Multifunctional Magnetic Silver Nanoshells with Sandwichlike

plating method, forming a multiplayer sandwichlike nanostructure. The plasmon resonance peaks can shift across a wide wavelength range by tuning the A...
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8870

J. Phys. Chem. C 2008, 112, 8870–8874

Multifunctional Magnetic Silver Nanoshells with Sandwichlike Nanostructures Minghai Chen, Yong Nam Kim, Hyeok Moo Lee, Cuncheng Li, and Sung Oh Cho* Department of Nuclear and Quantum Engineering, Korea AdVanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea ReceiVed: January 23, 2008; ReVised Manuscript ReceiVed: April 7, 2008

We present a novel multifunctional magnetic Ag nanoshell with sandwichlike nanostructure, which is composed of a yolk-egg-like magnetic silica core and a continuous Ag nanoshell coating. First, yolk-egg-like magnetic silica was prepared by a facile one-step sol-gel method to embed the magnetic nanoparticles at the boundary between inner silica spherical core and outer silica shell. Then Ag nanoshells were coated by electroless plating method, forming a multiplayer sandwichlike nanostructure. The plasmon resonance peaks can shift across a wide wavelength range by tuning the Ag shell coverage and thickness. The magnetic Ag nanoshells fuse the broad NIR absorption property and superparamagnetic property into one particle, possessing promising biomedical applications such as magnetic-field targeted photothermal therapy agents and multimodal molecular probes. 1. Introduction Noble metallic nanostructures have been widely used for biolabeling,1 cell detectors,2 biosensors,3 and photothermal therapy4 because of their remarkable size- and shape-dependent optical and electronic properties as well as excellent stability and biocompatibility.5 Magnetic nanoparticles (NPs)6 have also gained great success in biological and biomedical applications such as targeted drug delivery,7 magnetic cell sorting,8 magnetic resonance imaging (MRI) contrast agents,9 and cancer treatments by hyperthermia and immunoassays.10 Recently, synthesis of multifunctional nanostructures based on the combination of metallic and magnetic nanostructures have received attentions due to the increasing requirements of the multifunctional nanostructures for many technical applications, especially in biomedicine and electronics.11 Among a variety of noble metallic nanostructures, nanoshells12 that are composed of dielectric cores covered with thin metal shells have appealing optical and electronic properties. The surface plasmon resonance (SPR) of metal nanoshells can be continuously tuned from ultraviolet to infrared by changing the core radius or the shell thickness.12e Particularly, the fact that the SPR absorption peak of the nanoshells can be tuned to near-infrared (NIR) wavelength region of 700-1000 nm is attractive for biomedical applications because biological tissue is nearly transparent to the so-called “water-window” region.13 These appealing properties of metal nanoshells, when combined with magnetic NPs, can provide multifunctional nanostructures because magnetic NPs can provide MRI contrast and direct the nanostructure to the target by exterior magnetic field. Gold (Au) nanoshells with magnetic NPs have been synthesized, where magnetic NPs were entrapped in the center of silica sphere by well-known Sto¨ber method.14 However, it is not easy to achieve such core-shell magnetic silica spheres with a well-defined shape and size. This method usually leads to a large polydispersity of the produced magnetic core-shell particles and a nonuniform distribution of the magnetic NPs in the silica matrices. This problem can be overcome if magnetic NPs are * Corresponding author. E-mail: [email protected]. Tel: +82-42-8693823.

