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2008, 112, 3175-3178 Published on Web 02/12/2008
In situ Immobilization of Gold Nanoparticle Dimers in Silica Nanoshell by Microemulsion Coalescence Hailin Wang, Karola Schaefer, and Martin Moeller* DWI an der RWTH Aachen e.V. and Institute of Technical and Macromolecular Chemistry, RWTH Aachen, Pauwelsstr. 8, D-52056 Aachen, Germany ReceiVed: December 1, 2007; In Final Form: January 28, 2008
A novel method based on controlled microemulsion coalescence was developed for immobilization of gold nanoparticle dimers in silica nanoshell. Gold nanoparticles were prepared in water-in-oil microemulsion, and then the microemulsion was intentionally fused by ethanol to form gold nanoparticle dimers. Afterward, the gold-containing droplets were vitrified by silica to stabilize the dimers. Transmission electron microscopy and UV-vis spectra were used to study the coalescence process and demonstrated our ability to control the size of gold nanoparticles and the thickness of the silica coating. The encapsulation of gold nanoparticle assemblies into silica particles also prevents further agglomeration and makes additional functionalization possible.
Gold nanostructures have attracted broad interest for research because of their unique properties such as plasmon resonance and conductivity in combination with shape- and size-controlled preparation and chemical stability. This rather unique combination of properties forms the basis for advanced and further ongoing developments in nanoelectronics, nanodevices, and bioand chemosensors.1-8 The surface plasmon resonance wavelength of a metal nanoparticle is strongly affected by a number of factors, for example, the shape and size of the particles as well as the refractive index of surrounding medium. Furthermore, it is also affected by other metal nanoparticles that are present in the immediate environment. When two gold nanoparticles are sufficiently proximate, their plasmon resonance couples and the wavelength shifts depending on the distance between the two particles.9,10 A lot of efforts have been made to prepare gold nanostructures with different size and morphologies. However, the controlled particle-particle interaction and programmed assembly of gold nanoparticles still remain a significant challenge.11-15 Up to now, several methods have been developed to prepare gold nanoparticle assemblies.5,14 Among these architectures, dimers are of special interests because of their potential application such as substrates in surface enhanced Raman spectroscopy (SERS),16 photothermal therapy utilizing the tunable plasma resonance from visible to near-infrared region and single-molecule sensing and detection.17 The optical properties of these nanoparticle pairs were experimentally and theoretically studied.18,19 Electron beam lithography technique,18,19 DNA,20,21 organic-bridged ligands,22,23 polymer lamellar single crystals,24 and solid-phase mono- and asymmetric-functionalization approaches25-28 have been used to fabricate gold nanoparticle dimer, trimer, or tetramer assemblies. Coupling of gold nanoparticles has also been reported in mesoporous silica.29 In this letter, we report a versatile liquid-phase method based * To whom correspondence should be addressed. Tel: +49 (0)241 80 233 00. Fax: +49 (0)241 80 233 01. E-mail:
[email protected].
10.1021/jp7113658 CCC: $40.75
SCHEME 1: Schematic Illustration of the Synthesis of Silica-Coated Gold Nanoparticles (Au NPs)
on microemulsion for assembling gold nanoparticles to dimers that are stabilized by incorporation into silica nanoparticles. Formation of aggregate micelles containing two or three gold nanoparticles has been reported in a study on synthesizing gold nanoparticles in block copolymer microemulsion during a certain stage of the reaction.30 The intention of this study is to fuse microemulsion droplets to achieve controlled assembly of gold nanoparticles and use the microemulsion as a template to encapsulate the gold nanoparticle assemblies into silica. The dual core-shell gold-silica particles can be modified by binding other molecules at the silica surface according to procedures developed in silica nanoparticle chemistry. Here, the silica coating provides a means to manipulate the distance of the gold cluster to surface bound functional molecules such as a fluorescent dyes. The synthetic strategy used in this study is outlined in Scheme 1. First gold nanoparticles were synthesized in cyclohexane/ © 2008 American Chemical Society
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Figure 1. TEM images of (a) gold nanoparticles after coalescence of the micelles; dimers are indicated by circles and inset shows a magnified view of one dimer; (b) S-2, silica-coated gold nanoparticle dimers and inset is a detailed view of the particle (the bar represents 10 nm); (c) S-3, Au/silica core-shell particles with several gold particles inside; and (d) S-1, Au/silica core-shell particles with one gold particle inside. The concentration of ethanol used: (a,b) 0.4 mol/L; (c) 0.8 mol/L; (d) 0 mol/L.
