Gold Shell Submicrometer Spheres

Apr 7, 2009 - A method to synthesize Fe3O4 core/Au shell submicrometer structures with very rough surfaces on the nanoscale is reported. The Fe3O4 ...
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J. Phys. Chem. C 2009, 113, 7009–7014

7009

Fabrication of Iron Oxide Core/Gold Shell Submicrometer Spheres with Nanoscale Surface Roughness for Efficient Surface-Enhanced Raman Scattering Yueming Zhai, Junfeng Zhai, Yuling Wang, Shaojun Guo, Wen Ren, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Changchun 130022, Jilin, People’s Republic of China ReceiVed: December 01, 2008; ReVised Manuscript ReceiVed: March 12, 2009

A method to synthesize Fe3O4 core/Au shell submicrometer structures with very rough surfaces on the nanoscale is reported. The Fe3O4 particles were first modified with uniform polymers through the layer-by-layer technique and then adsorbed a lot of gold nanoseeds for further Au shell formation. The shell was composed of a large number of irregular nanoscale Au particles arranged randomly, and there were well-defined boundaries between these Au nanoparticles. The Fe3O4 core/Au shell particles showed strong plasmon resonance absorption in the near-infrared range, and can be separated quickly from solution by an external magnet. This kind of very rough Fe3O4 core/Au shell multicomponent was used in the adsorption of 4-aminothiophenol and as a substrate for detection by surface-enhanced Raman spectroscopy. Introduction Assembling several materials together is a useful method to obtain multicomponents deriving excellent properties from each building block. A successful strategy to create multicomponents is to prepare one material first, and then use it as a seed on which to deposit another component.1 Magnetic and noble metal nanoparticles are of particular importance due to their broad range of potential applications. Magnetic materials can be manipulated conveniently by a magnetic field, so that they are usually used to catch the target in solution and concentrate them together with the assistance of an external magnet. Iron oxide based magnetic materials are being studied intensively because of their potential applications for protein separation,2-4 high-density magnetic recording,5 drug delivery,6 catalysis,7,8 detection,9 and magnetic resonance imaging.10 Noble metals, especially Au nanoparticles, have also received great attention due to their attractive electronic,11 optical,12 and catalytic properties13,14 and wide applications in detection of the existence of certain biomolecules.15 From these viewpoints, the combination of magnetic nanoparticles and noble metals in one nanocomposite would form a unique multifunctional material.4,16,17 Core/shell nanoparticles with a magnetic core and metallic shell, such as Fe3O4/Au and Fe3O4/Ag, are even more interesting composite systems.18-22 Magnetic nanoparticles with a Au shell are more stable in corrosive conditions and are easily functionalized through the robust interaction between Au and thiol groups. Furthermore, the Au coating can also give the whole multifunctional material unique plasmonic properties.16,23 Because Au ions are very easily reduced, preventing homogeneous nucleation is a challenge in forming gold-based multicomponents. There are some successful synthetic methods to obtain Fe3O4-Au multicomponents, for example, reducing HAuCl4 onto Fe3O4 nanoparticles via iterative hydroxylamine seeding,24 decomposing Fe(CO)5 on the surface of the gold nanoparticles followed by oxidation in 1-octadecene solvent,25 coating the Fe3O4 core by a thick silica shell which can be * To whom correspondence [email protected]..

