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Various Au Nanoparticle Organizations Fabricated through SiO2 Monomer Induced Self-Assembly Ping Yang, Masanori Ando, and Norio Murase* Health Research Institute, National Institute of Advanced Industrial Science and Technology, 1-8-31 Midorigaoka, Ikeda-city, Osaka 563-8577, Japan Received August 6, 2010. Revised Manuscript Received December 9, 2010 A novel method has been developed to fabricate the assembly of Au colloidal nanoparticles (NPs) using SiO2 monomers. The key strategy was the use of a controlled sol-gel procedure including hydrolysis, deposition, and condensation of tetraethyl orthosilicate (TEOS). Namely, the assembly of Au NPs was created by the anisotropic deposition of SiO2 monomers and subsequent permanent fixing by the growth of a SiO2 shell. Various assemblies of Au NPs such as dimer, trimer, and pearl-chain morphology were fabricated by systematically changing the concentration and injection speed of TEOS. A longitudinal plasmon resonance band was observed as a result of the assembly of Au NPs and can be tuned from visible to near-infrared by altering the length of pearl-chain morphology. In addition, single Au NP was homogeneously coated with a SiO2 shell by means of controlling the deposition rate of SiO2 monomers during a St€ober synthesis without the use of a silane coupling agent or bulk polymer as the surface primer to render the Au surface vitreophilic. The Au NPs (mean size 11.4 nm in diameter) were thus encapsulated into SiO2 beads with a wide range of sizes (from 20 to 50 nm in diameter). These pure SiO2-coated Au beads with tunable shell thickness should be crucial for biosensors, particularly as Raman-tag particles.
Introduction Metal nanoparticles (NPs) have attracted much attention in recent years owing to their unique physical and chemical properties and their applications in catalysis, optoelectronics, and biological and chemical sensing.1-4 This is particularly true for Au nanostructures, and therefore the synthesis of Au nanostructures with well-controlled morphology and size is important for uncovering their morphology-dependent properties and for achieving their potential applications.5 The self-assembly of NPs has been identified as an important process where the building blocks spontaneously organize into ordered structures by thermodynamic and other constraints. Various approaches have been employed to assemble NPs in an ordered manner and to investigate the scope of potential applications.6,7 The self-assembly of NPs into useful morphologies is a much-anticipated development in nanotechnology as it offers the promise of creating materials of well-characterized, nanometer-scale heterostructures with interesting properties.8 The assembly of NPs can be induced by solvent evaporation or through sophisticated procedures involving chemical reaction,9 special molecular interactions between biotin-avidin,10 van der *To whom correspondence should be addressed. E-mail: n-murase@aist. go.jp. Fax: þ81-72-751-9647.
(1) Nam, J. M.; Thaxton, C. S.; Mirkin, C. A. Science 2003, 301, 1884. (2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. Adv. Mater. 2003, 15, 353. (3) Murphy, C. J. Science 2002, 298, 2139. (4) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (5) Xu, J.; Li, S.; Weng, J.; Wang, X.; Zhou, Z.; Yang, K.; Liu, M.; Chen, X.; Cui, Q.; Cao, M.; Zhang, Q. Adv. Funct. Mater. 2008, 18, 277. (6) Xing, S.; Tan, L. H.; Yang, M.; Pan, M.; Lv, Y.; Tang, Q.; Yang, Y.; Chen, H. J. Mater. Chem. 2009, 19, 3286. (7) Yang, M.; Chen, G.; Zhao, Y.; Silber, G.; Wang, Y.; Xing, S.; Han, Y.; Chen, H. Phys. Chem. Chem. Phys. 2010, in press (DOI: 10.1039/c0cp00127a). (8) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396. (9) Wei, Y.; Bishop, K. J. M.; Kim, J.; Soh, S.; Grzybowski, B. A. Angew. Chem., Int. Ed. 2009, 48, 9477. (10) Mirkin, C. A.; Retsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607.
