Facile Preparation Route toward Speckled Colloids via Seeded

Jan 30, 2013 - A facile method to prepare monodisperse speckled colloids has been developed via one-step seeded polymerization from noncross-linked la...
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Facile Preparation Route toward Speckled Colloids via Seeded Polymerization Xiaohui Meng,†,‡ Yinyan Guan,†,‡ Zhongwei Niu,§ and Dong Qiu*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100190, China § Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: A facile method to prepare monodisperse speckled colloids has been developed via one-step seeded polymerization from noncross-linked latex particles. It was found that both cross-linking agents in the added monomer mixture and charged initiation species are essential for the formation of speckles on composite latex particle surface in seeded polymerization. The size and number density of speckles on the surface are tunable by adjusting the concentration of surfactant. A possible mechanism for the formation of such speckled colloids has been proposed based on a series of control experiments. Speckled colloidal particles were used as substrates for the adsorption of tobacco mosaic virus, and a much stronger adsorption was observed compared to smooth particles, implying a potential application of these speckled particles in virus collection and more.

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INTRODUCTION The chemistry of colloidal particles has long been recognized to have an important effect on their surface properties; however, the effect of surface morphology is far from being well evaluated. One of the major reasons is the lack of facile ways to control particle surface morphology. Changing particle surface morphology can be advantageous compared to chemical modifications in certain areas, for example, in vivo applications where only limited chemicals are allowed1−3 or self-assembly by suppressing the depletion interaction to fabrication of patchy particles.4 It is well-known that superhydrophobicity or superhydrophilicity can be achieved by increasing the surface roughness in nano/micro scale without changing the overall chemistry.5−9 Molecular dynamics simulation showed that adsorption is related to the relative sizes of adsorbents and micro domains at a patterned interface.10,11 Recently, gold nanoparticles with rippled surfaces were shown to have extremely low protein adsorption,12−15 opening the possibility of controlling protein adsorption through surface patterning rather than chemical modification. Although various methods have been developed to control colloidal particle surface morphologies, such as chemical deposition,16 physical adsorption,17 plasma etching,18 and so on, the instability and low production yields severely limit their practical applications. Pickering emulsion polymerization based on nanoparticle stabilizer provides another method to fabricate particles with complex surfaces,19−25 where the surface morphology is determined by the particle stabilizers. However, the stabilizing © 2013 American Chemical Society

particles are normally packed close to each other, and it is almost impossible to regulate the packing density. On the other hand, a few studies have demonstrated that the surface morphology of colloidal particles could be influenced by cross-linking degree and the resultant surface pattern can be fairly stable under various physicochemical conditions.26−36 In those studies, we noticed that monodisperse colloidal particles with speckled surfaces can be obtained in some occasions by conventional emulsion or dispersion polymerization,26,30−34 implying that seeded polymerization may be an effective approach to obtain colloidal particles with stable speckled surfaces. However, the possibility of adjusting the size and number density of speckles on the surface of final particles has not been investigated yet. According to those works, the formation of speckles highly depends on the quantity and procedure of adding cross-linkers. In this study, we developed a facile one-step seeded polymerization method to prepare monodisperse colloidal particles with tunable speckled surfaces based on the in situ formation of speckles at the solid/liquid interface. By optimizing a few parameters, we can obtain more control over the speckle size and number density on composite particles, which has not been demonstrated before. The effect of speckled particles with different speckle sizes and number Received: November 15, 2012 Revised: January 24, 2013 Published: January 30, 2013 2152

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free TMV in supernatant is detectable before the further microscopic characterization. Characterization. TEM images were obtained using a Hitachi 800 transmission electron microscope operated at an accelerating voltage of 100 kV. SEM images were collected on a JEOL JSM-6700 scanning electron microscope operated at an accelerating voltage of 5 kV. Dynamic light scattering and zeta potential measurements were carried out on a Zetasizer-nanozs laser-scattering particle size distribution analyzer. UV absorbance spectra were collected on a SHIMADZU UV1601 spectrophotometer.

densities on the adsorption of tobacco mosaic virus (TMV) was also examined.