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functionalized with polyelectrolytes to facilitate the deposition of gold seeds and further covered with an outer shell of gold,26,27 and covalently attaching to the surface of Fe3O4 used molecules containing thiol groups for further connecting gold nanoparticles by Au-S bonding.2 Recently, Huang et al. reported a method to form a smooth gold shell on the magnetic particles. The magnetic cores were modified with polymers through copolymerization on the surface, which can tune the thickness of the polymers by varying the monomer concentration and further assemble gold nanoparticles as seeds to form the shell.28 In this case, the modification of the magnetic core is a key point for further gold shell deposition. The layer-by-layer (LBL) assembly is an efficient and simple method to construct high-quality and uniform coatings on the substrates, which varied from macroscopically platform charged surfaces to micrometer- and submicrometer-sized,chargedcolloidalparticlesandevennanoparticles.29,30 Polystyrene colloids and SiO2 particles are usually used as the substrates in many reported works, but a few researchers have used magnetic particles. There is a distinct advantage to using magnetic particles as a platform for the LBL self-assembly technique compared to polystyrene colloids and SiO2 particles, because the centrifugation procedures for frequent separation processes can be replaced by magnetic separation, which is easy, effective, and time-saving. The diverse properties of multicomponents are greatly influenced by the types of components, particle sizes, surface geometries, and distance between the nanoparticles. On the basis of these factors, gold-based systems can provide unique optical properties.31-34 For example, some kinds of gold shells exhibit significant absorbance in the near-infrared (NIR) region of the electromagnetic spectrum where tissue and blood are transparent and therefore can be used for therapeutic applications.31,33,35,36 For gold materials with rough surfaces, the enhanced local electromagnetic field is one mechanism that can contribute to the signal strength in surface-enhanced spectroscopies. Halas and co-workers have shown that the introduction of nanoscale texturing on the nanoparticle surface can result in interesting changes to both the far-field and near-field properties.34 Likewise, submicrometer Au spheres with nanoscale surface roughness and “meatball-like” morphology have unique optical

10.1021/jp810561q CCC: $40.75  2009 American Chemical Society Published on Web 04/07/2009

7010 J. Phys. Chem. C, Vol. 113, No. 17, 2009 properties and can be used as substrates for surface-enhanced Raman spectroscopy (SERS).32 In this study, we fabricated a submicrometer magnetic core/ gold shell structure with nanoscale surface roughness. The submicrometer magnetic particles were chosen as the cores, which could give the magnetic material a strong magnetic force after modification with other materials evenly coated with a gold shell. When silica was introduced to help construct the goldcoated magnetic silica spheres, the final products often showed a low saturation magnetization value, far from the saturation magnetization of the magnetic nanoparticles used as the core. This could be attributed to the diamagnetic contribution of the thick silica shell surrounding the magnetic cores and the low volume ratio taken up by the magnetic core compared with the whole magnetic silica spheres.34 The Fe3O4 particles were modified with polymers through the LBL method for assembling large numbers of gold seeds. Highly rough Fe3O4 core/Au shell particles with nanoscale roughness and gaps were formed after the final reduction with hydroxylamine hydrochloride. This material was then used to extract 4-aminothiophenol (4-ATP) from the sample solution, and a magnet was used to concentrate the microparticles onto the side of the sample vial, allowing detection by SERS. Experimental Section Reagents. Ferric chloride, ethylene glycol, sodium acetate anhydrous, chloroauric acid (HAuCl4), polyethylene glycol, and hydroxylamine hydrochloride were purchased from Beijing Chemical Reagent Factory (Beijing, China). Poly(methacrylic acid, sodium salt) (PMAA; average Mw ) 9500), branched poly(ethyleneimine) (BPEI; average Mw ) 25 000), and 4-aminothiophenol were purchased from Sigma-Aldrich. All reagents were used as received without further purification. The water used was purified through a Millipore system. Preparation of Gold-Nanoparticle-Modified Magnetic Spheres. The Fe3O4 nanoparticles were prepared according to Li’s method.37 In a typical experiment, FeCl3 · 6H2O (0.45 g) was dissolved in ethylene glycol (14 mL) to form a clear solution, followed by the addition of NaAc (1.2 g) and polyethylene glycol (0.3 g). The mixture was stirred vigorously for 30 min and then sealed in a Teflon-lined stainless-steel autoclave (15 mL capacity). The autoclave was heated to and maintained at 200 °C for 8 h and allowed to cool to room temperature. The black products were washed several times with ethanol and dried at 60 °C. The gold colloids were prepared according to Frens’s method.38 Briefly, 100 mL of 0.01% HAuCl4 solution was heated to boiling, and 3 mL of 1% trisodium citrate was added. The solution was kept boiling for 5 min. The modification of Fe3O4 with polyelectrolytes and gold nanoseeds was carried out by the LBL assembly technique. A 10 mL sample of a 1 mg/mL PMAA aqueous solution was added to 1 mL of Fe3O4 suspension (2 mg/mL). The PMAA was allowed to adsorb for 20 min while the solution was stirred. PMAA-coated Fe3O4 particles were then collected by a magnet, followed by washing with water several times. Finally, they were redispersed in 1 mL of water and sonicated for 1 min to prevent aggregation. Positively charged BPEI was then deposited onto the above negatively charged Fe3O4 particles in similar conditions and with similar procedures. The above processes were repeated one time. Then 10 mL of negatively charged gold nanoparticles was added to 1 mL of polymer-coated Fe3O4 solution with an outermost layer of positively charged BPEI. The composites were allowed to equilibrate overnight under

