Article pubs.acs.org/Langmuir
Dendritic Silica Nanoparticles Synthesized by a Block CopolymerDirected Seed-Regrowth Approach Junzheng Wang,†,§ Ayae Sugawara-Narutaki,†,‡ Atsushi Shimojima,†,∥ Minoru Osada,§ Renzhi Ma,§ and Tatsuya Okubo*,† †
Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
§
S Supporting Information *
ABSTRACT: A facile seed regrowth method is presented for the preparation of a new type of colloidal dendritic silica nanoparticles (DSNPs) with unique Konpeito-like morphology and high surface area (∼400 m2 g−1). Growth of silica nanoprotrusions on the surfaces of colloidal silica nanoparticles proceeds by hydrolysis and polycondensation of tetraethoxysilane (TEOS) in the presence of a PEO−PPO−PEO-type block copolymer (Pluronic F127) under controlled pH conditions. The polymers adsorbed on the seed surface play a crucial role in the formation of DSNPs. DSNPs with controllable size (28−85 nm) and narrow size distributions can be obtained by using monodisperse silica nanoparticles with various sizes as seeds. The surface morphology of DSNPs is tunable by changing the concentration of TEOS. Additionally, novel dendritic silica nanochains are prepared using one-dimensionally assembled silica nanoparticles as the seeds. in the suspension.18 Monodisperse, spherical silica NPs with sizes ranging from 14 to 550 nm were prepared by the regrowth of the SNSs using TEOS as a silica source.16,17 The nanostructure with dendritic shapes obtained from the regrowth of SNSs has never been reported. In this paper, a facile route is described to prepare colloidal DSNPs with unique Konpeito-like morphology. They are formed through regrowth of SNSs in the presence of block copolymer Pluronic F127 (PEO100−PPO70−PEO100) in a liquid−liquid (TEOS−water) biphasic system. The size of the DSNPs is widely tunable (28−85 nm) by changing the size of seed SNSs. Site-selective regrowth of silica on the F127adsorbed SNSs is likely the key to the formation of well-defined DSNPs. We recently reported that one-dimensional (1D) chains of SNSs are obtained through the liquid-phase selfassembly of the SNSs with the aid of F127 at appropriate pH (∼7.5) in the absence of additional TEOS.19,20 F127 adsorbs onto SNSs to modify the interparticle interactions to drive 1D self-assembly.20 In the present study, a new role of the adsorbed F127 is demonstrated. F127 acts as a surface modifier to form dendritic structure on the surfaces of silica seeds during siteselective silica deposition process. Additionally, higher order nanostructures, dendritic silica nanochains (DSNCs), are also obtained by using the 1D SNS chains as seeds.
1. INTRODUCTION There has been growing interest in the preparation of colloidal silica nanoparticles (NPs) with controlled structure and shape.1,2 A variety of silica NPs, such as hollow silica NPs,3,4 mesoporous silica NPs,5,6 silica nanospheres,7,8 anisotropic silica NPs,9,10 and Janus silica NPs,11 have been synthesized to date. Colloidal silica NPs with dendritic morphology are of particular interest for applications especially in catalysis and biomedicine because of their easily accessible high surface areas and wide pore mouths.12,13 Dendritic, fibrous silica NPs were synthesized using the microwave-assisted hydrothermal technique.12 They possess higher adsorption capacity for DNA than the corresponding MCM-41-based NPs with cylindrical channel pores and deliver the genes to cells effectively.13 However, to the best of our knowledge, simple and versatile methods to fabricate dendritic silica NPs (DSNPs) with diameters below 50 nm have not been reported, though such NPs are desired as nanocatalysts and nanocarriers. A seed regrowth strategy has proved to be an efficient approach to precisely control the size and shape of NPs.14−17 Small NPs are synthesized first and used as seeds to form larger sized NPs. This strategy has been successfully applied for the shape control of noble metal NPs with narrow size distributions.14 Dendritic gold nanostructures were synthesized using covalently capped gold NPs as seeds.15 Silica nanospheres (SNSs), which are synthesized by using basic amino acid as a catalyst in a tetraethoxysilane (TEOS)−water biphasic system, are promising candidates as seed NPs because of their small sizes (< 30 nm), narrow size distributions, and colloidal stability © 2015 American Chemical Society
Received: April 7, 2014 Published: January 21, 2015 1610
DOI: 10.1021/la504955b Langmuir 2015, 31, 1610−1614
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Langmuir
