. is grateful for financial support from the China Scholarship Council (CSC) and the Global Center of Excellence for Mechanical Systems Innovation (GMSI, The University of Tokyo).

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