Zhai et al. SCHEME 1: Schematic Depiction of the Fabrication Procedure of the Fe3O4 Core/Au Shell Nanostructure

stirring. The excess gold nanoparticles were subsequently removed by three magnetic separation/redispersion cycles. Formation of the Gold Shell. Au shells were formed using a modification of Natan’s method by reduction of HAuCl4 with hydroxylamine.39 A 1 mL sample of gold-nanoparticle-modified Fe3O4 microspheres (2 mg/mL) was diluted to 40 mL with water. A 0.4 mL portion of 1% sodium citrate and 0.2 mL of 1% HAuCl4 were added. Then hydroxylamine solution (80 mM, 0.2 mL) was introduced slowly under vigorous mechanical stirring and occasional ultrasonication. The products were collected with a magnet, washed with water at least three times, and then dispersed in water (40 mL). The same procedure described above was repeated three times to give rough Au shells. Instruments. An XL30E SEM scanning electron microscope equipped with an energy-dispersive X-ray (EDX) analyzer was used to determine the composition of the products. SEM images were taken on a JEOL JXA-840 scanning microanalyzer scanning electron microscope, and the accelerating voltage was 20 kV. ζ potential measurements were performed using a Zeta sizer NanoZS (Malvern Instruments). UV-vis-NIR spectra were collected on a CARY 500 scan UV-vis-NIR spectrophotometer. The Raman instrument includes an FT-Raman spectrometer (Thermo Nicolet 960) equipped with an InGaAs detector and a Nd/VO4 laser (1064 nm) as an excitation source. The laser power used was about 0.3 mW. The as-prepared Fe3O4 core/Au shell particles were dispersed in 1 mL of 10-4 M 4-ATP solution, separated by a magnet after 6 h, and transferred to a glass slide for detection. Magnetic measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer at 300 K. Results and Discussion Multicomponent structures of the Fe3O4 core/Au shell with a rough surface were fabricated as schematically outlined in Scheme 1. In brief, Fe3O4 particles were modified with polymers through the LBL method. Then gold nanoparticles were confined on the surfaces through electrostatic interaction, and the gold shell was introduced to form the core/shell structure. As shown in the SEM image (Figure 1), the Fe3O4 particles had relatively smooth surfaces, and they were used as the template. PMAA adsorbed on the surface of the Fe3O4 particles first to enhance the negative charge of the surface, improving the stability of the particles. The large amount of negative surface charge was therefore favorable for the deposition of positively charged BPEI with a driving force of electrostatic

Fe3O4 Core/Au Shell Spheres with Surface Roughness

Figure 1. SEM images of the Fe3O4 nanoparticles.

Figure 2. ζ potential of the polymer-modified Fe3O4 nanoparticles as a function of the polyelectrolyte layer number for PMAA/BPEI coatings. The odd layer numbers correspond to PMAA deposition. The even layer numbers correspond to BPEI adsorption, and zero corresponds to the Fe3O4 core.

interactions, while amine groups were introduced to the particle surface. Negatively charged PMAA and positively charged BPEI were alternatively deposited onto the surface of Fe3O4 particles, forming uniform polyelectrolyte multilayers, and the polyelectrolyte could provide a positively charged outer surface to facilitate the adsorption of the negatively charged gold seeds. ζ potential measurements were employed to monitor the formation of each polyelectrolyte layer deposited sequentially on the Fe3O4 particles. Polyelectrolyte depositions caused the ζ potential to alternate in sign, depending on whether the outermost layer was positively or negatively charged. Figure 2 shows the ζ potential as a function of the polyelectrolyte layer number for Fe3O4 particles coated with PMAA and BPEI. The original Fe3O4 particles had a slightly positive ζ potential of 6.7 mV. The presence of a single layer of adsorbed negatively charged PMAA on the Fe3O4 particles caused a reversal in the ζ potential to negative values (-18.3 mV), and subsequent deposition of a positively charged BPEI reversed the ζ potential to positive values (42.9 mV). Further deposition of the PMAA