2. EXPERIMENTAL SECTION 2.1. Materials. TEOS was obtained from Tokyo Chemical Industry Co., Ltd. and used without further purification. Arginine (Arg) was purchased from Wako Pure Chemical Industries Ltd. and used as received. Pluronic F127 was purchased from Sigma-Aldrich. Milli-Q water (18.2 MΩ cm−1) was used for all experiments. 2.2. Synthesis of SNS Seeds. In a typical synthesis, 54.6 mg of Arg was dissolved in 41.4 g of water, followed by the addition of 3.13 g of TEOS. The biphasic reaction was carried out at 60 °C for 24 h under stirring at a rate of 500 rpm in a water bath. SNS seeds with a size of ca. 22 nm were obtained. Larger silica seeds with a size in the range of 50−80 nm were obtained by regrowth of the SNS seeds in a mixture containing TEOS, water, and Arg. 2.3. Preparation of DSNPs with Tunable Size. DSNPs were prepared by the Pluronic F127-directed seed-regrowth method. An appropriate portion of the SNS seed suspension (SiO2 2.0 wt %) was added to an Arg−water solution (Arg 0.13 wt %) containing Pluronic F127 (1.5−8.0 wt %). The final SiO2 concentration was 0.2−0.56 wt %. The required amount of TEOS (0.5−4.0 g) was added to the above mixture stepwise (≤ 1.0 g) over 24−96 h. The typical composition for the preparation of DSNPs was as follows: 10 g of the SNS seed suspension (SiO2 2.0 wt %), 33.6 mg of Arg, 0.55 g of Pluronic F127, 25.5 g of water, and 2.0 g of TEOS. The reaction was conducted at 60 °C for 24 h under stirring at ca. 500 rpm. 2.4. Preparation of Dendritic Silica Nanochains. 1D silica nanochains were prepared by the method developed by us.19,20 In a typical synthesis, 10 g of the suspension of silica nanochains (SiO2 2.0 wt %, F127 2.0 wt %) was added to the 25.5 g of Arg−water solution (Arg 0.13 wt %) containing Pluronic F127 (2.0 wt %). Subsequently, 1.0 g of TEOS was added to the system. After addition of TEOS, the reaction was allowed to proceed at 60 °C for 24 h under constant stirring at ca. 500 rpm. 2.5. Characterization. Field-emission scanning electron microscopy (FE-SEM) images were obtained using Hitachi S-900 and Hitachi S-4800 at accelerating voltages of 6 and 10 kV, respectively. SEM samples were prepared by spin coating a suspension of DSNPs onto cleaned silicon wafer. Organic species in SEM samples were removed by UV-ozone treatment. The samples were coated with a Pt layer prior to SEM observation. The transmission electron microscopy (TEM) image was recorded on a JEM-3100FEF with an accelerating voltage of 300 kV. Nitrogen adsorption and desorption isotherms were obtained at 77 K using Autosorb-iQ (Quantachrome Instruments). Samples were obtained by drying the as-synthesized DSNP suspensions at 60 °C and then calcined at 550 °C for 5 h. The uncalcined and calcined samples were degassed at 150 and 300 °C for 8 h, respectively. The Brunauer−Emmett−Teller (BET) specific surface areas of samples were calculated from adsorption data at a relative pressure range of 0.05−0.2. The pore size distributions were obtained with the adsorption branches of the isotherms by the Barrett−Joyner−Halenda (BJH) method.
Figure 1. SEM images of DSNPs synthesized in the presence of 0.56 wt % of silica seeds (22 nm) and 1.5 wt % of F127 with different concentrations of TEOS: (A) 0, (B) 1.64, (C) 5.25, and (D) 9.98 wt %. Upper insets show the high-magnification SEM images. Lower inset in D shows the photograph of a suspension of DSNPs.
obtained. The suspension is transparent light blue, showing excellent colloidal stability of the DSNPs (inset of Figure 1D). The TEM analysis clearly shows that nanoscale protrusions less than 10 nm in size are present on the surface of DSNPs (Figure 2), which were prepared at 5.25 wt % TEOS. The high-
3. RESULTS AND DISCUSSION Syntheses of DSNPs have been carried out at different concentrations of TEOS, while the other conditions remained unchanged (SNS seed 0.56 wt %, F127 1.5 wt %, Arg 0.13 wt %; concentrations before TEOS addition). SNSs with an average size of 22 nm are used as seeds (Figure 1A). It has been revealed that the TEOS concentration has a major effect on the size and morphology of DSNPs. FE-SEM images of DSNPs prepared by the Pluronic F127-directed seed-regrowth method are shown in Figure 1B−D. At 1.64 wt % TEOS (Figure 1B), DSNPs with a rough surface and an average diameter of ca. 28 nm are observed. Upon increasing the TEOS concentration to 5.25 wt % (Figure 1C), DSNPs with nanoscale protrusions and a size of ca. 34 nm are found. By further increasing TEOS concentration to 9.98 wt % (Figure 1D), DSNPs with Konpeito-like morphology and a size of ca. 38 nm are
Figure 2. Representative TEM image of DSNPs prepared in the presence of 0.56 wt % of silica seeds and 1.5 wt % of F127 with 5.25 wt % of TEOS. Upper inset shows the high-magnification TEM image.