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Waals forces,11 electrostatic force,12 dipole-dipole interaction,13 solvent exchange,14 modification of selected facets of NPs with organic molecules,15 magnetic fields,16 lipid (surfactant) assembly,17 and polymer assistance.18,19 One key in the self-assembly of NPs is to control the thermodynamic balance of interaction between NPs, which includes longrange electrostatic repulsion and short-range van der Waals attraction.20 An anisotropic interaction is required for creating one-dimensional (1-D) assembly of NPs. Since metallic NPs did not have intrinsic dipolar interactions, their assembly has to be driven through tuning the isotropic electrostatic interaction into an anisotropic one by partially replacing the stabilizer of NPs with other chemicals. Most of the reported studies have been focused on creating the 1-D assembly by adjusting the concentration of ligands which could induce the assembly. However, these assemblies are usually structurally unstable during storage and purification. They become gradually aggregated into larger structures with time.19 Therefore, it is expected to develop a new strategies for permanently fixing the structures and thus their optical properties. In this article, we describe a sufficient method to control the assembly of Au NPs into pearl-chain morphology with adjusted lengths by tuning the deposition rate of SiO2 monomers on the surface of the NPs. These assemblies were then fixed permanently by further coating with a SiO2 shell. (11) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515. (12) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420. (13) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (14) Zhang, H.; Wang, D. Angew. Chem., Int. Ed. 2008, 47, 3984. (15) Glotzer, S. C.; Solomon, M. Nature Mater. 2007, 6, 557. (16) Ahniyaz, A.; Sakamoto, Y.; Bergstrom, L. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 17570. (17) Li, X.; Li, Y.; Yang, C.; Li, Y. Langmuir 2004, 20, 3734. (18) Kang, Y.; Erickson, K. J.; Taton, T. A. J. Am. Chem. Soc. 2005, 127, 13800. (19) Cho, E. C.; Choi, S. W.; Camargo, P. H. C.; Xia, Y. Langmuir 2010, 26, 10005. (20) Zhang, Z.; Wu, Y. Langmuir 2010, 26, 9214.
Published on Web 12/28/2010
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SiO2-coated colloidal particles are a class of materials widely used in many fields of colloid and materials science. SiO2-coated Au NPs have exhibited important applications for biosensors. For example, the SiO2 coating makes the Raman-tag particles more stable and easily functionalized.21 However, unlike metal oxide NPs or semiconductor quantum dots, which can be easily coated with SiO2,22-26 Au NPs were considered not to be suitable for direct SiO2 coating due to their low chemical affinity to SiO2. Various routes have been reported to prepare such SiO2-coated nanocomposites.27 Amino-terminated silane coupling agents are often used as the primer to make the Au surface vitreophilic for subsequent SiO2 coating. Mulvaney and co-workers first reported a three-step procedure for coating Au NPs with a SiO2 shell using a silane coupling agent as the surface primer.28 Alternatively, Graf et al. developed a general method to coat colloidal particles (including Au and Ag) with SiO2 using a nonionic polymer, poly(vinylpyrrolidone) (PVP), instead of a silane coupling agent as the surface primer.29 Although Au NPs can be homogeneously coated with a SiO2 shell by using primers to make Au surface vitreophilic, they are not suitable for several applications. SiO2 coating includes surface activation for silanization with silane coupling agents, which results in a weak affinity of the Raman molecules to the Au surface because of competition between the Raman reporter and the coupling agent on the Au surface.21 Polymer-mediated route requires a proper choice of polymer, whose length strongly influences the homogeneity and smoothness of the SiO2 coating.29 Therefore, much effort has been devoted to prepare SiO2-coated Au NPs without using a silane coupling agent or bulky polymer stabilizer. Ying’s group reported on direct SiO2 coating on Au and Ag particles by a reverse micelle method.30 A slow deposition of SiO2 monomers during preparation resulted in Au NPs coated with a homogeneous SiO2 shell. A slightly modified St€ober process could be applied for the SiO2 coating of Au or Ag NPs without the use of any surface primers.31-33 However, this straightforward strategy can only be used for coating large Au particles (∼50 nm in diameter) at a low NP concentration. Mine and co-workers reported on the small Au NPs coated with a SiO2 shell.34 In that case, the size of the beads containing Au NP-free SiO2 beads is more than 100 nm in diameter. A higher concentration would give rise to the formation of irregularly SiO2-coated particles with aggregated Au cores.30 Therefore, the controlling of sol-gel procedure for small Au NPs directly coated with a pure SiO2 shell by St€ober synthesis is a challenge. Recently, we reported on SiO2 beads with multiple semiconductor quantum dots and magnetic NPs prepared by modifying (21) Wang, C.; Chen, Y.; Wang, T.; Ma, Z.; Su, Z. Adv. Funct. Mater. 