SCHEME 1: Schematic Illustration for the Fabrication Procedure of the Magnetic Ag Nanoshells

selectively adsorbed on preformed colloidal cores through electrostatic attraction or coordination following a proper surface modification of the cores.15 Here, we introduce a novel magnetic silver (Ag) nanoshell with a sandwichlike nanostructure, where a layer of magnetic NPs, an intermediate silica layer, and an optical Ag nanoshell are successively coated on a silica spherical core. The fabrication procedure of the sandwichlike magnetic Ag nanoshell is shown in Scheme 1. First, magnetite (Fe3O4) NPs are prepared by a solvothermal method using high temperature coprecipitation of Fe3+ and Fe2+ in diethylene glycol (DEG) and diethanolamine (DEA) mixture solvent. Second, as-prepared magnetic NPs are captured on the surface of preformed silica spheres through the electrostatic attraction between positively charged Fe3O4 NPs that are activated by acid solution and negatively charged silica beads. Simultaneously, a silica layer encapsulates the Fe3O4coated silica sphere by a modified Sto¨ber method. As a result, yolk-egg-like stable magnetic silica beads are created by this one-step method. Third, the magnetic silica beads are modified with -NH2 groups followed by the adsorption of Au NPs via electrostatic attraction. Finally, Ag nanoshells are coated on the magnetic silica beads by an electroless plating method using Au NPs as seeds. Ag+ ion from silver nitrate (AgNO3) was reduced into elemental Ag by formaldehyde (HCHO). Compared to the Au nanoshells, Ag nanoshells show strong optical absorption and wide tunable plasmon resonance band.12b Since

10.1021/jp800649b CCC: $40.75  2008 American Chemical Society Published on Web 05/28/2008

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Figure 1. TEM images of the samples at different fabrication stage of the magnetic Ag nanoshells: (a) silica spheres, (b) magnetic silica beads, (c) magnetic silica beads with Au seeds attached on the surface, (d and e) Ag nanoshells with different coverage, and (f) final magnetic Ag nanoshells with continuous Ag coating.

TABLE 1: Zeta Potentials of the Sample at Different Stages

an Ag nanoshell and a magnetic NPs layer are separated by a silica layer, disturbance between the two shells can be prevented. 2. Experimental Section Chemicals. Iron(III) chloride hexahydrate (FeCl3 · 6H2O), iron(II) chloride tetrahydrate (FeCl2 · 4H2O), DEG, DEA, sodium hydroxide (NaOH), aqueous ammonia (NH3 · H2O, 28%), hydrochloric acid (HCl, 35%), HCHO (37%), potassium carbonate (K2CO3), trisodium citrate (TSC), and AgNO3 were purchased from Junsei Chemical Co., Ltd., Japan. Tetraethyl orthosilicate (TEOS, 98%), tetrakis-(hydroxymethyl)-phosphonium chloride (THPC, 80% solution in water), (3-aminopropyl) trimethoxysilane (APTMS, 97%), and hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 · 3H2O) were purchased from Aldrich. All of the chemicals were of analytical grade and used without further purification. In the whole experiment, pure water and absolute alcohol were used, which were purchased from DC Chemical Co., Ltd., Korea. Preparation of Magnetic Silica Beads. The magnetic NPs were synthesized by a solvothermal coprecipitation method, where 4 mmol of FeCl3 · 6H2O, 2 mmol of FeCl2 · 4H2O, 16 mmol of NaOH, 10 mL of DEA, and 60 mL of DEG were loaded into an autoclave and maintained at 180 °C for 8 h. The black products were collected and redispersed in water to form a 0.5 wt% suspension. The detailed characterizations of the magnetic nanoparticles were provided in Supporting Information. Silica spheres were prepared by the hydrolysis and condensation of 3 mL TEOS in 280 mL of alcohol-ammonia mixture solution (V/V ) 1/9). These SiO2 spheres were redispersed in alcohol and formed a 0.8 wt% suspension. Then another silica shell was grown on the silica spheres with magnetic NPs embedded in the outer shell by mixing magnetic NPs suspension (1-5 mL), silica spheres (5 mL), HCl (0.2 mL, 0.1 M), and TEOS (0.12 mL) in an alcohol-ammonia mixture (V/V ) 1/20). Preparation of Magnetic Nanoshells. An electroless plating method12,13 was used for the preparation of magnetic nanoshells. The negatively charged Au seeds were synthesized through reduction of HAuCl4 by THPC. Typically, 0.5 mL of NaOH (1 M) and 1 mL of THPC diluted water solution (0.95 wt%) were

zeta potential (mV)

bare SiO2

Fe3O4

SiO2-NH2a

Au seeds

-48.74

+53.82

+13.23

-6.02

a SiO2-NH2 is the magnetic silica beads after grafted with -NH2 groups using APTMS.