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EXPERIMENTAL SECTION

Materials. Styrene (St, Sigma-Aldrich), tert-butyl acrylate (TBA, Sigma-Aldrich), glycidyl methacrylate (GMA, TCI), and divinyl benzene (DVB 55%, J&K Chemistry) were treated by passing through basic alumina columns to remove inhibitors before use. Recrystallized azo-bis-isobuyronitrile (AIBN, Sinopharm chemistry) and potassium peroxodisulfate (KPS, Alfa-Aesar) were used as initiators. Methanol, ethanol, tetrahydrofuran (THF, Beijing Chemistry), polyvinylpyrrolidone (PVP K30, Shantou, China), sodium dodecyl sulfate (SDS, Aladdin, China), poly(diallyldimethylammonium chloride) (PDDA, 400 000−500 000 g mol−1, 20 wt % aqueous solution, Sigma-Aldrich), and tobacco mosaic virus (TMV, 10 mg mL−1, separated from tobacco leaves) were used as received. Pure water was generated by an ELGA Purelab system, with a resistance greater than 18.2 MΩ. Methods. Synthesis of Spherical Polystyrene (PS) Seed Particles. The seed particles used in this study were prepared by dispersion polymerization (with a diameter of 800 nm and polydispersity index of 1.04, Figure S1). In a typical experimental set, a mixture of St (10 g), AIBN (100 mg), PVP (2 g), and methanol (100 g) was kept at 70 °C for 12 h with magnetic stirring under nitrogen atmosphere. PS seed particles were isolated by centrifugation (8000 rpm, 5 min) after the polymerization. These particles were washed twice with ethanol followed by water before further use. Centrifugation was repeated after each wash. Finally, the resultant particles were dispersed in water. Seeded Polymerization for Composite Particles with Tunable Speckled Surface. In a typical experiment, 2.2 mL of seed dispersion (with a solid content of 0.18 g mL−1) was added into a 50 mL flask equipped with a reflux condenser, followed by the addition of 0.5 mL (variable) of SDS aqueous solution (10 mg mL−1), and then the blend solution was diluted to 20 mL with pure water. After 5 min of agitation (300 rpm), the mixture of TBA (or GMA) and DVB was injected. After deoxygenation by bubbling with nitrogen for 30 min, the system was heated to 70 °C. Then 200 μL of KPS aqueous solution (10 mg mL−1) was added to initiate polymerization. The reaction was allowed to proceed for 12 h with magnetic stirring (350 rpm). The final colloidal dispersions were characterized without any further purification. Removal of Seed Particles. A total of 0.5 mL of as-prepared composite colloidal particle dispersion was washed by ethanol for two cycles of centrifugation and redispersed in 5 mL mixture of water and THF (1:3, v:v). After 6 h, the mixture was centrifuged (10 000 rpm, 8 min) and redispersed in 5 mL of mixture of water and THF (the same ratio as above). The centrifugation and redispersion cycles were repeated twice. Finally, the particles were dispersed in water. Hydrolysis of Speckled Particles. The speckled particles were partially hydrolyzed through a method modified from Furukawa.37 0.5 g of speckled particles were dispersed in 30 mL of ethanol/water mixture (5:1, v:v), and then 6 mL of formic acid (88 wt %) was charged. After a 12 h of heat treatment in 50 °C, the particles were isolated by centrifugation (8000 rpm, 5 min) and washed thoroughly with water. Adsorption of TMV. Before the adsorption of TMV on the speckled surface, the preproduced partially hydrolyzed speckled particles were incubated with PDDA aqueous solution (1 wt % of water) for 6 h in 0.01 M potassium phosphate buffer (pH 7.2). The excess PDDA were removed by centrifugation-washing cycle. Then the PDDA-coated speckled particles were concentrated to about 20 mg mL−1 by centrifugation (10 000 rpm, 5 min). A total of 0.1 mL of the above PDDA-coated particle dispersion was added to 4.9 mL of 0.1 mg mL−1 TMV aqueous solution (0.01 M potassium phosphate buffer, pH 7.2). After 2 h of incubation, the excess TMV were removed by centrifugation (10 000 rpm, 5 min, 20 °C). The supernatant was kept for the further assay to quantify the TMV adsorbed on speckled particles by UV absorbance measurement at 260 nm using an 38 The TMV-coated particles were extinction coefficient (E0.1% 1cm ) of 3.0. washed with potassium phosphate buffer (pH 7.2) thoroughly until no

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RESULTS AND DISCUSSIONS Seeded polymerization is often used to prepare monodisperse colloidal particles with larger sizes. In a typical seeded polymerization, noncross-linked seeds are first swollen by monomer, then the polymerization takes place in seed particles, and usually, core−shell spherical particles with a larger overall size39−42 or anisotropic particles43−45 will be obtained. The final morphology of the particles highly depends on the degree of phase separation between two different phases. Many works have been carried out to fabricate anisotropic particles with different chemical and physical properties by facilitating the phase separation.46−50 However, only core−shell particles were always obtained in the absence of cross-linker under otherwise identical conditions, showing the important role of cross-linker in determining composite particle morphology. On the basis of those findings, we propose a strategy to prepare monodisperse composite colloidal particles with surface speckles by seeded polymerization from noncross-linked PS latex particles (Scheme 1). Scheme 1. Illustration of the Formation of Speckled Colloidal Particles