2008, 18, 355. (22) Philipse, A. P.; van Bruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92. (23) Bechger, L.; Koenderink, A. F.; Vos, W. L. Langmuir 2002, 18, 2444. (24) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676. (25) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4991. (26) Yi, D. K.; Lee, S. S.; Ying, J. Y. Chem. Mater. 2006, 18, 2459. (27) Cong, H.; Toftegaard, R.; Arnbjerg, J.; Ogilby, P. R. Langmuir 2010, 26, 4188. (28) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (29) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693. (30) Han, Y.; Jiang, J.; Lee, S. S.; Ying, J. Y. Langmuir 2008, 24, 5842. (31) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524. (32) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Nano Lett. 2002, 2, 785. (33) Liu, S.; Han, M. Adv. Funct. Mater. 2005, 15, 961. (34) Mine, E.; Yamada, A.; Kobayashi, Y.; Konno, M.; Liz-Marzan, L. M. J. Colloid Interface Sci. 2003, 264, 385. (35) Yang, P.; Murase, N.; Suzuki, M.; Hosokawa, C.; Kawasaki, K.; Kato, T.; Taguchi, T. Chem. Commun. 2010, 46, 4595.
896 DOI: 10.1021/la103143j
Yang et al. Scheme 1. Self-Assembly of Au NPs Induced by SiO2 Monomer
reverse micelle route and St€ober synthesis.35-37 In this paper, we found out an outer SiO2 coating offers new possibilities for the shape control of the assembly of Au NPs. SiO2 monomers can be anisotropically deformed in a controlled way by selected deposition on the surface of Au NPs. This method was successfully applied to adjust the morphologies of Au NP assemblies. The control of assembling processes is a key for adjusting the morphology of Au NP organizations. When the injection speed of tetraethyl orthosilicate (TEOS) was optimized, single Au NP was directly coated with a SiO2 shell without using a silane coupling agent as the primer by St€ober synthesis. Namely, a very high injection speed resulted in the formation of Au NP-free SiO2 beads while the slow injection speed made an assembly of the Au NPs. These Au NP assemblies may find important application in surface-enhanced Raman scattering because small molecule Raman probes can readily approach Au surface due to the SiO2 shell is porous.38
Experimental Section Chemicals. All chemicals were obtained from Sigma-Aldrich and used as received. Chemicals were of analytical grade or of the highest purity available. The pure water was obtained from a Milli-Q synthesis system. Preparation of Au NPs. Citrate-stabilized Au NPs were synthesized according to the as-reported method with slight changes.39 Briefly, 90 mg of sodium citrate was dissolved in 100 mL of H2O with vigorous stirring and heated to 100 °C. 1 mL of 50 mM HAuCl4 was injected into the solution with a speed of 0.2 mL/min. The reaction mixture was maintained at the boiling temperature for further 3 min before allowing to cool down to room temperature. The initial colloidal solution of Au was condensed by a 3000-MWCO filter at 6000 rpm (4800g) for several times to remove unreacted materials for the purpose of preventing the NPs from agglomeration. For comparison, the sample was centrifuged at 15 000 rpm (30 000g) for 10 min to get aggregated one. Self-Assembly of Au NPs by a Sol-Gel Process. The assemblies of Au NPs with different morphologies were prepared by a controlled sol-gel reaction using a St€ ober synthesis. Scheme 1 indicates the self-assembly procedure of Au NPs by a sol-gel process. Typically, 1 mL of 0.1 μM redispersed Au colloid was added in the mixture of 8 mL of ethanol and 0.1 mL of 25% ammonia with vigorous stirring. The concentration and injection speed of TEOS were precisely adjusted. Table 1 summarizes the (36) Yang, P.; Murase, N. ChemPhysChem 2010, 11, 815. (37) Yang, P.; Ando, M.; Murase, N. New J. Chem. 2009, 33, 1457. (38) Jin, R. Angew. Chem., Int. Ed. 2010, 49, 2826. (39) Ojea-Jimenez, I.; Romero, F. M.; Bastus, N. G.; Puntes, V. J. Phys. Chem. C 2010, 114, 1800.