added to 25 mL of water under vigorous magnetic stirring. After 5 min, 25 mL of HAuCl4 (2 mM) was added. Within 30 s, the color of the mixture changed from yellow to dark brown, indicating the formation of THPC-gold nanoparticles. In the meantime, the surfaces of magnetic silica beads were grafted with -NH2 groups by mixing of magnetic silica beads with APTMS in alcohol for 12 h and following refluxing at 80 °C for 1 h. Then, Au seeds and APTMS modified magnetic silica beads were mixed together to induce the adsorption of Au seeds onto silica surfaces through electrostatic interaction. The final Au-attached magnetic silica beads were dispersed in water to form a suspension with a concentration of 1.12 × 108 particles/ mL for following growth of noble metal nanoshells. To grow Ag nanoshells, 2 mL of Au-attached magnetic silica beads and 0.5 mL of AgNO3 (0.1 M) were diluted to 20 mL by water under a gentle agitation. 50 µL HCHO and 25 µL aqueous ammonia were added into the mixture in order. Ag+ ions were reduced into elemental Ag by HCHO at pH ∼10, which is tuned by aqueous ammonia.12c The color of mixture gradually changed from weak brown to deep brown in 10 min, indicating the formation of Ag nanoshells. Characterizations. The crystal structure of the as-prepared samples was characterized by X-ray diffraction (XRD) (Philips X’pert PRO) analysis. The morphologies and microstructures of the samples were observed using a JEOL 2100F transmission electron microscope (TEM) in Korea Basic Science Institute (KBSI). Energy dispersive spectroscopy (EDS) patterns and mapping are also taken from the same TEM. Zeta potentials were measured in a Zeta potential analyzer (Zetaplus, Brookhaven Instruments). The magnetic hysteresis loops were measured on a vibrating sample magnetometer (Lakeshore model 7304) at room temperature, using powder sample packed in a capsule. The UV-vis spectra were recorded in a V-530 UV-vis spectrophotometer.

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Figure 2. EDS characterizations of magnetic silica beads with Au seeds attachment: (a) EDS mapping image, (b) Au and Fe, (c) Fe, and (d) Au.

Photothermal Conversion Experiment by NIR Lasers. An SDL-808-LM-5000T Diode-Pumped Solid-State Laser system (Shanghai Dream Lasers Technology Co., Ltd., China) with a center wavelength of 808 nm was used as the NIR laser source. The size of the laser spot is 10 mm × 5 mm and the output power is 5.4 W. In the experiment, 1 mL of particle aqueous suspension stabilized by TSC with different concentrations was filled into a quartz cell (10 mm × 10 mm × 40 mm) and positioned 2.5 cm far away from the laser source. The temperature of the solution was detected by a HT3500C sensitive thermometer. 3. Results and Discussion Figure 1 shows the TEM images of the samples at each stage displayed in Scheme 1. Silica spheres were prepared by the hydrolysis of TEOS in alcohol-ammonia mixture solution and the spheres had the diameter of ∼170 nm with very narrow size distribution (Figure 1a). Their diameters can be precisely controlled by tuning the TEOS concentrations. Figure 1b shows the fabricated magnetic silica beads that have a core-shell structure, which can be explicitly distinguished by the contrast difference. Careful observation reveals that small NPs were embedded at the boundary between inner silica sphere core and outer silica layer shell, indicating that the magnetic silica beads formed a unique yolk-egg-like nanostructure. The high magnification image shows the magnetic nanoparticles entrapped in the silica matrix with an average diameter of ∼8 nm (inset in Figure 1b). Figure 1c and its inset display the magnetic silica beads coated with small Au NPs of 2-3 nm in size. The electrostatic interaction of the nanoparticles with silica surface results from the nature of surface charge, which can be measured by zeta potentials, as shown in Table 1. Then, continuous Ag nanoshells were grown on the Au NPs. Ag NPs grow on Au seeds by the reduction of AgNO3 and the Ag coverage can be controlled by changing the AgNO3 amount, as shown in Figure 1d-f. Isolated Ag NPs were formed on the magnetic silica beads when 0.2 mL of AgNO3 solution was added (Figure 1d). Ag coverage on the Au seeds was enhanced with increasing the Ag+ amount (Figure 1e, 35 mL of AgNO3). Finally, when 0.5 mL of AgNO3 was added, a complete layer of Ag nanoshell