TABLE 1: Sample Specifications and Reagents Concentration Used sample
NP-5 (M)
HAuCl3 (µM)
citrate (mM)
Ra
ethanol (M)
NH3 (mM)
TEOS (mM)
S-1 S-2 S-3 S-4 S-5 S-6 S-7
0.1 0.1 0.1 0.1 0.1 0.1 0.2
15 15 15 20 20 20 50
10 10 10 50 50 50 25
3.3 3.3 3.3 4.2 4.2 4.2 1.7
0 0.4 0.8 0.4 0.4 0.4 0.4
0.8 0.8 0.8 1.0 1.0 1.2 1.2
10 10 10 10 12 12 15
a
yieldb (%) 43 31 29 38 33
figure 1c 1b 1d 3a 3b 3c 3d
R ) [water]/[surfactant]. bThe yield of silica-coated dimers.
NP-5 (IGEPAL CO-520)/water ternary microemulsion system. Two portions of microemulsion containing chloroauric acid (HAuCl4‚3H2O) and sodium citrate aqueous solution, respectively, were mixed, and then the microemulsion was stirred gently at room temperature for 12 h to allow completion of the reaction. The specification of samples and concentration of reagents is presented in Table 1. In this study, the concentration ratio of water to surfactant (R) is not too large, and the size of water droplets is small enough to ensure that only one gold nanoparticle was formed in each water droplet. Afterward, an accurately designed amount of ethanol was added to the system. It has been reported that ethanol increases the spontaneous curvature of the interface, favors the formation of interconnections among aqueous channels and thus induces an effective interaggregate attraction.31 Therefore, ethanol can destabilize the microemulsion and thereby makes the water droplets
coalesce to a certain degree. The gold-containing micelles would fuse more drastically with increasing concentration of ethanol. Thus, it is expected that the majority of micelles would contain two gold nanoparticles when a certain amount of ethanol is added. Subsequently tetraethyl orthosilicate (TEOS, silica precursor) and a microemulsion containing aqueous ammonia solution (catalyst) were added to the gold nanoparticle microemulsion to form a layer of silica outside the gold assemblies. The obtained particles can be precipitated from the microemulsion by addition of excessive ethanol and redispersed in polar solvents such as water or ethanol after the surfactant has been washed off. Figure 1a is a typical transmission electron microscopic (TEM) image of the gold nanoparticle dimers formed after the microemulsion has coalesced (the sample was prepared by putting one drop of the microemulsion on a TEM grid). It can
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TABLE 2: Statistical Results of Percentage of Silica Containing Different Amount of Au NPsa sample
ethanol
0 Au NP
1 Au NP
2 Au NPs
>2 Au NPs
S-1 S-2 S-3
0M 0.4 M 0.8 M
6% 1% 0
92% 40% 10%
2% 43% 24%
0 16% 66%
a
Au NPs means gold nanoparticles.
be seen clearly that most gold nanoparticles exist as a pair on the TEM grids (indicated in circles on the image). Figure 1b shows the obtained silica-coated gold nanoparticle dimers. Almost all of the particles have a core-shell structure that is composed of 20∼40 nm silica particles containing one, two, or several 4∼6 nm gold nanoparticles as cores. Figure 1b inset is a detailed view of one silica-coated gold nanoparticle dimer. By analysis of 500 particles, it is estimated that 43% of the silica particles contain gold particle pairs, 40% contain one gold particle, and the rest contain more than two gold particles. Table 2 gives the statistical results of samples obtained with different amounts of ethanol. These results prove that the proportion of silica-coated gold nanoparticle dimers can be controlled by the addition of ethanol. As mentioned above, the coalescence of microemulsion caused by ethanol is a key step of the gold dimer formation. UV-vis spectroscopy and TEM were employed to monitor this process. When the gold nanoparticles are formed in the microemulsion at the beginning, they produce a plasmon resonance peak (λmax) at 530 nm in microemulsion (Figure 2A, curve a). The characteristic red color of gold nanoparticles can be observed. At this stage, only one gold particle exists in each water droplet in the microemulsion, which is also proved by the experiment of silica coating as described above, but without the addition of ethanol. Only silica particles with one gold nanoparticle inside were observed (Figure 1d). With the addition of ethanol into the system, the red color fades and then turns to a bluish color gradually. In a typical procedure used to prepare particles depicted in Figure 1b, 0.4 mol/L ethanol (concentrations mentioned in this communication are calculated by the whole volume of the microemulsion) is added. The plasmon resonance band broadens, displays a red