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Article Table 1. Preparation Conditions and Properties of Au NP Assemblies Coated with SiO2 Shell
sample
TEOS amount/μL
TEOS injection speed/(μL min)
reaction time/h
morphology of Au assembly
others
1 2 3 4 5 6 7 8 9 10
2 6 4 6 8 8 8 3 5 9
0.1 0.1 0.05 0.05 0.05 0.3 0.2 0.2 0.2 0..2
2 3 3 3 3 3 3 3 3 3
dimer and trimer dimer and trimer dimer, trimer, and pearl chain pearl chain pearl chain single bead single bead single bead single bead single bead
assembly consisted of 2-3 Au NPs length adjustment of pearl chain suppression of SiO2 nucleation control of SiO2 shell thickness
Figure 1. TEM images (a: condensing; b: centrifuging) and UV-vis spectra (c) of Au NPs. Inset in (c) shows picture of redispersed Au colloidal solutions: (1) condensed Au NPs; (2) centrifuged Au NPs. Because of aggregation of Au NPs induced by centrifuged force at high speed, the color of redispersed colloidal solution of Au NPs separated by centrifuging was blue-shifted.
preparation conditions and properties of the Au NP assemblies. The as-prepared assembly was centrifuged at 15 000 rpm for 10 min and then redispersed in H2O for further characterization. Preparation of SiO2 Beads with Single Au NP. Single Au NP coated with a homogeneous SiO2 shell was carried out by using a similar procedure with the method of the assembly of Au NPs. The injection speed of TEOS was adjusted for controlling the SiO2 monomers deposition during St€ ober synthesis. The preparation conditions and properties of samples are summarized in Table 1. Apparatus. Observations by transmission electron microscopy (TEM) were carried out using a Hitachi EF1000 electron microscope. The UV-vis spectra of samples were taken using a conventional spectrometer (Hitachi U-4000).
Results and Discussion Aggregation of Citrate-Stabilized Au NPs Induced by Centrifuging. Au NPs were prepared by the citrate reduction of HAuCl4. The size of Au NPs depended strongly on the preparation conditions, such as the concentration of HAuCl4, the injection speed of HAuCl4 solution, and reaction time. When the injection speed is more than 0.5 mL/min, the size distribution of Au NPs increased. When the reaction time is more than 15 min, individual Au NPs were aggregated. In this case, the color of Au colloidal solution changed into blue from deep red. For the separation of Au NPs from initial reaction solution, the unreacted materials were removed by condensing at low speed instead of centrifuging at a high speed to avoid the aggregation of the NPs. Figure 1 shows the TEM images (a: condensing; b: centrifuging) and UV-vis spectra (c) of Au NPs. The mean size of Au NPs is 11.4 nm in diameter. The result clearly show the aggregation of the Au NPs created when the initial colloidal solution were centrifuged at 15 000 rpm. This phenomenon attributed to the stability of colloidal particles in an aqueous solution. A similar phenomenon was observed in the literature.40 (40) Roca, M.; Pandya, N. H.; Nath, S.; Haes, A. J. Langmuir 2010, 26, 2035.