Figure 3. (a) XRD pattern of magnetic Ag nanshells (9-Ag); (b and c) EDS patterns of samples with different Ag coverage, which correspond to the samples shown in Figure 1, panels d and f, respectively.

was created on each bead, forming the sandwichlike magnetic Ag nanoshells (Figure 1f). The shell thickness was ∼30 nm. In this strategy, the size of the silica core can be controlled by changing the preformed silica sphere size, which can be easily achieved by Sto¨ber method. Furthermore, the thickness of Ag shell can also be readily tuned by controlling the addition amount of AgNO3 solution. The chemical composition of the magnetic Ag nanoshells was characterized by EDS. Figure 2 shows the EDS mapping images of the magnetic silica beads decorated with Au NPs, which corresponds to the sample shown in Figure 1c. The elemental distribution map of Fe (Figure 2c) combined with the TEM image (Figure 1b) suggests that Fe3O4 NPs were embedded in the boundary between inner silica core and outer silica shell. The dispersed Au signals (Figure 2d) on the spherical matrix confirm that tiny Au NPs were attached on the surface of silica beads. Figure 3 displays the XRD pattern and the EDS spectra of the samples shown in Figure 1, panels d and f. All of the diffraction peaks match well with those of fcc Ag crystal (PDF card No. 040783) except the peak at 55° that comes from the silicon (Si) substrate. If the magnetic silica was not completely coated with Ag nanoshells (Figure 1d), elemental signals of Si and Fe as well as Ag appeared in the EDS spectrum (Figure 3b). However, the EDS spectrum for the sample of Figure 1f shows that the Fe signal disappeared and Si signal was

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Figure 5. Measured magnetic hysteresis loops of the fabricated (a) magnetic silica beads and (b) magnetic Ag nanoshells.

Figure 4. (a) UV-vis spectra of the magnetic Ag nanoshells with different Ag coverage. The Ag coverage gradually increases from curve I (no coverage) to curve VIII (Ag shell thickness of ∼30 nm). (b) NIR heating curves with different particles concentrations in water (unit: particles/mL).