Fabrication of Speckled Colloids. First, noncross-linked PS particles are partially swollen by the mixture of vinyl monomer and cross-linker DVB. When initiated by KPS, polymerization may take place at two sites: the partially swollen seed particles and the continuous phase (as what observed in surfactant-free emulsion polymerization51−53). Thus both cross-linked shells on seed particles and water-soluble oligomers (due to the charge from KPS radical) will be produced. As the polymerization proceeds, the oligomers grow and become less soluble and, thus, will form microaggregate at the core−shell particle surface, similar as surfactant molecules may do at interfaces. Monomers diffusing from monomer droplets will continue to grow on these microaggregates as they have higher curvature as well as surface tension compared to 2153

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Table 1. Recipes for the Production of Speckled Colloidsa sample

1−1

2−1

2−2

3−1b

3−2

3−3

4−3

5−1

5−2

5−3

TBA/μL GMA/μL DVB/μL SDS/g L−1 KPS/g L−1 speckle size/nm speckle coverage/% edge-to-edge distance/nm polydispersity index (PDI)

275 0 32 0.375 0.1 22 12 35 1.06

0 275 35 0.50 0.15 30 30 32 1.05

0 275 35 0.375 0.1 36 23 35 1.03

275 0 32 0.25 0

294 0 0 0.25 0.1

1.05

1.04

294 0 64 0.25 0.1 30 18 38 1.20

250 0 32 0.25 0.1 18 9 40 1.04

275 0 32 0.125 0.1 35 4 135 1.03

275 0 32 0.25 0.1 26 10 50 1.07

275 0 32 0.50 0.1 20 21 22 1.06

In all cases, the final volumes were kept as 20 mL with pure water. The terminology x−y corresponds to a SEM image shown in Figure x obtained by this recipe, for instance, recipe 2−2 corresponds to Figure 2b. bThe initiator for seeded polymerization (case 3−1) was AIBN with a concentration of 1 wt % of the monomer. The speckle size and average edge-to-edge distance of neighboring speckles were calculated from highmagnification SEM photos. The coverage of speckle was calculated roughly by the fraction of the projection of speckles in the normal direction in a randomly selected area with the assumption that the spheres are semisphere and taking the selected area as a flat. More details are available in the Supporting Information (Figure S3). a

the shell nearby, and the speckles are formed and increased. It is worth noting that, the concentration of emulsifier should be lower than its critical micelle concentration (CMC = 2.3 g/L for SDS), which might effectively avoid the formation of large secondary particles. The proposed strategy is tested by the following experiments. Monodisperse speckled colloidal particles were readily obtained by one-step seeded polymerization from noncrosslinked PS latex particles according to the design in Scheme 1. The morphology of typical speckled colloidal particles, produced by recipe 1−1 listed in Table 1, was shown in Figure 1a. The overall diameter of speckled particles increased

Figure 2. SEM images of colloidal particles with tunable surface morphology in the presence of GMA. The concentrations of SDS were (a) 0.5 and (b) 0.375 g L−1, respectively. The concentrations of KPS were (a) 0.15 and (b) 0.1 g L−1, respectively. The charged amount of GMA and DVB were 275 and 35 μL, respectively.

Role of Cross-Linker and Initiator Attribute in the Formation of Speckles. As mentioned earlier, cross-linking in the particle shell and amphiphilic oligomer aggregating onto the seed particle surface are two essential factors for the formation of speckled colloidal particles. The introduction of the crosslinking agent DVB not only restricts the penetration of oligomer aggregates deeply into the seed particle but also prevents the newly formed speckles from being dissolved by monomers. Amphiphilic oligomers aggregating on the particle surface are supposed to result from the ionic radicals from KPS, a process similar to surface micellization of surfactant molecules. To confirm the above hypothesis, two control experiments were carried out. In one experiment, the polymerization was initiated by AIBN, which does not produce ionic radicals, under otherwise identical conditions (Figure 3a), where no speckled particles were formed. More importantly, the possibility that the speckles on the composite particle surface may be due to the phase separation during the polymerization of swollen monomer can be excluded by this control experiment. If the formation of the speckles was resulted from phase separation, similar speckled particles should also have been obtained by recipe 3−1, with AIBN as initiator in the presence of DVB. In the other experiment, the polymerization was initiated by KPS but in the absence of DVB (Figure 3b), where again no speckled particles were formed. These two control experiments supported the validity of the above hypotheses. Nevertheless, when the concentration of DVB is too high, the final speckled particles tend to be caved (Figure 3c), a common phenomenon due to the stress arisen from the uneven cross-linked network.24,28,34 With the increase