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Generally, the stability of colloidal particles in an aqueous solution is described by the classical Derjaguin-Landau-VerveyOverbeek (DLVO) theory. For monodisperse spherical NPs, the total interaction potential, VT, is the sum of the electrostatic repulsive potential (Velec) and the van der Waals attractive potential (VvdW).20 Here, VvdW is related to the ratio of the center-tocenter distance of neighboring NPs and their radius whereas Velec is ascribed to the surface potential of NPs (Ψ0). The value of Ψ0 is related to the amount of surface charge that could be obtained in several ways, the preferential adsorption of ions, the dissociation of surface groups, isomorphic substitution, the adsorption of polyelectrolytes, and the accumulation of electrons. For citratestabilized NPs, the surface negative charges are generated from deprotonated -COOH groups in citrate ions. When the Au colloidal solution was centrifuged at high speed, the distance between NPs decreased. As a result, the decrease of electrostatic repulsive potential and the increase of the van der Waals attractive potential resulted in a low stability of Au NPs. Figure 1c shows the UV-vis spectra of Au NPs separated by condensing (1) and centrifuging (2). The inset in Figure 1c shows the picture of redispersed colloidal solutions of Au NPs prepared by condensing (1) and centrifuging (2). Au NPs show an intense surface plasmon absorption band in the visible range (typically around 520 nm). A longitudinal plasmon resonance band was observed as a result of the aggregation of Au NPs (b). Assembly of Au NPs Induced by a Sol-Gel Process. As summarized in Table 1, the injection speed of TEOS drastically affects the morphology of Au NP assemblies and SiO2 nucleation. TEM analysis of samples prepared using various preparation conditions is presented in Figures 2 and 3. Figure 2 shows the TEM images of the dimers and trimers of Au NPs (sample 1 with a thin SiO2 shell and sample 2 with a thick SiO2 shell). For sample 1, the statistical yield of single Au NPs, dimers, and trimers was 1:4:1 estimated by several TEM images. The result indicates the assembly of Au NPs revealed same morphology for Samples 1 DOI: 10.1021/la103143j
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Figure 2. TEM images of dimers and trimers of Au NPs: (a) sample 1 with thin SiO2 shell; (b) sample 2 with thick SiO2 shell as shown in Table 1.
and 2 regardless of the thickness of SiO2 shell. The self-assembly of Au NPs was created at the beginning of St€ ober synthesis because samples 1 and 2 were prepared by using same TEOS injection speed. The dimer and trimer morphology of Au NPs were fixed permanently by further deposition of SiO2 monomers with time. The increased thickness of a SiO2 shell for sample 2 was ascribed to a large amount of TEOS added. The TEOS injection speed played an important role for such self-assembly. Lower TEOS injection speed was preferable to assemble Au NPs as shown below. Figure 3 shows the TEM images of the assembly of Au NPs with a long pearl chain (samples 3-5 shown in Table 1) prepared using a low TEOS injection speed (0.05 μL/min). For sample 3, the statistical yield of dimers, trimers, and n-mers was 0.8:1:4 estimated by several TEM images. The length of pearl-chain morphology increased with increasing the amount of TEOS. For sample 4, the statistical yield of dimers, trimers, and n-mers was 0:0.5:6 estimated by several TEM images while no dimer and trimer were observed for sample 5. Because these three samples were prepared by using the same 898 DOI: 10.1021/la103143j
Figure 3. TEM images of assembly of Au NPs: (a) sample 3; (b) sample 4; (c) sample 5 shown in Table 1. Pearl-chain morphology was created under a TEOS injection speed of 0.05 μL/min.
TEOS injection speed, the amount of TEOS affected the morphology of the assembly of Au NPs under such low TEOS injection speed. The original pearl-chain structures were coated with a SiO2 shell as shown in Figure 3. We also compared UV-vis spectra of these pearl-chain assemblies of Au NPs with different length. Figure 4 shows the UV-vis spectra of the assemblies of Au NPs (samples 1, 3, and 4 shown in Langmuir 2011, 27(3), 895–901
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Figure 4. UV-vis spectra of assemblies of Au NPs (samples 1-4 shown in Table 1). Inset shows picture of samples. The UV-vis spectrum of colloidal solution of Au NPs is shown for comparison.
