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A facile method to prepare a series of SiO2@Au


Materials Chemistry and Physics 105 (2007) 419–425

A facile method to prepare a series of SiO2@Au core/shell structured nanoparticles
Junguo Xue a , Chungang Wang a , Zhanfang

Ma a,b,?
a

Chemistry Department, Northeast Normal University, Changchun 130024, PR China b Department of Chemistry, Capital Normal University, Beijing 100037, PR China

Received 31 March 2006; received in revised form 27 April 2007; accepted 3 May 2007

Abstract This paper reports a systematic investigation of the attachment of different sized gold nanoparticles (3.5–32 nm) to the polyethyleneimine (PEI) and 3-aminopropyltrimethoxysilane (APTMS)-functionalized, respectively, surfaces of different sized silica nanoparticles (60–340 nm) at room temperature. Functionalization of the surfaces of silica nanoparticles with PEI or APTMS has a profound in?uence on the attachment gold nanoparticles. While PEI-functionalized silica surfaces bound the midnanometer gold nanoparticles (13–32 nm), APTMS-functionalized silica surfaces did not. A series of different silica–gold core–shell composites were characterized using transmission electron microscopy (TEM) and UV–vis absorbance spectra. ? 2007 Elsevier B.V. All rights reserved.
Keywords: Core–shell composite; Polyethyleneimine; Silica nanoparticle; Gold nanoparticle

1. Introduction Nanoparticles show properties that are different from those of their corresponding bulk materials [1–3], especially the design and controllable fabrication of core–shell composites have received extensive scienti?c and technological interests due to their applications in various ?elds such as photonic crystals [4–6], self-assembled mesoscopic wires [7], nanoengineering of optical resonance [8], surface-enhanced Raman scattering (SERS) [9], catalysis [10] or biochemistry [11]. Up to now, a variety of approaches have been developed to deposit metal particles on dielectric cores, such as metal precursor reduction [12], thermal evaporation techniques or sputtering methods [13,14], in situ chemical reduction [15], seed method [16], self-assembly [17], electroless plating [18], the inverse micelle [19] and the sol–gel method [20]. All these methods can be divided into two types, one is that the pretreatment of the core surface is unnecessary and metal particles are synthesized or deposited directly

? Corresponding author at: Chemistry Department, Northeast Normal University, Changchun 130024, PR China. Tel.: +86 431 5098597; fax: +86 431 5098597. E-mail address: mazhanfang@yahoo.com (Z. Ma).

on the core. The other is that the modi?cation or functionalization of the core surface has been performed, then adding the functionalized dielectric colloidal core into metal nanoparticles solution or metal salt solution. Almost all of these are formed with low or nonuniform particle densities in the shell layer. Shell density can be affected by the strength of attraction between the core and shell particles, and also by the balance of attractive and repulsive forces between particles in the shell layer. These factors are in turn dependent on extrinsic parameters such as particle size and electrostatic charge. As far as we know, numerous examples of small (<10 nm) metal nanoparticles assembled on oxide [21] or polymer [15] microspheres have been reported, but the reports of midnanometer-sized (20–100 nm) metal assembled on oxide or polymer are not many, with notable exception of the work reported by Caruso et al. [22] and Sadtler and Wei [23]. Colloidal metal particles with midnanometer dimensions have a special appeal because of their intense plasmon responses and large scattering cross sections [24], so their ensemble properties merit particular attention. Although PEI is an effective linking agent to prepare core/shell composite, the reported method using PEI as linking agent is usually at re?ux temperature [23]. In this paper, a series of the silica–gold core/shell composites using PEI as a linking agent were synthesized at room temperature instead of re?ux temperature. We found

0254-0584/$ – see front matter ? 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.05.010

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Scheme 1. The physical principle of the fabrication of PEI-functionalized SiO2 @Au core–shell composite.