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7011 and the BPEI also changed the ζ potential. The alternation in the ζ potential qualitatively demonstrated a successful recharging of the particle surface with the deposition of each polyelectrolyte layer and suggested the stepwise layer growth occurring during the fabrication of the polymer-modified Fe3O4 particles. The positively charged Fe3O4 particles were then mixed with the citrate-stabilized gold nanoparticles, and the gold nanoparticles were confined on the Fe3O4 particle surfaces via electrostatic interactions. As shown in Figure 3a,b, many small gold nanoparticles are closely and evenly immobilized on the surface of the polymer-coated Fe3O4 surface. This suggests that highquality, quite uniform, and positively charged coatings of the polymer on the Fe3O4 particles formed through the LBL assembly technique. Gold seeds supply numbers of small, randomly oriented crystalline domains in the following seedmediated growth of the Au shell. The gold nanoshell was formed by selectively reducing chloroauric acid onto the surface of the gold seeds through a surface catalytic reaction using chloroauric acid and hydroxylamine hydrochloride. Repeated reductive deposition led to a rough gold nanoshell. It should be pointed out that the existence of the Fe3O4 core/Au nanoparticles is necessary for hydroxylamine to reduce HAuCl4. After reduction, no obvious color of the solution can be seen when the products are separated by a magnet, which indicates that very little new nucleation of gold happened in solution. Parts c and d of Figure 3 show the SEM image of the as-made Fe3O4 core/Au shell structures, which are monodisperse particles with highly rough gold shells. The surface of each particle is composed of large fissures and well-separated Au islands, which consist of a large number of irregular nanoparticles. Citrate as the protective agent does not have as strong a selective adsorbing ability in the seedmediated growth of the gold shell as cetyltrimethylammonium bromide (CTAB) or poly(vinylpyrrolidone) (PVP), which can help to create nanoscale bumps on the gold particles to form a rough surface.36 The roughness may be mainly caused by the high concentration of the HAuCl4 and higher temperature caused by the ultrasonication. The formation of the Fe3O4 core/Au shell structures was further characterized by EDX, which confirmed the existence of the gold shells on the surface of the Fe3O4 particles (Figure 4). UV-vis absorption spectroscopy experiments were carried out to confirm the preparation procedures of as-prepared core/ shell particles (Figure 5). All samples were dispersed in deionized water for absorption experiments. From Figure 5, it is clear that the absorption spectrum of Fe3O4 particles shows an absorption peak around 670 nm (Figure 5a). After deposition of the gold seeds on the polymer-modified Fe3O4 surface (Figure 5b), the plasmon resonance band shifted obviously to higher wavelengths, which is attributable to the strong interactions and coupling of the surface plasmons between neighboring gold nanoparticles.34 When chloroauric acid was further reduced onto the existing surface of the gold seeds to form gold nanoshells, the NIR absorption increased (Figure 5c). The surface topography of a particle can significantly influence its optical properties. Peaks corresponding to higher order multipole resonances such as quadrupole and octupole can be observed in the spectrum. Two plasmon resonance peaks at around 953 and around 793 nm are identified as quadrupole and octupole resonances, respectively. Magnetic properties of the Fe3O4 core/Au shell submicrometer spheres were characterized by using a SQUID at 300 K. As shown in Figure 6A, the magnetization of Fe3O4 submicrometer particles was about 75 emu/g (Figure 6A-a), and Fe3O4 core/

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Figure 3. SEM images of the as-prepared nanoparticles: (a) Fe3O4 core/gold nanoparticles; (b) high-resolution SEM micrograph of Fe3O4 core/ gold nanoparticles; (c) Fe3O4 core/Au shell nanoparticles; (d) high-resolution SEM micrograph of Fe3O4 core/Au shell nanoparticles.