Table 1). The inset in Figure 4 shows the pictures of samples 1 and 4 and initial Au NPs. The UV-vis spectrum of initial Au colloidal solution is shown for comparison. As expected, a longitudinal plasmon resonance band was observed as a result of the assembly and can be tuned from visible to near-infrared by altering the length of pearl-chain morphology. The inset shows the color difference of Au NP assemblies with different pearl-chain length. Sample 1 had a broad plasmon resonance band containing two peaks, namely, at 520 and 590 nm (shoulder). Sample 4 had a new absorption peak that appeared at 660 nm. The new absorption peak appears due to the interparticle separation in the chains.20 The exact position of this plasmon band is extremely sensitive to particle size, shape, distance, and the optical and electronic properties of the medium surrounding the particles. Colloidal Au NPs is usually composed of single-crystalline, singly twinned, or multiply twinned structures. According to TEM observation, the Au NPs we prepared are a mixture of various morphologies such as decahedrons, tetrahedrons, truncated tetrahedrons, triangle, and cubes. These nonideal spherical NPs have different crystalline facets with different chemical activities. This makes the Au surface have low chemical affinity to SiO2. In addition, the strength of the binding force of the stabilizer on different crystalline facets of NPs is not uniform. As a result, the stabilizers with a weak binding force on some facets will desorb first under the same treatment. Consequently, some citrate ions with a low binding affinity to Au NPs were removed first and an anisotropic distribution of surface charge formed. The anisotropy of the surface charge distribution transforms isotropic electrostatic repulsion between Au NPs into anisotropic styles. Therefore, the deposition of SiO2 monomers on the Au NPs is anisotropic as we explained in Scheme 1. This slow anisotropic deposition of small amount SiO2 monomers resulted in the formation of the pearl-chain morphology of Au NPs. In contrast, the quick anisotropic deposition of large amount SiO2 monomers resulted in an inhomogeneous SiO2 coating on the Au NPs as explained later. With the decomposition of SiO2 monomer and further condensation, the Au NPs were assembled into a dimer. Since the electrostatic potential of an Au NP dimer is higher at its lateral side than its end side, the Au NPs preferred attaching to the end of the dimer. This was also experimentally supported by recent work of Xia and co-workers.19 According to the DLVO theory,20 the stability of colloidal Au NPs in an aqueous solution is extremely sensitive to particle size, shape, surface charge, the distance between Au NPs, and the optical and electronic properties of the medium surrounding the NPs. In current experiment, the selfassembly of Au NPs induced by SiO2 monomers was observed Langmuir 2011, 27(3), 895–901
Figure 5. TEM image of Au NPs encapsulated in SiO2 beads: (a) sample 6 and (b) sample 7 shown in Table 1. In the case of higher TEOS injection speed (0.3 μL/min for sample 6), bare SiO2 beads were formed.
during a St€ober synthesis. Because of SiO2 monomers gradually generated by the hydrolysis of TEOS, the thickness of SiO2 shells on the NPs increased with time. During this process, the distance between the NPs was determined by simultaneous assembly. As a result, the amount and injection speed of TEOS determined the morphology of the assembly of Au NPs. The self-assembly of Au NPs was terminated at a critical SiO2 shell thickness. With further sol-gel procedure, the assembly of Au NPs was fixed permanently by the growth of a SiO2 shell. After fixing, the structures of pearl-chain assemblies could be preserved for a long period of time, during which their characteristic optical properties remained unchanged. In addition, once coated with SiO2, the assemblies of Au NPs were reported to be stable in aqueous solutions when their pH, temperature, or salt concentration were varied.21 We expect that these core-shell type structures can be applied as molecular probes. In addition, we used similar technique to assemble CdSe/ZnS quantum dots. The results will be published elsewhere. Single Au NP Encapsulated in SiO2 Bead. High injection speed of TEOS prevents the assembly of Au NPs and results in DOI: 10.1021/la103143j
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Figure 7. Influence of SiO2 shell thickness on UV-vis spectra of Au NPs encapsulated in SiO2 beads (samples 8-10 shown in Table 1). The UV-vis spectrum of Au NPs is shown for comparison.