that silica particles functionalized with APTMS were much less effective at forming core–shell ensembles than PEI, especially gold nanoparticles with midnanometer, whose high amine content allowed both greater and more uniform gold particle coverage.
2. Experimental 2.1. Materials and instruments
Hydrogen tetrachloroaurate (HAuCl4 ·3H2 O, 99.99%), 3-aminopropyltrimethoxysilane (APTMS, 95%) and tetraethyl orthosilicate (TEOS, 98%) were purchased from Sigma (USA), Triton X-100 (TX-100) was purchased from Aldrich, cyclohexane, n-hexanol, trisodium citrate, sodium brohydride (NaBH4 , 98%), ammonia water (NH4 OH, 25%), absolute ethanol and polyethylenimine (PEI) with average molecular weight 600,000 g mol?1 were obtained from Beijing Chemical Reagents Company (Beijing, China). All reagents used were analytical grade and without further puri?cation. All the glassware was cleaned with aqua regia (v/v, HCl/HNO3 , 3:1; caution: aqua regia is very toxic chemical and should be handled carefully!) and thoroughly rinsed with Millipore water (18.0 M cm?1 ) prior to the experiments. UV–vis spectroscopy was performed on U-3010 spectrophotometer (Hitachi, Japan). Transmission electron microscopy (TEM) was performed with a JEOL-100CX electron microscopy under 80 kV accelerating voltage. Formvar-coated copper grids (200 meshes) were used as support carrier.

2.2. Preparation of colloidal gold
2.2.1. Preparation of 3.5 ± 0.7 nm colloidal gold Au nanoparticles were prepared according to the reported protocol [25] with slight modi?cations. A 20 mL aqueous solution containing 2.5 × 10?4 M HAuCl4 and 2.5 × 10?4 M trisodium citrate was prepared in a conical ?ask. Next, 0.6 mL of ice-cold, freshly prepared 0.1 M NaBH4 solution was added into the above solution while stirring. The solution turned pink immediately after adding NaBH4 indicating particle formation. 2.2.2. Preparation of 13 ± 1 nm colloidal gold In a 1 L round-bottom ?ask equipped with a condenser, 500 mL of 1 mM HAuCl4 was brought to rolling boil with vigorous stirring. Rapid addition of 50 mL of 38.8 mM sodium citrate to the vortex of the solution resulted in a color change from pale yellow to burgundy. Boiling was continued for 10 min, the heating mantle was then removed, and stirring was continued for an additional 15 min. Transmission electron microscopy (TEM) indicated a particle size of 13 ± 1 nm. 2.2.3. Preparation of 32 ± 2 nm colloidal gold The colloidal Au was prepared by rapidly adding 1.2 mL of 1% trisodium citrate to 100 mL of 0.01% HAuCl4 aqueous solution when HAuCl4 solution got a rolling boil under vigorous stirring. The color changed from pale to blue, then turned to burgundy. Boiling was continued for 15 min, stirring until it reached room temperature. TEM image indicated that the particle size was ca. 32 ± 2 nm.

Scheme 2. Contrastive procedure for coating PEI and APTMS-functionalized silica colloids with gold.

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2.3. Preparation of the silica colloid
2.3.1. Preparation of 60 ± 2 nm silica colloid The microemulsion was formed by mixing cyclohexane (7.5 mL), n-hexanol (1.8 mL), Triton X-100 (1.77 mL) and TEOS (0.1 mL). After mixing for 20 min, 60 L of NH4 OH was added to initiate the polymerization. The reaction was allowed to continue for 24 h. When the polymerization was complete, an equal volume of acetone was added, and the mixture was vortexed to break the microemulsion state. The solidi?ed silica NPs were collected by centrifugation at 3000 × g for 10 min and washed three times with 95% ethanol. Between the washes, the nanoparticles were dispersed by vortex and sonication. 2.3.2. Preparation of 105 ± 3.5 nm silica colloid The silica colloid (105 nm) was prepared by seed-mediated growth method according to a typical procedure. The seed solution was prepared by ?rst mixing absolute ethanol (50 mL), deionized water (1 mL) and TEOS (98%, 1.5 mL), then adding ammonium hydroxide (25%, 3.0 mL) under slow stirring at 40 ? C for 3 h. TEOS (98%, 1 mL) was brought to the above solution under the same condition, and reacted for 3 h. The method of washing is similar to the above. 2.3.3. Preparation of 340 ± 9 nm silica colloid In a typical procedure, the solution I was prepared by mixing absolute ethanol (46 mL) with ammonium hydroxide (25%, 10 mL), and the solution II was prepared by mixing TEOS (98%, 1 mL) with absolute ethanol (4 mL), then the solution II was added quickly into the solution I in an Erlenmeyer ?ask under rapid stirring (>150 rpm) at 21 ? C and reacted for 2 h.