Figure 1. SEM (a) and TEM (b) images of speckled colloidal particles before and after THF extraction. The recipe for speckled particle fabrication is available in Table 1 as 1−1.

from 800 to 850 nm with a polydispersity index of 1.06 after the seeded polymerization. The size of speckles is about 22 nm, and around 12% of the colloidal particle surface is covered with speckles. Collapsed particles were obtained after THF extraction (Figure S2). TEM image shows that those collapsed particles were hollow and the speckles were embedded in the shells (Figure 1b). Compared with the SEM image (Figure 1a), no significant difference was observed in the size and number density of speckles, confirming that both the shells and speckles were actually cross-linked. Interestingly, no free secondary particles were observed in either SEM or TEM images, indicating the speckles do not result from the adsorption of secondary latex particles but the amphiphilic oligomer aggregates at the interface. To explore the universality of the above method for speckled particles, another vinyl monomer GMA was used. As shown in Figure 2, speckled particles with different speckle size and number density were obtained following recipes 2−1 and 2−2 listed in Table 1. 2154

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Figure 3. SEM images of colloids obtained by seeded polymerization initiated with AIBN in the presence of DVB (a), initiated with KPS without DVB (b), and initiated with KPS with excess DVB (c). The monomer and SDS charged amount for those three cases were 275 μL/0.25 g·L−1 (a), 294 μL/0.25 g·L−1 (b), and 294 μL/0.25 g·L−1 (c), respectively.

Figure 4. SEM images of speckled colloidal particles produced at different polymerization intervals: (a) 10 min, (b) 3 h, and (c) 12 h. The recipe was 250 μL of TBA, 32 μL of DVB, and 0.25 g L−1 SDS.

Figure 5. SEM images of colloidal particles with tunable surface morphology by adjusting the concentration of SDS: (a) 0.125, (b) 0.250, and (c) 0.50 g L−1. The charged amounts of TBA and DVB were 275 and 32 μL, respectively.

in cross-linker concentration, the force to maintain the morphology of the outer swollen soft shell will not be strong enough to counteract the stress arisen from the uneven crosslinked network; then cavity on the surface of composite particles will form. Evolution of Speckles. In Scheme 1, we proposed that further polymerization of monomer supplied from monomer droplets would take place in oligomer microaggregates on the composite particle surface, to result in the growth of speckles. The morphology of colloidal particles at different polymerization intervals was investigated in detail (Figure 4). It is clearly seen that the overall particle size did not change too much after the first 10 min interval (Figure 4a); however, the speckles become more and more evident with polymerization going on (Figure 4b) and finally keep the number density constantly while the size of speckle increasing (Figure 4c), strongly indicating that the further polymerization actually took place on the speckles. Subtle Adjustment of Speckle Size and Number Density by Surfactant Concentration. At this stage, it is quite clear that the speckles resulted from amphiphilic oligomer microaggregates on the particle surface. In principle, the oligomers aggregation should be adjustable with surfactant concentration. In particular, the speckle number density should be controlled by the surfactant since the aggregation of oligomers at the interface may be facilitated by SDS molecules,

resulting in an increase in the number density of speckles and a decrease in speckle size. SDS concentration was therefore chosen as the only variables to control the surface morphology of final colloidal particles (Figure 5). As expected, the speckle size and number density of the final colloidal particles can be controlled simply by varying SDS concentration. Figure 5 shows the SEM photos of colloidal particles with tunable surface morphologies by different concentrations of SDS. The speckle size decreases from 35 nm (Figure 5a) to 20 nm (Figure 5c) with the increase of SDS concentration from 0.125 to 0.50 g L−1, whereas the coverage of speckles increases from 4% to 21%. It is worth noting that, the final speckle particles are still monodispersed for all cases in Table 1 (except for the case 3−3, Figure 3c, due to the shrink of the surface) after the seeded polymerization. TMV Adsorption on Speckled Surfaces. TMV, a rod-like bionanoparticle, has a helically arranged protein shell and a genomic single-strand RNA core. The diameter of TMV is about 18 nm while with a length of 300 nm. Considering the diameter of TMV is comparable with the average edge-to-edge distance of neighboring speckles in the case of 5−3 (22 nm, in Table 1), tentative studied were carried out to illustrate the influence of speckled surface on adsorption behavior of TMV. Speckled particles with different average edge-to-edge distances produced by recipes 5−1 and 5−3 were chosen as model particles. The specific surface areas of model particles 5−1 and 2155