magnification image (Figure 2, inset) shows no gap between the seed particles and the regrown part, suggesting that newly formed particles are attached to the surface by Si−O−Si bonds. Although the above system contains F127, neither random aggregation nor 1D assembly of SNSs19 occurs because the pH of the suspension was maintained at 10, where sufficient electrostatic repulsion between SNSs was expected. 1D assembly of SNSs in the presence of F127 is typically observed at pH near 7.5.19,20 It should also be noted that F127 does not serve as template for producing mesoporous silica21 under our experimental conditions. The reaction under acidic conditions 1611
DOI: 10.1021/la504955b Langmuir 2015, 31, 1610−1614
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Langmuir rather than basic conditions is generally required for strong interaction between PEO chains and silicate species. Thus, the adjustment of pH value is important for the formation of DSNPs. Stepwise addition of TEOS is also important to obtain DSNPs with narrow size distributions. For example, DSNPs with a size of 34 nm (Figure 1C) were prepared by adding 2.0 g of TEOS (5.25 wt %) twice: 1.0 g of TEOS was added to the seed suspension and allowed to react for 24 h, followed by the addition of another 1.0 g of TEOS. When 2.0 g of TEOS was added at one time to the seed suspension, growth of DSNPs and nucleation of new particles occurred simultaneously, leading to the formation of small silica nanospheres in addition to DSNPs (Figure S1, Supporting Information). This result indicates that relatively slow supply of silicate species is crucial for promoting growth of the seed SNSs exclusively. To monitor the time-dependent regrowth of the DSNPs, the sample was observed by FE-SEM every 2 h after adding 1.0 g of TEOS to the DSNPs suspension, whereas the final concentration of TEOS reached 5.25 wt %. As shown in Figure S2, Supporting Information, the sizes of the newly formed particles on DSNPs increased gradually while retaining their dendritic shapes. The concentration of SNS seeds affects the diameter of DSNPs. The SNS concentrations have been set at 0.2 or 0.4 wt %, while other parameters remained constant: 1.5 wt % F127, 0.13 wt % Arg, and 6.8 wt % TEOS. The diameter of DSNPs decreases from 44 to 38 nm (Figure 3). The decrease in the size
Figure 4. SEM image of silica nanospheres prepared from the growth of silica seeds in the absence of F127.
Figure 5. (A) Nitrogen adsorption−desorption isotherms of the DSNPs with different sizes (a) 38 nm DSNPs before calcination, (b) 38 nm DSNPs after calcination, and (c) 51 nm DSNPs after calcination. Adsorption and desorption points are marked by solid (●) and empty (○) circles, respectively. Isotherms b and c are offset vertically by 70 and 470 cc g−1, respectively. (B) Corresponding BJH adsorption pore size distributions curves.
Figure 3. SEM images of DSNPs synthesized in the presence of 0.13 wt % of Arg, 1.5 wt % of F127, and 6.8 wt % of TEOS with different SNS seed concentrations: (A) 0.2 and (B) 0.4 wt %.