at room temperature. After 3 h, the centrifugation/wash cycles were operated to remove unadsorbed PEI. This was followed by the addition of the amount of gold nanopartiles (raising the concentration of citrate ions in gold particle dispersions by 0.5–1.0 mM prior to addition) into aqueous solutions of PEI coated silica particles. After 2 h, the centrifugation/wash cycles were operated to remove unadsorbed gold nanoparticles.

2.5. Synthesis of APTMS-functionalized SiO2 @Au core–shell composite
The above synthesized silica nanoparticles were functionalized with 3aminopropyltrimethoxysilane (APTMS) in the aqueous solution (pH 8.0, containing 0.1 M KCl) at room temperature. After 3 h, the APTMS modi?ed silica nanoparticles were rinsed by several cycles of centrifugation to remove unadsorbed APTMS. The fabrication of metal-modi?ed silica was similar to that of PEI-functionalized SiO2 @Au core–shell composite.

3. Results and discussion The physical principle underlying the method and its fabrication is illustrated in Scheme 1. PEI can easily adsorb on SiO2 surface through the hydrogen bonding between –NH group of PEI molecules and –OH group of SiO2 surface. Gold nanoparticles were adsorbed on SiO2 surface by electrostatic self-assembly on preadsorbed PEI layer, which has high amine content to provide more active sites to adsorb gold nanoparticles than APTMS, especially for midnanometer gold nanoparticles as shown in Scheme 2. Fig. 1A–C shows that TEM images of PEI-functionalized 340 nm silica colloids coated with different colloidal Au 3.5,

2.4. Synthesis of PEI-functionalized SiO2 @Au core–shell composite
The silica colloids were subsequently coated with positively charged polyethyleneimine (PEI) in the aqueous solution (pH 8.0, containing 0.1 M KCl)

Fig. 1. TEM images of PEI-functionalized 340 nm silica colloids coated with different colloidal Au (A) 3.5 nm, (B) 13 nm, (C) 32 nm and (D) the corresponding UV–vis spectra.

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Fig. 2. TEM images of PEI-functionalized 105 nm silica colloids coated with different colloidal Au (A) 3.5 nm, (B) 13 nm, (C) 32 nm and (D) the corresponding UV–vis spectra.

Fig. 3. TEM images of PEI-functionalized 60 nm silica colloids coated with different colloidal Au (A) 3.5 nm, (B) 13 nm, (C) 32 nm and (D) the corresponding UV–vis spectra.

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13 and 32 nm, respectively. The UV–vis absorption spectra of the silica colloids with an average diameter of 340 nm at different colloidal Au coating are shown in Fig. 1D. There is a peak ca. 534 nm in curve A, which red-shifts so greatly in comparison with the bare colloidal Au (506 nm). In curve B, a peak ca. 545 nm appears, which red-shifts comparatively with curve A, and red-shifts greater than its coating colloidal Au (518 nm). In a similar way, the plasmon peak of curve C redshifts more than that of the curve B. This is because that the cores of the samples are same, while different monolayer colloidal Au were coated on it, namely the thicknesses of shell are different. So the thicker the shell is, the greater the plasmon peak red-shifts. Fig. 2A–C shows that TEM images of PEI-functionalized 105 nm silica colloids coated with different colloidal Au 3.5, 13 and 32 nm, respectively. Fig. 2D represents the UV–vis absorption spectra of the silica colloids with an average diameter of 105 nm at different colloidal Au coating, which is similar to the above (Fig. 1D). Fig. 3A–C shows that TEM images of 60 nm PEI-functionalized silica colloids coated with different colloidal Au 3.5, 13 and 32 nm, respectively. Fig. 3D indicates the UV–vis absorption spectra of the silica colloids with an average diameter of 60 nm at different colloidal Au coating, the UV–vis absorption spectra of the composite SiO2 @Au (60@32 nm, curve C) is not very smooth due to the adsorption of colloidal Au not very good (see Fig. 3C). This is because the size of the silica colloid is closed to that of colloidal Au. The UV–vis absorption spectra of colloidal Au with 3.5 nm diameter and colloidal Au (3.5 nm) at different silica colloids coated are shown in Fig. 4. From Fig. 4, we can ?nd that the plasmon peaks of these composites SiO2 @Au all became red-shifted in comparison with their corresponding bare colloidal Au, which make the gold colloid possible to be used for wide biological applications (the utility of Au nanoparticles is limited because their plasmon resonance is con?ned to relatively narrow wavelength ranges and cannot be readily shifted), and the plasmon peak of the composite SiO2 @Au (340@3.5 nm) red-shifts more than that of the com-