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5−3, obtained by BET adsorption, are 8.5 and 10.1 m2 g−1, respectively. To ensure effective adsorption of TMV (zeta potential −38 ± 5 mV in potassium phosphate buffer, pH 7.2) on the surface of speckled particles, the speckled particles from TBA were hydrolyzed with formic acid first as described in experimental section. Then surface electrostatic potential inversion was achieved by PDDA adsorption on the negative speckled particle surface. The initial zeta potentials of the model particles were −33.8 ± 6 (5−1) and −37.6 ± 4 mV (5−3) and inversed to 29.0 ± 5 and 33.6 ± 4 mV, respectively, after PDDA adsorption. Panels a and b in Figure 6 are the SEM photos

composite particle, however, the size matching between the average edge-to-edge distance of speckles and the diameter of TMV is helpful for providing discrete anchoring sites. Therefore, one would also expect that the adsorption of TMV on these speckled colloidal particles should be much stronger. The adsorption strength of TMV on the above model colloidal particles was demonstrated by washing with potassium phosphate buffer (Figure 6c,d). After the wash procedure, hardly any bare particle surface were observed in the case of 5− 3 (Figure 6d) while most of the adsorbed TMV in the case of 5−1 were washed away (Figure 6c), supporting that the speckles can help to anchor adsorbed TMV particles.

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CONCLUSION We have developed a facile method to prepare monodisperse composite colloids with speckles on their surfaces. The size and number density of the speckles can be controlled by varying the surfactant concentration. It was found that the use of both cross-linking agents in the added monomer mixture and initiator bringing about ionic radicals is essential for the formation of speckles on particle surfaces. The formation of the speckles was proposed to result from the aggregation of amphiphilic oligomers on seed particle surface and following further polymerization. This method in principle can be applied to prepare speckled colloids with other chemical compositions. The adsorption of TMV particles on the speckled particles surface indicates that proper edge-to-edge distance between two neighboring speckles can significantly enhance adsorbed amount and adsorption strength of TMV on the particles, illustrating the potential application in bionanoparticle collection and immobilization.

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Figure 6. SEM images of model speckled particles (a, 5−1 and b, 5−3) and TMV-adsorbed model speckled particles (c, 5−1 and d, 5−3) after three times washing with potassium phosphate buffer. The rod-like particles shown in background of c and d are the TMV desorbed from the surface of model particles.

ASSOCIATED CONTENT

S Supporting Information *

SEM photos of PS seed particles obtained by dispersion polymerization, speckled colloids after solvent extraction with THF, and statistical data on speckles. This material is available free of charge via the Internet at http://pubs.acs.org.

of model particles 5−1 and 5−3 after PDDA adsorption. Obviously, no significant changes on the surface morphology of model particles can be seen compared with panels a and c in Figure 5. The equilibrium adsorption amounts of TMV on particles surface are about 110.7 and 153.2 mg.g−1 for model particles 5−1 and 5−3, respectively, which correspond to 13.0 (5−1) and 15.2 mg m−2 (5−3). According to geometry of TMV, closely arranged TMV monolayer on a flat substrate would be about 12.5 mg m−2, which is comparable with TMV adsorption on model particles 5−1 (13.0 mg m−2), implying probably a monolayered TMV adsorption in this case. However, multilayered adsorption might be the case for model particle 5−3 (15.2 mg m−2) considering the free surface of model particle 5−3 without speckle is less than 80% (less than 8.0 m2 g−1). TMV is relatively rigid and with a size comparable to the speckles; therefore, TMV would not wrap the speckles, which means that the extra surface area brought about by speckles would not significantly contribute to TMV adsorption. In this case, any increment in adsorption would be resulted from the match of dimensions between TMV and interstices between speckles. As listed in Table 1, the edge-toedge distances between neighboring speckles on the surface of model particle 5−3 is about 22 nm, which matches the diameter of TMV well. Maybe it is impossible for a TMV particle positioning itself between the speckles entirely considering the random distribution of those speckles on the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Project No. 91027032, 51173193), State Key Development Program of Basic Research of China (Project No. 2012CB933200), National Key Technology R&D Program of China (Project No. 2011BAI02B05), and Chinese Academy of Sciences (Grant No. KJCX2-YW-H19).

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