shift (λmax ) 554 nm), and decreases in intensity (Figure 2A, curve b), indicating the formation of gold nanoparticle dimers as can be observed in the TEM image of the microemulsion at this stage (Figure 1a). When 0.8 mol/L ethanol is added to the microemulsion, the UV-vis absorption curve becomes very broad and the maximum shifts to 616 nm (Figure 2A, curve c). That is caused by the formation of larger gold nanoparticle assemblies. The silicacoating experiment is also performed at this stage. The TEM photo shows that most silica particles contain several gold nanoparticles (Figure 1c). Figure 2B depicts the UV-vis spectra of these samples after silica coating, which show the obvious red shift of the plasmon resonance band caused by the formation of gold nanoparticle assemblies. When the ethanol concentration reaches above 1 mol/L, the microemulsion turns to the bluish color at once. However, within half an hour the blue color turns colorless, and black precipitation is observed as the excessive ethanol destabilizes the microemulsion extremely so that the gold nanoparticles aggregate rapidly and precipitate. In this procedure, sodium citrate was chosen as the reducing agent. The surfaces of formed gold nanoparticles are negatively charged due to the adsorbed citrate groups, which makes it difficult to coat the gold nanoparticles with silica. Therefore, a little amount of 3-aminopropyltriethoxysilane (APTES) is necessary, which replaces the citrate anions and adsorbs on the gold surface driven by the high complexation constant for gold
Figure 2. (A) UV-vis spectra of gold nanoparticles in microemulsion before silica coating: (a) without ethanol; (b) with 0.4 M ethanol; (c) with 0.8 M ethanol. Curves b and c were measured 0.5 h after ethanol was added. (B) Corresponding spectra of gold nanoparticles after silica coating: (d) S-1; (e) S-2; and (f) S-3.
Figure 3. TEM images of silica-coated gold nanoparticle dimers with different size. The diameters of silica/gold nanoparticles are the following: (a) S-4, 58/15 nm; (b) S-5, 90/15 nm; (c) S-6, 137/15 nm; (d) S-7,140/26 nm. The bar represents 50 nm.
amines.32 In our experiments, 0.2 mmol/L APTES was used. In the absence of APTES, most of the gold particles were not coated with silica. It can also be observed that the silica particles formed in the microemulsion without ethanol are much more spherical and monodisperse than those formed after the addition of ethanol. This is an obvious consequence of the destabilization of the micelles. The size of the gold core and silica shell can be adjusted through changing the microemulsion parameters and the concentration of reactants. Larger gold particles can be obtained by changing the concentration of chloroauric acid and sodium citrate and the concentrate ratio of water to surfactant of the microemulsion, while thicker silica shell can be prepared by a higher concentration of ammonia and TEOS. Figure 3 shows some silica-coated gold nanoparticles of different size. The plasmon spectra of silica-coated 15 and 26 nm gold dimers are shown in Figure 4.
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Figure 4. UV-vis spectra of silica-coated 15 nm gold nanoparticle monomer (a) and dimer (b, S-6) and 26 nm gold nanoparticle monomer (c) and dimer (d, S-7). The diameter of silica is 137 and 140 nm, respectively.
In conclusion, the preparation of silica-coated gold nanoparticle dimers can be achieved by a combination of standard microemulsion procedures for preparing gold and silica nanoparticles with a controlled coalescent process. The practicability of microemulsion as a potential technique for controlled assembly of nanoparticles has been demonstrated. This method is not only limited to gold nanoparticles but can also be adapted to any other types of nanoparticles that can be synthesized in appropriate microemulsion systems. Work on other microemulsion system and destabilization agents are being undertaken to improve the yield of dimers. Acknowledgment. The authors thank the research association Forschungskuratorium Textil e.V. for financial support of the research project (AiF-No. 14512 N) provided within the program of funding, “Industrial Cooperative Research (IGF)” from funds of Bundesministerium fu¨r Wirtschaft und Technologie (BMWi) via a grant of Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke” e.V. (AiF). References and Notes (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (2) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078-1081.
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