Figure 6. TEM image of Au NPs encapsulated in SiO2 beads with adjusted SiO2 shell thickness: (a) sample 8, (b) sample 9, and (c) sample 10 shown in Table 1.
single Au NP coated with a SiO2 shell. Two parameters are important for the first SiO2 coating: pH and silicate concentration. At a pH range of 8-10, condensation/deposition must occur at a sufficient rate to homogeneously coat the particles but still slow enough to avoid the formation of SiO2 nuclei. At the same time, the silicate concentration also plays a role in determining the deposition rate which plays an important role for the morphology of the assembly of Au NPs. Figure 5 shows the TEM image of Au NPs encapsulated in SiO2 beads: (a) sample 6 and (b) sample 7 shown in Table 1. The self-assembly of Au NPs did not occur by using these high TEOS injection speeds. However, in the case of higher TEOS injection speed (0.3 μL/min for sample 6), bare SiO2 beads without Au NPs were formed as shown in Figure 5a. This is ascribed to large 900 DOI: 10.1021/la103143j
amount TEOS led to SiO2 nuclei formed. To avoid coprecipitation of SiO2 nuclei, 0.2 μL/min is preferable as shown in Figure 5b (sample 7). The amount of TEOS was adjusted for tuning the thickness of SiO2 shells on Au NPs for samples 8-10 in Table 1. Figure 6 shows the TEM image of Au NPs encapsulated in SiO2 beads with adjusted thickness of SiO2 shells: (a) sample 8, (b) sample 9, and (c) sample 10. In Figure 6a, inhomogeneous SiO2 coating was clearly observed. With increasing the amount of TEOS, inhomogeneous part was filled out and become homogeneous. This phenomenon is related to the prior inhomogeneous deposition of SiO2 monomers. Figure 7 shows the influence of the thickness of SiO2 shells on the UV-vis spectra of Au NPs encapsulated in SiO2 beads (samples 7-9 shown in Table 1). The UV-vis spectrum of initial Au NPs is shown for comparison. The uncoated and SiO2 coated Au NPs were dispersed in H2O. Au NPs show a very intense surface plasmon absorption band in the visible range (around 520 nm). The exact position of this plasmon band is extremely sensitive to particle size, shape, and the optical and electronic properties of the medium surrounding the particles. SiO2 is electronically inert (it does not exchange charge with the Au particles), but its refractive index is different from that of both water and ethanol (and of course from that of Au). Initially, as the shell thickness is increased, the plasmon absorption band becomes intense. This is due to the increase in the local refractive index around the Au NPs. However, when the SiO2 shell is sufficiently large, scattering becomes significant, resulting in a strong increase in the absorbance at shorter wavelengths. However, there was no significant change in the peak wavelength, and the colors of the dispersions before and after coating were essentially the same. Further increasing the SiO2 thickness did not change the peak position any more. This observation was consistent with that of Mulvaney et al.28
Conclusions We have demonstrated here the self-assembly of Au NPs into dimer, trimer, and pearl-chain morphology by a controlled sol-gel procedure. The anisotropic deposition of SiO2 monomers on the Au NPs resulted in the self-assembly of Au NPs. The pearlchain morphology of Au NPs with different length was fixed permanently by further growing a SiO2 shell on it. The amount and injection speed of TEOS play important roles for controlling the morphology of the assembly. A longitudinal plasmon resonance band from visible to near-infrared was observed by altering the length of the pearl chain. With an optimal injection speed, Langmuir 2011, 27(3), 895–901
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single Au NP was directly coated with a SiO2 shell without the use of a silane coupling agent or bulk polymer as the surface primer to render the Au surface vitreophilic. These beads should be crucial for molecular probes (such as for DNA). The results shown here should be utilized for further research in plasmonic enhanced fluorescence and the fabrication of multiple functional nanocomposite materials. We expect that these core-shell type structures can be applied as molecular probes or as building blocks of
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photonic crystals whose optical properties can be finely tuned by varying the structures of the chainlike assemblies. Furthermore, we will focus on the assembly of Au NPs with different sizes such as less than 10 nm and several tens of nanometers. Acknowledgment. This work was supported in part by Core Research for Evolutional Science and Technology (CREST) from the Japan Science and Technology Agency (JST).
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