Figure 4. EDX analysis of the Fe3O4 core/Au shell nanoparticles.

Au shell spheres reached a 26 emu/g saturation moment (Figure 6A-b). The obvious decrease in magnetization revealed the diamagnetic contribution of the thick Au shell, but the value of the magnetization was still lager than those in some reported works.16,27 This could be explained by the fact that the mass ratio of Fe3O4 in the core/shell submicrometer structure was larger than that of some reported structures, especially when silica was used as the interlayer between Fe3O4 and Au. There were no significant changes in the coercivity of Fe3O4 particles after they were coated with the Au shell. Upon placement of a magnet beside the vial, materials were quickly attracted to the side of the vial within a few seconds, leaving the solution transparent, as shown in Figure 6B, which illustrates their

Figure 5. Vis-NIR absorption spectra: (a) Fe3O4 nanoparticles; (b) Fe3O4/gold nanoparticles; (c) Fe3O4 core/Au shell structures.

magnetic nature, and the particles can be well redispersed again by shaking and ultrasonic vibration. This kind of highly rough Fe3O4 core/Au shell multicomponent is thought to be useful as an SERS substrate because of the large number of irregular nanoscale gold particles arranged randomlyandthegapsformedbetweenthesegoldnanoparticles.25,40,41 Taking advantage of their magnetic properties, we dispersed the Fe3O4 core/Au shell particles in the solution containing 10-4 M 4-ATP and separated the materials by a magnet for the detection of SERS. Figure 7 shows FT-SERS spectra of 4-ATP

Fe3O4 Core/Au Shell Spheres with Surface Roughness

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7013 that the thiol group in 4-ATP directly contacts the gold surface.42,43 It can also be clearly observed that the SERS spectra are dominant with a1 vibration modes at 1077.9, 1583.2, and 1174.4 cm-1, and the vibration mode at 387.6 cm-1 is assigned to one of the vibrational modes of the C-S bond. Here, the enhancement at 1064 nm excitation is presumed to mainly derive from the electromagnetic mechanism because the surface plasmon resonance of the materials is at the near-infrared region.42 The enhancement may come from the strong localized electromagnetic field produced by the gaps between two neighboring particles and the rough nanoshell possessing larger local field enhancements than the smooth surface.34 It has been well-recognized that the junctions of the adjacent two particles provide SERS-active sites, due to the existence of so-called “hot spots” having intense local electromagnetic fields in which highly efficient Raman scattering can be obtained.44-46 Conclusions

Figure 6. (A) Hysteresis loops recorded at 300 K of (a) Fe3O4 and (b) Fe3O4/Au. (B) Photographs of the Fe3O4 core/Au shell particle suspension before (a) and after (b) magnetic separation by an external magnet.

In this paper, we reported the synthesis of Fe3O4 core/Au shell particles with highly rough surfaces in nanoscale. The LBL technique was used to construct high-quality coatings of polymers on the magnetic particles, which can easily assemble a lot of gold nanoparticles as the seeds for further formation of the gold shell. The shell is composed of a large number of irregular nanoscale gold particles arranged randomly, and there were well-defined boundaries formed between these gold nanoparticles. It is shown that the Fe3O4 core/Au shell particles can be used as substrates for SERS because of the strong localized electromagnetic field produced by the highly rough gold surface. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grants 20675076 and 20820102037). References and Notes

Figure 7. (a) SERS spectra of 4-ATP on Fe3O4 core/Au shell nanoparticles. (b) Normal Raman spectrum of 4-ATP in a solid sample.

on the Fe3O4 core/Au shell particles (Figure 7a) and the normal Raman spectrum of solid 4-ATP (Figure 7b), which indicate that the 4-ATP can effectively assemble onto the Fe3O4 core/ Au shell particles via standard thiol chemistry. Compared to the spectrum of the solid 4-ATP, the noticeable differences in the FT-SERS spectrum on these Fe3O4 core/Au shell particles are the frequency shift and some changes in the relative intensity for most of the bands. Comparing part b and a of Figure 7, the C-S band shifts from 1085.7 to 1077.9 cm-1 and the C-C band shifts from 1590.9 to 1583.2 cm-1. These changes indicate

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