Fig. 5. UV–vis spectra of (d) 13 nm colloidal Au and different PEIfunctionalized silica colloids, (a) 340 nm, (b) 105 nm and (c) 60 nm coated with 13 nm colloidal Au.

posite SiO2 @Au (105@3.5 nm). Similarly, the plasmon peak of the composite SiO2 @Au (105@3.5 nm) red-shifts in comparison with the composite SiO2 @Au (60@3.5 nm). This is because that the shells of these samples are same. However, the sizes of the cores are different, which means that the bigger the core is, the greater the plasmon peak red-shifts. The UV–vis absorption spectra of colloidal Au with 13 nm diameter and colloidal Au (13 nm) at different silica colloids coated (Fig. 5) and the UV–vis absorption spectra of colloidal Au with 32 nm diameter and colloidal Au (32 nm) at different silica colloids coated (Fig. 6) are both similar to the above (Fig. 4). Functionalization of the surfaces of silica nanoparticles with PEI or APTMS has a profound in?uence on the attachment gold nanoparticles. While PEI-functionalized silica surfaces bound the midnanometer gold nanoparticles (13–32 nm), APTMSfunctionalized silica surfaces did not. The results were shown in Figs. 7 and 8.

Fig. 4. UV–vis spectra of (c) 3.5 nm colloidal Au and different PEIfunctionalized silica colloids, (a) 340 nm, (b) 105 nm and (d) 60 nm coated with 3.5 nm colloidal Au.

Fig. 6. UV–vis spectra of (a) 32 nm colloidal Au and different PEIfunctionalized silica colloids, (d) 340 nm, (b) 105 nm and (c) 60 nm coated with 32 nm colloidal Au.

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Fig. 7. TEM images of (A and C) 32 nm colloidal Au attached on APTMS-functionalized 340 and 105 nm silica surfaces, respectively, (B and D) 32 nm colloidal Au attached on PEI-functionalized 340 and 105 nm silica surfaces, respectively.

Fig. 8. TEM images of (A and C) 13 nm colloidal Au attached on APTMS-functionalized 340 and 105 nm silica surfaces, respectively, (B and D) 13 nm colloidal Au attached on PEI-functionalized 340 and 105 nm silica surfaces, respectively.

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4. Conclusion In conclusion, we have prepared gold monolayerencapsulated silica particles by PEI as a linker between silica surface and gold nanoparticles at room temperature instead of re?ux temperature. Because the PEI molecules were easily deposited on hydroxylated silica particles, and higher amine content allowed both greater and more uniform gold particles coverage. The plasmon peaks of the composites silica–gold core–shell all red-shifts so greatly in comparison with their corresponding bare colloidal Au, with the increasing of gold nanoparticle diameter of the composite SiO2 @Au (the silica colloid not change), the plasmon peak red-shifts, and with the increasing of silica nanoparticle diameter of the composite SiO2 @Au (the gold nanoparticles not change), the plasmon peak also shifts red. Moreover, we have made a comparison that the ability to attach the colloidal gold nanoparticals between the PEI and APTMS-functionalized different sized silica colloid. Acknowledgements This study was supported by Analysis and Testing Foundation of Northeast Normal University and Natural Science Fundation of China (NSFC). References
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