dramatically decreased compared to Ag signal (Figure 3c), indicating that thick enough Ag nanoshells were formed around the magnetic silica cores. The optical properties of the magnetic Ag nanoshells were characterized by UV-vis spectra (Figure 4a), which show a continuous red shift toward the NIR region with increasing the Ag coverage. The spectrum of the magnetic silica beads coated with Au NPs (sample 1c) exhibits the absorption peak at 520 nm, which is assigned to the SPR peak of Au NPs (curve I in Figure 4a). However, when isolated Ag NPs were formed on the magnetic silica beads (sample 1d), the absorption peak shifted to ∼480 nm (curve II). As the Ag coverage on the beads was increased, the absorption spectra red-shifted and furthermore that the peak was broadened to cover the whole visible and NIR wavelength region (curves III-VIII). To investigate the applicability to photothermal therapy, the heating behavior of the fabricated magnetic Ag nanoshells was measured. In the experiment, 1 mL aqueous suspension of magnetic Ag nanoshells stabilized by TSC was irradiated by a diode-pumped solid-state laser with the output power density of 10.8 W/cm2 and a center wavelength of 808 ( 10 nm. The measured temporal behavior of the suspension temperature is shown in Figure 4b. The temperature of the suspension linearly increased with time. The increasing rate of the temperature was proportional to the particle concentration of magnetic Ag nanoshells. The temperature increased rapidly to 80 °C within 100 s at the particles concentration of 6.0 × 107 particles/mL (curve VIII in Figure 4b). Even at low concentration of 9.6 × 106 particles/mL (curve III), the temperature was increased to 42 °C within 120 s, which is still high enough to kill cancer cells.13c–e Comparatively, pure water without the magnetic Ag nanoshells has no obvious temperature elevation even after being exposed to the NIR laser for 5 min (curve I).

The magnetic property of the fabricated magnetic silica beads and nanoshells were characterized by a vibrating sample magnetometer at room temperature. The measured magnetic hysteresis loops shown in Figure 5 clearly reveals that both the magnetic silica beads and magnetic Ag nanoshells have superparamagnetic behaviors. The Fe3O4 fraction of the magnetic silica beads prepared according to the proportion of 3.5 mL Fe3O4, 5 mL SiO2 and 0.12 mL TEOS in the experimental stage was calculated to be about 30 wt%. Its saturation magnetization was measured to be 6.35 emu/ g, as shown in curve a. After coated with Ag nanoshells, the corresponding saturation magnetization decreased to 2.6 emu/g (curve b). The superparamagnetic property of the magnetic Ag nanoshells offers prospective biomedical applications such as magnetic-field targeting and MRI contrast combining with unique optical properties. 4. Conclusion In summary, multifunctional magnetic Ag nanoshells with sandwichlike nanostructures were successfully fabricated. The magnetic Ag nanoshells integrate unique optical properties of Ag and superparamagnetic property of Fe3O4 NPs into one particle. Especially, the broad NIR absorption combining with superparamagnetic behavior makes it possess promising biomedical applications such as magnetic-field targeted photothermal therapy agents and multimodal molecular probes. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. 2007-00543). Supporting Information Available: Detailed characterization of Fe3O4 nanoparticles, including XRD, XPS and TEM. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Huang, X. H.; El-Sayed, I.; Qian, H. W.; El-Sayed, M. A. Nano Lett. 2007, 7, 1591. (b) Eghtedari, M.; Oraevsky, A.; Copland, J. A.; Kotov, N. A.; Conjusteau, A.; Motamedi, M. Nano Lett. 2007, 7, 1914. (c) Lee, K. S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 19220. (d) Beissenhirtz, M. K.; Elnathan, R.; Weizmann, Y.; Willner, I. Small 2007, 3, 375. (2) (a) Zayats, M.; Baron, R.; Popov, I.; Willner, I. Nano Lett. 2005, 5, 21. (b) J. Haes, S. L; Zou, G. C; Schatz, R. P.; Duyne, V J. Phys. Chem. B 2004, 108, 6961. (3) (a) Cao, Y. W.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536. (b) Yu, C. X.; Nakshatri, H.; Irudayaraj, J. Nano Lett. 2007, 7, 2300. (c) Stone, J. W.; Sisco, P. N.; Goldsmith, E. C.; Baxter, S. C.; Murphy, C. J. Nano Lett 2007, 7, 116. (d) Beissenhirtz, M. K.; Elnathan, R.; Weizmann, Y.; Willner, I. Small 2007, 3, 375. (4) Chen, J.; Wang, D.; Xi, J.; Au, L.; Siekkinen, A. R.; Warsen, A.; Li, Z. Y.; Zhang, H.; Xia, Y.; Li, X. Nano Lett. 2007, 7, 1318.

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