Figure 2. Although the pore size of DSNPs before and after calcination almost remains unchanged, the BET surface area and pore volume increase dramatically (Table 1). This is
of DSNPs with an increase in the seeds concentration is reasonable because TEOS are evenly consumed for the growth of the different number density of silica seeds. No obvious changes in size and morphology are found when changing the F127 concentration in the range of 1−8 wt % (SNS seed 0.56 wt %, Arg 0.13 wt %, TEOS 5.25 wt %). It is noted that silica nanospheres with smooth surfaces are obtained from the growth of silica seed in the absence of block copolymer F127 as shown in Figure 4. These results indicate that F127 plays a key role in the formation of DSNPs, but excess does not affect their morphology. Nitrogen adsorption analysis gives information on the porous nature of this new type of DSNPs. The nitrogen adsorption− desorption isotherm of the DSNPs obtained by drying exhibits a capillary condensation step at a relative pressure of P/P0 = 0.75−0.85, which is characteristic of a type IV isotherm according to IUPAC classification (Figure 5A). The pore sizes calculated from the adsorption data using the BJH model are ca. 15.1−23.5 nm (Figure 5B), suggesting the pores are interparticle mesopores. No peak corresponding to the voids between the protrusions can be observed, which is probably due to their irregular arrangement on the surface, as shown in
Table 1. Pore Characteristics of DSNPs with Different Sizes sample name 38 nm DSNPs (before calcination) 38 nm DSNPs (after calcination) 51 nm DSNPs (after calcination)
BET surface area (m2/g)
BJH pore size (nm)
total pore volume (cc/g)
32
15.6
0.14
421
15.1
0.94
363
23.5
1.15
because a large amount of F127 is adsorbed on the DSNPs surfaces during the drying process, and thereby the smaller BET surface area is generated before calcination. It should be noted that the BET surface areas of DSNPs are much higher than nonporous SNSs. The BET surface area of the nonporous SNSs with a size of about 13 nm is 228 m2 g−1,18 whereas our DSNPs have higher surface areas even though their sizes are larger (421 and 363 m2 g−1 when the sizes are 38 and 51 nm, respectively). This is attributed to the nanosized protrusions formed on the seed SNSs. Although the BET surface areas of DSNPs are still 1612
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Langmuir smaller than those of mesoporous silica NPs (∼1000 m2 g−1),22 the dendritic morphology of DSNPs should be advantageous for applications where higher surface accessibility is required. Much larger DSNPs can be prepared by using larger SNSs as seeds. For example, when 50 and 80 nm silica nanospheres were used as seeds, DSNPs with average diameters of 55 and 85 nm were obtained, respectively (Figure 6). Uniform DSNPs
On the basis of the above investigations, it has been shown that F127 is essential for the formation of dendritic structures on silica seeds. F127 may serve as the surface modifier for siteselective silica deposition. It is well known that F127 can be adsorbed on a silica surface through hydrogen bonding between ether oxygens of PEO and silanol groups of silica.24 The nanosegregated structures of F127 might be formed on the surfaces of SNS seeds because of the amphiphilic character of this polymer. Silica deposition near the hydrophilic PEO parts is more favorable during the regrowth of silica in the system. Such a site-selective silica deposition leads to the formation of well-defined DSNPs finally.
4. CONCLUSIONS A polymer-mediated seed-regrowth approach is demonstrated to synthesize a new type of DSNPs with controllable diameter in the range of 28−85 nm. Monodisperse silica nanospheres are used as seeds, and Pluronic F127 is used as a surface modifier. The obtained DSNPs possess large external surface area (∼400 m2/g) and high colloidal stability. It is possible to create nanolevel irregularities (convex/concave shapes) on the silica nanoparticles surface. This new type of DSNPs holds exciting applications in the biological area (e.g., high-resolution bioimaging) and catalysis support.25
Figure 6. SEM images of DSNPs synthesized in the presence of larger silica seeds with 1.5 wt % of F127 and different concentrations of TEOS and silica seeds: (A) 2.70 wt % of TEOS and 1.43 wt % of silica seeds (50 nm); (B) 1.37 wt % of TEOS and 1.59 wt % of silica seeds (80 nm).
possess a number of tiny silica nanoparticles on their surfaces. The shape of these DSNPs is similar to a type of mesoporous silica nanoparticles assembled from primary silica nanoparticles, which were synthesized with a cationic surfactant in TEOS and water mixture under basic conditions.23 Compared with the smaller DSNPs (Figure 1), the larger DSNPs (Figure 6) regrown from the larger SNS seeds have much more newly formed tiny particles on their surfaces. The number density of the larger SNS seeds is relatively low; therefore, one DSNP can earn more tiny particles. We further demonstrated the feasibility of the F127-directed seed-regrowth method by using 1D assembled silica nanochains19 as seeds. The silica nanochains before regrowth show relatively smooth surfaces (Figure 7A). Highly dispersed
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of DSNPs. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Addresses ‡
Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. ∥ Department of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO) and Grant-in-Aid for Scientific Research on Innovative Areas of “Fusion Materials” (Area No. 2206) from MEXT.
Figure 7. (A) SEM image of silica nanochains before regrowth. (B) SEM image of DSNCs synthesized in the presence of 0.56 wt % of silica seeds and 2 wt % of F127 with 2.74 wt % of TEOS. Upper inset shows the high-magnification SEM image.
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REFERENCES
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DSNCs with an average diameter of 33 nm are prepared successfully after regrowth of silica nanochains in the presence of F127 and TEOS (Figure 7B). With careful observation of a high-magnification SEM image (inset in Figure 7B), one can find many tiny particles on the surface of DSNCs, which is apparently different from the morphology of silica nanochains.19 Similar to the DSNPs system, the diameter of DSNCs can be controlled by the concentrations of TEOS and seeds, while the concentration of Pluronic F127 has a negligible effect on the diameter of DSNCs under constant TEOS concentration (data not shown). 1613
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